Revisiting the impact of Co at high voltage for advanced nickel-rich cathode materials

The utilization of high nickel content NCM cathode material is now viewed as the promising strategy to alleviate the gap between the demand and supply in the commercialized Li-ion batteries (LIBs), especially in the electric vehicles (EVs) and portable electronic. Both LiNi0.8Co0.1Mn0.1O2 (NCM811) and LiNi0.8Co0.1Al0.05O2 (NCA) that successfully applied in the LIBs industry has a tendency to decrease the content of cobalt from the cost and environmental concerns. Despite a lot of efforts have been made, the high nickel content oxides cathodes still suffered from the fast capacity fading thus leading to the large and time consuming debates on the Co effect. Herein, the different Co content Ni-rich layered oxide compound, LiNi0.8Co0.1Mn0.1O2 (NCM811) and LiNi0.8Co0.15Mn0.05O2 (NCM8155), were systematically studied to disclose the influence of Co on the battery performance especially in the high voltage regions. Electrochemical tests showed when the cut-off voltage is 4.7 V, the capacity retention of the NCM8155 was much higher than that of the NCM811 material i.e. 85.70 % and 67.83 % after 100 cycles, respectively. This can be ascribed to the reduce of the crack growth supported by the SEM, STEM and multiple characterization techniques. In situ XRD unveiled the high Co could effectively inhibit the expansion of the lithium interlayer spacing under high voltage, which in turn lessen the lattice expansion of the material after cycling. This was further confirmed by the theoretical study using DFT calculation from atomic points of view. Our research provides a novel insight into the influence of Co content in the Ni-rich layered oxide materials thus accelerating the development of advanced energy storage systems.