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Page 22 of 27 Yang et al. Microstructures 2023;3:2023013 https://dx.doi.org/10.20517/microstructures.2022.30

mechanisms of the potassiation and depotassiation processes have been deeply discussed and analyzed. For
an elementary substance, the mechanism is a simple alloying reaction. For compound materials, the
reaction process is mainly a conversion-alloying reaction. The various formations of the intermediate
product in the potassium ion for the same material are mainly due to the various nanostructures and grain
sizes of the materials. Modifications of the materials have also been explored and investigated. The
approaches can be classified as the hybridization of active materials with high conductivity and architectural
engineering. Highly conductive materials, including graphene, carbon nanotubes, graphite, N-doped carbon
and carbon nanosheets. The architectural engineering methods, including the design of one-dimensional
nanotubes, 2D nanosheets and 3D structural materials, such as core-shell structures and their combinations.
By using these modification methods, the significant volume change and sluggish reaction kinetics can be
effectively solved.

The electrochemical performance of alloy-based electrodes has now been greatly improved and the reaction
processes have also been deeply analyzed. Further research can be carried out on the following aspects:

(1) Low initial Coulombic efficiency is the main problem that remains for anode materials, which might be
ascribed to the irreversible insertion of potassium ions and the decomposition of the electrolyte. In the full
cell, the maximum cell energy is obtained when the anode irreversible capacity exactly matches that of the
+
cathode material. The low initial Coulombic efficiency (ICE) indicates the large consumption of K
provided from cathode, which results in lower energy density in the full cell and faster capacity drop.
Improving electrolytes with higher ion conductivity will increase the ICE.

(2) Although fabricated nanostructures and hybrids with carbon will significantly hinder the volume
changes, alloy-based anode materials still face the problem of volume expansion and pulverization during
cycling. Furthermore, this problem may bring the severe side effect of the reaction between the electrolyte
and the new surface of the electrode, leading to the formation of the SEI on the new surface, which results in
a capacity decrease and instability of the cycling performance. This side effect may also result in the
maldistribution of electrons, leading to dendrite growth and the polarization of electrodes. This will limit
the application and manufacturing of PIBs. Electrolyte and electrode interface engineering or controlling
the content and structure of the SEI layer or designing an artificial SEI layer can make up for the shortage.

(3) Safety problems are still an issue for future development. Alloy-based anode materials are currently
limited in their application at high and low temperatures. Aqueous electrolyte and flame-retardant
electrolyte systems could be promising designs for future applications. In addition, non-flammable
carbonate electrolytes can also be used to address battery safety issues.

In light of the abundance of potassium resources and the significant progress that has been made in the
research on alloy-based anodes for PIBs, these anodes will be promising for commercialization in the near
future.

DECLARATIONS
Acknowledgments
The authors thank Dr. Tania Silver for her critical reading of the manuscript.

Authors’ contributions
Characterizing, writing original draft: Yang Q