| Lithium-ion battery dominates the supply of portable electronic devices.Andit is also increasing in power-typed application such as electric vehicles to ensure sustainable development.Increasing the energy density and power density of lithium-ion battery are the key to improving the mileage.Thus,researchers have been looking for an anode material with high specific capacity,good safety and excellent electrochemical performance to replace commercial graphite negative.Cobalt phosphides have attracted much attention because of their relatively small polarization,high ionic conductivity of Li3P and high specific capacity.However,they also have problems of large volume expansion and poor conductivity.As for that,a multi-phase synergistic phosphorus-cobalt compound and its carbon composite are produced by mechanical ball milling in order to obtain excellent electrochemical performance.Firstly,the synergic nano-CoP/CoP3 is prepared by mechanical ball milling.The results get that when the phosphorus-cobalt ratio is 1:2,the milling time is 40h,and the heat treatment temperature is 600℃,the symbiotic growth phosphorus-cobalt compound shows better electrochemical performance.Actually,the minimum capacity retention is up to 87.8%at 0.2 A/g.Even if at a high current density of 5 A/g,the reversible specific capacity of 373.3 mAh/g is maintained after 1000 cycles,and its capacity retention is as high as 91.49%compared to the third cycle.The results present that the phosphorus-cobalt ratio affects the category of phase,the milling time influences the relative content of the phase,and the heat treatment has effect on the crystallinity and the stability of structure.These factors adjust the phase synergy of the phosphorus-cobalt compound to further improve the electrochemical performance of materials.Then,CoP/CoP3 with intercalated CNT composite is prepared by one-step ball milling of cobalt,phosphorus and CNT.When the content of CNT is 20%,the CoP/CoP3/CNT has best electrochemical performance.Actually,the polarization of the CV curves with different scan rates is reduced by 0.138V compared with CoP/CoP3,and the relative difference of the CV redox peak is reduced from 0.52V to 0.47V.Meanwhile the added CNT increases the discharge capacity of the first cycle and retards the upward trend of capacity.Even though at a current density of 5 A/g,the cycle capacity of the CNT-added sample remaines stable,and it is 265.8 mAh/g higher than that of the CNT-free material after 400 cycles.The results show that CNT increases the depth of charge/discharge at low current,promotes the rapid transmission of ions and improves the stability of electrode material.Finally,the phosphorus-cobalt compound is coated with phenolic resin by soft template method to obtain the CoP/CoP3@C of core-shell structure.The coated carbon layer acts as stress buffer layer and conductive layer.After optimization of carbon content,the CoP/CoP3@C has higher and more stable coulomb efficiency.And its specific capacity after 300 cycles at 5A/g is 616.2 mAh/g,which is nearly 1.6 times that of no carbon coating.Moreover as the scanning voltage rate increases,the potential offset is 0.036 V lower than CoP/CoP3.Then the core-shell structure shows better rate performance.The calculation shows that this is due to the improvement of the ion diffusion performance of the battery properties after the carbon coating. |
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