Design Of High Performance Anode Materials For Lithium-ion Batteries

Li-ion batteries(LIB)have attracted great attention in the applications for hybrid electric vehicles,electric vehicles,and smart grids due to their high energy density and long cycle life.However,the most used commercial anode material,graphite,is unable to keep pace with the increasing demands for high energy and power density because of its low theoretical capacity(372mAh g-1).Consequently,substitutes with high capacity,long cycle life,and high rate capability are needed.In this regard,SnO2 based electrode materials,which have shown numerous appealing features such as high theoretical capacity(1494 mAh g-1),abundance,and low cost,have been considered as one of the most promising alternative anode materials for the next generation LIBs.However,SnO2-based anodes suffer from pulverization problem caused by large volume change(>300%)and serious aggregation of tin particles formed during lithiation process,which would lead to the loss of electrical conductivity and consequently severe capacity fading in the cycling.Much effort has been made to improve the electrochemical behavior of SnO2-based anodes.One of the most effective approaches is to use nanostructured SnO2-based materials,which could not only shorten the path lengths Li+ transports,but make the oxidation-reduction reaction Sn + 2Li2O(?)SnO2 + 4Li+ + 4e-reversible.This reversible reaction increases the theoretic capacity of SnO2 from 781 mAh g-1(Sn + xLi+ +xe-1(?)LixSn(0<x>4.4))in bulk materials to 1494 mAh g-1 in nanostructured materials.Additionally,fixing the SnO2 nanoparticles with different carbon materials was one of the key factors to improve the anode conductivity and capacity stability.In this work,we designed a pomegranate-like SnO2@C composite to fix this problem.With this anode,we demonstrate a high stable capacity of 928 mAh g-1 based on the total mass of the composite at a current density of 500 mA g-1.At high current density of 2 A g-1,this composite anode shows a capacity of 853 mAh g-1 in the first charge,in such high current density,we can even get a capacity retention of more than 91%(779 mAh g-1)after 1000 cycles.We’ve also designed a SnO2@graphene composite.we anchored the SnO2 nanocrystals into three dimensional graphene gel network to tackle this problem.As a result of the three dimensional(3-D)architecture,the huge volume change during cycling was tolerated by the large free space in this 3-D construction,resulting in a high capacity of 1090 mAh g-1 even after 200 cycles.What’s more,at a higher current density 5 A g-1,a reversible capacity of about 491 mAh g-1 was achieved with this electrode.Although,the problem caused by drastic volume change can be improved by making the materials into nanoscale,the low density and low initial coulombic efficiency are still the biggest obstacles that prevent the high-performance anode materials from practical application.Thus,we developed a cold-flow process to improve the low density caused by nano-sized anode.In this work,a much denser packing of tin dioxide(SnO2)@Carbon nanostructured electrode is achieved via a cold flow process of the soft SnO2@carbon precursor composite.After carbonization,the density of 3.14 g cm-3(bulk form)and the tap density of 1.93 g cm-3(power form)are obtained.For the low initial coulombic efficiency,we designed a galvanic cell process to solve this problem.After this process,a high initial coulombic efficiency of 92.7%was obtained.