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Yang et al. Energy Mater 2023;3:300029 https://dx.doi.org/10.20517/energymater.2023.10 Page 3 of 18

In this regard, optimizing electrolytes by using interfacial film-formation functional additives to
simultaneously regulate Ni-rich cathode and Li anode interfacial properties is a promising strategy [43,44] .
Among the diverse electrolyte additives that have been reported so far, borate (B)-containing additives,
especially B-containing lithium salts, have been employed as effective salts or additives for high-voltage
cathodes or alkali metal anodes [45-60] . For example, Zhang et al. introduced trace (~2%) lithium
bis(oxalato)borate (LiBOB) as an electrolyte additive to improve the cyclic stability of Li-rich layered oxide
(LRLO) cathode by optimizing the cathode interfacial structure and eliminating the detrimental
[61]
nucleophilic superoxide attack under the synergy effect of LiPF . Chen et al. demonstrated lithium
6
difluorobis(oxalato)phosphate (LiDFBOP) as the multifunctional additive can build a stable and robust
organic/inorganic hybrid interphase on Na anode and Na V (PO ) F cathode surface by the preferential
3
2
4 2 3
-
reduction and oxidation of DFBOP , realizing excellent cyclic stability for high-voltage Na||Na V (PO ) F
2
4 2 3
3
[62]
system . Mao et al. configured 1 M lithium difluoro(oxalate) borate (LiDFOB)-based electrolyte, where the
LiDFOB can not only in-situ construct a hierarchical and robust cathode/electrolyte interface (CEI) film on
the LiNi Co Mn O (NCM811) cathode surface but also induce the uniform plating of Li, resulting in
0.8
0.1
2
0.1
[63]
greatly improved cyclic stability of Li||NCM811 batteries . While B-containing lithium salts are widely
utilized in batteries, a systematic investigation of the differences in the interfacial films formed from their
decomposition on the anodes and cathodes is rarely reported. It is crucial to have a comprehensive
understanding of the working mechanism of key components in the electrolyte for the rational design of
electrolytes to improve the performance and stability of the batteries.
Herein, LiDFOB, as a B-containing lithium salt type additive incorporating the advantages of both lithium
tetrafluoroborate (LiBF ) and LiBOB, was selected for high-energy-density Li||LiNi Co Mn O (NCM85)
0.1
0.05
2
0.85
4
battery. Theoretical calculations and experimental studies systematically demonstrated that DFOB is
-
preferentially reduced on the Li anode surface to form a highly electrochemically stable, electronically
insulating, and ion-conductive solid electrolyte interface (SEI) film under the synergistic effect of ethylene
carbonate (EC), which effectively promotes the homogenous plating of Li and suppresses the continuous
decomposition of the electrolyte and the formation of inactive lithium. On the NCM85 cathode surface, the
DFOB is preferentially oxidized and spontaneously captures the harmful hydrofluoric acid (HF) and O to
-
-
form an antioxidant and electronically insulating CEI film, which can effectively suppress the oxidative
decomposition of the electrolyte and the erosion of HF to electrodes. Meanwhile, the B element in CEI film
will form a strong bond with the lattice O of the cathode surface, thereby stabilizing the phase structure of
NCM85 and inhibiting the subsequent irreversible structural transition and O release. Consequently, the
2
cyclic stability and rate capability of Li||NCM85 batteries are significantly improved, even at a high charge
cut-off voltage of 4.6 V. The comprehensive examination of LiDFOB in this work offers valuable insights
and helpful guidance for the design of B-contenting electrolyte additives or lithium salts for high-energy-
density lithium metal batteries.
EXPERIMENTAL
Preparation of electrolytes and electrodes
The electrolytes and electrodes were both prepared in the Mbraun glove box (Ar atmosphere) with moisture
and oxygen levels less than 0.1 ppm. 1 M LiPF in EC and ethyl methyl carbonate (EMC)
6
(EC:EMC = 3:7 wt%) (provided by Shenzhen Capchem Technology Co., Ltd.) was used as the base
electrolyte. A range of electrolytes was prepared by dissolving various concentrations of LiDFOB (obtained
from Guangdong Canrd New Energy Technology Co., Ltd.) additive into the base electrolyte (abbreviated
as "base" in the figures). The cathode was prepared by mixing NCM85 (provided by Ningbo Ronbay New
Energy Technology Co., Ltd.), polyvinylidene fluoride (mixed in N-methyl2-pyrrolidone), and acetylene
black with the weight ratio of 8:1:1 uniformly. After coating the slurry onto the aluminum current collector
(12-mm diameter) and drying in vacuum for 8 h at 110 °C, the cathode was then stored in a glove box for