LiNi0.8Co0.1Mn0.1O2 has a higher capacity but has inferior cycle stability, especially at high and low temperatures. In this work, 1-propylphosphonic acid cyclic anhydride (PACA) is designed as a functional additive of electrolyte for increasing the rate performance and long cycle stability of the LiNi0.8Co0.1Mn0.1O2/graphite full cell. Additive PACA helps to form interface films on both electrodes and a stable electrode–electrolyte interface to reduce the dissolution of the transition metal of the cathode materials and prevent the parasitic interfacial reactions of LiNi0.8Co0.1Mn0.1O2. Compared with the baseline electrolyte, these batteries with additive PACA exhibit higher capacity retention and excellent rate performance within a voltage range of 2.75–4.2 V. The batteries with 0.5 wt % additive PACA under a current density of 1 C have a capacity retention of 92% after 550 cycles at room temperature. It especially demonstrates excellent electrochemical performance (a capacity retention of 87% after 350 cycles) at high and low temperatures (45 and −20 °C), and the discharge capacity retained 51.2% theoretical capacity at 1 C.

LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM 811) has superior capacity but meanwhile also faces continuous electrolyte decomposition and transition metal dissolution induced by unstable cathode electrolyte interphase (CEI) films. The poor compatibility of the traditional electrolyte and lithium anode and the uncontrollable growth of the lithium (Li) dendrite also hinder the application of the lithium anode. Based on this, we designed the 1-methyl-1-cyanopropylpyrrolidine bisfluoromethanesulfonimide salt (PYR 1(4CN) TFSI) to reduce the interface impedance on the cathode side and enhance the compatibility between the electrolytes and the lithium metal anode. A uniform and thin CEI was formed on the cathode side, and a dense and stable solid−electrolyte interphase was formed on the anode side, which effectively inhibited the side reaction and further promoted the electrochemical performance. The symmetrical cells with the addition of 0.5 wt % PYR 1(4CN) TFSI can be maintained from 400 h to more than 750 h at a current density of 0.5 mA cm −2 . After 200 cycles, the discharge specific capacity delivered by Li||LiNi 0.8 Co 0.1 Mn 0.1 O 2 cells was 107 mA h g −1 at 1 C.

No 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.