The electrochemical properties of the rutile-type TiO2 and Nb-doped TiO2 were investigated for the first time as Na-ion battery anodes. Ti(1-x)Nb(x)O2 thick-film electrodes without a binder and a conductive additive were prepared using a sol-gel method followed by a gas-deposition method. The TiO2 electrode showed reversible reactions of Na insertion/extraction accompanied by expansion/contraction of the TiO2 lattice. Among the Ti(1-x)Nb(x)O2 electrodes with x = 0-0.18, the Ti(0.94)Nb(0.06)O2 electrode exhibited the best cycling performance, with a reversible capacity of 160 mA h g(-1) at the 50th cycle. As the Li-ion battery anode, this electrode also attained an excellent rate capability, with a capacity of 120 mA h g(-1) even at the high current density of 16.75 A g(-1) (50C). The improvements in the performances are attributed to a 3 orders of magnitude higher electronic conductivity of Ti(0.94)Nb(0.06)O2 compared to that of TiO2. This offers the possibility of Nb-doped rutile TiO2 as a Na-ion battery anode as well as a Li-ion battery anode.

ZnO nanoparticles were prepared by laser ablation of a zinc metal plate in a liquid environment using different surfactant (cationic, anionic, amphoteric, and nonionic) solutions. The nanoparticles were obtained in deionized water and in all surfactant solutions except the anionic surfactant solution. The average particle size and the standard deviation of particle size decreased with increasing amphoteric and nonionic surfactant concentrations. With the increase of the amphoteric surfactant concentration, the intensity of the defect emission caused by oxygen vacancies of ZnO rapidly decreased, while the exciton emission intensity increased. This indicates that anionic oxygen in the amphoteric surfactant molecules effectively occupied the oxygen vacancy sites at the ZnO nanoparticle surface due to charge matching with the positively charged ZnO nanoparticles.

We evaluated the charge−discharge performance of a Sn 4 P 3 negative electrode in an ionic liquid electrolyte comprised of Nmethyl-N-propylpyrrolidinium bis(fluorosulfonyl)amide (Py13-FSA) and NaFSA. We also conducted cyclic voltammetry and transmission electron microscopy for the Sn 4 P 3 electrode to reveal the reaction mechanism. It was suggested that Na 15 Sn 4 and Na 3 P are formed via phase separation in the first sodiation and that elemental Sn and elemental P formed by following a desodiation reaction with Na ions in the subsequent cycles. The Sn 4 P 3 electrode exhibited a high Coulombic efficiency of 99.1% at the fourth cycle and an excellent cycling performance with a high reversible capacity of 750 mA h g −1 even at the 200th cycle. We demonstrated that there are two important factors to improve the performance: (i) higher volume fraction of Sn than P and (ii) uniform dispersion of Sn nanoparticles in a P matrix. The ionic liquid electrolyte showed good applicability to the Sn 4 P 3 negative electrode due to its superior electrochemical stability.

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.

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.

Propylene carbonate (PC) and an ionic liquid consisting of 1-[(2-methoxyethoxy)methyl]-1-methylpiperidinium (PP1MEM) and bis(trifluoromethanesulfonyl)amide (TFSA) were used as electrolyte solvents for Li-ion batteries, and the anode properties of Si electrodes were investigated using a thick film prepared by gas deposition without any binder or conductive additive. The Si electrode in PP1MEM-TFSA exhibited good cycle performance with a reversible capacity of 1050 mA h g–1 even at the 100th cycle, whereas the Si electrode in PC showed a capacity of only 110 mA h g–1. It is noteworthy that the electrode performance was significantly enhanced just by changing the electrolyte solvent to an ionic liquid even with the same Si used as the active material. Raman mapping analyses of the Si anodes after cycling were conducted to clarify the deterioration mechanisms of the electrodes. It was revealed that, in the case of PC, crystalline Si locally remained in the electrode after cycling, and Li–Si alloying and dealloying reactions occurred in limited regions. This led to the generation of intensive stress accumulation due to the extreme volume changes of Si in the regions inside the electrode, causing severe disintegration of the Si electrode. Consequently, the anode property of the Si electrode in PC resulted in very poor performance. In contrast to the behavior in the organic electrolyte, Li–Si reactions uniformly took place over the entire electrode in PP1MEM-TFSA, which relatively avoided any stress accumulation that could lead to electrode disintegration. This is considered to be the reason for the significant improvement in the cycle performance of the Si electrode using the ionic liquid instead of the conventional electrolyte.

As anode materials of Li-ion and Na-ion batteries, the electrochemical insertion/extraction reactions of Li and Na were investigated for a rutile-type Nb-doped TiO2 synthesized by a sol–gel method. We changed the particle and crystallite sizes of the Nb-doped rutile TiO2 powders by annealing at various temperatures between 100 and 1000 °C and prepared thick-film electrodes consisting of the powders. The anode performances were remarkably improved not only in the Li-ion battery but also in the Na-ion battery with a reduced annealing temperature of 400 from 1000 °C. We revealed that the Nb-doped TiO2 showing better high-rate performances exhibited a larger ratio of crystallite size to particle size. The size-dependent enhancement in the performance of rutile TiO2 was much more drastic than that of anatase TiO2. These results suggest that rutile’s potential diffusivity of Li and Na appeared more obviously when increasing the ratio because its diffusion coefficient is anisotropic and significantly high.

We hydrothermally synthesized various-element-doped rutile TiO2 for Na-ion battery anodes. Sn- and In-doped TiO2 electrodes showed poor performances. In contrast, Ta- and Nb-doped TiO2 electrodes exhibited larger discharge capacities, which is attributed to an expanded diffusion path and improved electronic conductivity. Among them, the Ta-doped one delivered excellent cycling performance and better rate capability. A first-principles calculation revealed that Ta doping reduced electron charge density in rutile’s Na+-diffusion path because Ta5+ has larger effective nuclear charge to attract strongly outermost electron. Therefore, Na+ was not bound by electron to easily diffuse in TiO2 particle, which leading to the enhanced capacity.