Study On The Electrolyte For Si/C Anode Of Lithium-ion Batteries

Rechargeable lithium-ion batteries have been world widely investigated and used because of their high energy density, long lifespan, no memory effect and environmental benignity. Silicon is considered to be one of the most attractive and promising anode material for the next generation lithium-ion batteries. Silicon anode has advantages over other secondary batteries, such as high theoretical specific capacity, low lithium intercalation voltage and low cost. However, its application is seriously restricted by its poor cycling performance, which is ascribed to the large volume change （>300%） during lithium intercalation and deintercalation process.The solid electrolyte interphase （SEI） film is formed by the reductive decomposition of electrolyte solution on the silicon anode surface during lithium intercalation process. Stable and dense SEI film can prevent the further decomposition of electrolyte and protect the silicon anode from cracking. The component of electrolyte plays on an important role on the properties of SEI film. However, the research about the electrolyte for lithium-ion batteries is mainly for carbon anode. Here, we improved the electrochemical performance of silicon anode by optimizing the electrolyte components and additives.The effect of the ratio of ethylene carbonate （EC）/diethylene carbonate （DEC） on the electrochemical performance of Si/C was studied. Results showed that with the increasing of the content of EC, the cycling stability and rate capability increased first and then decreased. When the ratio of EC/DEC was 40:60, the electrode showed the best cycling stability with a capatity retention of 46.2% after 100 cycles. The electrode exhibited the best rate performance as the EC/DEC was 35:65. It was ascribed to that the components of SEI film changed and the cracks on the electrode surface decreased. The electrode in electrolyte with 35% EC showed the lowest charge transfer impedance. The initial charge capacity increased with the increase of the concentration of LiPF6. The electrode showed the best rate capability as the concentration of LiPF6 is 1.2 mol/L.The effect of organic film-forming additive fluorinated ethylene carbonate （FEC） and vinylene carbonate （VC） on the electrochemical performance of Si/C anode were analyzed. The mechanism was researched at the same time. Results indicated that FEC could obviously improve the electrochemical performance of Si/C anode. After the addition of FEC, the coulombic efficiency increased from 97% to 99% after 20 cycles. Moreover, the initial charge capacity, cycling stability and high rate capability increased with the increasing of the content of FEC. But the electrochemical performance deteriorated again when FEC amount passed through 10%. As the content of FEC was 15%, the Si/C anode showed the highest initial charge capacity of 2681 mAh/g, with a growth of 10% compared with that of the electrode in FEC-free electrolyte. After 100 cycles, the capacity retention of the electrode in 15% FEC-containing electrolyte was 60.7%. The charge capacity was as high as 1549 mAh/g at a current density of 12.5 A/g with a growth of 4 times compared with that of the electrode in FEC-free electrolyte. During the lithium intercalation process, FEC was reduced prior to electrolyte and formed a SEI film which was more stable, denser and thinner. The SEI film stabled the electrode structure. The decrease of SEI resistance and electronic transfer impedance could be seen. The analysis indicated that the SEI film on the electrode surface in FEC-containing electrolytes was composed of ROCO2Li, Li₂CO₃, LiF and poly（FEC）. The addition of VC effectively improved the cycle life of Si/C anode. The cycling stability increased with the increasing of the content of VC. When the content of VC was over 3%, the cycling stability of the electrode decreased again. The charge capacity retention of the electrode in 3% VC-containing electrolyte was 80.7% after 100 cycles, which was much higher than that of the electrode in VC-free electrolyte. It was mainly due to the denser SEI film with better mechanical properties after the addition of VC. The cracks on the electrode surface decrease. The SEI film mainly consisted of ROCO2Li, Li₂CO₃ and LiF and polycarbonate.K₂CO₃ was evaluated as an inorganic film-forming additive for Si/C anode. It was found that the addition of K₂CO₃ improved the electrochemical performance of Si/C anode. The initial charge capacity increased from 2377 mAh/g to 2671 mAh/g after the addition of 10% K₂CO₃. Furthermore, the cycling stability and high rate performance increased at the same time. After 100 cycles, the capacity of the electrode with 10% K₂CO₃ was 1379 mAh/g, with a capacity retention of 51.6% and the electrode showed capacity of as high as 1358 mAh/g at high current density of 12.5 A/g. The analysis results indicated that the content of Li₂CO₃ increased and denser SEI film was formed on the surface of Si/C electrode.