Understanding the rate capability of high-energy-density Li-rich layered Li_1.2Ni_0.15Co_0.1Mn_0.55O₂ cathode materials

The high-energy-density, Li-rich layered materials, i.e., xLiMO ₂(1-x)Li₂MnO₃, are promising candidate cathode materials for electric energy storage in plug-in hybrid electric vehicles (PHEVs) and electric vehicles (EVs). The relatively low rate capability is one of the major problems that need to be resolved for these materials. To gain insight into the key factors that limit the rate capability, in situ X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) studies of the cathode material, Li_1.2Ni_0.15Co_0.1Mn _0.55O₂ [0.5Li(Ni_0.375Co_0.25 Mn _0.375)O₂·0.5Li₂MnO₃], are carried out. The partial capacity contributed by different structural components and transition metal elements is elucidated and correlated with local structure changes. The characteristic reaction kinetics for each element are identified using a novel time-resolved XAS technique. Direct experimental evidence is obtained showing that Mn sites have much poorer reaction kinetics both before and after the initial activation of Li₂MnO₃, compared to Ni and Co. These results indicate that Li₂MnO₃ may be the key component that limits the rate capability of Li-rich layered materials and provide guidance for designing Li-rich layered materials with the desired balance of energy density and rate capability for different applications. In the cathode material Li_1.2Ni_0.15Co_0.1Mn _0.55O₂ [0.5Li(Ni_0.375Co_0.25Mn _0.375)O₂·0.5Li₂MnO₃] the capacity contributed from different components and elements is elucidated and correlated with the local structure changes. The reaction kinetic characteristics for each element are been identified and differentiated. It is observed that Li₂MnO₃ may be the key component determining the rate capability of the Li-rich layered materials.