Enhancing Li-rich Cathode Materials by Ammonia Treatment – Insights from Solid-state NMR Spectroscopy

Li-rich cathode materials of the formula xLi₂MnO₃·yLiNi_aCo_bMn_cO₂ (x + y = 1, a + b + c = 1) suffer capacity and average voltage fade during cycling in Li-ion batteries that prohibits their commercialization despite their high discharge capacity, ca. 250 mAh/g. Treatment of these materials with NH_3(g) at high temperatures produces materials with enhanced electrode capacity and a stabilized average voltage during cycling. The present study provides insights into the changes occurring in the materials via ⁶Li and ⁷Li solid state NMR investigations of the untreated and treated 0.35Li₂MnO₃·0.65LiNi_0.35Mn_0.45Co_0.20O₂ material, as well as its constituent phases, Li₂MnO₃ and LiNi_0.4Co_0.2Mn_0.4O_2, separately. Treatment with the gas for the right duration increases the content of desirable surface products (Li₂O, LiOH, Li₂CO₃, etc.) to enhance the surface film formation with minimal Li leach out from the bulk, while longer treatment results in destruction of the surface film and further Li leaching from the bulk of the electrode, leading to its deterioration.