Potassium-ion Nickel Metal Hydride No. 5 battery research and commercialization have broad prospects

The information-based and automated modern society is constantly progressing, and electrochemical energy storage devices have played an important role in it. Since the 1990s, with the commercial application of lithium-ion batteries, lithium-ion batteries have become a part of our lives since the 21st century. Lithium-ion Nickel Metal Hydride No. 5 battery portable devices and powered vehicles have been everywhere in our lives. However, unlike other commercial products that become cheaper the more they are sold, the scarce lithium resources make the cost of lithium-ion batteries continue to rise in the future. Seeking an alternative low-cost electrochemical energy storage device has become an urgent problem to be solved. The electrochemical properties of potassium and sodium are similar to those of lithium, and the earth's reserves are abundant, making them the only choice to replace lithium-ion batteries in the future.

The research on sodium-ion batteries has made great progress in recent years. However, due to its standard electrode potential (-2.71V, vs SHE.) and large ion radius limitation, the energy density and power density of sodium-ion batteries are still far behind those of lithium-ion batteries. The standard electrode potential of potassium (-2.93V, vs SHE.) is closer to that of 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 inferior to that of lithium ions, current research on potassium ion negative electrode carbon materials shows that the power density of potassium ion batteries is higher than that of sodium and closer to that of lithium ion batteries, and the rate performance is also better. However, at this stage, research on potassium ion positive electrode materials is minimal, and most studies use aqueous solutions as electrolytes, which limits the voltage window. Recently, the research group of Professor Lei Yong from the Technical University of Ilmnau in Germany and the research group of Shanghai University have cooperated to prepare a low-cost dye nanoparticle: Prussian blue, and elaborated on its electrochemical properties as a potassium ion positive electrode material in an organic electrolyte. At the same time, using it as a potassium ion positive electrode material, a high-performance potassium ion full Nickel Metal Hydride No. 5 battery was matched for the first time. The test results show that Prussian blue as a potassium ion positive electrode material presents a high discharge platform (3.1–3.4V) and a stable reversible specific capacity. At a charge and discharge rate of 50mA/g, it still has a cycle specific capacity of 73.8mAh/g, and the degradation rate is extremely slow, only 0.09% per-cycle. At the same time, by analyzing the electrochemical storage mechanism of Prussian blue molecules, they found that this framework molecular structure is extremely conducive to the storage and release of potassium ions with a larger radius, and its main active position is on C-FeⅡ/FeⅢ. Finally, they used this positive electrode material and combined it with the commercially used superP as the negative electrode material to design and match a potassium ion full Nickel Metal Hydride No. 5 battery for the first time. At a charge and discharge rate of 100mA/g, the full Nickel Metal Hydride No. 5 battery has a reversible specific capacity of up to 68.5mAh/g and a long cycle life. It still retains 93.4% of the specific capacity after 50 cycles of charge and discharge. For potassium ions with a larger radius, such a breakthrough is invaluable.

The research on low-cost Prussian blue dye as a potassium ion cathode material and its matching design for the whole Nickel Metal Hydride No. 5 battery have made lithium-ion batteries find a better alternative. This research provides broad prospects for the future research and commercial application of potassium ion batteries. This paper has been published online in Advanced Functional Materials (DOI: 10.1002/adfm.201604307) and is briefly introduced in the current Back Cover.

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