Understanding mechanical failure behaviours and protocol optimization for fast charging applications in Co-free...

Currently, it is a significant challenge to achieve long-term cyclability and fast chargeability in lithium-ion batteries, especially for the Ni-based oxide cathode, due to severe chemo-mechanical degradation. Despite its importance, the fast charging long-term cycling behaviour is not well understood. Therefore, we comprehensively evaluate the feasibility of fast charging applications for Co-free layered oxide cathodes, with a focus on the extractable capacity and cyclability. The cathodes with a Ni content of over 80% attain 80% of their nominal capacity, along with superior cyclability under fast charging due to the suppression of the following two mechanical failure modes: (i) Li-ion concentration shock fracture (CSF) and (ii) H2-H3 phase shock fracture (HSF). In particular, CSF produces stronger stress than HSF and causes severe crack penetration in mid-Ni cathodes under fast charging. Meanwhile, HSF induces mild internal stress, but prolonged exposure accelerates mechanical degradation. To maximize the fast charging application of high-Ni cathodes, we evaluated a 5C constant current constant voltage protocol to deliver 180 mAh g^-1 in 35 min, improving the cycle life by up to 89% over 100 cycles with LiNi_0.90Mn_0.10O₂. This study provides insights into the fast charging applications of high-Ni cathodes, thereby advancing the understanding of their behaviour and optimization.