Momdification Of Silicon Based Anode Materials For Lithium-ion Batteries

The developing of next generation of lithium-ion battrries with higher energy density is mostly detemined by finding and matching better cathode and anode materials.Among anode materials,silicon-based materials?including Si and SiO_x?attracted a great deal of attention due to their ultra-high specific capacity and low voltage platform.However,huge volume expasion?Si⁴20%,SiO²70%?occurred during the?de?lithiation process,resulting in a series of problems,such as the pulverization of active materials,electrical contact failure among solid-solid interface and the continuous formation of solid-liquid interface?SEI?,and so on.Finally,it leads to the serious decline of cycling stability.Although the modification of silicon-based anode materials is widely studied at present,there are still few technologies that can achieve high cycling stability with low-cost and mass-production.In addition,SiO_x also has low initial Coulomb efficiency?ICE?,which seriously limits its industrialization development.In order to improve the cycle stability,reduce the synthesis cost of silicon-based negative materials and improve the initial Coulomb efficiency of SiO_x anodes,the modification approaches and techniques has been studied in this paper.The main contents are as follows:?1?Graphene nanoscroll-wrapped silicon composite?Si@GNS?The composite of one dimensional flexible graphene nanoscroll-wrapped silicon nanoparticles?Si@GNS?was realized by a simple cold quenching method followed by a facial annealing process.The silicon nanoparticles are uniformly and firmly attached to the interlayer galleries of GNS nanosheets assisted by adding sodium citrate.Supported by the excellent physical properties of GNS,such as high conductivity,strong flexibility and high mechanical strength,as well as special one-dimensional porous milti-walled carbon tube structure to accomadate the huge volume expansion of silicon,the as obtain Si@GNS materials exhibit ultra-high cycle stability and excellent rate performance.For example,at0.2 A·cm^-2 current density,the reversible capacity of 1000 cycles is kept stable at 1300mAh·g^-1,and the capacity retention rate is more than 83%.?2?Dual-carbon modified SiO_x composite?SiO_x/C@C?A kind of dual-carbon modified SiO_x composite?SiO_x/C@C?was successfully prepared by a three-step of ball-milling,spray-drying and traditional CVD carbon coating process.The first carbon modification layer was obtained by the carbonization of organic carbon source,and the second layer was obtained from the cracking of ethylene.Profitted from the dual-carbon modification layers not only greatly improve the conductivity of SiO_x anode material,but also effectively buffer the huge volume effect of SiO_x,the obtained SiO_x/C@C composite exhibits excellent cycling performance.Furthermore,we also studied the effects of different organic carbon sources on the electrochemical performance of SiO_x/C@C,including macromolecular aromatic-structured pitch?SiO_x/Pit-C@C?and micromolecular ring-structured glucose?SiO_x/Glu-C@C?and macromolecular chain-structured polyvinylpyrrolidone?SiO_x/PVP-C@C?.The results showed that the performance of the SiO_x/Pit-C@C composite is the most excellent.?3?Tin-modified SiO_x?SSO?compositeTin-modified SiO_x composite was obtained by a two-step high energy ball-milling?HEBM?method.The first step of HEBM process was to reduce the size of SiO_x particles,and the second step of HEBM process was to prepare SSO composite with nano-Sn particles uniformly distributed in SiO_x phase.Compared to nSiO_x,SSO composite exhibited not only an effective improvement on the cycle stability,but also,more importantly,a great improvement?13%¹8%?on the initial coulomb efficiency?ICE?.For this reason,we have made an intensive investigation on the mechanism of great ICE enhancement of SSO composite by means of non-in-situ XRD and XPS techniques.