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Page 2 of 18 Yang et al. Energy Mater 2023;3:300029 https://dx.doi.org/10.20517/energymater.2023.10

film on the surface of both the Li anode and NCM85 cathode. This film effectively suppresses the consumption of
active lithium and the severe decomposition of the electrolyte. Furthermore, the presence of B elements in the
cathode-electrolyte interfacial film, such as BF , BF OH, and BF OBF compounds, can coordinate with the lattice
2
2
2
3
oxygen of the cathode, forming strong coordination bonds. This can significantly alleviate lattice oxygen loss and
mitigate detrimental structural degradation of the Ni-rich cathode. Consequently, the Li||NCM85 battery cycled in
LiDFOB-containing electrolyte displays superior capacity retention of 74% after 300 cycles, even at a high charge
cut-off voltage of 4.6 V. The comprehensive analysis of the working mechanisms of LiDFOB offers valuable insights
for the rational design of electrolytes featuring multifunctional lithium salts or additives for high energy density
lithium metal batteries.

Keywords: Lithium metal battery, lithium difluoro(oxalate) borate, Li anode, Ni-rich cathode, SEI/CEI film

INTRODUCTION
Lithium-ion batteries (LIBs) are widely used in various electronic equipment as energy storage devices,
while the rapid development of electric vehicles (EVs) has put forward higher requirements for LIBs in
terms of energy/power density and cyclic stability . To this end, the development of high-specific energy
[1-6]
LIBs with lithium metal as the anode (currently the largest energy density) and a suitable cathode is the key
to improving the overall energy density of LIBs [7-10] . Among all the identified cathode materials, nickel (Ni)-
rich-layered oxide Li[Ni Co Mn 1-x-y ]O (x ≥ 0.8) with suitable Ni, Co, and Mn atomic ratios is considered
y
x
2
-1
one of the most promising cathode materials due to their high specific capacity of more than 200 mAh g
and low cost [11,12] . However, the practical application of the Ni-rich cathode material is limited by severe
capacity decay and thermal instability . Several possible reasons have been proposed to explain the
[13]
performance degradation of Ni-rich materials, mainly including irreversible structural changes and
interfacial degradation . On the one hand, at a highly delithiated state, the irreversible phase transition
[14]
from the second hexagonal phase (H2) to the third hexagonal phase (H3), accompanied by abrupt lattice
contraction and rise of internal stress, will result in partial structure collapse at the near-surface area of
crystal and serious grain intergranular or intragranular cracking of particles [15,16] . Meanwhile, the structure
collapse accompanied by oxygen evolution and transition metal dissolution will cause the spread of this
collapse and cracks into the particle core, which leads to fast capacity loss. Additionally, the electrolyte can
penetrate into these cracks, which can be catalyzed by highly active Ni and superoxide ions (O ) to form an
-
4+
undesired phase transition layer and excessive deposition of electrolyte decomposition products. This
results in a substantial reduction in the kinetics of Li-ion (Li ) migration and electron transfer of the
+
4+
2+
-
electrode [17-19] . In addition, Ni ions are easily reduced into stable Ni ions in the presence of O , resulting in
2+
+
capacity loss and O release . Meanwhile, due to the similar size of Ni and Li , it will cause more serious
[20]
2
Li /Ni mixing, which further reduces the stability of the structure [21,22] . Therefore, achieving a stable Ni-rich
+
2+
cathode necessitates the high stability of surficial/interfacial structure, especially at a higher voltage.
To solve these problems, several strategies have been explored in recent studies, such as coherent surface
coating [20,23-30] and near-surface element doping or substitution [31-37] , to better stabilize the near-surface
structure of the material (especially the stability of surface lattice oxygen) and effectively suppress electrolyte
decomposition. However, these approaches usually involve complicated synthetic processes, and the
introduction of inert compositions reduces the specific capacity of cathodes. More importantly, these
methods can only target the cathode alone but cannot simultaneously protect the anode, especially the
hyperactive lithium metal anode. Detrimental and persistent interfacial side reactions occur between lithium
metal and the electrolyte, which induces the generation of lithium dendrites and inactive lithium, resulting
in short cycle life and causing safety issues [38-42] .