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Page 4 of 27 Yang et al. Microstructures 2023;3:2023013 https://dx.doi.org/10.20517/microstructures.2022.30

Figure 2. Theoretical and volumetric capacities of P, Bi, Sb, Ge and Sn.

These conversion-alloying type reactions can be expressed as M N + (xn + ym)K + (xn + ym)e ↔ xK M +
-
+
x
n
y
yK N. Similarly, the metallic compounds that go through conversion-alloying type reactions also deliver
m
-1
high theoretical capacities. For example, Sn P delivers a high capacity of 585 mAh g while the experimental
4 3
capacity is ~384 mAh g .
-1
Challenges
Although alloying-type anode materials deliver high theoretical capacities, their practical reversible
capacities are far below their theoretical capacities. The severe volume change causes capacity decay, poor
cycle life, inferior rate performance, sluggish kinetics and limited cycling lifespans.
The initial capacity is a key factor, especially for the anode material, which contributes to the energy density
of the full cell. The significant volume changes during the discharge-charge processes cause pulverization of
the active materials, which results in discontinuous particles. Due to the large resistance within the particles,
the potassium ions cannot be fully extracted, which results in irreversible capacity and low Coulombic
efficiency. In addition, the stress generated in the electrode during the discharge process damages the SEI,
resulting in its breakdown and the reformation of a new SEI film. In addition, the pulverized particles
inevitably go through crystallization and aggregation, which increase the diffusion length of the potassium
ions and also lead to irreversible capacity loss. The decreasing reversible capacity results in a rapid capacity
drop and short cycling life.

ALLOY-BASED ANODES FOR PIBS
Phosphorus-based anode materials
Among alloying-typed anode materials, phosphorus is very attractive because it has a high theoretical
-1
+
capacity of 2596 mAh g in LIBs and SIBs and a low work potential (~0.3 V vs Na/Na ). In PIBs, it has a
-1
high theoretical capacity of 2590 mAh g based on the three-electron alloying mechanism.
Mechanism of phosphorus and metal phosphides in PIBs
There are three main types of phosphorus in nature, namely, white phosphorus (WP), red phosphorus (RP)
and black phosphorus (BP). WP is toxic and has a low ignition point, so it is unsuitable as an electrode. RP
-1
exists in a non-crystalline form and has a low conductivity of 10 S m . BP is a layered structure
-12
semiconductor material, which has a wide interlayer spacing of 5.2 Å and a higher conductivity at 300 S m .
-1
-1
BP has a high theoretical capacity of 2600 mAh g for LIBs and NIBs and also has a low diffusion barrier of
+
0.035 eV for Li and 0.064 eV for Na , which makes it a promising anode material to explore for PIBs. There
+