Synthesis And Electrochemical Properties Of Transition Metal Selenide For Potassium Ion Batteries Anodes

With the accelerated popularization of digital electronic products and new energy vehicles,lithium-ion batteries have increasing market share of the secondary batteries in the world.Recently,the limited and unevenly distributed lithium resources（0.0017 wt%）or cobalt resources（0.001 wt%）in the earth brings about the climbing cost of batteries,greatly hindering the sustainable development of lithium-ion batteries.Due to the similar chemical properties to lithium,abundant reserves,low cost and excellent ion diffusion kinetics,potassium-ion batteries（PIBs）have been considered as one of the potential candidates for next-generation storage devices.Unfortunately,PIBs also suffer from huge volume expansion,slow reaction kinetics and poor cycling stability upon K⁺intercalation and deintercalation due to larger iorn radius of K-ion,consequently hindering their commericial application in the future.Therefore,it is highly desirable for developing new electrode materials which could be beneficial for fast K⁺intercalation and deintercalation.Transition metal selenides have a crystal structure similar to graphite,in which each metal atom（M）and Se atom forms a six-coordinated octahedron with M-Se-M bonding.In addition,the weak van der Waals forces between layers are favorable for large-size K-ions intercalation and deintercalation,leading to high energy density and excellent performances.Unfortunately,transition metal selenides suffer from low intrinsic electronic conductivity and large volumetric expansion upon the insertion/extraction.Nanostructure building and carbon hybriding are regarded as effective ways to improve K⁺-ion diffusion kinetics and cycling stability.In our work,building heterojunction interface and coating nitrogen-doped carbon are carried out to enhance K⁺-ion diffusion kinetics in transition metal selenide anodes.The main works are as follows:（1）Mo Se₂/Mo O_3-x@r GO composites with heterostructure were synthesized by a facile solvothermal method and subsequently thermal-annealed in Ar/H₂ atmosphere.The reduced graphene oxide provided a three-dimensional conductive network for the composites,and the amorphous Mo O_3-x containing oxygen vacancies could not only effectively buffer the self-aggregation of Mo Se₂ nanosheets but also built a polarized electric field to promote the migration of potassium ions.Therefore,the composites deliver248.2 m Ah·g^-1 after 50 cycles at 0.2 A·g^-1 with a capacity retention of 84.6%and 182.9m Ah·g^-1 after 150 cycles at 1.0 A·g^-1 with a capacity retention of almost 61.2%,and the specific capacity was maintained at 124.1 m Ah·g^-1 at a current density of 5 A·g^-1.（2）Fe₃Se₄ anchored on nitrogen-doped carbon compostes are prepared by sol-gel method using ferric nitrate,urea,and citric acid as precursors,followed by high-temperature calcination and selenization process.The nano size of Fe₃Se₄ shortens the K-ion diffusion path and nitrogen-doped carbon nanosheets（NC）as conductive framework could enhance electron transportation in composites.In addition,nitrogen-doped carbon would be coated on the surface of Fe₃Se₄and effectively buffer the volume expansion of Fe₃Se₄ upon cycling.Comparing to pristine Fe₃Se₄ and NC composites,Fe₃Se₄@NC composites exhibit better cycling stability and rate capability,delivering 196.6 m Ah·g^-1after 136 cycles at a current density of 0.2 A·g^-1 and 119.2 m Ah·g^-1 after 200 cycles at 1.0A·g^-1.Analysis from the electrochemical measurements reveals that the synergistic effect between Fe₃Se₄ and NC would be responsible for the enhanced electron/ion migration across the interface.