The Implications of Particle Morphology on the Capacity Retention, Side Reactions, and Impedance Build-Up of Nickel-Rich NCMs upon Cycling in Full-Cells: Poly- vs. Single-Crystalline NCM851005

The rising interest in single-crystalline NCMs (LiMO₂, M = Ni, Co, Mn) has generated numerous publications which feature promising results in terms of cycle-life improvement when compared to the conventional polycrystalline analogues. To elucidate the effect of the two morphologies on the capacity retention and the internal resistance, this study aims to discriminate the effect of different degradation phenomena of polycrystalline and single-crystalline NCM851005 (LiNi_0.85Co_0.10Mn_0.05O₂) in coin full-cells cycled against graphite anodes. The impact of the particle morphology is analyzed over the course of more than 200 charge/discharge cycles for two temperatures of 25 and 45 °C, applying 4.1 or 4.4 V as upper cutoff voltages. The morphology-dependent surface area changes, resulting mainly from the tendency of polycrystalline NCMs towards particle cracking upon calendering, charging, and extended cycling, are quantified via krypton-gas physisorption, and the consequences of particle cracking regarding the amount of gas evolution, transition-metal dissolution, loss of lithium inventory, and resistance build-up are evaluated. In particular, the pronounced cathode impedance build-up of polycrystalline NCMs, investigated by electrochemical impedance spectroscopy using a micro-reference electrode in full-cells, exposes the impact of particle cracking and the induced electronic resistances within a secondary agglomerate on the rate capability.