Metal lithium anodes have been a commonplace. We have also done a lot of reports on metal lithium anodes. The research on metal lithium anodes can basically be summarized as a sentence 'How to avoid the generation of lithium dendrites'. Advantages of lithium metal anodes Needless to say, the specific capacity is up to 3860mAh/g, and the voltage platform is -3.05V (vs standard hydrogen electrode), which can be said to be an ideal choice for negative electrode materials. But the metal lithium negative electrode also faces a very difficult problem. Lithium dendrites, which is a common phenomenon in the electro-metallurgy of metals. The growth of dendrites can cause short circuits inside the battery, resulting in serious safety accidents. Therefore, how to avoid the growth of Li dendrites during charging becomes a problem. The core issue of all metal lithium anode research.
Recently, Maohui Bai (first author) and Keyu Xie (corresponding author) of Northwestern Polytechnical University have produced a layer of reduced graphite on the surface of metal Li by directly reducing graphene oxide by using metal Li. The presence of graphene layer can be very Good inhibition of the growth of Li dendrites, while stabilizing the SEI ink to improve the coulombic efficiency, the presence of the graphene layer can also significantly improve the rate performance of the metal Li. Experiments show that the electrode can be in LiPF6 at a current density of 5 mA/cm2. The carbonate electrolyte is circulated 1000 times without short circuit.
The above figure shows the preparation process of graphene-coated metal lithium anode. First, the graphene oxide is dispersed in tetrahydrofuran (THF), and then lithium metal is placed in the above dispersion. We can observe the color of the solution gradually. The transition from brown to black indicates that the graphene oxide in the solution gradually transforms the graphene, and the reduced graphene is deposited on the surface of the metal Li to form a protective layer. By controlling the reaction of the metal Li in the graphene oxide dispersion Time, can effectively control the thickness of the graphene layer on the surface of the metal lithium negative electrode, so as to obtain the best electrochemical performance.
In order to verify the ability of the metal Li anode prepared by the above process to inhibit the growth of Li dendrites, Maohui Bai prepared two metal Li anodes into a button cell and tested them at different current densities (the results are shown below). It can be seen that the metal Li anode which has not been subjected to the graphene coating treatment has a large voltage fluctuation during the cycle, and an apparent internal short circuit occurs substantially less than 100 times, and the metal Li anode subjected to the graphene coating treatment is The voltage is very stable during the cycle, and the cycle life exceeds 1000 times without obvious short circuit phenomenon. This is the metal Li negative electrode with the longest cycle life in LiPF6-carbonate electrolyte.
From the following figures d and e, we can compare the voltage platform of the metal Li anode at different current densities, when the current density is from 1 mA/cm. 2Increase to 5mA/cm 2The metal Li anode voltage platform without graphene coating greatly increased from 54.3mV to 449.2mV, while the metal Li anode protected by graphene coating only increased from 19.2mV to 79.3mV, indicating that the graphene coating can be significant. Reduce the polarization of the metal Li negative electrode.
The reason why the graphite coating enhances the cycle performance of the metal Li anode can be obtained from the EIS analysis by comparison at 1 mA/cm. 2At the current density of 1, 100, 200 and 500 EIS test results, we were able to find that the graphene-coated metal Li anode surface SEI membrane resistance Rf and charge exchange impedance RCT were significantly lower than uncoated, and cycled The increase rate of the resistance of the metal Li anode protected by graphene coating is also slower than that of the common metal Li anode, indicating that the graphene coating can help the metal Li anode to form a more stable SEI film, and slow down the SEI film during the cycle. Growth.
The recycled metal lithium anode is taken out, and the surface of the electrode of the metal Li anode without the graphene coating is very rough (Fig. b), and a lot of Li dendrites are grown. The metal with graphene coating protection The surface of the Li negative electrode is still very smooth (Fig. c below). No obvious growth of dendrites of metal Li is observed. From the side cut of the electrode, it can be seen that the surface of the electrode of the common metal Li is very variable after the cycle. Sparse porous, thickness reached 54um after 40 cycles, thickness variation reached 170%. The thickness of SEI film on the surface of metal Li negative electrode with graphene protection was only 13um after 40 cycles, showing very good stability. .
In order to verify the practicality of the above electrodes, Maohui Bai also uses LiFePO. 4As a positive electrode, a full battery was fabricated for electrochemical testing using a metal Li or a graphene layer to protect the metal Li as a negative electrode. From the following figure a, we can see that the battery capacity of the metal Li negative electrode protected by the coating after 300 cycles is almost no. Decay, and the battery capacity of the ordinary metal Li negative electrode is reduced to 69% of the initial capacity, indicating that the graphene coating can significantly improve the cycle performance of the metal Li negative electrode. From the magnification test results in Figure b below, it can be seen that The graphene-coated metal Li anode battery has a significantly higher discharge capacity at a large rate than the ordinary metal Li anode battery, which indicates that the graphene coating is also helpful for improving the rate performance of the battery.
The graphene coating developed by Maohui Bai protects the metal Li anode from inhibiting the growth of Li dendrites, allowing the metal Li anode to circulate more than 1000 times without short circuit, and the presence of graphene coating can also improve the metal Li anode. The structural stability of the surface SEI film improves the coulombic efficiency of the battery and improves the capacity retention rate of the battery in the long-term cycle. The most important thing is that this process has the potential for large-scale application, and the graphene deposition process can be changed to the spray method. Thereby greatly improving production efficiency, making this technology extremely practical.
