| The choice of electrolyte determines the working mechanism of the battery and affects the specific energy,safety,cycle performance,charge-discharge performance,storage performance,and cost of the battery.Inorganic solid electrolytes greatly improve the safety of lithium-ion batteries.However,which is not high.The room temperature ionic conductivity of the inorganic solid electrolyte is increased by optimizing,modifying,doping or compounding with the polymer.In particular,the filling of inorganic solid electrolytes into polymer aggregates to prepare composite polymer electrolytes has become the focus of research in recent years.To prepare composite electrolyte with excellent electrochemical properties,it is first necessary to select a suitable inorganic fast ion conductor material.Generally,inorganic fast ion conductors with stable structure,high density,good thermal stability,and high ionic conductivity at room temperature are the preferred materials for researchers.NASICON-type inorganic solid electrolyte has a stable cubic structure with three-dimensional network skeleton[A2P3O12]composed of AO6 octahedron and PO4 tetrahedron.It has many advantages such as easy transmission of lithium ions,good density and thermal stability,high ionic conductivity at room temperature and so on.In addition,the choice of polymer matrix is particularly important.Polymers with low crystallinity,high dielectric constant,good thermal stability and high mechanical strength are ideal matrix frameworks for preparing composite electrolytes.Polyvinylidene fluoride(PVDF)and polyacrylonitrile(PAN)have attracted great attention because of their high dielectric constant,good electrochemical stability,strong hydrophobicity and good thermal stability.In this paper,the inorganic fast ion conductor Li1+xAlxTi2-x(PO4)3(LATP)was prepared by the simple solution method,and the effects of different Al doping concentration,calcining temperature and time about its conductivity were investigated.The inorganic fast ion conductor was filled into the PVDF and PAN polymer matrix.The inorganic-organic composite electrolyte LP and nanofiber composite electrolyte LPN were successfully prepared by simple casting method and electrostatic spinning method.The material characterization and electrochemical performance tests were carried out to explore the best mass ratio of LATP to PVDF and the optimal filling amount of LATP inorganic nanoparticles.The specific research content is as below:1.The effect of different Al doping concentration(x)on the conductivity of synthetic LATP was explored.With the increase of A1 doping concentration,the conductivity of the LATP ions was first increased and then decreased.When the doping concentration of Al was x=0.3,the room temperature ionic conductivity of LATP was the highest,and the activation energy was the lowest.The influence of calcination temperature on the conductivity of the LATP in the range of 800?950? had been investigated.The LATP solid electrolyte obtained at 900? has the most uniform grain size,the clearest grain boundary and the highest ionic conductivity.Therefore,900? is the most suitable calcination temperature for the LATP electrolyte.After determining the Al doping concentration of the LATP electrolyte and the optimum sintering temperature,the effect of sintering holding time on the conductivity of LATP was studied.The calcination time is too short,the reaction is not completed and unnecessary impurities are appeared,which affect the ionic conductivity.When the reaction time is too long,the densification degree of electrolyte is decreased,which hindered the lithium ion migration.The experiment shows that the best calcination reaction time is 4 h.Taking all factors into account,the optimum process conditions for the determination of the LATP synthesis are that the Al doping concentration is x=0.3(Li1.3Al0.3Ti1.7(PO4)3),the calcination temperature is 900?and the reaction time is 4 h.2.With PVDF as the matrix,inorganic fast ion conductors LATP with different mass ratios were added,and the inorganic-organic composite electrolyte LP was prepared by direct casting film formation method.SEM analysis showed that the inorganic fast ionic conductor LATP was uniformly dispersed in the PVDF matrix and the electrolyte membrane was dense inside.XRD and FTIR analysis showed that LP composite electrolyte film had similar diffraction peak of LATP and good crystallinity.With the increase of LATP filler ratio of inorganic fast ionic conductor,the crystallization peak of polymer membrane decreases gradually,but the position of the main absorption peak of FTIR in electrolyte membrane almost does not change,which shows that the combination of polymer and inorganic particles is only a physical mixture without any chemical reaction.TG and DSC analysis showed that with the increase of LATP inorganic content,the thermal decomposition temperature of LP composite electrolyte gradually increased,melting temperature and crystallinity gradually decreased.The anodic decomposition voltage of all the samples is greater than 5.02%(vs.Li/Li+),which can meet the practical requirements of all lithium ion batteries.When m(LATP):m(PVDF)=2:1,the electrochemical window of LP-2 is the widest,which is 5.67 V(vs.Li/Li+).the electrolyte uptake is the best,which is 171%,the ionic conductivity is the highest at room temperature and the interface stability is the best.The first discharge specific capacity of Li/LP-2/LiFePO4 battery assembled by LP-2 can reach 161.6 mAh g-1 at 0.1 C rate,and the corresponding capacity retention rate is 95.7%after 50 cycles,and its rate stability and cycle stability are better.3.Using PAN as the electrospinning matrix,the LATP were added to the system,the composite polymer electrolyte LPN was prepared by electrospinning.SEM analysis showed that the fiber diameter distribution of the LPN composite electrolyte membrane was smooth and uniform,XRD analysis showed that the LPN composite electrolyte membrane had the superimposed diffraction peaks of LATP and PAN,and the main characteristic peaks did not change in any way,and the crystallinity was good.TG and DSC analysis showed that the thermal decomposition temperature of the LATP composite electrolyte was increased slightly in the second stage,and the melting temperature was first decreased and then increased with the increase of LATP inorganic content.The anodic decomposition voltage of all the samples is greater than 5.12 V(vs.Li/Li+),which can meet the practical requirements of all lithium ion batteries.When the mass fraction of the LATP is 4%,the electrochemical window of LPN-4 is the widest of 0-5.68 V(vs.Li/Li+),and the electrolyte absorbency is the best of 231%,while the ionic conductivity at room temperature is the highest.The stability of the interface is the best and basically stabilized after 120 h.The stable impedance of the interface is about 365 ?,and the mechanical property is good.The first discharge specific capacity of Li/LPN-4/LiFePO4 battery assembled by LPN-4 is 165.2 mAh g-1 at 0.1 C rate.After 50 cycles,the capacity retention rate is 98.2%,the rate and cycle stability performance were excellent. |
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