Page 85 - Read Online

Page 18 of 27 Yang et al. Microstructures 2023;3:2023013 https://dx.doi.org/10.20517/microstructures.2022.30

other alloy-based anodes, which amount to ~272% in LIBs and 120% in SIBs. Similarly, Ge undergoes a
significant volume change in the discharge/charge process in PIBs. Although the volume changes of
germanium are less compared to other alloy-based materials, they can cause pulverization and result in a
capacity decrease in the same manner.

In order to improve the electrochemical performance of germanium-based anode materials, the ordinary
methods include constructing nanostructures combining Ge with carbon materials [110-114] in LIBs and SIBs.
Li et al. designed hollow carbon spheres with germanium encapsulated inside by introducing a germanium
[114]
precursor into the hollow carbon particles and then followed this with a thermal reduction . The hollow
carbon spheres served as a physical matrix that could effectively protect the germanium core from
coalescing or pulverization. Similarly, Mo et al. designed a 3D-interconnected porous graphene foam with
[112]
germanium quantum dots doped into it by a facile approach . This structure provided close contact
between the electrode materials and the current collector, and the yolk-shell structure effectively alleviated
the significant volume changes and provided a stable SEI. Designing nanostructures in combination with
carbon materials are also an efficient method to improve the electrochemical performance of Ge-based
[106]
materials in PIBs. Liu et al. synthesized a dual carbon structure with germanium encapsulated inside . The
as-prepared dual carbon matrix was composed of mesoporous carbon and an amorphous carbon layer, as
shown in Figure 9. Using this structure, the dual carbon effectively alleviated the expansion of germanium.
Yang et al. designed a nanoporous structure Ge with small ligaments and interconnected porous prepared
[107]
by a chemical-dealloying method . The nanoporous germanium delivered a high initial capacity of
[107]
-1
290 mAh g and a stable capacity of 120 mAh g over 400 cycles .
-1
Using active or inactive elements to form Ge-based binaries or composites is another effective method to
[115]
[116]
improve the electrochemical performance. The inactive metals alloyed with Ge include Co and Cu ,
which can improve the conductivity. The active materials have been applied in the formation of Ge-based
[117]
compounds are Si , Sn , Sb , Te and Se , which have high theoretical capacities. As discussed
[118]
[120]
[119]
[121]
above, phosphorus has the highest theoretical capacity in PIBs and can increase the capacity of the total
capacity by the formation of GeP . Zhang et al. prepared GeP , which delivered a stable capacity of
x
5
-1
213.7 mAh g in PIBs for 2000 cycles at a current density of 500 mA g -1[51] . The active Se metal can form
layered metal selenides with Ge, which has a large interlayer distance of 5.41 Å. The GeSe/CNT composite
synthesized by a simple ball-milling method delivered a stable cycling performance with 311 mAh g
-1
retention after 400 cycles. Furthermore, the electrode delivered a capacity of ~200 mAh g at a high current
-1
density of 5 A g -1[122] . Ge-based anode materials exhibit larger volume changes in PIBs compared to the
volume changes in LIBs and SIBs because of the larger size of potassium ions. The construction of 3D
porous and yolk-shell structures in combination with carbon materials, such as carbon spheres, graphene or
amorphous carbon, can efficiently ameliorate the volume changes and improve the electrochemical
performance.
Sn-based anode materials for PIBs
Sn has been an attractive anode material for LIBs and SIBs for a long time and has high theoretical
-1
capacities of 991 and 845 mAh g via the formation of Li Sn and Na Sn , respectively. The study of Sn in
15
4
4.4
PIBs started in 2016 , with the formation of KSn. Sn has a theoretical capacity of 226 mAh g in PIBs.
-1
[123]
Mechanism of Sn-based anode materials
Based on the K-Sn phase diagram, K Sn, KSn, K Sn , KSn and K Sn can form at different temperatures.
2
2
2
4
3
23
Wang et al. were the first to study the reaction mechanism of Sn in PIBs using in-situ TEM and XRD
methods . They revealed a two-step process corresponding to Sn → amorphous K Sn → KSn. A similar
[124]
4
9