Insights into Enhancing Electrochemical Performance of Li-Ion Battery Anodes via Polymer Coating

Abstract

Due to the ever-growing importance of rechargeable lithium-ion batteries, the development of electrode materials and their processing techniques remains a hot topic in academia and industry. Even the well-developed and widely utilized active materials present issues, such as surface reactivity, irreversible capacity in the first cycle, and ageing. Thus, there have been many efforts to modify the surface of active materials to enhance the electrochemical performance of the resulting electrodes and cells. Herein, we review the attempts to use polymer coatings on the anode active materials. This type of coating stands out because of the possibility of acting as an artificial solid electrolyte interphase (SEI), serving as an anode protective layer. We discuss the prominent examples of anodes with different mechanisms: intercalation (graphite and titanium oxides), alloy (silicon, tin, and germanium), and conversion (transition metal oxides) anodes. Finally, we give our perspective on the future developments in this field.

1. Introduction

The development of rechargeable lithium-ion batteries (LIBs) began in the early 1990s and received remarkable attention, as they are employed in many modern portable electronic devices, as well as hybrid electric vehicles (HEVs) and electric vehicles (EVs) because of their high energy densities [1]. The progress is very fast regarding the development of new active anode and cathode materials, electrolyte formulation, electrode composition, and cell design with consideration of safety and costs [2,3]. The interest in developing active materials for LIBs has been considerably increasing to enhance specific capacity and enable higher rate capability and long cycle life stability—the critical parameters for practical applications. Various material modification methods have been introduced in academia and industry to improve the general properties of active materials without changing the elemental or crystal structure, and thereby, the intrinsic properties. One of the well-known methods is surface architecture, which represents surface coating and etching, which protects the active material particles from direct contact with the electrolyte. In LIBs containing liquid (or polymer) electrolytes, the electrode surface is covered by a passivating layer called the solid electrolyte interface (SEI) on the anode materials and the cathode electrolyte interface (CEI) [4] on the cathode particles. The SEI in particular has been regarded as a crucial interface in the battery related to the capacity fade, cycle life, and other key performance parameters. The SEI has a protective role, blocking the electrons that could further reduce the electrolyte, but it also consumes the valuable electrolyte and Li-ions, leading to irreversible capacity loss [5,6]. During the first few cycles, the SEI film forms from electrolyte decomposition and reduction reactions with the lithium salts on the anode particle surface [7]. However, the SEI film is usually not stable, particularly for anode materials with massive volume expansion during the lithiation step. Therefore, the electrolyte ions are consumed over the cycles, resulting in capacity degradation [8].

Coating the thin layer of “protective” material onto the active material particle can increase its micro-structure stability, resulting in improved electrochemical properties. Many modifications to the electrode [9,10,11,12,13] and active material particles have been investigated, such as surface modification by carbon, metal oxides, and polymers [14,15,16,17,18,19,20,21,22,23,24]. Coatings consisting of inorganic materials, such as Al₂O₃, MgO, ZrO₂, SiO₂, TiO₂, and others, commonly provide a Li⁺-conducting protective layer on the active material, which reduces the chemical reaction between the active material and the electrolyte. However, their deposition usually requires a high-temperature treatment, and the formation of uniformly distributed coating is very challenging.

The SEI should allow the rapid Li-ion transfer between the electrolyte and the electrode without blocking the electron pathway on the electrode current collector interface. Ideally, this layer should self-heal when the changes in the electrode surface occur due to volume expansion. A thin polymer layer on active anode materials can theoretically fulfill these criteria and could act as an artificial SEI. The properties of this layer, such as its thickness, ion-diffusion capability, chemical composition, and mechanical properties, are vital for having a stable electrode. In this review paper, we attempt to analytically address the progress of polymer coating on anode electrodes/particles to enhance the electrochemical performance of LIBs.

2. Polymer Coating on Anode Materials

In the following sections, we discuss the effect of polymer coating on the electrochemical performance and morphology of different anode materials with mainly three different electrochemical (de)lithiation mechanisms, the advantages and drawbacks for these anode materials are summarized in

Table 1. An overview of the (dis)advantages of different types of anodes.

Types of Anodes	Benefits	Drawbacks
Intercalation-type anodes	Low-cost materials, high electronic conductivity, good safety for Ti-based anodes, long cycle life, high power capability	Low specific capacity, safety issue for Gr, low energy density
Alloy-type anodes	High specific capacity, good safety, low-cost materials and abundance for Si, high energy density	Large volume changes, low electronic conductivity, poor CE, poor cycling
Conversion-type anodes	High specific capacity, low-cost materials, low operation potential	Poor CE, unstable SEI, poor cycling

:

−

Intercalation anodes, such as Graphite (Gr) and Titanium oxides.

−

Alloy anodes, for instance, Silicon (Si), Germanium (Ge), and Tin (Sn).

−

Conversion anodes, for example, transition metal oxides (Fe₃O₄, Co₃O₄, CuO, etc.).

Figure 1 illustrates the advantages and drawbacks of these three types of materials and the connection between working potentials and the specific capacity of the anode materials [25]. Generally, depending on the anode active material, polymer coating on the particles could help resolve some critical challenges, such as poor cycle life and C-rate capability, low Coulombic efficiency (CE), unstable SEI, and high irreversible capacity, which will be addressed here in detail for the specific polymers and anode materials.

A thin polymer layer on active anode materials could act as an artificial SEI. This layer can be fabricated for mechanical flexibility to maintain the passivation of active anode materials. The polymer film could be synthesized via different techniques [26,27,28,29,30,31,32,33,34,35,36,37,38,39]. Generally, thin polymer films could accommodate volume expansion (unlike glass or ceramic layers) while simultaneously demonstrating good chemical and structural stabilities during (de)lithiation processes. The polymer film thickness is typically about 2–25 nm [10,40,41,42,43,44,45,46,47,48,49,50,51,52,53].

Conductive polymers are attractive additive materials for LIBs because of their outstanding electrochemical properties: enhancing the electronic conductivity, inhibiting the phase transition, increasing structural stability, decreasing active material dissolution, leading to a remarkable improvement in reversible capacity, rate capability, and cycle stability. Conductive polymers, such as polypyrrole (PPy) [44], polyaniline (PANi) [54], poly(3,4-ethylenedioxythiophene) (PEDOT) [55], PEDOT:poly(styrenesulfonate) (PEDOT:PSS) [31], and polythiophene (PT) [33], but also other polymers, such as polyvinylidene fluoride (PVDF) [12,32] and Polydopamine (PDA) [56], have been used as attractive coating agents for active anode materials to improve the mechanical flexibility and the electrochemical performance of LIBs. Figure 2 demonstrates the contribution of polymers reported in the literature chosen here for coating active anode materials. The category “others” comprises the literature using polyvinylpyrrolidone and polyacrylonitrile [49], poly(1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane) [51], poly(vinyl alcohol) [52,57], polyether, polyethylene glycol tert -octylphenylether and polymer polyallyl amine [17], poly(diallyl dimethylammonium chloride) and poly(sodium 4-styrenesulfonate) [58], polyacrylic acid and polymethacrylic acid [9], PVDF [12,32], 1,3,5-trivinyl-1,3,5-trimethylcyclotrisiloxane [13], PEDOT:PSS [31,59], PDA [56], poly(dimethyldiallylammonium chloride) and poly(methyl methacralate) (PMMA), poly(sodium-p-styrenesulfonate) [60], 1,3,5-trimethylcyclotrisiloxane (V3D3) [13] and poly(ethylene oxide) [61]. These polymers may serve as a host for Li-ion (de)intercalation and enhance the electron transfer in the electrode, particularly with the electrodeposition method. In the following sections, we will discuss the effects of polymer coating on the electrochemical performance of anode materials.

3. Intercalation Anodes

3.1. Graphite (Gr)

Gr, as a state-of-the-art anode material of LIBs, has been investigated [62,63,64], and it has been on the market for almost three decades, since the beginning of the commercialization of LIBs. Although Gr has a low capacity, it is a reliable and stable anode material, but some significant challenges still remain:

• Rate capability: safety issues because of lithium (Li) plating on the electrode surface.
• First cycle irreversible capacity, or initial CE towing to the electrolyte decomposition, and therefore, the consumption of Li-ions as the charge carrier.
• Ageing, prompted by cycling and calendar ageing, and the resulting safety concerns because of the low (de)lithiation potential close to the Li plating.

These challenges potentially result in accelerated capacity fading. During recent years, many improvements have been made to Gr morphology and particle sizes, but there have also been extensive investigations into the functionalization of Gr-based anodes via polymer, carbon, and nano-oxide coating layers [9,12,13,38,39,65,66,67,68,69,70,71,72]. Surface modification of the Gr particles is one strategy to enhance the electrochemical performance and solve some of the challenges mentioned earlier [68]. The reported studies about polymer coating on Gr particles/electrodes of LIBs are discussed in the following paragraphs.

When an ethylene carbonate (EC)-based electrolyte is used, the stable SEI is formed during the first charging, enabling the reversible Li (de)intercalation [9,69,73]. However, these electrolytes are unsuitable for low operating temperatures. Propylene carbonate (PC) with low freezing temperature (−49 °C), higher ionic conductivity at low temperatures than EC electrolytes, and a high melting point (37 °C) is one of the alternatives for EC-based electrolytes and could be used in LIB applications. However, PC-based electrolytes experience a major drawback, since Li-ions solvated with PC molecules co-intercalate into crystalline Gr; this causes the exfoliation of graphene layers and the continuous decomposition of the electrolyte, low Columbic efficiency, and finally, capacity fading due to lack of a suitable SEI [74,75]. To address this issue, many electrolyte additives are studied to improve the electrochemical performance and formation of a stable SEI to only pass Li-ion without solvents [9]. Another approach to suppress the decomposition and co-intercalation of PC molecules is the surface modification of Gr using polymers [9,69,73]. Komaba et al. [9] have studied Gr electrode modification using polyacrylic acid (PAA), polymethacrylic acid (PMA), and polyvinyl alcohol (PVA) as binders to modify the electrode surface. The electrochemical properties of polymer-coated Gr as anodes of LIBs were investigated in LiClO₄ dissolved in PC (Figure 3). The sample using PAA did not show visible exfoliation of Gr after the first cycle in LiClO₄ in PC, whereas for the sample with PVDF, a partial exfoliation of Gr was confirmed (Figure 3a,b). The polymer-coated Gr particles showed a thin SEI layer due to the enhanced SEI formation process caused by the interaction of oxygen in the polymers and Li-ions, resulting in a much better electrochemical performance (Figure 3c,d).

The Wu group [17] has developed an artificial SEI (A-SEI) on the surface of natural Gr (NG) and artificial Gr (AG) particles via the design of a multifunctional polymer coating to achieve promising electrolyte/A-SEI/electrode interfacial properties. The coated NG electrode demonstrated a capacity of 336 mAh g⁻¹. It showed 95% of full capacity even at a 10 C rate, which was one of the highest rate capabilities reported for an NG electrode. The A-SEI was designed via binary polymeric coating (P&P), including polyallylamine (PAAm) and polyethylene glycol tert-octylphenyl Ether (PEGPE). The achievement of a robust adhesion on the Gr surface was enabled by π−π attractive interaction of aromatic rings in the PEGPE structure and NH₂ in PAAm, creating strong hydrogen bridge bonding with O in the PEGPE, and nitrogen electrons of the amin groups were utilized to direct Li⁺ ions, resulting in an outstanding rate capability of P&P coating Gr electrodes [17] (Figure 4). In another study [76], this group demonstrated β-phase PVDF coating on Cu and Li substrates in ether-based electrolytes, which effectively mitigated Li dendrite formation. They took advantage of using β-phase PVDF and applied it for coating on the Gr electrodes [12] containing a Li dendrite suppression under severe lithiation conditions in a typical carbonate-based electrolyte and under Li-plating conditions (either 20% over-lithiation or fast lithiation at up to 10C). The polymer-coated Gr electrodes demonstrated good cycling stability with high Coulombic efficiencies (Figure 5) and also offered a new approach to improving the battery safety of LIBs. The PVDF coated on the Gr anode also reduced the charge-transfer resistance due to over-lithiation [12].

Initiated chemical vapor deposition (iCVD) is a low-energy thin film processing tool without using any solvent, which produces thin polymeric films by directly converting one or more monomer vapors to a polymer film on a substrate [77,78]. Thin polymer layers of 1,3,5-trivinyl-1,3,5-trimethylcyclotrisiloxane (V3D3) were coated on the surfaces of Gr electrodes via iCVD [13]. Due to a more steady interface between the active Gr surface and liquid electrolyte and lower charge-transfer resistance, the poly(V3D3) coated Gr electrodes with a coating thickness of about 40 nm acted as a uniform SEI layer consisting of desirable Li-containing compounds, showing higher cycling stability and CE and remaining more steady than the uncoated electrodes. Recently, Seo et al. [39] developed and optimized the thickness of coated PDA on the surface of Gr particles using a wet-based coating method to form a uniform PD layer on the Gr particles as an ion-conducting path for Li-ions. The PD-coated Gr particle improved the electrochemical stability of Gr for both the rate capability and cycling life.

3.2. Titanium Oxide Anodes

Titanium-based anode materials include TiO₂, [79] Li₄Ti₅O₁₂ (LTO), [80] and MLi₂Ti₆O₁₄ (M = 2Na, Sr, Ba, Pb) [81] as a group of alternative materials to the conventional Gr. As their operation potential is above 0.8 V vs. Li⁺/Li, there is consequently no issue in having a stable SEI layer on the active particles [81]. Additionally, a significant safety advantage and superior thermal stability compared to the Gr anode make them attractive and interesting anode materials for LIB applications [81]. However, polymer coating on titanium-based particles could improve rate capability. For instance, Yan Liu et al. [31] have developed a synthesis method to coat LTO particles with a layer of PEDOT:PSS with an average thickness of 3 nm, resulting in an amount of about 1 wt.% of the composite particles. The polymer-coated LTO particles efficiently enhanced electrical conductivity, and the fabricated electrodes were more homogeneous, resulting in a better reversible capacity and rate capability than the pristine LTO electrode. The PEDOT:P PSS-coated LTO delivered a reversible capacity of about 177 and 161 mAh g⁻¹ at 0.1 C and 10 C. Nevertheless, LTO particles coated with conductive PTh [33] via an in situ oxidative polymerization method demonstrated a reversible discharge capacity of 172 and 151 mAh g⁻¹ at 0.1C and 10C. By adding nitrogen-doped graphene nanosheets to the LTO coated with 10 wt.% PPy anode composites, capacities of about 186 and 152 mAh g⁻¹ at 0.1C and 10C were delivered [48]. Slightly lower capacity was achieved for PTh and PPy coated LTO composite anodes compared to the PEDOT:PSS due to the larger amount of PTh or PPy (about 10 wt.% and thickness of 10 to 12 nm) in the composite. Generally, the excellent rate performances for the polymer-coated LTO anodes could be attributed to the conductivity of polymers, which can facilitate the electron and Li⁺ transmission during the charge/discharge process tests.

Regarding TiO₂ as an anode material of LIBs, many studies have been conducted [82]. However, coating TiO₂ particles with conductive polymers improved the electrochemical performance of mesoporous TiO₂ [83] and TiO₂ nanoparticles [84] by adding 8 wt.% and 15 wt.% PANi in the composites, respectively.

4. Alloy Anodes

4.1. Silicon-Based Anodes

Silicon (Si), as a low-cost and abundant material, has been considered one of the most studied and attractive anode materials in the pure [10,45,85,86,87,88], oxide [89], and composite [71] form for LIBs due to its high gravimetric (4200 mAh g⁻¹) and volumetric capacities (2400 mAh cm⁻³). However, the diffusion coefficient of Li in Si is low (about 10⁻¹³ cm² s⁻¹) [90], and Si has a low electrical conductivity (10⁻³ S cm⁻¹, which can be increased to about 10⁻² S cm⁻¹ after alloying with Li) [91]. These drawbacks can be solved by adding conductive carbon additives. However, fast capacity fading and high volume expansion, about 300%, during the charge/discharge processes seriously hinder the application of Si anode materials. Until now, several efforts have been focused to solve the mentioned challenges. For instance, some effective methods to solve these challenges include producing Si nanostructures [92,93] and Si/Carbon nanocomposites [71,94,95]. Nevertheless, as an important cause of the failure of Si anodes, the formation of a stable SEI on the surface of Si cannot be achieved with the mentioned methods. Hence, surface modification with a polymer coating on the Si-based anodes is one of the attempts to control SEI formation, which is discussed in the following paragraphs.

PPy, PANi, and PTh have been applied as effective coating layers on Si particles because of their high electrical conductivity, flexibility, and environmental friendliness [44,91,96]. Since PPy can form a conductive matrix, it has been used as a conductive binder and also as an active material in battery and supercapacitor applications [97,98,99,100,101]. Additionally, it acts as a host matrix to avoid significant volumetric expansions during the charge/discharge processes. PPy has been used for coating Si particles with different structures [44,45,86,102,103]. Herein, the important findings will be addressed. Du et al. [44] have studied PPy coating on porous Si hollow nano-spheres via in situ chemical polymerization. The PPy coating significantly enhanced the surface electronic conductivity of Si with excellent structural stability. Having porous channels in this nanocomposite buffered large volume changes. Additionally, it facilitated the diffusion of electrolyte and Li⁺ into the electrode, resulting in a high capacity (1772 mAh g⁻¹) and cycling stability (88% capacity retention after 250 cycles) and a high rate capability (Figure 6). Luo et al. [45] have studied thin PPy coating (thickness of about 6 nm) on Si nanoparticles (NPs) and found that the PPy-coated particle electrodes increased the critical size of Si NPs from 150 nm to about 380 nm.

The Bao group [10] demonstrated an interesting synthetic self-healing polymer coated on low-cost Si micro-meter particles, about 3–8 µm. The mechanical damages and cracks of the coated polymers due to the volume expansion of Si particles during the charge/discharge processes could be repaired naturally via the branched hydrogen bonding of the coated polymer, enhancing the Si lifetime anode electrodes with particle sizes in micro-meters. They increased the conductivity of the coated polymers to about 0.25 S cm¹ by adding uniformly conductive carbon nanoparticles into the polymer. CE is critical for the commercialization of every electrode material; the self-healing polymer coating electrode shows an initial CE of about 80%, which is comparable with those electrodes prepared with Si nanoparticles [104,105,106].

PANi is also an important polymer for coating Si particles [87,96] due to its high conductivity (16 S m⁻¹) [107]. Wu et al. [96] have developed in situ polymerization coating with conductive polymer hydrogel, PANi, placed into Si-based anode electrodes, creating a three-dimensional connection network between the Si nanoparticles and PANi. This hydrogel framework showed a highly conductive PANi network structure, having porous space for Si volume expansion and forming a thin and stable SEI layer. As a result, the fabricated PANi/Si anode electrodes demonstrated 90% capacity retention after 5000 cycles at a high current density of 6.0 A g⁻¹. Additionally, the stable SEI formation on the electrode materials was the reason for the high CE in the half-cell battery tests. The partial carbonization of PANi to carbon on Si nanoparticles was studied by Mu et al. [108]. They obtained a Si/PANi/C composite, which demonstrated better electrochemical performance; although the CE after three cycles reached 99%, the initial CE was about 60%, which is a drawback of this composite. The PANi/LiClO₄ film was used for coating on Si@Carbon composite (with a capacity of about 700–800 mAh g⁻¹) materials by the Takeda group [40]. This coating increased the electronic contact of the Si@Carbon composite, held good mechanical integrity, and tolerated the micro-structural change from Si upon the cycling tests, resulting in a stable cycling life compared to uncoated composites.

Pan et al. [109] recently used PANi to coat Si nanoparticles. This composite exhibited a reversible capacity of 1000 mAh g⁻¹ at 1 A g⁻¹ after 300 cycles, also showing good cycling stability at an areal capacity of 3 mAh cm⁻² after 150 cycles and an excellent rate performance of 942 mAh g⁻¹ at 5 A g⁻¹. Zheng et al. [91] have developed PTh coating on porous Si particles by a simple chemical oxidative polymerization approach. PTh represented a flexible layer to hold Si particles during volume expansion/shrinkage processes and decreased the SEI layer. The Si@PTh composite electrodes demonstrate a longer cycle life with a reversible capacity of 1130.5 mAh g⁻¹ at 1 A g⁻¹ after 500 cycles. The reason could be an excellent structural stability of the composites and the use of porous Si in the electrodes; however, the main drawback of using porous Si would be low volumetric capacity.

Attia et al. [49] have used a self-assembling polyvinylpyrrolidone (PVP) and polyacrylonitrile (PAN) mixture on the surface of Si NPs, followed by a slow heat treatment process. A robust SiO₂ shell was produced around the Si nanoparticles because PAN improved the oxidation of Si. In contrast, the decomposition of PAN produced a nitrogen-rich carbon coating on the SiO₂ layer, leading to enhanced stability and reversibility of the electrodes. The thickness of the mixed polymers/SiO₂/nitrogen-rich carbon layer was about 3–4 nm. Further improvements were achieved by combining graphene as a conductive network with these particles to produce electrodes with higher stability and electrochemical performance. Using PAN coating also improved Si nanoparticle capacity and cycling life, as shown by Yoon et al. [110]; the composites delivered a specific capacity of 2000 mAh g⁻¹, with a capacity retention of 95% and 75% after 100 and 1000 cycles at 0.5 C, respectively. Fu et al. [50] have developed a coated polyimide on Si NPs by a mechanical stirring process. The polyimide film provided high ionic conductivity and wettability, and the polyimide-coated Si composites revealed much lower Li⁺ diffusion resistances than pure Si. However, the polyimide-coated Si anode in the full cell with LiCoO₂ as cathode showed poor cycling stability of 50% after only 50 cycles.

A thin, cross-linked polymer film on Si electrodes via iCVD has been studied by Tenhaeff’s group using poly(1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane) (pV4D4) as the polymer [51]. pV4D4 films with a thickness of about 25 nm on Si electrodes improved the initial CE and capacity retention over 100 cycles and demonstrated higher rate capability than uncoated electrodes. However, even after 100 cycles, the CE of the coated electrode was low (about 94%), making it difficult for practical application. Using high-energy ball-milling, Si/poly(vinyl alcohol) (PVA) composite particles with a size of about 200 nm were achieved from PVA and micron-sized Si particles [52]. A high initial CE of about 86% and a capacity of 1526 mAh g⁻¹ after 100 cycles were obtained with 5 wt.% PVA in the Si composite anodes. The better electrochemical performance could be attributed to using ball-milling. At the same time, the polymer coating effectively reduced the size of Si particles and kept them in nanometer size, thereby shortening the diffusion pathway of Li⁺. By applying both polyethylene glycol tert–octylphenyl ether and polymer polyallyl amine (this was explained in the section on graphite), the Wu group [17] succeeded in increasing the capacity of Si from 240 mAh g⁻¹ (for uncoated Si electrode) by six-fold to 1630 mAh g⁻¹ at 2 C. Although they obtained interesting results, the drawback of this work could be the costly polymers for practical applications.

Recently, Wang et al. [111] synthesized a Si/carbon composite by ball-milling using a waste Si slag, and they used poly(hexaazatrinaphthalene) (PHATN) for coating on Si/carbon composites. The benzene rings in the PHATN could be the active centers for accepting Li-ions, forming stable Li-rich PHATN thin and influential SEI films, which change the LiF’s formation path, thereby reducing the consuming electrolytes. Additionally, PHATN molecules expand owing to the change of molecular configuration, providing more space for the volume changes of the Si particles during (de)lithiation processes. This composite anode demonstrated a specific capacity of about 1120 and 417 mAh g⁻¹ after 500 cycles at 1.0 and 16.5 A g⁻¹, respectively. PDA coating on SiO_x developed by Gu et al. [112] showed better wetting properties with water and electrolyte solution. It delivered an initial Columbic efficiency of 80.48% and excellent specific capacities of about 1270 and 1140 mAh g⁻¹ at 0.05 and 3 C, respectively, and a capacity retention of about 80% after 150 cycles at 1 C.

4.2. Silicon@Graphite (Si@Gr)

Wu’s group [58] has developed a mixed polymer, including poly(diallyl dimethylammonium chloride) and poly(sodium 4-styrenesulfonate) (PDDA–PSS polymer) on the surface of Si/Gr composite materials containing Si NPs. They showed that the PDDA–PSS polymer, in addition to surface coating, also improved the uniformity of Si NPs distribution on the Gr surface and maintained the Si particles during volume changes without separation, thus allowing the NPs to maintain close contact with Gr micro-particles. The coated electrodes demonstrated higher Coulombic efficiencies throughout the cycling, a specific capacity of 450 mAh g⁻¹ at a lower C-rate, with 95% retention after 200 cycles, and an outstanding rate capability of 96% capacity retention (426 mAh g⁻¹) at 10 C. However, the uncoated Si/Gr electrode presented poor performance, less than 200 mAh g⁻¹ at 10 C. Furthermore, the PDDA–PSS-coated Si/Gr exhibited the same volume expansion as the pristine Gr electrode. Kim et al. [57] investigated poly(vinyl alcohol)-PO₄ (PVA-PO₄) coating on Si/Gr composites. Si/Gr materials were prepared by ball-milling with 30% Si in the composite, and the PVA−PO₄ on the surface of Si/Gr was produced again by ball-milling of PVA, NH₄H₂PO₄, and Si/Gr particles. Electrochemical improvements were pronounced for the PVA-PO₄ coated Si/Gr composites and delivered a discharge capacity of about 1500 mAh g⁻¹, and stable CE throughout the cycles thus provided a particular improvement. Here, the PVA–PO₄ acted as a binder in addition to lithium polyacrylate, reducing the electrode’s pulverization. Furthermore, it preserved stable Li₂O with high ionic conductivity during cycling, forming a stable SEI layer.

Recently, Abdollahifar et al. [38] developed a chitosan coating on Si/Gr particles as an artificial SEI layer using a simple solution method. The coating was made of sulfonated chitosan and cross-linked with glutaraldehyde, promoting good ionic conduction and sufficient mechanical strength of the polymer layer. The coated Si/Gr composite anodes demonstrated a capacity of 600 mAh g⁻¹, with excellent cycling stability of up to 1000 cycles. The coated polymer improved CE by reducing active Li losses and electrolyte consumption. A significantly enhanced cycling stability of the chitosan-coated anodes could be ascribed to the artificial SEI function of the polymer coating. It exhibited a cation (Li⁺)-selective behavior to facilitate the diffusion of Li-ions between the electrolytes and the Si/Gr composite material.

4.3. Tin (Sn)-Based Anodes

Several high-capacity anode materials have been reported and studied widely as promising candidates for LIBs applications, such as Tin (Sn), Si, metal oxides, and Li metal, due to their high theoretical capacity and natural abundance. High electrical conductivity of Sn, make it an attractive anode condidate for LIB applications. However, this material experiences low initial CE, which could be avoided using a Li_xSn alloy, which also serves as a potent Li-containing anode material [113,114]. Li_xSn alloys have gained much attention due to the high theoretical specific capacity of Li_4.4Sn, which equals 788 mAh g⁻¹ [113,115]. Like Si anodes, Sn also has a major drawback due to its significant volume changes during cycling, which can cause severe mechanical damage to the anode, eventually leading to capacity fading of the cell, and gradual aggregation of Li_xSn/Sn. Some of the efforts that have been introduced to solve the above issues include nanosizing the active material and introducing porosity into the structure, as well as the surface coating [113,116,117]. Surface polymer coating is an exciting method for protecting the micro-structure of the electrode that experiences internal stress while protecting the surface of the active material from side reactions. Nevertheless, the role of surface polymer coating in the particle networks of anodes remains challenging. The following section will discuss polymer coating on Sn-based anode materials.

Fan et al. [118] reported that the ultra-thin 3D PPy coating (3–4 nm) generated in situ on the surface of Sn nanoparticles provided structural integrity and increased capacity. The 3D-PPy-coated Sn anodes result in a higher specific capacity of 766 mAh g⁻¹ (0.2 A g⁻¹) and 583 mAh g⁻¹ (2 A g⁻¹). It was shown that the conductive polymer had multiple functions and acted as a binder and a conductive network in which the Sn nanoparticles are dispersed. The reference electrodes without a 3D conductive network demonstrated a capacity of 375 mAh g⁻¹ after similar cycles. In a study accomplished by Li et al. [119], the various thicknesses (8–40 nm) of PPy coating on Sn nanoparticles were used as protection during the lithiation step. The optimized coating thickness of 20 nm stabilized the Li_xSn nanoparticles (Li_xSn@PPy), and this composite anode showed an excellent specific capacity of 534 mAh g⁻¹ after 300 cycles, with a capacity retention of 86%. Moreover, the optimized PPy coating offers less volume expansion of the active anode material and prevents the agglomeration of Li_xSn particles. It is claimed that the PPy conductive polymer coating provides the channels of fast electron diffusion and enables quicker reaction kinetics. Cao et al. reported on the PEO-coated Sn anodes, where the polymer should act as a passivation layer that neutralizes the volume expansion of the active material during cycling [120].

4.4. SnO₂

One of the most promising candidates for anode materials among the metal oxides is SnO₂ owing to the high theoretical capacity, safe working potential, and natural abundance. The oxide of Sn also suffers from significant volume change (>300%) during cycling [117]. The electrochemical reactions of Li and SnO₂ can be seen below Equations (1) and (2). Assuming that the reactions are fully irreversible, a reversible theoretical capacity of 782 and 1494 mAh g⁻¹ can be obtained.

SnO₂ + 4Li⁺ + 4e⁻ → Sn + 2Li₂O

(1)

Sn + xLi⁺ + xe⁻ → Li_xSn (0 ≤ x ≤ 4.4)

(2)

Yuan et al. reported on the SnO₂-PPy composite anodes through the chemical polymerization method [26]. The SnO₂-PPy (18.25% of SnO₂ amount of loading) composite anode and bare SnO₂ anode delivered a discharge capacity of 562 (1st cycle) and 450 mAh g⁻¹ (20th cycle), and 570 (1st cycle) and 250 mAh g⁻¹ (20th cycle), respectively. The proposed mechanism for improved cycling stability of the SnO₂-PPy composite anode is that the concentration of polymer PPy acts as a conductive matrix and buffers the volume change reductions in the active composite anode material, resulting in Li_xSn alloying and de-alloying reactions. Cui L et al. reported on one-dimensional (1D) nanocomposites of 79 wt.% of SnO₂@PPy, which exhibit a capacity of 430 mAh g⁻¹ after 20 cycles, which is three times higher than bare SnO₂ [121]. Shao Q.-G et al. designed SnO₂ with a multi-walled carbon nanotube (MWCNT) and as a protective layer of the surface coating of PPy (SnO₂@MWCNT@SnO₂@PPy). The composite anode materials with higher conductivity effectively suppressed mechanical stress and prevented the aggregation of SnO₂ nanoparticles, delivering 600 mAh g⁻¹ for 30 cycles [27].

Liu et al. prepared the core-shell hollow SnO₂@PPy with the hydrothermal method, after which the in situ chemical polymerization took place [28]. The nanocomposites of SnO₂@PPy (21 wt%) result in a specific capacity of 1036 and 331 mAh g⁻¹ at the current density of 0.1C and 2.0C, respectively. The polymer coating prevents the pulverization of the hollow SnO₂ and prevents them from aggregating. Zhou M et al. reported on the double-shelled hollow micro-spheres with poly(ethyleneglycol dimethacrylate-co-methacrylic acid as SnO₂/P(EGDMA-co-MAA)composite anodes, which showed a high capacity retention of 49.5 wt%. The initial discharge and charge capacity of SnO₂/P(EGDMA-co-MAA) with 35.3 wt.% SnO₂ (18 nm) and 49.5 wt.% SnO₂ (24 nm) showed a capacity of 1170, 896, 689, and 470 mAh g⁻¹, respectively [122]. After 430 cycles, the SnO₂/P(EGDMA-co-MAA) composite anode displayed a discharge capacity of 711 mAh g⁻¹. Cao et al. reported that the PPy polymer coating played an interesting role in SnO₂ nanoparticles (SnO₂@PPy) and resulted in an initial discharge/charge of 935 and 723 mA h g⁻¹ with a CE of 77%. The effects between the PPy-coated SnO₂ and the hollow structure can decrease the pulverization of SnO₂ [29].

Li et al. have used the in situ transmission electron microscopy (TEM) to study the role of interparticle connections in SnO₂ nanoparticles with different coating layers of PPy and metal oxide MnO₂ [123]. The PPy coating layers in SnO₂@PPy provide a large contact area and robust adhesion between SnO₂ nanoparticles, demonstrating a better cycling stability and fast kinetics. The SnO₂ NPs@MnO₂ layer showed a specific capacity of 680 mAh g⁻¹ after 10 cycles. However, SnO₂@PPy (20 nm) delivered a stable capacity of 780 mA h g⁻¹ with a current rate of 0.1 C over 400 cycles. Recently, Zhang et al. [37] studied the PPy polymer coating on SnO₂ nanoparticles coated with the sol–gel method. The SnO₂ encapsulated by the conductive PPy showed specific capacities of 930 and 560 mAh g⁻¹ at 0.2 and 2 A g⁻¹, respectively. Li et al. introduced the hierarchical SnO₂ spheres coated with PPy, which improved conductivity and accommodated the structural integrity during cycling [124]. This composite anode resulted in a stable cyclability of 782 mAh g⁻¹ (at 0.25 C) after 650 cycles and 580 mAh g⁻¹ (at 4.0 C) after 5000 cycles.

Arif et al. reported a ternary-type SnO₂ nanocomposite as an anode incorporated with reduced graphene oxide (rGO) and PEDOT:PSS as a conductive polymer [125]. This work reports that 5 wt.% of PEDOT:PSS coated on the SnO₂/rGO composite delivers an improved capacity of 980 mAh g⁻¹ (0.1 C) with a CE over 99% after 160 cycles; both the rGO and the PEDOT:PSS act as the conductive medium and buffer the volume change of SnO₂ NPs. The PDA-coated SnO₂ [117] with a layer thickness of 5 nm exhibited significantly improved cycling stability and demonstrated good cyclability for over 300 cycles. Li et al. reported a facile approach combined with hydrothermal treatment and polymerization to fabricate the core-shell-structured SnO₂@C/PEDOT:PSS micro-sphere anodes, as seen in Figure 7a [126]. For a better understanding of structural changes during cycling, the TEM images of the SnO₂@C anode after 200 cycles at a current rate of 0.5 C are depicted in Figure 7b,c. The TEM images show a stable micro-sphere morphology that helps avoid aggregation during the cycling process. However, without polymer coating, the bare anode materials exhibited cracks and fractures, leading to an unstable structure and degrading the electrochemical performance. The group reported the initial specific capacity of 1170 mAh g⁻¹ at a current rate of 0.1 C and 441 mAh g⁻¹ for 1200 cycles at a high current rate of 2C (Figure 7d). The PEDOT:PSS polymer coating accommodates the volumetric change of SnO₂. Guo et al. reported that the in situ polymerization of SnO₂-Fe₂O₃@PANi results in anodes with an excellent capacity of 1000 mAh g⁻¹ (current density of 400 mA g¹) after 380 cycles [127]. Adding a carbon layer can improve the electronic conductivity of the anode, and in situ polymerization of PANi coating prevents agglomeration and particle growth during the heat treatment. This method ensures homogeneous carbon coating, thus providing good conductive contact between SnO₂-Fe₂O₃ and the carbon shield surrounded by the anode.

Table 2. Various polymer coatings applied to SnO₂-based anodes and their electrochemical performances.

Polymer	Synthesis Method	Coating on Particle/Electrode	Thickness	Cycling Performance (mAh g−1)/Current Rate (mA g−1)	Ref.
Polypyrrole (PPy)	Chemical polymerization method	Electrode	-	450 (20 cycles)/50 250 (20 cycles for bare SnO2)/50	[26]
	Chemical polymerization method	Particle	-	600 (30 cycles)/100	[27]
	In situ chemical polymerization	Electrode	-	430 (20 cycles)/50	[121]
	Hydrothermal method and in situ chemical-polymerization	Particle	25 nm	448.4 (100 cycles)/78	[28]
	Chemical vapor-phase polymerization	Electrode	20 nm	646 (150 cycles)/100	[29]
	In situ coating method	Particle	20 nm	760 (400 cycles)/0.1 C	[123]
Poly(3,4-ethylenedioxythiophene-poly(styrenesulfonate) (PEDOT:PSS)	Wet chemical method	Particle	2–3 nm	980 (160 cycles)/80	[125]
	Hydrothermal method and polymerization	Particle	6 nm	441.5 (1200 cycles)/2 C	[126]
Poly(ethyleneglycol dimethacrylate-co-methacrylic acid)	Hydrothermal method and polymerization	Particle	24.5 nm	711.9 (430 cycles)/200	[122]
Polydopamine (PDA)	Wet chemical method	Electrode	5 nm	1502 (300 cycles)/160	[117]

summarizes the effect of polymer coating on the performance of some reported SnO₂ anodes.

4.5. Other Tin (Sn)-Based Anodes

Sn−Ni-alloy-type anode materials are promising candidates for LIBs, with a simple fabrication method, higher capacity, and conductivities. Sn-Ni nanotubes have boosted mechanical strength and efficient electron transfer in the nanotube, but they also partly improved the mechanical stress during Sn volume expansions. A study conducted by Fan et al. used PEO to control the formation of SEI and stabilize the anode structure despite the volume changes, as well as to stabilize the electrode interface [128]. It was shown that adding a thin PEO coating to the Sn−Ni alloy nanotubes improves the cycling and rate performance. However, the 1D nanotube morphology may not be optimal for battery applications, and a three-dimensional (3D) configuration could be more beneficial due to the larger active surface area of the electrodes. Gowda et al. introduced the conformal coatings of PMMA (25–30 nm), which were deposited on the Sn–Ni alloy anode materials and served as a gel-type electrolyte after being soaked with liquid electrolyte for the application of LIBs (Figure 8) [129]. The 3D Sn−Ni@PEO composite exhibited reversible capacities of 939 and 533 mAh g⁻¹ at the current densities of 0.2 and 5 A g⁻¹, respectively [128]. Over 80 cycles, the composite anode retained its specific capacities of 619 and 170 mAh g⁻¹, with current rates of 0.2 and 5 A g⁻¹, respectively (Figure 8). The nanotube array anode Sn/Ni@PEO shows a stable discharge capacity of 806 mAh g⁻¹ over 200 cycles. On the other hand, the Sn/Ni nanotube array without the PEO coating resulted in a capacity of 600 mAh g⁻¹ after 200 cycles [130]. The composite system Sn/Ni@PEO combines the 3D hollow nanostructure of the active material and the surface coating of PEO for accompanying the volume expansion of Sn, delivering higher cycle stability, better contact with the current collector, and high rate performance. Tolganbek et al. recently introduced a layer-by-layer method (LBL) for coating a thin polymer electrolyte onto 3D Ni₃Sn₄ anode particles (Figure 8). This method has the advantage of controlling the thickness of the deposited polymer layer on the 3D active material structure. Figure 8c represents the Ni₃Sn₄ anode/gel-like polymer electrolyte (GPE)/LiFePO₄ cathode 3D cells scheme, fabricated using the LBL deposition method [36]. With this design, the cells showed a capacity of 143 mAh g⁻¹ over 100 cycles (Figure 8c,d).

4.6. Germanium (Ge)

In terms of Ge, with a theoretical capacity of 1600 mAh g⁻¹ and a volumetric capacity of about 8500 mAh cm³ and with a better electronic conductivity and transmission rate of Li⁺ than Si, much attention has been paid to the anodes of LIBs [131,132]. However, Ge displays volume expansion up to 300% during cycling, delivering low initial CE and poor cyclability [133,134,135]. Polymer coating on the particles [59] and electrodes [32] can be used to improve these issues. Regarding electrode coating, Sun et al. [32] established a method for producing high-performance Ge film electrodes grown on a 3D current collector (CuO) and an in situ formation of PVDF−hexafluoropropene/SiO₂ (PVDF-HFP/SiO₂) protective layer on the electrode surface. The coated polymer improved the mechanical and ionic/electronic transport properties of Ge, which led to high reversible capacity (about 1100 mAh g⁻¹) and cycling stability over 3000 cycles at 1 C rate with 95% retention and an excellent rate capability even at 10 C (974 mAh g⁻¹). Moreover, after a few initial cycles, the electrodes showed a high CE of 99.9%, indicating good reversibility for the (de)lithiation processes. The PEDOT:PSS coating (about 3 wt.%) on the surface of Ge nanoparticles (10–100 nm) was developed by Liu et al. [59]. The PEDOT:PSS coated Ge electrode improved the initial CE from 81% (for the pristine electrode) to 89% and demonstrated an excellent rate performance at 2 C (800 mAh g⁻¹) and 4 C (700 mAh g⁻¹), whereas the uncoated electrode showed almost no capacity at 2 C.

5. Conversion Anodes

Conversion anodes mainly include, but are not limited to, transition metal oxides, sulfides [136], phosphides [137,138,139,140], and nitrides [141,142,143,144]. Herein, polymer coating on the transition metal oxides will be discussed.

5.1. Iron Oxides

Iron oxide anodes, such as Hematite, α-Fe₂O₃ (paramagnetic mineral only in a Fe²⁺ oxidation state, with a theoretical capacity of 1007 mAh g⁻¹), and Fe₃O₄ (ferromagnetic material in both Fe²⁺ and Fe³⁺ oxidation states, with a theoretical capacity of 926 mAh g⁻¹), are non-toxic, natural abundance, easy-to-prepare, low-cost materials with high electronic conductivity and have been considered as one of the high-capacity anode candidates for LIBs. Except for the higher theoretical capacity and density (5.24 g cm⁻³), iron oxide anodes could be safer than Gr (2.16 g cm⁻³) owing to their higher potential, which reduces Li dendrite deposition on the anodes during charge/discharge cycling. However, iron oxide anodes suffer from poor rate capability and long-term cyclability because of the large volume change (over 200%) during Li-ion (de)intercalation (conversion reaction) processes [145,146,147,148,149,150,151,152,153,154]. Therefore, many approaches have been proposed to increase the iron-oxide-based anodes’ cyclability and rate capacity [155,156,157,158,159]. Polymer coating on the particular structures of iron oxide particles, in a study by Jeong et al. [42], for instance, could provide a unique electrode structure with a short diffusion distance for ions and a fast mass transport channel for the electrolyte and necessary void spaces for significant volume variations during the cycling tests. They prepared a hierarchical core-shell hollow structure of α-Fe₂O₃ coated with PANi as an anode material via in situ polymerization. α-Fe₂O₃@PANi hollow structure anodes delivered efficient and fast ion/electron pathways for electrochemical reactions. Furthermore, more space for significant volume expansions resulted in a large reversible capacity (958 mAh g⁻¹ at 0.1 C), high rate capability (793 mAh g⁻¹ at 5 C and 724 mAh g⁻¹ at 10 C), and better cycling stability (Figure 9). Liu et al. [160] used PPy for coating α-Fe₂O₃ particles directly on the iron foil as a current collector to produce a unique structure. The α-Fe₂O₃@PPy electrodes showed a specific capacity of 0.42 mAh cm² at 0.1 mA cm²; however, no improvement was achieved for the cycling stabilities compared to the uncoated electrodes. To enhance the cycling stability of Fe₂O₃ when PPy is used as a coating polymer, carbon coating on Fe₂O₃ before polymer coating is a critical step, as shown by Han et al. [43]. Due to the use of Fe₂O₃ NPs, a porous carbon matrix, and the formation of two protective layers of carbon and PPy, this unique nanostructure with improved electrical conductivity demonstrated better electrochemical performance. This composite showed a practical capacity of 1004 mAh g⁻¹ with a Fe₂O₃ loading (47 wt.%) close to the theoretical capacity. High volumetric capacity is another advantage of using iron oxide anodes—an essential parameter in cell packs. The volumetric capacity of the Fe₂O₃@Carbon/PPy composite was about 816 mAh cm⁻³, which is more than two times that of conventional Gr electrodes (about 370 mAh cm³) [161].

In order to protect Fe₃O₄ from structural variation, prevent the aggregation of NPs, and avoid direct contact between the active materials and the electrolyte, an effective polymer, PPy, was used to coat Fe₃O₄ NPs [162]. The Fe₃O₄@PPy composite demonstrated notable capacity retention of 98% of the initial capacity (544 mAh g⁻¹) after 300 cycles. In this study, the remarkable cycling stability could be due to the high content of PPy (about 40 wt.%) in the composite. Coating Fe₃O₄ hollow nano-sphere particles (about 200 nm) with PANi can also increase the reversible capacity (1090 mAh g⁻¹ at 50 mA g⁻¹) and improve its rate capability compared to uncoated and solid particles of Fe₃O₄ [163].

5.2. MnO₂

Although manganese (Mn) oxides are well-known materials for supercapacitor applications [164], they can also be applied as electrode materials for LIBs [165,166] and other types of batteries [167,168,169,170]. Among the Mn-oxide materials, MnO₂ received more attention for LIBs applications due to its low cost, natural abundance, environmental friendliness, low discharge voltage plateau, and, importantly, a high theoretical capacity of 1230 mAh g⁻¹. However, Mn-oxides exhibit low electrical conductivity (10⁻⁵ to 10⁻⁶ S cm⁻¹) [171] and significant volume variations during (de)lithiation processes [172]. Additionally, they suffer from Mn dissolution [173,174,175], leading to degradation of the electrochemical performance in terms of rate capability and cyclic stability. The polymer coating can relatively overcome the drawbacks mentioned above. For instance, Chen et al. [55] synthesized MnO₂ nano-boxes and demonstrated that PEDOT coated on the surface of MnO₂ nano-boxes (edge and shell of about 300 nm and 50 nm) by in situ polymerization of 3,4-ethylenedioxythiophene offers the paths for Li-ion diffusion and the space to buffer the volume variations. The conductive polymer guarantees structural stability and increases the electronic conductivity of MnO₂, resulting in improved electrochemical performances with a reversible capacity of 628 mAh g⁻¹ after 850 cycles at 1 A g⁻¹. Xiao et al. [41] studied ultra-thin MnO₂ nanosheets coated with PTh, which improved the capacity and cycling stability and delivered a reversible capacity of 500 mAh g⁻¹ after 100 cycles at 500 mA g⁻¹. However, the uncoated electrode only retained a capacity of 250 mAh g⁻¹ after the same cycling. PPy could also improve the MnO₂ electrochemical performance, which was shown by Garakani et al. [171]. They used MnO₂ nanowires@PPy core shell grown on graphene foam to increase conductivity and achieve a stable structure against volume expansion upon lithiation. The PPy-coated MnO₂ electrode demonstrated a three-times-faster lithiation speed than the uncoated MnO₂. It showed a reversible capacity of 945 mAh g⁻¹ at 0.1 A g⁻¹, while the uncoated electrode only showed 550 mAh g⁻¹ after 150 cycles.

5.3. Copper Oxide (CuO)

Copper oxide (CuO), another interesting and low-cost transition metal oxide, has been used as an anode for LIBs [176,177,178,179] because of its high theoretical capacity of 674 mAh g⁻¹ [178] and environmental benignity. However, this low conductive material shows a significant volume expansion (174%) upon lithiation [180] and low initial CE (35% to 65%) [181,182,183]. Therefore, these critical issues need to be resolved for practical applications. The PPy coating on CuO particles could minimize the mentioned challenges [34,46,184]. For instance, Yin et al. [46] synthesized various CuO particles with shell nano-belt structures by controlling the polymerization time of pyrrole. The shell played a vital role in Li storage properties of the final composite, along with using sodium dodecyl sulfate (surfactant) as a key factor in obtaining high-value core-shell nanostructures. Within three hours of polymerization, about 7 wt.% PPy was produced on the CuO composite, providing a high reversible capacity of 760 mAh g⁻¹ after 45 cycles. They also showed some improvements in another study [184]. Zhou et al. [34] developed an evaporation method for the in situ polymerization of PPy on CuO arrays for uniform polymer coatings. The uniform polymer coating could well preserve the stability of mechanical structures of the composite and provide rapid transmission of Li-ions and electrons during the cycling tests, leading to high Li storage and enhancing the specific capacities and cycling stability (up to 561 mAh g⁻¹ at 1 C after 100 cycles), which was higher than 30% compared to the pristine CuO anode.

5.4. Co₃O₄

Spinel Co₃O₄ is a transition metal oxide, which has been investigated as a competitive anode of LIBs [185,186,187,188,189] due to its high theoretical capacity (890 mAh g⁻¹) [185] and density (6.11 g cm⁻³, about three times that of Gr). Like many conversion anodes, such as iron-based anodes, Co₃O₄ has poor electronic conductivity and suffers from severe volume change and particle aggregation during cycling, thus leading to poor stability performance. A thin PPy coating layer on the Co₃O₄ nanowires improved conductivity and acted as a buffer layer to relieve the strain induced by volume variation upon cycling. A Co₃O₄ coated with a PPy anode [190] displayed notably improved cycle performance with a reversible capacity of 700 mAh g⁻¹ at 3 A g⁻¹ after 500 cycles. In contrast, an uncoated Co₃O₄ nanowire only showed 150 mAh g⁻¹ after 100 cycles. The electrochemical performance of CoCO₃ anode materials of LIBs can also be improved by thin PPy coating layers [191].

5.5. ZnFe₂O₄

ZnFe₂O₄ is a low-cost, easy-to-prepare, abundant raw material, with a high theoretical capacity (1000 mAh g⁻¹). However, ZnFe₂O₄ anode materials, like other ternary mixed transition metal oxides, suffer from poor cycle stability and rate capability because of low electronic conductivity and significant volume expansion during (dis)charge (conversion reactions) processes. PDA, a bionic ionic permeable film, was coated on the surfaces of ZnFe₂O₄ particles by the self-polymerization of dopamine to accommodate the volume expansions [56]. ZnFe₂O₄@PDA composites delivered higher capacity and improved the rate capability of composites at high current densities compared to uncoated electrodes. PPy coating on one-dimensional ZnFe₂O₄ [53] also improved the rate capability of composite electrodes; more than two times capacity was retained at high current densities, 2 A g⁻¹, compared to the pristine electrodes.

A polymer coating also improves the electrochemical performance of the uncommon anodes on the anode particles, such as Bi₂S₃ by PEDOT [192], NiO by PANi [47], hard carbons by double coating with poly(dimethyldiallylammonium chloride) and poly(sodium-p-styrenesulfonate) for high rate anodes [60], aluminum by poly(ethylene oxide) [63], phosphorus anode by PPy [193], 3D porous conductive framework by hyperbranched polyol [194], and Sb₂S₃ by PPy [195].

5.6. Vanadium-Based Anodes

Wang et al. [196,197] reported the PPy surface polymer coating on vanadium-based anodes Li_xV₂O₅, which could stabilize the cycling capacity of the anode and improve the cycle life. With this approach, the PPy-coated anode delivers a better CE of ~86% after 40 cycles compared to a bare anode, whose initial capacity decreases to as low as a CE of ~8%. The cell performance of LiMn₂O₄/LixV₂O₅ has a specific capacity of 43 mAh g⁻¹ at the voltage of 1.15 V [197]. The anode coated with a layer of PAN showed an improved electrochemical performance compared to the uncoated anode, with an initial capacity of 47 mAh g⁻¹ and a CE of 80%.

Another interesting anode material is VS₂, a capable substitute for conventional carbon anode materials because of its high theoretical specific capacity. Nevertheless, this type of anode active material has not been applied in practical applications, since it suffers from significant capacity decay and poor cycle life. Zhou et al. [198] prepared VS₄ through a solvothermal method; this anode material offered some interesting properties, such as high sulfur content and one-dimensional structure. Three different polymers (PEDOT, PPY, and PANi) were reported as coatings on the VS₄ anode material surface to improve the electron conductivity, decrease the diffusion of polysulfides, and modify the electrode/electrolyte interface (Figure 10a). The VS₄ anode without polymer coating had a capacity of 100 mAh g⁻¹; PEDOT- and PPY-coated VS₄ had a specific capacity of 318 mAh g⁻¹ (VS₄@PEDOT) and 448 mAh g⁻¹ (VS₄@PPy), as shown in Figure 10b. Among these polymer coatings, VS₄@PANi exhibited a CE of 86% in the first cycle and a reversible capacity of about 755 mAh g⁻¹ (100 mA g⁻¹) over 50 cycles. In comparison with PEDOT and PPy polymers, PANi exhibited better performances when coated on this anode material, which could be due to its strong interaction with VS₄ anode materials. Ding et al. studied PEDOT-PSS-coated VS₂ nanosheets prepared using an aqueous solution method [35]. The coated anode of 5VS₂@PEDOT-PSS delivered a higher reversible capacity of 569 mAh g⁻¹ (0.1 A g⁻¹) after 100 cycles. Apart from VS₂ and VS₄ anodes, copper vanadate (CVO) is a promising anode material for LIBs due to its layered structure and excellent kinetics. The PEDOT:PSS-coated CuV₂O₆ nano-belts were prepared through a dip-and-dry method [199]. The electrochemical performance of CuV₂O₆/PEDOT:PSS and bare CVO anodes delivered a specific capacity of 915 mAh g⁻¹ and 1142 mAh g⁻¹, respectively, in the first cycle. The reversible capacity after 100 cycles was 536.6 and 476.5 mAh g⁻¹, corresponding to coated and uncoated CVO, respectively. The PEDOT:PSS coated on CVO anodes offered better electron and ion conductivity, fewer structural changes, and decreased unwanted side reactions during cycling.

6. Summary and Outlook

LIBs technology provided a new door for energy storage for higher applications, especially in automotive EVs and HEVs, and stood out as an indispensable alternative for both present and future energy operations. In the past two decades, many studies on LIBs have been quite exciting, and more novel materials and strategies are being developed. There is a substantial demand for lightweight, reduced-size, space-efficient, low-cost, and high-capacity LIBs. This demand will continue to increase with technology maturation. Throughout the extensive material research and design, there should be an improvement in the development of various anode materials to enhance the capacity and cycle life of LIBs. This comprehensive review provides the various anode materials widely employed for LIBs and the polymer coating effects under investigation to increase their performance. The electrochemical reactions of LIBs were elaborated and overviewed with the advanced anode materials fabricated with a polymer coating.

Various coating strategies using polymer compounds are vital to improving the anode materials’ performance and SEI concerns. Plenty of progress has been accomplished in developing high-performance anode materials for LIBs. However, further investigation is required into the supporting mechanisms that limit their performance to support the chemically stable materials’ fabrication and development of easy processes for anode materials’ production. We mainly discussed research activities regarding polymer coating with different anode materials and showed the achievements of various high-performance anode materials, i.e., intercalation anodes (Gr and titanium oxides), alloy anodes (Si-, Sn-, and Ge-based compounds), and conversion-type anodes (transition metal oxide compounds). Each category had promising features and capacities, but some drawbacks limit the optimal electrochemical performance of LIBs. Undoubtedly, substantial challenges occur for each component, requiring significant research efforts in various fields to open up their full potential for the high-performance anode materials. The Si- and Sn-based anodes are alloy-type materials and are the most attractive because of their high capacity. However, the main challenges are significant volume changes during cycling, which have limited the electrochemical performances, thus frustrating their implementation. In addition, some of these anode materials experience poor conductivity, affecting their low capacity. To overcome these challenges, the polymer coating approach demonstrated and showed excellent electrochemical performance in LIBs. Generally, the critical challenges for anode materials are poor cycle life and C-rate capability, low CE, unstable SEI, and high irreversible capacity. Polymer coating on the anode active material particles could solve some of these critical challenges, depending on the anode material and specific polymer. For instance, in graphite anodes, the issue is Li plating with deep discharge at potentials near 0.0 V in a half cell (or fully charged for a full cell) and low initial CE. In Si-based anodes, the problem is volume expansion, resulting in unstable and thick SEI, poor cycling life, and low CE; transition metal oxides also have similar challenges. Therefore, a thin polymer coating (about 2–25 nm) can improve some of these issues via acting as an artificial SEI, and depending on the polymer, it could increase the conductivity of anode electrodes. Furthermore, maintaining the active anode materials’ passivation and accommodating volume expansion during (de)lithiation processes improves the mechanical flexibility. Conductive polymers, such as PPy, PAN, and PEDOT, are frequently used because of their outstanding electrochemical properties. These polymers may also serve as a host for Li-ion (de)intercalation and deliver excellent electronic contact between the mass loading and the current collector.

A quantitative understanding of the mechanisms, polymer coating thickness, nanoscale design, and properties is still needed. To better understand the nature of the SEI layer produced with polymers, its impact on the CE needs to be fully considered. Notably, more advanced in situ/operando characterization techniques (such as TEM, X-ray photon spectroscopy (XPS), and X-ray diffraction (XRD)) and theoretical and simulation studies are necessary to explore the in-depth analysis of polymer coating microscopic processes that occur during (de)lithiation processes at the atomic and particle levels. Additionally, low-cost fabrication strategies must be developed for polymer coating on anodes with desirable performance. Polymer coating could enhance the interparticle connections of alloy anodes and lightweight production and application of high-capacity anode materials for LIBs. Future developments in the controllable thickness of various polymer coatings and control of SEI formulations, such as carbon nanotube anodes, Si anodes, and high-capacity Sn anodes, are promising advancements for the future of LIBs. This review provides a holistic view of recent innovations and advancements in the various kinds of polymer coatings for anode materials for LIBs. It also provides a broad view of the prospects that the field of battery technology holds for energy conversion, storage, and applications.