Lithium-ion batteries are widely available commercially and attempts to extend the lifetime of these batteries remain necessary. The energy storage characteristics of NbO₂ with a rutile structure as a material for the negative electrode of lithium-ion batteries were investigated. When negative potential was applied to the NbO₂ electrode during application of a constant current in a nonaqueous solution containing lithium ions, these ions were inserted into the NbO₂. Conversely, upon application of positive potential, the inserted lithium ions were extracted from the NbO₂. In situ X-ray diffraction results revealed that the variation in the volume of NbO₂ accompanying the insertion and extraction of lithium was 0.14%, suggesting that NbO₂ is a zero-strain (usually defined by a volume change ratio of 1% or less) active material for lithium-ion batteries. Moreover, the highly stable structure of NbO₂ allows the corresponding electrode to exhibit excellent cycling performance and coulombic efficiency.

This study investigated the influence of variations in the mixing ratio of ethylene carbonate (EC) to ethyl methyl carbonate (EMC) on the composition and effectiveness of the solid electrolyte interphase (SEI) in lithium-metal batteries. The SEI is crucial for battery performance, as it prevents continuous electrolyte decomposition and inhibits the growth of lithium dendrites, which can cause internal short circuits leading to battery failure. Although the properties of the SEI largely depend on the electrolyte solvent, the influence of the EC:EMC ratio on SEI properties has not yet been elucidated. Through electrochemical testing, ionic conductivity measurements, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy, the formation of Li₂CO₃, LiF, and organolithium compounds on lithium surfaces was systematically analyzed. This study demonstrated that the EC:EMC ratio significantly affected the SEI structure, primarily owing to the promotion of the formation of a denser SEI layer. Specifically, the ratios of 1:1 and 1:3 facilitated a uniform distribution and prevalence of Li₂CO₃ and LiF throughout the SEI, thereby affecting cell performance. Thus, precise control of the EC:EMC ratio is essential for enhancing the mechanical robustness and electrochemical stability of the SEI, thereby providing valuable insights into the factors that either enhance or impede effective SEI formation.

This study aims to improve the electrochemical properties of lithium metal for application as a negative electrode in high-energy-density batteries. Lithium metal was heat-treated at varying temperatures to modify the native and solid electrolyte interphase (SEI) films, which decreased the interfacial resistance between the lithium electrode and electrolyte, thereby improving the cycling performance. Moreover, the influence of the native and SEI films on lithium metals depended on the heat-treatment temperature. Accordingly, X-ray photoelectron spectroscopy (XPS) was performed to investigate the chemical composition of the native and SEI films on the heat-treated lithium metals before and after immersion in an organic electrolyte solution. The XPS results revealed the high dependence of the chemical composition of the outer layer of the native and SEI films on the heat-treatment temperature, implying that the native and SEI films can be effectively modified by heat treatment.

This study examined the role of fluoroethylene carbonate (FEC) in stabilizing the solid electrolyte interphase (SEI) and enhancing the electrochemical performance of SiOx-based composite negative electrodes in lithium-ion batteries. Two electrolyte systems were used: 1.0 mol dm⁻³ (M) LiPF₆ in a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) with 0.5 wt.% VC, and 1.0 M LiPF₆ in a mixture of EC and EMC with 1.0 wt.% VC and 10 wt.% FEC. These systems enabled the investigation of how FEC contributes to SEI stabilization and cycling stability. FEC promotes the formation of a LiF-rich SEI layer, which mitigates volume expansion and enhances capacity retention. Additionally, the accumulation of Li₂CO₃ and Li₂O in the SEI was found to increase interfacial resistance, as observed through electrochemical impedance spectroscopy (EIS). Among the SiOx contents tested (0%, 3%, and 7.8%), the 3% SiOx content exhibited the best balance between SiOx and carbon nanotubes, resulting in improved SEI formation and enhanced electrochemical performance. These results offer insights into the optimization of electrolyte formulations for long-term cycling stability in SiOx-based lithium-ion batteries.