Construction of highly stable and fast kinetic interfacial films on the electrodes of graphite//LiNi0.5Mn1.5O4 cells by introducing a novel additive of 2-thiophene boric acid (2-TPBA)

A novel additive, 2-thiophene boric acid (2-TPBA), is investigated as a bifunctional surface film forming additive for both anode and cathode. By adding 1.0 wt% of 2-TPBA additive to the electrolyte (1.0 M LiPF6 in Ethylene carbonate (EC)/Diethyl carbonate (DEC)/Ethyl methyl carbonate (EMC) = 3/2/5, wt.%), the capacity retention of Li//LiNi0.5Mn1.5O4 and Li//graphite cells is significantly improved, specifically, from 37% to 93% after 200 cycles, and 31.9%-91.9% at 0.5C after 150 cycles, respectively. In addition, the cycling performance of graphite//LiNi0.5Mn1.5O4 full cell at 4.8 V is significantly improved from 48.7% to 70% after 160 cycles. The reaction mechanism of this additive is investigated using a combination of electrochemical methods, ex-situ spectroscopic characterization techniques and computational calculations. The improved performance results from the sacrificial reduction on the graphite anode and electron-polymerization of the 2-TPBA additive on the LiNi0.5Mn1.5O4 cathode surface to generate a stable and conductive surface film. S-based polymers and lithium fluorinated borate corresponding to the oxidation products of 2-TPBA additive are the main cathode electrolyte interphase (CEI) components on the LiNi0.5Mn1.5O4 cathode surface; while lithium thiophene borate is consid-ered as the solid electrolyte interphase (SEI) component on graphite anode surface in the additive containing electrolyte.