Study On The Electrochemical Performance Of Transition Metal Phosphides As Electrode Materials For Sodium/Potassium Ion Batteries

In order to reduce the consumption of non-renewable energy consumption,sustainable,low-cost energy storage devices have emerged.Among these energy storage devices,lithium-ion batteries have received extensive attention due to their excellent electrochemical performance.The continuous consumption of lithium resources and the uneven distribution of lithium resources have led to a rapid increase in its price.Sodium and potassium are abundant in the crust.Therefore,exploring sodium ion and potassium ion batteries has become a current research hotspot.Transition metal phosphate is a kind of phosphide composed of metal ions and phosphate anions.It has been widely studied in lithium ion batteries,but it exhibits serious volume expansion and slow electron migration rate in sodium/potassium ion battery applications.Based on this,a unique structural design or a specific synthesis strategy that enhances the potassium ion dynamics is required in the electrode design.In this paper,a simple hydrothermal method,high temperature annealing,in-situ synthesis of M₂P₂O₇（M=Fe,Co,Ni）and Ni₁₂P₅carbon composites,the microscopic morphology shows nano-flakes,alleviating the process of ion insertion and extraction The volume effect and enhanced electrical conductivity,it is applied to sodium/potassium ion battery to show good electrochemical performance:（1）Co₂P₂O₇/C composite materials are prepared through a solvothermal method using phenylphosphonic acid as the organophosphorus agent and high-temperature post-annealing process.Simply by changing the metal precursor,Ni₂P₂O₇/C and Fe₂P₂O₇/C composite materials can be obtained under the same synthesis conditions.Among the three types of carbon metal pyrophosphate,the designed Co₂P₂O₇/C composite has a capacity of 502 m Ah g^-1at 0.1 A g^-1,and has stable electrochemical cycling.What is impressive is that with a current density of 1 A g^-1,the Co₂P₂O₇/C electrode can maintain 170 m Ah g^-1after 900cycles.The high capacity and the stable cycling performance be attributed to their unique nanoparticles assembled nanoflake structure,which can afford enough active sites for K⁺intercalation.In addition,the robust pyrophosphate crystal structure and the in situ formed carbon composition also have positive effects on enhancing the long-term cycling performance and the electrode’s conductivity.（2）Ni₁₂P₅is prepared by changing the annealing conditions on the basis of the precursor of nickel pyrophosphate.The unique nano-sheet structure improves the specific surface area of the material,and the existence of small holes in the sheet ensures the full contact between the material and the electrolyte and the migration of sodium ions in the material.When it is assembled into a half battery,it shows a better rate performance.In addition,the reaction kinetic mechanism of the battery was explored by cyclic voltammetry,and the results showed that Ni₁₂P₅is controlled by diffusion and surface effects when applied to sodium ion batteries.