| At present,the main obstacles hindering the further development of lithium batteries are liquid electrolytes,which are highly flammable,leaky,and toxic,which lead to a significant increase in the thermal instability inside lithium batteries,and cause the many safety problems such as thermal runaway and explosion.Solid electrolytes will become an inevitable choice for the high requirements of future lithium-ion batteries because of its high safety characteristics.In light of many advantages,such as the high ionic conductivity,good stability and electrochemical performance,stable contact with lithium metal,wide electrochemical window etc,garnet-type electrolyte materials have become the preferred choice among inorganic solid electrolyte materials.However,garnet-type solid electrolyte posses the poor solid-solid interface contact after being applied to lithium-ion solid-state batteries,which would lead to excessive interface resistance and be easy to cracking during assembly.The solid polymer electrolyte represents thesignificant flexibility so that it maintains the better solid-solid interface contact with the electrodes,as well easy to machine into various shapes.Unfortunately,there is a low ionic conductivity at room temperature.In our work,based on the advantages of garnet solid-state electrolyte and virtues of polymer electrolyte PVDF,a composite electrolyte material with excellent performance was successful constructed and further assembled into solid-state lithium-ion battery for electrochemical performance study.The main research contents were included as follows:(1)Li6.5La3Zr1.5Nb0.5-xTaxO12(x=0-0.5)garnet-type solid electrolyte was prepared using super-valent Nb5+ and Ta5+occupying the Zr4+position in the Li7La3Zr2O12 structure.Results showed that the obtained electrolyte materials are cubic garnet-structure.Both of Nb5+and Ta5+ occupy Zr sites to regulate the lithium ion migration channel to make lithium ion easy to migrate.When x=0.25,the ionic conductivity of Li6.5La3Zr1.5Nb0.25Ta0.25O12 reached to be 2.57×10-4S/cm at 17?,and its activation energy in the range of 213K-253K is only 0.301 eV.The symmetrical battery was assembled by using of Li6.5La3Zr1.5Nb0.25Ta0.25O12 as electrolyte and Li as electrodes.The battery was measured at a current density of 0.2 mA cm-2 at 28?.It was found that the uneven current distribution of lithium metal at the interface of Li/LLZNTO was occurred,and the uneven deposition of Li metal on the surface of the solid electrolytecan be observed.The discharge specific capacity for full battery assembled by LiFePO4/Li6.5La3Zr1.5Nb0.25Ta0.25O12/Li was increased from 54.54mAh/g to the maximum value of 117.16mAh/g at 0.1C,and then decreased to 31.11mAh/g,indicating that the battery assembled with this electrolyte has poor charge-discharge cycling performances.(2)Two types of composite electrolyte materials were prepared by mixing PVDF/TiO2/LLZNTO or PVDF/LITFSI/LLZNTO.Experimental results showed that the combination of PVDF and LITFSI with a mass ratio of 3/2 and 10%LLZNTO presented the ideal performance.The electrical conductivity at 30? is 3.53 × 10-4 S/cm,and the stablilized window voltage for the battery contained of the composite electrolyte and Li is up to 5.0V.Meanwhile,the samples containing 5%TiO2 and 15%LLZNTO combined with the PVDF showed the best performance with the 2.6%porosity and 2.89× 10-4 S/cm at 30?.The stablilized window voltage for the battery composed of the composite electrolyte with Li is up to 4.5 V.The samples showed the good stability for lithium metal and the better of mechanical properties.The capacity was obtained to be 117.9mhA/g and the coulomb efficiency was 99.9%for the assembled LiFePO4|Li cell after charge-discharge for 25 cycles at 0.1 C. |
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