Controllable Fabrication And Lithium Storage Behavior Of Silicon-Based Anodes

Owing to its unique advantages such as high theoretical specific capacity(4200 mAh g^-1,Li₂₂Si₅),low working voltage,and abundant reserves,silicon（Si）is considered as one of the promising anode materials for next-generation high-energy-density lithium-ion batteries（LIBs）.However,there are still some crucial challenges like large volume effect,unstable solid electrolyte interface（SEI）,and slow reaction kinetics that restricts the commercialization of Si anodes.In this work,the gas-phase separation and in-situ reduction of CO₂ methods were designed to regulate and optimize the structure of Si anodes.As a result,the as-obtained Si anode exhibited rapid reaction kinetics,small volume effect,and stable SEI.Thus,the improved lithium storage performance is realized.The main research achievements obtained are as follows.（1）Two-dimensional Si（2DSi）was controllably synthesized from commercial CaSi2 by a green gas-phase separation method.In this way,the structure,morphology,and component of 2DSi could be regulated.The uniformly dispersed layered structure could inhibit volume effect.The large specific surface area may promote ion transport and electrolyte diffusion.At 5000 mA g^-1,the 2DSi-900 anode showed a discharge capacity of 835 mAh g^-1 after 3000 cycles with a capacity retention of 90.92%.（2）The layered Si/C composites（L-Si/C）were controllably prepared from commercial CaSi2 with CO₂ as a carbon source.The layered construction of Si as well as the inner structure of carbon coating layer could be regulated via process adjustment.When the temperature was increased from 550℃ to 750℃ the L-Si/C products showed a uniform layered structure with an increase in the content and graphitization degree of carbon coating layer.The homogeneous layered structure could promote reaction kinetics and reduce volume effect.In the meantime,the controlled carbon coating layer was able to improve electronic conductivity,stabilize SEI and reduce volume effect as a protective layer.As a result,L-Si/C750 showed improved Coulombic efficiencies,cycling property,and rate performance.（3）The nanoporous Si@C composites（NPSi@C）were controllably constructed from commercial Mg₂Si with CO₂ as a carbon source.Thus,the porous structure of Si and the content and graphitization degree of carbon coating layer could be regulated.With temperature increased between 500℃ and 700℃,the NPSi@C products displayed uniform porous structure,improved specific surface area,and enhanced porosity,as well as increased content and graphitization degree of carbon coating layer.The reduced volume effect,promoted electrolyte diffusion,and accelerated ion transport were achieved with the continuous porous structure.In addition,the homogeneous carbon coating layer could not only promote reaction kinetic by improving electronic conductivity,but also reduce volume effect and stabilize SEI as a protective layer.As a result,the NPSi@C-700 anode exhibited improved Coulombic efficiencies,cycling property,and rate performance.（4）The nanoporous Si@N-doped carbon nanosheets frameworks（NPSi@NCNFs）were controllably and environmentally friendly fabricated by gas-phase separation method from commercial Mg₂Si with g-C₃N₄ as both carbon source and nitrogen source.The porous structure of Si and the inner structure of N-doped carbon coating layer could be regulated with different reaction conditions.As the temperature was increased from 600℃ to 800℃,the porosity,specific surface area,and carbon layer content of NPSi@NCNFs products increased and then decreased.In the meantime,the graphitization degree was improved and the N doping amount of carbon coating layer was reduced.The controlled carbon coating layer could improve electronic conductivity to promote reaction kinetics and function as a protective layer to reduce volume effect and stabilize SEI.Plus,the small volume effect,rapid electrolyte diffusion and fast ions transport were accomplished with the homogeneous and continuous porous structure.At 5000 mA g^-1,the NPSi@NCNFs-700 anode exhibited a discharge capacity of 822.2 mAh g^-1 with a capacity retention of 95.68%.