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Page 4 of 13 Park et al. Energy Mater 2023;3:300005 https://dx.doi.org/10.20517/energymater.2022.65

Figure 2. Schematic illustration of Li metal dendrite growth process in conventional electrolyte (A) without and (B) with additives.

electrode, they form branch-shaped structures known as dendrites, more localized and non-uniform Li-ion
flux occurs, and as a result, dendrite formation is accelerated. To avoid dendrite growth by localized Li-ion
flux, intensive research has been undertaken to introduce additives into the electrolyte to improve the ionic
conductivity. Figure 2B shows the process of Li deposition with an artificial SEI, which has a high ionic
conductivity (> 10 S cm ). The localized Li-ion flux caused by the low ionic conductivity of the SEI layer is
-5
-1
diminished, and as a result, the Li metal grows uniformly, because Li ions are evenly deposited in all
areas [37-42] .

Ionic conductivity measurements
Uneven Li deposition is well known to occur due to the heterogeneous ionic conductivity of the native SEI
on a Li metal surface. The ionic conductivity (σ ) can be calculated by alternating current impedance
SEI
spectroscopy [27,43] . Experimentally, in this method, a sinusoidal potential is applied over a wide frequency
range to an electrochemical cell with blocking electrodes as ideal capacitors and its response is recorded. As
shown in Figure 3, a typical equivalent circuit for Li metal with an SEI can be classified into six components:
(i) bulk resistance of the cell (R ); (ii) SEI resistance of the interfacial layer (R ); (iii) charge transfer
SEI
Bulk
resistance (R ); (iv) capacitance of the interfacial layer (CPE ); (v) double-layer capacitance (CPE Electrode );
CT
SEI
and (vi) Warburg component, reflecting the diffusional effects of Li on the host materials. The ionic
conductivity (σ ) is calculated by
SEI

where ρ is the specific ionic resistivity, R is the specific ionic resistance, d is the thickness and A is the
SEI
SEI
act
active cross-sectional area. It should be noted that ρ and R are the only material properties that are
SEI
SEI
independent of geometry. Han and co-workers calculated the ionic conductivity of a lithium-rich
antiperovskites (LiRAP) film (10 S cm ), which was considered as an artificial SEI. A LiRAP-ASEI with a
-1
-4
thickness of 1 μm on copper (Cu, 1.6 cm in diameter) disks was used for EIS measurements and R was
SEI
estimated to be 13 Ω . Such values are not easy to evaluate from direct experiments [45,46] and many
[44]
researchers have therefore suggested alternative approaches, i.e., theoretical calculations.
Notably, the σ is governed by the accumulated components of the SEI. The σ is estimated to vary from
SEI
SEI
-4
-12
10 to 10 S cm -1[47-49] . A relatively low value is known to cause dendritic Li growth in Li metal anodes.
Among the components, LiF, Li CO , Li O and Li N are representative and correspond to ionic
3
2
2
3