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Page 2 of 13 Park et al. Energy Mater 2023;3:300005 https://dx.doi.org/10.20517/energymater.2022.65

Keywords: Electrolyte additive, ionic conductivity, lithium dendrite, lithium metal battery, solid electrolyte
interphase (SEI), solid electrolyte

INTRODUCTION
Lithium (Li)-ion batteries play an important role in applications for extending the operating hours of small
[1-3]
information technology devices and the driving mileages of electric vehicles . In particular, although high-
energy-density batteries are desirable, commercial lithium-ion batteries based on a graphite anode cannot
provide sufficient energy density. Even if a newly developed structure or material is applied in the
electrodes, it is not easy to achieve a gravimetric energy density as high as 300 Wh kg . One method to
-1
[4]
overcome this challenge is to replace the existing graphite or silicon additive anode . Li metal is an ideal
anode material for achieving high energy density, owing to its high theoretical capacity (3860 mAh g ), low
-1
redox potential (-3.04 V vs. a standard hydrogen electrode) and low density (0.534 g cm ) . Moreover, it is
-3 [5]
known that the performance of Li metal batteries can be further enhanced by adapting high-voltage lithium
nickel cobalt manganese oxide cathodes . During Li plating and stripping, however, inherent dendritic
[6]
growth, uneven solid electrolyte interphase (SEI) formation and Li volume expansion are unavoidable issues
that trigger internal short-circuiting in Li metal batteries .
[7]
The electrolyte has a profound impact on the electrochemical cycling of the Li metal anode. On the surface
of Li metal, an SEI layer is formed by accumulating various decomposition products created by a chemical
reaction with the organic electrolyte [8-10] . This SEI layer provides passivation to prevent Li metal corrosion by
[11]
preventing contact between the electrolyte and the Li metal . However, it is difficult to predict the
electrical properties of SEI layers because their heterogeneous phases vary depending on the type of organic
electrolyte [12-14] . The primary issue is that Li electrodeposition is locally concentrated along any cracks in the
SEI layer, leading to various side reactions inside the electrode [15,16] . During repeated plating and stripping,
particles of inactive dead Li are gradually accumulated, resulting in performance degradation. Given these
difficulties, the utilization of metallic Li anodes still faces many challenges for commercial viability,
especially dendritic Li growth .
[17]

So far, various approaches have been developed to suppress dendritic Li growth. For example, electrolyte
additives have been used to achieve high ionic conductivity, separator design has been proposed to improve
dendritic blockage, interlayer coatings have been utilized to stabilize the Li metal surface and host
architectures that can store Li metal have been developed. Li is a highly active material that reacts with all
organic electrolytes, leading to SEI layer formation [18-23] . In particular, the thickness of the SEI layer shows
unlimited growth until both the organic electrolyte and the metallic Li are entirely consumed. SEI growth
mechanisms are difficult to understand due to the variety of organic electrolytes utilized . We know that
[24]
the high ionic conductivity (σ ) of an SEI effectively suppresses the dendritic Li growth during Li plating
SEI
[22]
and stripping. Nevertheless, there remains doubt regarding which component entirely governs the σ SEI . In
this mini review, we summarize the σ of each SEI phase and provide insights to understand and predict
SEI
the phenomenon of dendritic Li growth.
Solid electrolyte interphase
The SEI model was first suggested by Dey and further developed by Peled [25,26] . The SEI as a passivation layer
is known to be a unique feature with simultaneous ionic conductivity and insulating properties. It consists
of different heterogeneous components, such as semi-carbonates, polyolefins, lithium oxide (Li O), lithium
2
carbonate (Li CO ) and lithium fluoride (LiF), as shown in Figure 1 [12,27] . Significant effort has been devoted
3
2