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Yang et al. Microstructures 2023;3:2023013 https://dx.doi.org/10.20517/microstructures.2022.30 Page 9 of 27

K-ion storage mechanism of Bi-based anodes
Based on the K-Bi equilibrium diagram with the KBi , K Bi , K Bi(α), K Bi(β) and K Bi phases, Huang et al.
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3
2
first studied the potassium-ion storage mechanism in Bi microparticles . They revealed stepwise Bi → KBi 2
[57]
→ K Bi → K Bi dealloying-alloying electrochemical processes after the initial surface potassiation. Similarly,
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2
a bulk Bi anode delivered a reversible three-step reaction during cycling, with K Bi as the fully discharged
3
[58]
product . Bi microparticles have the same mechanism as shown in Figure 5A and B, with K Bi as the final
3
discharged product. As shown in Figure 5C, the observation of K Bi during the potassiation process was
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5
[59]
first reported by the group of Guo . They found a different transition process, in which the potassiation of
Bi nanoparticles proceeds through a solid-solution reaction, followed by a two-step reaction, corresponding
to Bi → Bi(K) and Bi(K) → K Bi → K Bi. Xie et al. constructed dual-shell-structured Bi box particles and
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microsized Bi, which had different appearances during the transformation from K Bi to K Bi under a low
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2
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current density . In the case of nanostructured Bi, the K Bi phase went through a transformation to K Bi,
[60]
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as shown in Figure 5D. In comparison, the microstructure of Bi retained the K Bi phase and no significant
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2
K Bi phase was formed. Interestingly, when the current was increased, no significant K Bi or K Bi phase was
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observed, indicating that the main mechanism was a surface-driven adsorption reaction under a high
current.
The study of the potassiation mechanism in Bi-based alloys has also attracted significant attention. The
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reaction process includes two stages. The first step was an intercalation reaction: Bi S + xK + xe → K Bi S .
+
x
2 3
2 3
The second step was a conversing-alloying reaction: K Bi S + (6-x)K + (6-x)xe → 3K S + 2Bi and Bi +3K +
+
-
+
2 3
x
2
-
3e →K Bi . Chen et al. also studied the reaction mechanism of Bi Se using in-situ operando XRD . The
[61]
[62]
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results indicated that the potassiation process also undergoes an intercalation reaction in the first steps, with
a conversion-alloying reaction in the following step. The electrochemical process was summarized as
follows: 2Bi Se + 4xK + 4xe → 4K BiSe , K BiSe + (6-x)K + (6-x)e → 3K Se + Bi and i + 3K + 3e → K Bi .
-
-
+
[62]
+
-
+
2
3
x
3
2
x
3
3
The above results illustrate the diverse potassiation mechanisms. The differences in the potassiation and
depotassiation processes were mainly because of the following reasons: (1) the mechanisms are strongly
dependent on the sizes of the materials; (2) the unique structure of the Bi-based anodes; and (3) the current
density of the electrochemical reaction. The small particle sizes, well-constructed nanostructure and low
current density resulted in full potassiation and transformation that involved several transition phases.
Modification strategies for Bi-based anodes
The main challenge for Bi-based anode materials is the pulverization and fracturing of the electrode during
the cycling process that are driven by the significant volume changes, resulting in capacity fading.
To improve the electrochemical performance of Bi, various methods have been applied. One method is to
combine Bi with carbon materials. Various porous carbon materials have been applied, such as porous
graphene and carbon nanosheets . Both of these porous carbons were synthesized using freeze drying
[63]
[64]
assisted by a pyrolysis method. The Bi/macroporous graphene composite delivered an excellent rate
-1
-1
performance of 185 mAh g at a high current density of 10 A g . This was because the 3D interconnected
macroporous graphene framework could provide robustness to maintain the structural stability.
N-doped carbons were demonstrated to simultaneously improve the conductivity and electrochemical
activity of carbon materials and were applied in combination with Bi [63,64] , as shown in Figure 6A-F.
Similarly, Shi et al. designed a multicore-shell Bi-N nanocomposite using a facile self-template method. The
anode delivered a stable performance of 266 mAh g after 1000 cycles at 20 A g , as shown in Figure 6G-I .
[65]
-1
-1
Li et al. used hollow N-doped carbon to coat bismuth nanorods, which showed the best long-cycling