The effect of lithium-excess on Ni-rich LiNi0.6Co0.2Mn0.2O2 cathode materials prepared by a Taylor flow reactor

Abstract In this study we investigated the effect of the Li-excess on electrochemical properties of LixNi0.6Co0.2Mn0.2O2 cathode materials, which was obtained by sintering Ni0.6Co0.2Mn0.2(OH)2 with a various amounts of LiOH (samples with Li-excesses of 5, 10, 20, and 25 mol% are denoted herein as NCM-5, NCM-10, NCM-20, and NCM-25, respectively). The LixNi0.6Co0.2Mn0.2O2 samples retained their excellent crystalline ordering in the rhombohedral layered structure, with the space group R3-m. When the Li-excess increased, Rietveld refinement revealed that cation mixing occurred, the lattice parameters decreased, the transition metal slab thickness increased, and the inter-slab Li space thickness decreased. Nevertheless, an appropriate degree of cation mixing could retain the structural stability and improve the rate capability of the electrodes. It was found that the sample containing the 20 mol% Li-excess (NCM-20) achieved the best cyclic stability, with a capacity retention of 90% at a current rate of 1C/1C for 200 cycles between 2.8 and 4.5 V at room temperature. In situ X-ray diffraction confirmed the greater stability of the crystal phase and physical structure of NCM-20 upon initial cycling. In operando microcalorimetry revealed that the thermal stability of NCM-20 was greater than that of the other cathode materials; it exhibited markedly less heat-generated flux and prevented thermal runaway.