Electrolyte Design And Interface Regulation Of High-Safety Lithium-Ion Batteries Using Microsized-Si Anodes

The rapid development of new energy vehicles and consumer electronics puts forward higher requirements for the energy density,safety,and operating temperature range of lithium ion batteries.Silicon（Si）is considered one of the most promising anode materials for next-generation high-energy-density lithium-ion batteries due to their high theoretical specific capacity,low operating voltage,and abundant resources.However,the huge volume change of Si negative electrodes during the lithiation/delithiation processes severely deteriorates the electrode structure,accompanied by continuous rupture/regeneration of the solid electrolyte interface（SEI）and electrolyte consumption,seriously limiting the battery cyclability and safety.Compared to the structural manipulation of Si-based anode materials,electrolyte design is considered a more economical and effective strategy to solve the problems of electrode structure instability,interface instability,and safety risks.In this dissertation,an electrolyte design（e.g.,solvent,lithium salt,additive design）strategy is proposed to regulate the interfacial composition and structure and lithium-ion（Li⁺）transport behavior of commercial micron-sized Si.The correlation of the electrolyte microstructure,electrode interfacial behavior,and battery performance was systematically investigated,which paves the way for developing electrolytes with high safety and wide-temperature-range operability for lithium-ion batteries using micron-sized Si anodes.（1）Interface regulation and thermal safety of microsized Si anodes using ionic liquid electrolytes.Taking full advantages of the ionic liquid 1-butyl-1-methylpyrrolidinium bis（fluorosulfonyl）imide(Pyr₁₄FSI)in terms of thermal stability and conductivity,an ionic liquid-based electrolyte 2 m Li FSI/Pyr₁₄FSI that can enable stable cycling of micron-sized Si anodes in high-temperature environments was obtained by optimizing the concentration of lithium salt bis（fluorosulfonyl）imide（Li FSI）.This ionic liquid electrolyte is abundant in FSI^--rich solvated clusters,which favors the formation of a uniform and inorganic SEI on the surface of Si anodes,and improve the stability of interfacial chemical reactions in high-temperature environments.The Si anode with this ionic liquid electrolyte shows a high capacity retention of 95.0%after 200 cycles at 2 A g^-1.Excellent cycling stability can still be maintained at a high temperature of 80°C.The effects of electrolyte and temperature on the composition and physicochemical properties（mechanical properties,ionic conductivity,thermal stability,chemical stability）on SEI were investigated.It is shown that the inorganic species-rich SEI has intrinsic thermal stability,excellent mechanical strength,and fast Li⁺transport ability.Associated with the stable interfacial electrochemical reactions at high temperatures,it can effectively inhibit side reactions and electrode structure degradation under high-temperature conditions,alleviate thermal runaway,and improve the high-temperature safety of micron-sized Si batteries.（2）Regulating the solvation structure of ionic liquid electrolyte and SEI for fast-charging microsized Si anodes.Sulfolane（SL）was introduced to solve the problem of poor rate performance of micron-sized Si anodes based on ionic liquid electrolytes.The Li⁺solvation structure,desolvation behavior,and SEI formation process of SL/ionic liquid electrolytes were systematically studied at a molecular level.The results show that SL in the Li⁺solvation sheath can significantly reduce the Li⁺desolvation energy barrier.The addition of 10wt%SL can not only promote the formation of stable inorganic species-rich SEI,but also accelerate the charge transfer at the interface,significantly improving the rate performance and long cycling stability of micron-sized Si anodes.The specific discharge capacity of the micron-sized Si anode can still remain～780 m Ah g^-1 after 500 cycles at a high rate of 12 A g^-1,showing capacity retention of 71.7%.This work proves that through rational solvation structure regulation,ionic liquid-based electrolytes can meet the requirement of fast charging and discharging of microsized Si anodes.（3）Enhancing interface stability of microsized Si anodes based on sulfolane-ether electrolytes.To address poor wettability of ionic-liquid electrolytes,the feasibility of high-safety electrolytes for micron-sized Si anodes using lithium salts（lithium difluorooxalate borate（Li DFOB））and molecular solvents（SL and tetraethylene glycol dimethyl ether（G4））featuring with good thermal stability was explored.The microscopic interactions within various components and their effects on the physical and chemical properties of the electrolyte was systematically explored.A new type of electrolyte composed of Li DFOB,SL,G4,and fluoroethylene carbonate（FEC）additive was successfully developed（Li DFOB/SL/G4=1/3/3（molar ratio）,with an FEC mass ratio of 10 wt%）.This electrolyte exhibits nonflammability and good wettability.The structure-activity relationship of the electrolyte microstructure and the composition and properties of the electrode interface was explored.Benefiting from the synergistic effects of G4 and SL solvents,the dissociation of Li DFOB is promoted,further assisted with the regulation ability of FEC to the solvation structure.DFOB^-is apt to be reduced on the surface of the Si anode and form a uniform and dense SEI,thereby improving the cycling stability of micron-sized Si anodes.In addition,the electrolyte can also facilitate the formation of a stable cathode electrolyte interface on the surface of the nickel-rich cathode of Li Ni_0.8Co_0.1Mn_0.1O₂（NCM811）.The specific discharge capacity of the micron-sized Si|NCM811 full cell with this electrolyte delivers a high specific discharge capacity of 176.0 m Ah g^-1 at 0.5 C（based on the NCM811 cathode）,demonstrating the feasibility of achieving the practical application of the micron Si negative electrode and its full battery through the collaborative design of the lithium salt,solvent,and additive.