Improving Cyclability and Structural Stability of Co-Free Layered Cathode by Controlling Porosity and Cracks in Secondary Particles for Low-Cost and High-Energy LIBs

The Co-free Li[Ni_0.75Mn_0.25]O₂ (NM7525) cathode offers the significant advantage of low production costs. However, it requires a high-voltage charging process (≥4.5 V vs Li/Li⁺) to achieve the energy density level of high-Ni Li[Ni_xCo_yMn_z]O₂ (x ≥ 0.8) cathodes, which leads to severe structural and morphological degradation in the secondary particles during prolonged cycling. Herein, it is demonstrated that the formation of a stable and homogeneous cathode-electrolyte interface (CEI) can effectively suppress large porosity and crack propagation in the secondary particles of the NM7525 cathode, as well as undesirable structural changes and microstrain in the crystal structure, with a high charging cut-off voltage of 4.45 V and an elevated temperature of 45 °C in the full-cell system. For stabilization of the CEI layer at the high-voltage operation, an optimized electrolyte system is applied, containing the additive with low highest occupied molecular orbital (HOMO) energy. This controlled porosity and crack formation in secondary particles enhances the electrochemical performances of the NM7525-based full cell. In the case of cyclability, the CEI-stabilized full-cell delivers a high-capacity retention of 90% after 100 cycles under the harsh operation conditions of high charging cut-off voltage and elevated temperature, whereas the as-prepared full-cell shows a retention of just 78%.