【作者】 李昊；

【导师】 高翔； 李娜；

【作者基本信息】 浙江大学 ， 工程硕士（专业学位）， 2020， 硕士

【摘要】 锂离子电池是未来社会最重要的储能器件,它能为各种便携移动设备以及交通工具提供动力,促进人类社会的发展。然而它的发展进程则十分缓慢,目前已经遭遇了能量密度瓶颈,科学家们正通过各种方法来提升锂离子电池的能量密度。使用高理论比容量的材料来替换传统正负极材料是提升电池能量密度的一种有效手段,硅作为一种新型负极材料在完全嵌锂时理论比容量高达4200 m Ah g^-1,极具应用价值,然而纯硅在实际使用时具有体积变化较大和导电性较差等缺点,因此在实际应用过程中,通常采取折衷的方案,将硅和传统石墨负极混合形成硅碳负极材料,带来了应用价值。然而硅碳负极材料并没有完全消除体积变化给电池性能造成的影响,此时粘结剂的选择给硅碳负极的应用带来转机,在硅碳负极完全发挥性能的过程起着重要作用。粘结剂作为锂离子电池的重要组成部分,尽管所占比例较小,但是在维持电极结构的稳定,保证电池的性能发挥过程中起着不可或缺的作用。粘结剂是一种聚合物,分子量较高,它通过与活性物质等一起混合成浆料,再涂布制备成电极片,在此过程中,粘结剂与电极内的其他组分发生物理或者化学反应起到粘结作用。通常硅碳负极要求粘结剂的侧链具有大量功能性基团,能与活性物质等相互吸引,形成较强的界面作用力,其次粘结剂应当具有精细的链结构,能够贯穿整个电极,维持电极结构的稳定,最后粘结剂应当尽量减少对电解液的溶胀,这样在硅碳负极循环的过程中,体积膨胀所带来的应力变化才不至于削弱界面作用力和电极整体的机械强度,维持完整的导电网络,保证锂离子的传输。基于以上的要求,本文先是通过RAFT乳液聚合技术制备了聚苯乙烯-b-聚丙烯酸-2-乙基己酯-b-聚苯乙烯（SEHAS）三嵌段共聚物乳液作为粘结剂。丙烯酸-2-乙基己酯（2-EHA）的引入在保证聚合物拉伸性能的同时,极大地降低了聚合物的电解液溶胀度。剥离力测试表明两种嵌段共聚物胶乳的在电极中的粘结力均大于SBR;在硅碳负极半电池中,100圈循环后的放电容量及容量保留率均高于SBR,说明嵌段结构和低溶胀的特点在维持电极结构的稳定中起到重要作用;倍率性能方面,由于两种嵌段的电解液溶胀度仍然稍大于SBR,因此倍率性能稍好,阻抗测试也表明二者的R_ct小于SBR,说明锂离子传输阻力小,大电流放电能力强。本文又以反应型乳化剂制备了自增稠型粘结剂剂ANC,丙烯腈单体的引入降低了溶胀度（10.8%,70℃/72h）,同时提升了粘结力,对增稠剂的交联进一步降低溶胀度,形成网状聚合物结构。将其制备的浆料和水溶液进行流变性能测试,并与CMC作对发现,相同质量分数的增稠剂所制备的浆料的初始粘度分别为4460 m Pa·s和3710 m Pa·s,说明ANC的增稠效果较CMC好;ANC水溶液粘度随剪切速率的变化较CMC的更为平缓,表现出更好的浆料稳定性;ANC的触变恢复率较CMC更高,分别为68.0%和61.8%,因此ANC更适合浆料涂布;ANC在低应变时的G’（储能模量）>G"（损耗模量）,表现出比CMC更高的屈服应力,具有更佳的悬浮稳定性。剥离力测试表明在硅碳负极极片中ANC产生的粘结力要高于CMC;循环性能方面,ANC为增稠剂制备的硅碳负极软包电池在100圈1C循环后仍有389m Ah的容量,容量保留率为91.12%,均高于CMC的370.9 m Ah和88.23%;高温储存性能方面,ANC的电池在经历7×24h的高温（60℃）储存后容量保留率和容量恢复率分别为92.6%和98.1%,均高于CMC的83.7%和90.8%;倍率性能方面,ANC的电池在2C和3C下的放电容量分别为368.4和248.8 m Ah均略低于CMC的396.2和262.6 m Ah,其余倍率放电容量基本相近,甚至高于CMC。总体来说ANC在硅碳负极的软包电池中的循环性能和高温存储稳定性方面相较于CMC已经显现出一定的优势,但是倍率性能较CMC仍有一定的差距。更多还原

【Abstract】 Lithium-ion battery is one of the most important energy storage devices,which can provide power for various portable mobile devices and vehicles,and promote the development of human society.However,its development is very slow.At present,it has encountered a bottleneck in energy density.Scientists are using various methods to increase the energy density of lithium-ion batteries.By using a material with a high theoretical specific capacity to replace the traditional positive and negative electrode materials,the energy density of the battery can increase effectively.Silicon,as a new type of negative electrode material,has a theoretical specific capacity of up to 4200m Ah g^-1 when fully embedded by lithium,which is extremely promising.However,pure silicon has the disadvantages of large volume change and poor conductivity in practical use.Therefore,a compromise solution is usually adopted to mix silicon and traditional graphite anodes to form silicon-carbon anode materials in practical applications.However,the silicon carbon anode material does not completely eliminate the impact of volume changes on battery performance.So the choice of binder brings a turning point to the application of silicon carbon anode,which plays an important role in the process of silicon carbon anode affecting its performance.As an important component of the lithium ion battery,the binder plays an indispensable role in maintaining the stability of the electrode structure and ensuring the performance of the battery,although its proportion is small.The binder is a polymer with a high molecular weight.It is mixed with the active material to form a slurry,and then coated to prepare an electrode sheet.During this process,the binder and other components in the electrode bind with each other physically or chemically.Generally,the silicon carbon negative electrode requires that the binder has a large number of functional groups,which can attract each other with the active material and form a strong interfacial force.Secondly,the binder should have a chain structure that can penetrate the entire electrode.To maintain the stability of the electrode structure,the binder should minimize the swelling of the electrolyte,so that during the intercalation/deintercalation of lithium ions of silicon-carbon anode,the stress changes caused by volume expansion will not weaken the interface force and the overall mechanical strength of the electrode to maintain a complete conductive network ensuring lithium ion transmission.Based on the above requirements,this paper prepared PS-PEHA-PS by RAFT emulsion polymerization.The introduction of 2-EHA,while ensuring the tensile properties of the polymer,greatly reduced the polymer’s electrolyte swelling degree.The peeling force test showed that the adhesion strength of the two block copolymer in the electrode was greater than SBR;in the silicon carbon anode half-cell,the discharge capacity and capacity retention after 100 cycles were higher than SBR,indicating that the block structure and low swelling characteristics play an important role in maintaining the stability of the electrode structure.In terms of rate performance,because the electrolyte swelling of the two blocks is still slightly larger than SBR,the rate performance is better.EIS tests also show that both R_ct are less than SBR,which means that the lithium ion transmission resistance is small and the high current discharge capacity is better.In this paper,a self-thickening agent ANC was prepared with a reactive emulsifier.The introduction of acrylonitrile monomer reduced the swelling degree（10.8%,70°C/72h）,and at the same time improved the adhesion.The crosslinking process of the thickener reduces the swelling degree,forming a network polymer structure.The rheological properties of the prepared slurry and aqueous solution were tested and compared with the CMC.The initial viscosity of the slurry prepared by the thickener with the same mass fraction was 4460 m Pa·s and 3710 m Pa·s,indicating better thickening effect of ANC than that of CMC;The change in viscosity of ANC aqueous solution with shear rate is more gentle than CMC,showing better slurry stability;the thixotropic recovery rate of ANC is higher than CMC,which are 68.0%and 61.8%,Therefore,ANC is more suitable for slurry coating;G’（storage modulus）>G"（loss modulus）at low strain shows a higher yield stress than CMC and has better suspension stability.The peeling test shows that the adhesion force produced by ANC in the silicon-carbon negative electrode is higher than that of CMC;in terms of cycle performance,the silicon-carbon anode soft-pack battery prepared by ANC as a thickener still has 389m Ah after 100 cycles at 1C,the capacity retention rate is 91.12%,both higher than CMC’s 370.9 m Ah and 88.23%.In terms of high temperature storage performance,the capacity retention rate and capacity recovery rate of ANC batteries after 7×24 h high temperature（60°C）storage are respectively 92.6%and 98.1%,both higher than CMC’s83.7%and 90.8%;In terms of rate performance,the discharge capacity of ANC batteries at 2C and 3C is 368.4 and 248.8 m Ah,respectively,which are slightly lower than CMC’s 396.2 and 262.6 m Ah.The other rate discharge capacity is basically similar,even higher than CMC.In general,ANC has shown some advantages compared to CMC in terms of cycle performance and high-temperature storage stability in silicon carbon anode,but the rate performance is not satisfying compared with CMC.更多还原