Capacity Fading of Ni-Rich Li[Ni_<i>x</i>Co_<i>y</i>Mn_{1–<i>x</i>–<i>y</i>}]O₂ (0.6 ≤ <i>x</i> ≤ 0.95) Cathodes for High-Energy-Density Lithium-Ion Batteries: Bulk or Surface Degradation?

Abstract

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 allowed the microcracks to propagate to the surface, providing channels for electrolyte penetration and subsequent degradation of the exposed internal surfaces formed by the microcracks. Further developments in particle morphology are required to dissipate the intrinsic lattice strain, stabilize the surface, and modify the composition to attain a satisfactory long-term cycling stability, and hence battery life.