Systematic Evaluation of Li ₃ PO ₄ Coatings on LNMO for Enhanced Cycling Stability using NMR‐Based Interfacial Probes

ABSTRACT High‐voltage cathodes such as LiMn _1.5 Ni _0.5 O ₄ (LNMO) offer promising energy density but suffer from interfacial degradation accelerated at elevated voltages and temperatures. Here, we present a comprehensive comparative study of three Li ₃ PO ₄ coating methods (precipitation, sol–gel, and dry sol–gel routes) applied to commercial LNMO powders. Coating quality and intimacy are systematically assessed using a correlative, multitechnique approach including ⁷ Li and ³¹ P solid‐state NMR, X‐ray diffraction, and electrochemical testing. A key insight from this study is the use of ssNMR relaxation behavior as a sensitive probe of coating intimacy to the active phase. The methodology is validated on commercial LNMO and reproduced in a lab‐synthesized LNMO to demonstrate reproducibility across particle morphologies. Among all methods, the sol–gel route produced a uniform ∼20 nm coating with optimal surface contact, translating to improved rate capability and outstanding high‐temperature cycling stability (87% retention after 100 cycles at 50 °C compared to 29% for the non‐coated LNMO), while retaining rate capability. These findings establish a practical framework for designing robust interfacial coatings in high‐voltage lithium‐ion battery materials.