Studies On The Positive Electrode Reaction Process And Product Structure Regulation Of Potassium-oxygen Battery

Since their invention in the 1990s,Lithium-ion batteries(LIBs)are gradually approaching their theoretical energy density limit.Therefore,lithium-oxygen batteries(LOB)with high energy density(theoretical value≈3505 Wh kg-1)have attracted intensive attention from researchers.However,due to the high chemical reaction inertness of the discharge product lithium peroxide(Li2O2),LOB requires a high charging voltage(>4.0 V vs.Li/Li+),which tends to cause severe side reactions and low round-trip efficiency(<60%).Potassium-oxygen batteries(KOB)are a promising system for large-scale energy storage due to the highly reversible O2/O2-redox reaction and low charging overpotential,which enables high energy density(≈ 935 Wh kg-1)and round-trip efficiency(>90%).However,due to the low conductivity(10-13 S cm-1)of the discharge product potassium superoxide(KO2),the positive discharge process of KOB mainly depends on the solution-mediated O2-nucleation and growth,while the surface-mediated progress leads to low capacities.Moreover,in the electrolyte with high discharge capacity,KOB shows a slow positive electrode reaction rate and a high overreaction potential.To address the dilemma between the discharge capacity and reaction kinetics for KOB,this paper systematically studied the influences of cationic additives and the electrolyte composition on the property of the discharge product and the cathode electrochemical process,thus achieving simultaneous improvement of the discharge capacity and reaction kinetics for KOB batteries.The main conclusions are as follows:(1)We developed a cation-composite organic electrolyte(0.5 M MTFSI DME(K:Cs=1:9,molar ratio)),which could improve the KOB cathode discharge capacity and reaction kinetics.At a current density of 0.25 mA cm-2,the composite electrolyte significantly reduced the overpotential during the charge-discharge process of the cathode(from 120 mV to 50 mV),and increased the discharge capacity(from 2.58 mAh cm-2 to 4.96 mAh cm-2).The analysis of the nucleation and growth mechanism of the product shows that the nucleation and growth of KO2 grains happened throughout the discharge process,and Cs+ doping into the bulk phase of KO2 grains changed their morphology.In the deposition/stripping of KO2,the cycling Coulombic efficiency and cycling stability in the cation-composite organic electrolyte are improved compared to that in the single-cation electrolyte(0.5 M KTFSI DME).(2)We also developed a solvent-composite organic electrolyte(0.5 M KTFSI DME:DMSO=6:4(volume ratio,v:v)),which can significantly improve the KOB cathode discharge capacity and reaction kinetics.At a current density of 0.25 mA cm-2,the solvent-composite electrolyte significantly reduced the overpotential during the charge-discharge process of the cathode(from 120 mV to 40 mV),and increased the discharge capacity(from 2.58 mAh cm-2 to 4.97 mAh cm-2).According to the results of infrared,Raman and NMR analysis,both the high-donor number solvent(DMSO)and the low-donor number solvent(DME)participate in the formation of the K+solvation structure in the composite electrolyte.Therefore,the cathode processes benefit from the fast reaction kinetics and solution-phase growth process assisted by DMSO molecules,as well as the quick desolvation ability and better oxygen dissolution assisted by DME molecules.At a current density of 0.25 mA cm-2,the deposition/stripping reversibility of KO2 is significantly improved,with a long cycle life of 244 cycles and an average Coulombic efficiency as high as 99.5%.Compared with the single-solvent electrolyte(0.5 M KTFSI DME),the novel solvent-composite organic electrolyte can significantly improve the KO2 deposition/stripping efficiency(>10%)and cycle life(5～6 times)during the cathode reaction processes.