Enhancing Power Density of Lithium-Ion Batteries through Controlled Reaction Mechanism of Lithium Insertion Material of LiNi 0.5 Mn 1.5 O 4

Throughout the last two decades, numerous studies have been performed to improve the power capability of LiNi 0.5 Mn 1.5 O 4 (LNMO), such as via element substitution, surface modification, and particle morphology control. The reaction mechanism is another factor affecting rate capability; single-phase homogeneous versus two-phase coexistence reactions. In this study, to improve the rate capability of LNMO by controlling the reaction mechanism, we synthesized LiNi 0.5 Mn 1.3 Ti 0.2 O 4 (Ti-LNMO), in which a part of Mn 4+ in LNMO was replaced by Ti 4+ [1]. During charging, the cubic lattice constant of Ti-LNMO decreased linearly from 8.20 Å to 8.05 Å (Fig. a). This result reveals that the lithium extraction of Ti-LNMO proceeds in a single-phase reaction over the entire range, while LNMO proceeds two-phase reaction [2]. To evaluate the inherent rate capability of LNMOs, the diluted electrode method [3,4], in which replacing a part of the active material with an inactive material makes the Li-ion transport process in the solid phase rate-determining, was applied to rate-capability tests (Fig. b). The Ti-LNMO exhibited superior rate capability than the LNMO without Ti-substitution, which demonstrated that high-power capabilities could be achieved by controlling the reaction mechanism. Furthermore, the solid-state Li-ion diffusion coefficient in the LNMOs was determined from the relationship between the discharge capacity and the current density using following equation [5], Q = ( a 2 j W )/(15 D Li ) where a is the particle radius, and D Li is the diffusion coefficient of Li ions in the particle. From the linear relationship between the discharge capacity and current density (Fig. c), D Li of Ti-LNMO and LNMO was calculated to be 9.5 × 10 −11 and 3.6 × 10 −11 cm 2 s −1 , respectively. This is the reason that the rate-capability of Ti-LNMO was more than three times greater than that of the LNMO. These results reveal that the lithium insertion mechanism is important for the kinetics of lithium insertion reaction. References [1] K. Ariyoshi and J. Sugawa, Electrochim. Acta , 455 , 142425 (2023). [2] K. Ariyoshi et al ., J. Electrochem. Soc. , 151 , A296–A303 (2004). [4] K. Ariyoshi et al ., J. Electrochem. Soc ., 167 , 140517 (2020). [3] K. Ariyoshi et al ., J. Power Sources , 509 , 230349 (2021). [5] K. Ariyoshi and J. Sugawa, Electrochemistry , 89 , 157–161 (2021). Figure 1