In response to the imminent problems of energy storage applications, the development of high-energy density battery systems has become an important topic of concern to the scientific and industrial communities in the past 20 years. Lithium metal is the "holy grail" material for the negative electrode of lithium batteries. It has an ultra-high specific capacity (3860 mAh g-1) and the lowest redox potential (-3.040V vs. standard hydrogen electrode). In the future, a high energy density energy storage system (All-solid-state lithium batteries, lithium-sulfur, and lithium-oxygen batteries) play an important role. At present, liquid lithium secondary batteries with lithium metal as the negative electrode and ternary high-nickel material as the positive electrode are one of the best candidate materials for achieving the short-term energy storage goal of 500 Wh kg-1. However, lithium metal does not match the most widely used carbonate-based electrolytes in thermodynamics and has poor kinetic performance. It is easy to form a physicochemically unstable interface film (Solid Electrolyte Interphase, SEI film) on the surface of the lithium metal negative electrode Dendrite growth and interfacial side reactions, especially failure under high-rate cycling conditions, are more severe. Therefore, the development of lithium metal anode protection technology and the search for an electrolyte with excellent power performance and stability with lithium metal anodes are the key issues in the current industry development. In response to the above problems, the new energy storage materials and devices team of the Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences has conducted a large number of interface protection structure designs for a long time, and has made some progress in the early stage (Nano Energy 2017, 39, 662-672; Adv . Funct. Mater. 2018, 28, 1805638; Nano Energy 2019, 59, 110-119; Adv. Energy Mater. 2019, 9, 1802912). On this basis, the team carried out more in-depth basic and applied research based on the interface cycling mechanism of lithium metal anodes, and has made a series of progress in the near future.
In terms of negative electrode protection technology, the research team collaborated with Professor Zhang Jiguang and Xu Wu of the Pacific Northwest National Laboratory in the United States to prepare a silver-fluoride on the surface of lithium metal based on a simple and effective ion replacement reaction Lithium artificial interface (Figure 1a). Lithium ions have a high adsorption energy on the surface of silver particles, which can effectively reduce the mass transfer energy barrier of lithium ions during the reduction process. The resulting lithium metal nucleation overpotential is 2.2 mV, which is only on the surface of conventional lithium anodes. Depositing 3% of the overpotential can realize the orderly nucleation of lithium metal during the deposition process and avoid local dendrite growth. Thanks to the protection of the artificial interface layer cross-linked by silver particles and lithium fluoride, in the case of using a conventional carbonate electrolyte, the 1.8 mAh cm-2 ternary material (LiNi1 / 3Mn1 / 3Co1 / 3O2) is used as the positive electrode The lithium metal secondary battery achieved a stable cycle of more than 500 weeks, and the capacity retention rate at the 500th week was higher than 80%. It is worth noting that this type of protection method is also applicable to the protection of negative electrodes of other alkali metals, and the effective protection of sodium metal negative electrodes is also confirmed in this work (Adv. Energy Mater. 2019, 9, 1901764). The team also collaborated with the Institut Europeen des Membranes of France to obtain an Al2O3-SiO2 core-shell structure through the bulk eutectic transformation in the silicon-aluminum alloy during the heating process (Figure 1b), and based on The special interface and bulk composition of this structure quickly form an aluminum-based host structure with a large number of LixAl sites during the lithiation process for efficient storage of lithium metal, and it is compatible with excessive metal oxides with a load of up to 4.5 mAh cm-2 The positive electrode constitutes a stable lithium metal full battery (Nano Energy 2020, 73, 104746). In addition, the team also cooperated with Jiangxi University of Science and Technology professor Ouyang Chuying's team and found that it is easy to deposit a layer of metallic lithium on the surface of graphene, and it has an electronic disturbance effect on the subsequent deposition reaction, which significantly increases the polarization of lithium metal deposition, forcing lithium ions Defects are deposited on the bottom layer of graphene and an artificial SEI film is formed in situ (Figure 1c). Based on this theoretical understanding, the developed three-layer graphene-three-layer lithium storage structure can not only significantly improve the cycle stability of lithium metal under liquid electrolyte, but also improve the interface stability of sulfide solid electrolyte and lithium metal (Adv. Sci. 2020 , 2000237).
In terms of electrolyte, the oxidation window of nitrile organic solvents can reach ~ 5V, which can cover the working voltage window of existing mainstream battery materials, and has high dielectric constant, low viscosity, good dissociation effect, and can form excellent dynamic performance. The electrolyte system has been widely used in supercapacitors. However, nitrile solvents are extremely corrosive to lithium metal and have been unable to be applied to lithium metal batteries. Recently, the team worked with Zhang Jiguang and Xu Wu of the Pacific Northwest National Laboratory in the United States to develop the first nitrile electrolyte with high salt concentration for high specific energy lithium metal batteries (Figure 1d). This kind of electrolyte with high salt concentration not only has the high oxidation stability of nitrile solvents, but also has a coulombic efficiency of more than 99.2% for lithium metal, and has excellent high current performance, capable of 4 mA cm-2 current Density lithium metal deposition at a density. The use of nitrile high-salt electrolytes can achieve a stable cycle of high-load (2 mAh cm-2> above) 4.5V lithium metal batteries (Adv. Funct. Mater. 2020, 2001285).
The above work was supported by the Ningbo 2025 project (2018B10061) and the National Key Research and Development Plan (2018YFB0905400).
Figure 1 Lithium metal anode protection based on (a) silver-lithium fluoride artificial interface, (b) LixAl-rich aluminum-based host structure and (c) few-layer graphene-three-dimensional lithium storage structure; (d) stable to lithium New type nitrile electrolyte
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