Rational Design And Intensive Study Of Electrolytes For Lithium-air Batteries Application

As a new energy storage system in recent years,rechargeable Li-air battery has a high energy density and it is a promising electrochemical energy storage device in the future.In this field,researchers have made remarkable achievements in the studies of the reaction mechanisms,the design of electrode materials and the characterization of discharged and recharged products.However,they still face the following problems and challenges:Firstly,the Li-air battery is usually designed as a semi-open structure to enable the entry of reactive gases(oxygen,carbon dioxide and nitrogen)from the air.Although the diffusion of these gases is realized,the stability of electrolyte materials is greatly challenged:the entry of oxygen and carbon dioxide may cause the decomposition of electrolyte solvent,the change of electrochemical window of electrolytes as well as their physical properties,then affects the rate capability,cycling stability,round-trip efficiency of Li-air batteries.Secondly,as known from the reaction mechanism of Li-air battery,solid-liquid-gas phases are involved during the formation and decomposition of Li₂O₂ and Li₂CO₃.Thus,the kinetic reaction rate,the migration rate of every component in electrolyte,catalytic activity of the electrode and the decomposition routes of the discharged products will have important influence on the charge and discharge overpotential of Li-air battery.At last,as an important component of Li-air battery,Li metal anode and its cycling stability have attracted more and more attention from researchers.Such as reactions between Li anode and the diffused O₂/CO₂,reactions between Li anode and additives,the conduction rate of Li⁺as well as the uniformity of the surface of Li anode,they are key scientific questions for the cycling life of Li anode.In order to solve these challenges faced by Li-air battery,researchers tried to optimize the electrochemical performance by designing special catalysts and modifying the surface of Li anode.However,more efficient approaches are lacking.Electrolyte is an important medium connecting the positive and negative electrodes,it is also essential in solid-liquid-gas phases.From the perspective of the electrolyte,it can effectively control the kinetic rate of electrode reactions,the optimization of the Li metal anode can also be realized.Meanwhile,the selection and design of electrolyte solvent,catalysts,and additive with different physical properties(relative dielectric constant,saturated vapor pressure,conductivity,solubility of gas and discharged product)can also alleviate these challenges faced by Li-air battery.Considering the following key scientific questions of Li-air battery:the decomposition of electrolyte,low kinetic rate of electrode reactions and the instability of Li anode,a series of solutions are put forward in this dissertation.Firstly,we developed a versatile halide ester additive which can concurrently improve the stability of Li-anode and the rate capability of Li-air battery.Meanwhile,in allusion of the complexity of gas-liquid-solid reactions for Li-air battery,we studied the influence of partial pressures and components from gases.At last,in order to overcome the limitation from the organic electrolyte in various temeratures,as well as the poor stability and safety,we developed and investigated two molten salt electrolytes for different temeratures,and corresponding electrochemical performance of Li-air battery in these electrolytes were also investigated.The major innovations in this dissertation can be summarized as follow:(1)Investigations of 2,2,2-Trichloroethylchloroformate(TCCF)as a versatile additive in Li-air battery:In the initial stage of the research,we explored the relations between molecular structures and functions of the additives reported in Li-air battery,then,we summarized the mechanism of these additives;meanwhile,we designed a novel additive with required functional groups which can overcome the scientific challenges in current Li-air battery.With the experimentally study of the molecular structures and electrochemical properties of TCCF,we found that TCCF can effectively reduce the overpotential during charge.Based on some electrochemical measurements and calculations,we discovered that this TCCF ester can improve the diffusion rate of O₂ in the conventional electrolyte,these properties created the conditions for the improved rate capacity in Li-air battery.Then,we took a detailed analysis on the surface of Li metal electrode,resuts showed that TCCF can promote the formation of a solid-electrolyte interphase layer on the surface of the Li metal,which restrains the loss and volume change of the Li electrode during stripping and plating,thereby achieving a improved cycling stability.Herein,we further explored the reaction mechanism of TCCF in this electrolyte,then spontaneous chemical reactions between TCCF and Li metal were found,and spontaneous termination of such reactions with the change of the surface composition of Li metal were also proved.This result not only proposed a new idea with specific functional design of electrolyte additives,it also linked up the electrochemical performance of solid-liquid-gas phases.(2)The effect of gas on the discharge reactions of Li-air battery under different compositions and partial pressures:For solid-liquid-gas phases,the dissolution of the gas in electrolyte is another important factor.Herein,we prepared CO₂,O₂/CO₂(1:3)and Ar/CO₂(3:1)as reactants,then corresponding electrochemical behaviors in different gases were studied,and the influence of trace O₂in Li-air battery was discovered.On this basis,we found that the partial pressure of CO₂ in electrolytes will also impact the electrochemical behaviors of Li-air battery.The discharged products were studied by various in-situ and on-line techniques,Li₂O was observed as a product with a low partial pressure of CO₂ in Li-air battery.Then,we put forward several hypothesises for the discharge reactions in this battery.Benefit from the comparisons between theoretical thermodynamic potential and the actual discharge potential,as well as the detected discharged product,we proposed another discharge reaction of Li-air battery with lower pressure in CO₂.(3)Electrochemical behavior of high-efficiency molten salt electrolyte in Li-air battery:Based on the last chapter,in order to reduce the high charge overpotential of Li-air battery with CO₂,Li NO₃ and KNO₃ were studied as novel electrolyte in Li-air battery and corresponding physical and electrochemical performances were also investigated.Results showed that this molten salt electrolyte has high ionic conductivity and they can effectively reduce the decomposition potential of Li₂CO₃ during the charging process.Based on corresponding thermodynamic calculations,we found that the charging overpotential can be reduced to 1.0V at a high operating temperature of140?.In the meanwhile,XRD,XPS,Raman and GC-MS characterizations were employed to examine the structures and components of their discharged and charged products,then the electrochemical reaction process of this molten salt-based Li-CO₂battery was determined.However,the limitation of Super P cathode in this battery was also noticed.To further optimize the electrochemical performance,we prepared a Ru catalyst on Super P cathode.Benefit from the synergistic effect of molten salt electrolyte and Ru catalyst,the charging potential of Li-air battery was successfully reduced to 3.2V,and corresponding cycling stability was also improved.(4)Electrochemical behavior of low-temperature molten salt electrolyte in Li-air battery:On the basis of the previous chapter,we developed another low-temperature molten salt electrolyte at a wider operation temperature(55-110?).By regulating the proportion of each component in Li FSA/KFSA mixture,Li-CO₂ battery with this electrolyte can maintain a long life of 50 cycles at 60?,this result provides a probability for the application of Li-CO₂ battery in various environments.The electrochemical measurement showed that Li-CO₂ battery with this molten salt electrolyte possessed a charge potential of 3.84V,which is much lower than that in conventional organic electrolyte.In the meanwhile,the discharged products were also studied by XRD,and the reasons for the improvements of electrochemical performance were also explored.We hold the opinion that the high ionic conductivity and improved solubility of discharge products in this molten salt are the main reasons for the reduction of charge potential,as well as the improved rate capability.From the above,with the optimization,investigation and exploitation in electrolyte material,a series of solutions are put forward for current challenges faced by Li-air battery.Benefit from these studies,many notions and research directions can be noticed to further improve the poor rate capability,low energy efficiency,poor cycling stability and instability of Li anode in Li-air battery.Therefore,it represents an important significance in the development of practical Li-air batteries.