| With the rapid development of electric vehicles and the increasing demand for grid energy storage,the demand for batteries with high energy density and high safety is increasingly urgent.All-solid-state lithium metal batteries are considered as the next generation energy storage equipment due to their safety and potential high energy density.Solid-state electrolytes,as the most critical component of the solid-state battery,will lead the development of future batteries to a large extent.Among the various types of solid electrolytes,composite solid electrolytes can combine the advantages of inorganic electrolytes(such as superionic conductors)with polymer electrolytes and are considered to be the most likely candidate for commercialization of solid lithium metal batteries.However,the ionic conductivity of the composite solid electrolyte is still insufficient compared with the traditional liquid electrolyte.In the composite solid electrolyte,lithium ions can be transported through multiple paths,including the superion conductor phase,the polymer phase,and the superion conductor/polymer interface.However,the ion transport mechanism of composite solid electrolytes depends largely on the type,structure,morphology of inorganic electrolyte and the interface between inorganic electrolyte and polymers.Heteroepitaxial phases are formed on the surface of superionic conductor by surface functionalization.In order to improve the electrochemical performance of the composite solid electrolyte,the ion transport mechanism in the composite solid electrolyte was systematically studied.Firstly,the superionic conductor Li6.4La3Zr1.4Ta0.6O12(LLZTO)shows the characteristics of high conductivity,wide electrochemical window and stability against lithium.The polymer PVDF has strong polar C-F bonds and a dielectric constant of up to 8.4,giving it the ability to dissolve lithium salts.Therefore,PVDFLLZTO composite solid electrolyte membrane was prepared by pouring method.The influence of the interaction between superionic conductor and polymer on the comprehensive performance of the composite solid electrolyte is systematically studied when LLZTO is used as the main lithium-ion conduction phase.With the increase of LLZTO concentration in the composite solid electrolyte,the superionic conductor forms a continuous inorganic conductive network.At this time,the migration of lithium ions is more inclined to the superionic conductor region,while the insulated(i.e.lithium free)PVDF only acts as a binder to provide shape support.The gradual increase of ionic conductivity and ionic migration of the composite solid electrolyte also brings the problems of film forming difficulty and poor contact with the electrode.When the mass fraction of LLZTO reaches 90%and 80%,respectively,the lithium-ion conductivity of the composite solid electrolyte membrane is 2.16×10-5 S cm-1 and 1.06×10-5 S cm-1,and the lithium-ion transference number is 0.88 and 0.83,respectively.However,considering the film forming property,mechanical property,and compatibility with electrodes of the composite solid electrolyte membrane,PVDF-80%LLZTO composite solid electrolyte showed relatively good comprehensive performance.At 2C,the discharge capacity of the solid-state battery is 85.3 mAh g-1,and the capacity retention rate is 93.6%at 0.2C cycle 150 cycles.Secondly,the transport of lithium ions in the superionic conductor is mainly affected by the vacancy concentration in the lattice,the types of surrounding ions,and the configuration and distance of doped cations.In order to improve the lithiumion conductivity of the composite solid electrolyte,the La2Sn2O7 interface layer with pyrochlore structure is constructed on the surface of LLZTO using the surface functionalization strategy.The formation of La2Sn2O7 interface layer successfully introduces defects in the superionic conductor,changed the environment around lithium ions in the crystal structure,and accelerated the transport of lithium ions.In addition,the OH-in LiOH is consumed during the formation of the interface layer,which increases the concentration of free lithium ions.Under the synergistic action of many aspects,the electrochemical performance of the composite solid electrolyte is obviously improved.By constructing 0.9wt%La2Sn2O7 on the surface of LLZTO,the lithium ion conductivity of the composites electrolyte is 1.30×10-4 S cm-1.In addition,the high impedance at the interface between the anode and solid-state electrolyte has been one of the main challenges faced by solid-state battery technology.The integration of positive material and composite solid electrolyte is realized by transfer printing technology,which reduced the interface resistance of the two from 296 Ω to 213 Ω.The solid-state battery with LiFePO4 as the cathode,lithium metal as the anode,0.9wt%La2Sn2O7 modified composite material as the electrolyte interlayer has a discharge capacity of 128.1 mAh g-1 at the rate of 2C,and a capacity retention rate of 86%at 0.2C after 200 cycles.Thirdly,the morphology of superionic conductor in polymer matrix and the transport mechanism of lithium ions are designed by surface functionalization.The LaRuO3 interface layer is constructed on the LLZTO surface to transform LLZTO from 0D nanoparticles into a 3D inorganic skeleton structure.The formation of the LaRuO3 interface layer in addition to the introduction of defects in the inorganic filler;The induced Li2Zr2O7 phase can connect the inorganic filler into a 3D inorganic skeleton structure,prolong the permeation path of lithium ions,and increase the active site between the superionic conductor and the polymer matrix.Therefore,the electrochemical performance of the composite solid electrolyte is further improved.The 3wt%LaRuO3 is synthesized on the surface of LLZTO,and the lithium-ion conductivity of the composite electrolyte is up to 6.06×10-4 S cm-1.The corresponding solid-state battery discharge specific capacity at 2C is 132.2 mAh g-1.After 200 cycles at 0.2C,the specific discharge capacity of solid-state battery is 136 mAh g-1.After 400 cycles,the specific discharge capacity of the solidstate battery remained at 129.6 mAh g-1.Finally,in addition to the superionic conductor itself,the superionic conductor/polymer interface undoubtedly provides a transport path for lithium ions.The Lix-CeO2 composite phase has lattice characteristic parameters much larger than the diameter of lithium ions and chemical/electrochemical properties similar to those of CeO2.The Lix-CeO2 composite phase with an efficient continuous ion transport pathway is constructed at the polymer/superionic conductor interface.The Lix-CeO2 composite phase not only widens the transport path of lithium ions,but also the oxygen vacancy contained in it can anchor the OH-in LiOH and increase the concentration of migrating lithium ions.When the mass fraction of the Lix-CeO2 composite phase is 2%,the lithium-ion conductivity of the composite solid electrolyte is increased to 2.325 ×10-3 S cm-1,and the discharge specific capacity of solid-state battery is as high as 134.8 mAh g-1 at 2C,and the discharge specific capacity remains at 135 mAh g-1 at 0.2C for 200 cycles. |
PDF Full Text Request