Preparation And Low-temperature Electrochemical Performance Of Amorphous Carbon Cathode Materials For Lithium-ion Batteries

The electrode material is an important factor affecting the low-temperature performance of lithium-ion batteries.In the anode,the diffusion of lithium ions in graphite anode is one of the important bottlenecks limiting the low-temperature performance of lithium batteries.Compared with graphite material,amorphous carbon material has good ion diffusion performance and is a preferred class of low-temperature lithium-ion battery cathode material.However,its specific surface area,layer spacing,and structural defects can affect the low-temperature performance of the material,so it is necessary to explore the diffusion pattern of lithium ions in amorphous carbon materials for anode material development.In this paper,coal bitumen was used as raw material and charred at different temperatures to make coal bitumen carbon materials with different charring temperatures;then nitrogen-doped porous carbon materials were synthesized with coal bitumen as the charcoal source,urea as the porogen and nitrogen source,and Si O₂as the template;finally,highly nitrogen-doped amorphous carbon materials were obtained by bitumen-assisted g-C₃N₄ charring;the preparation process and the influence of elemental doping on the three carbon materials were studied.The effect of the preparation process and elemental doping on the microstructure and low-temperature electrochemical properties of the three carbon materials were investigated,which showed thatAs the charring temperature increases,the layer spacing gradually decreased,the amorphous degree gradually decreased,the specific surface area and pore volume also gradually decreased,the first Coulomb efficiency and capacity retention gradually increased,and the discharge specific capacity decreased.The difficulty of diffusion of lithium ions between layers at low temperature was the main reason for the capacity decrease of lithium-ion battery anode materials at low temperature.Among the four specimen,asphalt charred at 800℃had the highest charge/discharge specific capacity at low temperature,and the discharge specific capacities at 25℃,0℃,-20℃and-40℃were 335,272,232 and 187 m Ah/g,among which 55.8%of the room temperature discharge specific capacity could be retained at-40℃,and the amorphous char material had better charge/discharge performance at low temperature.With the increase of urea addition,the amorphous degree increased,the layer spacing gradually decreased,and the pore volume and specific surface area first increased and then decreased.The maximum specific surface area of 167.64 m²/g was obtained when the addition amount was 20%,and the number of microcrystalline layers～5 layers,which resulted in a porous carbon material with few microchip layers and large specific surface area,but the nitrogen doping amount was less through urea doping.With the increase of urea addition,the discharge specific capacity of the material at low temperature showed a trend of rising and then decreasing,which was similar to the change law of specific surface area,that is,the large specific surface area was beneficial to the performance of the material in charging and discharging at low temperature.The highest specific capacity of PN-2 at low temperature was obtained at 20%urea addition,and the average specific capacities at 25℃,0℃,-20℃and-40℃were 372,295,254 and 197 m Ah/g.With the increase of g-C₃N₄ addition,the intensity of diffraction peak gradually decreased,the degree of amorphous gradually increased,and the layer spacing gradually increased;the nitrogen content gradually increased,and the highest nitrogen doping content reached 10.93%,and the presence of nitrogen was mainly in the form of pyridine nitrogen,pyrrole nitrogen and graphite nitrogen;the specific surface area and pore volume showed a trend of first increasing and then decreasing,in which the asphalt to g-C₃N₄ ratio of 1:1 has the largest specific surface area and pore volume,which were 106.38 m²/g and 0.295 cm³/g,respectively.With the increase of g-C₃N₄ addition,the specific capacity of discharge at low temperature showed a trend of increasing and then decreasing,which was similar to the change of specific surface area.Among them,the highest discharge specific capacity was achieved at low temperature with the ratio of g-C₃N₄ to asphalt of 1:1,and the discharge specific capacities at-40°C and-60°C reached 235 and 161m Ah/g,respectively.the increase of the first coulomb efficiency was beneficial to the capacity of the carbon material at low temperature,and the high nitrogen element doping increased the first coulomb efficiency to a certain extent in addition to the increase of the lithium storage capacity.The paper has 46 figures,13 tables and 119 references.