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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.

The reversible capacity is not dependent on the amount of Si included in the silicide; the capacity was found to be independent on the composition of silicide even in the case of a given transition metal making up the silicide.

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.

We investigated the Li storage properties of spindle single-crystalline rutile TiO2 fine particles synthesized by a large-scale sulfate process. Their anode properties were compared with those of p...

We investigated the impact of electrolyte difference on lithiation and delithiation properties of a Li1.00Si 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 Li1.00Si desorbed by simply immersing the electrode in an ionic-liquid electrolyte, that is, the phase transition of Li1.00Si 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 Li1.00Si did not elute into the electrolyte. The addition of vinylene carbonate (VC) to the ionic-liquid electrolyte suppressed the phase transition of Li1.00Si to Si. Although the Li1.00Si 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 Li1.00Si contributed to the superior cycle life; thus, the characteristic ductility, malleability, and high electrical conductivity of lithium silicide should improve the electrochemical performance.

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Understanding the distribution of various LixSi phases is essential for producing lithium-ion batteries with Si-based electrodes. Herein, we developed a facile and rapid analysis method by utilizin...

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.

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.

We investigated the impacts of MnO2 crystal structures and Fe doping into the MnO2 crystal structures on photoelectrochemical charge–discharge properties of composite electrodes composed of TiO2 an...

Rapid quenching is one of the methods for preparing silicide/Si composite alloy as an active material for negative electrode in lithium-ion batteries. In this study, we focused on the method’s control over the positional relation between the Si and silicide phase by changing the additive elements. Various Si-alloys and the relationship between their lithiation and delithiation properties and their arrangement was investigated.

Single-crystalline Nb-doped TiO2 particles were hydrothermally synthesized for Na-storage materials. Annealing in air changed the particle color from blue to white. The successful introduction of o...

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.

Photoelectrochemical charge–discharge properties of TiO2/MnO2 composite electrodes were investigated in Na2SO4 aqueous solutions by using the photosynthesis-related electrolyte additives of nicotinamide adenine dinucleotide phosphate (NADPH) and adenosine triphosphate (ATP). The photovoltage significantly increased from 230 to 700 mV by adding only NADPH because it efficiently consumes photoexcited holes on TiO2 to enhance electron accumulation. By adding both ATP and NADPH, the photovoltage was further enhanced to 1400 mV. The drastic synergy effect successfully boosted the photoelectrochemical Na+ storage because the enhanced photovoltage of TiO2 could increase the Na+ adsorption amount on MnO2, demonstrating a new photoelectrochemical energy-storage system inspired by the photosynthesis.

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.

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.

1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide (EMI-TFSA) is one of the promising ionic liquids as electrolyte solvent to enhance the electrochemical performance of Si electrode for Li-ion batteries (LIBs) because of its low viscosity and high conductivity. However, it has low stability against reduction and its reductive decomposition product inhibits Li+ insertion to electrodes, leading to poor cycling stability. To exert a potential of EMI-TFSA, we employed vinylene carbonate (VC) as film-forming additive. Si electrode exhibited very high cycling stability and rate capability in 20 vol.% VC-added EMI-TFSA-based electrolyte. In addition, by replacing TFSA anion with bis(fluorosulfonyl)amide (FSA) for Li salt and ionic liquid solvent, an excellent cycling performance and outstanding rate capability was achieved. VC cannot only fabricate a good surface film but also lower the interaction between Li+ and FSA-, providing smooth desolvation of FSA- to obtain better high-rate performance. Non-flammability of the VC-added electrolytes was confirmed by fire resistance test in closed-system: no ignition was observed even at 300°C. Consequently, we found that mixture electrolyte consisted of EMI-based ionic liquid and VC, especially 1 M LiFSA/EMI-FSA with 20 vol.% VC, is a prospective candidate for simultaneously enhancing the electrochemical performance of Si electrode as well as safety of LIBs.

Mesoporous spherical aggregates of Nb-doped anatase TiO2 nanoparticles were synthesized by a simple one-pot solvothermal method in supercritical methanol and were evaluated as anode materials for L...

To achieve electrode performance with both high capacity and long cycle life, we investigated the effect of the anion structure in an ionic liquid electrolyte on the electrochemical performance of an annealed Ni-P/(etched Si) negative electrode for Li-ion batteries. The electrode maintained a discharge capacity of 1890 mA h g-1 after 250 cycles in bis(fluorosulfonyl)amide-based ionic liquid electrolyte, which was approximately three times higher than that in bis(trifluoromethanesulfonyl)amide-based electrolyte.

We investigated the lithiation and delithiation properties of pure binary silicide electrodes in an ionic liquid electrolyte as novel anodes for lithium‐ion batteries. Some electrodes maintain a high reversible capacity in the electrolyte, whereas they show a poor cycling performance in an organic electrolyte. The superior performance results from the high affinity for the transition metal that composes the silicide with Li. Based on reaction behavior analysis, the crystal structure of silicide is maintained during the cycling, and phase separation does not occur. The ionic liquid electrolyte suppresses the formation of cracks and exfoliation of the silicide layer from a substrate. In addition, a surface film formed on the silicide electrode through the reductive decomposition of the electrolyte has different components than that on a Si electrode, even in the same ionic liquid electrolyte. Soft X‐ray emission spectroscopy demonstrates that the pure silicide itself reacts with Li. The obtained results will provide significant insights into novel alloy‐based anode materials for lithium‐ion batteries.

We have evaluated the anode properties of the silicide/Si composite electrodes for lithium-ion batteries and revealed that ternary silicide made by elemental substitution improved the electrochemical performance of the electrodes. In particular, a Cr0.5V0.5Si2/Si composite electrode exhibited a good cycle stability. Herein, we attempted mechanical grinding of the Cr0.5V0.5Si2/Si composite and addition of fluoroethylene carbonate (FEC) into the electrolyte to further improve the performance of the electrode. The electrode showed a superior cycling performance by these attempts as expected. The mechanical grinding should cause the formation of amorphous Si phase and fine dispersion of Cr0.5V0.5Si2 in the Si phase, which suppresses the pulverization of the Cr0.5V0.5Si2/Si composite particle during charge-discharge. It is considered that the addition of FEC suppresses the continuous reductive decomposition of the electrolyte, which contributes to the improvement in the cyclability.

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.

We prepared composite electrodes by using rutile TiO 2 particles and γ-MnO 2 particles, and evaluated their photoelectrochemical capacitor properties based on Na + adsorption by light irradiation in aqueous electrolytes. By employing different synthesis method for TiO 2 particles, we synthesized TiO 2 particles with various particle sizes and crystallite sizes. An electrode of sol–gel-synthesized TiO 2 showed higher photovoltages compared with an electrode of commercial TiO 2 . This probably originates from a larger contact area between electrode surface and electrolyte because of its smaller particle size than commercial TiO 2 's size. A further enhancement in photovoltage was attained for an electrode of a hydrothermally-synthesized TiO 2 with good crystallinity. We consider that electron−hole recombination was suppressed because hydrothermal TiO 2 has a lower density of lattice defect trapping the photoexcited carriers. As photoelectrochemical capacitor, a composite electrode consisting of hydrothermal TiO 2 and MnO 2 exhibited a 2.4 times larger discharge capacity compared with that of commercial TiO 2 and MnO 2 . This result is attributed to an increased amount of Na + adsorption induced by the enhanced photovoltage of TiO 2 .

Sodiation–desodiation behaviors were investigated for electrodes composed of various binary phosphides, InP, CuP2, GeP, SiP, and LaP, as anode materials of a Na-ion battery. Although LaP electrode did not react with Na, the other electrodes showed reversible sodiation–desodiation reactions in the initial cycles. Rapid capacity decays were observed for CuP2, GeP, and SiP electrodes. In contrast, a better cyclability was attained for the InP electrode. These results indicate that binary phosphides (M–P) require four properties for improving cyclability: (i) low thermodynamic stability of M–P, (ii) high electronic conductivity of M, (iii) low hardness of M, and (iv) reactivity of M with Na.

To evaluate the properties of TiO2/MnO2 composite electrodes for use as photoelectrochemical capacitors, Zn-doped TiO2 with a rutile-type structure hydrothermally synthesized as a photoelectric conversion material. The substitutional solid-solution of 2 at.% Zn in TiO2 successfully increased the oxygen vacancy amount and the electronic conductivity. A wider optical band gap was observed for the Zn-doped TiO2 compared with an undoped TiO2, indicating that Zn doping increased the carrier concentration due to the generation of oxygen vacancies. The photovoltage of the TiO2 electrode was drastically enhanced from 250 to 1600 mV by 2 at.% Zn doping. The charge-discharge properties of the Zn-doped TiO2/MnO2 composite electrodes were evaluated by photoirradiation of Zn-doped TiO2, where Na+ adsorption−desorption reactions occurred on the MnO2 surface. The Zn-doped TiO2/MnO2 composite electrodes afforded a larger discharge capacity of 13 mA h cm−2 compared to 7 mA h cm−2 for the TiO2/MnO2 composite. Plausibly, the higher carrier concentration increased the Fermi level above the conduction band of TiO2, enabling conducting electrons to overcome the energy barrier between the TiO2 particles. Doping Fe into MnO2 in the composite electrode further enhanced these properties, and Fe doping increased the capacity of the composite from 13 to 25 mA h cm−2. These results demonstrate that the commonly used approach of doping with impurity elements can improve the properties of various electrode materials for use as photoelectrochemical capacitors.

We evaluated the Na-storage properties of Fe2O3/Sb composite electrodes prepared using α-Fe2O3 with different particle sizes and Sb, and investigated the effect of Sb addition on their charge−discharge cycling performances as Na-ion battery anodes. With reducing the Fe2O3 particle size, the initial reversible capacity of the Fe2O3 electrode was increased, whereas the capacity decay became more pronounced. The 10 wt% addition of Sb could significantly improve the rapid capacity decay which was observed for the electrode of Fe2O3 with the smallest particle size. These results suggest that Sb improved the conductions of electron and Na ions in the electrode. The overpotential of desodiation reactions of Fe2O3 was obviously suppressed by the Sb addition. This result indicates that Sb maintained the electron percolation network between the sodiated Fe2O3 particles to improve the reversibility of Fe2O3 conversion reaction. Consequently, we could demonstrate that Sb has a remarkable effect of drawing out the potential anode performance of abundant and inexpensive Fe2O3.

The electrochemical performance of variously treated Ni-P-coated Si (Ni-P/Si) negative electrodes for lithium-ion batteries was investigated. X-ray diffraction analysis revealed the formation of nickel silicide (NiSi and NiSi2) after annealing, which improved the adhesion between the Ni-P coating layer and Si particles. Spotty Ni-P particles did not aggregate on an etched Si surface due to an anchor effect, even after annealing, whereas the particles aggregated on an untreated Si surface. An annealed Ni-P/(etched Si) negative electrode maintained a discharge capacity of 2000 mA h g-1 even at the 100th cycle in an organic electrolyte, which can be attributed to Ni-P particles remaining on the surface of the annealed Ni-P/(etched Si) electrode even after the charge-discharge test. The annealed Ni-P/(etched Si) electrode also exhibited superior cycle performance with a reversible capacity of 1000 mA h g-1 over 750 and 1100 cycles in an organic electrolyte containing film-forming additive and an ionic liquid electrolyte, respectively. Consequently, the annealed Ni-P/(etched Si) electrode achieved both high reversible capacity and long cycle life.

Concentration changes of electrolyte salt in practical lithium ion batteries occur due to various factors during operation, and the changes causes serious degradation of battery performance. It is important to identify elementary factors and how each of the factors induces the concentration changes in batteries by using a simplified system. The concentration of ions in the electrolyte solution between a highly oriented pyrolytic graphite (HOPG) electrode and a lithium foil electrode in a model battery was studied during charge/discharge cycles by in situ microprobe Raman spectroscopy. The concentration of ions decreased during de-intercalation of Li+ from HOPG. The decreased concentration recovered during subsequent rest time with a time constant of several hours. The results of impedance spectroscopy showed that the cycle dependence of the resistance of the surface film on the graphite was similar to that of the time constant. On the other hand, the cycle dependence of the capacitance at the graphite surface and the cycle dependence of the charge transfer resistance at the graphite surface were similar to that of the degree of concentration change. The results suggest that the resistance of the surface film can be evaluated from the rate of the recovery.