Construction Of Three-Dimensional Co₉S₈/Carbon Free-Standing Electrodes And Their Lithium-Ion Storage Performances

The excessive depletion of fossil fuels leads to a series of environmental and energy issues.With rapid development of energy industry and advent of wearable electronic devices,lithium-ion batteries（LIBs）have been widely considered as one of the most promising energy storage devices.However,the commercialized graphite anode is limited by its low theoretical capacity(372 m Ah g^-1)and powder form.Hence,designing free-standing anode with high capacity will be key to improving the performances of LIBs.Cobalt sulfide（Co₉S₈）,as a typical transition metal sulfide,has high theoretical capacity because its lithiation mechanism is featured with conversion reaction,which promisingly makes it replace graphite.However,the poor electrical conductivity and serve volume expansion effect of cobalt sulfide hinder its practical application in LIBs.Low-cost melamine foam can maintain high flexibility after carbonization.Moreover,carbonized melamine foam（CMF）has unique three dimensions（3D）structure and abundant nitrogen-doped carbon,which can improve the electrical conductivity of electrode.Hence,in this paper,a series of studies are based on Co₉S₈/CMF in lithium-ion storage.Firstly,in order to alleviate the volume expansion effect of Co₉S₈ nanoparticles during the insertion/extraction of lithium-ion,Co₉S₈ nanoparticles were anchored on the CMF by solvothermal method and coated polydopamine（PDA）derived nitrogen-doped carbon layer.This synthesis route not only improves the crystallinity of Co₉S₈ nanoparticles,but also constructs sandwich-type CMF-Co₉S₈-C composite.In the hybrid composites,the 3D structure of CMF provides the sufficient active sites and promotes the contact of electrolyte.Meanwhile,the outer carbon layer not only further accelerates the transmissions of e^-/Li⁺but also accommodates volume expansion effect of Co₉S₈ nanoparticles.As a consequence,the sandwich-type CMF-Co₉S₈-C exhibits an excellent reversible capacity(645 m Ah g^-1 at 0.5 A g^-1 after 200 cycles)and stable long-term cycling performances(520 m Ah g^-1 at 2 A g^-1 after1000 cycles).Secondly,in order to keep the flexibility of electrodes,ZIF-L was used as template and anchored on the CMF.Then,CMF@Co₉S₈-C hybrid was successfully constructed.Due to the mild synthesis conditions of ZIF-L,it can in-situ grow on the skeleton of CMF by the catechol groups of PDA in the room temperature.After the sulfurization with sublimed sulfur,Co₉S₈nanoparticles were successfully embedded in hierarchical porous carbon foams.The mechanical strength of composite is significantly improved because of the direct reaction with sublimed sulfur in Ar.The ZIF-L derived porous carbon enhances the BET surface area of the electrode and provides sufficient macro-and mesopores,which facilitates the diffusion of electrolyte and transfer of e^-/Li⁺.Similarly,the porous carbon wrapping around Co₉S₈ nanoparticles can effectively moderate the volume change.Thus,due to the above-mentioned merits,CMF@Co₉S₈-C not only shows good cycling and rate performances but also can be directly used as free-standing electrode for LIBs,which also lays a foundation for the construction of lithium-ion storage device with high energy and power densities.Thirdly,in order to further simplify the synthesis route of the composite.Melamine foam was directly used as substrate and 3-Mercaptopropionic acid（MPA）was used as a bridge to construct electrode materials.MPA not only chelates the cobalt-ions on melamine foam but also offers sufficient sulfur source during the subsequently annealing process.Because of the chelation of MPA and Co²⁺,ultrafine Co₉S₈ nanoparticles（～5 nm）are evenly distributed on the CMF without obvious aggregation.The M-Co₉S₈@NCF composite also has excellent flexibility and can be used as free-standing electrode for LIBs.Benefitting from the synergistic effect of unique 3D carbon network and ultrafine Co₉S₈ nanoparticles,volume expansion effect is relieved and reaction kinetic is significantly improved.Hence,M-Co₉S₈@NCF exhibits superior rate performance(1024 mAh g^-1 at 0.1 Ag^-1 and 468 mAh g^-1 at 2 Ag^-1)and stable cycling performance(617 mAh g^-1 at 1 Ag^-1 for 500 cycles).Moreover,porous nitrogen-doped carbon foam（PNCF）was synthesized as the cathode of lithium-ion capacitor（LIC）.M-Co₉S₈@NCF//PNCF LIC device is fabricated to combine the advantages of battery-type anode and supercapacitor-type cathode,which achieves a high energy density(166 Whkg^-1),high power density(7674 Wkg^-1)as well as satisfactory capacitance retention(83.5%at 5 Ag^-1after 5000 cycles).In this paper,based on the Co₉S₈/CMF,flexible free-standing electrodes were successfully consrtucted and their lithium-ion storage abilities were significantly improved by the simple and rational design.This work offers an efficient and stable energy storage material for advanced wearable electronic devices.