Interface Modification Of NASICON Solid Electrolyte And Synthesis Of New Solid Electrolyte

Lithium-ion batteries are widely applied in portable electronic equipment,electric vehicles,sensors and other fields.However,the organic electrolyte used in traditional lithium batteries is flammable and corrosive,and it is easy to cause safety accidents.At the same time,the complex battery structure greatly limits the further improvement of energy density.Inorganic solid electrolytes have been widely studied in recent years due to their advantages such as high safety,stable electrochemical window,strong stability,high mechanical strength and high energy density.NASICON-type solid electrolyte Li_1.3Al_0.3Ti_1.7?PO₄?₃?LATP?with high ionic conductivity,stability in air,and a wide electrochemical window endows it with a huge potential in all-solid-state battery applications.However,LATP as a solid electrolyte has a very fatal defect in all-solid-state lithium batteries.Metal Li and Ti⁴⁺in the electrolyte continuously undergo a reduction reaction,which causes the electrolyte to deactivate and the performance to decline so sharply that greatly hinders its development and application.To address this problem,we modified the surface of the LATP solid electrolyte to improve the stability between the electrode and the electrolyte interface,and then assembled it into an all-solid-state lithium battery and a symmetrical battery for electrochemical cycling.In this paper,we focus on the application of interface-modified LATP-based solid electrolyte to develop all-solid-state lithium metal batteries and new solid electrolytes,and have achieved the following research results:?1?Sericin with multifunctional polar functional groups is used as the electrolyte interface modification layer,and its abundant lithium-philic sites?-OH,-COOH and-CO-NH-?facilitate the free conduction of lithium ions in three dimensions.At the same time,sericin film as a"barrier layer"can hinder electrons,solving the problems of direct contact between Li metal and LATP solid electrolyte surface and the continuous conduction of electrons to reduce Ti⁴⁺to Ti³⁺effectively which causes the electrolyte structure to collapse.In Li/LFP all-solid-state batteries with sericin-coated LATP solid electrolyte,a long electrochemical cycle life and an average high Coulomb efficiency of 99.3%can be obtained.Under the test condition of the current of 0.05 mA cm^-2,the Li/Li symmetrical battery has a cycle stability of more than 850 h,while the symmetrical battery with unmodified LATP has a greatly increased polarization after200 h of cycling.SEM results showed that the surface morphology and thickness of the sericin membrane remained stable before and after cycling.The successful operation of Li/Fe₃ full battery and its good electrochemical performance indicate that the sericin film has a good"tolerance"to the volume change of the electrode.?2??-CD,which has a strong encapsulation of compounds,was used as the LATP solid electrolyte interface modification layer to increase the stability of LATP to photothermal and oxygen.?-CD has abundant-OH,which can react with Li ions to form-OLi to conduct inside,and at the same time,it can hinder the transfer of electrons,effectively solving the direct contact between Li metal and LATP solid electrolyte due to the characteristics of its inorganic substances,which reaction caused the problem of failure of the solid electrolyte.During battery cycling,?-CD will gradually infiltrate into the upper surface structure of LATP and complex with LATP,enhancing the stability of the LATP interface.In Li/LFP all-solid-state batteries with?-CD modified LATP solid electrolyte,a long electrochemical cycle life and an average Coulomb efficiency of 99%can be obtained.Under the test conditions of 0.05 mA cm^-2,the Li/Li symmetrical battery has a cycle stability of more than 1360 h.In a Li/LFP full battery,at a current density of 0.2 mA cm^-2,after 200 cycles of the full battery,the discharge specific capacity can still remain at 141.5 mAh g^-1.The discharge specific capacity can still remain at113.9 mAh g^-1 even at a current density of 0.4 mA cm^-2 after 110 cycles.?3?K₂C₆O₆,as a potential cathode material for sodium batteries,has good conductivity for Li,Na,and K ions.At the same time,it is extremely difficult to conduct electrons as an organic matter.Thus,using K₂C₆O₆ as an LATP solid electrolyte interface modification layer can be very effective at improving the interface stability between electrolyte and electrode during battery cycling.The electrochemical cycling performance of batteries with K₂C₆O₆ modified LATP solid electrolyte has been significantly improved.Li/Li symmetrical batteries cycled more than 1200 h at a current density of 0.05 mA cm^-2.And Li/LFP full cells also show good cycle performance and extremely high average Coulomb efficiency.?4?Na metal is abundant,cheap and has similar physical and chemical properties to Li.In recent years,all-solid-state sodium batteries have been widely studied.The high conductivity solid electrolyte is a key part of the development of Na-based solid-state batteries.However,prototypes of electrolyte structures and corresponding synthetic methods are still lacking.Conventional oxide and sulfide electrolytes have expensive raw materials?such as Na₂S?or high energy consumption?such as synthesis at 1000°C?in sintering synthesis.Here,we propose a new type of sodium-enriched antiperovskite solid electrolyte?Na₃SO₄F?,which is rich in Na and fluoride,and it is synthesized by solid-phase reaction with low-cost raw materials at moderate temperature?500°C?.We further co-doped Mg and Cl ions at 60°C to increase their ionic conductivity by three orders of magnitude,close to 10^-4 S·cm^-1.The generation of Na vacancies?by doping with magnesium ions?and lattice expansion?by replacing F sites with larger-sized Cl ions?are the main reasons for the increase in conductivity.Na-Sn/Fe[Fe?CN?₆]₃solid-state battery based on Na_2.99Mg_0.01SO₄F_0.95Cl_0.05 electrolyte can be operated reversibly,with the first discharge capacity up to 91.0 mAh g^-1,and 77.0 mAh g^-1 reversible capacity.