A research team led by Hui Kwun Nam, associate professor in the Institute of Applied Physics and Materials Engineering (IAPME), University of Macau (UM), has recently made important progress in the research of anode materials for potassium-ion batteries, which is expected to provide solutions for poor cycling stability problems for the development of the next generation of energy storage systems. The team has also further optimised the materials to achieve higher energy density while improving the stability and safety of the batteries. The two research results have been published in the international journal Advanced Energy Materials and Advanced Functional Materials and have applied for patents in China.

At present, the use of traditional fossil fuels has led to the deterioration of the global environment and the depletion of non-renewable resources, which is a major problem facing mankind. It is imperative to vigorously develop new renewable energy sources, such as solar energy, wind energy, geothermal energy, and tidal energy. However, most renewable energy sources have intermittent and regional characteristics. The construction of a smart grid is an indispensable component for the large-scale and rational use of the above renewable resources. For the construction of a smart grid, a stable, efficient, and inexpensive energy storage system is essential. It seems to be a good choice to use the currently widely used lithium-ion battery as an energy storage system. However, the uneven distribution and low overall abundance of lithium resources restrict its large-scale application. In contrast, potassium is highly abundant with a more balanced distribution and its electrode potential is similar to that of lithium. For this reason, in the past five years, the research of potassium-ion batteries has received much attention.

Anode materials are critical to the performance of batteries. Among the many anode materials of potassium-ion batteries, phosphorus and its metal phosphides are favourable competitors for the anode electrodes of the batteries because of their high specific capacity and reasonable discharge voltage. However, these kinds of electrode materials are based on conversion or even alloying reaction mechanisms. They will undergo huge volume changes after cycling and will result in extremely poor cycling stability, which makes it difficult to meet the actual application requirements. In order to solve the abovementioned problems, some solutions, including complicated morphology control and expensive flexible substrates, have been reported, and significant improvements have also been made, but complex and high-cost solutions remain difficult to put into practice. By analysing previous studies, the research team found that the oxidation of phosphorus and its metal phosphides seems to be unavoidable in the synthesis of materials or in the preparation of electrodes. However, phosphide electrodes have a high electrochemical stability and phosphates are susceptible to amorphisation. For this reason, in situ-formed amorphous phosphate impurities can act as buffers to reduce the volume expansion of phosphide electrodes. To this end, the team took a different approach by using metal oxide as the raw material and synthesising amorphous phosphate-doped metal in one step through a simple ball milling method. This in situ-formed amorphous phosphate has a uniform distribution and can be used as a buffer to effectively reduce the volume expansion of the metal phosphide electrode. As a general synthesis strategy, the team synthesised a total of ten metal phosphides (VPx, CrPx, MnP_x, FeP, FeP₂, FeP₄, CoP, NiP₃, CuP₂, and ZnP₂) containing amorphous phosphate-doped metal phosphides. Among them, ZnP₂ as a representative was tested and analysed.

Based on the above analysis, the research team carried out further investigation. First, the controllable zinc phosphate content was achieved simply by controlling the ratio of Zn, ZnO, and P raw materials and three zinc phosphide composites with different contents of zinc phosphate were prepared. The mass fractions of zinc phosphate are 10, 20, and 30 wt%, respectively. Secondly, non-flammable triethyl phosphate (TEP) was used as the electrolyte to improve the safety of the battery. High capacity (the highest capacity was 571.1 mAh g⁻¹ at a current density of 0.1 A g⁻¹), high stability (94.5% capacity retention after 1000 cycles), and high safety (no burning under open flame conditions) were realised for the zinc phosphide composite electrodes.

In the two studies above, Ji Shunping, a PhD student in the IAPME is the first author. Prof Hui and Assistant Professor Chen Shi in the IAPME, as well as Prof Hui Kwan San at the University of East Anglia and Prof Shao Zongping of Curtin University are the corresponding authors. IAPME Chair Professor Tang Zikang and Associate Professor Li Haifeng as well as PhD students Li Junfeng and Wang Shuo also made important contributions to the supplementation and analysis of experiment data in the studies. The project was funded by the Science and Technology Development Fund of Macao (File no: 0041/2019/A1, 0046/2019/AFJ, 0021/2019/AIR) and UM (File no: MYRG2018-00192-IAPME and MYRG2020-00187-IAPME).

The full version of the articles can be viewed at https://onlinelibrary.wiley.com/doi/10.1002/aenm.202101413

https://onlinelibrary.wiley.com/doi/10.1002/adfm.202200771?af=R