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The anode properties of Nb-doped rutile TiO2 electrodes were investigated in an ionic liquid electrolyte comprised of N-methyl-N-propylpyrrolidinium cation and bis(fluorosulfonyl)amide anion for use in a safe Na-ion battery. Although the electrolyte’s conductivity was lower than that of a conventional organic electrolyte at 30 oC, it showed high conductivity comparable to that of the organic electrolyte at 60 oC. The Nb-doped TiO2 electrode showed excellent cyclability in the ionic liquid electrolyte at 60 oC: a high capacity retention of 97% was observed even at the 350th cycle, which is comparable to value in the organic electrolyte (91%). In a non-flammability test in a closed system, no ignition was observed with the ionic liquid electrolyte even at 300 oC. These results indicate that combination of a Nb-doped TiO2 anode and ionic liquid electrolyte gives not only an excellent cyclability but also high safety for a Na-ion battery operating at a temperature below the sodium’s melting point of 98 oC., source:This research was published by Elsevier inc.: H. Usui, et al. Niobium-doped titanium oxide anode and ionic liquid electrolyte for a safe sodium-ion battery. Journal of power sources. 2016, 329, 428-431, source:http://www.sciencedirect.com/science/article/pii/S0378775316310485

We investigated the influence of the anions in ionic liquid electrolytes on the electrochemical performance of a silicon (Si) negative electrode for a lithium-ion battery. While the electrode exhibited poor cycle stability in tetrafluoroborate-based and propylene carbonate-based electrolytes, better cycle performance was achieved in bis(fluorosulfonyl)amide (FSA–)- and bis(trifluoromethanesulfonyl)amide (TFSA–)-based electrolytes, in which the discharge capacity of a Si electrode was more than 1000 mA h g–1 at the 100th cycle. It is considered that a surface film derived from FSA–- and TFSA–-based electrolytes effectively suppressed continuous decomposition of the electrolyte. In a capacity limitation test, a discharge capacity of 1000 mA h g–1 was maintained even after about the 1600th cycle in the FSA–-based electrolyte, which corresponds to a cycle life almost twice as long as that in TFSA–-based electrolyte. This result should be explained by the high structural stability of FSA–-derived surface film. In addition, better rate capability with a discharge capacity of 700 mA h g–1 was obtained at a high current rate of 6 C (21 A g–1) in FSA–-based electrolyte, which was 7-fold higher than that in TFSA–-based electrolyte. These results clarified that FSA–-based ionic liquid electrolyte is the most promising candidate for Si-based negative electrodes., source:This research was published by Elsevier Inc.: K. Yamaguchi, et al. Influence of the structure of the anion in an ionic liquid electrolyte on the electrochemical performance of a silicon negative electrode for a lithium-ion battery. Journal of Power Source, source:http://www.sciencedirect.com/science/article/pii/S0378775316315208

The effect of pre-lithiation on the electrochemical performance of Si negative electrodes for lithium-ion batteries was studied. A pre-volume-expansion of Si induced by the pre-lithiation is expected to decrease the relative change in volume of Si during charge-discharge reactions. Suppression of change in volume of Si should improve the cycle performance, the Coulombic efficiency, and so on. When the mechanically pre-doped Li was extracted from Li-Si alloy during the first cycle, an alloy electrode exhibited the poor cycle performance just like a Si-alone electrode. On the other hand, the Li1.00Si electrode exhibited the improved cycle performance and higher Coulombic efficiency under the discharge cut-off potential of 0.7 and 0.5 V, respectively. The electrode also showed 1.5 times longer cycle life compared to the Si-alone electrode when the charge capacity and the cut-off potential were limited. These results demonstrated that the appropriate mechanical pre-lithiation helps to improve the poor electrochemical performance of the Si negative electrodes., source:Yasuhiro Domi, Hiroyuki Usui, Daichi Iwanari, and Hiroki Sakaguchi Effect of Mechanical Pre-Lithiation on Electrochemical Performance of Silicon Negative Electrode for Lithium-Ion Batteries Journal of the Electrochemical Society 2017 164: A1651-A1654., source:http://jes.ecsdl.org/content/164/7/A1651

Effect of film-forming additives, such as vinylene carbonate (VC) and fluoroethylene carbonate (FEC), on electrochemical performance of Si negative-electrode in lithium-ion batteries was investigated using a thick-film prepared by gas-deposition without any binder and conductive additive. In VC-containing electrolyte, Si electrode exhibited better cycle performance with a discharge capacity of 1000 mA h g-1 beyond 750th cycles at a constant charge capacity of 1000 mA h g-1. In FEC-containing electrolyte, in addition, Si electrode showed good rate performance with a discharge capacity of 2000 mA h g-1 even at a high current rate of 10 C. When the current rate was returned to an initial low current rate of 0.4 C, more than 90% of initial discharge capacity was maintained. Film-forming additives relatively suppressed change in the surface morphology of Si thick-film electrode after charge-discharge cycling; disintegration of Si electrode was prevented by the additives., source:International Journal of Electrochemical Science. 2015, 10(11), 9678-9686, source:http://www.electrochemsci.org/papers/vol10/101109678.pdf

Excellent cycling performance of an electrode composed of silicon alone was achieved in a bis(fluorosulfonyl)amide (FSA)‐based electrolyte, with a high discharge capacity of 950 mA h g−1 observed even at the 500th cycle. To elucidate the reaction behavior of the Si electrode in an FSA‐based ionic liquid electrolyte, we investigated the change in the cross‐sectional morphology of the Si‐active material layer, the distribution of Li in the layer, and the crystallinity of Si on the electrode surface. By cross‐sectional scanning electron microscopy, we confirmed that the electrode thickness increased with the cycle number. The increase in thickness was less noticeable in the FSA‐based electrolyte than in an organic electrolyte. An elemental analysis of the electrode material revealed that a film derived from the electrolyte was formed not only on the surface but also inside of the electrode. Soft X‐ray emission spectroscopy demonstrated that the distribution of Li in the FSA‐based electrolyte was more uniform for the cross‐section of the cycled electrode compared to that in an organic electrolyte. The results of Raman spectroscopy indicated that domains of amorphous Si were homogeneously distributed on the electrode surface in the FSA‐based electrolyte. The uniform distribution of the lithiation−delithiation reaction should help to suppress disintegration of the active material layer., source:This is the peer reviewed version of the following article: K. Yamaguchi, Y. Domi, H. Usui, H. Sakaguchi, Citation for: Elucidation of the Reaction Behavior of Silicon Negative Electrodes in a Bis(fluorosulfonyl)amide‐Based Ionic Liquid Electrolyte. ChemE, source:https://onlinelibrary.wiley.com/doi/full/10.1002/celc.201700724

Composite electrodes comprising TiO2 and MnO2 particles were prepared as hotoelectrochemical capacitor electrodes that enable both photoelectric conversion and energy storage. A MnO2 electrode was also prepared for characterization of MnO2 alone, and its electrochemical capacitor property was evaluated. The MnO2 electrode showed higher specific capacitance than a RuO2 electrode. Results of the photoelectrochemical measurements demonstrated that the discharge capacity of the TiO2/MnO2 composite electrode was slightly inferior to that of the TiO2 and RuO2 electrodes using. By reducing MnO2 particle size, the composite electrode exhibited increased surface roughness and enhanced capacity. We consider that the smaller MnO2 particles can efficiently store Na+ ions even at a low photo-charge voltage of TiO2., source:This research was published by Elsevier Inc.: Y. Usui et al. Light energy storage in TiO2/MnO2 composite electrode for photoelectrochemical capacitor. Materials letters. 2017, 186, 338-340, source:https://www.sciencedirect.com/science/article/pii/S0167577X1631610X

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 Cr0.5V0.5Si2/Si electrode exhibited much better cyclability compared with CrSi2/Si and VSi2/Si electrodes. Herein, we investigated the electrochemical performance of a CrxV1–xSi2/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 CrSi2/Si and Cr0.5V0.5Si2/Si was smaller than that of VSi2/Si, and VSi2 was significantly pulverized compared with the other silicides. It is considered that VSi2 deformed easily by the stress from Si expansion and pulverized because the hardness of VSi2 was the smallest among the silicides used in this study. These results reveal that Cr0.5V0.5Si2/Si has great potential as an anode material for next-generation LIBs and hardness is an important property for compositing silicide with Si.

We investigated the applicability of ionic-liquid electrolytes to FeSi2/Si composite electrode for lithium-ion batteries. In conventional organic-liquid electrolytes, a discharge capacity of the electrode rapidly faded. In contrast, the electrode exhibited a superior cycle life with a reversible capacity of 1000 mA h g(Si)−1 over 850 cycles in a certain ionic-liquid electrolyte. The difference in the cycle life was explained by surface film properties. In addition, the rate performance of the FeSi2/Si electrode improved in another ionic-liquid electrolyte. Remarkably, lithiation of only Si in FeSi2/Si composite electrode occurred whereas each FeSi2- and Si-alone electrode alloyed with Li in the ionic-liquid electrolyte. FeSi2 certainly covered the shortcomings of Si and the FeSi2/Si composite electrode exhibited improved cycle life and rate capability compared to Si-alone electrode.

We have previously disclosed that the ionic-liquid electrolyte sodium bis(fluorosulfonyl)amide (NaFSA)/1-methyl-1-propylpyrrolidinium bis(fluorosulfonyl)amide (Py13-FSA) can significantly improve the cycling stability of Sn4P3 negative electrodes for Na-ion batteries (NIBs). However, the strong electrostatic interaction between Na+ and FSA− in the electrolyte leads to high viscosity and low conductivity. In this study, we have tried to improve the conductivity of the electrolyte and enhance the rate capability of the Sn4P3 electrode by introducing an ether group in the side-chain of the ionic liquid cation to reduce said electrostatic interaction. Ether-substituted ionic liquid 1-methoxymethyl-1-methylpyrrolidinium (PyMOM)-FSA showed higher conductivity than Py13-FSA and the Sn4P3 electrode exhibited a higher rate capability. The differential capacity vs. potential plots suggest that the reaction between Na+ and Sn or P is promoted in the ether-substituted ionic liquid electrolyte. These results demonstrate that introduction of an ether moiety is an effective approach to improve the rate capability of the Sn4P3 electrode in NIBs.

K-ion battery is a potential candidate as a next-generation battery with a high energy density, long cycle life, and high safety. To commercialize the battery, the improvement of safety is absolutely essential. We apply a nonflammable ionic-liquid electrolyte to a Sn4P3 electrode as negative-electrode for K-ion battery. The electrode achieves the excellent cycling performance with a discharge capacity of 365 mA h g−1 over 100 cycles in the ionic-liquid electrolyte. Rate capability in the ionic-liquid electrolyte is almost the same as that in the organic-liquid electrolyte.

Annealed Ni–P–coated Si (Ni–P/Si) anodes for lithium-ion batteries have shown a superior cycle life with discharge capacity of 1000 mA h g−1 over 1100 cycles in some ionic-liquid electrolytes. However, the annealing temperature has yet to be optimized for Ni–P/Si electrodes. We investigated the electrochemical performance of Ni–P/Si electrode annealed at various temperatures in this study. The Ni–P/Si electrodes annealed at 800 ± 20 °C exhibited a superior cycle life with a reversible capacity of 1000 mA h g−1 over 1000 cycles, whereas the capacity of the electrodes annealed at temperatures of 750 °C and 850 °C faded at approximately 500 cycles. At 800 °C, a newly formed NiSi2 phase was theorized to significantly contribute to improving adhesion between the Ni–P coating layer and the Si particles. The Ni–P coating particles tended to aggregate at 850 °C, leading to a reduction in the coating effect, that is, a decline in their reactivity with Li+, acceleration of electrode disintegration, and a reduction in electrical conductivity. On the other hand, Ni–P/Si electrodes annealed at 850 °C exhibited a superior rate performance. The amount of available NiSi2 which ultimately contributed to higher reactivity with Li should increase.

It is reported that silicon (Si) anodes with a smaller crystallite size show better electrochemical performance in lithium-ion batteries (LIBs); Si particles with different diameters are also used. However, it is yet to be clarified whether the better performance is attributed to crystallite size or particle diameter. The effect of Si crystallite size on its anode performance using Si particles having the same diameter and different crystallite sizes is investigated. Longer cycle life is obtained for smaller crystallite size, due to the small amount of the amorphous Li-rich Li—Si phase formed during charging. The phase is likely to form in a greater amount in Si particles with larger crystallite size, leading to degradation of the Si electrode at an early stage. Furthermore, Si electrodes with larger crystallite size show superior rate performance because of the high Li diffusion rate into the broader grain boundary; on the other hand, Si with smaller crystallite size should limit Li diffusion due to the narrower grain boundary. Therefore, smaller crystallite size helps improve the cycle life but deteriorates the rate performance of LIBs.

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.

A GexSi1−x alloy electrode is useful for addressing the shortcomings of a Si negative electrode for lithium-ion batteries. To further improve the electrochemical performance of a GexSi1−x negative electrode, a film-forming additive and the formation of a composite with LaSi2 were applied. A Ge0.1Si0.9 electrode exhibited better cyclability in the additive-containing electrolyte with a discharge capacity of 1240 mA h g−1 at the 400th cycle. In addition, a Ge0.1Si0.9/LaSi2 composite electrode showed better cycle performance than a Ge0.1Si0.9 electrode.

The degradation mechanism of an Sn4P3 electrode as Na-ion battery anode was investigated by using a transmission electron microscopic observation. At the first desodiation, we confirmed that Sn nanoparticles with 6 nm in size were dispersed in an amorphous-like P matrix. Compared to this, we observed aggregated Sn particles with sizes exceeding 50 nm after the drastic capacity fading. The capacity fading mechanism was for the first time confirmed to be Sn aggregation. To improve the capacity decay, we carried out the two kinds of charge−discharge cycling tests under the reduced volume changes of Sn particles and P matrix by limiting desodiation reactions of NaSn and Na3P, respectively. The Sn4P3 electrode exhibited an excellent cyclability with the discharge capacity of 500 mA h g−1 for 420 cycles under the limited desodiation, whereas the capacity decay was accelerated under the limited sodiation. The results suggest that the Sn aggregation can be improved by the reduced volume change of the P matrix, and that it is very effective for improving anode performance of Sn4P3 electrode.

We hydrothermally synthesized In-doped rutile TiO2 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 TiO2. In the case where no surfactant was used, the best anode performance was obtained for 0.8 atom % In-doped TiO2 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 TiO2 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 TiO2 particles because of its reduced particle length.