Interfacial Study And Electrolyte Design For Li,Na,Ca Metal Anodes

Compared with lithium-ion batteries,rechargeable metal batteries(Li,Na,Ca)possess higher energy density providing them with great advantages and application prospects in the field of electrochemical energy storage.However,there are still some problems for metal anodes,which seriously hinder their commercial application,mainly including the following aspects:1)The low electrochemical potentials of metal anodes cause side reactions with most electrolytes to form the irreversible solid electrolyte interphase(SEI)on the surface of the anodes.The structural instability and inhomogeneity of the SEI lead to the extremely complex electrochemical behavior of the anodes,which in turn triggers the continuous occurrence of side reactions and the growth of metal dendrites;2)The metal anodes have the problem of dendrite growth,which will not only aggravate the side reactions and the formation of inactive "dead"lithium,but also has the possibility to pierce the separator,resulting in the short circuit of the batteries;3)The huge volume change of the metal anodes will cause the pulverization of the metal anodes and the repeated breakdown and repair of the SEI,which further aggravates the occurrence of side reactions and the electrochemical polarization of the anode interface.And these above challenges are ultimately attributed to the electrochemical interfacial problems of the metal anodes.The regulation of electrolytes is one of the effective means to solve the interfacial problems of metal anodes.Since SEI is formed by the chemical and electrochemical reactions between the metal anodes and the electrolytes,modifying the composition of the electrolyte can directly improve the physical and chemical properties of the SEI and regulate the electrochemical interfacial behavior.Therefore,this thesis mainly studies the physical and chemical properties of SEI,and regulates the properties of SEI and the interfacial electrochemical behavior of Li,Na,and Ca metal anodes through electrolyte modification and additives,further realizing stable and highly reversible Li,Na,and Ca metal anodes.The specific content includes the following parts:In chapter 1,we briefly introduced the basic knowledge of metal anodes,mainly including the working principle of metal batteries and the types,advantages,and challenges of metal anodes.Combined with the basic theoretical research and the interfacial modification strategies of metal anodes,the latest research progress on metal anodes is summarized.Finally,the design ideas and the research content of this thesis are expounded.In chapter 2,we briefly introduced the basic information of the chemical reagents and instruments used in the experiments,and the assembly of the coin cells,the electrochemical tests,and the characterizations of the metal anode interface were also introduced.In chapter 3,we systematically investigated the effects of temperature on the interfacial of Li metal anodes.Increasing temperature could reduce the deposition overpotential of lithium metal anode and regulate the deposition of lithium metal in large size.At the same time,increasing temperature helps to construct an organic-rich SEI on the surface of the lithium metal anode.This suggests that the temperature regulates the nucleation size of Li metal by affecting thermodynamics on the one hand,and modifies the composition of SEI through electrochemical kinetics manipulation.As a result,the highly reversible and stable Li metal anodes are achieved at the elevated temperature.In chapter 4,LiNO3 was introduced into carbonate electrolyte to regulate the SEI of lithium metal anode.We developed N-methylpyrrolidone(NMP)as a cosolvent to greatly improve the solubility of LiNO3 in carbonate electrolyte.The added NO3-could be preferentially reduced on the surface of lithium metal anode to form nitrogen-rich SEI,which effectively inhibits the decomposition of carbonate electrolyte.Meanwhile Li3N with high ionic conductivity also contributes to improve the interfacial reaction kinetics.Finally,the coulomb efficiency and cycle life of lithium metal anode in carbonate electrolyte are improved.In chapter 5,we developed a novel fluorine-free electrolyte(NaBH4-G2),and constructed the fluorine-free SEI of sodium metal anode.This electrolyte can build a stable SEI rich in B-O organic polymer on the surface of Na metal anodes,which greatly improves the elasticity of the SEI,so that the SEI can effectively relieve the stress caused by the huge volume change of Na metal anode,preventing the SEI and the Na metal anode from cracking and pulverization.At the same time,the SEI also have high ionic conductivity and uniformity,which can effectively avoid the dendrite growth of sodium metal.As a result,a cycle life of 1000 cycles and an average coulombic efficiency of 99.91%were achieved.In chapter 6,we introduce LiBH4 salt into Ca(BH4)2-THF electrolyte to regulate the solvation structure of calcium ions,and then regulate the interface electrochemical behavior of calcium metal anode.The introduction of LiBH4 salt significantly reduces the coordination number of calcium ions in the first solvation shell,thus lowering the desolvation energy of calcium ions at the Ca metal anode interface and improving the interfacial reaction kinetics.This strategy increased the cycle life of the calcium metal anode to more than 200 times,with an average Coulombic efficiency of 97.6%,the highest level reported at the time.In chapter 7,we briefly summarized the innovative works in this thesis,and look forward to the future research works.