Preparation And Properties Of Ni-Co Hydroxides And Nickel Sulfides As Electrode Materials

With the increasing energy demand and the emerging environmental concerns,energy production and storage become the most important issues for human's economy and industry.Supercapacitor has demonstrated promising application in energy storage field over other storage devices.RuO2 and carbon-based materials are the industrial prospective electrode materials.However,it is difficult to achieve industrialization because of the high price and resource scarcity of RuO2 and the low capacitance of carbon materials.Therefore,it is important to develop non-noble metal electrode materials with low-cost and high capacity.Ni-Co hydroxides and nickel sulfides have the advantages of low cost and high capacity.In this paper,we studied the controllable synthesis and electrochemical performance of Ni-Co hydroxides and nickel sulfides as electrode materials in supercapacitors.Firstly,we designed a controllable synthesis scheme,through chemical treatment of Ni-Co phosphite,to obtain Ni-Co hydroxides for the first time.We prepared a series of Ni-Co hydroxide-based electrode materials with different microstructures and excellent electrochemical performance by changing the oxygen content in the alkali solution.Based on the above research,the structure-property relationship between Ni-Co hydroxides with different microstructures and electrochemical performance was revealed and the synthesis process of Ni-Co hydroxides was illuminated.The adjustment of oxygen content was introduced into the preparation process of Ni-Co hydroxide for the first time.The effect of synthetic conditions such as Ni:Co ratio in raw materials was discussed.Three-dimensional structure interconnected by nanosheets was obtained by chemical treatment of precursor in mixed film.Anthemia-like Ni-Co hydroxide/Ni foam and nano array Ni-Co hydroxide/Ni foam were synthesized by chemical treatment of precursor/Ni foam.The electrochemical characterization results showed that the above samples delivered high capacity and rate capacity.The Co³⁺obtained by the chemical oxidization enhanced the conductivity of electrode materials.The interconnected flower-like structure and nanosheet array structure provided efficient and fast transportation pathways for both ion and electron.And the in suit growth further reduced the contact resistance between Ni foam and hydroxides.The influence of the oxygen content in alkali solution on the formation rate,microstructure of hydroxides,valence state of Co and electrochemical performance of Ni-Co hydroxides were further investigated.The results showed that Ni-Co hydroxides can be synthesized by ion changing during chemical treatment.At the same time,Co²⁺was oxidized into Co³⁺by the oxygen in alkali solution.The oxygen content is an important factor.Secondly,we synthesized NiS₂ hollow spheres using the L-cysteine without addition of template.The NiS₂ showed high gravimetric capacity due to the hollow structure.And we designed a controllable synthesis scheme to synthesize nickel sulfides with excellent electrochemical performance using thiourea.Furthermore,the phase-property relationship between nickel sulfides with different composition and electrochemical performance was revealed.The effect of synthetic conditions including the amount of thiourea,reaction temperature and reaction time on the phase and size of the nickel sulfides were discussed.The influence of the phase composition and particle size of the nickel sulfides on the electrochemical performance was further investigated.The sample NiS/Ni₃S₄ with favorable particle size and composition delivered high gravimetric capacity and impressive circling stability,whose nanoparticles with small size could buffer the volume changes during the long-term cycling.Moreover,the introduction redox counterpart of Ni²⁺/Ni³⁺into NiS/Ni₃S₄ composite may favor the reversible transition of different states and enhance charging/discharging stability.And the synergistic effect deriving from an intimate contact of NiS and Ni3S4 may inhibit nickel sulfide particles from aggregating and avoid electrode materials dissolving into the electrolyte.