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Introduction of high-performance lithium metal battery electrolyte

by:dcfpower     2021-03-24

The theoretical specific capacity of lithium metal batteries can reach 3860mAh/g, the electric potential is only -3.04V (vs standard hydrogen electrode), and it has very excellent conductivity. It is an ideal high specific energy lithium ion battery negative electrode material. However, during the charging process, the lithium metal anode will cause the growth of lithium dendrites due to local polarization, which will lead to the reduction of Coulomb efficiency and the loss of active lithium, and even cause safety accidents, which greatly restricts the metal lithium anode. Applications.

In the design of metal lithium secondary batteries, the performance of the battery is highly dependent on the amount of electrolyte and the charge and discharge current Density, the interaction between the lithium metal negative electrode and the NCM positive electrode when the amount of electrolyte is small, will accelerate the decomposition of the electrolyte in the lithium metal negative electrode and form a high-impedance SEI film. However, too much electrolyte will not only reduce the energy density of the battery, but also increase the cost of the battery. Therefore, it is particularly critical to develop a more stable electrolyte to reduce the amount of electrolyte.

The metal lithium anode comes from FMC, and the electrolyte cycle performance of adding FEC and DFEC has been significantly improved. FEC+DFEC still maintained good cycle stability after 300 cycles. The cycle temperature is increased to 45℃, and the FEC+DFEC mixed solvent electrolyte still maintains the best cycle performance.

The rate performance of the electrolyte using FEC is better than that of DFEC and FEC+DFEC electrolyte.

The electrolyte using the FEC+DFEC hybrid system shows better cycle stability, which can be stably cycled for more than 250 times, and the battery does not show the phenomenon of FEC battery capacity decline and then recovery. This is mainly because the mixed electrolyte system of FEC+DFEC can form a more stable SEI film on the surface of the negative electrode.

When the battery adopts the FE+DFEC system electrolyte at the beginning of the cycle and after 200 cycles, the battery not only maintains good stability in capacity, but also the battery's charge and discharge voltage does not appear to decline significantly. .

In the EC-based electrolyte, the SEI film is thicker and contains more PEO-like polymer components, which indicates that the reduction polymerization of EC on the surface of lithium metal during the cycle is The main reason leading to the decline of lithium metal batteries.

Compared with EC solvent, the SEI film formed in FEC solvent contains some organic components as well as some inorganic components such as lithium carbonate, lithium fluoride, and lithium alkyl carbonate. The composite SEI film not only gives the SEI film good elasticity, can adapt to the volume change of metallic lithium, but also allows Li+ to quickly pass through the SEI film.

There are a large number of lithium dendrites on the surface of the lithium metal negative electrode using the EC-based electrolyte. The dendrites greatly increase the surface area of u200bu200bthe lithium metal negative electrode and accelerate the decomposition of the electrolyte. In contrast, the surface of the metal lithium negative electrode of the FEC and DFEC electrolytes shows sheet-like metal lithium. Compared with needle-like lithium dendrites, the electrode has a smaller specific surface area, which effectively improves the cycle performance of the electrode. .

Research shows that compared with EC-based electrolytes, FEC-based and DFEC-based electrolytes can form a composite SEI film of organic and inorganic components on the surface of the metal lithium negative electrode, thereby ensuring good mechanical stability , It also ensures the migration of Li+, thereby effectively improving the cycle stability of the metal lithium battery. At the same time, the FEC+DFEC composite electrolyte system in the metal lithium battery pack, DFEC will first decompose to form a stable SEI film, reducing the decomposition of FEC, and the remaining FEC can continuously repair the damage of the SEI film during the cycle, thereby further improving the battery cycle stability.

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