Preparation Of Silicon Carbon Anode Materials And Research On Pre-lithiation Technology

Silicon-based materials have become ideal anode materials for lithium-ion batteries because of their ultra-high theoretical specific capacity（3587 m Ah/g）,suitable voltage platform and abundant reserves.However,the severe volume change of silicon-based materials during charge and discharge,which can cause problems such as material pulverization,active material losing contact with current collecotrs,and SEI crack and re-growth,will lead to cell failure.The silicon-carbon composite is an effective way to solve the above problems.On the one hand,carbon materials can be used as a matrix to ensure the conductivity of the electrode materials.On the other hand,carbon materials can also buffer the volume expansion of silicon-based materials.Therefore,the design of silicon-carbon composites with different structures has become the focus of current research.In this paper,a silicon-carbon composite material with better cycle stability was prepared by optimizing experimental conditions,and then the first coulombic efficiency of the material was improved by the prelithiation method,which provide a good experimental basis for the improvement of the energy density and cycle performance of the full battery based on the silicon anode.A silicon/hard carbon/soft carbon composite was firstly constructed using nano silicon,corn stalks,and pitch as raw materials.Small particle size nano silicon can effectively solve the problem of material pulverization,the pore structure of hard carbon materials can buffer volume expansion,and soft carbon can avoid direct contact between nano silicon and electrolyte.Using Ca Cl₂ and KOH to treat the carbonization process of corn stalks,hard carbon with different pore structures can be obtained.The hard carbon treated with Ca Cl₂ is mainly mesoporous,and the silicon/hard carbon/soft carbon composite is further constructed to obtain the best electrochemical performance.The initial reversible capacity of this silicon-carbon material is 969 m Ah/g.After 100 cycles at a current density of 0.5 A/g,the capacity retention rate is as high as 74.3%.Further explore the influence of silicon content on the electrochemical performance of the material.Although the specific capacity of the first few cycles has significantly improved after the silicon content is increased,the subsequent capacity decays rapidly,and the final optimized silicon content is 20%.The capacity retention rate of pure nano-silicon is only 29.7%,and under the same（dis）charge conditions,the silicon-carbon material designed in this paper has better cycle stability.Since the initial coulombic efficiency of silicon-carbon materials is low,various pre-lithiation technologies are required to compensate active lithium prior to cell operation to realize the commercialization of silicon-carbon materials.Chemical prelithiation has the most practical value due to its simple operation,but the most commonly used chemical prelithiation reagent at present is lithium-biphenyl/1,2-dimethoxyethane（Li-Bp/DME）.The prelithiation using Li-Bp/DME on the silicon-based negative electrode is poor,which is limited by its high redox potential.In order to solve this problem,this paper uses electrolysis for the first time to successfully achieve the prelithiation of silicon-based materials using Li-Bp/DME.With Cu as the anode,Si/C as the cathode,and Li-Bp/DME as the electrolyte,lithium foils are added between the cathode and anode to inhibit the transfer of intermediate Bp molecules from the anode to the cathode.Feasibility of this scheme is verified by a three-electrode system.Further,the degree of prelithiation of silicon-based materials can be simply controlled by adjusting the electrolysis voltage.With the increase of the electrolysis voltage,the initial coulombic efficiency has significantly improved,but due to the formation of irreversible products during the prelithiation process,the specific capacity of the half-cell has slightly decreased.But in the full cell,after pre-lithiation,the specific capacity has increased from 80 m Ah/g to 110 m Ah/g,and the energy density has therefore increased by 37.5%.This new pre-lithiation scheme can control the degree of pre-lithiation by adjusting the voltage.Compared with chemical pre-lithiation,which control the degree of pre-lithiation by controlling the reaction time,this method is obviously more precise and thus has good practical value.