Recently, PNNL of the Pacific Northwest National Laboratory of the United States released a heavy message on its official website. According to reports, PNNL developed a high-performance lithium metal battery electrolyte that can increase the service life of lithium metal batteries by more than seven times. PNNL said The project is part of the 'Battery500 consortium' program, which aims to develop a highly reliable, long-life, low-cost lithium metal battery with a specific energy that is more than three times that of current lithium-ion batteries, allowing the specific energy of the battery pack to reach 500Wh/kg. Above. However, many domestic media have interpreted it as 'PNNL developed an electrolyte to increase the battery life by 7 times' and never mentioned the lithium metal battery. It is obviously misleading to readers.
The theoretical specific capacity of metallic lithium electrode reaches 3860mAh/g, and the potential is only -3.04V (vs standard hydrogen electrode). It is a very ideal negative electrode material, but the metal lithium negative electrode has a fatal defect - lithium metal dendrite. To solve the problem of lithium dendrites, various solutions have been proposed. Electrolyte optimization is a common method by adding some F-containing compounds to the electrolyte, such as (C2H5)4NF(HF)4, fluoroethylene carbonate. Such as can significantly improve the stability of metal Li surface SEI film, high concentrations of Li salt has also proved to be a very effective method, such as high concentrations of LiTFSI electrolyte can significantly inhibit Li-S battery lithium dendrite growth. Although high-concentration electrolytes are beneficial to improve the performance of metallic Li anodes, they also have negative effects such as increased electrolyte viscosity, reduced ionic conductivity, and increased electrolyte costs.
Recently, Shuru Chen et al. of PNNL of the Pacific Northwest National Laboratory proposed a solution for partial dilution, that is, adding a partially electrochemically stable diluent to a high-concentration electrolyte, and the Li salt in the electrolyte will not dissolve. Among these diluents, the solvent in the high-concentration electrolyte can be dissolved with the diluent. Therefore, the 'dilution' electrolyte will form a local high-concentration region and a local low-concentration region, thus retaining a high concentration. In the case of excellent electrolyte properties, it solves the problem of high-concentration electrolytes. Under the guidance of this concept, Shuru Chen et al. designed a stable working electrolyte in the negative electrode of metal Li and the positive electrode system of 4V. The inhibition of the growth of the negative Li dendrites, the cycle life of the metal Li/NCM111 battery is increased by more than 7 times, and the practicality of the metal Li battery is greatly improved.
Shuru Chen used bis(2,2,2-trifluoroethyl)ether (BTFE) to dilute 5.5M LiFSI/DMC electrolyte to obtain local diluted electrolytes with different LiFSI concentrations. The following figure shows different electrolysis. The comparison of Coulomb efficiency of liquid Li/Cu battery shows that the Coulomb efficiency of the 1.2M LiFSI/DMC electrolyte is very low, only about 9%. If the LiFSI concentration is increased to 5.5M, the coulombic efficiency of the battery is immediately With the increase to 99.2%, it can be seen that the high concentration of LiFSI electrolyte has a significant effect on improving the performance of the metallic Li negative electrode. When a part of BTFE is added to the electrolyte, even the concentration of LiFSI is reduced to 2.5M and 1.2M. The ability to maintain high Coulombic efficiencies (99.5% and 99.2%, respectively) suggests that locally diluted electrolytes have a significant effect on inhibiting the growth of Li dendrites and increasing Coulombic efficiency.
The following figure shows the SEM images of electrodes after different electrolyte cycles (Figure a, e is the traditional LiPF6 electrolyte, Figure b, f is 1.2MLiFSI/DMC, Figure c, g is 5.5M LiFSI/DMC electrolyte, Figure d H is 1.2M LiFSI/DMC-BTFE electrolyte. From the figure we can see that in the traditional LiPF6 electrolyte and 1.2M LiFSI electrolyte, the metallic Li shows a loose, porous state, and is accompanied by Li Zhi. Crystal growth, but in the electrode of the partially diluted electrolyte 1.2M LiFSI/DMC-BTFE we can observe that mainly composed of Li particles with a diameter of about 5um, there is no growth of Li dendrites. Cross section from these electrodes we The influence of different electrolytes on the negative electrode of Li can also be seen. The thickness of the electrode in the electrolyte of 1.2M LiFSI/DMC-BTFE is significantly lower than that of the negative electrode of Li in other electrolytes (the areal density is the same). Diluting the negative electrode of metal Li in the electrolyte can form a more dense structure, thereby reducing the occurrence of side reactions and improving the Coulomb efficiency and cycle life.
In order to verify the stability of the above electrolyte under high voltage system, Shuru Chen uses metal Li as negative electrode and NMC111 material as full battery made of positive electrode (2mAh/cm2, 4.3V). The following figure shows full battery with different electrolytes. Electrochemical performance. From Fig. a, we can see that under the 1C charge-discharge rate, the battery with traditional electrolyte shows a rapid increase in polarization, and the life is rapidly declining (100 cycles, capacity retention rate is only 40% High-strength 5.5M LiFSI/DMC electrolytes are helpful for improving the coulombic efficiency of metallic Li anodes, but continuous polarization increase and capacity decline still occur in the cycle, and the final cycle 100 times capacity retention rate. It is only about 76%, which may be due to excessively high Li salt concentration leading to increased electrolyte viscosity, reduced ionic conductivity, and poor wettability. The partially diluted electrolyte shows excellent cycle performance in the cycle. (Circulation 300 times, the capacity retention rate can reach about 95%, the capacity retention rate of cycle 700 is >80%).
The study on the mechanism of action of the above-mentioned electrolytes found that the force between LiFSI and BTFE was significantly weaker than the force between LiFSI and DMC. Therefore, LiFSI prefers the solvation reaction of DMC, which is formed in the electrolyte. The local high-concentration LiFSI-DMC region guarantees the performance of metallic Li cells. In addition, the addition of a part of BTFE to a high concentration of LiFSI-DMC can increase the diffusion capacity of Li+ and reduce the diffusion capacity of FSI-, thereby increasing the electrolyte. Rate performance. Theoretical calculations of the frontier orbit also show that FSI will decompose on the negative electrode surface before DMC, resulting in a higher LiF content SEI film, thereby stabilizing the interface between the metal Li negative electrode and the electrolyte, and improving the cycling stability of the metallic Li battery. .
ShuruChen et al. from a unique point of view, through the local dilution method, retained the local high concentration of Li salt region in the low-concentration electrolyte, the advantage of this is not only to maintain a high concentration of Li salt in the inhibition of Li dendrite growth, The advantages of increasing the Coulomb efficiency of Li batteries are avoided, and the disadvantages of high viscosity, low ionic conductivity, and high cost of high-concentration electrolytes are also avoided, achieving a significant achievement of 700 cycles of Li/NMC batteries for stable development. 2. The ability to increase the cruising range of electric vehicles with lithium metal batteries is of great significance.