Si-and Ge-based Materials As A High Performance Anode For Lithium-ion Batteries

To meet the ever-growing demands for the electric vehicles?EVs?,portable electronics and large-scale renewable energy storage,it is extremely urgent to develop the next generation Lithium-ion Batteries?LIBs?with low cost,high energy density,and long cycle life.In the case of anodes,the group IV elements?silicon and germanium?materials provide high specific theoretical capacities(Si,Ge are 3579 mAh g^-1,1600 mAh g^-1,respectively)which are much higher than Today`s commercial graphite anode(372 mAh g^-1).Especially,Silicon has been attracted much more attention due to its highest theoretical capacity except Li metal anode,offering potential for significantly improving performance of LIBs.However,the bulk silicon anode suffers from over 300% volume expansion during lithiation/delithiation process,which leads to particle pulverization,loss of interparticle electrical contact and instability of solid electrolyte interphase?SEI?,resulting in repeating chemical side reactions with the electrolyte and fast capacity fading.Meanwhile,Ge-based material as anode has been attracted much attention due to its high theoretical capacity and lithium ion diffusivity at room temperature than Si,which could exhibit higher rate performance than Si in LIBs.However,Ge-based anodes face significant challenge due to the 260% volume expansion during lithiation/delithiation process,which limits its practical application in LIBs.Therefore,the key is to design a suitable structure to mitigate volume expansion and overcome particle fracture of the Si-and Ge-based anodes with a low-cost simple synthetic route.Hence,we design and synthesize micro-sized Si-and Ge-based materials as high-performance anodes for lithium ion batteries,and study their structures and electrochemical performance.The results are as follows:?1?We present a facile and large-scale approach for preparing micro-sized porous silicon by acid etching the abundant and inexpensive metallurgical Fe-Si alloy as high-performance anode in LIBs.Profiting from the unique micro-sized structure,it exhibits a fantastic first-cycle Coulombic efficiency of 88.1% and an excellent reversible capacity of 1250 mAh g^-1 at 500 mA g^-1 after 100 cycles.Furthermore,the micro-sized porous silicon without carbon coating can deliver a reversible capacity of 558 mAh g^-1 at a high current density of 5 A g^-1 due to the unique porous structure.This work provides a promising route for a large-scale production of high-performance micro-sized Si as anode materials in LIBs.?2?We report an industrially established spray-drying process for synthesis of micro-sized durable three-dimensional?3D?network structure GeO_x/multi-walled carbon nanotubes?MWCNTs?composite spheres consisting of 5?15 nm nanoparticles for LIBs anodes.The 3D GeO_x/MWCNTs composite sphere structure not only provides high ability of Li-ion intercalation,but also enhances electronic conductivity of the composite spheres for LIBs,increasing the rate capabilities.Meanwhile,the 3D network structure provides adequate space for GeO_x expansion resulting in good long-term cycling stability.The interconnected 3D network structure GeO_x/ MWCNTs spheres exhibit over 1000 mAh g^-1 at 500 mA/g after 300 deep charge-discharge cycles and provide a capacity of 550 mAh g^-1 at 5 A/g.Even at a high current density of 10 A/g,it can deliver a reversible capacity of 365 mAh g^-1.The results display that when the active material loading reached ?1.2 mg/cm2,the discharge capacity can also retain 723 mAh g^-1 after 150 cycles.This scalable and industrially method provides attractive future prospects for high-performance Ge-based anodes with excellent electrochemical performance with a high active materials loading in the next generation LIBs.