Synergistic ion diffusion mechanism was used in potassium ion batteries for the first time

Potassium-ion batteries are an emerging energy storage system, which is still in the early stage of development. Due to the relative lack of knowledge about potassium-ion batteries, it still takes time and effort to explore further. Among the many lithium-ion battery replacements, potassium-ion batteries have certain advantages in terms of price and energy density due to the low cost of potassium (K) and lower standard potential.

Research on Potassium Ion Battery Technology

The standard electrode potential of potassium (-2.93V, vs SHE) is closer to lithium (-3.04V, vs SHE). Since the ionic radius of potassium and sodium ions is larger than that of lithium ions, although the energy density is less than that of lithium ions, at present, research on potassium ion anode carbon materials shows that the power density of potassium ion batteries is higher than that of sodium and closer to lithium ion batteries, and the rate performance is better. Recently, research on potassium ion batteries has emerged in an endless stream, but most of them focus on the development of new electrode materials, but there is still a lack of understanding of the mechanism of potassium ion storage and transmission. However, this is precisely the key to breakthroughs in potassium-ion battery technology.

Preparation of potassium-type birnessite potassium ion battery electrode by ion exchange method

Birnessite is a layered transition metal oxide, which is ubiquitous in nature. Its sheet is composed of manganese-octahedral MnO₆ co-edge or co-angle. The layers are filled by water molecules and K⁺, and the interlayer spacing is about 0.7 nm, and varies with its water content and alkali metal ion content. Layered transition metal oxides have been widely used as positive electrode materials for rechargeable batteries (lithium, sodium ion batteries), however, such a compact crystal structure is difficult to accommodate large potassium ions, resulting in a low initial potassium content of the initial positive electrode material. Such as P2-K_0.3MnO₂, P2-K_0.6CoO_2, P3-K_0.5MnO₂. Potassium-ion batteries rely on potassium ions to be deintercalated between the positive and negative electrodes to store the released energy, but the negative electrode materials are mostly potassium-free materials. Therefore, the positive electrode material is the main supplier of potassium ions, and the potassium-depleted material is obviously not the ideal positive electrode material. K-Birnessite is also limited to this factor. As an extensive soft chemical synthesis method, ion exchange method has been used to decouple potassium ions in K_0.55CoO₂. Therefore, the ion exchange method can increase the potassium ion content in potassium-type birnessite to improve the amount of potassium ions in the process of charge and discharge, to achieve the purpose of improving its electrochemical performance.

Recently, researchers at Beijing University of Chemical Technology used traditional solid-phase reactions to explore the synthesis conditions of potassium-type birnessite. In view of the low potassium content of this material, the ion exchange method was used to increase the potassium content. The optimized potassium ion battery electrode material K-Birnessite (s-Kbir) has a specific capacity of 125 mAh g^-1 at a magnification of 0.2 C. The in-situ XRD technique demonstrates the reversible structural change of this material during charge and discharge, and has a deeper analysis of the potassium storage mechanism of the electrode material.

Potassium ion diffusion in potassium ion battery

The traditional ion diffusion is based on the model of single ion jumping between adjacent vacancies. However, the crystal structure of the potassium-depleted material has a large number of potassium ion vacancies, which leads to the complication of potassium ion diffusion, such as gap diffusion, group diffusion, and synergy ion diffusion, etc. Combined with first-principles calculation (DFT), the researchers first introduced a synergistic ion diffusion mechanism into potassium-ion batteries, revealing that in the presence of a large number of vacancies, potassium ions will diffuse in a synergistic manner, successfully explaining K-Birnessite as the origin of the excellent electrical properties of potassium ion battery electrode materials. At the same time, this theory also deepens the understanding of the dynamics of solid-state ions and battery electrodes.

Edited by Suzhou Yacoo Science Co., Ltd.