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Page 2 of 27 Yang et al. Microstructures 2023;3:2023013 https://dx.doi.org/10.20517/microstructures.2022.30

future. It is necessary to combine electrical energy storage devices with these renewable energies.
Rechargeable lithium-ion batteries (LIBs) are high-energy electrical energy storage devices that have been
commercialized for around three decades. LIBs cannot only be used with natural clean renewable energies
but are also ubiquitous in our daily lives for powering electronics, including cell phones, electric cars and
laptops. However, their high-cost resources and the uneven distribution of lithium in the Earth’s crust make
it imperative to develop alternatives to LIBs with comparable performance [10-12] .

Potassium-ion batteries (PIBs) are possible alternatives to LIBs. Compared to lithium resources in the
Earth’s crust, potassium resources are significantly more abundant in the Earth’s crust at ~1.5 wt.%. The
price of potassium salts, such as K CO , is far less compared to Li CO . In addition to the lower cost of
2
3
2
3
potassium resources, inexpensive aluminum current collectors can be used together with PIBs to offer a
low-cost method based on economical salts [13-15] . In addition, potassium ions exhibit much weaker Lewis
acidity, which results in smaller solvated ions compared to lithium and sodium ions. Therefore, the ionic
+
conductivity of solvated K is higher than that of lithium and sodium ions [16,17] . In addition, the lower energy
required to dissolve potassium ions also results in their fast diffusion kinetics.
Similar to LIBs and sodium-ion batteries (SIBs), the study of cathode materials for PIBs mainly includes
layered transition metal oxides, Prussian blue analogs (PBAs) and polyanionic compounds. Layered
transition metal oxides based on K MO (x ≤ 1, M = Co, Cr, Mn, Fe or Ni) deliver high capacity but face the
x
2
critical problems of multiple plateaus and large structural changes during potassium-ion
intercalation/deintercalation [18,19] . The chemical formula of PBAs is represented as K M1[M2(CN) ] H O (0
2
6 n
x
≤ x ≤ 2), where M1 and M2 represent various metals, such as Fe, Cr, Co and Ni [20-23] . One advantage of PBAs
is their three-dimensional (3D) open frameworks that are available for large K to diffuse. Another
+
advantage of PBAs in PIBs is their high average working potential of 3.5 V. Currently, the disadvantages of
PBAs are their low conductivity and bulk density [24,25] . Polyanionic compounds also have 3D open channels
that are available for the fast diffusion of large K ions [26,27] . The study of PIB cathode materials makes the
development of full-cell PIBs possible and promising.

The search for anode materials is also an important part of PIB research and development. Commercialized
graphite has been widely applied in LIBs; however, it is not an ideal anode candidate. Even though graphite
has a theoretical capacity of ~280 mAh g from the formation of KC 8 [28,29] , the large radius of the potassium
-1
ions results in sluggish diffusion kinetics and the formation of an unstable SEI. Therefore, graphite anodes
deliver limited experimental capacity and cycling life in PIBs. As a result, it is crucial to develop high-
performance anode materials with high specific capacity and long cycling life for practical application. In
the past five years, there has been a large volume of research regarding electrode materials for PIBs,
including metal-organic structure design [30,31] for electrodes and the modification of electrode surfaces [32,33] .
However, there have been few review papers that focus on anode materials for PIBs, especially on high-
performance alloy-based anode materials, including their modification and mechanisms in PIBs . In this
[34]
review, we comprehensively summarize the current understanding of alloy-based anode materials and their
composites for PIBs, as shown in Figure 1, including their mechanisms, modification strategies and recent
research progress for potassium storage. The challenges and future perspectives corresponding to these
materials are also presented.

Alloy-based elements can deliver high-capacity anode materials via the formation of potassium-rich
materials. For example, Bi has a high theoretical capacity of 385 mAh g PIBs. Sb has a high theoretical
-1
-1
capacity of 687 mAh g . P has the highest theoretical capacity among alloy-based anodes in PIBs of
865 mAh g . Ge has a high theoretical capacity of 369 mAh g and Sn has a theoretical capacity of
-1
-1