Preparation And Electrochemical Performance Of Transition Metal-based Sulfate Cathode Materials For Sodium-ion Batteries

As energy storage devices similar to lithium-ion batteries?LIBs?,sodium-ion batteries?SIBs?have drawn much attention because of abundant resources and low-cost feature.Developing high performance,especially,high voltage cathode materials is a key issue in terms of commercializing SIBs.Sulfate cathode materials,showing advantages in abundant raw materials,easy processing and superb operating voltage,are promising for SIBs.In this paper,a new high voltage sulfate cathode material Na2Fe?SO4?2 for SIBs was synthesized successfully and a series of research on it was carried out,including the investigation into structure,studies on the electrochemical properties,the exploration of charge-discharge mechanism and failure analysis.In addition,the voltage of sulfate cathode was improved further by transitional metal substitution.The main content and results of research are as follows:?1?A new cathode Na2Fe?SO4?2?NFS?has been synthesized by ball milling and solid-state reaction using Na2SO4 and FeSO4?7H2O as raw materials.The cathode crystallizes in monoclinic space group C2/c,with a=12.6756?7??,b=12.7992?6??,c=6.5332?5??,?=115.585?5?°,V=955.85?5??³.Its structure is composed of Fe₂O₁₀ dimer units connected by SO4 tetrahedral that forms the typical alluaudite-type framework,in which sodium ions occupy three different kinds of sites.NFS contains tunnels formed by the association of Fe2O10 dimer unit and SO4 tetrahedra along the c-axis,constructing the fast pathway for the diffusion of sodium ions located at Na?2?and Na?3?sites.The theoretical specific capacity of NFS is calculated to be 91 mAh g^-1 based on the full usage of sodium ions at Na?2?and Na?3?sites.Coated with carbon using ascorbic acid as carbon source,NFS/C displays a high operating voltage at³.6 V based on Fe²⁺/Fe³⁺redox couple.The cathode exhibits a reversible capacity of 82 mAh g^-1 at0.1C under nonoptimal carbon coating and maintains over 60%capacity retention at2C.The excellent sodium storage ability at 0°C and 55°C further demonstrates the advantages of Na2Fe?SO4?2 for future energy storage applications in a wide temperature range.In addition,NFS shows high structural stability,superior thermal stability and good resistance to moisture.?2?Based on Na2Fe?SO4?2,other transitional metal elements have been used to substitute for Fe site to improve the voltage of cathode further.The influence of different substitution quantity on electrochemical performance of Na2Fe1-y My?SO4?2?y=0.1?0.2?0.3?0.4?has been studied.The impurity Na2Ni?SO4?2 would generate with Ni introduced,which decreases both capacity and voltage;As for Co,when y?0.2,the structure maintains the same but with crystallinity reduce,which results in the deterioration of electrochemical performance.When y=0.4,the impurity Na2Co?SO4?2would form and hinder the electrochemical reaction;With Mn introduced,the structure can remain and the voltage can be elevated.When y=0.4,the voltage of Na2Fe0.6Mn0.4?SO4?2 can be increased to 3.8 V.However,Mn²⁺is inactive,which leads to capacity decrease.