Insights into the enhanced cycling stability of cobalt-free single-crystal layered oxide cathodes at elevated voltage

Co-free single-crystal cathodes Li(Ni_xMn_1−x)O₂ have been proposed as promising candidates due to remarkable thermal stability and low cost. Unfortunately, insufficient in-depth understanding on the failure mechanisms hinders the development of these cathodes. Here, we compare the electrochemical performance of two model materials, that are, single-crystal cathodes LiNi_0.6Mn_0.4O₂ (NM64) and LiNi_0.8Mn_0.2O₂ (NM82) under the same SOC (rather than the same cut-off voltage) in order to shed light on the capacity fading mechanism of Li(Ni_xMn_1−x)O₂ from a different point of view. By modulating the SOC, we find that NM82 undergoes faster capacity decline than NM64, even when its cut-off voltage is lower (4.5 V for NM82 and 4.6 V for NM64). The more serious Li/Ni mixing in NM64 helps mitigate its anisotropic lattice contraction, while the more highly active Ni⁴⁺ ions in NM82 induce serious irreversible phase transition, more serious loss of lattice oxygen and undesirable reactions. This work highlights that the damage of “high” voltage should be re-assessed together with other factors, such as anisotropic lattice contraction and the amount of highly reactive Ni⁴⁺. And developing high voltage single-crystal Li(Ni_xMn_1−xO₂) cathodes with moderate Ni content is an effective route towards high stability Co-free single-crystal cathodes with a low cost.