Preparation Of Transition Metal Phosphide/Carbon Composites And Study On Their Potassium Storage Property

Potassium ion battery has attracted more and more attention due to its low production cost.Graphitic carbon material is the most commonly used anode material for alkali metal batteries,but the theoretical specific capacity is low,so it is urgent to develop a material with high theoretical specific capacity and good cyclic performance.The transition metal phosphide（TMP）is a gap compound with high theoretical specific capacity and has attracted the attention of many researchers.However,its low electrical conductivity and large volume expansion during cycling are still the main obstacles limiting its application.In order to solve the above problems,TMP can be combined with a carbon source to ensure the integrity of the material during the cycles.Meanwhile,carbon with good electrical conductivity can provide a fast channel for ion transport.In this study,FeP@CNTs,C@Fe₂P₄O₁₂@CNTs and M_xP_y@BNCNTs composites were prepared by combining transition metal phosphates with carbon nanotubes.Through adjusting the ratio of the TMP and carbon nanotubes to confirm the samples with the best electrochemical performance.The specific research contents as follows:（1）With hydrothermal synthesis,FeP and Fe₂P₄O₁₂ nanoparticles were designed to attach to the surface of carbon nanotubes,and Fe₂P₄O₁₂ was coated with a thin layer of carbon,thus FeP@CNTs and C@Fe₂P₄O₁₂@CNTs materials were prepared.We adjusted the ratio of FeP,Fe₂P₄O₁₂ and carbon nanotubes and compared the structure and electrochemical performance.The combining FeP with carbon nanotubes can repress the agglomeration phenomenon of the composite material during the cycle,but after 140 cycles,the specific capacity of FeP@CNTs composite is only 46.6 m Ah g^-1.By coating Fe₂P₄O₁₂@CNT with carbon layer,the volume expansion of the material during potassium ion embedding/removal is effectively alleviated.The C@Fe₂P₄O₁₂@CNTs can still maintain a high specific capacity of 141.8 m Ah g^-1 after 1000 cycles under the current density of 0.5 A g^-1.By comparing the above two experimental results,it could be found that only attaching iron-based phosphide on the surface of carbon nanotubes cannot effectively improve the cycle stability,and the volume expansion of material during the cycle process will still reduce the cycle life of the battery.The improvement method is to coat the outer layer of iron phosphide with a thin layer of carbon,so as to inhibit the volume change of the material when potassium ions are embedded and removed.（2）Through using Fe（NO₃）₃·9H₂O,Co（NO₃）₃·6H₂O and Ni（NO₃）₃·6H₂O as metal sources,boric acid and urea as boron and nitrogen sources,Fe,Co and Ni nanoparticles were confined in boron and nitrogen doped carbon nanotubes.Then,FeP@BNCNTs,Co P/Co₂P@BNCNTs and Ni₂P@BNCNTs materials were synthesized by phosphating in double temperature zone.The nanotubes act like the outer skin of a pea to protect the transition metal phosphide nanoparticles inside,which effectively suppresses the material’s volume expansion during the cycles.By adjusting the ratio of M_xP_y to carbon nanotubes,the sample with the best electrochemical performance was confirmed.under the current density of 1 A g^-1,after 1000 cycles,the specific capacity of FeP@BNCNTs is 123.6 m Ah g^-1,and the Ni₂P@BNCNTs is 112.3 m Ah g^-1.Co P/Co₂P@BNCNTs has the best cycle performance,which still maintains 163.2 m Ah g^-1,and the average coulomb efficiency is close to 99%.The overall experimental results show that the design of confining transition metal phosphide to the interior of carbon nanotubes can well ensure the integrity of the material during the cycling process,thus M_xP_y@BNCNTs composite materials exhibit better cycling performance.