Bulk and Surface Reactivity of LiNi0.5Mn1.5O₄ in Contact with (Acidic) Water

Lithium-ion battery cathode materials such as LiNi0.5Mn1.5O4 (LNMO) are very sensitive to water, which has so far hindered the successful commercialization of aqueous electrode processing strategies. Herein, a detailed investigation of the surface and bulk reactivity of ordered LNMO with water and an aqueous solution of phosphoric acid to decipher the reaction mechanism and the impact on the eventual electrochemical behavior is presented. The comprehensive analysis via, for instance, neutron diffraction and synchrotron X-Ray diffraction, X-Ray absorption spectroscopy, magic-angle spinning nuclear magnetic resonance spectroscopy, thermogravimetric analysis coupled with mass spectrometry, high-resolution transmission electron microscopy, and X-Ray photoelectron spectroscopy reveals that the (acidic) water treatment particularly affects a very thin layer at the particle surface, while the bulk material remains largely unaffected. Nonetheless, when processed classically with N-methyl-2-pyrrolidone and polyvinylidene fluoride into electrodes, the significant impact of this layer on the electrochemical behavior highlights the important impact of the material surface on the eventually achievable performance in battery cells.