The electrolyte is an important part of the lithium ion battery, and it plays the role of conducting ions between the positive electrode and the negative electrode. However, the traditional carbonate electrolyte has high flammability, and the combustion of the electrolyte is important in thermal runaway. The source of heat production, according to NASA engineers' test 18650 battery, in the thermal runaway, if the electrolyte decomposition heat is not included, the material decomposition will release 29-49kJ energy in the whole thermal runaway, but once the electrolyte is burned out Energy calculation, the energy released by the decomposition reaction in the thermal runaway of lithium-ion battery can reach 119-175kJ (see link: "NASA Space Lithium-Ion Battery Thermal Runaway Analysis"), showing the safety of electrolyte for lithium-ion battery Important influences. In order to solve the problem of solving the flammability of carbonate electrolytes, ionic liquids, fluorinated solvents, etc. have been developed, but these electrolytes have not been widely used because of cost, electrical conductivity, etc., Ziqi of Wuhan University. Zeng et al. developed a high concentration (Li: solvent molecule = 1:2) phosphate electrolyte (see link: "Wuhan University to develop high-safety non-combustible electrolyte"), Portion of the solvent molecules and solvated Li + form the housing, while maintaining the characteristics of the electrolytic solution incombustible, greatly improves the coulombic efficiency and cycle stability.
Although the electrolyte developed by Wuhan University solves the problem of flammability, its solvent requires the use of a phosphate ester electrolyte and a high concentration of lithium salt LiFSI, which increases the cost of the electrolyte. Recently, Hieu Quang Pham of Chungnam National University, Korea A non-combustible electrolyte has been developed on the basis of a conventional carbonate electrolyte. The method is to add fluorocarbonate DFDEC to a conventional electrolyte (1M LiPF6, solvent is PC). When burning, the F ions in the electrolyte will combine with the H ions to achieve the purpose of suppressing combustion.
Generally speaking, when PC is used as a solvent, there is a problem of co-insertion of solvent molecules. However, if a stable SEI film can be formed, the problem of co-intercalation of PC can be well suppressed, so Hieu Quang Pham adds 1% to the electrolyte. FEC additive to help form a better SEI film on the surface of the negative electrode, inhibiting the co-embedding problem of PC.
It can be seen from the following figure c that the electrolyte can be easily ignited only with the PC solvent, but we added different proportions of DFDEC to the above electrolyte (PC: DFDEC=1:9, 2:8, After 3:7 and 4:6), the electrolyte will not burn.
The electrochemical stability of the electrolyte is also a concern for us. The HOMO energy of DFDEC is -13.11eV, lower than EC (-12.86eV) and EMC (-12.71eV). Therefore, the oxidation resistance of DFDEC solvent on the positive electrode surface is better than that of EC. The traditional organic solvent such as EMC, linear polarization scanning also confirmed this point. The electrolyte with PC, DFDEC solvent showed the first weak oxidation peak at around 4.32V, and no large oxidation occurred until 5.7V. Peak, electrochemical stability is much better than traditional carbonate electrolyte.
Figure c below shows the cycle performance curves of different ratios of PC/DFDEC mixed solvent electrolyte between 2.0 and 5.0V. It can be seen that PC: DFDEC=1:9 has poor cycle performance and capacity retention after 50 cycles. The rate is only 49% (positive material is Li1.13Mn0.463Ni0.203Co0.203O2; LMNC, negative electrode is metal Li, button battery), and the electrolyte ratio of 3:7 is better, the capacity can reach 280mAh/ g, the capacity retention rate can reach 93% after 50 cycles, and the first Coulomb efficiency reaches 79%.
In order to verify the performance of the above electrolyte in the whole battery, Hieu Quang Pham uses LMNC as the positive electrode and graphite as the negative electrode to prepare the full battery, and uses the 3:7 ratio electrolyte which is better in the button battery, as shown in the figure below. According to the test results, the total efficiency of the whole battery using the electrolyte is increased to 72%, and the capacity retention rate of the cycle is about 66% (2.5-4.85V), which is very large compared to the traditional carbonate electrolyte. Lifting, but still decaying faster, mainly because graphite cannot form a good SEI film in PC solvent, so PC co-embedding problems occur, leading to delamination and spalling of graphite. To solve this problem Hieu QuangPham Adding 1wt% FEC to the above electrolyte helps the surface of the negative electrode to form a more stable SEI film. It can be seen from the following figure that the first cell efficiency of the whole battery is increased to 73% after the addition of FEC, and the capacity retention rate of the cycle is 100 times. Increased to 80%.
In order to analyze the factors that DFDEC promotes the cycle performance of lithium-ion batteries under high voltage, Hieu Quang Pham performed XPS element valence analysis on the surface of LMNC before and after the cycle (as shown in the figure below). From the following figure A, it can be seen in traditional carbonic acid. The surface of LNMC circulating in the ester electrolyte contains about 31% of Mn2+ ions. This is caused by the disproportionation reaction of Mn4+ on the surface of LNMC particles after reduction to Mn3+ to form Mn4+ and Mn2+. With the decrease of the valence state of Mn element, In order to maintain charge balance, the LMNC material also loses part of O, resulting in a material transition from a layered structure to a spinel structure. However, when PC: DFDEC=3:7 electrolyte is used, we can only observe the surface of LNMC. To 26% Mn3+, the addition of 1wt% FEC will further reduce the proportion of Mn3+ to 18%, indicating that the new DFDEC and PC mixed solvent electrolyte improves the interface stability of LMNC materials at high voltage. .
As can be seen from Figure B-2 below, the surface of the LMNC material after circulation in the conventional electrolyte forms an uneven surface layer, which mainly contains OP-F3−y(OR)y, containing PF-compound, Esters and carboxylates, etc. At the same time, under the transmission electron microscope, we also observed the region showing the spinel structure near the surface. Mn, Ni and other elements were also detected on the surface of the negative electrode, indicating that LMNC is in the traditional electrolyte. The stability is poor at high voltage. However, in the mixed electrolyte of PC and DFDEC (added to FEC), a thin (9nm), uniform and smooth surface film is formed on the surface of LMNC material, and the layered structure of LMNC material It also has a good retention. This shows that compared to the traditional, the new electrolyte can better stabilize the structure of LMNC under high voltage, reduce structural decay and dissolution of transition metal elements, and improve cycle performance.
Generally, flame retardant additives have a negative impact on the performance of lithium ion batteries, so they are rarely used in practice. Hieu Quang Pham makes carbonate electrolytes by adding DFDEC solvent to a conventional carbonate (PC) electrolyte. It also has non-combustible properties, while maintaining good electrochemical performance, and by adding a small amount of FEC to help form a better SEI film, inhibiting the problem of PC co-embedding, further improving the electrolyte The performance, and the use of DFDEC additives also improve the cycle stability of the electrolyte at high voltage, which is of great significance for the application of next-generation high-voltage materials.
