Research Of The Preparation Of Nano-silicon By Mechanical Grinding In-situ Coating And Its Electrical Properties

Silicon(Si) anode material have received widespread attention due to the excellent specific capacity(theoretical specific capacity:4200 m Ah/g),and it is a popular choice for a new generation of anode materials.However,the huge volume change during the charging and discharging process easily causes the electrode material to powder and fall off,and the unstable SEI will continue to consume Li⁺during the cycles,resulting in rapid capacity degradation.Nanometerization of silicon materials and surface coating with carbon materials can limit the volume expansion of silicon during charging and discharging,and can also avoid direct contact with other electrolytes,effectively improving battery capacity and cycle stability.In this thesis,the nano-silicon dispersion was prepared by the mechanical grinding method,and the carbon material was used for composite coating during grinding.The conclusions are as follows:Firstly,the mechanical grinding method was used to prepare nano-silicon dispersion.The influence of different dispersant types,different grinding processes and different dispersant dosages on the preparation process of nano-silicon dispersion were systematically studied.Used the best dispersant(A03 acetamide dispersant)and the segmented grinding method within the test range.The first stage 0.6 mm grinding media was used to coarsely grind the particles,and then 0.3 mm grinding media was used for fine grinding.Dispersant content was 40%of the solid content,and the filling rate and energy consumption of the two-stage grinding were 80%,0.04 k Wh/g and 90%,0.09 k Wh/g,and finally a stable nano-silicon dispersion with a D₉₀ of143 nm could be obtained.The package was modified to improve the cycle stability of the battery.In order to improve the cycle stability of the nano-silicon negative electrode,the 0-dimensional carbon material acetylene black were used as the wrapping material,and the carbon black dispersion was prepared by the mechanical grinding method.Grinding was carried out with 0.3 mm grinding media and B01 dispersant at a filling rate of 90%.When the grinding energy consumption reached 0.05 k Wh/g,a dispersed and stable acetylene black dispersion with D₉₇=96 nm was obtained.By successively mixing nano silicon dispersion,acetylene black dispersion and artificial graphite in a sand mill,a spatial structure of nano silicon/acetylene black/artificial graphite silicon carbon composite material(Si@C@graphite)was prepared.At a current density of 1C(1200 m Ah/g),after 50 cycles,the capacities of SAG 01-04 were 374.5m Ah/g,414.3 m Ah/g,436.7 m Ah/g and 467.8 m Ah/g,which were 93.44%,92.23%,90.36%and 89.68%of the second cycle capacity.In order to further improve the capacity retention rate of the silicon-carbon anode material,it was necessary to select a more suitable carbon material as the wrapping layer.In further work,in order to improve the cycle stability and capacity retention rate,one-dimensional carbon material multi-walled carbon nanotubes were used as the wrapping material,and the carbon nanotube dispersion was prepared by the mechanical grinding method,and the B02 dispersant and the segmented grinding method were used.In the first stage,0.6 mm grinding media was used to coarsely grind the particles,and then 0.3 mm grinding media was used to coarsely grind the particles.The amount of dispersant is 60%of the solid content.When the total energy consumption of grinding reached 0.11 k Wh/g,we could obtain a well-dispersed carbon nanotube dispersion liquid.By successively mixing nano silicon dispersion,multi-wall carbon nanotube dispersion and artificial graphite in a sand mill,a spatial structure of nano silicon/multi-wall carbon nanotubes/artificial graphite silicon carbon nanotube composite material(Si@SWCNTs@graphite).At a current density of 1C(1200 m Ah/g),the first discharge specific capacity of the SCG01-03 were 371.4 m Ah/g,398.3 m Ah/g and 409.2 m Ah/g;after 50 cycles,the capacities of SCG 01-03 were 345.1 m Ah/g,366.7 m Ah/g and 379.4 m Ah/g,which were 96.67%,92.06%and 92.71%of the second cycle capacity.The 0-dimensional or 1-dimensional carbon material coating of the silicon particles can ensure the good electrochemical performance of the battery by inhibiting the volume expansion and preventing the repeated growth of the SEI caused by the continuous contact between the silicon surface and the electrolyte.With the increase in silicon content,the circulation capacity of the sample has also been continuously increased.Compared with the Si@C@graphite series samples,the Si@SWCNTs@graphite samples have a higher capacity retention rate at the same silicon content,because the meshed carbon nanotube structure makes the electrode material more stable,but the stable capacity decreases slightly because the lithium insertion of the carbon nanotubes consumes part of the lithium ions during the cycle.