Research On The Electrochemical Performance Of Metal Sulfides-carbon Composites As The Anode Material For Sodium/potassium Ion Batteries

Lithium-ion batteries have been widely penetrated and used in various fields of human life.However,some problems such as low natural abundance,and high cost,have seriously hindered their application in large-scale energy storage systems.Sodium/potassium ion batteries,which are rich in sodium/potassium resources,have attracted extensive attention to be a good choice as a new energy storage system in the future.However,in order to obtain high-performance sodium/potassium ion batteries,the design and development of suitable electrode materials are indispensable.For the research of electrode materials,the main problems currently existing are slow kinetics of Na+/K+ with a larger ion radius in the electrode and large volume change caused by the electrochemical reaction between Na+/K+ and the electrode material.To address the above problems,this thesis takes metal sulfides/carbon composites as research objects to study their sodium/potassium storage performance as anode materials.The details are as follows:(1)Heterostructured Bi2S3/MoS2 coated with nitrogen-doped carbon layer(Bi2S3/MoS2@NC)was obtained through the high-temperature solvothermal method and the subsequent high-temperature calcination process.The amorphous carbon coating layer effectively relieved the structural strain caused by the large volume change generated within the processes of Na+/K+intercalation/de-intercalation,and also improved the electrical conductivity of the composite material.More importantly,the internal electric field formed on the heterogeneous interface accelerated the transfer of ions and electrons.Therefore,the Bi2S3/MoS2@NC composite exhibited good sodium storage performance(the capacity of 381.5 mA h g-1 achieved at a current density of 5.0 A g-1 and 412 mA h g-1 at 0.5 A g-1 after 400 cycles)and potassium storage performance(a high specific capacity of 382.8 mA h g-1 achieved after 100 cycles at 0.1 A g-1).At the same time,we used in-situ diffraction of X-rays and ex-situ High Resolution Transmission Electron Microscope to study the sodium storage mechanism of Bi2S3/MoS2@NC electrode.The results showed that Bi2S3 underwent a reversible conversion and alloy reactions,namely:Bi2S3 ? Bi ?Na3Bi.While MoS2 corresponds presented a reversible conversion process of MoS2 ?NaxMoS2 ? Mo.(2)The CoS2 nanoparticle supported by the sulfur-doped carbon nanosheet framework(CoS2/SC)was efficiently synthesized by the high-temperature decomposition and high-temperature gas-phase vulcanization method.The carbon framework served as a scaffold to support CoS2 nanoparticles,which effectively improved the electrical conductivity of the composite,and relieved the volume change caused by the intercalation/de-intercalation of potassium ions.Therefore,the CoS2/SC electrode exhibited excellent potassium storage properties.At a current density of 0.2 A g-1,the capacity still maintained at 363.6 mA h g-1 after 100 cycles.When tested under a larger current density(2 A g-1),the capacity was 273 mA h g-1 after 300 cycles.Furthermore,the composite also presented good rate capability,delivering a high discharge capacity of 286 mA h g-1 at 5 A g-1.Furthermore,Galvanostatic Intermittent Titration Technique(GITT)and electrochemical impedance method were applied to further research and analysis on the reaction kinetics of the CoS2/SC electrode in the process of potassium storage.