【作者】 王强；

【导师】 沈湘黔；

【作者基本信息】 江苏大学 ， 材料科学与工程， 2019， 硕士

【摘要】 随着能源需求的不断上升,传统锂离子电池受限于自身的理论能量密度(420Wh kg^-1)已经逐渐不能满足市场的需求。尤其是新能源电动汽车的发展,迫切需求具有安全、环保、高能量密度的二次储能系统。锂硫电池因具有2600 Wh kg^-1的理论能量密度,且对环境友好,可以作为破解电动汽车“里程焦虑”的候选者之一。锂硫电池最早在1962年被提出,但迄今为止,锂硫电池的应用依然不成熟,不能投入商业化大规模的生产和使用中。主要是因为锂硫电池依然存在很多问题,包括活性物质硫和反应产物LiS/Li₂S₂不导电;充放电过程中体积膨胀严重;反应中间产物多硫化物的穿梭效应和电池自放电现象等。本文以成本较低的葡萄糖为原料,采用水热法,通过控制不同的水热温度和时间制备了三种不同粒径的葡萄糖水热碳微球（CSs）。再利用KOH对其进行活化,作为硫负载的基体。研究了不同粒径,比表面积和孔体积的多孔碳微球对锂硫电池电化学性能的影响。水热碳微球的平均粒径分别为674 nm,364 nm和130nm,活化后多孔碳微球（ACSs）的比表面积分别为1633 m² g^-1,431.98 m² g^-1和431.5 m² g^-1,孔隙体积分别为2.291 cm³ g^-1,0.655 cm³ g^-1和0.608 cm³ g^-1。实验表明,比表面积和孔体积的增大,有助于提高电池的电化学性能。在0.5 C下,首圈库伦效率分别为95.42%,93.97%和92.58%。碳微球比表面积和孔体积最大的电极电池初始放电比容量为713.6 mAh g^-1,循环200圈后的容量保持率为68.7%。与科琴黑、碳纳米管这些高导电碳相比,上述多孔碳微球的导电性较差。并且多孔碳微球与硫复合后,部分硫未进入碳微球的孔洞中,而是分布在碳微球表面,不利于正极材料对多硫化物的束缚。为了提高正极材料的导电性和对多硫化物的束缚效果,在正极材料中加入多层石墨烯和碳纳米管。多层石墨烯占正极材料总质量的10%,而碳纳米管则作为导电添加剂使用。多层石墨烯具有片状结构,可以有效固硫和抑制多硫化物穿梭,而碳纳米管可以形成三维网络,为正极提供离子电子传输通道,大大提高正极导电性。硫负载量为2.35 mg cm^-2时,加入多层石墨烯和碳纳米管的电池在0.5 C下,初始放电比容量为605.1 mAh g^-1,循环200圈后的放电比容量为507.1 mAh g^-1,容量保持率为83.8%。在高硫负载(S=3.2mg cm^-2)条件下,加入多层石墨烯和碳纳米管的电池在0.3 C下,循环100圈后的容量保持率高达91.1%。与未添加多层石墨烯和碳纳米管的电极电池相比,容量保持率有了明显提高。这也说明了多层石墨烯和碳纳米管的使用对多孔碳微球/硫正极起到了良好的改性作用。更多还原

【Abstract】 With the increasing demand for energy,traditional lithium-ion batteries limited by their own theoretical energy density(420 Wh kg^-1)have been unable to meet the market requirement.In particular,the development of new energy electric vehicles requires a safe,environmentally friendly and high-energy density secondary energy storage system.Lithium-sulfur batteries have a theoretical energy density of 2600 Wh kg^-1,and are very environment-friendly,which could be a candidate to crack the range anxiety of electric cars.Although Li-S batteries have been proposed since 1962,its application is still not mature,so it cannot be put into commercial mass production and use.Mainly because that Li-S batteries still have many problems,including the insulation of active material and reaction products LiS/Li₂S₂;Severe volume expansion during charging and discharging;the shuttle of intermediate polysulfide and self-discharge phenomenon of the batteries etc.Hydrothermal method was used to prepare carbon microspheres using cheap glucose as the raw material.Three kinds of carbon microspheres with different particle sizes were prepared by controlling different hydrothermal temperature and time.After being activated by KOH,these carbon microspheres were used as the matrix to load with sulfur.And the effects of porous carbon microspheres with different particle sizes,specific surface area and pore volume on the electrochemical performance of lithium-sulfur batteries were investigated.The average particle sizes of hydrothermal carbon microspheres were 674 nm,364 nm and 130 nm respectively.The specific surface areas of porous carbon microspheres correspondingly are1633.95 m² g^-1,431.98 m² g^-1 and 431.5 m² g^-1 respectively after activation.Pore volumes are 2.291 cm³ g^-1,0.655 cm³ g^-11 and 0.608 cm³ g^-1 respectively.The increase of specific surface area and pore volume is helpful to improve the electrochemical performance.At 0.5 C,the coulomb efficiencies of the first cycle are respectively95.42%,93.97%and 92.58%.The initial discharge specific capacity of the battery with the largest specific surface area and pore volume of carbon microspheres in the electrode is 713.6 mAh g^-1,and the capacity retention is 68.7%after 200 cycles.However,compared with Ketjen-black and carbon nanotubes,which are highly conductive carbon,the above porous activated carbon microspheres have relatively poor electrical conductivity.Moreover,when porous carbon microspheres loaded with sulfur,some sulfur does not enter the pores of the carbon microsphere but on the surface,which is not conducive to the adsorption of polysulfide.In order to improve the conductivity of the cathode and the adsorption of polysulfide,multilayer graphene and carbon nanotubes were added to the porous activated carbon microspheres.Multilayer graphene accounts for 10%of the total mass of the cathode material,while carbon nanotubes are used as conductive additives.Multilayer graphene has a sheet structure,which can effectively consolidate sulfur and inhibit the shuttle of polysulfide.While carbon nanotubes can form a three-dimensional network,providing ion electron transport paths for the cathode and greatly improving the electrical conductivity of the cathode.When the content of sulfur is 2.35 mg cm^-2,the initial discharge specific capacity of the battery with multilayer graphene and carbon nanotubes is 605.1 mAh g^-1 at 0.5 C and the discharge specific capacity was 507.1mAh g^-1 after 200 cycles.The capacity retention rate was 83.8%.When the sulfur content is 3.2 mg cm^-2,the battery maintains a capacity of 91.1%after 100 cycles at0.3 C.Compared with batteries without multilayer graphene and carbon nanotubes in the electrode,the capacity retention is significantly improved.These also indicate that the use of multilayer graphene and carbon nanotubes have good modification on porous activated carbon microspheres/sulfur cathode.更多还原