高性能锂离子电池用N掺杂C-Sn交联纳米纤维自支撑电极的理性设计

Rational Design of Cross-Linked N-Doped C-Sn Nanofibers as Free-Standing Electrodes towards High-Performance Li-Ion Battery Anodes

为了提高碳材料作为锂离子电池负极材料的比容量,将氮掺杂的碳纤维与高容量的Sn进行复合。通过静电纺丝及低温碳化制备了均匀镶嵌Sn纳米颗粒的氮掺杂碳纳米纤维(C-Sn)复合膜。该复合膜直接用作自支撑锂离子电池负极时表现出较好的电化学性能,Sn的引入显著提高了碳纳米纤维膜的电化学性能。碳均匀包覆Sn后形成的纤维结构可以促进离子电子的传导,并能有效缓冲Sn纳米粒子在循环过程中的体积变化,从而有效抑制粉化与团聚。Sn含量约为25.6%的CSn-2电极具有最高的比容量和更优异的倍率性能。电化学测试结果表明,在2A·g^(-1)的电流密度下,充放电循环1000圈后充电(放电)比容量为412.7(413.5)mAh·g^(-1)。密度泛函理论(DFT)计算结果表明,N掺杂非晶碳与锂具有良好的亲和性,有利于将合金化反应之后形成的SnxLiy合金锚定在碳表面,进而缓解了充放电过程中的Sn的体积变化。本文为高性能储锂材料的设计提供了一种切实可行的策略。

Li-ion batteries(LIBs)have been considered as one of the most promising power sources for electric vehicles,portable electronics and electrical equipment because of their long cycle life and high energy density.The free-standing electrodes without binder,current collector and conductive agent can effectively obtain lager energy density as compared to the traditional electrodes where the addition of inactive components is required.In addition,the freestanding electrode plays an important role in developing flexible electronic devices.Currently,conventional graphite is still the main commercial anode material,but its theoretical specific capacity is limited,and the rate performance is poor.In recent years,the high temperature pyrolytic hard carbon has attracted wide attention due to its higher theoretical specific capacity and more defects than graphite carbon.Moreover,polymer polyacrylonitrile(PAN)can be used as the raw material for preparation of free-standing anodes without any conductive additives or binders by electrospinning technique.Meanwhile,it is beneficial to reduce the production cost and simplify the manufacturing procedures of electrode.However,PAN-based hard carbon anode materials also have certain problems,such as low conductivity,poor rate performance,unsatisfactory cycling stability,and inferior initial Coulombic efficiency(CE).In addition,soft carbon has advantages of high carbon yield,good conductivity,superior cycling stability,high initial CE and relatively low price,but its specific capacity is generally lower than that of hard carbon materials.Based on above analysis,carbon anode materials with good electrochemical performance can be obtained by combining hard carbon and soft carbon,but the specific capacity of carbon materials is still low.Tin(Sn),as an anode material for LIBs,has a high theoretical specific capacity(994 mAh·g^(-1))and a low lithium alloying voltage.Nonetheless,the practical use of Sn anode has been limited by its huge volume change(theoretically∼260%)during the repeated alloying-dealloying process,resulting in large pulverization and cracking,which triggers the rapid capacity fading.Hence,in order to increase the specific capacity of carbon anode materials of LIBs,the C-Sn composite film with uniform Sn nanoparticles embedded in N-doped carbon nanofibers was prepared by electrospinning method following by a low-temperature carbonization process.The film was directly used as a free-standing electrode for LIBs and exhibited good electrochemical performance,and the introduction of Sn significantly improved the electrochemical properties of the carbon nanofiber film.The formed fibrous structure after Sn was uniformly coated with carbon can promote the conduction of ions and electrons,and effectively buffers the volume change of Sn nanoparticles during cycling,thus effectively preventing pulverization and agglomeration.The C-Sn-2 electrode with a Sn content of about 25.6%has the highest specific capacity and best rate performance among all samples.The electrochemical test results show that,the charge(discharge)capacity reaches 412.7(413.5)mAh·g^(-1)at a current density of 2 A·g^(-1)even after 1000 cycles.Density functional theory(DFT)calculations show that N-doped amorphous carbon has good affinity with lithium,which is conducive to anchoring the SnxLiy alloy formed after alloying reaction on the carbon surface,thereby relieving the volume change of Sn during charge-discharge.This article provides a feasible strategy for the design of highperformance lithium storage materials.