PNNL

Abstract

With the rapidly growing demand for batteries with higher energy density and extended cycle life, lithium-ion batteries based on single-crystal LiNi1-x-yCoxMnyO2 (NCM, 1-x-y = 0.6) cathode materials are gaining increasing attention due to their improved structural stability re-sulting in superior cycle life compared to batteries based on polycrystalline NCM. However, an in-depth understanding of the less pronounced degradation mechanism of single-crystal NCM is still lacking. Here we present a detailed post-mortem study, comparing pouch cells with single-crystal vs. polycrystalline LiNi0.60Co0.20Mn0.20O2 (NCM622) cathodes after 1375 dis-/charge cycles against graphite anodes. Transmission electron microscopy reveals that the thickness of the cation-disordered layer forming in the near-surface region of the cathode par-ticles does not differ significantly for single-crystal and polycrystalline particles. In contrast, cracking is pronounced for polycrystalline particles, but practically absent for single-crystal particles. Transition metal dissolution as quantified by time-of-flight mass spectrometry on the surface of the solid electrolyte interphase (SEI) of the graphite anode is much reduced for sin-gle-crystal NCM622. Similarly, Li2CO3 and LiOH gas evolution during the first two cycles as quantified by online electrochemical mass spectrometry is much reduced for single-crystal NCM622. Benefitting from all these advantages, we demonstrate a graphite/single-crystal NMC622 pouch cells with a nominal cathode areal capacity of 6 mAh/cm2 with an excellent capacity retention of 83% after 3000 cycles to 4.2 V at room temperature, emphasizing the po-tential of single-crystalline NCM622 as cathode material for next-generation lithium-ion bat-teries.