Structural Manipulation And Properties Of NASICON-type Lithium-ion Solid-state Electrolytes

Lithium-ion batteries have become the important development direction of the next-generation lithium-ion batteries due to the high safety and high energy density.Lithium-ion solid-state electrolytes have thus been the Achille heel of the all-solid-state batteries that affect the final electrochemical performance greatly.High ionic conductivities are the most important property,and improving ionic conductivities has always been the research focus.From the perspective of structural design,the ionic conductivity is determined by grain structure and grain boundary,in this paper,the widely concerned NASICON-type solid-state electrolytes Li_1.3Al_0.3Ti_1.7（PO₄）₃（LATP）and Li Zr₂（PO₄）₃（LZP）have been selected as the research targets,when combining other structural characterizations such as Electrochemical Strain Microscopy（ESM）and Transmission Electron Microscopy（TEM）,the influence of grain and grain boundary structure on the ionic conduction behavior can be studied.On the other hand,synchrotron radiation X-ray has been applied to investigate the changing of solid-state surface structure under different atmosphere,and the corresponding influence on ionic conductivity performance.Firstly,the influence of grain boundary structure on ionic conductivity performance has been investigated.Grain boundary is the dominant factor that limits the total ionic conductivity.However,the influence factors on grain boundary conductivity are complicated,and the research about grain boundary structure needs deep understandings.In that case,the influence of sintering temperature on LATP glass-ceramic phase structure and microstructure was investigated.Through Spark Plasma Sintering（SPS）,a short-term ordered phase structure formed between glass and crystalline phase interlayer.Combining ESM technology,the local electrochemical response in the microstructure can be obtained.When comparing the difference of grain boundary structures and ionic conductivities of two samples that synthesized through conventional sintering and SPS,it is suggested that the major difference of ionic conductivities comes from the existence of the short-term ordered phase.Moreover,B element was doped in LATP glass-ceramic,the glass phase around grain boundary structure was optimized to improve the grain boundary ionic conductivity,thus leading to the higher total ionic conductivity.On the other hand,the crystal structure would affect the prefactor and activation energy during the ions transport,thus affecting the final ionic conduction behavior.In this case,a novel solution-based synthetic method has been developed to synthesize room temperature stable rhombohedral LZP ceramic electrolytes.Through optimizing preparation parameters such as lithium content,the ionic conductivity can be increased up to 1.91×10^-4S/cm,this conductivity is the highest one reported up to now.Besides,Ga³⁺was selected to be doped in Li site and Zr site in LZP structure and obtained Li_1-3xGa_xZr₂（PO₄）₃（x=0,0.02,0.05,0.1）and Li_1+xGa_xZr_2-x（PO₄）₃（x=0,0.02,0.05,0.1）,respectively,which have totally different microstructure and ionic conduction behaviors.Their ionic conductivities were determined by prefactor and activation energy,respectively.When combining structural characterizations,frequency-dependent impedance spectra and thermodynamic analysis,the intrinsic structural influencing factors on ionic conductivities were investigated,and the competition relationship between migration entropy and migration enthalpy was emphasized.Furthermore,multivalent elements were doped in LZP structure,respectively.Based on multiple excitation entropy theory,the origin of migration entropy and the intrinsic relationship in Meyer-Neldel law were studied.It has guiding significance for the understanding of ionic transportation mechanisms and optimizing ionic conductivities.Finally,the influence of the surface structure changes under different atmosphere on ionic conduction performance was investigated.The environment stability of solid-state electrolytes determines the condition of synthesis,transport and usage,thus determines the microstructure and final ionic conduction properties.Nevertheless,most research works about environment stability were based on through ex-situ characterizations,it’s inevitable to be exposed to ambient environment and thus some contradictory results were obtained.In this case,synchrotron radiation Ambient Pressure X-ray Photoelectron Spectroscopy（APXPS）was applied to test the surface of LATP and LZP materials and obtained the surface structural information.In-situ APXPS under Ar and H₂O of LATP and LZP samples were carried out,and the relationship between the changing of Full Width at Half Maximum（FWHM）and the structural ordering has been stablished.It is found that LZP exhibits lower FWHM under 5 Torr H₂O,indicating that the H₂O environment would lead to the increases in structural ordering.Combining other structural characterizations and electrochemical performance,the increases in the bottleneck size during ions transport is the major factor of the ionic conductivity enhancement,which can up to 10^-4 S/cm.Based on the in-situ characterization,the possible reaction mechanism under H₂O can be put forward,providing new insights for future structural optimization and design.