Effect of Silicon Crystallite Size on Its Electrochemical Performance for Lithium-Ion Batteries

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

Applicability of an Ionic Liquid Electrolyte to a Phosphorus‐Doped Silicon Negative Electrode for Lithium‐Ion Batteries

We investigated the applicability of an ionic liquid electrolyte to a phosphorus‐doped Si (P‐doped Si) electrode to improve the performance and safety of the lithium‐ion battery. The electrode exhibited excellent cycling performance with a discharge capacity of 1000 mA h g-1 over 1400 cycles in the ionic liquid electrolyte, whereas the capacity decayed at the 170th cycle in the organic electrolyte. The lithiation/delithiation reaction of P‐doped Si occurred a localized region in the organic electrolyte, which generated a high stress and large strain. The strain accumulated under repeated charge‐discharge cycling, leading to severe electrode disintegration. In contrast, the reaction of P‐doped Si proceeded uniformly in the ionic liquid electrolyte, which suppressed the electrode disintegration. The P‐doped Si electrode also showed good rate performance in the ionic liquid electrolyte; a discharge capacity of 1000 mA h g-1 was retained at 10 C

Degradation mechanism of tin phosphide as Na-ion battery negative 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

Anode Properties of CrxV1-xSi2/Si Composite Electrodes for LithiumIon Batteries

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

TiO2/MnO2 composite electrode enabling photoelectric conversion and energy storage as photoelectrochemical capacitor

We prepared composite electrodes by using rutile TiO2 particles and γ-MnO2 particles, and evaluated their photoelectrochemical capacitor properties based on Na+ adsorption by light irradiation in aqueous electrolytes. By employing different synthesis method for TiO2 particles, we synthesized TiO2 particles with various particle sizes and crystallite sizes. An electrode of sol-gel-synthesized TiO2 showed higher photovoltages compared with an electrode of commercial TiO2. This probably originates from a larger contact area between electrode surface and electrolyte because of its smaller particle size than commercial TiO2's size. A further enhancement in photovoltage was attained for an electrode of a hydrothermally-synthesized TiO2 with good crystallinity. We consider that electron-hole recombination was suppressed because hydrothermal TiO2 has a lower density of lattice defect trapping the photoexcited carriers. As photoelectrochemical capacitor, a composite electrode consisting of hydrothermal TiO2 and MnO2 exhibited a 2.4 times larger discharge capacity compared with that of commercial TiO2 and MnO2. This result is attributed to an increased amount of Na+ adsorption induced by the enhanced photovoltage of TiO2

Lithiation and Delithiation Properties of Si-based Electrodes Pre-coated with a Surface Film Derived from an Ionic-liquid Electrolyte

Ionic-liquid electrolytes can enhance battery performance and safety but are expensive. To reduce the use of ionic-liquid electrolytes, we investigated the charge/discharge properties of Si-based electrodes in an organic-liquid electrolyte, where the electrode surface was pre-coated with a film derived from an ionic-liquid electrolyte. No improvement in the electrode performance was observed compared to that of a nonmodified Si electrode. Once the modified film was broken down, a stable surface film could not be reformed in the organic-liquid electrolyte

Improved Electrochemical Performance of a GexS1-x Alloy Negative Electrode for Lithium-Ion Batteries

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

Indium-Doped Rutile Titanium Oxide with Reduced Particle Length and Its Sodium Storage Properties

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

Reaction Behavior of a Silicide Electrode with Lithium in an Ionic-Liquid Electrolyte

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

Enhanced Performance of Sn4P3 Electrode Cycled in Ionic Liquid Electrolyte at Intermediate Temperature as Na‐Ion Battery Anode

Charge-discharge performances of Sn4P3 anodes for Na‐ion battery were evaluated in an ionic liquid electrolyte using N‐methyl‐N‐propylpyrrolidinium bis(fluorosulfonyl)amide at intermediate temperatures of 60 and 90 oC. At these temperatures, the anode showed extra capacities based on the full sodiation of Sn in a potential range below 0.2 V vs. Na+/Na because its slow kinetics was improved by elevating operation temperature. Under the current density of 0.1 A g-1 (0.08 C), the Sn4P3 anode at 60 oC exhibited a large capacity of 750 mA h g-1 at the 120th cycle and high Coulombic efficiencies above 99% after the 5th cycle. On the other hand, the efficiency degraded at 90 oC by the electrolyte decomposition. At 60 oC, the anode attained an excellent rate performance with capacity of 250 mA h g-1 even at 3 A g-1 (2.65 C). These results demonstrated the promising operation at intermediate temperature at around 60 oC for Sn4P3 anode in ionic liquid electrolyte