Page 102 - Read Online

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

Figure 1. Schematic illustration of a Li dendrite with various SEI components. Owing to the cracks generated during the
charge/discharge process, Li metal is exposed to the electrolyte and a localized Li-ion flux occurs.

to further understanding the formation of the heterogeneous components. Spotte-Smith and co-workers
reported the formation mechanism of the key components of the SEI and gaseous byproducts through
computational reaction networks containing over million reactions and kinetic Monte Carlo simulations .
[28]

Interestingly, the mechanisms of the formation of SEI components are affected by the potential of the Li
metal anode. Sun and co-workers verified that a bilayer SEI was formed when the potential of the Li metal
+ [29]
anode was below 0.1 V (vs. Li/Li ) . The inner layer was composed of more inorganic compounds, such as
Li O, lithium nitride (Li N), LiF, lithium hydroxide (LiOH) and Li CO (4.4% for SEI of Li metal anode at
2
3
2
3
0 V), whereas the outer layer was composed of more organic compounds, such as ROCO Li, ROLi and
2
RCOO Li (16.8% for SEI of Li metal anode at 0 V). Furthermore, when the potential of the Li metal anode
2
was below 0 V (vs. Li/Li ), inorganic components were primarily generated on the Li metal anode . Ideally,
+
[29]
the SEI requires high Li-ion conductivity, low electronic conductivity and high thermal and mechanical
stability for fast Li-ion kinetics, reduced electrolyte depletion and minimal volume expansion. As the
thickness of the SEI layer gradually increases during Li plating and stripping, its weak mechanical properties
are insufficient to accommodate the significant volume expansion of up to 300% [10,30,31] . If tiny cracks, known
as “hot spots”, exist on the SEI layer, they are directly exposed to the organic electrolytes, such as carbonate
and ether bases [15,16] . The uneven Li-ion flux then has a tendency towards local penetration and subsequently
accumulates at these spots, leading to inactive or dead Li and then to capacity and cyclability losses. In
addition to dead Li, dendritic Li growth is an unavoidable feature, which must be suppressed, because the
introduction of dendritic growth consumes both the Li anode and the electrolyte until cell failure .
[32]

In addition, the ionic conductivity of the SEI (σ ) determines the morphology of the Li metal anode by
SEI
affecting the pathways of Li ions in the SEI layer, owing to the potential field caused by its low
[33]
conductivity . Ma and co-workers reported the Li-ion transport mechanism in inorganic SEI components
such as LiF, Li N, Li O, LiOH and Li CO . Interestingly, the bulk ionic conductivity of inorganic SEI
2
3
2
3
components is extremely low. However, relatively high ionic conductivities were found at the interface of
different types of inorganic SEI components. In particular, at the interface between LiF and Li O, the ionic
2
conductivity (1.96 × 10 S cm ) is extremely improved compared to the bulk materials (5.2 × 10 and
-10
-4
-1
10 S cm , respectively). This indicates that the Li ions migrate through grain boundaries to increase the
-1
-9
ionic conductivity of the SEI [34-36] .
Figure 2A shows the process of Li deposition with a conventional electrolyte-deposited SEI with poor ionic
conductivity (< 10 S cm ). Initially, the SEI layer is formed owing to an unintended and unavoidable side
-1
-6
reaction between the electrolyte and the Li metal anode. Due to the formation of the SEI layer with low
ionic conductivity, Li ions intensively penetrate specific areas. Once Li ions are deposited on the Li metal