The Synthesis Of NASICON Type Solid Electrolyte And Its Application In Li-Air Battery

Nowadays,conventional Li-ion batteries with liquid organic electrolyte are important for a wide variety of applications ranging from the portable electronics to electric vehicles(EV).However,the safety issues of Li-ion batteries that originate from liquid electrolytes are of greater concern in large batteries for EV and the lithium intercalation/de-intercalation materials determine the limited energy density.Rechargeable Li-Air batteries(aqueous and non-aqueous)are attracting increasing attentions in the past decade due to its remarkably high theoretical energy densities,3460 Wh·kg-1 for non-aqueous Li-Air battery and 3860 Wh·kg-1 for aqueous Li-Air battery,which is comparable to gasoline.In an aqueous Li-Air battery,a water stable and impermeable lithium ionic conductor is sandwiched between the lithium anode and the air cathode to prevent the lithium metal anode from reacting with H20 and 02.Generally speaking,non-aqueous Li-Air battery is composed of a Li metal anode,a porous cathode and an organic electrolyte.However,the solubility of 02 in electrolytes results in the inevitable crossover of 02 from the cathode to theLi anode to react with the Li anodes,which lead to a degradation and consumption of the Li anode.Therefore,using solid state electrolytes protected lithium metal anode,can not only improves the safety but also increases the energy density and stability of next-generation batteries.NASICON type lithium ionic conductors continue to attract considerable interest because of its relatively high Li ionic conductivity,large range of operating temperature,high mechanical strength and steady chemical stability in ambient amtosphere and water.The NASICON type Li1.4A10.4Ti1.6(P04)3(LATP)micro-crystal powder was synthesized by citric acid assisted sol-gel method.The LATP powder was firstly crystalized at 850? for 4h,and the XRD patterns showed that the NASICON structure was obtained and without any impurity phases.The LATP films were prepared by tape casting method by using as-synthesized LATP powder and subsequently recrystalized at various temperature for 5h.The Impedance spectra of LATP film recrystalized at various temperature indicated the film sintered at 950? had highest lithium ionic conductivity and it was demonstrated that no impurity existed in LATP films recrystalizated at 950?.Structural refinement was carried out with Rietveld technique(GSAS program),and the lattice parameters were a=b=8.49647 A and c=20.79251 A.The high-purity LATP-Epoxy films were prepared by modification with epoxy resin.The fracture surface topography of LATP film and LATP-Epoxy film showed,some micron-size holes exist in LATP film and the dense structure was found in LATP-Epoxy film.The EDX mapping of the fracture of LATP film and LATP-Epoxy film indicated carbon uniformly distributed in LATP-Epoxy films,which means epoxy resin had soaked into LATP film.The relative density of 89.5%was obtained for LATP film,which was increased to 93.0%for LATP-Epoxy film.The difference in the relative densities for LATP film and LATP-Epoxy film indicated the epoxy resin had been immersed in LATP film successfully.The total,bulk,and grain boundary lithium ionic conductivities for the LATP film at 25? were 9.35×10-4 S cm-1,2.69×10-3 S cm-1 and 1.46×10-3 S cm-1,respectively.The total,bulk,and grain boundary lithium ionic conductivities for the LATP-Epoxy film at 25? were 3.35×10-4 S cm-1,1.84×10-3 S cm-1 and 4.09×10-4 S cm-1,respectively.The high lithium ionic conductivity for both LATP film and LATP-Epoxy contributed to homogeneous mixture at sol-gel process and the decreasing grain boundary impedance for this special structure.The activation energy for LATP film and LATP-Epoxy film were 0.35 eV and 0.34 eV respectively,based on Arrhenius equation.The NASICON type Li1.45Al0.45Ge0 2Ti1.35(PO4)3(LAGTP)micro-crystal powder was syn-thesized by solid state reaction.The LAGTP lumps were firstly crystalized at 900? for 5h,and the LAGTP films were recrystalized at 900? for another 5h which were also prepared by tape casting method.The XRD patterns of LAGTP lumps and films showed that NASICON structure were obtained and without any impurity phases.The lattice parameters were a=b=8.47843 A and c=20.77428 A,which were a bit smaller than LATP's due to the substitution of Ti4+(ionic radius 0.605 A)by Ge4+(ionic radius 0.530 A).A special structure,which composed of LGATP micro-crystal clusters with the long crack and micron-size hole,was found inside the LAGTP films by the fracture surface topography.The total,bulk,and grain boundary lithium ionic conductivities for the LAGTP film at 25? were 7.82×10-4 S cm-1,1.60 x 10-3 S cm-1 and 1.53×10-3 S cm-1,respectively.The activation energy for LAGTP film were 0.34 eV based on Arrhenius equation.The water permeation test proved the LATP-Epoxy films can prevent water penetrating for 15d,which indicated that epoxy resin filled the holes in LATP films.The electrochemical impedance spectra(EIS)of aqueous Li-Air battery with LATP-Epoxy film as separator for various time af-ter assembled,indicated without any H2O penetrate from cathode to anode.The resistances of each part of aqueous Li-Air battery were measured by EIS and fitted by Zview software.Gal-vanostatic discharge-charge measurements of aqueous and non-aqueous Li-Air batteries indicated LATP-Epoxy film can used as a protective layer for Li metal anode of novel high energy density batteries.Future work is needed to decrease the interface resistance between solid state electrolyte and catholyte and improve the cycle performance of Li-Air battery.