Studies On Preparation And Interface Modification Of Inorganic Solid-state Electrolyte Materials

Restricted by the flammable organic liquid electrolytes,the conventional lithium-ion batteries suffer from safety issues,which limits their further applications.Using solid-state electrolytes to replace organic liquid electrolyte,solid-state batteries are considered as one of the most promising next generation energy storage devices because of their high safety and wide electrochemical window.Moreover,great improvement in energy density is expected when paired with Li-metal anode.Developing the solid-state electrolyte with high ionic conductivity,excellent thermal and electrochemical stability and good interfacial compatibility with cathode and anode materials is the key issue to design the solid-state batteries.However,the preparations of solid-state electrolyte and interphase in previous reports are complicated and costly,which could limit the practical applications of the solid-state batteries in the future.Therefore,the thesis focuses on the developing simple,convenient and efficient strategies to prepare solid-state electrolyte and interface layer for solid-state batteries.The thesis includes the following three parts:Firstly,NASICON-type material NaxZr2Six-1P4-xO12 is considered as an ideal option of solid-state electrolyte materials for solid-state Na-based batteries.However,the ionic conductivity of this material essentially needs to be further improved.Herein,a simple strategy is proposed to enhance the ionic conductivity of NASICON material by adding NaF into precursors,improving the ionic conductivity of classical Na3Zr2Si2PO12 from 0.45 mS/cm to 1.7 mS/cm at room temperature.Furthermore,both qualitative and quantitative analyses are conducted by employing XRD,XPS,SEM and solid-state NMR techniques to investigate the long-range and local structures as well as compositions for the as-prepared Na3Zr2Si2PO12-xNaF.It is revealed that with the addition of NaF into precursors,the monoclinic phase of NASICON grains gradually transforms into rhombohedral phase with in-situ formation of a binder-like glassy phase around them,which indicate NaF plays an important role in the modification of grain boundary and the improvement of the ionic conductivity.Furthermore,previous works mainly focus on the preparation and improvement of the Na3Zr2Si2PO12 and rarely notice the material with another Si/P proportion.Herein,NASICON-type electrolytes with different Si/P proportion are prepared to fill the gap and explore the optimal composition.Low-temperature EIS revealed that the bulk and grain boundary conductivity both increase with higher Si/P proportion,and the optimum conductivity of 3.2 mS/cm at room temperature is achieved for the Na3.3Zr2Si2.3P0.7O12,which is an order of magnitude higher than that of classical Na32r2Si2PO12.An interesting phenomenon was firstly observed that parts of NASICON electrolyte is not air-stable.The ionic conductivity of Na3.4Zr2Si2.4P0.6O12 decrease from 3.2 mS/cm to 1.5 mS/cm after in the air for a while.The results of NMR spectra demonstrate that the Na3.4Zr2Si2.4P0.6O12is unstable in the air and transforms into the Na3.3Zr2Si2.3P0.7O12 along with the generation of the Na2CO3 and silicate impurity,which cause the deterioration of the grain boundary conductivity and the decrease in total ionic conductivity.Combined with NaF complex strategy,the optimum conductivity of 3.6 mS/cm at room temperature is achieved for the Na3.2Zr2Si2.2P0.8O12-0.5NaF,which is the highest ionic conductivity among as-reported NASICON-based materials.Finally,yet the interfacial issues with Li-metal limit their applications,an ultra-simple and effective strategy is proposed to enhance the interfacial connection between garnet-type solid-state electrolyte and Li-metal just by drawing a graphite-based soft interface with a pencil.Both experimental analysis and theoretical calculations confirm that the reaction between graphite-based interfacial layer and metallic lithium forms a LiC6 interface with good ionic and electronic conductivity,which may ensure the uniform distribution of lithium ions in the interfacial layer.With improvement of graphite-based interface,the symmetric Li cells can stably cycle for over 1000 hours under a current density of 300 ?A/cm2 at room temperature.Based on the graphite-based interface and toothpaste-like cathode fabrication method,a solid-state lithium battery with excellent performance is successfully designed with NCM523 cathode.The cathode material achieved a reversible specific capacity of 175 mAh/g at the current density of 0.1C,which is consistent with the performance of the materials in battery with organic liquid electrolyte.There is no obvious capacity decay at 0.5C and more than 80%capacity retention was obtained after 500 cycles.Overall,the thesis is focused on designing the sample and inexpensive strategy to prepare the solid-state electrolyte and interphase materials.The excellent performance of Na3.2Zr2Si2.2P0.8O12-0.5NaF electrolyte and graphite-based interphase realized by the simple strategy reported in this thesis provides valuable insights for the further research and applications of solid-state batteries.