Mechanical Design Strategies in Facilitating Durable Ni-Rich Layered Cathodes for Lithium-Ion Batteries

Along with the growth of the electric vehicle market, the Ni-rich layered oxides(x≥0.8) for cathode active materials have attracted great attention because their high Ni content provides high-energy-density to meet the needs of high-energy-density applications. However, the large amount of Ni often results in severe micro-crack propagation that causes capacity degradation due to structural instability such as heterogeneous phase transformations and anisotropic structural changes under high charging voltage conditions. Until recently, many previous studies have focused on inhibiting micro-crack formation due to structural instability, specifically the anisotropic lattice variation that leads to mismatch of primary particles in Ni-rich layered oxides upon repeated (de)intercalation. While the degradation mechanisms by which anisotropic lattice variation act between primary particles have been identified, the mechanism inside the particles is not yet fully understood. In this regard, a more systematic and comprehensive study could be required to explore the mechanisms of anisotropic behavior inside particles in Ni-rich layered oxides. Additionally, through advanced experimental analysis and computational calculations, a comparison of the thermodynamic features of Ni-rich layered oxides (Li[NixCoyMz]O2, where M represents the transition metal) reveals similar phase transitions (H1 (hexagonal 1)→M (monoclinic)→H2→H3) regardless of the transition metal M. However, the cycle retention varies significantly depending on the composition of the transition metal.<br/><br/>In this work, to understand these undesired phenomena, we systematically apply thermodynamic, chemo-mechanic, and physicochemical perspectives to analyze the correlation between (in)coherent phase separation and volume conservation behavior to propose an opposite view of the traditional aspect of anisotropic lattice variation and a new design strategy for single-crystal Ni-rich layered cathodes that reduces the internal stress between separated phases to zero grain internal stress. First, we analyzed the (in)coherent phase separation of Li1-x[Ni10/12Co1/12Mn1/12]O2 (NCM) and Li1-x[Ni10/12Co1/12Ti1/12]O2 (NCT) by comparing the thermodynamic phase stability during the whole delithiation process along the formation energy of mixing enthalpy and found that NCT was thermodynamically more stable and relatively coherent phase separation occurred than commercial NCM. Furthermore, for a direct understanding of the cycle degradation mechanism by (in)coherent phase separations, we calculate the directional lattice strain and volumetric variation for NCM and NCT combined with phase transition. Interestingly, upon volume change, both oxide models exhibited volume-conserving regions due to anisotropic lattice changes, which resulted in a relative decrease in internal stress. Moreover, the NCT exhibits near zero volumetric lattice variation. Based on the correlation between thermodynamics and the volumetric lattice misfit, the oxygen bond populations for both oxide models are identified, and a severe reduction is obsreved across the entire oxygen framework in the incoherent phase separation model NCM, whereas a preserved feature is observed in the coherent phase model NCT. These systemically novel concepts underpin the Ti-induced “zero volumetric misfit” concept makes coherent phase separation. These concepts are expected to play a global role in the layered oxide cathodes, and it provides a crucial design strategy leading to the enhancement of cycle retention for durable high-energy-density LiBs.