生物质多孔碳/硫复合锂硫电池正极材料的可控制备与改性研究

【作者】 张辉；

【导师】 姚耀春；

【作者基本信息】 昆明理工大学 ， 冶金与能源工程， 2023， 博士

【摘要】 近年来,锂硫电池因为其高的理论比容量及能量密度、硫的资源广泛且价格低廉等优点受到广泛关注,是目前锂离子电池的最佳替代品,极具发展前景。然而,锂硫电池因其固有的缺点严重制约了其商业化的进程:硫及还原产物的低电导率会导致较差的电化学性能及较慢的反应动力学;多硫化物易溶于液体电解质而产生穿梭效应,导致自放电及容量的衰减;硫在氧化还原过程中高的体积变化率会引起电极材料不可逆的结构破坏和快速的容量衰减;透过隔膜到达负极表面的多硫化物会造成锂表面的腐蚀及钝化等等。对含硫正极材料的合理设计与制备是一项值得研究的工作。本论文以制备成本低廉、合成方法简单、性能优良的碳/硫复合正极材料为目标,采用具有特殊微观结构的生物质原料为碳源制备多孔碳,根据第一性原理的计算结果选择合成方法,通过对硫晶体形态调控、碳基底掺杂改性、隔膜改性等手段提升电池的电化学性能。具体研究内容及成果如下:1、制备出适合锂硫电池使用的多孔碳基底,并合成了碳硫复合正极材料。采用具有天然管道形状的玉米须为碳源,对比不同活化剂、活化剂用量、活化温度等实验条件,通过分析多孔碳的表面结构,筛选出最优的玉米须基多孔碳制备方案为采用K₂CO₃为活化剂,活化剂用量为3:1,活化温度800℃。通过第一性原理研究多孔碳表面与硫的相互作用力得知,硫更倾向吸附在碳表面而不是自身聚集,从而选择简单高效的溶解-结晶法制备碳/硫复合正极材料。探究不同硫负载量对碳/硫复合正极材料的电化学性能影响,确定最优的硫负载量为70%,0.1C时初始放电容量可以达到980 m Ah g^-1。2、研究了硫负载形态对复合材料性能的影响。利用不同搅拌速度、合成温度、真空环境等条件调控碳/硫复合物制备过程,通过对材料物相、形貌及电化学性能等方面的对比,明确600 rpm的搅拌速度、40℃合成温度、真空环境时负载硫晶体可以获得纳米级粒径且均匀地负载在碳表面,复合正极材料能够表现出更好的电化学性能。3、通过对碳基底掺杂氮、铁以及铁氮共掺杂来提升复合材料的电化学性能。通过第一性原理计算分析出掺杂对材料性能影响的机理,结果表明氮掺杂后碳材料与Li₂S₆的相互作用力提升0.548 e V,铁掺杂可以降低Li₂S的分解能垒5 e V及改变Li⁺的扩散路径。通过对比物相、形貌以及电化学性能,确定掺入原料1/2的三聚氰胺、1/3的FeCl₃·6H₂O时碳/硫复合正极材料在0.1C电流密度下容量可分别提升187 m Ah g^-1及115 m Ah g^-1。4、对隔膜进行改性以提升电池的电化学性能。以玉米须基多孔碳为原料,通过在普通隔膜上增加涂覆层的方法建立物理屏障,阻止Li PSs的穿梭效应,0.1C时电池容量可达到1286 m Ah g^-1。通过用氧化石墨烯替代部分导电剂的方法制备多孔碳改性隔膜,除物理屏障外还增加了对Li PSs的化学吸附位点,进一步提升电池容量至1497 m Ah g^-1,硫的利用率达到90%。更多还原

【Abstract】 In recent years,lithium-sulfur batteries have received much attention because of their high theoretical specific capacity and energy density,economic and extensive sulfur resources.They are the best alternative to lithium-ion batteries and have great development prospects.However,the commercialization of lithium-sulfur batteries is severely hindered by their inherent drawbacks:the poor conductivity of sulfur and its reduction products leads to a poor electrochemical performance and slow down the reaction kinetics;polysulfides are easily soluble in liquid electrolytes,leading to shuttle effect and resulting in self discharge and capacity attenuation;the huge volume expansion of sulfur during charging process will cause irreversible structural damage of the electrode and rapid capacity decay;the polysulfide that reaches the anode surface will corrode the lithium surface.Therefore,the rational design and preparation of composite cathode is worth studying.In this thesis,we aim to prepare a porous carbon/sulfur composite cathode with lower price,facile composite method and excellent electrochemical performance.An appreciate carbon skeleton was prepared using biomass materials with special microstructure.The composite method was determined under the guidance of first principles study.The electrochemical performance of the battery was improved by doping and modification.The results of the study are as follows.1.Porous carbon skeleton suitable for lithium sulfur battery was prepared,and then composite cathode were synthesized.Porous carbon was prepared by using natural pipe shape corn silk as carbon source under the experimental conditions of different activators,different amounts of activators and different temperatures.The surface morphology of the prepared porous carbon were compared to determine the most appropriate manufacture conditions:K₂CO₃was used as the activator with a3:1 ratio at 800°C.The first principle was used to study the sulfur adsorption capacity of carbon surface.The result indicated that the S₈molecule tends to be adsorbed onto the carbon porous surface rather than agglomerate themselves.So a facile and effective dissolution-crystallization strategy of composite carbon to sulfur was selected.The influence of different sulfur content on the electrochemical performance of carbon/sulfur composite cathode was investigated to assure the 70%sulfur content.The initial discharge capacity can reach 980 m Ah g^-1at 0.1C.2.The influence of sulfur crystal morphology on the properties of composites was studied.The synthesis of sulfur carbon composites under different stirring speeds,temperatures and vacuum circumstance was experimented to clarify that sulfur crystals loaded at stirring speeds of 600 rpm,synthesis temperatures of 40°C,and vacuum environments can yield nanometer-sized particles and be distributed uniformly on the surface of the carbon.The composite cathode material exhibits higher electrochemical performance.3.The electrochemical properties of the composites were improved by doping the carbon skeleton with Fe,N and Fe-N co-doping.The mechanism of doping’s effect on material properties was examined by using the first principle study.The findings demonstrate that nitrogen doping can enhance the ability of carbon materials to adsorb Li₂S₆of 0.548 ev,and iron doping can reduce the decomposition energy barrier of Li₂S of 5 ev and alter the diffusion path of Li⁺.The amount of doping was determined by comparing the physical phase,morphology and electrochemical properties.The capacity of the composite cathode rose by 187 m Ah g^-1and 115 m Ah g^-1at 0.1C when doped with half melamine and one third FeCl₃·6H₂O.4.The separator was modified to further improve the electrochemical performance of the battery.The commercial separator was modified with corn silk-based porous carbon.The shuttle effect of Li PSs was prevented by establishing a physical barrier,and the specific capacity of the battery were improved to 1286m Ah g^-1.Another porous carbon modified separator was prepared by replacing part of the conductive agent with graphene oxide,and the composite cathode expressed the specific capacity of 1497 m Ah g^-1because of the further chemical adsorption of Li PSs based on the physical barrier.The utilization of sulfur reached 90%.更多还原