摘要
随着环境问题和能源问题的日益突出,传统汽车逐渐走向新能源化。锂离子电池具有放电电压平台高、自放电小、环境友好等优点,被认为是最有前景的新能源汽车动力之一。然而,随着人们对新能源汽车续航能力要求的逐渐提高,进一步提高汽车动力电池的能量密度成为当今社会研究的热点。目前,商业化车用动力锂离子电池的正极材料以磷酸铁锂(LiFePO4)和三元材料(Li(Nix Coy Mn1-x-y)O)为主,负极以石墨为主,其能量密度仅为200~300 Wh·kg^-1。因此,提高汽车动力电池的能量密度,研发高能量密度的正负极材料是动力电池的研究方向之一。硅具有4200 mAh·g^-1的超高理论比容量,是制备车用高能量密度型锂离子电池最有前景的负极材料之一。然而,硅在充放电反应中的剧烈体积变化严重阻碍了其商业应用。硅采用合金化反应方式储存锂离子,合金化反应在提供高比容量的同时伴随着300%的体积膨胀。剧烈的体积变化导致活性物质脱落、SEI膜持续形成等问题,进而导致实际使用时电池容量的快速衰减。此外,纯硅属于半导体,本征载流子浓度很低,无法满足电极对导电性的要求。解决上述问题最常用的方法有以下三种:(1)硅的纳米化。锂离子在固体中的扩散较为困难,在外加电场作用下,锂离子在硅中的扩散速度依然很慢。通过硅纳米化的方式可以缩短锂离子从硅表面到中心的扩散距离,有效缩短电池充电时间。(2)硅/碳复合。碳材料具有良好的循环稳定性和导电性,将硅与碳复合,碳可以缓冲硅在合金化反应中剧烈的体积变化,提高整个负极的电子电导率,外层碳壳能阻止硅和电解液的直接接触,形成稳定的SEI膜。(3)微观结构设计。中空核-壳结构、3D多孔结构等特殊结构可以缓解硅的体积膨胀效应,有效抑制电极材料的脱落。研究中经常综合使用上述三种方法来制备高性能纳米硅/碳负极材料,如3D多孔纳米硅/碳材料、中空核-壳纳米硅/碳材料等。本文先阐述了硅锂合金的电化学反应机理与容量衰减的原因,以及纳米硅的制备方法,然后从表面包覆、结构制备、掺杂、MOFs改性等方面对硅/碳复合材料的常见修饰方法进行了综述,并进一步分析了中空核-壳结构、多孔结构等在提高电化学性能上的优势。最后,本文总结了纳米硅/碳作为负极材料的优点与当前遇到的问题,归纳并分析了不同包覆材料、不同包覆方法和不同离子掺杂带来的性能差异及原因,提出未来纳米硅/碳产业化道路上的关键突破点,并展望了其在纯电动汽车领域的应用前景。
With the increasing environmental and energy problems,traditional cars are gradually moving toward new energy.Lithium-ion batteries are considered to be one of the most promising new energy vehicle powers due to their high discharge voltage platform,low self-discharge rate and friendly environment.However,with the increasing demand for the driving distance of new energy vehicles,improving the energy density of batteries has become a hot topic.Therefore,improving the energy density of batteries for pure electric vehicles and developing high energy density positive and negative materials are one of the research directions of batteries.At present,the cathode materials of commercial pure electric vehicles are mainly LiFePO 4 and Li(NixCoyMn1-x-y)O.The anode material is graphite.This type of battery has an energy density of only 200—300 Wh·kg-1.Silicon has a high theoretical specific capacity of 4200 mAh·g^-1,which is one of the most promising anode materials for high energy density lithium ion batteries.However,the dramatic volume change of silicon in the charge and discharge reaction hinders the commercial application of silicon materials.The alloying reaction of silicon can store lithium ions.The alloying reaction provides a high specific capacity accompanied by a 300%volume expansion.A drastic volume change causes the active material to fall off,and the SEI film continues to form,which causes a rapid decay of the battery capacity in actual use.In addition,pure silicon is a semiconductor with low intrinsic carrier concentration,so it can not meet the requirements for conductivity.The most common methods for the above problems are listed below:(ⅰ)Synthesis of nano-sized silicon.The diffusion of lithium ions in solids is difficult.Although with the help of the electric field,the diffusion rate of lithium ions in silicon is still very slow.Silicon nanocrystallization can shorten the diffusion distance of lithium ions from the silicon surface to the center,which effectively shortens the battery charging time.(ⅱ)Pre-paration of silicon/carbon composite.Carbon materials have good cycle stability and electrical conductivity.If silicon is combined with carbon,car bon can buffer the volumetric change of silicon in the alloying reaction and increase the electronic conductivity of the negative electrode.The outer carbon shell can prevent direct contact between silicon and electrolyte to form a stable SEI film.(ⅲ)Design the microstructure of the material.Special structures such as hollow core-shell structure and 3D porous structure can alleviate the volume expansion effect of silicon,which will effectively inhibit the falling off of the electrode material.The above three methods are often used in combination to prepare high-performance nano-sized silicon/carbon anode materials,such as 3D porous nano-silicon/carbon materials,hollow core-shell nano-silicon/carbon materials.This paper first explains the electrochemical reaction mechanism and the reasons for capacity decay of silicon-lithium alloys.Next,the preparation method of nano-sized silicon is given.Then,the common modification methods of silicon/carbon composites are reviewed from surface coa-ting,structure preparation,doping and MOF modification.Furthermore,the advantages of hollow core-shell structure and porous structure in improving electrochemical performance are analyzed.Finally,this paper summarizes the advantages and disadvantages of nano-silicon/carbon as a negative electrode material and analyzes the performance differences and causes of different covering materials,different coating methods and different ion doping.The key point in nano-silicon/carbon industrialization is proposed and the fascinating application prospect of nano-silicon/carbon is envisioned in pure electric vehicles.
作者
赵立敏
王惠亚
解启飞
邓秉浩
张芳
何丹农
ZHAO Limin;WANG Huiya;XIE Qifei;DENG Binghao;ZHANG Fang;HE Dannong(School of Material Science and Engineering,Shanghai Jiao Tong University,Shanghai 200240,China;National Engineering Research Center for Nanotechnology,Shanghai 200241,China)
出处
《材料导报》
EI
CAS
CSCD
北大核心
2020年第7期26-35,共10页
Materials Reports
基金
国家重点基础研究发展计划(973计划)项目(2015CB931901)
上海市高新技术领域项目(18511110000)
上海市基础重大项目(18JC1410604)。
关键词
硅/碳负极
锂离子电池
高能量密度
包覆掺杂
硅基材料
silicon carbon anode electrode
Li-ion battery
high energy density
coating and doping
silicon based material
