Role of LaNiO₃ in suppressing voltage decay of layered lithium-rich cathode materials

Lithium-rich cathode materials possess poor cycle stability and severe voltage decay during cycling. The main reason is that they suffer severe structure transformation during (I) Li₂MnO₃ activation in the initial charge process accompanied with the release of O₂ and the extraction of Li-ions from transition metal (TM) layer; (II) the migration of TM-ions (Mn⁴⁺, Ni²⁺) from TM layer to Li layer during cycling. Both of them accelerate phase transformation from the layered to LiNi_xMn_2−xO₄ (0 ≤ x ≤ 2) spinel structure. In order to solve this problem, LaNiO₃ surface reorganization layer is proposed in this work. In the process of surface modification, the La salts are decomposed and then bond with Ni ions that diffused from the bulk of particles to the surface at high-temperature calcinations. Due to the strong La–O bond energy, there is less Li₂O removed and less lithium vacancy formed in TM layer during every charge processes. In addition, the stable existing form of Ni³⁺ in material surface inhibits the migration of TM-ions from TM layer into Li layer. LaNiO₃ surface can protect the electrode from the erosion by electrolyte, and effectively impede the electrode/electrolyte interface side reactions. Owing to the positive effects of LaNiO₃ surface reorganization layer, the modified Li_1.2Mn_0.6Ni_0.2O₂ samples exhibit superior high capacity retention (more than 87.7% after 200 cycles) with a significantly decrease in voltage decay, which exhibit an improvement over the state of art.