In-situ Transmission Electron Microscopy Study On The Failure Behavior And Mechanism Of Electrode Materials In Potassium-ion Batteries

Lithium-ion batteries（LIBs）have been widely employed for electrochemical energy storage.However,the tiny amount and uneven distribution of lithium ore in the crust make it difficult to satisfy the larger application demands in the future owing to rising costs of mining,transport,and import.It is anticipated that the demand for lithium resources will decline with the development of new alkali metal ion batteries.Potassium-ion（PIBs）batteries are a new type of electrochemical device developed based on LIBs,which have significant advantages such as small solvation ion radius,abundant production,stable discharge,and low cost.It is probably the best substitute for affordable and durable batteries in the future.It is anticipated to be extensively used in portable equipment and new energy vehicles.PIBs have some failure issues that constrain their development:（1）dissolving of the electrode materials reduces PIBs’cycling life;（2）reduction in first cycle capacity results in a reduction in specific capacity;（3）electrode materials expansion causes stress cracking,etc.As a result,researching the failure behavior and mechanism of PIBs electrode materials can aid in advancing battery performance research and development.The thesis focuses on the anode materials,such as antimony（Sb）nanowires and copper oxide（Cu O）nanowires,and the cathode materials,such as elemental sulfur（S）and selenium（Se）in PIBs.In-situ transmission electron microscopy and diffraction techniques are used as the main research methods to systematically explore the evolution phenomena and laws of the above electrode materials during potassiation/depotassiation,revealing their energy storage mechanisms,and elucidating their failure behavior and mechanism during charge and discharge processes.The following are the primary research findings:Firstly,the mechanism of PIBs electrode material dissolution leading to failure is investigated.A microscopic platform for the electrochemical reaction between potassium ions（K⁺）and selenium（Se）as the anode material in a liquid state is set up using In-situ transmission electron microscopy（TEM）.When potassium ions are embedded in Se nanowires,a multi-step reaction occurs.Firstly,the Se nanowire is transformed from a solid state to potassium ployselenide（K₂Se_x）which is dissolved in the electrolyte.Then the Se nanowire is transformed into insoluble potassium selenide（K₂Se）which precipitates from the electrolyte.Solid flocculent selenium is formed when K⁺is de-embedded by applying the reverse voltage.K₂Se_xis produced while the K-Se batteries cycling,consuming up the Se electrode and shortening its cycling life.In contrast,K-S and Li-S nano batteries are constructed in-situ TEM using elemental sulfur（S）as cathode material.It is found that the porous structure of carbon materials is conducive to reducing the volume expansion and improving the conductivity of sulfur electrodes in the cycling process.At the same time,the doping of nitrogen and oxygen will transform non-polar carbon into polar carbon,which is conducive to reducing the"shuttle effect".Based on this,Li-S batteries’performance is improved by obtaining 3D-NOPC@S electrodes.After500 cycles,the specific capacity of 543 m Ah g^-1 is still maintained,with a decay rate of only 0.023%per cycle at 2 C.Even if the sulfur loading reaches 4 mg cm^-2,it can still maintain 579 m Ah g^-1 with high cycle stability,providing a theoretical basis for improving the performance of Li-S batteries.Second,research the mechanism of failure is brought on by PIBs’declining first cycle capacity.Building Li-Sb,Na-Sb,and K-Sb nano batteries with Sb nanowires as alloy anode materials.The electrochemical reactions and the failure issue of various alkali metal ions embedded in Sb nanowires using in-situ TEM are examined.The experimental results show that when a small amount of Li⁺is embedded in Sb,the lithium antimony（Li₃Sb）alloy is formed,and the delithiation can be restored to Sb single crystal.Li₃Sb is still generated when a significant quantity of Li⁺is embedded in Sb or after several cycles;however,the delithiation recovery result is polycrystals,which are small particles of Sb.When Na⁺is embedded into Sb nanowires with directionality,it is first embedded along the nanowires’axis,then embedded along the radial direction to form fine Na₃Sb polycrystals.The crystal structure of the reaction changes slightly,but the morphology changes seriously.Sb nanowires are embedded in the K⁺on the discharge process,which also causes axial reactions with violent expansion.The formation of K₃Sb polycrystals with porous structures and poor crystallinity is resulted in the pulverization of Sb electrodes.Providing a new explanation for the decrease in the first cycle capacity of alloy anode materials.Finally,the mechanism of failure caused by the volume expansion of PIBs electrode materials is studied.Utilizing Cu O nanowires as the anode material,nanoscale PIBs are constructed in the in-situ TEM and numerous cycles were completed.The results of the experiment indicate that Cu O nanowires undergo a two-step process of potassiation（discharging process）.Potassium copper oxide（KCu O）is first produced with the fast increase of the nanowires’volume.A secondary reaction takes place when K⁺insertion increases in which small particles of copper（Cu）and potassium oxide（K₂O）are created and their volume expansion is reduced.The process of depotassiation results in the creation of big cuprous oxide（Cu₂O）particles,and the volume of nanowires decreases due to the transformation of crystal structure.Compared with other electrode materials,the unique volume shrinkage will lessen the risks associated with using PIBs,including volume expansion,stress overload,and short circuits.Cu O has the potential to be used in commercial PIBs since it maintains its structural stability throughout numerous cycles.