碳包覆的蛋黄壳结构氧化锰锂离子电池负极材料

Yolk-shell manganese oxide nanostructures for lithium-ion battery anodes

基于锂离子电池在循环过程中产生的体积效应严重影响整个电池的循环稳定性的问题,本研究设计了一种利用聚吡咯包覆金属有机框架的简单方法,来合成蛋黄壳结构的碳包覆氧化锰材料,并用于锂离子电池的负极材料.所制备的碳包覆氧化锰纳米颗粒在锂离子电池充放电过程中表现出良好的比容量,在0.1,0.5和2Ag^-1的电流密度下分别表现出723,651,374 mAh g^-1的比容量.在具有优异的倍率性能的同时,该材料还具有优异的稳定性.在上述3个电流密度下,该材料循环200圈后容量没有明显的衰减.该纳米结构MnOx的制备方法和电化学理解也可以推广到其他过渡金属氧化物,最终实现高性能的锂离子电池.

Nowadays,anodes based on graphitic materials are struggling with the ever-stringent requirements on lithium ion batteries(LIBs)in terms of energy and power densities,life-span,and deformability,from their extended applications such as automobiles,power grids and wearable electronics.As one of the promising alternates,transition metal oxides(TMOs)hold great perspectives.Unlike the lithium insertion mechanism of graphite,the lithiation/delithiation of TMOs is through redox conversion,imparting higher capacity and better safety.Among the numerous TMOs that have been extensively studied for LIB anodes,manganese oxides(MnOx)are particularly attractive due to their high natural abundance,environmental benignity,tunable oxidation states,and low fabrication costs.However,as common to most of the TMOs,MnOxare essentially non-conductive and suffer from severe volume expansion upon lithiation,which greatly limit their rate capability and cycle performance.To overcome the above problems of MnOx(as well as other TMOs),many solutions have been attempted and can be majorly classified into two strategies:nanostructuring and carbon-compositing.The former based on developing diverse nanostructures brings in numerous benefits such as shortened ion diffusion path,enlarged electrochemically active surface,promoted electrolyte infiltration,as well as relieved lattice stress.The later based on hybridizing with various kinds of carbonaceous materials enables to greatly enhance electronic conductivity,buffer the cyclic volume fluctuation,and reinforce the electrode stability by providing structural supports for the active materials.Of the explored nanostructures,the yolk-shell structure has gained increasing attentions due to the prominent advantages in buffering the volume expansion and promoting the utilization of active materials.On one hand,when compared to solid nanoparticles the void spaces in the yolk-shell structure can accommodate the cyclic volume change during charge/discharge,effectively preventing the pulverization of active materials.On the other hand,in comparison to the hollow nanospheres and nanocages,the volumetric specific capacity of the yolk-shell structure is obviously superior.Nevertheless,despite of these apparent benefits,previous reported fabrications of yolk-shell nanostructures have been quite complex and typically involved multi-step synthesis with both high material and energy inputs.Besides,the structural stability and integrity of the yolk-shell structure itself pose an issue,in which the nanostructures might crack and collapse due to mechanical stress,especially in the prolonged charge/discharge process.Therefore,a rational design and facile fabrication of yolk-shell nanostructures for optimizing their utilization and stability in LIB anodes are highly desired for achieving the outstanding electrochemical and cycle performance.In this paper,a simple method involving polypyrrole-coated metal organic framework is adopted to synthesize manganese oxides with yolk-shell structure and use them as the anodes for lithium ion batteries.The as-prepared yolk-shell MnOxnanoparticles exhibit good specific capacity during charge/discharge,and demonstrate high specific capacities of723,651,384 m Ah g^-1 at current densities of 0.1,0.5 and 2 A g^-1,respectively.Great stability and rate performance are also achieved with minimal capacity attenuation for over 200 cycles.Both the fabrication methodology and electrochemical understandings gained here for nanostructured manganese oxides can also extend to the other TMOs towards their ultimate implementation in high-performance LIBs.