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Yang et al. Microstructures 2023;3:2023013 https://dx.doi.org/10.20517/microstructures.2022.30 Page 19 of 27

Figure 9. Schematic illustration of preparation of Ge-CMK and Ge@C-CMK composites [106] . Copyright 2021, Elsevier.

potassiation process was evaluate, as shown in Figure 10A-C. The results revealed that the tetragonal K Sn 4
4
phase was formed at a voltage of ~0.01 V. Their study indicated that K Sn and KSn are overall identical
4
4
[125]
phases in terms of their crystal structure. In the de-alloying process, K Sn decomposed at 0.98 V .
4
4
The study of the mechanism of Sn-based alloys has also attracted significant attention. The teams of Ma and
Ci have both studied the potassium-ion storage mechanism in SnS 2 [126,127] and their results are similar. In the
discharge process, SnS → SnS + K S → Sn + K S + K Sn → K S + KSn, as shown in Figure 10D. Like the
23
2 5
2
4
2 5
2 5
alloying process of crystalline Sn in PIBs, the final product, KSn phase, is formed within the voltage range of
0.20-0.01 V. For SnSe, based on the study of Verma et al., the potassiation process is SnSe → Sn + K Se →
5
2
K Sn + K Se , which is reversible . The KSn phase was not detected, as shown in Figure 10E.
[128]
23
4
5
2
Modification strategies for Sn-based anode materials
Sn-based alloys suffer from significant volume expansion in PIBs, which results in pulverization and
capacity drop. Various methods have been applied to ameliorate the volume change and improve the
electrochemical performance. To solve these drawbacks, hierarchical nanostructural design is an effective
strategy. 2D nanosheet structures have been applied to improve the electrochemical performance of Sn-
based anode materials. Lakshimi et al. studied an SnS /graphene composite in PIBs, which delivered a high
2
capacity of 350 mAh g -1[129] . Qin et al. designed hierarchical polyaspartic acid-modified SnS nanosheets
2
embedded into carbon . The as-prepared electrode enlarged the interlamellar space of 6.8 Å and delivered
[126]
a high-rate performance of 273 mAh g at a current density of 2 A g -1[126] . Cao et al. also designed a 2D SnS
-1
[130]
nanosheet composite that exhibited an ultralong lifespan . Sun et al. used a nanosheet structure with
strong interactions between the layers, which can efficiently accelerate electron and ion transfer and hinder
the volume change . The as-prepared composite delivered a stable long-term cycling performance of
[131]
[131]
-1
165 mAh g at a current density of 10 A g after 5000 cycles .
-1
The group of Yang also designed a nanosheet structure, which delivered a high capacity of 206.1 mAh g
-1
after 800 cycles . Zhou et al. designed a sheet-like tin sulfide composite, as shown in Figure 11A, which
[132]
delivered a rapid rate capacity of 460 mAh g at a current density of 2A g and an excellent cycling stability
1
-1
of over 500 cycles at a current density of 1 A g -1[133] . The utilization of the 2D structure efficiently hinders the
volume change and improves the electronic and ionic conductivity. Combining Sb-based alloys with 3D
structures has been an effective method to improve the electrochemical performance of PIBs. Yolk-shell 3D
carbon boxes were designed as a matrix to accommodate SnS , as shown in Figure 11B. Introducing interior
2
void space has been an effective strategy to accommodate the volume changes. The composite delivered a
-1
stable cycling performance of 352 mAh g at 1 A g , as shown in Figure 11C .
[134]
-1