Preparation Of Two-dimensional Nickel Disulfide Composite Electrode Material And Its Sodium Storage Properties

The development of renewable energy and highly efficient energy-storage technologies is the key to peak carbon dioxide emissions and achieve carbon neutrality.Sodium-ion batteries share the similar charge-storage mechanism with lithium-ion batteries and attract more attentions in recent years,due to the low cost and global availability of sodium resources.Graphite anodes in lithium-ion batteries are not suitable for sodium-ion batteries owing to the limited intercalated Na⁺.The development of high-performance anode materials for sodium-ion batteries is vital to meeting the requirements for large-scale applications.Transition metal sulfides are deemed to be promising anode materials,especially nickel disulfide(NiS₂),due to its high theoretical capacity of 873 m A h g^-1.However,the inherent poor conductivity and volume expansion during charging and discharging seriously jeopardize the electrochemical performance of NiS₂.In this work,two-dimensional porous NiS₂nanoflakes were synthesized by means of the topological sulfurization of precursor Ni(OH)(OCH₃).The electrochemical sodium-storage properties of NiS₂ in ether-based(1 M Na PF₆ in DME)and ester-based(1M Na PF₆ in DMC:EC(1:1(V:V))and 5%FEC)electrolytes are studied,and the voltage failure mechanism in the NiS₂anode in ether-based electrolyte for sodium storage is unraveled.To improving its long cycle life,reduced graphene oxide or conductive polypyrrole was combined with NiS₂to fabricate hierarchical structures.The main findings are as follows:(1)Two-dimensional Ni(OH)(OCH₃)was prepared by solvothermal method and calcined with sublimated sulfur to obtain porous NiS₂ nanoflakes.The electrochemical properties of NiS₂ were evaluated in ether-based and ester-based electrolytes respectively.It was found that NiS₂ had better electrochemical properties in ether-based electrolyte,which achieved a reversible capacity of 419 m A h g^-1 at 1.0 A g^-1 after 90 cycles.The CV curves and XPS results of NiS₂in ether-based electrolyte show that NiS₂ undergoes a highly reversible gradual intercalation-conversion reaction,while NiS₂ in ester-based electrolyte shows a poor reversibility.Although NiS₂ has a high capacity in ether-based electrolyte,undervoltage failure occurs within 100 cycles.XRD and SEM characterization showed that sodium polysulfide species were easy to form in ether-based electrolyte,and caused shuttle effect,resulting in failure phenomenon.(2)The Ni(OH)(OCH₃)@reduced graphene oxide(Ni(OH)(OCH₃)@r GO)composite was fabricated by the solvothermal method,in which the in-situ growth of Ni(OH)(OCH₃)nanosheets on graphene oxide and the reduction of graphene oxide occur in parallel.The oxygen groups and defects on graphene oxide serve as the active sites for the nucleation of Ni(OH)(OCH₃),resulting in the decrease of the size and thickness of Ni(OH)(OCH₃)compared with the bare Ni(OH)(OCH₃).The calcination of Ni(OH)(OCH₃)@r GO with sublimed sulfur leads to the formation of NiS₂@S-r GO.Compared with the bare NiS₂,the specific capacity,rate capability,and cycling stability of NiS₂@S-r GO are significantly enhanced,owing to the enhanced charge transport and increased active sites on NiS₂@S-r GO and the high conductivity of r GO.Furthermore,r GO can also inhibit the diffusion of polysulfides,and thus alleviates the voltage failure.As a result,NiS₂@S-r GO can deliver a stable reversible capacity of 378 m A h g^-1 at 1.0 A g^-1 for 1800 cycles.(3)The NiS₂@PPy composite with a core-shell structure was obtained by in-situ polymerization of pyrrole on NiS₂.The polypyrrole coating can not only improve the conductivity of the material,but also accommodate the volume change of NiS₂ during the cycling process.More importantly,the lone-pair electrons in nitrogen atoms on the polypyrrole framework can effectively adsorb sodium polysulfides,and thus inhibit the shuttle effect.As a result,the as-prepared NiS₂@PPy presents a high reversible capacity of 393 m A h g^-1 at 2.0 A g^-1 over 1700 cycles.In addition,the NiS₂@PPy electrode shows great rate capability.A high capacity of 373 m A h g^-1 can be obtained even at 5.0 A g^-1 over 1700 cycles.The consecutive CV analysis demonstrates the substantial capacitive contributions to charge storage,which was up to 96.6%at a sweep rate of 1.0 m V s^-1.The above result provides a good explanation for good reversibility and rate capability of NiS₂@PPy.