3 lithium negative modificationPeled in 1979, the concept of SEI film was first proposed. The multi-phase chemical structure of the SEI membrane had a direct impact on electroplating, peeling behavior and cycle life of metal lithium batteries. It is an important aspect of solving the challenge of metal lithium secondary battery.
According to the current research, the modified idea of the Lithium negative electrode by modifying the SEI film is mainly: (1) The main component of the SEI film is made of decomposing reaction product of negative electrode lithium and electrolyte, and modified the SEI film by improving the electrolyte. Yes * Direct and efficient method; (2) artificial non-in-situ forms a layer of SEI film in lithium negative surface, more flexible, and the plan is more simple and economical.
3.1 Modified SEI film by electrolyte in situ
Typically, the lithium battery electrolyte is mainly composed of an organic solvent, a lithium salt, and an additive, and almost all of the electrolyte components have an important effect on the formation of the SEI film, and the stable SEI film is formed directly by improving the component of the electrolyte. Direct efficiency.
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3.1.1 Liquid electrolyte
Usually used carbonate electrolytes do not inhibit the growth of lithium dendrites, and the couluminous efficiency of the battery is also low. Usually used ether electrolytes have better inhibitory effects of lithium derinating crystals, but their ionically conductivity is poor, and there is a poor antioxidant capacity, which seriously limits its application in high voltage positive electrodes. Therefore, exploring the new electrolyte system and looking for new organic solvents and lithium salts is the main task in the research work of electrolyte.
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The researchers found that "high concentration lithium salt" pair forms a stable SEI film, thereby improving the lithium negative electrode has a significant effect. Hu et al. Proposed "solvent dissolved in lithium salt" research ideas, organic solvents are 1,3-dioxolane (Dol) + ethylene glycol dimethyl ether (DME) (volume ratio 1: 1), add high concentration lithium When salt litfsi, the surface of the lithium negative electrode formed a stable SEI film, and the solution had a high lithium ion migration (0.73), which effectively inhibited the growth and shape of lithium dendrites, and coulomb efficiency reached 100%.
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The double lithium salt also has excellent effects on SEI membrane modification, and Xiang et al. Takes Litfsi and LiBob as a double lithium salt, Ec + EMC (mass ratio 4: 6) is an organic solvent. When the LI-LINI0.8CO0.15Al0.05O2 battery is high, the SEI film formed in the electrolyte of LiPF6 is thicker, the impedance is large, and the capacity is faster. In the electrolytic solution of lithium and LiBobobis, the lithium negative electrode surface forms a layer of sulfur-rich high-conductivity SEI film, and the battery is better during large current cycles.
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Previous studies have proven that the organic solvent plays a main impact on the SEI membrane component, looking for a new organic solvent, the concentration of the solvent and the lithium salt is also critical to the negative electrode of the modified lithium. MIAO et al. Proposes a novel ether-based electrolyte to indicate the superior performance of the mixed electrolyte fluid. 1,4-dioxane (DX) and DME do organic solvent, lifsi make lithium salt. DX has better antioxidant properties, and DME also has high oxidative stability, so such mixed electrolytes have a wide electrochemical stable window (≈4.87 V). DX also has a lower reduction potential in a variety of ether-based solvents, so it is low in reactive activity with lithium negative electrodes, 200 times with low current density, and coulomb efficiency can be maintained at 98%, and there is no dendritic crystal.
3.1.2 Electrolyte Additives
The electrolytic solution additive has a reduction potential than the solvent and salt, and can be quickly reacted with a lithium anode, and form a SEI film that is more stably, densely stabilized than the organic solvent and lithium salt. The additive has two major categories of inorganic additives and organic additives. The previously studied inorganic additives mainly include acid gas such as CO2, SO2, N2, HF, Mg2 +, Zn2 +, I-, Ga3 +, Bi3 +, SN4 + and other iodine and SNI3, Ali3 and other iodine. Chemicals. Organic additives mainly have fluorocarbonate (FEC), vinyl carbonate (Vc), alkylene glycol ester (ES), tetrahydrofuran, and derivatives, naphthoalkane, benzene, gyridine compound, polyethylene oxide (PEO), PEO of dimethyl ether, dimethylsilane and the copolymer of propylene oxide.
Today's researchers have invested a certain amount of work with fluorine-containing compounds, and Kanamura et al. Proposed HF can improve the surface morphology of lithium deposition and inhibit the growth of lithium dendrites. A small amount of HF and H2O can cause the SEI membrane to generate a dense LIF / Li2O thin layer in the carbonate solvent, which causes lithium to be smooth hemispherical deposition, and the current density is uniformly distributed, inhibiting the growth of lithium dendrites. * Near zhang et al. To the additive FEC, the FEC additive and negative electrode lithium reactive activity can achieve the formation of LIF-rich SEI film, which can effectively inhibit lithium dendrites. Growth.
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The organic additive is easily reduced and polymerized to form a SEI film, which has a better flexibility, adhesion, strong mechanical properties, high ionic conductivity and uniform current distribution than inorganic additives. The organic additives in earlier studies include VC and FEC, INABA, etc., the additive FEC and Vc are compared in the electrolyte LiClo4 / carbonate (PC), and the SEI film formed in the additive FEC is more dense, more It has elastic, small impedance, and more uniform current distribution, effectively inhibiting the growth of lithium dendrites.
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3.1.3 Ion Liquid
The room temperature ionic liquid has a wide electrochemical stable window, non-flammable, low vapor pressure, good electrical conductivity, good thermal stability, non-volatility, etc., is expected to replace conventional carbonates and ether electrolytes. Current studies have more ionic liquids, mainly imidazole, pyrroles, pyridine, quaternary amiodic ions and hexafluorophosphate, fluoroboric acid, sulfonic acid, and derivative anion thereof.
Recently, Li et al, the effect of PY13TFSI and ether-based electrolyte as a mixed ionic liquid electrolyte on the stability of lithium metal anode electroplating and peeling, the reversibility of lithium deposition and disengagement can be synergistic between PY13TFSI ionic liquid and lithium salt concentration. The effect is significantly increased, and the mixed electrolyte increases the stability of the SEI layer through the in-situ passivation process, effectively suppresses the growth of lithium dendrites and the corrosion of the lithium battery negative electrode during the lithium battery cycle.
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