Design And Electrochemical Performance Of Cobalt-Based Sulfide Anode Materials For Sodium Ion Batteries

Sodium-ion batteries（SIBs）are considered as one of the most promising candidates for lithium-ion batteries（LIBs）in the future.As an important component that affects the performances of SIBs,anode materials still need to be explored and studied.Cobalt based sulfides possess a series of advantages including relatively high theoretical capacity and conductivity,abundant active reaction sites,good thermal and mechanical stability,and low cost,suggesting great application potential in SIBs.However,cobalt based sulfides are prone to significant volume changes upon the cycling process,which can result in rapid capacity fading and poor cycling stability,thereby deteriorating the Na-storage performance of battery.In this work,structural design for the anode materials was focused,and various synthesis routes were conducted to obtain high-performance cobalt based sulfides anode materials for Na-storage via nanomaterialization of active materials,combination with carbon materials,and space reservation for volume expansion.Firstly,porous carbon networks immobilized Co₉S₈ composite（Co₉S₈@NPC）was designed via sol-gel,pyrolysis carbonization and in-situ sulfidation.Benefiting from the yielded porous feature and space confinement of Co₉S₈ nanoparticles（NPs）in conductive carbon sheet networks,the Co₉S₈@NPC anode material can deliver enhanced Na-storage performance.When test at 50 mA g^-1,large initial charge/discharge capacities of 325.3/752.5mAh g^-1 are obtained.Even at 3000 mA g^-1,a considerable rate capability of 138.5 mAh g^-1 can be achieved.After 100 cycles at 50 mA g^-1,the discharge capacity is 275.3 mAh g^-1.After 3000 cycles at 500 mA g^-1,the discharge capacity is 120.4 mAh g^-1.Besides,pseudocapacitive behavior,reaction dynamics and related theoretical simulation based on firstprinciples calculations are also combined to initially elucidate the Na-storage mechanism.Secondly,CoS_1.097@C core-shell fibers was designed via coaxial electrospinning,stabilization and carbonization.CoS_1.097 powders are distributed in the inner shell of the carbon fibers and sufficient pore spaces are present among themselves.The unique encapsulation structure,porous characteristics,and onedimensional conductive carbon shell can enable this anode material itself high initial specific capacity,excellent rate capability,and long cycle life.The initial charge and discharge capacities at 50 mA g^-1 are 386.0 and 830.9 mAh g^-1,respectively.After 2000 cycles at 500 mA g^-1,the discharge capacity is 216.3 mAh g^-1.Even at 3000 mA g^-1,the rate capacity can be maintained at 83.3 mAh g^-1.Thirdly,flexible porous CoS_1.097@carbon nanofibers was designed via electrospinning,stabilization and carbonization,and in-situ sulfidation.CoS 1.097 NPs are confined into porous N-doped carbon nanofibers,and thus accommodated volume expansion of active material and high electrical conductivity for the electrode can be achieved.Besides,the excellent flexibility of the anode material can also endow with desirable potential for the assembly of wearable batteries.When used as flexible and binder-free electrode,it delivers high initial charge/discharge capacities(329.7/490.3 mAh g^-1 at 50 mA g^-1),enhanced rate capability(76.6 mAh g^-1 at 3000 mA g^-1),and long-term cycling life(181.8 mAh g^-11 at 200 mA g^-1 after 300 cycles,171.1 mAh g^-1 at 500 mA g^-1 after 900 cycles,141.4 mAh g^-1 at 1000 mA g^-1 after 1600 cycles).