With the vigorous development of electric vehicles and mobile electronic devices, the energy density of lithium-ion batteries that provide power energy has put forward higher requirements. The energy density of the lithium-ion battery based on the plug-in principle is close to its upper limit energy density, and there is little room for improvement. In comparison, lithium metal batteries with lithium as the negative electrode have unparalleled advantages in improving energy density. However, lithium metal batteries based on traditional liquid electrolytes have many challenges such as repeated cracking and generation of SEI, volume expansion of lithium anodes, growth of lithium dendrites, dead crystals, etc., low Coulomb efficiency, increased battery impedance, and safety issues. All of these Limited the rapid development of high-performance lithium metal batteries.
Recently, relying on the Qingdao Energy Storage Industry Technology Research Institute (hereinafter referred to as "Qingdao Energy Storage Institute") built by the Qingdao Institute of Bioenergy and Processes of the Chinese Academy of Sciences, in order to solve the above-mentioned key bottleneck problems of lithium metal batteries, polymer electrolytes are the core breakthrough Point, systematically explore solutions from three aspects: (1) the protection of the lithium anode by the large anion lithium salt; (2) the construction of an artificial organic / inorganic composite interface film (SEI); (3) consideration of both high voltage resistance and lithium anode protection The development of multi-functional polymer electrolytes and a series of innovative research work have played a major role in promoting and leading the development of high-performance lithium metal batteries.
In order to solve the lithium dendrite problem from the perspective of lithium salts, researchers at the Qingdao Energy Storage Institute designed and synthesized a new type of perfluoro-tert-butoxy lithium trifluoroborate (LiTFPFB) with a large anion structure. The new lithium salt retains On the one hand, the main structure of LiBF4 anion can improve its stability to aluminum current collectors; on the other hand, the presence of large anions can form a lithium anode protective film in situ to improve the electrochemical performance of lithium metal batteries. The study found that the ionic conductivity of the lithium salt is significantly higher than that of LiBF4, and has better stability for the aluminum current collector. A protective film can be formed on the lithium metal anode to effectively inhibit the further reaction of the lithium metal with the electrolyte, and then Effectively protect the lithium anode. Related achievements were published as back cover articles in the international magazine Chem. Sci. (Chem. Sci., 2018, 9, 3451-3458).
In view of the unfavorable factors such as unstable SEI, serious side reactions, low Coulomb efficiency, and short cycle life when using lithium metal as a negative electrode, the researchers designed an artificial SEI film from the perspective of polymer interface modification to effectively protect the lithium metal negative electrode (Figure 1a), the SEI film is composed of polycyanoacrylate and lithium oxide derivatives dispersed therein (Chem. Mater., 2017, 29, 4682-4689). The study found that the synergistic effect of the organic / inorganic layer makes the interface conduct lithium ions at the same time, and ensures that SEI is not easy to fall off, and also significantly inhibits the side reactions occurring in the interface area (Chem. Mater., 2016, 28 , 3578-3606), which gives metal lithium batteries excellent interface stability and long cycle stability (Chem. Mater., 2018, 30, 4039-4047).
In order to solve the instability of the positive electrode solid electrolyte interface (CEI) in high-voltage (4.45 V) lithium cobaltate / lithium metal batteries, electrochemical oxidation side reactions are likely to occur at the interface at high potentials, and lithium negative electrodes exist at high current and large capacity Lithium dendrites and other issues, Qingdao Energy Storage Institute under the guidance of the "rigid and flexible" polymer electrolyte design concept (Small, 2018. DOI: 10.1002 / smll.201800821; Adv. Sci., 2018, 5, 700503) , Using bacterial cellulose as a rigid framework support material, prepared polyvinyl methyl ether-maleic anhydride multifunctional polymer electrolyte (Energ. Environ. Sci., 2018, 11: 1197-1203). The experimental results show that the polymer electrolyte can have both the multifunctional functions of stabilizing the positive electrode interface and protecting the lithium negative electrode, thereby synergistically improving the long cycle stability of the 4.45 V lithium cobalt oxide / lithium metal battery. At the same time, the article clarified the multifunctional mechanism of action of the polyvinyl methyl ether-maleic anhydride polymer electrolyte (Figure 1b). Due to a series of work on high-voltage lithium cobalt oxide / lithium metal batteries, researchers were invited to write a review titled Reviving lithium cobalt oxide-based lithium secondary batteries-toward higher energy density for Chem. Soc. Rev (Chem. Soc. Rev ., 2018, DOI: 10.1039 / C8CS00322J), which discussed in detail the research progress, challenges and future opportunities and development directions of high energy density lithium cobalt oxide batteries.
Related series of researches have won the National Natural Science Foundation Outstanding Youth Science Fund, the National Key R & D Program, the Nano Pilot Project of the Chinese Academy of Sciences, the Shandong Natural Science Fund, the Qingdao Municipal Energy Research Industry Think Tank Joint Fund, and the Qingdao Energy Institute "One Three Five" project And so on.
Figure 1 (a) Artificially constructed organic / inorganic composite electrolyte membrane (SEI) modified high-performance lithium metal anode; (b) Polyvinyl methyl ether-maleic anhydride multifunctional polymer electrolyte
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