【作者】 王迪；

【导师】 何平； 唐月锋；

【作者基本信息】 南京大学 ， 材料科学与工程， 2020， 博士

【摘要】 锂空气电池作为近年来备受关注的新型电池体系,具有超高的能量密度,是一种极具发展前景的电化学储能装置。针对这一储能体系,研究人先后在反应机理的探究、电极材料的优化、电池结构的设计等方面开展了大量研究并取得了显著成果。尽管如此,目前锂空气电池仍面临以下几方面的挑战:首先,为了使空气中的氧气、二氧化碳或氮气能够进入电池并在电池正极参与电极的放电反应,锂空气电池需为半开放式结构。虽然这一结构可以实现充放电过程中反应气体的参与,却也给电解液的稳定性带来了极大挑战。氧气在电解液中的溶解与迁移极易导致电解液溶剂的分解,进而影响锂空气电池的能量转换效率、循环寿命等电化学性能。其次,从锂空气电池的反应机理可知,放电产物过氧化锂及碳酸锂的形成与分解涉及固-液-气三相,因此电池充放电过程中的反应速率,以及各反应组分在电解液中的迁移速率、电极的催化活性、放电产物的分解路径等都将对电池的充放电过电位产生重要影响。最后,锂金属负极作为锂空气电池中的重要组件,近年来研究人员将越来越多的注意力转移到了影响其循环稳定性的副反应方面,如锂金属与扩散至其表面的氧气或二氧化碳的反应、锂金属与电解液溶剂或添加剂之间的反应、负极表面锂离子的传导速率及锂沉积/溶解过程中锂表面的形貌等。上述科学问题都是决定锂空气电池最终电化学性能的重要因素。为了解决锂空气电池面临的这一系列科学问题,研究人员曾尝试从锂金属负极表面的修饰和催化剂材料的设计等方面对电池的电化学性能进行优化,但仍缺少更有效的方法来实现锂空气电池整体电化学性能的提升。由于电解液既是构成锂空气电池的重要组件,也是锂空气电池固-液-气三相组分中的重要介质,因此,从电解液入手,不仅能够更有效地调控固-液界面和气-液界面中与反应气体及放电产物相关的电极反应动力学速率,还能够对金属锂负极表面进行优化;与此同时,选择和设计具有不同物理性质（相对介电常数、饱和蒸汽压、电导率、气体及放电产物的溶解度等）的电解液溶剂、液相催化剂或添加剂也可以更加有效的缓解锂空气电池面临的上述挑战。本论文针对前锂空气电池面临的电解液易分解、动力学反应速率慢、锂金属负极稳定性差等科学问题,从电解液入手,为这一系列科学问题的解决提出了多种方案。首先,针对性的设计并寻找了一种具有多功能的电解液添加剂,既能够改善正极的倍率性能,又能够增强锂金属负极的稳定性;同时,考虑到固-液-气三相体系中,反应气体在电解液中的溶解至关重要,本工作中详细考察了气体组分及分压对锂空气电池反应路径和电化学性能的影响;此外,为了克服传统有机电解液稳定性差、安全性差以及在不同温度下应用的局限性,本工作中又分别开发并研究了两种具有不同温度区间的熔融盐电解液,并考察了锂空气电池在熔融盐电解液体系中电化学性能的变化,取得了以下成果:（1）电解液添加剂2,2,2-氯甲酸三氯乙酯（TCCF）在锂空气电池正极倍率性能及负极稳定性方面的研究:首先,本工作中根据锂空气电池中已报道的各类添加剂的分子结构和主要功能对添加剂分子的作用进行了总结;同时针对目前锂空气电池中存在的主要科学问题,确定了具有相应结构的TCCF为本论文中的电解液添加剂。通过这一针对性的选择与设计,以及对TCCF的分子结构和电化学性能所进行的实验研究,发现TCCF可以有效降低锂空气电池的充电过电位;同时,借助相关的电化学测试和计算,发现TCCF的引入成功提升了O₂在该电解液中的溶解度及迁移速率,为电池倍率性能的提升创造了动力学条件。随后,研究还发现通过在金属锂表面构筑一层稳定的SEI保护层,TCCF可显著改善锂金属负极的循环稳定性。为此,本工作中对TCCF在电解液中的作用机理进行了探究,发现了其与金属锂之间自发的化学反应以及随着金属锂表面组分的变化该自发反应的自发性终止,探明了该反应过程所引起的电解液组分的变化及锂空气电池电化学性能的提升。该研究结果不仅提出了对电解液添加剂进行针对性功能设计的思路,还将锂空气电池固-液-气三相间（金属锂负极-电解液-正极活性气体）的电化学性能联系起来,揭示了电解液组分在锂负极稳定性和放电产物形成与分解过程中的重要作用。（2）气体组分及分压对锂空气电池放电反应路径的影响:在锂空气电池的固-液-气三相体系中,反应气体在电解液中的溶解对电池的电化学性能同样具有至关重要的作用,本工作中分别以纯CO₂气体、O₂/CO₂混合气体（1:3）、Ar/CO₂混合气体（3:1）为反应气体比较了不同气体组分下锂空气电池的电化学行为,发现了电解液中痕量O2的存在对CO₂气体在锂空气电池中放电反应路径的影响。在此基础上,本工作中又考察了不同CO₂气体分压对锂空气电池放电过程的影响,研究了电池在传统有机电解液体系中的电化学反应过程,借助多种在线表征技术和原位测试方法对不同条件下所对应的放电产物进行了分析对比,发现了锂空气电池在CO₂低分压反应条件下,其放电过程中Li₂O的存在。通过对该过程中可能存在的电化学反应路径进行假设,以及理论热力学电位与实际电位之间的比较、实验过程中放电产物的检测,最终给出了特殊条件下以CO₂为反应气体的锂空气电池的电化学反应路径。（3）高效熔融盐电解液在锂空气电池中的电化学行为研究:针对以CO₂为反应气体的锂空气电池所存在的充电过电位大等问题,提出了由无机盐硝酸锂和硝酸钾组成的电解液材料,研究表明,该熔融盐电解液在以CO₂为反应气体的锂空气电池中具有较高的离子电导率并能够有效降低放电产物Li₂CO₃的分解电位,在该锂空气电池的运行温度（140℃）下,电池的充电过电位可降低到1.0 V左右。同时,借助XRD、XPS、Raman、GC-MS等表征技术对该电池的充放电产物进行了分析,确定了该锂空气电池的电化学反应过程,并发现了纯碳正极在该电解液体系中的局限性。为了进一步优化该熔融盐锂空气电池的电化学性能,本工作中以单质钌催化剂对碳电极进行了修饰。通过熔融盐电解液和贵金属催化剂的联合作用,成功地将电池的充电电位降低到了3.2V左右,进而提升了电池的循环稳定性。（4）低温熔融盐电解液在锂空气电池中的应用及电化学行为研究:提出了适用于55-110℃温度下的锂空气电池熔融盐电解液材料—双氟磺酰亚氨锂/双氟磺酰亚氨钾混合物。通过各组分比例的调控,以CO₂为正极反应气体、以该熔融盐为电解液的锂空气电池可以在60℃条件下实现长达50周的电化学循环,为电池在中高温环境下的应用创造了条件。通过电化学测试,发现同样以Super P为电池的正极,锂空气电池在该熔融盐电解液中的充电电位可降至3.84 V,明显低于其在传统有机电解液中的充电电位。通过对放电产物的初步分析,最终认为,该以CO₂为反应气体的锂空气电池过电位的降低及倍率性能的提升来自于Li₂CO₃在该熔融盐电解液中溶解度的提升和该熔融盐电解液的高离子电导率。综上,本论文从锂空气电池电解液的设计入手,通过电解液组分的优化、分析及新材料的开发,对目前锂空气电池面临的主要科学问题提出了多种解决方案。为电池倍率性能、能量转换效率、循环寿命、负极稳定性的提升提供了全新的解决思路和方向,对实现锂空气电池的实际应用具有十分重要的意义。更多还原

【Abstract】 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.更多还原