【作者】 徐琦；

【导师】 郭孝东； 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】 受到环境危害及资源价值等属性的影响,废旧锂离子电池（LIBs）的回收利用对于可持续发展具有重要意义。废旧LIBs的大部分元素现在都可以回收,而石墨负极的利用关注较少。事实上,尽管原材料成本低、储量丰富,石墨的制备过程却条件苛刻而且能耗高。目前废旧石墨（SG）的回收方式主要包括两种,一种是通过火法或湿法工艺将其回收,另外一种是根据材料特性将其转化成其他功能材料（主要是石墨烯）或直接应用于其他领域。相较于回收和转化的工艺,直接利用具有污染小、回收成本低和能耗低等优点。SG由于其在电池循环过程中形成的固有性质,如独特的碳结构、大量的缺陷和丰富的表面官能团等,已被广泛应用于吸附剂等领域。石墨本身作为一种具有良好的导电性、柔韧性和润滑性的碳材料,已被证明可以起到吸附多硫化锂（LiPSs）、改善硅（Si）负极的低电导率和体积膨胀的作用。若将SG用于锂硫（Li-S）电池和Si负极,有望利用其独特性质更好地改善Li-S电池和Si负极性能,从而实现SG的资源化、短程化及高值化利用。基于此,本文对SG的理化性质进行了系统的研究,并根据其固有特性,将其应用于Li-S电池隔膜包覆和硅/石墨（Si/G）负极。结果表明,SG不仅能吸附LiPSs并促进其催化转化,而且能有效缓解Si负极的电极膨胀。主要研究工作如下:（1）SG的理化性质研究。将SG从废旧LIBs中回收,对其理化性质进行了详细的表征,并与人造石墨（AG）对比。研究结果表明,由于充放电过程中正极材料的溶解,导致SG表面存在一些过渡金属离子。同时,SG在反复的充放电过程中发生了结构的坍塌,形成了多孔结构,比表面积增大,还引入了一些缺陷和有机官能团。由于Li⁺的反复脱嵌,还造成SG层间距的扩大和部分石墨的剥离。（2）SG改性Li-S电池隔膜研究。将SG涂覆在Li-S电池商业隔膜上,与AG对比。研究结果表明,SG和AG包覆的隔膜均能提升Li-S电池的性能,但是SG的改善作用要大于AG。这主要是因为SG的多孔结构、缺陷和极性官能团可通过物理吸附和化学吸附的结合限制LiPSs的穿梭,而表面过渡金属元素也可以通过S-TM键固定LiPSs,并改善隔膜的电导率和LiPSs的转化动力学。在0.1、0.2、0.5、1和2 C(1 C=1675 m Ah g^-1)的电流密度下,基于SG包覆隔膜的电池放电比容量分别达到1317.7、1111.3、1010.9、904.1和813.1 m Ah g^-1。在1 C下经历500个循环后平均每圈衰减率仅为0.08%,平均库仑效率超过98%。（3）SG合成Si/SG负极。将SG用作硅碳负极的基体,与AG对比。相较于Si/AG负极,Si/SG负极的电化学性能得到明显改善。这主要是由于SG表面的过渡金属离子能够让其和Si纳米颗粒通过静电作用力结合,因此可以更好地缓解循环过程中Si纳米颗粒的体积应力。另外,富含缺陷和剥离的石墨可以有效地增强导电性,促进电子和Li⁺在电极中的传输。SG上的含氧官能团还可以诱导有机固体电解质界面（SEI）膜组分的形成,从而提供可持续的机械韧性以适应长循环过程中电极的膨胀。在0.1、0.2、0.5、1、2和3 A g^-1的电流密度下,Si/SG电极的放电比容量分别达到1055.8、1014.4、956.0、864.3、741.6和646.4 m Ah g^-1,在1 A g^-1下经历400个循环后容量保持率仍有69%。本文通过对SG固有性质的研究将其成功应用于Li-S电池和Si/G领域,不仅为利用废旧资源设计Li-S电池功能性隔膜和改善Si/G负极界面稳定性提供了指导,也为SG的高效利用提供了思路。更多还原

【Abstract】 Affected by attributes including environmental hazards and resource value,the recycling of spent lithium-ion batteries（LIBs）is of great significance to sustainable development.Most of the elements in spent LIBs can be recycled now,but little attention has been paid to the utilization of graphite anode.Actually,the preparation process of graphite is complex and energy-intensive,despite the low cost and abundance of raw materials.At present,the recycling of spent graphite（SG）mainly includes two methods.One is to recycle it through pyrometallurgical or hydrometallurgical methods,and the other is to convert it into other functional materials（mainly graphene）or directly apply to other fields according to the characteristics of the material.Compared with the recycling and conversion process,direct utilization possesses the advantages of low pollution,low recycling cost and low energy consumption.SG has been extensively applied in adsorbents and other fields owing to its inherent properties,such as unique carbon structure,abundant defects and surface functional groups,which were formed during the battery cycling process.Graphite,as a carbon material with good conductivity,flexibility and lubricity,has been proven to be able to adsorb lithium polysulfides（LiPSs）,improve the conductivity and mitigate the volume expansion of the silicon（Si）anode.If SG is applied in lithium-sulfur（Li-S）batteries and Si anodes,it is expected to improve the performance of Li-S batteries and Si anodes based on the unique properties,thereby realizing the resource,short-range and high-value utilization of SG.Based on the above reasons,this work has conducted a systematic study on the physical and chemical properties of SG,and applied it to the modification of Li-S separator and Si/graphite（Si/G）anode according to its inherent properties.As a result,the SG can not only adsorb LiPSs and promote their catalytic conversion,but also effectively alleviate the electrode expansion of the Si anode.The main research work is as follows:（1）Investigation on the physical and chemical properties of SG.The SG was recycled from the spent LIBs,and its physical and chemical properties were characterized in detail and compared with artificial graphite（AG）.The results show that some transition metal ions existed on the surface of SG,which were derived from the dissolution of cathode material during the battery cycling process.Meanwhile,the structure of SG collapsed,forming a porous structure,enlarging the specific surface area,and introducing some defects and organic functional groups.Owing to the repeated lithiation/delithiation,the graphite interlayer distance was expanded and some graphite was exfoliated.（2）Investigation on the modification of Li-S separator by SG.The SG was coated on the commercial Li-S separator and compared with AG.The results show that both SG and AG-coated separators can improve the performance of Li-S batteries,but the improvement of SG is better than that of AG.This is mainly because the porous structure,defects and polar functional groups of SG can restrain the shuttle of LiPSs through a combination of physical and chemical adsorption.The surface transition metal ions can also anchor LiPSs by S-TM bonds,and improve the electrical conductivity of the separator and the conversion kinetics of LiPSs.At current densities of 0.1,0.2,0.5,1 and 2 C(1 C=1675 m Ah g^-1),the cell with SG-coated separator exhibits discharge capacities of 1317.7、1111.3、1010.9、904.1 and 813.1 m Ah g^-1,respectively.After 500cycles at 1 C,the average decay rate per cycle is only 0.08%,and the average coulombic efficiency exceeds98%.（3）Synthesizing Si/G anode using SG.The SG was used as the matrix for the Si/carbon anode and compared with AG.As compared to the Si/AG anode,the electrochemical performance of the Si/SG anode has been significantly improved.This is mainly because the transition metal ions on the surface of the SG can make it integrate with the Si nanoparticles through electrostatic force,so that the volume stress of the Si nanoparticles upon charging/discharging can be better restrained.In addition,defect-enriched and exfoliated SG can effectively enhance the electrical conductivity,facilitating the transport of electrons and Li⁺in the electrode.The oxygen-containing functional groups on SG can also induce the formation of organic solid electrolyte interphase（SEI）components,thereby providing sustainable mechanical toughness to accommodate the electrode expansion during prolonged charge/discharge cycles.At current densities of 0.1,0.2,0.5,1,2 and 3 A g^-1,the Si/SG electrodes can deliver discharge capacities of 1055.8、1014.4、956.0、864.3、741.6 and 646.4 m Ah g^-1,respectively,with a capacity retention of 69%at 1 A g^-1 after 400 cycles.In this paper,SG was successfully applied to Li-S batteries and Si/G anode by exploring its inherent properties,which not only provided guidance for designing functional separator of Li-S batteries and improving the interface stability of Si/G anode based on waste resources,but also threw light on the efficient application of SG.更多还原