In-situ Transmission Electron Microscopy Study On Li/Na/K-Se Batteries

The development of cathode materials with high energy density is the currently a hotspot in the research for high energy density alkali secondary batteries,and it is also one of the important ways to solve energy problems and achieve efficient use of energy.Sulfur-based electrode materials have received extensive attention from researchers due to their ultra-high theoretical energy density.However,sulfur cathodes face key issues such as poor conductivity,easy dissolution of intermediate products during cycling,and material volume expansion,resulting in low specific capacity of sulfur electrodes.Poor rate performance and short cycle life limit its commercial application.Selenium and sulfur have similar chemical properties,but selenium has better conductivity than sulfur.Using selenium as a positive electrode material can provide a volume energy density equivalent to that of a sulfur electrode.However,the selenium positive electrode also faces similar problems with sulfur,including the shuttle effect of intermediate products and Volume expansion issues,etc.Reasonable design and construction of composite selenium-based electrode materials are the key to achieving their high energy density.Aiming at the problems of selenium-based cathode materials,this paper studies the charging and discharging phenomena of lithium metal,sodium metal and potassium metal and synthetic carbon-selenium materials through in-situ spherical aberration corrected environmental transmission electron microscopy,revealing the energy storage mechanism of selenium-based materials.This study revealls the electrochemical reaction mechnisms of selenium-based materials reacting with lithium,sodium,and potassium.The specific content is as follows:（1）The research and development of high-performance lithium-ion batteries is the current research focus in the field of new energy,so the construction of Li-Se@CNT nanobatteries,using transmission electron microscopy（TEM）in situ observation of the kinetic reaction and morphology changes of nano-tubes in the process of lithium charge and discharge,using selected electron diffraction（SAED）,Electron energy loss spectroscopy（EELS）and other techniques revealed the reaction mechanism of lithium-selenium batteries.The results show that the original sample nanotubes expand from 296 nm to 355 nm in diameter and the volume expands by 43%during lithium discharging.At the same time,lithium ions will react with selenium,and Se with hexagonal crystal structure will be transformed into Li₂Se with face centered cubic structure.This reaction is transformed in one step,and the formation of polylithium-selenium is not found in this solid phase reaction.However,the experimental results show that the lithium ions are not separated from the Li₂Se,and this irreversible phenomenon is the reason for the low coulomb efficiency of the lithium-selenium battery.（2）Because of the advantages of abundant sodium resources,low manufacturing cost and no pollution to the environment,sodium ion battery has become the focus of researchers.We used sodium metal as the cathode,carbon selenium-containing Se@CNT nanotubes as the cathode material,and sodium oxide as the solid electrolyte to construct nano-sized cells.By using TEM,the mechanism of the sodium discharge and charge reaction,as well as the changes of the interface and structure of the electrode material,were studied in situ.The results show that the sodium discharging reaction process is Se+Na⁺+e^-→NaSe,NaSe+Na⁺+e^-→Na₂Se;It can be found that the sodium reaction of Se@CNT material is divided into two steps:at first,a small concentration of sodium ions was embedded,and the hexagonal crystal structure of Se is alloying to form the hexagonal crystal structure of NaSe.This reaction is thermodynamically feasible,rapid reaction and accompanied by a small volume expansion.Subsequently,high concentration of sodium ions continued to enter the nanotubes,and the NaSe with hexagonal crystal structure under went transformation reaction to form face-centered cubic crystal Na₂Se,accompanied by a large volume expansion.The desodium reaction process is 4NaSe→2Na⁺+Na₂Se₄+2e^-;2Na₂Se→2Na⁺+2e^-+Na₂Se₄;Under positive pressure,sodium ions detach from NaSe and Na₂Se to form the tetragonal Na₂Se₄ phase.We think that the incomplete reaction of the denatrification process and the formation of nanocrystals in the electrode material are one of the reasons for the serious degradation of the performance of the Na-Se battery.（3）The K-Se@CNT nanocells were constructed by TEM and recorded by in-situ video to observe the kinetic reaction and morphology change of the nanoparticle nanotubes in the process of potassification and depotassification.The SAED and EELS techniques were used.The electrochemical reaction mechanism of potassium-selenium cell was revealed.The results show that the volume expansion of Se@CNT nanotubes is as high as 200%during the potassium inlaying process,which is related to the large potassium ion radius.At the same time,the alloying reaction between potassium ions and selenium is also a multi-step reaction,resulting in the formation of polyselenides.The potassium discharge reaction process is 5Se+2K⁺+2e^-→K₂Se₅;3K₂Se₅+4K⁺+4e^-→5K₂Se₃;Although Se@CNT nanotubes expand greatly,the structure of the nanotubes remains smooth and defect-free,mainly because of the presence of the polyvalent state of Se in the potassium selenium compound.Selective electron diffraction patterns showed that potassium ions did not escape from K₂Se₅ and K₂Se₃ when the positive pressure was applied to the nanocrystal cell,which was due to the large radius of potassium ions and slow kinetic reaction.