煤基石墨烯在锂离子电池中的应用

Application of coal-based graphene for lithium-ion batteries

以煤为前驱体,通过物理、化学耦合方法,可制备高附加值的石墨烯材料,且可提高产品振实密度,具备一定的揉曲结构,但工艺流程长,需改善煤炭纯化脱灰、催化石墨化、氧化还原工序或热解CVD技术。石墨烯作为一种新型碳纳米材料,具有高导电性、高导热性、比表面积大和机械性能稳定等优势,在锂离子电池行业中应用前景广阔。针对复合石墨烯对锂离子电池正极、负极和导电剂的影响,总结石墨烯在锂离子电池中的应用进展和最新研究成果。石墨烯通过与活性材料和传统导电剂复合,可实现“面-点”和“点-点”充分接触,从不同空间跨度上在活性材料周围形成高效稳定的导电网络,同时发挥材料各自独有的特点,并且能够抑制充放电时活性材料产生的体积膨胀效应,稳定结构,从而提高整个体系的比容量、充放电性能、循环性能、倍率性能等电化学特性,改善电池体系的散热性能,提高使用温度上限,与传统的炭材料相比具备明显优势。煤分子结构、元素组成、矿物质组分、显微组分和变质程度的差异,会影响石墨烯产品的性能。变质程度高的煤种,可以获得石墨化度较高的产品,最终石墨烯微晶尺寸更大,片层更少;若以褐煤、烟煤为原料,必须经过深度破碎,石墨化条件更为苛刻。以锂离子电池应用为导向,宜采用煤直接氧化还原工艺,原料以变质程度较深的无烟煤为主,可生成石墨化度更高、微晶尺寸更大、片层更少的石墨烯。煤中本身含有一部分氧元素,在氧化还原过程中会脱除大量含氧官能团,会在石墨烯片层留下部分缺陷与无序的碳空位,会在锂电池充电过程中充当储电穴位,增加可逆容量。加强煤预处理研究,优化煤大分子基本单元的构造,改善芳烃结构单元,使有机多环芳烃分子较容易转化为层状石墨结构,利于后续氧化插层;在现有电池体系中,石墨烯直接作为锂离子电池负极存在电压滞后、库伦效率低的缺点,需进一步研究嵌锂脱锂过程中石墨烯的微观变化。而电极厚度、活性材料颗粒的尺寸差异、石墨烯表面性质、官能团含量、层数及片层结构等因素影响石墨烯优势的发挥。

High value-added graphene with high tap density and crumpled texture could be produced from coal via various physical or chemical methods.However,the technological process is complex,purification of deashing,catalytic graphitization,oxidation-reduction and CVD need to be promoted.Graphene,as a novel carbon nanomaterial,possesses unique properties,such as high electri⁃cal conductivity,high thermal conductivity,large specific surface area and stable mechanical properties.Coal-based graphene has very broad application prospects in lithium-ion batteries.The application and latest research achievements of graphene in lithium-ion batteries was reviewed.The modification of anode and cathode of batteries by graphene and the preparation of graphene composite conductive agent were focused.A conductive network with a"surface-point"and"point-point"contact mode could be constructed by graphene and differ⁃ent spatial spans could be formed with active materials and conventional conductive agents.Formation of efficient and stable conductive networks around the active materials from different spatial spans,while taking advantage of the unique characteristics of each material.In addition,it can suppress the effect of volumetric expansion,and stabilize the structure,which can improve the specific capacity,charge-discharge properties,cycle performance,rate capability and heat dissipation performance of the whole system.It can also improve the heat dissipation performance of the battery system and increase the upper limit of service temperature,which has obvious advantages compared with the traditional carbon material.The molecular structure,elemental composition,mineral content,maceral components,and degree of metamorphism of coal would affect the characteristics of coal-based graphene.Products with high graphitization degree could be pro⁃duced from high degree of coal meta-morphism,which had large crystallite dimension and few layer.Graphitizing and particle size is more stringent with using lignite coal and bituminous coal as raw materials.For lithium-ion battery applications,the approach of graphene based coal should use direct oxidation-reduction with high degree of coal.The raw material is mainly anthracite coal with deeper metamor⁃phism,which can generate graphene with higher graphitization,larger microcrystal size and fewer lamellae.And there are a few defects and carbon vacancy by removing oxygen element which could provide extra storage points in the charging process of lithium batteries and increase the reversible capacity.It is important to enhance pretreatment research which could improve basic structural units of coal and ar⁃omatic structure.Organic polycyclic aromatic hydrocarbons are easily transformed to layered graphite structure which is good for oxide in⁃tercalated.In the conventional battery system,directly using graphene as the anode of lithium-ion battery had the disadvantages of voltage lag and low coulomb efficiency.It is necessary to further elucidate the microscopic changes of graphene in the process of lithium inserting and extracting.In addition,the electrode thickness,size distribution,surface properties,functional groups,layer number,and lamellar structure of graphene would affect its advantages.