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Yang et al. Microstructures 2023;3:2023013 https://dx.doi.org/10.20517/microstructures.2022.30 Page 5 of 27

are two interlayer migration paths: zigzag- and armchair-type migration paths. The zigzag path has a much
lower energy barrier. Based on calculations, K has the lowest energy barriers for both paths compared to Li
and Na, which endow PIBs with fast discharging and charging. The corresponding voltage can be calculated
based on the following equation:

where V, μ, q, G, e and N are the voltage, chemical potential, charge, absolute electron charge, Gibbs free
energy and the number of K ions, respectively.

Thus, the calculated potassium-ion insertion process is BP → K P → KP. A previous study of the
2 3
potassiation mechanism indicated that the final product was KP [36-39] , which was first revealed by the group
of Glushenkov. Compared to this result, a further study by Jin et al. used X-ray absorption near-edge
structure and ex-situ X-ray diffraction (XRD) methods to analyze the mechanism . The results
[38]
+
-
demonstrated that the potassiation process of BP was K + P + e → KP. An RP-based nanocomposite was
studied by the group of Xu . The composite was synthesized by anchoring RP nanoparticles on a 3D
[39]
nanosheet framework. The reaction mechanism of the composite was explored by transmission electron
microscopy (TEM) and selected area electron diffraction (SAED). Based on the first cycle reaction results,
-1
KP was proposed to be the final product, corresponding to a capacity of 865 mAh g , which is lower than
the theoretical capacity. Yu et al. synthesized an RP/carbon nanocomposite by embedding RP into free-
[40]
standing nitrogen-doped porous hollow carbon . Using in-situ Raman spectroscopy and ex-situ XRD, the
final product in the discharge process was directly proved to be K P .
4 3
One challenge for BP and RP in the potassiation process compared to lithiation is the lower capacity. A BP-
graphite composite had only 42% of the capacity for lithiation. The other issue is their large volume
expansion. BP-graphite showed a 200% volume expansion when discharged to 0.01 V .
[41]

The large volume expansion during the potassiation process and low conductivity of RP severely limit the
application of phosphorus-based anode materials in PIBs. To overcome this, active (Sn, Ge and Se) and
inactive metals (Co, Fe and Cu) have been hybridized with P to form phosphides. During the discharge
process, the decomposed nanocrystals form a conductive and elastic matrix to enable faster charge transfer
and hinder volume expansion. In addition, the active metals become alloyed with potassium ions and also
make contributions to the capacity. Metal phosphides can be classified into two categories based on active
and inactive metals. For the inactive metals, the storage mechanism reaction can be summarized as follows:

-
M P + zK + ze = xM + K P (2)
+
z y
x y
For the active metal phosphides, the reaction can be summarized as follows:
M P + (xn + ym)K + (xn + ym)e = xK M + yK P (3)
+
-
m
x y
n
The original phosphide M P decomposes and the phosphorus is converted into K P, while inactive metal
x y
3-x
M is dispersed as a matrix and active metal M is also alloyed with K to produce K M.
n