Engineering interface upgrade of LiNi0.8Co0.1Mn0.1O2 cells from PYR1(4CN)(2O2)TFSI with cyano and ether groups as dual functional pyrrolidine electrolyte

Abstract

It is a concern that cells with lithium (Li) metal anodes and LiNi0.8Co0.1Mn0.1O2 (NCM 811) cathodes exhibit high energy density. However, the chemical and electrochemical properties of an original solid electrolyte interphase (SEI) film formed by the Li metal reaction are unstable, resulting in uneven plating and rapid growth of Li dendrites. Due to the high nickel content of NCM 811, Ni4+ dissolved at the electrode interface leads to side reactions and irreversible rock salt structure, forming an unstable cathodic electrolyte interphase (CEI) film. Ion liquid (IL) electrolytes provide a strategy for forming stable SEI/CEI and keeping NCM 811 structural stability, but their high viscosity has limited their electrochemical performance. Functionalize pyrrolidine with ether and cyano groups is introduced, the high flexibility of the ether group can reduce the viscosity of the IL-based electrolyte, and the oxygen atom can provide Li+ coordination sites to accelerate Li+ transport. The strong electron absorption ability of the cyano group shows the strong coordination ability with transition metal ions to inhibit the erosion of CEI by side reactions. Under the dual function of cyano and ether groups, more TFSI− participate in the formation of the SEI film, which leads to the increase in beneficial components with high ionic conductivity, further inhibiting dendrite growth and promoting uniform plating. Thus, LiǁLi cells, with 0.5 wt. % 1-cyanopropyl-1-diethyl ether pyrrolidine bisfluoromethanesulfonimide salt [PYR1(4CN)(2O2)TFSI], revealed excellent plating voltage stability for more than 450 h. After 200 cycles, the discharge specific capacity of LiǁNCM 811 cells was 123 mAh g−1 and an excellent capacity retention of 62.1% at 1 C. This work shows a strategy of improving SEI/CEI from the electrolyte with cyano and ether groups and provides a feasible horizon in the long-term cycle performance of lithium metal batteries.