New potassium salt KFSI boosts potassium ion battery

With the rapid development of today's technology, electric energy storage technology has also followed the trend, moving toward high efficiency and low cost. Lithium-ion secondary batteries (LIBs), as the most mature energy storage systems in the field of renewable energy, have achieved great success both in technical research and in practical applications. However, the lack of lithium resources will inevitably lead to high costs. Among the elements having the same electrochemical properties as the main group, potassium has a working voltage and energy density closer to that of lithium, and potassium is rich in resources and low in price, so potassium ion batteries are receiving more and more attention.

At present, reports on negative electrode materials for potassium ion secondary batteries (PIBs) are mainly concentrated on various types of carbon materials. The second is the alloying anode material using the alloying/de-alloying reaction as the potassium storage mechanism. However, due to the relatively large potassium ion radius (1.38 Å), large volume changes will occur during charge and discharge, making the electrode material easy to powder and aggregate. At the same time, potassium is very active, and it is easy to produce side reactions during battery cycling. , resulting in poor battery cycle life. Recently, Prof. Guo Zaiping from the University of Wollongong in Australia replaced the traditional potassium hexafluorophosphate (KPF6) with bis-sulfonylimide potassium salt (KFSI), which greatly improved the performance of alloy anode materials in potassium ion batteries. They reported for the first time the electrochemical performance of metal ruthenium materials in potassium ion batteries. At the same time, it was found that using the traditional KPF6/ester electrolyte system, the negative electrode material destroyed the SEI that had formed in the first circle due to the intense volume change during charge and discharge. The membrane, which continuously consumes the electrolyte, generates a new SEI membrane, resulting in a rapid decay of capacity. In contrast, in the case of using an ester solvent, the SEI film formed by switching to KFSI has better stability, can reduce side reactions, and effectively avoid the capacity decay caused by volume change of the alloy negative electrode material. . At the same time, they also found that KFSI electrolyte can also effectively improve the electrochemical performance of other alloy-based anode materials such as tin and antimony. This article was published in Advanced Energy Materials. Dr. Zhang Qing is the first author of the paper, and Dr. Mao Jianfeng and Professor Guo Zaiping are the co-authors of the paper.

Figure 1. Electrochemical performance of Bi-based materials: (a) Cyclic voltammetry using KFSI electrolyte and (b) KPF6 electrolyte; (c) KFSI electrolyte and (d) Constant current using KPF6 electrolyte Charge and discharge curve; (e) cycle performance and corresponding coulombic efficiency; (f) rate performance.

Figure 2. TEM image after cycling with KPF6 electrolyte: (a) 2 turns, (b) 5 turns, (c) 10 turns; TEM image after KFSI electrolyte cycle: (d) 2 turns, (e) 5 cycles, (f) 10 cycles; distribution of fluorine after 10 cycles in the material (g) KPF6, (h) KFSI; XPS diagram after 10 cycles in KPF6 and KFSI electrolytes respectively: i) F 1s, (j) C 1s, (k) O 1s.

Figure 3. (a) Load-displacement depth curves of Bi/rGO electrodes after cycling in KPF6 and KFSI electrolytes; photographs of separators cycled 10 times in (b) KFSI and (c) KPF6 electrolytes, respectively; Bi The surface height map of the /rGO electrode after cycling in (d) KPF6 and (e) KFSI electrolytes, and the surface potential map after (f) KPF6 and (g) KFSI electrolyte cycles.

Figure 4. (a) Schematic diagram of SEI film formation and stabilization effect of Bi material in different electrolyte salts. Other alloy-based anode materials such as (b, c) Sn/C and (d, e) Sb/C use KFSI and KPF6 electrolytes respectively to compare the electrochemical cycle performance and coulombic efficiency.

【summary】

This work improves the electrochemical performance of such materials in potassium ion secondary batteries by optimizing the potassium salt (KFSI) which is more suitable for the anode material based on the alloying reaction. The study found that thanks to the preferential reduction of KFSI salt, the SEI film produced by KFSI electrolyte is more uniform and stable, which can better protect the electrolyte decomposition, improve the interface stability of the electrode material during charge and discharge, and improve the Cycle performance. This research work provides a new and simple solution for improving the cycle stability of potassium ion batteries, and is expected to be used in other potassium ion battery electrode materials.

Qing Zhang, Jianfeng Mao*, Wei Kong Pang, Tian Zheng, Vitor Sencadas, Yuanzhen Chen, Yajie Liu and Zaiping Guo*, Boosting the Potassium Storage Performance of Alloy-Based Anode Materials via Electrolyte Salt Chemistry, Adv. Energy Mater. 2018, 1703288.