Controlled Synthesis And Lithium-ion Storage Performances Of Porous Micro-Sized Silicon Based Anode Materials For Advanced Lithium-ion Batteries

Lithium-ion batteries?LIBs?are widely used in various fields of energy storage due to higher specific power/energy,longer cycle life,and competitive costs.With the rapid development of electric vehicles and consumer electronic products in recent years,the energy density of LIBs is required to be higher.Silicon?Si?has a high theoretical specific capacity of 4200 mAh/g,which is considered as a promising candidate anode material in next-generation high performance LIBs.However,Si anode materials suffer from poor conductivity,huge volume expansion and serious pulverization during lithiation process,which results in the rapid deterioration of cycle and rate performance.Nanostructured Si materials can effectively suppress the pulverization of Si and enhance the cycle performance.However,nanostructured Si anode materials have high specific surface area and low tap density,which causes a low initial Coulombic efficiency?ICE?and low volumetric energy density.Those disadvantages greatly hinder the commercial application of nanostructured Si.The micro-sized porous Si?MPSi?assembled by nano-Si units have the merits of both micro and nano-Si materials,such as high tap density and low specific surface area.MPSi can effectively restrain the pulverization of Si and alleviate the volume expansion of Si,which is a promising candidate as anode material for high energy density LIBs.Herein,this paper designed and prepared various high-performance MPSi anode materials and explored their energy storage properties from the aspects of low-cost and large-scale preparation,structural design,performance improvement and electrochemical test.The main research work and innovations are as follows:?1?Rice-ball-like micro-sized porous silicon?NS-MPSi?is prepared by de-alloying Mg-Si alloy to remove Mg at high temperature under vacuum.NS-MPSi is assembled by stacking nano-Si particles,which has a low specific surface area.Nano-Si particles in NS-MPSi can avoid pulverization,and the porous structure can effectively alleviate the volume expansion of micro-sized Si to ensure cycle stability.NS-MPSi shows a high ICE of 81.7%and high specific capacity of 3370 mAh/g.The reversible capacity of 997 mAh/g can be maintained after 250 cycles at a current density of 0.2 C.?2?Ant-nested-like porous micro-sized Si?AMPSi?is synthesized on a large scale by thermal nitriding Mg₂Si in nitrogen followed by the removal of the Mg₃N₂ by-product in an acidic solution.AMPSi is consisted of the interconnected Si nano-ligaments and 3D penetrating channels.The interconnected nano-Si skeleton can avoid pulverization of Si,and 3D bicontinuous channels enable the internal expansion of AMPSi during lithiation,resulting in low electrode swelling.AMPSi has low specific surface area and high tap density?0.84 g/cm³?and high ICE of 86.6%.The capacity of AMPSi@C is 1271 mAh/g even after 1000 cycles at 0.5 C with a capacity retention of 90%.AMPSi@C shows an outstanding volume capacity of 1712 mAh/cm³.The electrode swelling is only 17.8%at a high areal capacity of 5.1 mAh/cm².Full cell consisting of commercial ternary cathode and AMPSi@C anode still has a high energy density of 502 Wh/kg and capacity retention of 84%after 400 cycles.?3?Carbon cage encapsulating marine-sponge-like Si yolk-shell composite?MS-Si@void@WG?is synthesized by thermal nitriding polydopamine-coated Mg₂Si composite followed by acid etching.Polydopamine is carbonized into carbon cage during this process.In MS-Si@void@WG,the continuous Si skeleton is completely encapsulated by carbon cage to form yolk-shell structure,which shows high tap density and enhanced structural stability.Yolk-shell structure can alleviate the volume expansion of MS-Si,maintain high activity of Si and prevent direct contact between Si and electrolyte,avoiding cycle performance deterioration.MS-Si@void@WG has a capacity of 826.6 mAh/g at 0.5C upon 1200 cycles with a capacity retention of 86.1%.At the areal capacity of 2.8 and5.2 mAh/cm²,the electrode swelling is only 7.6%and 14.6%,respectively.?4?Besides,to further improve the conductivity and structural stability of Si,we introduce CNT or GO as high conductive agent to combine with nano-sized Si by self-assembly method to construct secondary micro-sized Si/C composites.Nano-sized Si can provide a high capacity,porous structure can alleviate the volume expansion of Si and CNT or GO can act as the structural skeleton and provide conductive network to enhance the structure stability and conductivity of composites.The Si/CNT composite shows a capacity of is 989.5 mAh/g even after 1000 cycles at 0.5 C with a capacity attenuation of0.035%per cycle and has a rate capacity of 345 mAh/g at 3 C.The Si/GO composite demonstrates a high capacity of 1034 mAh/g after 1000 cycles at 0.5 C with a capacity retention of 79.5%and shows a rate capacity of 497 mAh/g at 2 C.