Preparation And Performance Of Single Lithium-ion Conducting Polymer Electrolyte For Lithium-ion Batteries

Lithium-ion batteries?LIBs?are widely used in portable electronic devices and hybrid electric vehicles in our daily life owning to their high working voltage,wide working temperature,high energy density,long storage life,durable cycling life and so on.Electrolyte is the key part of lithium-ion batteries,which plays the role of lithium ion transportation between electrodes.Great progress on the development of lithium-ion batteries based on conventional organic liquid electrolyte has been made over recent decades.However,its potential safety problems cannot be ignored.The organic liquid electrolytes are inflammable and easy to leak,which limit the high-temperature use of lithium-ion batteries and pose a security risk in practical application,what's worse,this will lead to poor cycling stability and the current technology of lithium-ion batteries with organic liquid electrolytes can not meet the actual demand for energy density.Thence,it is a task brooks no delay to explore a more reliable electrolyte,and polymer electrolytes have attracted extensive interests.Polymer electrolytes provide a possibility to improve the safety and energy density of lithium-ion batteries.The flexibility of polymer materials determines that they are easier to form films and polymer lithium-ion batteries can be designed into different forms.The basic requirements for polymer electrolytes are ionic conductivity,lithium-ion transference number,safety,electrochemical stabilities,mechanical properties,interface stabilities and so on.In conventional dual-ion conducting polymer electrolytes,anions and lithium ions move in opposite directions under the applied voltage.Anions are easy to accumulate at the anode,leading to concentration gradient in the system,which results in low lithium-ion transference number as well as poor electrochemical performance.It has been reported that polymer electrolytes with a high lithium ion transference number have better electrochemical performance than those with a low lithium ion transference number,even the ionic conductivity of the latter is ten times that of the former.To overcome this problem,the best solution is to take full advantage of single lithium-ion conducting polymer electrolytes?SIPE?.In single lithium-ion conducting polymer electrolytes,anions are fixed on the polymer backbone through forming covalent linking.The most common anions are carboxylate,sulfonate,sulfonyl imide,boron centered and phosphorus centered anions.Different from conventional dual-ion conducting polymer electrolyte,in single lithium-ion conducting polymer electrolyte,the only mobile species are lithium ions.The lithium-ion transference number of SIPE is close to unity and thus can eliminate the concentration polarization to a certain extent,and then further improve battery performance.In the present work,we have introduced fluorine-containing acrylates into polymer electrolyte systems of lithium-ion batteries for the first time.We have designed two novel single lithium-ion conducting polymer electrolytes based on the copolymerization of hexafluorobutyl methacrylate?HFMA?and sulfonate-containing olefin monomers.Sulfonate-containing olefin monomers provide fixed anions and removable Li+.The strong electron-withdrawing group?-C-F?in the flexible side chain enables fluoropolymer to present a great electrochemical stability,which can also promote the dissolution and dissociation of alkali metal salts,thus creating a proper delocalization of negative charges of the system and thereby increasing carrier concentration.Lewis base group in the flexible side chain of HFMA plays the role of solvation and desolvation to promote the migration of lithium ions.?1?P?HFMA-co-ASLi?was firstly synthesized by copolymerizing hexafluorobutyl methacrylate?HFMA?and sodium allyl sulfonate?SAS?with a subsequent lithiation process.The simple one-step designed SIPE has outstanding thermal and electrochemical stability as well as high lithium-ion transference number.Structure of P?HFMA-co-ASLi?is confirmed through FTIR and NMR,and the average Mw is 1.04×10⁵ g mol^-1,which satisfied the requirements of forming self-standing film.Thermal properties and mechanical behavior are studied as well.The P?HFMA-co-ASLi?membrane is stable up to 300 ?.The breaking strength is 7.1 MPa and elongation at break is 14.8%.The lithium-ion transference number is higher than 0.9.The electrochemical window can reach 4.6 V,proving a high electrochemical stability,and the ionic conductivity can reach 10^-4 S cm^-1 at 80 ?,thus the battery made up with SIPE membrane exhibits excellent electrochemical properties during charge-discharge cycles at high temperatures.?2?P?HFMA-co-AMPSLi?was synthesized through copolymerizing hexafluorobutyl methacrylate?HFMA?and 2-acrylamido-2-methylpropanesulfonate?AMPS?,following a lithiation process.The simple one-step designed SIPE has outstanding thermal stability.Structure of P?HFMA-co-AMPSLi?is confirmed through FTIR and ¹H-NMR,and the average Mw is 2.11×10⁵ g mol^-1,which satisfied the requirements of forming self-standing film.Thermal properties are studied as well.The P?HFMA-co-ASLi?membrane is stable up to 290 ?.A high ionic conductivity of 10^-3 S cm^-1 at 40 ? can be achieved.