【作者】 熊仕昭；

【导师】 谢凯； 洪晓斌；

【作者基本信息】 国防科学技术大学 ， 材料科学与工程， 2010， 硕士

【摘要】 锂硫电池具有能量密度高、硫资源丰富等优点,在未来化学电源发展中具有应用优势,但放电性能和循环性能差制约着其进一步发展和应用。本文通过研究正极形貌和锂硫电池阻抗在充放电过程的变化以及飞梭效应对锂硫电池充电过程的影响,探索制约锂硫电池放电性能和循环性能的原因。论文进一步研究了温度、电解液粘度和电导率等液相传质因素对锂硫电池放电过程的影响,并采用正极包覆改性和电解液添加剂的途径来抑制飞梭效应,提高锂硫电池的放电性能和循环性能。研究表明,在锂硫电池的充放电过程中,高价态聚硫离子的溶解和低价态的Li₂S₂和Li₂S形成的钝化层会造成离子在电解液中的扩散和迁移阻力增加。高价态聚硫离子的溶解扩散形成锂硫电池的飞梭效应,造成锂硫电池充放电效率低,温度和锂盐浓度是影响飞梭效应的重要因素,温度越高、锂盐浓度越低,飞梭电流越大。实验表明,当电解液中锂盐浓度大于1 mol·L^-1时,电解液粘度会急剧上升,电解液的扩散系数变小。电解液粘度的上升导致离子在电解液中的离子淌度变小,造成电解液电导率降低。锂硫电池放电过程中单质硫全部转化为S₄^2-溶解在电解液中时,电解液的粘度增大为原来的6倍,电解液电导率不足原来的50%。实验表明,液相传质过程限制锂硫电池的放电过程。温度降低引起离子在电解液中的扩散系数降低,扩散流量变小。当温度低于10℃时,锂硫电池的放电性能明显恶化。加入高介电常数溶剂PC可以降低电解液粘度,提高电解液电导率,加入量以vol6%为宜。采用PEO包覆改性碳硫活性材料可以有效抑制聚硫离子的溶解和扩散,同时由于PEO的低离子电导率使得离子在硫正极中的迁移阻抗增加,PEO包覆量为4% wt的锂硫电池首次放电比容量达1136 mAh·g^-1,循环20次后比容量保持为1023mAh·g^-1。采用聚苯胺包覆的硫正极,聚苯胺分子连接碳黑粒子和碳黑接触节点形成导电网络,提高了硫正极结构稳定性和导电性,聚苯胺包覆量为5.8%wt的锂硫电池首次放电比容量达1273 mAh·g^-1,循环20次后比容量保持为1029 mAh·g^-1。采用硝酸锂作为锂硫电池电解液的添加剂,通过化学反应的方式在锂负极表面形成具有钝化负极活性表面、保护锂负极的界面膜,抑制飞梭效应,避免在锂负极表面形成不可逆的硫化锂,提高锂硫电池的放电性能和循环性能。采用硝酸锂作添加剂的锂硫电池首次放电比容量为1172 mAh·g^-1,20次循环后比容量保持为1017 mAh·g^-1。采用LiBOB作为锂硫电池电解液的添加剂,在一定电位下诱发形成具有保护性的表面膜,阻止电解液和锂负极的接触,采用摩尔分数为4%LiBOB作添加剂,所形成的表面膜完整致密且离子迁移阻抗较低。采用摩尔分数为4%LiBOB作添加剂的锂硫电池首次放电比容量为1191mAh·g^-1,20次循环后比容量保持为892mAh·g^-1。更多还原

【Abstract】 The lithium-sulfur battery has many attractive properties, it has a high theoretical energy density and it is made with relatively cheap materials, it has special advantages in terms of batteries building for future. But the application and development of lithium sulfur battery is limited by discharge capability and cycle life. The morphology of cathode, the impedance change of battery during discharge and the effect of shuttle phenomenon on charge process of lithium sulfur were investigated to explore the causes which were responsible for poor discharge capability and cycle life of lithium sulfur battery. The effect of liquid transport factors such as temperature, viscosity and ionic conductivity of electrolyte on the discharge process of lithium sulfur were investigated in this dissertation. Cathode coating and electrolyte additive were adopted to restrain shuttle phenomenon and improve the electrochemical properties of lithium sulfur battery.The investigation of charge-discharge process showed that the dissolution of higher-order polysulfides and solid film formed by lower-order lithium sulfide caused the increase of diffusion and convection resistance of ion in electrolyte. Shuttle phenomenon formed by the dissolution of polysulfides caused poor charge-discharge efficiency of lithium sulfur battery. Temperature and concentration of lithium salt were factors which had vital effect on shuttle phenomenon. Shuttle phenomenon increased with higher temperature and concentration of lithium salt.The viscosity of electrolyte increased when the concentration of lithium salt in electrolyte was higher than 1M, then the diffusion coefficient of electrolyte was moved down. The increase of viscosity of electrolyte caused reduction of ionic mobilities and ionic conductivity. The viscosity was more than six times noumenon and the ionic conductivity was less than half of noumenon after elemental sulfur transforming into S₄^2-during the discharge process of lithium sulfur battery. It was showed that the discharge process of lithium sulfur battery was restrained by liquid transport process. The viscosity of electrolyte depressed after adding PC（propylene carbonate） who had high dielectric constant, then the ionic conductivity of electrolyte increased. The content of PC in the best cell was vol 6%.The dissolution and diffusion of polysulfides were prevented after the carbon-sulfur composite was coated by PEO, but low ionic conductivity of PEO resulted in high transfer resistances of cathode. The initial discharge capacity of lithium sulfur coated at 4%wt（PEO） is 1136 mAh·g^-1, a reversible capacity of 1023 mAh·g^-1 after 20 cycles. Carbon particle and conductive node were connected by PANI molecule and formed electronic conduction network, electronic conductivity and structural stabilization of cathode were improved. The initial discharge capacity of lithium sulfur coated at 5.8%wt（PANI） is 1273 mAh·g^-1, a reversible capacity of 1029 mAh·g^-1 after 20 cycles. LiNO3 adopted as additive of electrolyte reacted with Li electrode to form protective surface film. The film resulted in passivation at the surface of the electrode and prevented the shuttle phenomenon. Cycle life and capacity of lithium sulfur battery were improved because The film prevented form of irreversible Li₂S on Li electrode. The initial discharge capacity of lithium sulfur with LiNO3 is 1172 mAh·g^-1, a reversible capacity of 1017 mAh·g^-1 after 20 cycles. LiBOB（lithium bis（oxalato） borate） adopted as additive of electrolyte reacted with Li electrode to form protective surface film at a certain voltage to prevented the parasitic reaction between polysulfide and Li electrode. The film formed at 4 mol% LiBOB showed low transfer resistances and integrity. The initial discharge capacity of lithium sulfur with 4 mol% LiBOB is 1191 mAh·g^-1, a reversible capacity of 892mAh·g^-1 after 20 cycles.更多还原