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

Figure 1. Alloy-based anode materials for PIBs.

226 mAh g in PIBs. We calculated the theoretical volumetric and gravimetric capacities of P, Bi, Sb, Ge and
-1
Sn, as shown in Figure 2. The volumetric capacities are calculated based on the density of the materials and
their theoretical weight capacities. Based on the calculation, the volumetric capacities of P, Bi, Sb, Ge and Sn
-3
are 1574, 2753, 4597, 1964 and 1652 mAh cm . While the study of alloy-based anode materials in LIBs and
SIBs has been extensive in recent years, the study of PIBs remains at the early stage. Therefore, high-
capacity alloy-based anodes are worthy of further study.

Potassium storage mechanism of alloy-based anode materials for PIBs
Potassium storage in anode materials for PIBs can be classified into three categories: intercalation, alloying
and conversion. The intercalation reaction results in a smaller volume change and higher reversible capacity
than the other potassium storage mechanisms. During the interaction reaction, potassium ions are inserted
into the anode material and form a new phase. This reaction usually takes place in materials with a layered
structure, such as graphite [35,36] and K Ti O . The alloy-based anode material SnS undergoes an intercalation
9
4
2
2
reaction in the first step and conversion and alloying reactions in the following steps. The intercalation
reaction can be expressed as M N + aK + ae ↔ K M N . Typical intercalation reactions deliver high
-
+
y
a
x
y
x
reversible capacity because of the low volume change of the crystal during the electrochemical reaction. Due
to the large radius of the potassium ion, however, anode materials with an intercalation-type potassiation
process experience a larger volume change and have less reversible capacity in their performance in LIBs
and SIBs.
Compared to the intercalation reaction, alloying-reaction materials undergo a larger volume change and
have higher theoretical capacities. Alloying-type materials react with K to form the binary compound K M.
x
-
+
The reaction process can be expressed as aM + bK + be ↔ K M . In this reaction, M represents Sn, Bi, Sb, P
b
a
or Ge. These alloying-type materials can form binary metallic materials that undergo conversion-alloying
reactions, in which the compound decomposes and further alloys with potassium. For example, Sn P
4 3
undergoes the following reaction: Sn P + 11K ↔ 4KSn + 3K P . Similarly, Sb Se goes through the
[26]
3
2
3
4 3
[27]
following conversion-alloying reaction: Sb Se + 12K + 12e ↔ 3K Sb + 2K Se .
-
+
2
3
3
3
2