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Yang et al. Microstructures 2023;3:2023013 https://dx.doi.org/10.20517/microstructures.2022.30 Page 13 of 27

different reduction resistances. The decreased reduction resistance in the 5 M electrolyte depressed the
irreversible electrochemical reaction and formed less SEI compared to the less concentrated electrolytes .
[73]
A comparison of the electrochemical performance of Bi-based anode materials in PIBs is shown in Table 2.

Based on the current study of Bi-based PIBs, KFSI-based electrolytes have better electrochemical
performance compared to KPF -based electrolytes because of the higher ionic conductivity and the
6
formation of a more stable and uniform SEI. Some ether-based electrolytes have extraordinary performance
in half cells because their ether-derived SEI possesses better mechanical flexibility. The concentrated
electrolyte can improve the electrochemical performance to a certain extent due to the lower resistance of
the electrolyte.

Sb-based electrodes for PIBs
Antimony is a layered structure hexagonal element with a high electrical conductivity of 2.5 × 10 S·m .
-1
6
Studies of Sb as anode applied in batteries can be traced back to the 1970s when Weppner first studied its
[77]
kinetic parameters and thermodynamic properties in mixing conducting electrodes to be applied in a Li Sb
3
system. Theoretically, one mole of Sb can alloy with three moles of lithium, sodium or potassium. The first
study of Sb in PIBs was in 2015 . Sb is a promising anode material with a high theoretical capacity of
[78]
687 mAh g in PIBs, which makes it a novel potential anode material.
-1
K-ion storage mechanism of Sb
Based on the Sb-K phase diagram, there are four K-Sb binary phases going through K Sb, K Sb , KSb and
5
3
4
[79]
KSb with decreasing K content . The corresponding equilibrium potentials of KSb , KSb, K Sb and K Sb
2
3
2
4
5
are 0.890, 0.849, 0.439 and 0.398 V, respectively, based on DFT computations , which are shown in
[74]
Figure 7A-C. In-situ XRD experiments and cyclic voltammetry (CV) were carried out to analyze the phase
[80]
changes . In the discharge process, the first step was the transformation of hexagonal Sb to amorphous Sb.
As reported, the peak at 28.6° corresponding to the (012) phase of Sb gradually became weaker , as shown
[81]
in Figure 7D. In the amorphous region, KSb and KSb phases can form at the potential of 0.78 V and at the
2
potential of 0.23 V, K Sb phase can form based on the CV results. When fully discharged to ~0.2 V, the
5
4
cubic K Sb phase with Fm3m symmetry forms as the final potassiation product. Upon charging, the K Sb
3
3
phase gradually decreases by the formation of the intermediate phase K Sb. When further charging, the Sb
x
phase forms with the decomposition of intermediate K Sb. In addition, the cubic K Sb phase can be
x
3
[82]
observed in the second cycle, while no crystalline Sb can be observed .
One interesting observation is the formation of the cubic K Sb phase as the fully discharged product. There
3
are two polymorphs of K Sb, hexagonal K Sb (h-K Sb) and cubic K Sb (c-K Sb). Based on the DFT
3
3
3
3
3
calculations, h-K Sb is more stable than c-K Sb, as shown in Figure 7E . If we consider the crystalline
[83]
3
3
energy and the reaction activation energy, however, the results are different. The following equation
represents the activation barrier ΔE*(x):
2
ΔE*(x) = 16πγ /3(ΔE (x)/p(x)V ) . (5)
3
g
0
where γ represents the surface energy, ΔE represents the energy gain on passing from the crystalline to
g
amorphous phase and V is the molar volume of the crystalline phase, as shown in Figure 7E and F. Even
0
the molar energy gain of h-K Sb is higher than that of c-K Sb by ~0.12 eV and h-K Sb also has a higher
3
3
3
surface energy and lower density. As a result, h-K Sb has a higher activation barrier, which results in the
3
final formation of c-K Sb instead of h-K Sb . Thus, based on current reports, the reaction can be
[83]
3
3
concluded as Sb crystal → Sb amorphous , Sb amorphous + xK + xe ↔ K Sb amorphous and K Sb amorphous + (3-x)K + (3-x)e ↔
-
+
+
-
x
x
c-K Sb crystalline .
3