Concentrated Electrolyte Application For Potassium Battery

Potassium ion batteries?PIBs?have great potential application for large-scale electric energy storage systems to storage renewable energies,benefiting from its merits of the abundance and low cost of potassium resources as well as the nearby standard electrode potential of potassium with lithium.Although the electrode materials of PIBs have been made some progresses,the development of electrolytes are seriously lagging-behind and mainly relyed on potassium hexafluorophosphate?KPF₆?,which gravely restricts the rapid development of PIBs.Therefore,it is of great significance to explore new electrolyte systems,and analyze the influence of different electrolytes on electrode materials,as well as investigate the applications of electrolytes in the full batteries.In this dissertation,potassium bis?fluoroslufonyl?imid?KFSI?was used to prepare high concentration electrolytes due to its high solubility in organic solvents includin g of 1,2-Dimethoxyethane?DME?,Ethylene carbonate?EC?,Ethyl methyl carbonate?EMC?,and so on.We further analyzed the physical properties of high concentration electrolytes,and applied the high concentration electrolytes in carbon-based anode materials,organic cathode materials,dual ion batteries,as well as the full batteries,and they all exhibit excellent electrochemical performance.We also made thorough discussions on the function mechanisms of high concentration electrolyte in different electro de materials via various measurements,such as X-ray photoelectron spectroscopy?XPS?,ex-situ or in-situ X-ray diffraction?XRD?,Raman spectrum,and transmission electron microscope?TEM?.The results and conclusions may provide some guidances and references for the development and commercialization of PIBs.The details of this dissertation are summarized as follows:1.Soft carbon with ether-based functional groups was prepared?named as E-carbon?,and we found that the exsistence of ether-based functional groups make the E-carbon storage potassium with more favorable themodynamics process.Besides,a new type of high concentration electrolyte?3M KFSI@DME?was prepared to explore the different behavior on E-carbon electrode between this high concentration electrolyte and traditional electrolyte?0.8M KPF₆@EC:EMC,1:1,v/v?.It could be found that the solid-electrolyte interface?SEI?contents were quite different at E-carbon surface when using two types of electrolytes.For high concentration electrolyte,the SEI content on E-carbon surface is mainly composed of inorganic compounds due to the decomposition of KFSI salt.While for the traditional electrolyte,more organic compounds were found at E-carbon surface because of the severe decomposition of EC and EMC solvents.The different SEI content further result in quite different electrochemical performance of E-carbon when using two types of electrolytes.As a result,the E-carbon with high concentration electrolyte exhibits higher Coulombic efficiency,higher reversible capacity,superior rate performance and much more excellent cycle stability.In particular,the E-carbon could exhibit a long stability over 14000 cycles at 2000 mA g^-1 with high average Coulombic efficiency over 99.9%,which is the longest cycle stability of anodes for PIBs up to now.2.On the premise of not destroying the C=O bonds?used for storage potassium?of Perylene-3,4,9,10-tetracarboxylic dianhydride?PTCDA?,the electrical conductivity was enhanced to 5.32×10^-66 S m^-1 by simply annealing treatment,making the annealed PTCDA have the potential of high rate performance.The electrochemical performance of annealed PTCDA was investigated with several different electrolytes,and we found that the dissolution of annealed PTCDA into organi c electrolyte should responsible for the irreversible phase transformation?by ex-situ XRD?,wich could further result in rapidly capacity decay.Besides,we also found that KFSI is more easily solvation with DME solvent than KPF₆,this likely account for the electrochemical performance of annealed PTCDA is more stable in KFSI based electrolyte.Therefore,we utilize the high concentration electrolyte?3M KFSI@DME?to suppress the dissolution of annealed PTCDA,result in a high reversible phase transition,further leading to enhanced cycle stability.Specially,the annealed PTCDA with the high concentration electrolyte could operate long-term cycle life over 8months at high area capacity of around 2 mAh cm^-2?which is the longest cycle time of cathode for PIBs up to now?.3.The solvation effect of different salt-to-solvent?KFSI:DME,1:10,1:5,1:2.5,1:1.5?mole ratios was investigated by Raman spectrum,and found that the solvation effect is enhanced with increased salt concentration.We utilize this unique solvation effect to suppress the dissolution of Polyaniline?PANI?cathode,promoting the PANI electrode exhibit high energy density and power density as well as high stability.The stability of PANI at high concentration electrolyte could be further i nterrupted as follows,by increasing salt concentration in the organic electrolyte,some free solvent molecules are eager to interaction with the salt ions,and it will decrease the solvent molecules that could interaction with the organic electrode,thus leading to a suppressed dissolution of organic electrode,resulting in enhanced cycle stability.Consequently,the PANI electrode along with high concentrated electrolyte?KFSI:DME,1:1.5?could deliver high rate performance,long cycle stability over1000 cycles,high energy density of 300 Wh kg^-1 and high power density of 9800 W kg^-1.4.The commercial prospect of commercial graphite in PIBs was evaluated by high concentration electrolyte?KFSI:DME=1:2.5,mole ratio?and traditional electrolyte?0.8M KPF₆@EC:EMC,1:1,v/v?.The SEI of graphite with different electrolytes were investigated by operando XRD and XPS,and found that around 60%SEI was formed before the process of K⁺insert into the graphite for high concentration electrolyte,while that is only 28%for traditional electrolyte.Besides,the content of SEI for high concentration electrolyte is mainly composed of inorganic compounds,while mainly organic compounds for traditional electrolyte.The different SEI formation process and composition further determine their different performance on graphite electrode when using different electrolytes.Consequently,the graphite electrode with high concentration electrolyte could exhibit long stability over 1700cycles without obvious capacity decay,corresponding to a running time of over 15months,much superior than graphite electrode with traditional electrolyte?rapidly capacity decay within 300 cycles?.5.A nonaqueous potassium ion battery-supercapacitor hybrid device?BSH?was developed with soft carbon as anode,activated carbon as cathode,and 3M KFSI@DME as electrolyte.The charge-discharge mechanism of BSH was investigated by various measurements,and found that FSI^-in the electrolyte interacts with the activated carbon cathode while the K⁺in the electrolyte intercalates into soft carbon anode during the charge process;conversely,the FSI^-/K⁺will depart from cathode/anode and return to electrolyte during discharge process.This BSH reconcile the advantages of potassium ion batteries and supercapacitors,achieving high energy density(120 Wh kg^-1),high power density(599 W kg^-1),long cycle life?1500 cycles?,and ultrafast charge/slow discharge performance.