Characterization Assisted Lithium Ion Cathode Design: Application of Solid-State NMR to Understand Bulk and Interface Relation of Aluminum Coated and Doped NMC Cathodes

Abstract

Researchers worldwide are searching for new electrochemical energy storage materials that meet high energy density, life, cost, and safety requirements of batteries for electric vehicle applications. The strategies for developing new chemistries with higher energy density, stability and safety concerns involve mitigating the problems of existing materials and understanding the structural changes and their relations with materials activity which requires characterization assisted material synthesis. For lithium ion cathode design, some of the mitigation strategies for low electrochemical cycle life, stability and safety concerns involve, increase of lithium content, change transition metal composition and use of different dopants and surface coatings. However, each change brings additional challenge to understand the structural modifications induced and its relationship with materials activity.In this work, we synthesize a series of Al2O3-coated LiNixMnyCo1-x-yO2(NMC) materials with different transition metal compositions, various aluminum loadings, solvents and different annealing conditions. Then solid state NMR, electron microscopy, and high-resolution X-ray diffraction techniques are applied to examine the interfacial and bulk composition and structure of alumina-coated cathodes prepared under different annealing temperatures and with cathode compositions. Their chemical and structural evolution after electrochemical cycling are further studied with 7Li and 27Al solid state NMR.We find that higher annealing temperatures lead to a more homogeneous and strongly attached coating on the cathode surface, accompanied with the formation of the new LiAlO2 phase due to the interaction between the surface alumina coating and the surface lithium species from the cathodes. A strong diffusion of surface Al into the bulk after high-temperature annealing is also found for cathodes with low Mn content. Aluminum loading level also determines how effectively a dopant and coating develop in the material. Our electrochemical tests show that material performance strongly depend on right coating to dopant ratio and interface composition which should be tuned for every cathode composition. This brings the need for characterization assisted synthesis for development of next generation lithium ion cathodes1-3.