In Situ Formation of a LiBO2 Coating Layer and Spinel Phase for Ni-Rich Cathode Materials from a Boric Acid-Etched Precursor.

Ni-rich cathode materials exhibit superior energy densities and have attracted interest among both research and industrial fields; whereas, their practical application is hindered by the intrinsic drawbacks brought by the high nickel content such as structural instability and rapid capacity fading. Herein, in situ formation of a LiBO2 coating layer and spinel phase layer is achieved on the surface of a Ni-rich cathode material via a boric acid etching method at the precursor state. The spinel phase is considered to have a 3D lithium diffusion tunnel and hence faster diffusion kinetics. Moreover, the LiBO2 layer possesses excellent (electro)chemical inertness and can suppress electrolyte decomposition, resulting in a more inorganic and stable cathode-electrolyte interface. The surface reconstructed sample exhibits better cyclic stability (93.3% capacity retention vs 85.3% for the pristine sample at 1 C for 100 cycles) and rate performance. The superiority of this surface reconstruction is demonstrated by a series of electrochemical techniques and characterization methods including high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), post-mortem X-ray photoelectron spectroscopy (XPS) analysis, and density functional theory (DFT) calculations.

Stable Electrode/Electrolyte Interface for High-Voltage NCM 523 Cathode Constructed by Synergistic Positive and Passive Approaches.

Increasing the working voltage of lithium-ion batteries (LIBs) is an efficient way to increase energy density. However, high voltage triggers excessive electrolyte decomposition at the electrode-electrolyte interfaces, where the electrochemical performance such as cyclic stability and rate capability is seriously deteriorated. A new synergistic positive and passive approach is proposed in this work to construct a stable electrode-electrolyte interface at high voltage. As a positive approach, inorganic lithium sulfide salt (Li2S) is used as an electrolyte additive to build a stable cathode electrolyte interface (CEI) at the LiNi0.5Co0.2Mn0.3O2 (NCM523) cathode surface. In a passive way, acetonitrile (AN) is applied as a solvent additive to suppress oxidative decomposition of a carbonate electrolyte via preferential solvation with a lithium ion. Because of the synergistic interaction between the positive and passive approaches, the cyclic stabilities of NCM523/Li cells improved with a tiny amount of Li2S (0.01 mg mL-1) and AN (0.5 vol %). The capacity retention increased to 80.74% after 200 cycles compared to the cells with the blank electrolyte (67.98%) and AN-containing electrolyte (75.8%). What is more, the capacity retention of the NCM523/graphite full cell is increased from 65 to 81% with the addition of the same amount of Li2S and AN after 180 cycles. The mechanism is revealed on the basis of the theoretical calculations and various characterizations. The products derived from the preferential adsorption and oxidation of Li2S on the surface of NCM523 effectively increase the content of inorganic ingredients. However, the presence of AN prevents oxidation of the solvent. This study provides new principle guiding studies on a high-voltage lithium-ion battery with excellent electrochemical performance.