Optimization Of Silicon Anode And Preparation Of High-performance Silicon-based Lithium-ion Battery

In order to meet the explosive growth of the new energy vehicle market and the application needs of large-scale energy storage power stations,it is urgent to improve the energy density of lithium-ion batteries.Silicon has become the most promising anode material for next-generation high-energy lithium-ion batteries due to its ultra-high specific capacity.However,due to its alloyed lithium-deintercalation mechanism,it will inevitably cause a huge volume effect of the material,and the resulting stress will lead to the fragmentation of silicon particles and the continuous growth of the SEI film on the silicon surface,resulting in battery capacity decline and electrode polarization growth,which has become a key issue restricting the application of Si electrodes.This paper takes nano-spherical silicon as the research object.From the optimization of electrolyte and interface,the compatibility of silicon anode and electrolyte is improved to stabilize the SEI film on the silicon surface,so that the cycle stability and rate performance of silicon anode have been improved.Further,the influence of cathode types on Si anodes was investigated,and it was found that different types of cathode materials also had a significant effect on the surface SEI of Si anodes,which pointed out a new direction for the research of Si-based full batteries.The main contents and results of the paper work are as follows:(1)The influence of lithium salt types on battery performance was studied.We selected six common lithium salts to configure unary,binary and ternary electrolytes.Compared with the other four lithium salts,the silicon anode has better cycling stability and rate capability in LiTFSI and LiFSI electrolytes.Then we use LiPF6 as the main salt,LiTFSI and LiFSI as auxiliary salts,configure four di-salt electrolytes and three tri-salt electrolytes.By optimizing the lithium salts,it is found that when LiPF6,LiTFSI and LiFSI are used as the lithium salts in the molar ratio of 7:1:2,the Si anode exhibits the best electrochemical performance.Under the condition of 0.5 C charge and discharge,the Si anode after 500 cycles,a high specific capacity of 1750.82 mAh g-1 is still maintained.At 30 C rate,the specific capacity is still as high as 1851.4 mAh g-1,which is almost six times that in LiPF6 electrolyte.(2)The surface coating of organic functional molecules was developed,and the SEI technology was grown in situ.An organic small molecular acid was selected.The mucic acid was used to coat the surface of the Si material.The hydroxyl and carboxyl groups in the mucic acid were abundant on the surface of Si particles.The hydroxyl groups are bonded,so that the mucic acid grows uniformly on the surface of the Si particles,and the Si@MA composite material is obtained.Due to the flexibility of mucic acid,the MA coating effectively relieves the huge stress caused by the volume expansion of Si,and its lithium-ion storage property enables Si@MA material to have better rate capability;Nano-silver particles were introduced into the Si@MA*Ag electrode material.The addition of Ag particles improves the overall electronic conductivity of the material,and can better generate more inorganic components in the SEI,such as Li2CO3 and LiF,and the SEI becomes more stable.The results show that the Si@MA*Ag electrode has the most excellent electrochemical performance:at 0.5 C,the reversible capacity after 500 cycles is as high as 1567.8 mAh g-1.The specific capacities remain at 2078.8 mAh g-1 and 1741.3 mAh g-1 at the rate of 84 and 126 mAh g-1.(3)The influence of cathode types on Si-based full cells was studied.In view of the poor performance of Si-based full cells,it is difficult to be widely used.Through the research on Si-based full cells of different cathode types,we found that different types of cathodes matching,there are different electrochemical performances in Si-based full cells.By comparing the electrochemical performance of LFP,NCM523,LCO,and LMO half-cells with LFP//Si,NCM523//Si,LCO//Si,and LMO//Si full cells,as well as the verification of the corresponding commercial cathode sheets,it was found that different types of cathodes affect the distribution of electrolyte degradation products.Compared with several other cathodes,NCM523 is more suitable for assembling silicon-based full cells to achieve greater rate performance and long cycle life:at 0.2 C current density,the capacity retention rate after 80 cycles is the highest,62.2%,at 10 C rate,the capacity retention rate was 44.7%.