Capacity Fading of Ni-Rich Li[NixCoyMn1–x–y]O2 (0.6 ≤ x ≤ 0.95) Cathodes for High-Energy-Density Lithium-Ion Batteries: Bulk or Surface Degradation?

Ni-rich Li[NixCoyMn1–x–y]O2 cathodes (x = 0.6, 0.8, 0.9, and 0.95) were tested to characterize the capacity fading mechanism of extremely rich Ni compositions. Increasing the Ni fraction in the cathode delivered a higher discharge capacity (192.9 mA h g–1 for Li[Ni0.6Co0.2Mn0.2]O2 versus 235.0 mA h g–1 for Li[Ni0.95Co0.025Mn0.025]O2); however, the cycling stability was substantially reduced. Li[Ni0.6Co0.2Mn0.2]O2 and Li[Ni0.8Co0.1Mn0.1]O2 retained more than 95% of their respective initial capacities after 100 cycles, while the capacity retention of Li[Ni0.9Co0.05Mn0.05]O2 and Li[Ni0.95Co0.025Mn0.025]O2 was limited to 85% during the same cycling period. The relatively inferior cycling stability of Li[NixCoyMn1–x–y]O2 with x > 0.8 is attributed to the phase transition near the charge-end, causing an abrupt anisotropic shrinkage (or expansion during discharge), which was suppressed for compositions of x < 0.8. Residual stress stemming from the phase transition destabilized the internal microcracks and allowe...