Batteries, Volume 8, Issue 6 (June 2022) – 11 articles

Zn-based batteries have been identified as promising candidates for flexible and wearable batteries because of their merits of intrinsic safety, eco-efficiency, high capacity and cost-effectiveness. Polymer electrolytes, which feature high solubility of zinc salts and softness, are especially advantageous for flexible Zn-based batteries. However, many technical issues still need to be addressed in Zn-based batteries with polymer electrolytes for their future application in wearable electronics. Recent progress in advanced flexible Zn-based batteries based on polymer electrolytes, including functional hydrogel electrolytes and solid polymer electrolytes, as well as the interfacial interactions between polymer electrolytes and electrodes in battery devices, is comprehensively reviewed and discussed with a focus on their fabrication, performance validation, and intriguing affiliated functions. Moreover, relevant challenges and some potential strategies are also summarized and analyzed to help inform the future direction of polymer-electrolyte-based flexible Zn-based batteries with high practicability. Full article

Li-S batteries are ideal candidates to replace current lithium-ion batteries as next-generation energy storage systems thanks to their high specific capacity and theoretical energy density. Composite electrodes based on carbon microstructures are often used as a host for sulfur. However, sulfur lixiviation, insoluble species formation, and how to maximize the sulfur-carbon contact in looking for improved electrochemical performance are still major challenges. In this study, a nitrogen doped mesoporous carbon is used as a host for sulfur. The S/C composite electrodes are prepared by sulfur melting-diffusion process at 155 °C. The effect of the sulfur melting-diffusion time [sulfur infiltration time] (1–24 h) and sulfur content (10–70%) is investigated by using XRD, SEM, TEM and TGA analyses and correlated with the electrochemical performance in Li-S cells. S/C composite electrode with homogeneous sulfur distribution can be reached with 6 h of sulfur melting-diffusion and 10 wt.% of sulfur content. Li-S cell with this composite shows a high use of sulfur and sufficient electronic conductivity achieving an initial discharge capacity of 983 mA h g⁻¹ and Coulombic efficiency of 99% after 100 cycles. Full article

Solvent-free (SF) anodes with different carbon materials (graphite, hard carbon, and soft carbon) were fabricated to investigate the stability of different anodes with polytetrafluorethylene (PTFE) degradation. The graphite anode with large volume variation during the charge/discharge process showed poor cycle life performance, while hard carbon and soft carbon with low-volume expansion showed good cycle life. The SF hard carbon electrodes with a high loading of 10.7 mg/cm² revealed good long-term cycling performance similar to conventional slurry-casting (CSC) electrodes. It demonstrated nearly 90% capacity retention after 120 cycles under a current of 1/3 C with LiNi_0.5Co_0.2Mn_0.3O₂ (NCM523) as cathode in coin cell. The rate capability of the high-loading SF electrodes also is comparable to the CSC electrodes. The high stability of SF hard carbon and soft carbon anodes was attributed to its low-volume variation, which could maintain their integrity even though PTFE was defluorinated to amorphous carbon irreversibly. However, the reduced amorphous carbon cannot tolerate huge volume variation of graphite during cycling, resulting in poor stability. Full article

Sodium-deficient nickel-manganese oxides with three-layered stacking exhibit the unique property of dual nickel-oxygen redox activity, which allows them to achieve enormous specific capacity. The challenge is how to stabilize the oxygen redox activity during cycling. This study demonstrates that oxygen redox activity of P3-Na_2/3Ni_1/2Mn_1/2O₂ during both Na⁺ and Li⁺ intercalation can be regulated by the design of oxide architecture that includes target metal substituents (such as Mg²⁺ and Ti⁴⁺) and oxygen storage modifiers (such as CeO₂). Although the substitution for nickel with Ti⁴⁺ amplifies the oxygen redox activity and intensifies the interaction of oxides with NaPF₆- and LiPF₆-based electrolytes, the Mg²⁺ substituents influence mainly the nickel redox activity and suppress the deposition of electrolyte decomposed products (such as MnF₂). The CeO₂-modifier has a much stronger effect on the oxygen redox activity than that of metal substituents; thus, the highest specific capacity is attained. In addition, the CeO₂-modifier tunes the electrode–electrode interaction by eliminating the deposition of MnF₂. As a result, the Mg-substituted oxide modified with CeO₂ displays high capacity, excellent cycling stability and exceptional rate capability when used as cathode in Na-ion cell, while in Li-ion cell, the best performance is achieved for Ti-substituted oxide modified by CeO₂. Full article

The electrification of the air transport sector demands for an energy storage that adds as little volume and weight to the overall system as possible. Regarding this so-called structural battery, composites enable the storage of electrical energy in commonly used load bearing fibre composite structures. A structural battery composite can store electrical energy while bearing mechanical loads, thus reducing parasitic mass and volume. In this study, structural cathodes were prepared by slurry coating carbon fibres with lithium iron phosphate (LFP), polyethylene oxide (PEO), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and carbon black. For the structural anodes, the carbon fibres were utilised as active material and slurry coated with PEO and LiTFSI. These structural electrodes as well as a structural separator were characterised by electrochemical cycling. With 139 $\mathrm{mAh}{\mathrm{g}}_{\mathrm{AM}}^{-1}$, the structural cathodes demonstrated good utilisation of the active material. The carbon fibres used in the anode exhibited capacities of up to 92 $\mathrm{mAh}{\mathrm{g}}_{\mathrm{AM}}^{-1}$. High irreversible lithium losses were observed, which are attributed to the poor electrolyte wetting behaviour of the carbon fibres. A structural battery demonstrator with a lithium metal anode was realised and reached a maximum specific energy of 64 $\mathrm{Wh}{\mathrm{kg}}^{-1}$ with respect to electrode and separator weight. Full article

The design of novel and high-performance binder systems is an efficient strategy to resolve the issues caused by huge volume changes of high-capacity anodes. Herein, we develop a novel water-soluble bifunctional binder composed of a conductive polythiophene polymer (PED) and high-adhesive polyacrylic acid (PAA) with abundant polar groups. Compared with conventional conductive additives, the flexible conductive polymer can solve the insufficient electrical contact between active materials and the conductive agent, thus providing the integral conductive network, which is extremely important for stable electrochemical performance. Additionally, the polar groups of this composite binder can form double H-bond interactions with the hydroxyl groups of SiO₂ layers onto the silicon surface, keeping an integral electrode structure, which can decrease the continuous formation of SEI films during the repeated cycles. Benefiting from these bifunctional advantages, the Si electrodes with the composite binder delivered a high reversible capacity of 2341 mAh g⁻¹ at 1260 mA g⁻¹, good cycle stability with 88.8% retention of the initial reversible capacity over 100 cycles, and high-rate capacity (1150 mAh g⁻¹ at 4200 mA g⁻¹). This work opens up a new venture to develop multifunctional binders to enable the stable operation of high-capacity anodes for high-energy batteries. Full article

Many battery equalizers have been proposed to achieve voltage consistency between series connected battery cells. Among them, the multicell-to-multicell (MC2MC) equalizers, which can directly transfer energy from consecutive more-charged cells to less-charged cells, can enable fast balancing and a high efficiency. However, due to the limitations of the equalizers, it is not possible to achieve fast equalization and reduce the size of the circuit at the same time. Therefore, a MC2MC equalizer based on a full-bridge bipolar-resonant LC Converter (FBBRLCC) is proposed in this paper, which not only implements MC2MC equalization, but also greatly reduces the circuit size by reducing the number of switches by nearly half. A mathematical model and simulation comparison with conventional equalizers are used to illustrate the high-speed equalization performance of the proposed equalizer and excellent balancing efficiency. An experimental prototype for eight cells is built to verify the performance of the proposed FBBRLCC equalizer and the balancing efficiencies in different operating modes are from 85.19% to 88.77% with the average power from 1.888 W to 14.227 W. Full article

TiNb₂O₇ anode material with a Wadsley–Roth crystallographic shear structure was prepared by solid-state synthesis at a relatively low temperature (1000 °C) and a short calcination time (4 h) using preliminary mechanical activation of the reagent mixture. The as-prepared final product was then ball milled in a planetary mill with and without carbon black. The crystal structure and morphology of the samples were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Electrochemical performance was studied in a galvanostatic mode in varied voltage intervals and at different cycling rates in combination with in situ electrochemical impedance spectroscopy (EIS) measurements. The resistance measured using in situ EIS had the highest values at the end of the discharge and the lowest when charging. The lithium diffusion coefficient, determined by galvanostatic intermittent titration technique (GITT), in samples milled with and without carbon black was an order of magnitude higher than that for the pristine sample. It was shown that improved electrochemical performance of the carbon composite TiNb₂O₇/C (reversible capacity of 250 mAh g⁻¹ at C/10 with Coulomb efficiency of ~99%) was associated with improved conductivity due to the formation of a conductive carbon matrix and uniform distribution of submicron particles by size. Full article

This study aims to quantify the synergistic effect of Ni²⁺ and Mn²⁺ ions on the capacitive performance of oxide, hydroxide and phosphate electrodes in alkaline electrolytes. Three types of phases containing both nickel and manganese in a ratio of one-to-one were selected due to their stability in alkaline media: oxides with ilmenite and spinel structures (NiMnO₃ and Ni_1.5Mn_1.5O₄); hydroxides with layered structures (β-Ni_1/2Mn_1/2(OH)₂); and phosphates with olivine and maricite structures (LiNi_1/2Mn_1/2PO₄ and NaNi_1/2Mn_1/2PO₄). In the mixed hydroxides and phosphates, Ni²⁺ and Mn²⁺ ions randomly occupied one crystallographic site, whereas in the ilmenite oxide, a common face was shared by the Ni²⁺ and Mn⁴⁺ ions. The electrochemical parameters of the Ni–Mn compositions were evaluated in asymmetric hybrid supercapacitor cells working with alkaline electrolytes and activated carbon as a negative electrode. A comparative analysis of oxides, hydroxides and phosphates enabled us to differentiate the effects of nickel and manganese ions, structures and morphologies on their capacitive performance. Thus, the best performed electrode was predicted. The electrode composition should simultaneously contain Ni and Mn ions, and their morphologies should comprise spherical aggregates. This was an ilmenite NiMnO₃, which delivers high energy and power density (i.e., 65 W h kg⁻¹ at 3200 W kg⁻¹) and exhibits a good cycling stability (i.e., around 96% after 5000 cycles at a current load of 240 mA g⁻¹). Full article

Advanced electrochemical impedance spectroscopy (EIS) was applied to characterize industrial Ni-Cd batteries and to investigate the electrochemical redox processes. A two-term calibration workflow was used for accurate complex impedance measurements across a broad frequency range of 10 mHz to 2 kHz, resulting in calibrated resistance and reactance values. The EIS calibration significantly improved the measurements, particularly at high frequencies above 200 Hz, with differences of 6–8% to the uncalibrated impedance. With an electromagnetic finite element method (FEM) model, we showed that the impedance is strongly influenced by the cable fixturing and the self-inductance of the wire conductors due to alternating currents, which are efficiently removed by the proposed calibration workflow. For single cells, we measured the resistance and the reactance with respect to the state-of-charge (SoC) at different frequencies and a given rest period. For Ni-Cd blocks that include two cells in series, we found good agreement of EIS curves with single cells. As such, EIS can be used as a fast and reliable method to estimate the cell or block capacity status. For electrochemical interpretation, we used an equivalent electric circuit (EEC) model to fit the impedance spectra and to extract the main electrochemical parameters based on calibrated EIS, including charge-transfer kinetics, mass transport, and ohmic resistances. From the charge-transfer resistance, we computed the exchange current density, resulting in 0.23 A/cm², reflecting high intrinsic rates of the redox electron transfer processes in Ni-Cd cells. Full article

The impact of full prelithiation on the rate and cycle performance of a Si-based Li-ion capacitor (LIC) was investigated. Full prelithiation of the anode was achieved by assembling a half cell with a 2 µm-sized Si anode (0 V vs. Li/Li⁺) and Li metal. A three-electrode full cell (100% prelithiation) was assembled using an activated carbon (AC) cathode with a high specific surface area (3041 m²/g), fully prelithiated Si anode, and Li metal reference electrode. A three-electrode full cell (87% prelithiation) using a Si anode prelithiated with 87% Li ions was also assembled. Both cells displayed similar energy density levels at a lower power density (200 Wh/kg at ≤100 W/kg; based on the total mass of AC and Si). However, at a higher power density (1 kW/kg), the 100% prelithiation cell maintained a high energy density (180 Wh/kg), whereas that of the 87% prelithiation cell was significantly reduced (80 Wh/kg). During charge/discharge cycling at ~1 kW/kg, the energy density retention of the 100% prelithiation cell was higher than that of the 87% prelithiation cell. The larger irreversibility of the Si anode during the initial Li-ion uptake/release cycles confirmed that the simple full prelithiation process is essential for Si-based LIC cells. Full article