Unraveling the Nanoscopic Strain Evolution in Single Crystal Cathodes

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Abstract
Single-crystal layered oxides (SC-NMC) with grain boundary-free configuration, have effectively addressed the long-standing cracking issue of conventional polycrystalline Ni-rich cathodes (PC-NMC) for lithium-ion batteries, prompting a shift in optimization strategies. However, continued reliance on anisotropic lattice volume change—a well-established failure indicator in PC-NMC—as a metric for understanding lattice strain and guiding compositional designs for SC-NMC becomes contentious. Herein, leveraging multi-scale diagnostic techniques, we unraveled the distinct nanoscopic strain evolution in SC-NMC cathodes, challenging the conventional composition-driven strategies and mechanical degradation indicators used for PC-NMC. Through particle-level chemo-mechanical analysis , we reveal a decoupling between mechanical strain and lattice volume change of SC-NMC, identifying that the structural instability in SC-NMC is primarily driven by multiple-dimension lattice distortions induced by kinetics-driven reaction heterogeneity and progressively deactivating chemical phases. Consequently, the roles of cobalt (Co) and manganese (Mn) in SC-NMC have been redefined based on the newly established mechanical failure mode. Unlike cobalt’s detrimental role in PC-NMC, we find Co to be critical in enhancing the longevity of SC-NMC cathodes by mitigating localized lattice strain along extended diffusion pathway, whereas Mn exacerbates mechanical degradation. Our findings fundamentally redefine the compositional requirements for SC cathodes compared to conventional NMC cathode systems, offering new insights into developing mechanically robust electrode materials with high capacity and superior durability.

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