Study On Surface Modification Of Silicon Anode Materials For Lithium Ion Batteries

With the development of economy and technology,the popularity of smart phones and wearable devices has increased the expectations of lithium-ion batteries with high energy density for computers,communications and consumer electronics.Moreover,with the increasing environmental protection issues,governments have introduced policies to encourage the development of new energy vehicles,which stimulates the market demand for power lithium-ion batteries with higher energy density.Therefore,silicon?Si?materials have received much attention due to their high theoretical specific capacity of 4200 mAhg^-1.However,Si anode materials for commercial lithium-ion face many challenges because of the huge volume expansion?>300%?during cycled lithiation process and low electrical conductivity.In particular,Si particle materials easily formed a natural oxide layer in the air,resulting in an increase in contact resistance.Thus,in this paper,first of all the Si particles were modified by silver?Ag?to prepare the Si/Ag particles and then the different Si/Ag particles were fabricated by selecting Si particles with different resistivity to obtain low-resistivity Si/Ag particles and the effect of the matching degree of work function between Si and Ag on the properties of Si/Ag particles was studied.Si/Ag@SiOx particles were successfully synthesized by the self-selective electroless deposition and wet oxidation methods.?1?The p-type Si wafers with bulk resistivity of 1?·cm were refined into sub-micron silicon particles less than 1?m via ball-mailing methods.These particles were reacted in 300 ml mixed solution with 6 mM AgNO3 and 1 M HF for 5 min to prepared Si/Ag particles.Benefiting from the excellent conducting channel provided by Ag nanoparticles,the powder resistivity of Si/Ag particles is decreases by 4.7×10⁴ times compared with that of Si particles.Besides,Si/Ag particles hold 263 mAhg^-1 higher capacity than Si particles at the current density of 2 C.?2?The powder resistivity of 0.001-?·cm-Si/Ag particles obtained from p-type Si with bulk resistivity of 0.001-?·cm is only 23.5 m?·cm,resulting in the excellent electrochemical performance.At the rate of 2 C,the discharge capacity of 0.001-?·cm-Si/Ag particles is 302 mAhg^-1 higher than that of 0.001-?·cm-Si particles,and increased by 443 mAhg^-1 over 1-?·cm-Si particles.At the current density of 0.1 C,0.001-?·cm-Si/Ag particles hold 1267 mAhg^-1 in capacity after 100 cycles.?3?The matching degree of work function between Ag and n-type 1?·cm Si is completely different from that between Ag and n-type 0.001?·cm Si.Benefiting from the matching of work function between n-1-?·cm-Si and Ag,the discharge capacity of n-1-?·cm-Si/Ag particles is 683 mAhg^-1 at a current density of 2 C,which is 40%higher than that of n-0.001-?·cm-Si/Ag particles.Moreover,the powder resistivity of n-1-?·cm-Si/Ag particles is only half that of n-0.001-?·cm-Si/Ag particles.For the mismatch of work function between n-0.001-?·cm-Si and Ag,at the current density of0.1 C,n-0.001-?·cm-Si/Ag particles hold 250 mAhg^-1 lower capacity than n-1-?·cm-Si/Ag particles after 100 cycles.?4?For that Ag nanoparticles were embedded into SiOx layer and connected with Si-core,the core-shell Si/Ag@SiOx particles improve the conductivity.Thus,Si/Ag@SiOx particles hold the discharge capacity of up to 868 mAhg^-1 at a current density of 2 C.Due to that the buffering effect of the SiOx layer,Si/Ag@SiOx particles show a discharge capacity of 1453.2 mAhg^-1 for the current density of 0.1 C after 100cycles,which is 2.3 times of that of Si particles.