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Page 6 of 27 Yang et al. Microstructures 2023;3:2023013 https://dx.doi.org/10.20517/microstructures.2022.30

Unlike phosphides in LIBs and SIBs, to date, the reported phosphide potassiation mechanisms are
conversion-type mechanisms. For example, the electrochemical reaction of Sn P in PIBs is a typical
4 3
conversion reaction, as first studied by the group of Guo based on an in-operando synchrotron XRD
investigation. In the initial discharge stage, Sn P breaks into Sn particles and the P component precipitates
4 3
in an amorphous form to react with potassium. Sn is alloyed with K and the KSn phase is formed. K P
3 11
further reacts with K, starting from ~0.17 V. The reaction process could be divided into three steps, namely,
Sn P + (9-3x)K ↔ 4Sn + 3K P, 23Sn + 4K ↔ K Sn and K Sn + 19K ↔ 23KSn, as shown in Figure 3A .
[42]
23
4
4 3
3-x
23
4
-1
The Sn P @carbon fiber electrode delivered cycling stability and a high-rate capability of 160.7 mAh g after
4 3
1000 cycles at a current density of 500 mA g . Like Sn P , GeP also has a similar conversion reaction. Based
-1
5
4 3
on in-operando synchrotron XRD measurements, a two-step reaction was observed as follows: GeP + 20/3K
5
↔ 5/3K P + Ge, Ge + K ↔ KGe. These two steps can be summarized into one equation as follows: 3GeP +
5
4 3
23K ↔ 5K P + 3KGe . This potassiation process is shown in Figure 3B. Similarly, in the first stage of the
[43]
4 3
reaction, GeP decomposes into Ge and P particles and the P component reacts with K to form K P . In the
5
4 3
following stage of the reaction, Ge alloys with K to form KGe. Se is also an active element that can form
-1
K Se through a two-electron transfer reaction. Se P exhibited a high reversible capacity of 1036 mAh g in
2
3 4
PIBs . Based on ex-situ XRD and X-ray photoelectron spectroscopy (XPS) results, Se P delivered a
[44]
3 4
[44]
reversible conversion-type reaction as follows: Se P + (18-4x)K + 18e ↔ 4K P + 3K Se , as shown in
-
+
2
3-x
3 4
Figure 3C and D. The inactive material, such as Cu P, undergoes the reaction of 2Cu P + (3-x)K + (3-x)e ↔
+
-
3
3
K P + 6Cu + P(amorphous) and the final discharge product is also K P .
[45]
3
3-x
In summary, until now, the reported phosphide potassiation mechanisms have been conversion-type
mechanisms, which are different from phosphides in SIBs and LIBs. In the first discharge step, the
phosphide decomposes into metal and phosphorus. After the anode has been fully discharged, the active
metal reacts with K and forms K M compounds, with the phosphorus alloyed with K to form K P.
m
n
Modification strategies for P and phosphides
Carbon materials, including nanosheets , nanofibers and graphite , have been applied in phosphorus
[45]
[39]
[40]
and phosphides. The hybridization of phosphorus and phosphides with carbon materials has been proven
to be an efficient method to improve the electrochemical performance. The introduction of carbon can
enhance the electron conductivity, accommodate the volume change and also shorten the potassium-ion
diffusion length. Furthermore, the induced carbon can form covalent P-C interfaces to prevent edge
reconstruction and ensure ion insertion and diffusion . In addition, the formation of P-C bonds [46-49] by
[35]
hybridizing BP with carbon materials can afford high capacity and cycling stability in PIBs by connecting
particles. This can also be seen from the work of Verma et al., where the electrochemical performance of
SnP was efficiently improved by hybridizing with carbon . The electrode maintained a reversible capacity
[50]
3
of 225 mAh g after 80 cycles, which was an improvement compared to the previous rapid capacity drop of
-1
the SnP electrode in cycling performance. Similarly, the group of Zhu designed a flexible and
[51]
3
hierarchically porous 3D graphene/FeP composite via a one-step thermal transformation strategy. The
interconnected porous conducting network sufficiently buffered stress due to the nano-hollow spaces and
greatly promoted the charge transfer. Thus, the composite delivered a high-capacity retention of 97.2% over
2000 cycles at a high rate of 2 A g in PIBs.
-1
Synthesizing nanostructured phosphorus and phosphide materials, such as yolk-shell structures , hollow
[52]
structures and nanowires, is another efficient method to improve the electrochemical performance of
phosphide and phosphorus anodes. For example, Yu et al. designed a one-dimensional electrode by
embedding RP into free-standing nitrogen-doped porous carbon nanofibers . This design was favorable
[40]
for reducing the absolute strain and preventing pulverization and agglomeration. As can be seen from their