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Page 16 of 27 Yang et al. Microstructures 2023;3:2023013 https://dx.doi.org/10.20517/microstructures.2022.30

[86]
as precursors . As observed by in-situ TEM, this hybrid material, which consists of carbon fibers with
yolk-shell Sb@C, has structural advantages in the potassiation and depotassiation processes, as shown in
Figure 8A-D. The inner Sb nanoparticles suffer from significant volume expansion during the potassiation
process, while the void space effectively relieves the volume changes and the carbon fiber shell maintains the
integrity of the structure and improves the conductivity. As a result, it delivered a capacity of 227 mAh g
-1
after 1000 cycles and had a high Coulombic efficiency of ~100%. Liu et al. designed and constructed Sb
nanoparticles confined by carbon, which exhibited long cycling stability over 800 cycles with a capacity
[87]
retention as high as 72.3% , as shown in Figure 8E.
A variety of porous structures have been applied to hinder the volume change during cycling [88-91] . A
[88]
microsized nanoporous antimony potassium anode was designed with tunable porosity . The nanoporous
structure can accommodate volume expansion and accelerate ion transport. Similarly, Zhao also
encapsulated Sb nanoparticles within a porous architecture . The composite delivered a high capacity of
[89]
-1
392.2 mAh g at 0.1 A g after 450 cycles. Carbon nanofibers have also been applied as nanochannels to
-1
solve the issues of poor potassium-ion diffusion and significant volume variation. The Sb@CNFs delivered a
[90]
-1
reversible capacity of 225 mAh g after 2000 cycles . Cheng et al. utilized a single-crystal nanowire
structure to improve the electrochemical performance of a Sb S anode material . After full potassiation, no
[91]
2 3
obviously pulverization was observed, although the diameter of the as-prepared Sb S @C nanowires
2 3
increased from 83 to 120 nm with a 45% expansion. The overall expansion of Sb S @C is ~111%, which is
2 3
lower than the Sn-K alloying reaction (≈ 197%), indicating that the nanowire structure can effectively hinder
the volume change during the potassiation/depotassiation process. Similarly, Jiao and Yu [92,93] also utilized a
one-dimensional structure. A 2D structure was also applied to improve the electrochemical performance of
Sb-based anode materials. Wang et al. designed a Sb S nanoflower/MXene composite that exhibited a high
2 3
reversible capacity of 461 mAh g at a current density of 100 mA g -1[94] . Its structural stability was enhanced
-1
by the strong interfacial connection between Sb S and the matrix. A 3D structure was also applied in Sb-
2 3
based anode materials. A core-shell Sb@Sb O heterostructure was fabricated, which delivered an excellent
2
3
-1
capacity of 239 mAh g at 5 A g in PIBs . These methods efficiently improved the electrochemical
[95]
-1
performance of Sb-based anode materials.
Another important method is improving the binder for the electrodes. He et al. used a polyvinylidene
fluoride (PVDF) binder, which has a high capacity of 226 mAh g over 400 cycles . Compared to PVDF,
-1
[96]
sodium carboxymethyl cellulose (CMC) can improve the initial columbic efficiency due to the pre-formed
SEI. In addition to these traditional binders applied in PIBs, the group of Guo developed a CMC-polyacrylic
acid (PAA) binder for a Sb-based composite . The cycling performance of the CMC-PAA binder was
[97]
improved due to the condensation reaction between the hydroxyl groups of CMC and the carboxylic acid
moieties of PAA, which effectively increased the viscoelastic properties of the binder and increased the
mechanism properties of the electrodes.
In summary, the modification methods for Sb-based anode materials are mainly nanostructural engineering
by designing nanofibers, nanoflowers, box shell structures and nanoporous structures in combination with
carbon fibers, MXenes, carbon shells, and so on. Multistructural design efficiently hinders the significant
volume change and efficiently alleviates the structural degradation.

Ge-based anode materials
Ge has a diamond cubic crystal structure, which is the same as silicon, and it is in the IVA group.
Germanium is an attractive non-toxic alloy-based anode material. The original study of Ge-based anode
materials can be dated back to the 1980s when the formation of the Ge-Li binary was first discovered.