Synthesis And Performance Of Novel Free-Standing Electrode Materials For Potassium-Ion Batteries

The advantages of natural abundance and low standard electrochemical potential of potassium render potassium-ion batteries(KIBs)a promising energy storage device with low cost and high energy density for future large-scale energy storage applications.However,the big radius of K⁺ions causes large volume changes during charge/discharge processes and sluggish kinetics,leading to poor cycling stability and rate performance.In addition,the traditional electrodes use non-active components,such as current collectors,conductive agents and binders,which leads to low overall specific capacity and energy density based on the whole weight of electrodes.To solve these problems,a series of carbon nanofiber based free-standing electrode materials were designed and fabricated and used in KIBs.The electrochemical performance and potassium storage mechanism were systematically investigated.The main results are summarized below:(1)Nitrogen-doped cup-stacked carbon nanotubes were designed and synthesized by a chemical vapor deposition method,and free-standing electrodes were fabricated by vacuum filtrating.The open layer alignment structure not only allows facile insertion/extraction of large radius K⁺ions,but also accommodates the volume change during K⁺ion insertion/extraction.The free-standing electrodes demonstrated an excellent electrochemical performance with a high reversible specific capacity(323mAh g^-1)and a favorable rate capability(1 A g^-1@75 mAh g^-1).(2)Free-standing microporous carbon nanofiber mats were prepared by electrospinning method followed by high temperature carbonization.The carbon nanofibers interweave into a three-dimensional(3D)conductive interconnected network structure,which not only provides electron/ion transportation pathway with a reduced diffusion distance,but also alleviates the stress caused by the volume expansion during charge/discharge processes.As a result,fast reaction kinetics and good structural stability were delivered,which achieved a superior cycling stability with 88%capacity retention after 2000 cycles,and an excellent rate capability with a high specific capacity of 100 mAh g^-1 retained even at 7.7 A g^-1.(3)A thin layer of SnO₂ was coated on carbon nanofibers by atomic layer deposition method and potassium metal composite anodes were fabricated by a molten infusion process.SnO₂ shows a high binding energy with potassium,which could regulate uniform potassium nucleation/deposition and suppress dendrite growth.Furthermore,the 3D interconnected network could relieve the stress induced by the volume expansion during repeated plating/stripping processes.As a result,the potassium metal composite anode demonstrated an amazing cycling stability for 1700 h,which is superior to the reported potassium metal anodes.(4)Free-standing microporous carbon nanofiber/small-molecule sulfur composites were fabricated and directly used as cathodes in KIBs.The synergistic reaction of microporous physical confinement and strong chemical interaction between small molecule sulfur and carbon can effectively avoid the formation of soluble polysulfides and eliminate the shuttle effect during the potassiation/depotassiation processes.The cathodes achieved an exceptional electrochemical performance,a high reversible specific capacity of 1392 mAh g^-1,a high energy density of 569 Wh kg^-1 based on the whole weight of the composite cathode and an ultra-stable cycling performance of a high capacity retention of 88%after 2000 cycles.Furthermore,the potassium storage mechanism was systematically investigated,and the results reveal that the sulfur exists as small molecules and the final discharge product is K₂S,which improves the fundamental understanding of the reaction process in KIBs.