Tuning Effects Of Architecture And Defect Of Nano-boxes Composed Of Nickel And Cobalt Compounds On The Cathode Electrochemical Performance Of Lithium Sulfur Batteries

Lithium-sulfur（Li-S）batteries are one of the most competitive candidates in the next generation of energy storage systems owing to their high theoretical energy density and low cost.However,the sulfur cathode of the battery and its discharge product lithium polysulfides（Li PSs）are insulative;"shuttle effect"resulted from the solubility of Li PSs in electrolyte poisons the lithium anode and thus shorten the cycle life of the battery significantly;the large volume changes in sulfur cathode during charging and discharging damage the architecture of the cathode mechanically and therefore poses serious insecurity in application.These issues have hindered the commercial development of the Li-S batteries.Employing polar materials based on transition metals as sulfur hosts is an effective strategy to address the issues mentioned above.Plus the low cost and abundant reserves of the transition metals,their compounds afford polarity for chemical confinement for Li PSs to inhibit the shuttle effect and the inherited intrinsic catalytic ability for fast conversions of Li PSs.Through a reasonable architectural design,for example,hollow architectures such as nano-boxes or nano-spheres,or structural design,for example,alien atom doping or substitution,the compounds can accommodate the volume changes and deliver even more enhanced catalytic activity.In this paper,two types of compound nano-boxes based on nickel and cobalt,which are composed of cobalt nickel oxide and nickel cobalt phosphide,respectively,were designed and synthesized for application as sulfur hosts in Li-S batteries.Morphologies,crystal structures,chemical status of the nanostructures are characterized by SEM,TEM,HRTEM,XRD,XPS,respectively,these results combined with the analysis of the electrochemical performance of the Li-S batteries were employed for establishing a determining relationship between morphology/structure and performance.The core research contents of this paper include two parts:In the first part,the influence of morphological architecture of sulfur host on the electrochemical performance of Li-S batteries was investigated.With the identical stoichiometric and crystal structure,three-dimensional nano-box and two-dimensional nano-flake are selected as two typical types of the architectures of the cathode,which are all made up of vertically aligned CoNiO₂ nanosheets.The cathode based on the nano-boxes（CoNiO₂（B））delivers an electrochemical performance superior to its analogue,the cathode based on nano-flakes（CoNiO₂（F））:a high reversible discharge specific capacity of 1232 m Ah g^-1 at 0.05 C,and a low capacity decay rate of 0.1%per cycle lasting for 300 cycles at 1 C.The nano-box architecture can afford a specific surface area which is 49%higher than the two-dimensional analogue,which not only provides more and shorter channels for convenient ion transportation,but also results in the exposure of more active sites.As a consequence,the nano-box cathode acquires a promotion of the chemical adsorption and catalytic conversion kinetics toward Li PSs.Therefore,the construction of a sulfur host with hollow-architecture nanostructures is favorable for enhancing the performance of Li-S batteries.In the second part,the effect of cathodic alien atom substitution on the electrochemical performance of Li-S batteries was investigated.Phosphide was selected as substitution element,while CoNiO₂ nano-boxes mentioned above were selected as substitution subject.The cathode based on phosphatized nano-box（NiCoP（B））delivers an electrochemical performance superior to its oxide analogue（NiCo-LDH（B））:a high initial discharge specific capacity of 1102 m Ah g^-1 at 0.1 C,and a low capacity decay rate of 0.049%each cycle lasting for 1000 cycles at 1C.The phosphatized nano-boxes have higher electrical conductivity,which is beneficial for rapid electron migration and hence the fast conversion of Li PSs;Phosphatization afford a specific surface area 48%higher than its oxide analogue,which results in further enhancement of conversion of Li PSs with the exposure of more active sites;Additionally,the enhanced polarity aroused from phosphatization,strengthens the chemical confinement for Li PSs and consequently inhibit shuttle effect for prolonged cycle life of Li-S batteries.Therefore,phosphatization is beneficial for sulfur hosts based on transition metals to improve their electrochemical performance.