Underlying limitations behind impedance rise and capacity fade of single crystalline Ni-rich cathodes synthesized via a molten-salt route

Layered oxide LiNi x Mn y Co z O 2 (NMC) cathodes are often synthesized as polycrystalline secondary particles. Due to intergranular fracture stemming from volume changes of randomly oriented primary particles during charge/discharge, the synthesis of larger single-crystalline cathodes is of high interest. In this work, molten salt assisted growth of micron-sized Ni-rich crystals is achieved with excellent crystallinity, low cation mixing, and negligible impurities. However, electrochemical performance is compromised by high surface reactivity resulting in decomposition of electrolyte and subsequent formation of a thick CEI layer. While intergranular fracture is eliminated, planar gliding and severe intragranular fracture along the (003) plane occurs in the high voltage region within the first few cycles and is associated primarily with H2 to H3 structural transitions. In addition, H2 to H3 transitions are highly irreversible with cyclic voltammograms revealing polarization growth within <5 cycles. Subsequently, the single-crystalline material exhibited markedly reduced available capacity and enhanced capacity fade from sharp impedance growth compared to its polycrystalline counterpart. This work furthers a fundamental understanding into the limitations of single-crystalline Ni-rich cathodes, and the obstacles limiting the advantages offered by the single-crystalline morphology. • Molten-salt flux is effective for synthesizing single-crystal Ni-rich cathode. • High energy (012) facets facilitate the formation of a thick CEI layer. • Highly irreversible H2 to H3 transition leads to rapid capacity fade. • Rapid onset of intragranular fracture and planar gliding within a few cycles. • Overpotential/impedance sharply rises upon moderate delithiation.