Silicon is electrochemically alloyed and de-alloyed with Li at potentials close to Li⁺/Li and is widely recognized as the most promising candidate for the anode of LIBs with a high energy density (250–300 Wh kg⁻¹) in the next generation. The most serious issue of Si anode is poor capacity retention owing to large volume changes during charging and discharging. We developed Si LeafPowder^® with a nano-flake structure to overcome the poor capacity retention. However Si anodes still have some problems such as large irreversible capacity, ceaseless electrolyte decomposition, swelling of the electrode, etc. for use in high-energy density LIBs in the next generation. In this article, these problems and the challenges to mitigate them are overviewed based on our resent data obtained by Si nano-flakes (Si LeafPowder^®).

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Novel Fe₃C carbon composite was proposed as new low cost and low environmental impact anode for Li-ion battery. Although the delithiated capacity of Fe₃C without AB were 100 mAh g⁻¹ in the first cycle at a rate of 7 mA g⁻¹, that of Fe₃C with AB was improved to 230 mAh g⁻¹ at a rate of 50 mA g⁻¹. Moreover, nanoparticle-Fe₃C carbon composite including a relatively high-purity Fe₃C could be obtained from α-Fe₂O₃ and ion-exchange resin. The best initial delithiated capacity more than 320 mAh g⁻¹ at a rate of 50 mA g⁻¹ and improved anode performance were obtained in the Fe₃C-carbon composite sintered at 650°C.

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Methanol oxidation reaction (MOR) has been studied on the low index planes of Pd (Pd(111), Pd(100) and Pd(110)) in 0.1 M HClO₄ using voltammogram. Onset potential of methanol oxidation increases as Pd(110)∼Pd(100) < Pd(111). The activity for the MOR is estimated using anodic peak current density of the voltammograms. The order of the activity is Pd(111)∼Pd(110) ≪ Pd(100): (100) surface enhances the activity for the MOR on Pd electrodes.

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A self-standing polyacrylate based hydrogel sheet was examined as a separator of an alkaline zinc secondary battery. It suppressed the zinc dendrite formation and resultant internal short circuit. The morphology of electrodeposited zinc under a hydrogel sheet was plate-like which was different from that of electrodeposited under a conventional nonwoven fabric separator. The nickel-zinc battery using the hydrogel sheet as a separator achieved more than 50 charge-discharge cycles.

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A composite of nano-Fe₃O₄ and carbon nanofiber (CNF), which has been investigated to maximize iron utilization, is adversely affected by shortcomings such as hydrogen evolution by-reactions, high overpotential, self-discharge, and capacity fade by cycles. Hydrogen evolution from the Fe₃O₄/CNF electrode can be inhibited by coating with a polymer such as poly(ethylene glycol). Although self-discharge is not high, even modified Fe₃O₄/CNF electrodes are hindered by overpotential and cycle degradation deriving from partial dissolution of divalent iron and its subsequent aggregation.

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LiCoPO₄-carbon composites (LiCoPO₄/C) were synthesized via a solid state reaction with a new furnace black, CB1, which was developed by Asahi Carbon Co., Ltd. The CB1 used here has superiority physico-chemical properties, e.g., ultrafine particle sizes, high specific surface areas, and high “structure”, the degree of linkage among carbon particles. The particle growth of LiCoPO₄ was suppressed by the addition of CB1 to the LiCoPO₄ precursors and then followed by calcination. CB1 was dispersed among LiCoPO₄/C particles successfully to form conductive paths. Consequently, a high electrochemical capacity was achieved in aid of the presence of CB1.

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We investigated the thermal reactivity of sodium plating on Ni substrate with 1 M NaPF₆/organic carbonate electrolyte with and without FEC to evaluate the thermal safety of a sodium ion battery. Without FEC, the heat flow profile of sodium plating exhibits lower peak intensity and larger width than lithium one. On the other hand, with FEC, the heat flow profile dramatically changes, exhibiting higher intensity of more than three times than FEC-free one. The higher reactivity is attributed to the higher specific surface area and larger active metal amount at the collapse temperature of the protective film formed by FEC.

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Micropore-rich activated carbon with high surface area and pore volume was prepared by alkali activation of azulmic carbon (AZC) precursor as nitrogen-doped carbon; AZC was a carbonized product from azulmic acid. We successfully loaded a large amount of sulfur (S) into micropores of the activated AZC. Our two kinds of activated AZC (BET surface area: 1,747 and 2,319 m² g⁻¹) can include S up to 55 and 62 wt%, respectively. The former activated AZC including S can be charged and discharged with lithium (Li) ion transfer stably and reversibly in glyme-based and carbonate-based electrolytes. The latter activated AZC can be charged and discharged in a glyme-based electrolyte. These different characteristics are due to a difference in fine pore structure between them. Our microporous activated AZC with high S loading is promising material as positive S electrode for rechargeable Li-S batteries.

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To extend the cyclability of a hybrid capacitor using Li pre-doped Si negative electrodes (NEs) (Si-CAP), we applied a polyimide binder and fluoroethylene carbonate (FEC) as additives in the Li pre-doping step. This binder successfully improved the cyclability by fixing Si nanoparticles as the NE active material on the Cu foil current collector to resist the volume change caused by Li–Si alloying. After Li pre-doping, there were no cracks on the Si NE surfaces, which had been observed in previous work using sodium carboxymethyl cellulose binder. Furthermore, FEC addition into the electrolyte at Li pre-doping improved homogeneity of the Li–Si alloying. As a result, a large amount of Li was pre-doped to the Si NE and a lower open-circuit potential was maintained. A Si-CAP cell of [Li alloyed Si | activated carbon] using polyimide binder exhibited relatively stable charge/discharge behavior and cyclability was maintained up to 800 cycles. Stability of the charge/discharge performance of FEC-containing Si-CAP was improved by formation of a LiF-rich solid-state interphase film on the Si NE. X-ray photoelectron spectroscopy revealed suppression of decomposition of the propylene carbonate solvent by electrochemical reduction.

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We performed the heat-treatment of 0.5Li₂MnO₃-0.5LiMn_5/12Ni_5/12Co_1/6O₂ under vacuum and investigated its cathode performance, and the crystal, electronic and local structures. The coulombic efficiency of the initial cycle was improved by the vacuum heat-treatment. Based on the results of a Rietveld analysis, the Mn-ordering increased after the reductive treatments as the oxygen content and the cation-mixing decreased. From the EXAFS, it was demonstrated that the intensity of the Mn-O and Mn-M bonds of the samples changed at the discharge state of 3.3 V for the heat-treated sample under vacuum. In order to clarify the transition-metal ordering in detail, the local environment of the samples at a 3.3 V discharge was analyzed by a Pair Distribution Function analysis. Based on the analysis, it was found that the heat-treated 0.5Li₂MnO₃-0.5LiMn_5/12Ni_5/12Co_1/6O₂ under vacuum formed LiMn₆ arrangements in the transition metal layer and stabilized the local environments. The vacancies at the Li sites also increased around 3.3 V due to the vacuum heat-treatment to insert Li ions. Therefore, the reinsertion site of Li formed during the discharge process for the heat-treated sample because of the delithiation and oxygen desorption derived from the reductive heat treatment to obtain an initial enhanced discharge capacity and coulombic efficiency.

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Electrochemical deposition and dissolution of Li metal on a carbon fiber composite electrode were investigated in lithium bis(trifluoromethylsulfonyl)amide-tetraglyme solvate ionic liquid electrolyte. The carbon fiber composite coated on a Cu substrate was composed of vapor-grown carbon fiber (VGCF^®-H) and poly(vinylidene fluoride). The coulombic efficiency for dissolution of Li deposits on the VGCF^®-H modified Cu electrode was kept more than 98% at 100th cycle and higher than that on a Cu electrode in the solvate ionic liquid. Li was deposited in the porous structure of the VGCF^®-H modified Cu electrode and scarcely observed on the surface of the VGCF^®-H modified Cu probably because the overpotential for the reduction of Li is smaller on the Cu substrate and/or deposited Li than that on the cylindrical basal plane of VGCF^®-H. This result suggests that the voids in the carbon fiber network effectively act as the Li deposition sites at the electrode|separator interface in the coin cell.

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We prepared activated carbon (AC) and sulfur (S) composite cathodes. The present AC has developed micropores and a large pore volume. The composite could successfully include a large amount of S and exhibited stable cycle performance in a glyme-based electrolyte. An oxidation treatment of the AC with a concentrated nitric acid solution at 120°C was found to improve S utilization in charge-discharge cycling. A cathode with the oxidized AC including S showed stable cycle performance with a high capacity in not only the glyme-based electrolyte but also a carbonate-based electrolyte.

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Maricite NaFePO₄ (m-NaFePO₄) has long been regarded as an electrochemically inactive material because Na⁺ cannot overcome the activation energy of its diffusion pathways for sodium extraction and insertion in the structure. In this study, the charge-discharge behavior of m-NaFePO₄ ball-milled to the nano-size level was investigated in Na[FSA]-[C₃C₁pyrr][FSA] (C₃C₁pyrr = N-methyl-N-propylpyrrolidinium and FSA = bis(fluorosulfonyl)amide) ionic liquid electrolyte at 298 and 363 K. The charge-discharge performance was improved upon elevating the operational temperature, possibly because of the enhanced Na⁺ diffusion in the maricite structure and in the electrolyte, and improved electrode reaction. The reversible capacity at 363 K increased with consecutive cycles and reached 100 mAh g⁻¹ with nearly 100% coulombic efficiency in the 120th cycle at the C/2 rate. Ex-situ XRD results confirmed the preservation of the maricite phase after cycling, which may indicate that the practical capacity for maricite NaFePO₄ without amorphization. By reconsideration of electrochemically inactive materials for intermediate temperature operation, this study suggests the possibility of extending the range of available positive electrode materials for sodium secondary batteries.

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The electrochemical properties of a sulfur positive electrode in equimolar glyme-Li salt mixtures were investigated. Cyclic voltammograms indicated that the insertion of Li into the sulfur/carbon composite electrode took place over at least three steps during the reduction process. In contrast, the broad anodic current suggested that the electrode kinetics for the extraction of Li were relatively slow. Stable charge-discharge operation of the Li-S cell consisting of a Li negative electrode with [Li(triglyme)][bis(trifluoromethanesulfonyl)amide] as the electrolyte was achieved at 800 cycles. This indicates that dissolution of Li₂S_x into the electrolyte was effectively suppressed in this system.

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