With the economic globalization and rapid development of science and technology, human demand for energy is increasing. Especially in recent years, the vigorous development of electric vehicles and mobile electronic devices, high energy density energy storage materials have become the focus of scientific research. Although the traditional intercalated lithium-ion battery with graphite as the anode material occupies an important position in the electronic equipment product market, its energy density is already close to its upper limit, and it is gradually unable to meet consumer demand. Compared with intercalated lithium-ion batteries, lithium metal batteries (such as Li-S, Li-O2 and other battery systems) that use metallic lithium directly as the negative electrode show a unique advantage in terms of energy density, and have become a recent research hot spot. However, metal lithium anodes show many practical problems that need to be resolved during use. First of all, it has extremely high electrochemical reduction performance, and it easily reacts with the electrolyte during charging and discharging, consuming a large amount of active lithium and electrolyte. Secondly, uncontrollable dendrite growth and electrode volume changes as well as gradually accumulating side reaction products and "dead lithium" are always serious problems facing metal lithium anodes. Relying on the research of the Qingdao Energy Storage Industry Technology Research Institute built by the Qingdao Institute of Bioenergy and Processes of the Chinese Academy of Sciences, researchers deeply analyzed the characteristics of lithium metal. Considering the actual situation in practical applications, the artificial interface was first constructed from the perspective of in-situ real-time formation ( Chem. Mater. 2017, 29, 4682-4689), to achieve stable lithium deposition and extraction of the negative electrode; in addition, the staff optimized the electrolyte for lithium metal batteries and designed the double salt electrolytes containing additives (Small, 2019 , 1900269), modified polyvinylene carbonate-based high-voltage polymer electrolyte (J. Mater. Chem. A, 2019, 7, 5295-5304) and rigid-flexible composite electrolyte with high lithium ion mobility coefficient (Small , 2018, 14, 1802244), the effective modification of the interface of the metal lithium anode has a good guiding significance for the development of high-energy lithium metal secondary batteries. Among them, the additive used in the experiment was a new type of perfluoro-tert-butoxy lithium trifluoroborate (LiTFPFB) with a large anion structure independently developed by Qingdao Energy Storage Institute.
With the continuous deepening of the protection of lithium metal anodes, researchers are more concerned about the failure mechanism caused by lithium dendrites and "dead lithium" in lithium metal batteries, but due to the similar morphology of the two, how to observe and distinguish the two is A very challenging subject, and this issue is extremely important for understanding the battery failure mechanism and predicting the cycle life of lithium metal batteries. To describe the distribution of active lithium species on the surface of lithium metal anodes and distinguish between lithium dendrites and "dead lithium", researchers at the Qingdao Energy Storage Institute were inspired by the fluorescent probe method in analytical chemistry and designed a 9,10-dimethyl The DMA fluorescent probe completed this task by traditional visible optics. The technology was recognized by international peers. The related achievements have written a scientific paper titled Fluorescence Probing of Active Lithium Distribution for Lithium Metal Anode (Angewandte Chemie International Edition, 2019, DOI: 10.1002 / anie.201900105).
After the battery is charged and discharged, the surface of the metal lithium anode may accumulate by-products (a large amount of by-product coating will make the active lithium inactive, that is, "dead lithium"). Therefore, the researchers uniformly coated the fluorescent small molecule DMA on the lithium metal surface after cycling. Since DMA can undergo fluorescence quenching reaction with active lithium and remain stable on the surface of by-products, it is possible to characterize the distribution of active lithium and its by-products in various electrolytes on the anode surface of lithium-ion batteries. The selection provides an important reference basis; in the process of lithium deposition and dissolution, the accumulation of by-products is visually and semi-quantitatively identified, which can link the performance degradation of the battery with the amount of by-products, and realize the prevention and control of battery performance failure Early warning; the position of lithium dendrites and "dead lithium" can be clearly identified on the surface of the lithium anode after cycling, and the cause of the failed battery can be analyzed. This technology provides an idea and direction for the analysis of the failure mechanism of lithium metal batteries.
Relevant series of studies have received support from the National Natural Science Foundation Outstanding Youth Science Fund, the New Energy Vehicle Solid-State Battery Project, the Chinese Academy of Sciences Deep Sea Pilot Project, the Shandong Provincial Key R & D Program Fund, and the Chinese Academy of Sciences Youth Promotion Association Fund.
(A) Schematic diagram of the reaction between DMA and the surface composition of lithium metal; (b) The emission of 5 mg mL-1 of DMA in TEGDME / DME (1: 1) before and after being treated by metallic lithium (the blue line is before treatment and the red line is after treatment) Spectral curve; (c) DMA probe method to observe the distribution process of active lithium on the surface of lithium metal after cycling
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