【作者】 李倩；

【导师】 陈彦逍； 但勇；

【作者基本信息】 四川大学 ， 化学工程（专业学位）， 2021， 硕士

【摘要】 锂硫电池具有超高能量密度,S₈和Li⁺完全反应生成Li₂S的理论能量密度高达2600 Wh kg^-1。硫资源丰富、成本低,是非常具有商业前景的电池体系之一。然而,锂硫电池的实际应用依然面临着严峻挑战,比如硫和硫化锂的导电性差,反应中间产物（Li PSs）的穿梭效应导致电化学性能恶化,体积应变以及锂枝晶带来的安全问题。为了解决这些问题,本文开展了以下研究:首先,通过碳酸氢钠活化法结合高温碳化获得含N、O元素的藻基生物质碳。因其具有连续的导电骨架和丰富的Li PSs化学吸附活性位点,与升华硫通过熔融法混合制备出具有优异电化学性能的复合正极材料CBBC/S。对制备出的材料进行了物理表征及电化学性能测试,筛选了最优的合成条件。小球藻作为前驱体在800℃下煅烧制备的800-CBBC/S表现出卓越的电性能。结果表明,在0.5C下充放电400个周期之后,库伦效率依旧超过90%。800-CBBC/S电池的倍率性能也有良好的表现。当测试条件重新变为0.1 C时,电池容量恢复了96.43%。上述结果证明:藻基生物质碳作为正极材料中的硫载体能够缓解电极的体积膨胀,改善活性物质与放电产物的绝缘性。在充放电过程中还能利用杂原子强力地吸附中间产物,减少了活性物质的损失。其次,使用700-CBBC材料改性LSBs隔膜。中间层保留了700-CBBC材料原本的形貌,表面多孔而且拥有大比表面积,不仅为化学吸附提供了大量的活性位点,碳涂层也更加有利于电解液的渗透,良好的润湿性还有助于充放电过程中Li⁺的快速传输。在0.2 C电流密度下,60%-700-CBBC包覆隔膜的首圈放电比容量为1195 m Ah g^-1。测试周期150圈,电池比容量高达951 m Ah g^-1,单圈衰减率0.134%。当测试电流变为0.5 C时电池仍有卓越的循环性能,初始容量为1098 m Ah g^-1,200圈后放电比容量为889 m Ah g^-1。电流密度逐渐增大,电池容量的差异更加明显。电池在1 C下进行充放电测试,样品初始容量依旧超过1000 m Ah g^-1,经过250个周期的测试,容量保持率接近80%。当电池在更大电流2 C下测试时,60%-700-CBBC电池初始容量高达958 m Ah g^-1,每一圈的容量衰减率低至0.149%。显然,由于700-CBBC夹层具有电负性高的氮氧杂原子极性基团和独特的海绵状多孔结构,能够有效地排斥带负电荷的多硫离子,从而可以提高活性材料的利用率以及促进锂离子传输。实验设计将藻基生物质碳CBBC与金属氧化物TiO₂相结合,使TiO₂纳米颗粒均匀负载在碳层表面,作为隔膜包覆层一方面对Li PSs起到化学吸附的作用,一方面促进Li PSs在电化学过程中的转化,减少了活性物质损失。催化效果和吸附作用相结合,即使在大电流密度下电池的循环稳定性也得到了保持。在2 C下进行测试,700-CBBC与10%-TiO₂-CBBC样品的首圈容量差值接近一百。10%-TiO₂-CBBC电池循环了100圈后,单圈衰减率低至0.07%。10%-TiO₂-CBBC功能性中间层的表面孔隙利于中间产物的储存,活性物质均匀沉积在中间层表面,杂原子可以吸附中间产物,表面少量的金属氧化物可以加速反应动力学,导电性良好的中间层又可以作为二次集流体使活性物质得到充分利用。更多还原

【Abstract】 Lithium-sulfur batteries have ultra-high energy density.The theoretical energy density of the complete reaction of S₈ and Li⁺to produce Li₂S is as high as 2600 Wh kg^-1.In addition,sulfur itself has abundant reserves and low price,which has become one of the most promising rechargeable battery systems.However,the practical application of lithium-sulfur batteries still faces severe challenges,such as poor conductivity of reaction raw materials and products,and the shuttle effect of reaction intermediate products（Li PSs）that lead to deterioration of electrochemical performance,volumetric strain,and safety problems caused by lithium dendrites.To solve these problems,this paper has carried out the following research:Firstly,N,O co-doped chlorella-based biomass carbon（CBBC）is prepared by chemical activation and high-temperature carbonization.The carbon material is a continuous conductive framework with abundant Li PSs chemisorption active sites on the surface.Therefore,it is mixed with sublimed sulfur to form composite cathode material（CBBC/S）by melting method.The physical characterization and electrochemical properties of the prepared materials were tested,and the optimal synthesis conditions were selected.Positive electrode（800-CBBC/S）calcined at 800℃with chlorella as precursor showed excellent electrical performance.After400 cycles of charging and discharging at 0.5 C,the coulombic efficiency exceeds 90%.The rate performance of 800-CBBC/S battery is also good.When the test condition changed to 0.1C again,the battery capacity recovered 96.43%.The above results prove that the CBBC can relieve the volume expansion,improve the insulation between the active material and the discharge product,and strongly adsorb Li PSs during the reation.Secondly,700-CBBC material is used to modify LSBs separator.The interlayer surface retains the original morphology of the 700-CBBC material.The surface is porous.It not only provides many sites for chemical adsorption,but the carbon coating is also more conducive to the penetration of the electrolyte.At the current density of 0.2 C,the discharge specific capacity of the first circle of 60%-700-CBBC coated battery is 1195 m Ah g^-1.The battery still has excellent cycle performance when the test current is 0.5 C.With the increase of current density,the difference of battery capacity is more obvious.The initial capacity of the sample still exceeded 1000 m Ah g^-1 at 1 C.After 250 cycles of testing,the capacity retention rate was close to 80%.Obviously,because the 700-CBBC interlayer has N,O double-doped polar group and unique sponge-like porous structure,it can effectively repel negatively charged polysulfide ions,thereby improving the utilization of active materials and promoting Li⁺transmission.The experimental plan is to combine biomass carbon with metal oxide TiO₂.As membrane interlayer,it can chemically adsorb Li PSs and accelerate the conversion of Li PSs.The utilization of active materials has been improved,and the cycle stability is also maintained at high current densities.With the combination of catalytic effect and adsorption,the cycle stability of the battery was maintained even at high current density.When tested at 2 C,the discharge capacity of 10%-TiO₂-CBBC is 100 m Ah g^-1 higher than that of 700-CBBC.After 100 cycles,it still has the highest capacity retention rate of 92.97%.The surface pores of the 10%-TiO₂-CBBC functional interlayer are conducive to storage intermediate products;heteroatoms can adsorb intermediate products,and metal oxides can accelerate the reaction kinetics.Finally,the loss of the active material is reduced,the active material is uniformly deposited on the interlayer surface.The carbonized interlayer can be used as“secondary current collector”and the active material is fully utilized.更多还原