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Guo et al. Energy Mater. 2025, 5, 500041 https://dx.doi.org/10.20517/energymater.2024.214 Page 5 of 21

uniform transport of Li-ions, forming an electrochemically and chemically stable interface that is suitable
for high-voltage cathodes. When the charge cutoff voltage was set at 4.4 V, the assembled NCM622|B,
-1
F-CSE|Li cell exhibited an initial discharge capacity of 191 mAh g at 0.1 C [Figure 2E], and after 100 cycles
-1
the discharge capacity slightly decreased to 187 mAh g . The efforts should focus on searching for the
outstanding liquid electrolyte additives into SSBs, and eventually transforming the solid-liquid electrolyte
interphase (SLEI) from the so-called “real culprit” into the “savior”.

Quasi-solid layer between SPE and electrodes
Researchers typically modify the SPE surface by introducing functional polymers with low molecular
weights, as the superior contact compatibility between these polymers enhances lithium-ion interfacial
kinetics [40,41] . Yang et al. prepared a polymeric buffer layer [adaptive buffer layer (ABL)] using
polyacrylcarbonate, poly (ethylene oxide) (PEO) and LiTFSI, which was sandwiched between the SPE and
the Li anode, thereby improving the surface contact among the cell [Figure 3A] . According to the
[42]
reported results, after cell cycling, the R increased without the polymeric layer while there were no
t
significant changes in the R with polymeric layer [Figure 3B]. The cross-sectional scanning electron
t
microscopy (SEM) images [Figure 3C and D] revealed that, in the absence of a polymeric layer, the surface
between the SPE and the Li anode was separated; in contrast, when a polymeric layer was present, the SPE
and the Li anode were closely contacted with the polymeric layer. The Li/ABL/SPE/lithium iron phosphate
(LFP) battery exhibited nearly double the initial specific discharge capacity (110 mAh/g) compared to the
battery without ABL (60 mAh/g), due to improved interfacial contact between Li and SPE. Additionally, the
battery with ABL demonstrated greater stability in Coulombic efficiency during cycling. Based on previous
research works, quasi-solid layers can be tailored to specific battery chemistries and operating conditions.
By adjusting the composition and structure of the layer, researchers can optimize battery performance for
different applications.

Multi-layered solid electrolytes
All solid-state electrolytes, whether SPEs or inorganic SE, exhibit larger interfacial resistances between
electrodes and electrolytes, primarily arising from poor surface adhesion [43,44] . Liu et al. resolved the above
issues by adopting a 3-dimension Li anode with high surface area connected to the bulk SE via a flowable
polymer electrolyte interphase . A molten SPE was thermally infiltrated into 3D Li-reduced graphene
[45]
oxide (Li-rGO) [Figure 3E]. This work adopted various SEs for the comparative research, including a
composite polymer electrolyte combining PEO and silica, a cross-linked poly (ethylene glycol) diacrylate
(PEGDA) electrolyte, and cubic garnet-type LLZTO ceramic. After flowable poly (ethylene glycol) (PEG)
infiltration, it becomes evident that the polymer electrolyte has fully occupied the nanoscale pores within
the Li-rGO anode. The SEM results [Figure 3F and G] reveal that the pristine Li-rGO anode displayed a
uniform stacking of nanoscale Li particles and layered rGO, characterized by a high degree of porosity.
Following thermal infiltration, a distinct change in color is visually evident [Figure 3H and I], with the
Li-rGO electrode appearing notably darker, attributable to the filling of nanopores. The results
demonstrated that the flowable polymer electrolyte interphase was vital to withstand the interfacial
fluctuation and maintain excellent contact after experiencing cyclic charging and discharging. They
demonstrated superior capacity delivery at varied rates of 0.2, 0.5, 1, 2, and 5 C compared to cells utilizing
the Li foil anode [Figure 3J]. Notably, in both symmetric and full-cell configurations, the overpotential was
significantly reduced, and the cycling stability was improved.

Direct coating on SPE or electrodes
The atomic layer deposition (ALD) technology can build up condensed layers at atomic level through
[49]
multiple steps [46-48] . Additionally, it addresses the “surface-wetting” issues by plasma exposure . To solve
the “shuttle effects” of lithium polysulfide intermediates in the SPEs and the poor interfacial compatibility