Preparation And Electrochemical Study Of High-voltage Na₄Co₃?PO₄?₂P₂O₇ Cathode Material For Sodium-ion Batteries

Amidst the variety of energy storage technologies in the market,lithium-ion batteries have an important role to play in energy storage and conversion.Currently,they dominate the portable electronic device market.In addition,the global market for electric vehicles is rapidly expanding,thereby leading to an increase in demand for lithium.However,global lithium resources are limited,with prices having increased recently and a possible supply crunch looming.Both sodium and lithium are in Group I and have many similar physical and chemical properties.In addition,sodium is widely available and has a low price,while sodium-ion batteries have similar working principles to lithium-ion batteries,making them the focus of much ongoing research as extremely promising candidates for energy storage technologies to replace lithium-ion batteries.Among these,Na₄Co₃?PO₄?₂P₂O₇ has a high operating voltage and theoretical specific capacity.However,it has low electrical and ionic conductivity,which significantly impact its performance.As such,this paper focuses on improving its performance via optimisation of synthesis conditions and doping.To obtain a suitable synthesis method,ball-milling and a soft template method have also been used to produce the material.The influence of synthesis parameters on electrochemical performance is investigated,with XRD data showing that NCPP sample crystallinity is highest for a 50-to-1 ratio of carbon nanotubes to raw material,which manifests as superior electrochemical performance.For the soft template method,F127 is found to be the best surfactant,with citric acid being the best carbon source.The sample synthesised using F127 and citric acid exhibits significant improvements in performance relative to other tested samples.In addition,other methods have been employed to optimise capacity,including the use of different separators and increasing the voltage window.The results show that glass fibre separators improve performance relative to conventional PE/PP separators.Discharge capacity increases by 20 m Ah g^-1 and capacity retention rate increases by 15%.Increasing the voltage window to 4.75 V causes electrolyte decomposition to occur,which leads to a rapid decline in capacity retention rate.This matches what is observed during electrochemical testing when capacity retention rate increases in tandem with discharge current rate.The material Na₄Co₃?PO₄?P₂O₇ is also doped with magnesium to create Na₄Co_3-xMg_x?PO₄?P₂O₇,and the synthesised material undergoes physical characterisation via ICP-OES,Raman spectroscopy,XPS as well as Rietveld refinement.Magnesium doping is found to have no significant effect on the structureof the synthesised material.The material is assembled in a battery to undergo electrochemical characterisation,and results show that magnesium is effective at enhancing material performance.An x-value of 0.15 is found to yield optimal results,with a discharge capacity of 82 m Ah g^-1 at 1 C,70 m Ah g^-1 at 5 C,65 m Ah g^-1 at 10 C and 60 m Ah g^-1 at 20 C.This material is also found to have good performance at high discharge current,with a capacity retention rate of 81.3% after 500 cycles at 20 C.In addition,the capacity retention rate is observed to increase when current rate increases,which is explained and analysed using electrolyte decomposition.