Improved Kinetics in Spinel-Related 5 V Positive Electrode Materials by Changing Lithium Insertion Schemes for Lithium-Ion Batteries

Spinel-related 5 V positive electrode materials LiNi1/2Mn3/2O4 (LNMO), Fe–Ti-co-doped LNMO (LNMO-FT), and LiCoMnO4 (LCMO) were prepared, and their reaction kinetics were examined by a galvanostatic intermittent titration technique (GITT) measurement to understand the factors affecting the reaction kinetics for enhancing the power capability and the energy density. X-ray diffraction (XRD), Raman, and X-ray absorption near-edge structure (XANES) measurements of LNMO-FT indicate that Fe3+ and Ti4+ ions are substituted for Ni and Mn ions and the transition metal ions are randomly distributed at the 16(d) sites in a space group symmetry of Fd3̅m. The lithium insertion/extraction process of LNMO proceeds in two-phase reactions, and LNMO-FT exhibits single-phase reactions with two-phase ones in a limited region. Single-phase reactions of these materials give smaller polarizations associated with mass transfer by the GITT than those of two-phase ones, and there is one-to-one correspondence between the polarization increase and the state of charge (SOC) at which the two-phase reaction proceeds. LCMO, which proceeds in a single-phase reaction, gives small and moderate polarizations throughout the charge and discharge operations. Basic functions and energy densities of spinel-related 5 V materials are compared with those of layered materials. The operating voltage of 4.7 V, which is lower than 5 V, and the highly crystallized octahedral primary particles for LNMO-FT are appropriate for high-voltage stability in nonaqueous electrolytes. All-solid-state lithium-ion batteries may enable us to introduce LCMO because of the small change in the lattice parameter of about 0.7% during the operation and the high operating voltage of above 5 V. Although the energy densities of LNMO-FT and LCMO of 598 and 618 Wh kg–1, respectively, in lithium cells are lower than those of layered materials, the single-phase reactions of spinel-related materials are appropriate for improving the power capability and the energy density.