Unlocking the synergistic effects of gradient engineering, Mg doping, and in situ Li conductive coating for high-performance Ni-rich LiNi0.92Co0.04Mn0.04O2 cathode materials

Abstract

A Ni-rich, Co-less NCM cathode with a concentration gradient structure, featuring Ni enrichment at the core and Mn at the surface, offers a promising balance of high energy density and cost-effectiveness. This structure optimizes the Ni²⁺/Ni⁴⁺ redox reaction while mitigating parasitic surface reactions between the highly reactive Ni⁴⁺ ions and the electrolyte. However, challenges such as cation-mixing disorder, H2–H3 phase transitions, and Mn³⁺ disproportionation compromise both surface and structural stability, thereby limiting electrochemical performance. To overcome these issues, we propose a synergistic modification strategy that integrates a concentration gradient of Mg doping and Li₃PO₄ coating for LiNi_0.92Co_0.04Mn_0.04O₂ cathodes. A comprehensive evaluation of the physicochemical properties and electrochemical performance of these modified cathodes was conducted. Electrochemical performance testing demonstrated that the Mg-doped (0.02 mol%) and Li₃PO₄-coated concentration gradient NCM92 cathode (MP-2@CG-NCM92) significantly outperforms its pristine counterpart, achieving 90.72% capacity retention at 4.3 V and 80% at 45 °C after 100 cycles at 1C, as well as 96.3% capacity retention at 4.3 V after 50 cycles at 5C. In contrast, the pristine concentration gradient NCM92 (pristine CG-NCM92) showed only 83.74% capacity retention at 4.3 V and 66% at 45 °C after 100 cycles at 1C, and 75.7% at 4.3 V after 50 cycles at 5C. To investigate the underlying mechanisms, density functional theory (DFT) calculations were conducted in conjunction with a series of experimental techniques, including in operando X-ray diffraction (XRD), self-discharge evaluation, thermal stability testing, focused ion beam-field emission scanning electron microscopy (FIB-FESEM), time-of-flight secondary ion mass spectrometry (TOF-SIMS), and post-mortem analysis. The results demonstrate that our proposed modification strategy effectively enhances the structural stability, surface durability, and electrochemical performance of the cathode, positioning it as a promising solution for the development of next-generation high-energy-density lithium-ion batteries.