Preparation And Property Research Of Novel Electrode Materials For Potassium-ion Batteries

Lithium-ion batteries are difficult to meet the demand of large-scale energy storage due to the limited Li resources and their uneven distribution.Sodium-and potassium-ion batteries?KIBs?are the most promising low-cost candidates.Compared with sodium-counterpart,KIBs tend to display higher working potentials,better rate capability and their anodes are cheaper more accessible.Whereas,the larger size of K-ion has put forward stricter requirements on cathode material,which is critical in the promotion of K-ion technology.The development of K-storage cathode and their mechanism investigation are urgent.Previous studies have shown oxalate cathodes exhibit decent redox potentials.In the beginning of this research,we found that K₂TM?C₂O₄?₂?TM for transition metals?family has a theoretical capacity up to?170mAh g^-1,indicating its possibility as great K-storage cathode materials,but its electrochemical properties have never been reported before.Since Fe and Mn are relatively earth abundant and environmental friendly,this study explored K₂TM?C₂O₄?₂?TM=Fe,Mn?as KIBs cathode and analyzed corresponding mechanisms.Specially,?1?Syntheses and electrochemical studies of K₂TM?C₂O₄?₂?TM=Fe,Mn?High-quality K₂Fe?C₂O₄?₂ and K₂Mn?C₂O₄?₂ crystallites are synthesized by hydrothermal reactions.The crystallographic structure of Mn-is determined for the first time.Powder XRD and EDS characterization verified high purity of obtained samples.In electrochemical studies,Fe-exhibited excellent redox reversibility in 2.0-4.25 V,delivering a charge capacity of 79.5 mAh g^-1?0.92 K⁺?at 0.1 C,a capacity retention of85%upon 300 cycles at 0.2 C.Whereas the Mn-displayed an abnormal potential drop in first charge and irreversibility,which may be caused by the disruption of Mn local structures according to synchrotron EXAFS results.?2?Reaction mechanism investigation of K₂Fe?C₂O₄?₂ by synchrotron X-ray absorption spectroscopyThe electron compensation mechanism is probed based on synchrotron XANES analysis using Athena program,demonstrating that a reversible Fe²⁺/Fe³⁺redox reaction occurred during cycling.Further,the oscillation functions in momentum?k?and real?R?space,??k?and??R?of K₂Fe?C₂O₄?₂ cathode at different states are obtained by EXAFS analysis using FEFF program,confirming K^??25%?and K^??25%?are removable,in accordance with experimental results of capacity.In summary,the Fe and Mn analogies in K₂TM?C₂O₄?₂ family have been prepared by hydrothermal methods and their abilities of K-ion storage have been assessed.The Fe-is electrochemically active with the migration of K^?and K^?ions during Fe²⁺/Fe³⁺redox reactions,while its Mn-analogue is inactive since Mn-environments is destroyed during the first K-desertion.This study provides experimental evidence and theoretical support for the development of KIBs cathode.