Advancements in Metal-Ion Capacitors: Bridging Energy and Power Density for Next-Generation Energy Storage

Abstract

Metal-ion capacitors (MICs) have emerged as advanced hybrid energy storage devices that combine the high energy density of batteries with the superior power density and long cycle life of supercapacitors. By leveraging a unique configuration of faradaic and non-faradaic energy storage mechanisms, MICs offer a balanced performance that meets the diverse requirements of modern applications, including renewable energy systems, electric vehicles, and portable electronics. MICs employ diverse ions such as lithium, sodium, and potassium, which provide flexibility in material selection, scalability, and cost-effectiveness. For instance, lithium-ion capacitors (LICs) excel in compact and high-performance applications, while sodium-ion (NICs) and potassium-ion capacitors (KICs) provide sustainable and affordable solutions for large-scale energy storage. This review highlights the advancements in electrode materials, including carbon-based materials, transition metal oxides, and emerging candidates like MXenes and metal–organic frameworks (MOFs), which enhance MIC performance. The role of electrolytes, ranging from organic and aqueous to hybrid and solid-state systems, is also examined, emphasizing their influence on energy density, safety, and operating voltage. Additionally, the article discusses the environmental and economic benefits of MICs, including the use of earth-abundant materials and bio-derived carbons, which align with global sustainability goals. The review concludes with an analysis of practical applications, commercialization challenges, and future research directions, including AI-driven material discovery and integration into decentralized energy systems. As versatile and transformative energy storage devices, MICs are poised to play a critical role in advancing sustainable and efficient energy solutions for the future.

1. Introduction

Energy storage technologies are foundational to the advancement of modern societies, underpinning innovations in renewable energy integration, the electrification of transportation, portable electronics, and smart grids [1,2,3]. Within this spectrum, the search for efficient, reliable, and scalable energy storage systems continues to intensify. While traditional lithium-ion batteries (LIBs) dominate the market due to their high energy density, they often face limitations in power density, cycle life, and safety concerns. On the other hand, supercapacitors provide exceptional power density and prolonged cycle life but suffer from insufficient energy density. This dichotomy has catalyzed the development of hybrid energy storage systems, particularly metal-ion capacitors (MICs), which aim to synergize the advantages of batteries and capacitors, thereby bridging the gap between these two dominant technologies [3,4,5,6]. Metal-ion capacitors represent a novel category of hybrid electrochemical energy storage systems that utilize a battery-like faradaic electrode (e.g., lithium-ion intercalation material) paired with a capacitor-like electrode (e.g., carbon-based material). This hybrid configuration enables MICs to achieve a balance between energy density and power density while maintaining excellent stability and scalability [7,8,9]. Their working principle involves the reversible electrochemical storage of ions, typically lithium, sodium, or potassium, within the electrodes, facilitating fast charging, high-capacity retention, and competitive cycle life. Recent advances in materials science, electrode architecture, electrolyte formulations, and device engineering have propelled the performance of MICs, placing them at the forefront of next-generation energy storage technologies [10,11,12]. The significance of high-performance MICs extends beyond technical capabilities. As society transitions towards greener technologies, MICs hold the potential to support renewable energy systems, provide robust solutions for electric vehicles (EVs), and enable efficient energy storage in portable and flexible electronic devices. The rapid surge in demand for such applications has spurred researchers to explore diverse configurations of MICs, including lithium-ion capacitors (LICs), sodium-ion capacitors (NICs), and emerging potassium-ion capacitors (KICs). These systems leverage the inherent advantages of specific ions while addressing the challenges of material availability, environmental impact, and cost-effectiveness. Energy storage has become a pivotal component of the global push toward decarbonization and electrification. From stabilizing intermittent renewable energy sources like solar and wind to powering the growing fleet of EVs, energy storage technologies must meet increasingly stringent performance requirements. Batteries, particularly LIBs, have dominated the energy storage landscape for decades due to their ability to deliver high energy densities. However, LIBs face critical challenges, including limited charging speeds, reduced performance at high power output, and safety concerns arising from thermal runaway and flammability of organic electrolytes. Conversely, supercapacitors have proven to be indispensable in applications demanding rapid energy delivery, such as regenerative braking in vehicles and power backup systems. Despite their superior power density, supercapacitors struggle to meet the energy density demands of modern applications, making them unsuitable for standalone energy storage in most cases. The gap between these two technologies creates an opportunity for hybrid systems like MICs, which combine the energy-storage mechanisms of both faradaic reactions and electrostatic adsorption.

MICs are particularly advantageous because they capitalize on the fast ion kinetics of supercapacitors while leveraging the high-capacity retention mechanisms of batteries. This dual mechanism not only enhances the overall energy and power output of MICs but also prolongs their cycle life, making them an attractive option for applications requiring high-performance energy storage [13,14,15]. Moreover, the incorporation of diverse metal-ion chemistries, such as lithium, sodium, and potassium, further broadens the applicability and sustainability of MICs, particularly when considering resource availability and cost constraints. Metal-ion capacitors operate on a hybrid mechanism that combines faradaic (battery-like) and non-faradaic (capacitor-like) energy storage processes. Typically, the negative electrode (anode) is a faradaic material capable of ion intercalation or adsorption, while the positive electrode (cathode) is a capacitor-like material, such as activated carbon or graphene, that stores charge electrostatically [16,17,18]. During operation, ions from the electrolyte shuttle between the two electrodes, contributing to charge storage through fast surface reactions and intercalation processes. This mechanism enables MICs to achieve higher energy densities than supercapacitors while maintaining higher power densities and longer lifetimes than batteries. Metal-ion capacitors (MICs) rely on three critical components: electrodes, electrolytes, and device architectures, each contributing significantly to their performance and applicability. Advanced electrode materials, including porous carbons, transition metal oxides, and metal–organic frameworks (MOFs), play a vital role in improving energy and power densities by optimizing ion diffusion, conductivity, and charge storage capacity. Electrolytes are equally essential, ensuring efficient ion transport and stability. Both organic and aqueous electrolytes have been widely explored, with ionic liquids and solid-state electrolytes emerging as promising options for enhancing MIC safety, voltage range, and efficiency. Device architecture innovations, such as flexible and all-solid-state MICs, are further advancing their capabilities, enabling integration into diverse platforms like wearable electronics and grid storage systems. MICs offer several key advantages, including high energy density, achieved through the incorporation of battery-like electrodes that store significantly more energy than traditional supercapacitors [19,20,21]. Their capacitor-like electrodes facilitate rapid charging and discharging, making them ideal for high-power applications. Moreover, their hybrid nature ensures long cycle life, with excellent stability over thousands of charge–discharge cycles. Scalability has also improved, with advancements in material synthesis and manufacturing enabling MICs to be deployed in large-scale applications [22,23,24].

Recent technological advancements in MICs, driven by breakthroughs in materials science, nanotechnology, and electrochemistry, have further boosted their performance. For instance, porous carbon materials in cathodes offer higher surface areas and conductivity, while titanium-based oxides and hard carbons as anodes enhance ion storage capacity and cycling stability. Together, these developments position MICs as highly promising energy storage solutions for a wide range of applications. Lithium-Ion Capacitors (LICs): LICs are among the most extensively studied MICs, leveraging the high energy density of lithium-ion chemistries. Challenges in LIC development include improving the kinetics of lithium-ion diffusion and mitigating side reactions that degrade performance over time. Sodium-Ion Capacitors (NICs): With sodium being more abundant and cost-effective than lithium, NICs are gaining traction for large-scale energy storage. Recent studies have focused on enhancing the intercalation properties of sodium-compatible anodes and designing stable sodium electrolytes. Potassium-Ion Capacitors (KICs): KICs represent a newer frontier in MIC research, offering low operating voltages and high energy densities. However, the larger ionic radius of potassium poses challenges for electrode materials, necessitating innovative design approaches.

This review provides a comprehensive overview of the advanced studies conducted on high-performance MICs, focusing on their technologies, systems, and applications [25,26,27]. The following sections delve into the foundational principles of MICs, the latest advancements in electrode materials and electrolytes, emerging trends in device design, and their role in practical applications. This introduction highlights the relevance and urgency of pursuing research in MICs by framing their role in the broader landscape of energy storage and identifying key challenges that need to be addressed for their widespread adoption [28,29,30].

2. Applications of High-Performance Metal-Ion Capacitors

Metal-ion capacitors (MICs) are highly versatile energy storage devices, making them suitable for a wide range of applications due to their ability to deliver rapid energy bursts and maintain stability over extended use cycles (Figure 1). In the transportation sector, MICs play a critical role in electric vehicles (EVs) by supporting fast-charging infrastructure, regenerative braking systems, and hybrid powertrains, where the combination of high energy and power density is essential [31,32]. In grid energy storage, MICs are being explored to address load balancing and frequency regulation, particularly in renewable energy systems, where they help mitigate challenges associated with intermittent energy generation. Their miniaturization and flexibility also make MICs ideal for portable and wearable electronics, such as medical wearables and IoT sensors, where lightweight and adaptable designs are critical. Furthermore, MICs are valuable in high-performance industrial applications, providing rapid energy delivery and ensuring durability in environments that demand reliability, such as industrial machinery and robotics [33,34,35]. Despite their advantages, MICs face several challenges that need to be addressed to realize their full potential. Developing electrode and electrolyte materials with enhanced conductivity, capacity, and stability remains a priority, as material limitations continue to impact overall performance [36,37,38]. Cost and scalability are also significant hurdles, as the large-scale production of advanced materials, particularly for lithium-ion capacitors (LICs) and potassium-ion capacitors (KICs), must become more economically viable. Additionally, the sustainability of raw material sourcing and the environmental impact of production are key considerations, especially for lithium-based systems. Optimizing the balance between energy density, power density, and cycle life requires further advancements in device architectures and electrolyte formulations.

Emerging research offers promising solutions to these challenges. Solid-state MICs, 3D-printed electrodes, and machine-learning-assisted material discovery are paving the way for enhanced performance and cost reduction. Exploring alternative ions, such as magnesium and zinc, could expand the scope of MIC applications while alleviating concerns about resource availability and environmental impact. These advancements collectively position MICs as a transformative technology for the future of energy storage [39,40,41].

3. Requirement for the Metal-Ion Capacitor

The performance of metal-ion capacitors (MICs) depends heavily on the properties of the materials used in their electrodes, electrolytes, and device architecture. These components must meet stringent requirements to ensure high energy density, power density, cycling stability, and safety [42,43,44]. Electrode materials play a vital role in this performance, with the positive electrode typically relying on high-surface-area carbon-based materials like activated carbon or graphene for effective charge storage through electric double-layer capacitance (EDLC). To achieve optimal performance, these materials must offer properties such as a high specific surface area (>2000 m²/g), excellent electrical conductivity (>10³ S/m), and thermal and chemical stability to resist degradation under extreme conditions. Proper pore size distribution, with mesopores (2–50 nm) and micropores (<2 nm), ensures rapid ion diffusion and efficient charge storage. On the other hand, the negative electrode must support faradaic ion storage through materials like hard carbon, graphite, or transition metal oxides. These materials need high specific capacities (300–1000 mAh/g), fast ion diffusion coefficients, minimal volume expansion during cycling (<5%), and long-term stability over thousands of cycles. Electrolytes are equally crucial for facilitating ion transport and ensuring the stability of MICs. Properties like a wide electrochemical stability window (>4.5 V for organic and ~3 V for aqueous systems), high ionic conductivity (>10⁻³ S/cm), and thermal stability (−20 °C to 60 °C) are essential for efficient operation. Solid-state and ionic liquid electrolytes are emerging as promising options, offering non-flammability, low volatility, and compatibility with high-voltage systems, although their cost and viscosity require further optimization. The separator, which prevents short circuits, must also support high ionic conductivity, exhibit thermal and mechanical robustness, and maintain chemical compatibility with the electrodes. Thin and porous separators (~10–25 μm thick with 30–60% porosity) are ideal for reducing internal resistance while supporting fast charge–discharge cycles. General material requirements for MICs extend beyond individual components [45,46]. Achieving high energy and power densities requires materials with fast ion kinetics and low internal resistance. Long cycle life and low self-discharge rates are essential for reliable and consistent performance over time. Sustainability is another critical factor, with a focus on using earth-abundant materials like sodium and potassium and integrating bio-derived or recyclable components [47,48]. Safety features, such as non-flammable electrolytes and high thermal tolerance, are particularly important for applications in extreme or high-temperature environments. The specific requirements for each ion chemistry add another layer of complexity. Lithium-ion capacitors (LICs) demand materials with high lithium diffusion coefficients (>10⁻⁹ cm²/s) and thermal stability to prevent dendrite formation. Sodium-ion capacitors (NICs) must accommodate the larger sodium ion radius (1.02 Å) while maintaining structural and electrochemical stability. Potassium-ion capacitors (KICs) face similar challenges, with an even larger ionic radius (1.38 Å), but benefit from faster kinetics in suitable electrolytes. Emerging material trends further enhance MICs’ capabilities. Carbon-based materials, such as graphene and carbon nanotubes (CNTs), offer exceptional conductivity and flexibility, while transition metal oxides (e.g., TiO₂, MnO₂) and carbides like MXenes provide high specific capacities through faradaic reactions. Metal–organic frameworks (MOFs), known for their high porosity and tunability, are also being explored for ion storage and transport. Solid-state electrolytes and ionic liquids are advancing as safer and more stable alternatives to conventional systems [49,50].

The electrochemical performance of metal-ion capacitors (MICs) is strongly influenced by the pore characteristics and porous structures of electrode materials, as they directly impact ion transport, charge storage, and overall energy storage efficiency. Micropores (<2 nm) primarily contribute to electric double-layer capacitance (EDLC) by increasing the accessible surface area for charge accumulation, whereas mesopores (2–50 nm) facilitate rapid ion diffusion, reducing internal resistance and enhancing rate capability. Macropores (>50 nm) function as electrolyte reservoirs, improving ionic conductivity and ensuring efficient charge–discharge kinetics, particularly in high-power applications. Optimizing the hierarchical porosity of carbon-based and composite electrode materials enhances both energy and power density by balancing ion accessibility and charge storage capacity. Recent studies have demonstrated that tailoring the pore size distribution and connectivity can significantly improve the electrochemical stability and cyclability of MICs. Therefore, understanding and optimizing the interplay between micropores, mesopores, and macropores is essential for developing high-performance MICs with improved efficiency, stability, and scalability.

The charging mechanism of metal-ion supercapacitors (MISCs) is a hybrid process that combines electric double-layer capacitance (EDLC) and faradaic charge storage, enabling enhanced energy density compared to conventional supercapacitors. During the charging process, the positive electrode, typically composed of porous carbon materials, stores charge through EDLC, where electrolyte ions adsorb onto the electrode surface without undergoing chemical reactions. Simultaneously, the negative electrode, which consists of battery-type materials such as hard carbon, transition metal oxides, or alloy-based compounds, undergoes a faradaic process, where metal ions (e.g., Li⁺, Na⁺, K⁺, Zn²⁺, or Mg²⁺) intercalate into the electrode structure via redox reactions. This dual mechanism enables MISCs to achieve a higher capacitance and energy density than pure EDLC-based supercapacitors while maintaining superior power output compared to traditional batteries. The choice of electrode materials, electrolyte composition, and ion size influences the kinetics of ion adsorption, diffusion, and intercalation, ultimately affecting the charge–discharge rate and overall performance. Optimizing the electrode–electrolyte interface and tuning the porosity of materials can further enhance ion transport, ensuring efficient energy storage and prolonged cycling stability in MISCs.

Metal-ion supercapacitors (MICs) offer significant advantages and unique innovations over traditional carbon-based supercapacitors (SCs) and other types of SCs by integrating faradaic charge storage with electric double-layer capacitance (EDLC), resulting in superior energy and power characteristics. Unlike conventional carbon-based SCs, which rely solely on EDLC mechanisms and exhibit limited energy density (typically 5–10 Wh/kg), MICs achieve much higher energy densities (50–200 Wh/kg) by incorporating battery-like electrodes that enable reversible ion intercalation or redox reactions. This hybrid charge storage mechanism allows MICs to bridge the gap between supercapacitors and batteries, offering higher energy retention without significantly compromising power density. Additionally, MICs demonstrate longer cycle life and greater stability compared to pseudocapacitive SCs, such as transition metal oxide- or conducting polymer-based SCs, which suffer from structural degradation over extended cycling. Another key advantage of MICs is their adaptability to a variety of metal ions, including lithium, sodium, potassium, zinc, and magnesium, allowing for tailored electrochemical properties based on cost, availability, and sustainability considerations. Moreover, MICs can utilize environmentally friendly aqueous, organic, or solid-state electrolytes, enhancing their safety and application flexibility. These features make MICs highly attractive for applications requiring both fast energy delivery and long-term energy storage, such as electric vehicles, grid stabilization, and portable electronics, offering a compelling alternative to traditional SCs while maintaining scalability and cost-effectiveness.

Future research must focus on optimizing the electrode–electrolyte interface to reduce resistance and improve stability, exploring high-entropy materials for their multifunctional properties, and leveraging 3D nanostructures to enhance surface area and ion accessibility. Sustainability remains a priority, with bio-derived carbons and recyclable materials gaining traction as eco-friendly solutions. Together, these advancements will enable MICs to achieve superior performance and expand their applications in a sustainable and cost-effective manner.

4. Lithium-Ion Capacitors for the Potential Applications

Energy storage systems are fundamental to advancing modern technology, driving innovation in electric vehicles (EVs), portable electronics, and renewable energy systems. However, traditional energy storage technologies, such as lithium-ion batteries (LIBs) and supercapacitors, face distinct limitations. LIBs provide high energy density but lack fast-charging capability and long cycle life, while supercapacitors excel in power density and durability but offer insufficient energy storage. This performance gap has paved the way for hybrid energy storage solutions, particularly lithium-ion capacitors (LICs), which combine the strengths of both technologies [51,52]. A lithium-ion capacitor is a hybrid electrochemical energy storage device that features a capacitor-like positive electrode and a battery-like negative electrode. The positive electrode typically consists of high-surface-area activated carbon, which stores charge through electric double-layer capacitance (EDLC), while the negative electrode employs a lithium-ion intercalation material such as graphite or hard carbon. This unique configuration enables LICs to deliver high energy density, rapid charge–discharge cycles, and excellent cycling stability. Moreover, LICs can operate across a wide voltage range, making them a versatile and efficient solution for various applications. As the global demand for reliable, fast, and efficient energy storage grows, LICs have emerged as a promising alternative to bridge the gap between existing technologies. Their ability to provide high power density and long-term stability has positioned LICs as a key enabler for sustainable energy systems.

Li-ion hybrid capacitors (LIHCs) combine the energy-storage capabilities of redox-type anodes with the high-power output of double-layer cathodes, offering a synergistic blend of the advantages seen in conventional Li-ion batteries and supercapacitors. However, the widespread application of LIHCs is hindered by the sluggish kinetics of Li-ion storage in most battery-type anode materials, which limits their performance at high charge/discharge rates. To address this, vanadium nitride (VN) nanowires are explored for their pseudocapacitive Li-ion storage behavior. This property is further enhanced by constructing a 3D porous architecture with reduced graphene oxide (RGO), which improves both the conductivity and the accessible surface area for Li-ion interaction. The resulting 3D VN–RGO composite demonstrates excellent Li-ion storage capacity, fast charge/discharge rates, and stable performance over a wide potential window of 0.01–3 V (vs. Li/Li⁺). This combination has the potential to significantly enhance the operating potential, energy density, and power density of LIHCs. Furthermore, when paired with porous carbon nanorods featuring a high surface area of 3343 m² g⁻¹ as the cathode, the 3D VN–RGO composite anode enables the fabrication of a novel LIHC. This device achieves an impressive energy density of 162 Wh kg⁻¹ at a power density of 200 W kg⁻¹, while maintaining 64 Wh kg⁻¹ even at a high-power density of 10 kW kg⁻¹, highlighting its excellent performance at both low and high charge/discharge rates. (Figure 2i,a–f)

Lithium-ion capacitors (LICs) are emerging as promising hybrid energy storage devices that effectively combine the high-energy density of lithium-ion batteries with the high-power performance of capacitors. However, their practical application is hindered by the mismatch in charge storage kinetics between the intercalation anode and the capacitive cathode, which limits energy storage capabilities at high charge/discharge rates. To address this challenge, this work presents a rational design of nanostructured LIC electrodes that predominantly exhibit capacitive charge storage mechanisms (encompassing both double-layer and pseudocapacitive behaviors) while minimizing intercalation-based processes. The proposed electrode design features a 3D interconnected TiC nanoparticle chain anode, fabricated through the carbothermal conversion of graphene/TiO₂ hybrid aerogels, paired with a pyridine-derived hierarchical porous nitrogen-doped carbon (PHPNC) cathode (Figure 3i,ii). Detailed electrochemical characterization of the individual electrodes reveals exceptional high-rate performance, validating their suitability for LIC applications (Figure 3iii–v). When assembled into a full LIC device, the PHPNC//TiC configuration achieves remarkable performance metrics (Figure 3A–C), including an energy density of 101.5 Wh kg⁻¹, a power density of 67.5 kW kg⁻¹ (at 23.4 Wh kg⁻¹) (Figure 3D), and impressive cycle stability with ~82% capacity retention over 5000 cycles in a voltage range of 0.0–4.5 V (Figure 3E). These results highlight the effectiveness of the nanostructured design in bridging the energy-power tradeoff in LICs.

The proposed electrode design features a 3D interconnected TiC nanoparticle chain anode, fabricated through the carbothermal conversion of graphene/TiO₂ hybrid aerogels, paired with a pyridine-derived hierarchical porous nitrogen-doped carbon (PHPNC) cathode. Detailed electrochemical characterization of the individual electrodes reveals exceptional high-rate performance, validating their suitability for LIC applications. When assembled into a full LIC device, the PHPNC//TiC configuration achieves remarkable performance metrics, including an energy density of 101.5 Wh kg⁻¹, a power density of 67.5 kW kg⁻¹ (at 23.4 Wh kg⁻¹), and impressive cycle stability with ~82% capacity retention over 5000 cycles in a voltage range of 0.0–4.5 V. These results highlight the effectiveness of the nanostructured design in bridg. (i) Schematic of the formation process of 3D interconnected TiC nanoparticle chains. (ii) Illustration of the assembled LIC structure. (iii) CV curves at different scan rates. (iv) Capacitances at varying current densities. (v) Cycling performance of the LIC (0.0–4.5 V in organic electrolyte). Insets: charge/discharge peak current variation with scan rate and charging/discharging curves at different current densities. Electrochemical properties of the TiC electrode in a Li half-cell (0.005–3 V vs. Li): (A) CV curves at varying scan rates, (B) charge/discharge peak current vs. scan rate (0.2–1 mV s⁻¹), (C) voltammetric response at 0.5 mV s⁻¹ with capacitive contribution highlighted, (D) charging/discharging curves at 0.1 A g⁻¹, and (E) rate performance at different current densities in the energy-power tradeoff in LICs.

Lithium-ion capacitors (LICs) represent a cutting-edge category within metal-ion capacitors (MICs), integrating the ultrafast charging and high-power output of supercapacitors with the superior energy storage capability of lithium-ion batteries (LIBs). This hybrid design features a capacitor-like positive electrode, commonly composed of high-surface-area activated carbon, paired with a battery-type negative electrode, such as graphite or lithium titanate, allowing LICs to achieve remarkable electrochemical performance. However, unlocking the full potential of LICs necessitates overcoming several technical challenges. One of the most pressing issues is enhancing energy density, as the limited charge storage capacity of the positive electrode remains a bottleneck. Advancing LIC technology requires the development of next-generation carbon-based materials with significantly increased surface area, precisely engineered pore structures, and enhanced electrochemical reactivity to maximize charge storage and ion transport efficiency. On the negative electrode side, alternative materials like silicon-based composites and metal oxides with higher specific capacities than graphite offer exciting possibilities for improving lithium storage. Alongside energy density, increasing power density and charging speed is crucial for LICs. This can be achieved by designing 3D porous electrode architectures and nanostructured composites that minimize ion diffusion paths while maximizing charge transfer kinetics. Additionally, conductive additives and electrode coatings can further reduce internal resistance, allowing for faster energy exchange.

Electrolyte development plays a pivotal role in ensuring operational stability, voltage range, and safety for LICs. Future electrolytes must offer wide electrochemical stability windows (>4.5 V) to support higher energy densities. Ionic liquids and hybrid organic electrolytes are promising candidates, given their non-volatility and thermal stability. Solid-state electrolytes, which mitigate leakage risks and thermal runaway, represent another exciting direction for enhanced safety and durability. Similarly, improving the cycle life of LICs will require strategies like surface engineering of electrodes to prevent capacity loss due to lithium plating or dendrite formation, as well as optimizing the electrode–electrolyte interface to reduce side reactions and ensure consistent ion transport. Sustainability and affordability are also critical to the widespread adoption of LICs. The scarcity and rising cost of lithium necessitate exploration of lithium-free or reduced-lithium variants, such as hybrid systems integrating sodium-ion or potassium-ion technologies. Incorporating bio-derived carbons and recyclable materials into LIC designs can further enhance their environmental appeal while reducing production costs. Design innovations will also broaden the applicability of LICs, from flexible LICs for wearable electronics to high-capacity modules for grid-scale energy storage. Advanced manufacturing techniques like 3D printing and roll-to-roll production will enable scalable and customizable LIC designs to meet diverse application demands.

LICs are particularly well-suited for renewable energy systems, where their ability to rapidly store and release energy can stabilize grids and buffer intermittent sources like solar and wind. This capability also positions LICs as essential components in microgrids and decentralized energy systems. Beyond renewable energy, their unique combination of performance metrics makes LICs versatile for applications like electric vehicles, where they can handle high-power demands for acceleration and regenerative braking, and wearable electronics, where lightweight and flexible designs are critical. LICs’ stability under extreme conditions also makes them suitable for aerospace and satellite systems. Despite their potential, LICs face challenges related to material stability at high voltages, cost reduction for lithium-compatible components, and scalability. Ensuring thermal safety, especially for high-energy applications, remains a priority. Leveraging advanced tools like AI and machine learning for material design and performance modeling can accelerate innovation in this field, while experimental validation will remain key to commercialization. Lithium-ion capacitors hold tremendous promise as a hybrid energy storage solution that bridges the gap between batteries and supercapacitors. With continued advancements in materials, electrolytes, and device design, LICs are poised to become a cornerstone of next-generation energy storage systems. Their versatility and performance potential make them ideal for applications ranging from portable electronics to renewable energy storage, paving the way for a sustainable and efficient energy future.

5. Sodium Ion Capacitors

As the global demand for energy storage solutions grows, there is an urgent need for technologies that are efficient, scalable, and sustainable. While lithium-ion batteries and lithium-ion capacitors dominate the market, concerns over the scarcity and cost of lithium have driven research into alternative technologies. Among these, sodium-ion capacitors (NICs) have emerged as a promising option. By combining the high energy density of sodium-ion batteries with the high-power density of supercapacitors, NICs offer a hybrid energy storage solution that leverages the abundance and cost-effectiveness of sodium. A sodium-ion capacitor employs a dual-electrode configuration, with a capacitor-like positive electrode and a battery-like negative electrode. The positive electrode typically consists of porous carbon materials that store charge through electric double-layer capacitance (EDLC), while the negative electrode stores sodium ions through faradaic processes such as intercalation or adsorption. This hybrid mechanism enables NICs to achieve higher energy density than supercapacitors and greater power density than sodium-ion batteries. Furthermore, the use of sodium, which is approximately 1000 times more abundant than lithium, significantly reduces the environmental and economic costs of these devices. Sodium-ion capacitors are particularly well-suited for applications requiring both rapid charge–discharge cycles and long-term reliability, such as renewable energy systems and grid storage. Their ability to operate over a wide voltage range and maintain stability under various conditions makes them a versatile and scalable energy storage technology [53,54].

Sodium-ion capacitors (NICs) have emerged as a promising energy storage solution, leveraging the natural abundance and cost-effectiveness of sodium to develop more accessible and sustainable technologies. Unlike lithium-based systems, which face resource limitations and supply chain constraints, NICs utilize widely available materials, making them an attractive option for large-scale applications. Their unique electrochemical properties allow them to operate efficiently across various energy demands, from stabilizing power grids to enhancing the performance of fast-response industrial systems. NICs exhibit a well-balanced combination of high energy storage capacity and rapid power delivery, making them particularly useful in scenarios requiring both quick charge–discharge cycles and extended operational stability. Their integration into renewable energy infrastructure strengthens grid reliability by compensating for fluctuations in energy generation, while their durability and safety profile make them a competitive choice for emerging mobility and smart energy applications. Additionally, advancements in electrode materials and electrolyte formulations continue to improve their efficiency, cycle life, and environmental impact, further positioning NICs as a key player in the transition toward more resilient and eco-friendly energy storage solutions. Unlike lithium-ion systems, which often involve energy-intensive mining and processing, NICs rely on more sustainable materials, including sodium-based compounds and bio-derived carbons. Additionally, the ability to use aqueous electrolytes in NICs reduces toxicity concerns, making them safer and more environmentally friendly. The sodium-ion capacitors are an essential innovation for sustainable energy storage. Their affordability, versatility, and performance make them a key enabler for the transition to renewable energy systems, smart grids, and decentralized energy solutions, contributing to a cleaner and more sustainable future.

Metal ion capacitors store energy by facilitating the intercalation of cations into an electrode at rates faster than those observed in batteries, while also operating within a wider potential window. This mechanism enables them to achieve a higher energy density compared to electrochemical double-layer capacitors. While Li-ion capacitors are commercially available, the development of Na-ion capacitors remains limited due to a shortage of materials capable of enabling fast and efficient Na-ion intercalation. The electrochemical behavior of 2D vanadium carbide (V₂C), a member of the MXene family, is a potential electrode material for Na-ion capacitors (Figure 4i,ii). Using X-ray diffraction (XRD), we analyzed the Na-ion intercalation mechanism, which revealed promising electrochemical performance with a capacitance of approximately 100 F/g at a scan rate of 0.2 mV/s. Furthermore, a full-cell Na-ion capacitor was assembled, utilizing hard carbon—a well-known anode material for Na-ion batteries—as the negative electrode. The device exhibited a capacity of 50 mAh/g and operated at a maximum cell voltage of 3.5 V, demonstrating the potential of V₂C as a promising material for advancing Na-ion capacitor technology (Figure 4a–d).

The simultaneous integration of high energy output and high-power delivery remains a critical challenge in the development of electrochemical energy storage systems, as most devices struggle to achieve both attributes simultaneously. A quasi-solid-state sodium-ion capacitor (NIC) addresses this challenge by combining a battery-type urchin-like Na₂Ti₃O₇ (NTO) anode with a capacitor-type peanut-shell-derived carbon (PSC) cathode (Figure 5i–iii). Utilizing a sodium-ion-conducting gel polymer electrolyte, the device demonstrates exceptional performance, achieving both high energy and high-power densities. The NIC delivers an energy density of up to 111.2 Wh kg⁻¹ at a power density of 800 W kg⁻¹, while maintaining an impressive 33.2 Wh kg⁻¹ at a significantly higher power density of 11,200 W kg⁻¹ placing it among the best-performing NICs reported to date. The device also exhibits excellent cycling stability, retaining ~86% of its initial capacity over 3000 cycles, indicating its robustness for long-term operation (Figure 5a–f). Moreover, the successful assembly of a flexible quasi-solid-state NIC highlights its practical applicability, as the device shows no significant capacity loss under various bending conditions. These results demonstrate the potential of this design in advancing high-performance, flexible, and durable NICs for next-generation energy storage systems.

Sodium-ion capacitors (NICs) have gained considerable attention as sustainable and cost-effective alternatives to lithium-based systems, particularly for large-scale energy storage applications. By combining the fast-charging capabilities and high-power density of supercapacitors with the moderate energy density of sodium-ion batteries, NICs present a hybrid energy storage solution capable of addressing the growing global demand for scalable and efficient energy storage technologies. Their reliance on sodium, an earth-abundant and inexpensive element, makes NICs particularly appealing for applications where affordability and resource availability are critical. However, realizing their full potential requires overcoming several technical and material challenges. One of the major priorities for future research in NICs is improving energy density, which remains a limiting factor compared to lithium-ion capacitors. The development of advanced electrode materials is central to this goal. For the positive electrode, the focus lies on optimizing porous carbon materials to enhance specific surface area, pore structure, and electrochemical activity. Functionalized carbons and graphene-based composites are being explored for their ability to facilitate faster sodium-ion adsorption and storage. For the negative electrode, hard carbons have shown promise due to their ability to reversibly store sodium ions; however, advancements in alternative materials such as layered metal oxides, organic electrode materials, and alloy-based anodes could provide higher specific capacities and improved cycling stability. Enhancing power density and charge–discharge rates is another critical area of focus for NICs. Sodium ions are larger in size compared to lithium ions, which can result in slower ion diffusion and limited charge transfer kinetics. To address this, future electrode designs must incorporate nanostructured architectures that minimize ion diffusion paths and maximize the active surface area. Strategies such as integrating conductive nanomaterials, creating 3D porous structures, or synthesizing hybrid composites can significantly improve sodium-ion transport while maintaining electrode integrity. Furthermore, reducing internal resistance through better electrode–electrolyte compatibility will enable NICs to meet the rapid energy delivery requirements of applications such as grid storage and electric transportation.

Cycling stability and long-term performance are critical for the adoption of NICs in large-scale applications. Sodium ions can induce greater structural changes in electrode materials compared to lithium ions, leading to capacity fading over repeated cycles. To mitigate this, surface engineering techniques such as coating or doping electrode materials can be employed to stabilize structural integrity during charge–discharge processes. Advanced binders and additives that improve mechanical resilience and enhance electrode conductivity are also key to extending the lifespan of NICs. Innovations in device design and architecture will expand the range of applications for NICs. Flexible and lightweight NICs can be tailored for wearable electronics and portable devices, where compact energy storage solutions are needed. Scalable modular designs are ideal for grid-level energy systems, allowing NICs to be integrated into renewable energy networks for balancing supply–demand fluctuations. Advances in manufacturing technologies, such as 3D printing and high-throughput material synthesis, can facilitate the production of NICs with tailored properties and lower costs.

Looking ahead, NICs have significant potential to support the transition to a more sustainable energy infrastructure. Their rapid response capabilities make them suitable for stabilizing renewable energy systems like solar and wind farms, where energy generation is inherently variable. In microgrid systems, NICs can act as reliable energy buffers, ensuring a steady power supply even during fluctuations. Furthermore, their ability to deliver both high power and moderate energy density makes NICs a valuable asset in electrified transportation systems, particularly for applications like regenerative braking or supplementary energy storage in hybrid vehicles. Despite these promising advancements, challenges such as optimizing material performance, addressing scalability issues, and ensuring economic feasibility remain. Collaborative efforts between academia and industry, alongside the application of advanced computational techniques like machine learning for material discovery, will be critical in accelerating the development of NICs. As these challenges are addressed, NICs are poised to become a cornerstone technology in the global push toward sustainable and efficient energy storage solutions. Sodium-ion capacitors offer a compelling combination of affordability, environmental compatibility, and versatility. As ongoing research continues to refine their performance and broaden their application scope, NICs are likely to play a transformative role in supporting clean energy systems, grid stability, and electrified transportation, contributing significantly to a sustainable energy future.

6. Potassium-Ion Capacitors (KICs)

Potassium-ion capacitors (KICs) are gaining significant attention in the field of energy storage due to their unique electrochemical properties and the distinct advantages they offer over traditional lithium-ion and sodium-ion systems. KICs represent a hybrid energy storage technology that combines the high energy density of potassium-ion batteries (KIBs) with the high-power density of supercapacitors. Leveraging the natural abundance and affordability of potassium, KICs offer a sustainable and efficient solution for diverse applications, ranging from portable electronics to grid-scale energy storage. As the demand for energy storage technologies with improved performance, affordability, and environmental compatibility grows, KICs are increasingly recognized as a promising candidate for next-generation energy storage systems. A typical potassium-ion capacitor consists of two distinct electrodes: a capacitor-like positive electrode, often made of porous carbon materials, and a battery-like negative electrode designed to store potassium ions through faradaic processes. During operation, the positive electrode stores charge via electric double-layer capacitance (EDLC), while the negative electrode accommodates potassium ions through intercalation or conversion reactions. This hybrid mechanism allows KICs to balance energy density and power density effectively, making them highly versatile for various applications. One of the key electrochemical properties of potassium is its relatively low standard reduction potential (−2.93 V vs. SHE), which enables higher operating voltages in KICs compared to sodium-ion systems. This higher voltage significantly enhances the energy efficiency of KICs, making them a competitive option in the energy storage domain [55,56].

Potassium-ion capacitors also benefit from potassium’s high ionic conductivity in both liquid and solid electrolytes. While potassium ions have a larger ionic radius (1.38 Å) compared to lithium (0.76 Å) and sodium (1.02 Å), their smaller solvated ionic size in electrolytes and faster transport kinetics partially offset this limitation. This enables KICs to achieve rapid charge–discharge cycles, which are essential for applications requiring immediate energy delivery, such as regenerative braking in electric vehicles or frequency regulation in power grids. Additionally, the larger ionic radius of potassium minimizes the risk of dendrite formation, which is a significant safety concern in lithium-based systems. This feature improves the safety profile of KICs, making them a reliable option for high-performance applications. One of the most notable advantages of KICs is the natural abundance and low cost of potassium. Unlike lithium, which is limited in availability and geographically concentrated, potassium is widely distributed and inexpensive, making it an excellent candidate for scalable and sustainable energy storage. This advantage is particularly significant for grid-scale energy storage and renewable energy integration, where cost-effectiveness is critical. KICs are well-suited for storing and distributing energy from intermittent renewable sources like solar and wind, offering a reliable solution for grid stabilization. Their accessibility and affordability also make them ideal for deployment in developing regions, where cost-effective energy storage solutions are crucial for improving energy access and fostering economic development.

The development of suitable electrode materials is a crucial area of research for advancing KICs. Positive electrodes typically use carbon-based materials such as activated carbon, graphene, or carbon nanotubes, which provide high surface area and conductivity for efficient charge storage. Optimizing pore size distribution and surface functionalization in these materials is essential to enhance ion diffusion and improve energy and power densities. On the negative electrode side, hard carbons are among the most promising materials due to their reversible potassium-ion storage capabilities, structural stability, and low cost. Other potential materials, including transition metal oxides, sulfides, and organic compounds, are being actively investigated for their ability to improve energy density and cycling stability in KICs.

Despite their many advantages, KICs face several challenges that require further research and innovation. The large ionic radius of potassium can lead to structural strain and mechanical instability in electrode materials during repeated cycling, which contributes to capacity fading and reduces cycle life. Addressing these issues requires the development of advanced electrode architectures, such as nanostructured or composite materials, that can accommodate the volume changes associated with potassium-ion intercalation. Additionally, the relatively high atomic weight of potassium compared to lithium or sodium impacts the gravimetric energy density of KICs, necessitating further optimization of material properties and device designs to maximize energy efficiency. Potassium-ion capacitors are poised for a wide range of applications, from large-scale grid energy storage to portable electronics. In grid storage, KICs can stabilize power networks by balancing supply–demand fluctuations and storing excess energy generated by renewable sources. In transportation, KICs support the electrification of vehicles by providing fast-charging capabilities and handling high-power demands for acceleration and regenerative braking. Additionally, their lightweight and long-lasting nature make KICs ideal for powering wearable devices, IoT sensors, and flexible displays.

The sulfur- and nitrogen-codoped 3D porous carbon nanosheets (S-N-PCNs) were developed to address the kinetics mismatch in potassium-ion hybrid capacitors (PIHCs). Structural analyses revealed a 3D porous framework with uniformly distributed sulfur and nitrogen dopants, providing ample structural defects, redox-active sites, and expanded graphite interlayers to enhance potassium-ion storage. Electrochemical evaluations demonstrated superior performance, with S-N-PCNs delivering a high capacity of 107 mAh g⁻¹ at 20 A g⁻¹ and exceptional cycling stability over 3000 cycles. When integrated into a PIHC with an activated carbon cathode, the device exhibited a high energy density of 187 Wh kg⁻¹, a power density of 5136 W kg⁻¹, and 86.4% capacity retention after 3000 cycles (Figure 6a–e). The synergy of heteroatom doping and advanced structural design underpins these results, highlighting the potential of S-N-PCNs for next-generation energy storage systems. Further optimization of synthesis methods, doping strategies, and device configurations could drive additional improvements in performance.

Hybrid potassium-ion capacitors (KICs) offer significant promise for large-scale energy storage applications, particularly for power grids, due to their cost efficiency, the weaker Lewis acidity of K⁺, and the low redox potential of the K⁺/K system. However, designing efficient K⁺ storage materials remains challenging owing to the larger ionic radius and heavier mass of K⁺ compared to Li⁺ and Na⁺. This study presents a novel strategy for fabricating hierarchical Ca_0.5Ti₂(PO₄)₃@C microspheres using the electro spraying method (Figure 7i). By leveraging the abundant vacancies in the crystal lattice and a carefully engineered nanostructure, the hybrid Ca_0.5Ti₂(PO₄)₃@C electrode demonstrates an impressive reversible capacity (239 mA h g⁻¹) and exceptional rate performance (63 mA h g⁻¹ at 5 A g⁻¹). Furthermore, a KIC incorporating this anode and an activated carbon cathode achieves a high energy density of 80 W h kg⁻¹, a power density of 5144 W kg⁻¹ (in the potential range of 1.0–4.0 V), and an extended cycle life exceeding 4000 cycles (Figure 7a–e). In situ X-ray diffraction analysis reveals a two-phase transition in Ca_0.5Ti₂(PO₄)₃ during initial discharge, followed by solid-solution processes during subsequent K⁺ insertion/extraction. This work underscores the potential of cost-effective potassium-based energy storage systems with outstanding energy/power densities and long-term stability.

The future of KICs is promising, with advancements in materials science, electrolyte chemistry, and device engineering driving their development. As research addresses the challenges of electrode stability, electrolyte optimization, and large-scale manufacturing, KICs are expected to become a key technology in the energy storage landscape. Their combination of affordability, sustainability, and versatility positions KICs as a viable alternative to lithium-based systems, supporting the global transition to cleaner and more efficient energy solutions. With their potential to revolutionize energy storage, potassium-ion capacitors are poised to play a transformative role in renewable energy integration, grid-scale storage, and electrified transportation systems.

Expanding Metal-Ion Capacitors: Zinc, Magnesium, and Aluminum-Ion Capacitors

While lithium, sodium, and potassium-ion capacitors have gained widespread attention in energy storage research, other metal-ion capacitors, particularly zinc-ion capacitors (ZICs), magnesium-ion capacitors (MICs), and aluminum-ion capacitors (AICs), are emerging as viable alternatives due to their abundance, cost-effectiveness, and promising electrochemical properties. These multivalent metal-ion capacitors (MvICs) differ from monovalent systems by offering higher charge storage per ion, potentially leading to increased energy densities [57,58]. Their ability to utilize earth-abundant metals makes them a sustainable and economically attractive solution for large-scale applications such as grid storage, renewable energy integration, and industrial energy systems. However, challenges related to electrode compatibility, electrolyte stability, and ion transport mechanisms must be addressed to unlock their full potential.

Zinc-ion capacitors (ZICs) have garnered significant interest due to the natural abundance, high theoretical capacity, and low toxicity of zinc. Unlike lithium, zinc is widely available and can be sourced at a lower cost, making ZICs an attractive option for sustainable energy storage. Furthermore, zinc ions (Zn²⁺) possess a high charge density, allowing for the storage of more charge per ion, which contributes to improved energy density. ZICs utilize a dual charge-storage mechanism similar to other metal-ion capacitors, with a positive electrode storing charge through electric double-layer capacitance (EDLC) and a negative electrode undergoing faradaic reactions to reversibly intercalate zinc ions. One of the major advantages of ZICs is their compatibility with aqueous electrolytes, which enhances safety by eliminating the risk of flammability associated with organic electrolytes. Aqueous electrolytes also enable fast ion transport, facilitating rapid charge–discharge cycles. Additionally, the relatively low redox potential of Zn/Zn²⁺ (−0.76 V vs. SHE) allows ZICs to operate at moderate voltages, reducing side reactions and improving cycling stability. However, ZICs face several challenges, including zinc dendrite formation, which can lead to short circuits and capacity degradation over prolonged cycling. This issue is particularly problematic for zinc metal anodes, requiring innovative strategies such as surface coatings, alloying, or electrolyte additives to suppress dendrite growth. Another limitation is the relatively slow diffusion kinetics of Zn²⁺ due to its divalent nature, which can hinder power performance. The development of high-performance cathode materials, such as vanadium-based oxides, manganese oxides, and Prussian blue analogs, is a key research area aimed at improving ZIC efficiency. The integration of gel or solid-state electrolytes may further enhance the stability and longevity of these capacitors. ZICs are particularly well-suited for large-scale energy storage applications, including renewable energy grids and backup power systems. Their cost-effectiveness, safety, and scalability make them a promising candidate for applications requiring stable and long-term energy storage solutions.

Magnesium-ion capacitors (MICs) have emerged as another promising multivalent metal-ion capacitor technology due to the abundance, non-toxicity, and high volumetric capacity of magnesium (Mg²⁺). Magnesium has a theoretical capacity of 2205 mAh/cm³, which is significantly higher than lithium, sodium, and potassium, making it an attractive candidate for high-energy-density energy storage systems. Additionally, magnesium does not form dendrites in most electrolytes, eliminating one of the primary safety concerns associated with lithium and zinc-ion systems. MICs operate on a similar hybrid mechanism, with a capacitor-like positive electrode storing charge through EDLC and a battery-like negative electrode enabling Mg²⁺ intercalation. One of the main challenges in MIC development is the sluggish diffusion kinetics of Mg²⁺ ions, which have a strong electrostatic interaction with host materials due to their divalent charge. This results in higher migration barriers and reduced ion mobility, leading to lower power density and slower charge–discharge rates compared to monovalent systems. To overcome these challenges, researchers are exploring a variety of electrode materials with open-framework structures that can facilitate magnesium-ion diffusion. Transition metal oxides, chalcogenides, and organic electrode materials have demonstrated potential as suitable hosts for Mg²⁺ intercalation. In addition, efforts are being made to develop magnesium-compatible electrolytes with high ionic conductivity and wide electrochemical stability windows. Conventional organic electrolytes have shown limited Mg²⁺ transport capabilities, necessitating the exploration of ionic liquids, polymer electrolytes, and hybrid aqueous-organic systems for improved performance. Another key research direction for MICs is the development of high-surface-area carbon-based positive electrodes to enhance power density. Materials such as graphene, carbon nanotubes, and activated carbon have demonstrated excellent charge storage capabilities in MICs, complementing the intercalation-based storage mechanism of the negative electrode. MICs hold significant potential for applications requiring high energy density and long cycle life, such as electric vehicles and industrial power systems. Their inherent safety, dendrite-free nature, and high capacity make them an attractive alternative for next-generation energy storage. However, overcoming their limitations in ion diffusion and electrolyte compatibility will be crucial for commercial viability.

Aluminum-ion capacitors (AICs) represent an emerging class of metal-ion capacitors that leverage the trivalent nature of aluminum (Al³⁺) for enhanced charge storage. Aluminum is one of the most abundant elements in the Earth’s crust, making AICs a highly cost-effective and sustainable energy storage solution. The use of Al³⁺ ions allows AICs to store more charge per unit volume, potentially leading to higher energy densities compared to monovalent and divalent systems. Additionally, aluminum is highly stable and resistant to dendrite formation, ensuring safer and longer-lasting operation. AICs function similarly to other metal-ion capacitors, with a capacitor-like positive electrode and a battery-like negative electrode capable of Al³⁺ intercalation. The challenge with AICs lies in the strong electrostatic interactions between Al³⁺ ions and host materials, which can hinder ion mobility and slow down charge–discharge kinetics. This issue necessitates the use of specially designed electrode materials with open-framework structures or layered architectures to accommodate aluminum-ion transport. Carbon-based materials, such as graphene and porous carbons, have been widely explored as cathodes in AICs due to their high conductivity and stability. On the anode side, materials such as metal oxides, sulfides, and Prussian blue analogs are being investigated to facilitate Al³⁺ intercalation while maintaining structural integrity over repeated cycling. Electrolyte development is another crucial aspect of AIC research. Unlike lithium-ion capacitors, which commonly use organic electrolytes, AICs have shown promising results with ionic liquid and aqueous electrolytes. Chloroaluminate-based ionic liquids, for example, have demonstrated excellent Al³⁺ transport properties, allowing for high-voltage operation and improved energy storage performance. However, optimizing electrolyte viscosity and reducing side reactions remain key challenges that need to be addressed for commercial implementation. The potential applications of AICs span from consumer electronics to industrial energy storage. Their low cost, safety, and high volumetric capacity make them an appealing option for grid energy storage and backup power applications. However, further advancements in electrode design and electrolyte engineering will be necessary to unlock their full potential.

Zinc, magnesium, and aluminum-ion capacitors represent a new frontier in metal-ion capacitor technology, offering distinct advantages in terms of cost, sustainability, and electrochemical performance. While each system faces unique challenges, ongoing research into electrode materials, electrolyte formulations, and device architectures is steadily improving their viability for real-world applications. ZICs, with their aqueous electrolyte compatibility and high charge capacity, are particularly suited for grid-scale energy storage. MICs, known for their high volumetric capacity and dendrite-free operation, hold promise for high-energy-density applications such as electric vehicles. AICs, leveraging the trivalent nature of aluminum, offer a low-cost and sustainable alternative with excellent safety characteristics. Their integration into renewable energy systems, electric mobility, and industrial power applications has the potential to revolutionize how energy is stored and utilized, driving forward a cleaner and more energy-efficient future.

7. Advantages of MICs

Metal-ion capacitors (MICs) stand out as a highly promising hybrid energy storage technology, combining the strengths of batteries and supercapacitors to achieve a unique balance of energy density, power density, and long cycle life [59]. MICs operate on a dual mechanism: the battery-like electrode utilizes faradaic reactions for energy storage, while the supercapacitor-like electrode relies on electric double-layer capacitance (EDLC) for rapid charge and discharge. This hybrid configuration allows MICs to deliver reliable, efficient, and versatile energy storage solutions, making them suitable for a wide range of modern applications. One of the key benefits of MICs is their ability to achieve balanced energy and power densities, which is crucial for addressing diverse performance requirements. With energy densities of 50–200 Wh/kg and power densities of 10–100 kW/kg, MICs surpass traditional supercapacitors while remaining competitive with battery technologies. This makes them ideal for applications requiring both sustained energy output and immediate power delivery, such as renewable energy storage, electric vehicles (EVs), and industrial systems. For instance, in EVs, MICs enhance performance by providing high-power bursts for acceleration and fast energy recovery during regenerative braking. Rapid charging and discharging are another critical advantage of MICs, enabled by the fast ion kinetics in the supercapacitor-like electrode. MICs can be recharged within minutes, a significant improvement over traditional batteries, which often require hours. This capability is especially beneficial for high-demand applications like industrial machinery, robotics, and renewable energy grids, where sudden energy fluctuations need to be managed efficiently. MICs’ responsiveness also supports grid stabilization by addressing energy generation variability from sources like solar and wind.

The lifetime of metal-ion supercapacitors (MISCs) generally surpasses that of batteries while remaining competitive with conventional supercapacitors, though it varies depending on electrode materials, electrolyte stability, and cycling conditions. Traditional carbon-based supercapacitors (SCs), which rely solely on electric double-layer capacitance (EDLC), exhibit excellent cycle stability, often exceeding 100,000 cycles, due to their purely physical charge storage mechanism. In contrast, MISCs, which incorporate faradaic charge storage in their battery-like electrodes, typically achieve 10,000 to 100,000 cycles, depending on the ion chemistry and electrode composition. While lithium-ion capacitors (LICs) and sodium-ion capacitors (NICs) generally offer 10,000–50,000 cycles due to electrode degradation from intercalation and volume expansion, potassium-ion capacitors (PICs) and zinc-ion capacitors (ZICs) face additional challenges related to structural strain and dendrite formation, often resulting in slightly lower cycle life. However, advancements in material engineering, such as nanostructured electrodes, surface coatings, and solid-state electrolytes, are improving the longevity of MISCs, narrowing the gap with EDLC-based SCs while maintaining superior energy density. Durability and long operational life further differentiate MICs from conventional batteries. With cycle lives often exceeding 10,000–100,000 cycles, MICs are designed to provide reliable long-term performance with minimal capacity loss. This durability is largely due to the non-faradaic nature of the supercapacitor-like electrode, which minimizes structural degradation during cycling. As a result, MICs are well-suited for applications like grid energy storage, wearable electronics, and backup power systems, where consistent, maintenance-free operation is essential. The versatility of materials used in MICs enhances their adaptability. Electrode materials such as activated carbon, graphene, and transition metal oxides can be customized to meet specific performance needs. Additionally, MICs accommodate a variety of ions, including lithium, sodium, and potassium, allowing for cost-effective and resource-efficient alternatives to lithium-ion systems. Sodium-ion and potassium-ion capacitors, for example, offer affordable and sustainable solutions by utilizing earth-abundant resources, making MICs a scalable option for large-scale energy storage applications. Safety is another strong suit of MICs. Operating within stable voltage ranges, MICs reduce the risks of thermal runaway often associated with lithium-ion batteries. Aqueous electrolytes, frequently used in MICs, are non-flammable and enhance safety, even under extreme conditions such as high temperatures. This makes MICs ideal for outdoor applications like solar farms and industrial settings where operational stability is critical. In addition to performance benefits, MICs contribute significantly to environmental sustainability. By incorporating bio-derived carbons and recyclable materials, they reduce the environmental impact of energy storage systems. Their compatibility with renewable energy systems makes them a vital component in addressing energy intermittency and transitioning to a sustainable energy ecosystem. MICs’ scalability also supports applications ranging from small-scale IoT devices and wearable electronics to medium-scale drones and EVs, and even large-scale grid energy storage. With continuous advancements in materials, design, and manufacturing processes, MICs are poised to redefine the energy storage landscape. By combining superior performance, affordability, and environmental compatibility, MICs bridge the gap between traditional energy storage technologies and the demands of next-generation applications. These unique capabilities position MICs as a transformative solution for a cleaner, more efficient energy future.

8. Suggestions for Enhancing Metal-Ion Capacitor Research and Development

Metal-ion capacitors (MICs) stand at the forefront of next-generation energy storage technologies, offering a unique combination of high energy density, superior power density, and exceptional cycling stability. However, further advancements in specific areas are essential to fully realize their potential. One key avenue is the development of advanced materials, including high-entropy materials (HEMs), metal–organic frameworks (MOFs), and two-dimensional (2D) materials such as MXenes and transition metal dichalcogenides (TMDs). These materials possess excellent conductivity, tunable properties, and large surface areas, making them ideal candidates for improving the performance and durability of MIC electrodes. Future research should explore these cutting-edge materials to unlock higher energy densities and enhanced ion kinetics in MICs. Another promising direction is the incorporation of lifecycle analysis (LCA) and environmental impact assessments into MIC development. By examining the environmental footprint of MICs from material sourcing to end-of-life disposal, researchers can address sustainability challenges head-on. For instance, a comparative study of the environmental impact of lithium-ion and sodium-ion capacitors could provide valuable insights into their ecological advantages. Additionally, integrating recyclable or bio-derived carbon materials into MIC designs would significantly reduce their environmental impact, aligning the technology with global sustainability goals. Practical implementation remains a critical aspect of MIC development. Case studies of pilot projects, such as their use in renewable energy microgrids or their deployment in electric vehicle subsystems like regenerative braking, could highlight the real-world potential of MICs. Demonstrating their success in large-scale grid stabilization or portable electronics applications would provide further evidence of their versatility and effectiveness. These examples could also identify operational challenges and inform future advancements in MIC technology. The path to commercialization of MICs is not without obstacles, but several strategies can be employed to overcome them. Manufacturing costs, for example, remain a significant barrier, particularly for advanced electrode materials. Adopting scalable production techniques, such as roll-to-roll processing or 3D printing, could help lower costs and improve accessibility. Similarly, regulatory hurdles and market competition from well-established lithium-ion technologies must be addressed through strategic partnerships and incentives for adopting MICs in industrial and consumer sectors. In addition to established applications, MICs hold promise in emerging fields such as wearable medical devices, high-altitude drones, and space exploration. Their flexibility, stability under extreme conditions, and ability to deliver rapid energy bursts make them uniquely suited for these niche applications. By focusing on these specialized use cases, researchers and industries can further differentiate MICs from competing technologies.

The future of metal-ion supercapacitors (MISCs) lies in advancing electrode materials, optimizing electrolytes, and expanding metal-ion chemistries to enhance energy storage performance and sustainability. Innovations in carbon-based materials, transition metal oxides, and metal–organic frameworks (MOFs) will improve charge storage and ion transport, while high-voltage organic, ionic liquid, and solid-state electrolytes will enhance electrochemical stability and safety. Beyond lithium, sodium, potassium, zinc, magnesium, and aluminum-ion supercapacitors are gaining attention as cost-effective, earth-abundant alternatives, with multivalent ions offering greater charge storage potential. Flexible and wearable MISCs will enable next-generation electronics, while hybrid battery-supercapacitor systems will optimize power management in electric vehicles and grid storage. AI-driven material discovery and machine learning will accelerate these advancements, paving the way for more efficient, scalable, and eco-friendly energy storage solutions. The role of artificial intelligence (AI) and machine learning (ML) in advancing MIC development cannot be overstated. These technologies can accelerate material discovery by identifying optimal electrode and electrolyte combinations and predicting long-term performance. Additionally, AI-driven modeling of ion transport mechanisms and device configurations could refine MIC designs, ensuring enhanced efficiency and scalability. Incorporating AI and ML into manufacturing processes could further improve quality control, reduce waste, and optimize production costs. MICs also offer unique advantages when integrated into broader energy systems. For instance, pairing MICs with lithium-ion batteries or hydrogen storage systems in hybrid configurations could maximize the efficiency of energy systems. MICs could manage short-term high-power demands while other technologies provide long-term energy storage, making them an essential component of multi-technology energy grids and electric vehicle energy systems. Finally, a comprehensive roadmap for MIC development would serve as a guide for researchers and industries alike. In the short term, efforts should focus on improving energy density and optimizing electrode–electrolyte compatibility. Medium-term goals could include scaling up production processes and reducing material costs, while long-term objectives might involve integrating MICs into decentralized energy systems and next-generation smart grids. This phased approach would ensure that MICs evolve to meet the growing energy demands of the future while maintaining their competitive edge in the energy storage market. By addressing these research directions, practical challenges, and commercialization strategies, MICs can establish themselves as a transformative energy storage technology. Their ability to balance performance, sustainability, and scalability positions them as a key solution for the energy needs of the 21st century.

9. Conclusions

Metal-ion capacitors (MICs) represent a pivotal advancement in the field of energy storage, offering a unique combination of high energy density, superior power density, long cycle life, and enhanced safety. By bridging the performance gap between batteries and supercapacitors, MICs have emerged as a transformative solution capable of addressing the growing demands of modern energy systems. Their hybrid configuration, combining faradaic and non-faradaic energy storage mechanisms, has unlocked significant improvements in efficiency, scalability, and versatility. This makes MICs ideal for a wide array of applications, ranging from portable electronics to large-scale renewable energy integration. The choice of diverse metal ions, including lithium, sodium, and potassium, further broadens the applicability and sustainability of MICs. Lithium-ion capacitors (LICs) are particularly suitable for applications requiring compact, high-performance systems, while sodium-ion capacitors (NICs) and potassium-ion capacitors (KICs) offer cost-effective and environmentally sustainable alternatives, particularly for large-scale energy storage. These advancements leverage earth-abundant and recyclable materials, aligning MIC technology with the global shift toward sustainability and resource efficiency. Additionally, MICs’ compatibility with renewable energy systems addresses critical challenges such as intermittency, enabling a stable and reliable transition to cleaner energy sources. Despite their promise, the implementation of MICs is not without challenges. Material limitations, such as structural stability under repeated cycling and the need for cost-effective, high-performance electrode materials, remain critical areas of focus. Similarly, advancements in electrolytes, particularly in achieving wider electrochemical stability windows and enhanced safety, are essential for unlocking the full potential of MICs. Addressing these challenges will require a multidisciplinary approach, combining innovations in materials science, electrochemistry, and device engineering. Looking ahead, MICs have the potential to revolutionize energy storage systems. Their ability to provide rapid energy delivery, prolonged durability, and environmental compatibility positions them as a cornerstone of future energy technologies. As research continues to refine their performance and scalability, MICs will play an increasingly central role in supporting the energy needs of modern society, driving innovations in electric mobility, smart grids, and renewable energy storage. Overall, metal-ion capacitors are more than just an incremental improvement in energy storage; they are a transformative technology capable of redefining the boundaries of energy efficiency and sustainability. Their widespread adoption, supported by ongoing research and technological advancements, will significantly contribute to the global pursuit of a greener, more energy-efficient future.