Single-Crystal-like Durable LiNiO₂ Positive Electrode Materials for Lithium-Ion Batteries

Cobalt-free, nickel-rich positive electrode materials are attracting attention because of their high energy density and low cost, and the ultimate material is LiNiO2 (LNO). One of the issues of LNO is its poor cycling performance, which needs to be improved. Referring to a current study to show the improved stability of single-crystal-like high-nickelate materials, we fabricated single-crystal-like (SC-) LNO and the counterpart polycrystalline (PC-) LNO samples and examined their electrochemical properties. SC-LNO was nearly single-crystal-like, as proved by electron backscattering diffraction, and had more cation mixing than PC-LNO. Cycle tests under 2.5–4.2 V, a 2C rate, and 45 °C conditions showed that the capacity retention of SC-LNO after 500 cycles (63.5%) was significantly better than that of PC-LNO (36.1%) under the same conditions and even better than that of PC-LNO cycled between 2.5 and 4.15 V (50.7%) with the same initial capacity as SC-LNO. The derivative dQ/dV profile of PC-LNO became featureless during a long cycling time, suggesting the progress of cation mixing in PC-LNO, whereas that of SC-LNO was better maintained, in accordance with the serious particle cracking in PC-LNO and no particle cracking found in SC-LNO as the result of post-mortem analysis after 500 cycles. The electrode impedance increase of PC-LNO was considerably larger than that of SC-LNO, corresponding to the formation of rock-salt phases at the surface and the cracked interface of the PC-LNO and the formation of scattered spinel-like phases with a thick cathode electrolyte interphase at the surface of SC-LNO. Accordingly, SC-LNO is shown to be less degraded in both the bulk nature (stable dQ/dV profile and no cracking) and the surface characteristics (high rate capacity maintenance and less impedance increase), suggesting the importance of single-crystal-like particles as durable electrode materials