Research On Modification Technology Of Silicon-based Thin Film Anode Materials For Lithium Ion Batteries

Due to the rapid development of the new energy vehicle market,the widespread application of consumer electronic devices,and the rapid expansion of the energy storage market,the demand for lithium ion batteries has increased speedily.The current commercial application of lithium ion battery anode material is mainly graphite,its theoretical specific capacity is only 372 m Ah g^-1,has been unable to meet the high capacity and high rate of the market demand,limiting the development of lithium ion batteries.Silicon anode material has a high theoretical specific capacity of 4200 m Ah g^-1,as well as low lithium insertion potential and abundant resource reserves,so it stands out among many anode materials.However,silicon anode material has two serious problems,which limit its commercial application.First,the severely volume expansion of silicon during charging and discharging is severe,and it is easy to crush and fall off after multiple cycles,which leads to the instability of electrode structure and poor cycle performance.Secondly,as a semiconductor,silicon has poor conductivity and small li-ion diffusion coefficient,which leads to poor discharge capacity,slow electrochemical reaction and low reversible specific capacity.In view of the above two problems,scientists have explored two methods,nanocrystallization and compounding of silicon.In this research,in order to alleviate the volume expansion of silicon,nano-silicon thin films were prepared by plasma enhanced chemical vapor deposition（PECVD）,and sodium alginate surface modified silicon films（Si/SA）were prepared by simple surface coating process.On the other hand,to improve its conductivity,a sodium alginate coating modified silicon film（Si/SA-CNTs）material containing carbon nanotubes was prepared by constructing a three-dimensional conductive network on the surface of the silicon thin films.The structure and properties of the materials were analyzed by technique of material characterization and electrochemical performance test.The following conclusions were obtained:（1）Silicon thin films tightly bonded to copper current collector were prepared by PECVD.The thickness of the film has a significant effect on the electrochemical performance.The silicon thin film with a thickness of 160 nm has the best electrochemical performance,the initial discharge specific capacity is 4079 m Ah g^-1.After 200 cycles at 0.5 C(1 C=4.2 A g^-1),the discharge specific capacity is reduced to615 m Ah g-1,and the cycle stability still needs to be further improved.（2）Si/SA material was successfully prepared by surface coating process.Its first cycle discharge specific capacity is 3902 m Ah g^-1.After 200 cycles at 0.5 C rate,the discharge specific capacity is still 1360 m Ah g^-1,which is 745 m Ah g^-1 higher than the pure Si thin film.At the same time,it is better than Si thin film at 1 C,2 C,5 C and 10C rates.Especially at 10 C rate,the reversible specific capacity of Si/SA is 1143 m Ah g^-1,while the pure Si thin film is only 746 m Ah g^-1.The sodium alginate coating can not only effectively alleviate the volume expansion of silicon,ensure the structural integrity of the silicon electrode,improve its cycle stability,but also improve the rate performance.（3）Si/SA-CNTs material was successfully prepared by adding three-dimensional conductive network of carbon nanotubes on the surface of Si/SA.Its first cycle discharge specific capacity is 3716 m Ah g^-1,and the discharge specific capacity after200 cycles is 2028 m Ah g^-1,which is 1413 m Ah g^-1 higher than the pure Si thin film,and the capacity retention rate is increased by 35.4%.It has a discharge specific capacity of 1726 m Ah g^-1 at 10 C rate,showing excellent electrochemical performance.The synergistic effect of sodium alginate coating and three-dimensional conductive network not only inhibits volume expansion,but also improves the conductivity of silicon film,showing excellent electrochemical performance.This research revealed that surface coating can effectively alleviate the volume expansion of Si thin film.Furthermore,3D conductive network is constructed on the surface of Si thin film to improve its conductivity.This method has universal application significance to other thin film materials.