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Page 12 of 27 Yang et al. Microstructures 2023;3:2023013 https://dx.doi.org/10.20517/microstructures.2022.30

where represents the Gibbs free energy of dissolution and and represent the
Gibbs energies of the salt lattice and the solvation of salts, respectively .
[70]
To date, the reported potassium salts used in Bi-based anodes are KPF and potassium
6
bis(fluorosulfonyl)imide (KFSI). KPF has a high calculated Kapustinskii lattice energy of 564.9 kJ mol ,
-1
6
[70]
while KFSI has a lower lattice energy , indicating that KFSI has higher solubility compared to KFP . KFSI
6
also has higher ionic conductivity than KPF and KFSI-based electrolytes can form more stable SEI layers.
6
This is because the FSI anion has weak S-F bonds that make it easier to form KF, which is a main
-
component in the SEI layer .
[71]
Zhang et al. first used KFSI as the electrolyte salt in Bi-based anode materials with ethylene carbonate (EC)
[59]
and diethyl carbonate (DEC) as solvents . The results indicated that the KFSI-based electrolyte had better
cycling performance compared to the KPF -based electrolyte. The morphological and mechanical properties
6
of the KFSI and KPF electrolytes were investigated using atomic force microscopy, Kelvin probe
6
microscopy and TEM. The results demonstrated that the KPF -based electrolyte formed a thicker and more
6
heterogeneous SEI layer, while the SEI layer in the KFSI-based electrolyte was more uniform.
Ether solvents are the most used solvents for Bi-based anode materials. As discussed above, the highest
occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of
solvents or anions are lower when solvents or anions modify a cation through coordination. This is because
an electron pair is donated to the cation. Thus, an anode with chemical potential μ > E lumo can spontaneously
transfer electrons to the LUMO of the electrolyte and trigger reduction. In ether solvents, the HOMO values
of the ion-solvent complexes are of the order of Li > Na > K , while the LUMO values follow the order of
+
+
+
+
+
Na > K > Li . Therefore, the reduction and oxidation products in ether-based PIBs are complicated.
+[13]
Huang et al. first used dimethoxyethane (DME) as their ether-based solvent in Bi-based PIBs . Using XPS
[57]
and in-situ Raman spectroscopy to probe the SEI components, it was revealed that C-C(H), C-O, C=O and
K-O bonds were formed and the SEI consisted of organic and inorganic compounds, such as
(CH CH -O-) K, (CH CH -OCH -O-) K, (RCO K) and K O . In addition, the oligomers were from the
2
n
x
2
2
2
2
2
n
2
reduction of DME. This ether-derived SEI possessed better mechanical flexibility because of the strong
binding in alkoxy (O-K) edge groups and the elastic properties of the as-formed SEI. This SEI could
effectively restrain the volume change of the particles. Density functional theory (DFT) calculations were
performed to analyze the interaction between Bi and DME. Three adsorption models of a DME molecule on
the (012) crystal plane of Bi were applied. Based on the models, the adsorption energies were 1.76, 0.65 and
0.60 eV. These adsorption energies were higher than for ester-based propylene carbonate (PC) molecules on
Bi, which favored the formation of a 3D porous structure in potassiation and depotassiation .
[58]
Generally, electrolytes for Bi-based PIBs contain 1 M K salts. Based on recent reports, ~70% of the
electrolytes applied in Bi-based PIBs are 1 M K dissolved in DME. Increasing the salt content results in
enhanced interactions between cations and anions. Increasing the salt concentration also decreases the
content of free-state solvent molecules. When the concentration is increased (>3 M), however, the free
molecules decrease, leading to a change in the solution structure, which usually gives rise to extraordinary
electrochemical properties and shifts the location of the LUMO from the solvent molecules to the salt. Thus,
the reductive decomposition of salts takes place before the decomposition of the solvent, which results in
the formation of a stable SEI [69,72] . Zhang et al. first used a concentrated electrolyte in Bi-based PIBs . The
[73]
Bi@C anode delivered the highest capacity of 202 mAh g in a 5 M KFSI-diethylene glycol dimethyl ether
-1
-1
-1
-1
electrolyte, which was higher than those in 1 M (163 mAh g ), 3 M (153 mAh g ) and 7 M (93 mAh g )
electrolytes. Based on this study, the differences in the electrochemical performance were due to the