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To ensure optimal operation of electrochemical energy storage devices, a precise design of the interfaces formed between the electrodes and the electrolyte is essential. Si is a promising anode active material for Li-ion batteries owing to its high theoretical capacity; however, the large volume change associated with Li absorption and release hinders its practical applications. The development of high-performance storage batteries depends on the construction of an electrode-electrolyte interface that facilitates Li + movement. To this aim, the authors have developed Si-specific interface observation methods. In this feature article, we review and discuss recent advances in the reaction behavior of Si-based anodes, particularly at the Si electrode-electrolyte interface.

We prepared composite electrodes using TiO 2 coated with chlorophylls a and b as photoelectric conversion material and MnO 2 as energy storage material and investigated their photoelectrochemical capacitor properties. The coating with the combination of chlorophylls a and b improved the photoelectric conversion function of TiO 2 , compared with the coating with each alone. Na + adsorption on MnO 2 was enhanced with increasing the chlorophyll coating amount. The reason is that more chlorophylls a and b absorb visible light in different wavelengths to promote an easier photoexcited electron transfer to MnO 2 , just as they improve the efficiency of photosynthesis reactions in nature.

Lithium-ion batteries are used in various extreme environments, such as cold regions and outer space; thus, improvements in energy density, safety, and cycle life in these environments are urgently required. We investigated changes in the charge and discharge properties of Si-based electrodes in ionic liquid electrolytes with decreasing temperature and the cycle life at low temperature. The reversible capacity at low temperature was determined by the properties of the surface film on the electrodes and/or the ionic conductivity of the electrolytes. The electrode coated with a surface film formed at a low temperature exhibited insufficient capacity. In contrast, a Si-only electrode precoated with the surface film at room temperature exhibited a cycle life at low temperatures in ionic liquid electrolytes longer than that in conventional organic liquid electrolytes. Doping phosphorus into Si led to improved cycling performance, and its impact was more noticeable at lower temperatures., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)

Silicon oxide (SiO x ) has been placed into practical use as an anode active material for next-generation Li-ion batteries because it has a higher theoretical capacity than graphite anodes. However, the synthesis method is typically vapor deposition, which is expensive, and the poor electron conductivity of SiO x restricts high performance. In this study, we prepared M /SiO x active materials consisting of SiO x and a third element ( M = Al, B, Sn) using a low-cost mechanical milling (MM) method and investigated their electrode properties as Li-ion battery anodes. Also, the authors added a third element to improve the conductivity of the SiO 2 matrix. Al, B, and Sn were selected as elements that do not form a compound with Si, exist as a simple substance, and can be dispersed in SiO 2 . As a result, we confirmed that SiO x has a nanostructure of nanocrystalline Si dispersed in an amorphous-like SiO 2 matrix and that the third element M exists not in the nanocrystalline Si but in the SiO 2 matrix. The electron conductivity of SiO x was improved by the addition of B and Sn. However, it was not improved by the addition of Al. This is because Al 2 O 3 was formed in the insulator due to the oxidization of Al. The charge-discharge cycle tests revealed that the cycle life was improved from 170 cycles to 330 or 360 cycles with the addition of B or Sn, respectively. The improvement in electron conductivity is assumed to make it possible for SiO 2 to react with Li ions more uniformly and form a structure that can avoid the concentration of stress due to the volume changes of Si, thereby suppressing the electrode disintegration., Competing Interests: The authors declare no competing financial interest., (© 2021 The Authors. Published by American Chemical Society.)

A TiO 2 electrode was coated with chlorophyll a to regenerate nicotinamide adenine dinucleotide phosphate (NADPH), which can enhance the photovoltages of the electrodes for photoelectrochemical capacitors. The photovoltage of an uncoated TiO 2 electrode was high during the first cycle but then steadily reduced owing to the oxidization of NADPH in the electrolyte during the photo-charge-discharge cycling. By contrast, a chlorophyll a -coated TiO 2 electrode maintained high photovoltages for 100 cycles. Residual NADPH concentrations after 100 cycles increased from 73% to 90% because of the coating, demonstrating that NADPH was regenerated by photoexcited chlorophyll a similar to a photosynthetic reaction in nature.

We have reported the effects of substituting a transition metal in silicide on the electrochemical performance of the silicide/Si composite anode for lithium-ion batteries (LIBs); the Cr 0.5 V 0.5 Si 2 /Si electrode exhibited much better cyclability compared with CrSi 2 /Si and VSi 2 /Si electrodes. Herein, we investigated the electrochemical performance of a Cr x V 1- x Si 2 /Si slurry electrode for its application in LIBs, and the results obtained were compared to those of a gas deposition (GD) electrode, which was comprised of only active materials. The slurry electrode exhibited a superior cycling life as with the GD electrode. After charge-discharge cycles, the expansion of the electrode thickness of CrSi 2 /Si and Cr 0.5 V 0.5 Si 2 /Si was smaller than that of VSi 2 /Si, and VSi 2 was significantly pulverized compared with the other silicides. It is considered that VSi 2 deformed easily by the stress from Si expansion and pulverized because the hardness of VSi 2 was the smallest among the silicides used in this study. These results reveal that Cr 0.5 V 0.5 Si 2 /Si has great potential as an anode material for next-generation LIBs and hardness is an important property for compositing silicide with Si., Competing Interests: The authors declare no competing financial interest., (© 2021 The Authors. Published by American Chemical Society.)

We investigated the impact of electrolyte difference on lithiation and delithiation properties of a Li 1.00 Si electrode to improve the Coulombic efficiency (CE) of Si-based electrodes. The results of X-ray diffraction, Raman spectroscopy, and soft X-ray emission spectroscopy demonstrated that a portion of the Li in Li 1.00 Si desorbed by simply immersing the electrode in an ionic-liquid electrolyte, that is, the phase transition of Li 1.00 Si to Si occurred. In contrast, this phenomenon was not confirmed in an organic-liquid electrolyte. Instead, the desorbed Li was consumed for the formation of a surface film; thus, the Li in Li 1.00 Si did not elute into the electrolyte. The addition of vinylene carbonate (VC) to the ionic-liquid electrolyte suppressed the phase transition of Li 1.00 Si to Si. Although the Li 1.00 Si electrode showed a low initial CE and poor cycling performance in a VC-free electrolyte, the electrode exhibited a high CE and a remarkable cycle life in the VC-added electrolyte. It was considered that no desorption of the mechanically added Li in Li 1.00 Si contributed to the superior cycle life; thus, the characteristic ductility, malleability, and high electrical conductivity of lithium silicide should improve the electrochemical performance.

Silicides are attractive novel active materials for use in the negative-electrodes of next-generation lithium-ion batteries that use certain ionic-liquid electrolytes; however, the reaction mechanism of the above combination is yet to be clarified. Possible reactions at the silicide electrode are as follows: deposition and dissolution of Li metal on the electrode, lithiation and delithiation of Si, which would result from the phase separation of the silicide, and alloying and dealloying of the silicide with Li. Herein, we examined these possibilities using various analysis methods. The results revealed that the lithiation and delithiation of silicide occurred., Competing Interests: The authors declare no competing financial interest., (Copyright © 2020 American Chemical Society.)

We hydrothermally synthesized In-doped rutile TiO 2 particles in an anionic surfactant solution and investigated the influences of In doping and the particle morphology on the Na + storage properties. The solid solubility limit was found to be 0.8 atom % in In-doped TiO 2 . In the case where no surfactant was used, the best anode performance was obtained for 0.8 atom % In-doped TiO 2 electrode by the benefits of three doping effects: (i) expanded diffusion-path size, (ii) improved electronic conductivity, and (iii) reduced electron charge density in the path. Further enhancement in the performance was achieved for the In-doped TiO 2 with a reduced particle length by the synthesis in the surfactant solution. This electrode exhibited a better cycle stability and maintained a high discharge capacity of 240 mA h g -1 for 200 cycles. The reason is probably that Na + can be inserted in the inner part of TiO 2 particles because of its reduced particle length., Competing Interests: The authors declare no competing financial interest., (Copyright © 2020 American Chemical Society.)

Elemental Si has a high theoretical capacity and has attracted attention as an anode material for high energy density lithium-ion batteries. Rapid capacity fading is the main problem with Si-based electrodes; this is mainly because of a massive volume change in Si during lithiation-delithiation. Here, we report that combining an ionic-liquid electrolyte with a charge capacity limit of 1000 mA h g -1 significantly suppresses Si volume expansion, improving the cycle life. Phosphorus-doping of Si also enhances the suppression and increases the Li + diffusion coefficient. In contrast, the Si layer expands significantly in an organic electrolyte even with the charge capacity limit and even in an ionic-liquid electrolyte without the limit. We demonstrated that the homogeneously distributed Si lithiation-delithiation, phase-transition control from the Si to Li-rich Li-Si alloy phases, formation of a surface film with structural and/or mechanical stability, and faster Li + diffusion contribute to suppressing Si volume expansion.

Lithium-ion batteries have attracted considerable attention due to their high power density. The change in concentration of salt in the electrolyte solution in lithium-ion batteries during operation causes serious degradation of battery performance. Herein, a new method of in situ Raman spectroscopy with ultrafine multifiber probes was developed to simultaneously study the concentrations of ions at several different positions in the electrolyte solution in deep narrow spaces between the electrodes in batteries. The total amount of ions in the electrolyte solution clearly changed during operation due to the low permeability of the solid-electrolyte interphase (SEI) at the anode for Li + permeation. The permeability, which is a key factor to achieve high battery performance, was improved (enhanced) by adding film-forming additives to the electrolyte solution to modify the properties of the SEI. The results provide important information for understanding and predicting phenomena occurring in a battery and for designing a superior battery. The present method is useful for analysis in deep narrow spaces in other electrochemical devices, such as capacitors., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The degradation mechanism of a graphite negative-electrode in LiPF6-based electrolyte solution was investigated using the basal plane of highly oriented pyrolytic graphite (HOPG) as a model electrode. Changes in the surface morphology were observed by in situ atomic force microscopy. In the initial cathodic scan, a number of pits appeared at around 1.75 V vs. Li(+)/Li, and fine particles formed on the terrace of the HOPG basal plane at about 1.5 V vs. Li(+)/Li. The fine particles were characterized by spectroscopic analysis, such as X-ray photoelectron spectroscopy and attenuated total reflection Fourier transform infrared spectroscopy. We added one of the components to LiClO4-based electrolyte solution, and successfully reproduced the formation of pits and fine particles on the basal plane of HOPG. Based on these results, the formation mechanisms of pits and fine particle layers were proposed.

The effect of phosphorus (P)-doping on the electrochemical performance of Si negative electrodes in lithium-ion batteries was investigated. Field-emission scanning electron microscopy was used to observe changes in surface morphology. Surface crystallinity and the phase transition of Si negative electrodes before and after a charge-discharge cycle were investigated by Raman spectroscopy and X-ray diffraction. Li insertion energy into Si was also calculated based on computational chemistry. The results showed that a low P concentration of 124 ppm has a meaningful influence on the electrochemical properties of a Si negative electrode; the cycle performance is improved by P-doping of Si. P-doping suppresses the changes in the surface morphology of a Si negative electrode and the phase transition during a charge-discharge cycle. Li insertion energy increases with an increase in the P concentration; Li insertion into P-doped Si is energetically unfavorable, which indicates that the crystal lattice of Si shrinks as a result of the replacement of some Si atoms with smaller P atoms, and therefore, it is more difficult to insert Li into P-doped Si. These results reveal that suppression of the phase transition reduces the large change in the volume of Si and prevents a Si negative electrode from disintegrating, which helps to improve the otherwise poor cycle performance of a Si electrode.

The inclusion of inorganic anions such as SO(4)(2-), NO(3)(-), and HPO(4)(2-) into the cavity of β-cyclodextrin monolayers on Au was examined by X-ray photoelectron spectroscopy (XPS), a quartz crystal microbalance (QCM), and chronocoulometric measurements of the competitive inclusion with ferrocene. The inclusion amounts of ferrocence in 0.2 M Na(2)SO(4), NaNO(3), and Na(2)HPO(4) solutions were less than 6% of the adsorption amount of β-cyclodextrin on Au, resulting in the apparent inhibition of the ferrocene redox reaction. The surface association constants of these anions reached about 10 on a logarithmic scale and were much higher than those for the inclusion of common organic guest compounds. A stronger anion inclusion was also demonstrated by the QCM response corresponding to the replacement of a preincluded organic guest with sulfate upon the injection of the sulfate solution. Quantitative analysis of the XPS data suggested a 1:1 association for each of these anions per surface β-cyclodextrin. There was no detectable inclusion for ClO(4)(-), Cl(-), and Br(-)., (© 2011 American Chemical Society)

Mixed per-6-thio-beta-cyclodextrin (CD-SH)/pentanethiol (C(5)SH) monolayers were constructed by the sequential immersion of a Au(111) electrode into solutions of CD-SH, ferrocene, and a mixed solution of ferrocene and C(5)SH. Highly compact CD-SH self-assembled monolayers (SAMs) with the surface CD-SH density of 74.0 +/- 6.3 pmol cm(-2) on a true surface area basis were formed in 1 mM CD-SH with the immersion time of more than 48 h as confirmed by reductive desorption voltammetry. Based on the concentration dependence of the adsorption amount, a Langmuir adsorption coefficient was determined to be 1.9 x 10(7) M(-1). Chronocoulometry in a ferrocene solution at the CD-SH SAM and the mixed CD-SH/C(5)SH monolayers revealed the following inclusion properties. (1) All the CD-SH cavities can be used for the inclusion of a guest compound before and after the adsorption of C(5)SH, as shown by the fact that the maximum inclusion amounts of ferrocene, 68.0 +/- 3.4 and 73.0 +/- 2.0 pmol cm(-2) before and after the adsorption of C(5)SH, respectively, were very close to the surface CD-SH density. (2) The association constant between the surface-confined CD-SH and ferrocene (7.6 x 10(4) M(-1)) is greater than the corresponding association constant in solution. (3) The intermolecular vacancies between the adsorbed CD-SH molecules are completely filled with C(5)SH. This ensures that the CD cavities are the only accessible sites for guest compounds and any other reactants.