In recent years, with the rapid growth of demand for power batteries, it has also led to the rapid rise in the prices of upstream raw materials. In particular, raw materials such as Li, Co and Ni have risen sharply in 2017 prices, which have greatly compressed lithium ion batteries Profit margins, but also become a further impediment to the cost of lithium-ion battery.In order to reduce the production cost of lithium-ion batteries, including K-ion batteries, Na ion batteries and other new low-cost batteries have been rapid development in recent years, cathode materials, electrolytes Have made great progress, but due to K + and Na + ion radius is relatively large, so the available negative electrode material for embedded choice or stretched, hinder the development and application of these low-cost batteries.In order to solve the hard carbon and other traditional materials in Embedded in K, Na ions in the poor performance of the problem, Peking University Jiaxin Zheng et al on the basis of the traditional K-ion battery, through the optimization of the battery formula to achieve the battery positive and negative respectively different types of reactions: the positive is still K + Embedded and prolapse, but the negative electrode using graphite material for Li + embedding and prolapse, so that both K-ion battery Costs, but also take into account the high-performance Li-ion battery.
We know that lithium-ion batteries really participate in the electrochemical reaction has three main parts: the cathode, the negative electrode and electrolyte, lithium-ion battery charging, there are two reactions occur simultaneously: 1) positive Li + prolapse, into the electrolyte ; 2) Li + in the electrolyte is embedded in the negative electrode. It should be noted that these two reactions occur simultaneously, that is to say, Li + in the electrolyte is embedded in the negative electrode under the electric field and is simultaneously discharged from the positive electrode Of Li + will replenish the reduced Li + in the electrolyte, which provides a new idea for optimizing the structure of K-ion battery. Jiaxin Zheng proposed a solution that the positive electrode material of K-ion battery is still used, but the electrolyte needs to be used at the same time Containing K + and Li + mixed electrolyte, the negative electrode using the traditional lithium-ion battery anode material - graphite material, so that during charging K + from the positive electrode into the electrolyte into the Li + embedded in graphite (Due to K + ion radius is relatively large, can not be embedded in graphite), this design while maintaining the K-ion battery at low cost, but also ensure good battery cycle performance , The battery voltage reaches 3.6V, 5000 cycles without significant decline.
In the selection of cathode materials, Jiaxin Zheng selected K2NiFeII (CN) 6, a common cathode material of K + battery, which has different mechanism of interaction between alkali metal ions and O in the cathode materials of common Li-ion batteries. The positive electrode material of K-ion battery such as K2NiFeII (CN) 6) interacts with 12 CNs through p electrons, and it has been found through computational studies that increasing the ionic radius of the alkali metal in the case of interaction with p electrons can increase the concentration of alkali metal ions Voltage platform, which is also verified by Cyclic Voltammetry (as shown in the above figure). The voltage of K + embedded in the material (3.59V) is 0.28V higher than the voltage of Li + intercalation (3.31V) In the process of K + will first be embedded in the cathode material.In the negative because of the K + radius is relatively short, so the negative almost only embedded Li +, thus ensuring that Li + is only embedded into the negative, K + only embedded into the positive, Positive and negative will not affect each other.
As can be seen from the graph below, when K2NiFeII (CN) 6 is used as the positive electrode (LiPF6 with 1 mol / L electrolyte), K + inrush current peak near 3.59V can be seen in the cyclic voltammogram , Indicating that only K + will be embedded in the positive electrode material at this time, but after 30 cycles, a current peak also appears at the 3.3V negative electrode mainly because the K + concentration in the electrolyte is too low, resulting in partial Li + intercalation Cathode material.If the part of KPF6 in the electrolyte, can effectively prevent the Li + embedded in the cathode material (as shown in Figure c, the electrolyte is 0.5mol / L of KPF6 and 0.5mol / L LiPF6 mixed solute ).
In the negative electrode material, Jiaxin Zheng selected a common commercial lithium-ion battery cathode material - graphite, according to previous research results, due to Li + ion radius is significantly smaller than K +, Li + is more easily embedded in the graphite material, so when Graphite material as an asymmetric K / Li dual-ion battery negative electrode, to ensure that only the negative embedded Li +, and the positive only embedded in K +.
The following figure shows the electrochemical performance of the asymmetric K / Li dual-ion battery designed by Jiaxin Zheng. K2NiFeII (CN) 6 is used as the positive electrode and graphite is used as the negative electrode material (pre-lithiation has been performed to avoid SEI Li is consumed during film formation), 0.5M KPF6 and 0.5M LiPF6 as electrolyte.
As can be seen from the graph a, the voltage platform of this battery is in the vicinity of 3.6V, the corresponding K + reacts in and out of the positive electrode, and the capacity of the positive electrode remains at 71.2mAh / g after 100 cycles, and the theoretical value is 70.7mAh / g is very close. And compared to pure K-ion battery, asymmetric Li / K dual-ion battery magnification performance has been significantly improved (shown below b). The most crucial is that the battery also showed a very Good cycle performance, no significant decline in 5000 cycles at a magnification of 30C (blue curve shown in the following chart c) .We also noticed that the mass ratio of positive and negative electrodes has significant effect on the cycling performance of the battery (Mass ratio of positive: negative = 1: 2, at this time the negative electrode capacity of 8 times the positive) can significantly improve the battery cycle performance, Xiaobian lead to the main problem is the amount of negative electrode is too large The selectivity of graphite to two kinds of ions is not as good as expected. As a result, when the graphite negative electrode is too small, part of K + will be embedded during the cycling process, causing the cycling performance to deteriorate. The selectivity of the negative electrode can be improved by the means of artificial selective SEI film , Reduced amount of graphite negative electrode.
Jiaxin Zheng's work provides a new idea for K-ion battery design. If we can not find the suitable K + -embedded negative electrode material right now, then we simply use the Li-ion battery's negative electrode, the positive electrode and the negative electrode to work alone, Influence, and effectively ensure the battery performance.Although the technology is not yet mature, there are many problems to be solved, such as the cathode material capacity is low, the negative electrode capacity design requirements, but as the technology matures, these problems Can be gradually resolved, and this design ideas can help us better promote other such as sodium-ion battery design, promote the popularization and application of new batteries.
