Research On The Catalytical Effects Of Nickel-based Electrocatalysts For The Transformation Processes Of Lithium Polysulfides

Lithium-sulfur（Li-S）battery has attracted much attentions because of its super high energy density(2600 wh kg^-1),environmental friendliness and low cost.However,the faultiness of electronic insulation,volume change,"shuttle effects"and slow kinetics of electrochemical conversion process of cathode immensely hinder the practical application of Li-S battery.The cathode active materials of lithium sulfur battery undergo the phase transition of"liquid phase-solid phase"in the process of charge and discharge.This sluggish kinetics process will lead to the increase of battery polarization,the accumulation of intermediate polysulfides（Li PSs）in electrolytes and the formation of"shuttle effects"and"dead sulfur",resulting in the irreversible loss of active materials（Sulfur）.At the same time,the Li PSs that diffuse to the anode will corrode the lithium metal anode,causing the uneven deposition of Li⁺and the formation of lithium dendrites,which definitely damages the electrochemical performance of lithium-sulfur battery.Aiming at ameliorating the shuttle effects of lithium sulfur battery and the slow kinetics of Li PSs transformation process,carbon nanotubes（CNT）loaded transition-metal（nickel）catalysts were designed and synthesized.By catalyzing the electrochemical reaction in the conversion process of Li PSs,the kinetics of Li PSs transformation was accelerated,and the shuttle effects and other side reactions were alleviated.The main contents of this thesis are as follows:1.By using CNT as substrates,the CNT loaded NiOOH nanosheets（Ni OOH@CNT）were prepared and the defect of poor electronic conductivity was avoided to some extend.In addition,Ni OOH@CNT was combined with carbon sulfur composite（SCM）by zeta potential self-assembly method（SCM/Ni OOH@CNT）.DFT calculations and experimental tests show that Ni OOH@CNT not only provides the ability to anchor Li PSs,but also accelerates the sulfur reduction reaction process（SRR）by promoting the breaking of terminal S-S bonds of Li PSs,which significantly improves the kinetic performance of this process and reduces the activation energy of Li₂S₄to Li₂S process from 23.79 k J mol^-1to 9.06 k J mol^-1.Assembled SCM/Ni OOH@CNT battery could stably cycle more than 400 times at a high rate of0.5 C and still maintains a high capacity of 813.3 m Ah g^-1,the capacity decay rate is only 0.07%per cycle,and still shows excellent capacity reversibility at a high discharge rate of 5 C.To validate the practical performance of SCM/Ni OOH@CNT,we assembled a 3 Ah SCM/Ni OOH@CNT 320 Wh kg^-1pouch-type cell.After 50 cycles at the rate of 0.2 C,it can still maintain a capacity of 849.1 m Ah g^-1,which proves that SCM/Ni OOH@CNT has great potential for practical application.2.The carbon nanotubes loaded atom-level Ni-N₄active sites（CNT@SA-Ni）were prepared by joule heating method.Spherical aberration TEM and synchrotron radiation prove that all Ni elements exist as the coordination of Ni-N₄.DFT theoretical calculations and experimental tests show that the Ni-N₄catalytic active site would coordinate with Li PSs by"Li-N"bond and"Ni-S"bond.Besides,and SA-Ni@CNT could not only catalyze SRR process by significantly reduce the Gibbs free energy of speed control steps（Li₂S₄to Li₂S₂and Li₂S₂to Li₂S）,but also catalyze SOR（Lithium Sulfides Oxidation Reaction）process by effectively reduce the energy barrier of the speed control step（Li₂S dissociation）.Benefit from bifunctional catalysis of SA-Ni@CNT,the overpotential of Li-S battery with SA-Ni@CNT coated separator decreased from 0.0263 V to 0.0195 V in the 100^thdischarge process,and the overpotential of charge process also decreased from 0.0449 V to 0.005 V in the 100^thcycle.The SA-Ni@CNT battery can still maintains the capacity of 829.8 m Ah g^-1after420 cycles at a high rate of 1 C,with a capacity decay rate of 0.137%per cycle and an average coulombic efficiency of 98.76%.As a comparison,the control battery has a capacity of 496.2 m Ah g^-1after 420 cycles with a capacity retention rate of 38.8%and a capacity decay rate of 0.225%per cycle,and an average coulomb efficiency of 94.79%.