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Page 28 of 64 Rehman et al. Energy Mater 2024;4:400068 https://dx.doi.org/10.20517/energymater.2024.06

at 0.2 C after 100 cycles. An excellent mechanistic approach for capacity origination and fading has been
[170]
adopted by Shen et al. . The material demonstrated promising capacity retention and stability, as
shown in Figure 13A. The capacity fading issue was traced using in-situ HRTEM with allied
techniques to detect species causing capacity degradation during (de)sodiation cycles, as shown in
Figure 13B and C . An ICE of 88.61% and a capacity of 330 mAh g over 100 cycles were offered by the
[170]
-1
electrode in half cell. Expansion during sodiation was captured by TEM in a time-lapse experimental
setup, which showed GeP nanoflake expansion from 0.93 to 1.25 µm without any cracking in the
nanoflake. The SAED pattern also recorded anisotropic expansions along different planes, leading
to intermediate orthorhombic NaGe P that ultimately resulted in amorphous phase NaGe and
3 3
Na P. The in-situ TEM also revealed that after completing the first cycle of sodiation, the amorphous
3
phase could not revert back to the crystalline GeP upon desodiation.

Li et al. have trailed the path of sodiation in the GeP@C anode for SIBs . The self-healing anode has the
[171]
capability of structural reformation under a multistep sodiation process involving intercalation followed by
the conversion and, finally, the alloying step. This self-healing property was attained due to a low formation
energy (-0.19 eV) of the layered material, which further stabilized the graphitic incorporation, creating P-C
bonds that synchronously sustained more Na at improved kinetics and conduction. The GeP anode has the
+
-1
-1
potential to achieve a high ICE (93%) and a high sodiation rate capacity (360 mAh g at 2 A g ). Although
inspiring, such performance was lower than that of the GeP@C that sustained a high ICE (above 90%), a
-1
long cycling life with a capacity of 850 mAh g at 0.1 A g over 300 cycles and a sodiation rate capacity of
-1
-1
-1
533 mAh g at 2 A g . The high metallic conductivity of the intermediate Na GeP with metallic conductive
x
behavior and interlayer bonding compatibility of graphite and GeP created stronger P-C interactions that
alleviated capacity performance and stability.
Besides GeP, GeTe has also been demonstrated by many researchers for SIBs anode capabilities. A GeTe/C
composite proposed by Sung et al. can yield good gravimetric capacity (98.5% after 100 cycles) and rate
performances (704 mAh g at 1 C and 630 mAh g at 3 C) . The conversion/alloying mechanism in the
-1
-1
[172]
composite anode of GeTe/C material has been investigated using ex-situ XRD and extended X-ray
absorption fine structure (EXAFS) during different (dis)charging states. In the first sodiation state,
amorphization led to metallic Ge, followed by Na-Ge bond formation. In contrast, the desodiated state
showed reappearance of the Ge-Ge bond proceeded by the reappearance of GeTe, which was evident as
presented by the following reactions:

GeTe → Ge + Na Te → NaGe + Na Te (sodiation)
2
2
NaGe + Na Te → Ge + Na Te → GeTe (desodiation)
2
2
A mechanism-directed approach has revealed the significance of amorphous structures in nullifying the
influence of stress-induced limitations and interfacial inhomogeneities that plague the SIB anode
capacity . For this purpose, 2D porous GeS nanosheets with amorphous structures have been
[173]
2
constructed. They demonstrated highly stable capacity and rate performance, as shown in Figure 14A(a-d).
The electrode maintained a highly uniform electrode/electrolyte interface, ensuring fast and isotropic Na +
diffusion channels to assist in the complete conversion reaction for optimum capacity without particle
aggregation [Figure 14B(a-c)]. The amorphous GeS also tolerated a capacity of 512.8 mAh g at 10 A g -1
-1
2
after an extended 1,000 cycling period. The mechanism of stress forbearance was illustrated based on
Raman and TEM results [Figure 14B(d-i)], whereby the sheet-like morphology managed stresses incurred
during cycling.