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Yang et al. Microstructures 2023;3:2023013 https://dx.doi.org/10.20517/microstructures.2022.30 Page 17 of 27

Figure 8. (A) Illustration of TEM device and (B) potassiation/depotassiation processes of Sb@carbon nanofibers (CNFs) with Sb
nanoparticles confined in carbon shell. (C and D) Potassiation and depotassiation processes of Sb@CNFs [86] . Copyright 2020, Wiley-
VCH. (E) Schematic illustration of traditional Sb and Sb@graphene (G)@C electrodes during potassiation/depotassiation processes [87] .
Copyright 2018, Royal Society of Chemistry.

Mechanism of Ge-based anodes in PIBs
Ge has a high capacity of 1623 or 1384 mAh g by the formation of the lithium-rich compounds Li Ge and
-1
5
55
Li Ge , respectively [98-102] , which makes it a promising anode material in LIBs. In SIBs, germanium delivers a
4
15
high capacity of 389 mAh g by forming the binary compound NaGe [103-106] at a voltage plateau of
-1
0.15-0.60 V vs. Na/Na . Based on the formation of KGe as the final product, germanium has a theoretical
+
capacity of 369 mAh g in PIBs. To date, the study of the potassium-ion storage mechanism for Ge in PIBs
-1
has been limited. Based on the previous studies of the performance of Ge in SIBs and LIBs, the mechanism
-
+
of potassium-ion insertion obeys the following equation: Ge + K + e ↔ KGe. This mechanism was proved
+
[107]
using SAED . For Ge-based compounds, the mechanism can be simplified to Ge M + (x + zy)K +
y
x
[42]
(x + zy)e = xKGe + yK M . In this process, the compound first decomposes, the Ge reacts with K to form
-
z
KGe and the active material reacts with K to form a compound. When the Ge-based compounds are 2D
-
materials, such as GeSe, the reaction can be considered as GeSe + xK + xe ↔ K GeSe based on
+
x
calculations [108,109] .
Modification strategies for Ge-based anode materials
Compared to other alloy-based anode materials, germanium has a relatively lower theoretical capacity. It
experiences a limited volume change during ion insertion and extraction processes; however, compared to