Ammonia treatment of 0.35Li₂MnO₃·0.65LiNi_0.35Mn_0.45Co_0.20O₂ Material: Insights from solid-state NMR analysis

Li-rich cathode materials of the formula xLi₂MnO₃·yLiNi_aCo_bMn_cO₂ (x + y = 1, a + b + c = 1) boast very high discharge capacity, ca. 250 mAh/g. Yet, they suffer capacity decrease and average voltage fade during cycling in Li-ion batteries that prohibit their commercialization. Treatment of the materials with NH₃(g) at high temperatures produces improved electrodes with higher stability of capacity and average voltage. The present study follows the changes occurring in the materials upon treatment with ammonia gas, through ⁶Li and ⁷Li solid-state NMR investigations of the untreated and ammonia treated 0.35Li₂MnO₃·0.65LiNi_0.35Mn_0.45Co_0.20O₂ as well as its constituent phases, Li₂MnO₃ and LiNi_0.4Co_0.2Mn_0.4O₂. The NMR analysis demonstrates the biphasic nature of these materials. Furthermore, it shows that the Li₂MnO₃ component phase in the integrated material is the phase mostly being affected by the gas treatment. A thickening of a protective surface film in the integrated material, with the right exposure time to the reactive gas, is observed, which further precludes Ni leach out from the bulk and leads to improved electrode performance. Formation of minor electrochemically inactive oxide phases in the integrated material and similarly in the Li₂MnO₃ alone upon longer exposure to the gas suggests that the performance deterioration observed can be linked to the rearrangement of ions in the Li₂MnO₃ constituent phase in the integrated material.