High-Temperature Treatment of Li-Rich Cathode Materials with Ammonia: Improved Capacity and Mean Voltage Stability during Cycling
Li-rich electrode materials of the family xLi(2)MnO(3)center dot(1-x) LiNiaCobMncO2 (a + b + c = 1) suffer a voltage fade upon cycling that limits their utilization in commercial batteries despite their extremely high discharge capacity, approximate to 250 mA h g(-1). Li-rich, 0.35Li(2)MnO(3)center dot 0.65LiNi(0.35)Mn(0.45)Co(0.20)O(2), is exposed to NH3 at 400 degrees C, producing materials with improved characteristics: enhanced electrode capacity and a limited average voltage fade during 100 cycles in half cells versus Li. Three main changes caused by NH3 treatment are established. First, a general bulk reduction of Co and Mn is observed via X-ray photoelectron spectroscopy and X-ray absorption near edge structure. Next, a structural rearrangement lowers the coordination number of Co-O and Mn-O bonds, as well as formation of a surface spinellike structure. Additionally, Li+ removal from the bulk causes the formation of surface LiOH, Li2CO3, and Li2O. These structural and surface changes can enhance the voltage and capacity stability of the Li-rich material electrodes after moderate NH3 treatment times of 1-2 h.