Research On Structure Design And Pre-lithiation Technology Of Silicon Carbon Anode Materials For Lithium Ion Batteries

Due to its high theoretical specific capacity of 4200 m Ah g^-1,abundant sources and reserves,semi-conductor silicon material is regarded as the ideal negative electrode material for the next-generation lithium ion battery.However,the huge volume change and low intrinsic conductivity of silicon-based materials have brought about many unfavorable factors such as serious powdering of the material,active materials shedding of the electrode,low Coulomb efficiency and poor cycle performance.In order to solve these problems,many reports have proposed solutions from various aspects such as nanometerization,hollowing,and material cladding recently,but the complex structure design brings high preparation cost,which makes it difficult to be applied in practice.Therefore,the development of simple and low-cost micron Si/C composite preparation technology and pre-lithium technology as well as the improvement of cyclic stability has become the focus of current research.In this paper,commercial micron silicon is used as raw material,which is compounded with graphite through ball grinding process,and H2O2 is added during this process to carry out partial oxidation modification on the surface of silicon material.Subsequently,the Si/G/C composite material is obtained by covering the pyrolysis carbon of the pitch.The results show that adding proper amount of H2O2 in the ball mill can partially oxidize the surface of silicon material and contribute to the increase of particle uniformity.In addition,it also has a certain protective effect on the graphite matrix.In terms of electrochemical performance,the addition of a small amount of H2O2 increases the first coulombic efficiency of Si/G/C materials from 77.4% to 83.5%,and can exceed 99% in the seventh cycle;The first reversible specific capacity reached 974 m Ah g^-1,which increased the rate by 18.2%,and still provided a high specific capacity of 848 m Ah g^-1 after 50 cycles;For rate performance,compared with 0.1 A/g,the specific capacity of 49.2% can be retained in 5 A/g,and it still has a high specific capacity of 860 m Ah g^-1 after returning to 0.2 A/g,which indicates that it has excellent rate performance.Due to the low coulombic efficiency of this material for the first time,we chose the electrochemical method for lithium compensation of the Si/G/C negative electrode.The degree of pre-lithiation can be controlled by controlling the external circuit resistance and short circuit time.The results show that with an external resistance of 100 Ω,a high degree of pre-lithiation can be achieved in only 30 minutes.Considering the safety of the electrochemical pre-lithiation with lithium metal and the complexity of the device in practical applications,in this paper,a new Li₂S-PAN prelithium reagent was designed by combining with chemical pre-lithium process.Li₂S-PAN material has good compatibility of ester electrolyte system.It has a lithium compensation specific capacity of 668 m Ah g^-1 in the range of 2.5-4.0 V.After releasing the stored active lithium for the first charge,it will not adversely affect the electrode matrix.In addition,Li₂S-PAN material can be dispersed with KB in tetrahydrofuran solvent to prepare pre-lithiated slurry,which can be quantitatively coated on the surface of Li Fe PO4 electrode,combined with mechanical pressing process to enhance the bonding strength between pre-lithiated layer and substrate.The Li Fe PO4 cathode with 20 μL and 40 μL prelithiation slurry show a significantly increased first-cycle charge capacities,10.5% and 22.3% higher than of the pristine Li Fe PO4 cathode,indicating a high lithium compensation efficiency of Li₂S-PAN.The full-cell of Li Fe PO4/Li₂S-PAN cathode and micron-scale Si/G/C anode have a reversible capacity of 123 m Ah g^-1 for the first cycle,which was 15% higher than the pristine counterpart and this value can up to 44.1% after 30 cycles.Moreover,the prelithiation process is easy to be compatible with the roll process in the industrial preparation,which makes the scheme has a good application prospect.