High Energy Density Copper-Based Cathode Materials For Li/Na Ion Batteries:Fabrication And Properties

Various new clean energy storage technologies,which are developed to meet the requirement for sustainable development,are becoming the rigid demand of the times.Multiple energy storage techniques coexisting is the growing trend due to the different technical requirements in various application fields.Lithium-ion batteries(LIBs)and sodium-ion batteries(SIBs),as two essential parts in rechargeable batteries,are the critical technologies for electrical vehicles(EVs)and large-scale energy storage,respectively.The energy density of LIBs,however,still cannot meet the long-driving range requirement of EVs.Similarly,the energy density of SIBs still cannot satisfy the need for large-scale energy storage.Basically,enhancing the specific capacity and operating voltage of cathode materials is an effective strategy to improve the energy density of batteries.To develop high energy density cathode materials,this dissertation combines the subject of Condensed Matter Physics with Materials Science,and is focused on high potential and low cost copper-based materials,from the point of material molecule design,electrode structure design,electrolyte regulation and electrochemical mechanism analysis.Firstly,we designed and synthesized a metal-organic composite cuprous 7,7,8,8-tetracyanoquinodimethane(CuTCNQ).Using CuTCNQ as a cathode for SIBs,both Cu cations and TCNQ anions participate in the redox reaction thus deliver a three electrons redox reaction.The theoretical specific capacity of CuTCNQ is as high as 300 mAh/g.Benefited from the high redox potential of Cu+/Cu2+(3.8 V vs.Na+/Na),the operating voltage of CuTCNQ is improved to-3.0 V.Compared with common organic electrodes,CuTCNQ exhibits higher energy density.Further research demonstrated that CuTCNQ electrode shows better electrochemical performance after nanosizing and compositing with carbon nanofiber.CuTCNQ electrode synthesized by solution reaction always possesses poor electronic conductivity and large particle size,which will deteriorate the rate and cycling performance for the Na-storage.To nanosize the CuTCNQ particles and improve its electronic conductivity,we synthesized a CuTCNQ/carbon nanofibers(CNFs)composite electrode that anchoring CuTCNQ on conductive CNFs by combining the electrospun technique with one-step solution reaction.The nanosized CuTCNQ particles reduce the Na+diffusion distance while the CNFs network improves the electronic conductivity of the electrode.As a result,the CuTCNQ/CNFs composite electrode exhibits superior rate capability and better cycling salability compared with the CuTCNQ powders synthesized by solution reaction.CuTCNQ is a kind of organic electrode with small molecule weight,and the dissolution of its redox products in the electrolyte causes the serious capacity fading.High concentration electrolyte can suppress the dissolution by alternating the complexing structure of electrolyte solvent.In this dissertation,we demonstrated the electrolyte structure changes by alternating the electrolyte concentration.When the concentration is higher than 5 mol/L,the number of free solvent molecules is decreased rapidly,thus the dissolution of redox intermediates has been suppressed.As a result,the cycling stability of CuTCNQ electrode is obviously improved,from irreversible in 1 mol/L electrolyte to be stable up to 50 cycles in 7 mol/L electrolyte.To achieve high energy density cathode materials for LIBs,inspired by the redox reaction of Cu+/Cu2+in CuTCNQ electrode and the working mechanism of Zn/Cu primary battery,we demonstrated the feasibility of metallic Cu as the cathode for LIBs.In Cu/Li half cell,the Cu cathode delivers a discharge capacity of 635 mAh/g,and the output voltage is 3.61 V(vs.Li+/Li)in carbonate electrolyte.It also displays superior rate and cycling performance.With Li4Ti5O12 as an anode,a Cu/Li4Ti5012 full cell exhibits a discharge capacity of 601 mAh/gCu and an energy density of 1202 Wh/kgCu.Computational modelling and in-situ ultraviolet-visible spectra confirmed that Cu cathode experiences a two-electron redox reaction:Cu ??Cu2++2e-during charge/discharge process,and exhibits a theoretical capacity of 837 mAh/g.