The Controllable Preparation And Performance Research Of Highly Oriented Lithium Battery Materials

Low cost,long cycling stability,high capacity and power density,excellent electrochemical reversibility and low pollution are expected for an ideal electrochemical energy storage system,which plays an indispensable role in the sustainable energy development.Among all these features,power density is related to the release rate of electric energy,and critical to the functioning of the energy storage device.Decreasing the tortuosity of the microscopic composition in the battery materials will facilitate the ions to migrate and enter into the electrodes more easily.The improved ionic diffusion and conduction help to enhance the rate capability of the electrochemical device,leading to a new research direction for a better overall performance in recent years.In this thesis,based on the theoretical fundamentals of lithium-ion battery kinetics and tortuosity,improving the lithium ions conduction in the electrode and electrolyte materials for an enhanced battery electrochemical performance is discussed.Highly oriented structures for the LiCoO₂ cathode with high energy density,LAGP/PEO composite solid electrolyte for lithium-ion battery,and r GO conductive cathode matrix for lithium sulfur battery are designed and fabricated.The three battery materials are characterized and studied for the enhancement mechanisms of the battery performance behind the highly oriented structure.The systematic study advances the structure design as well as performance analysis,and establishes solid theoretical and practical foundations for the batteries with high rate capabilities.The main research topics in this thesis are as follows:The first section is aimed to increase the LiCoO₂ cathode energy density as well as ensure the fast diffusion and conduction of lithium ions for better charge transfer,by lowering the tortuosity of the pathways of lithium ions in the cathode material.The environment-friendly ice-templating method with a relatively simple process is utilized to fabricate a highly oriented structure of the LiCoO₂ cathode.The pore size of the highly oriented framework,the layer thickness along the axial direction of LiCoO₂ material,and the porosity of the material all can be tuned between 3 μm to 20 μm,10 μm to 30 μm,and 40% to 70% respectively.To have the optimal crystal structure of the LiCoO₂ material,the sintering temperatures after ice-templating should be between 850 °C to 900 °C for 6 hours.This highly oriented LiCoO₂ cathode has a positive effect to the battery kinetics compared with the traditional cathode in which the active LiCoO₂ material is fully packed together.To be more detailed,this highly oriented structure of the active material can ensure a satisfied wetting capability between the electrolyte and the thick cathode,decreasing the battery charge transfer and SEI resistance,to facilitate ions to arrive at the cathode more easily.Compared to the traditional LiCoO₂ cathode material,the highly oriented LiCoO₂ cathode shows a better cycling stability and a longer cycle life.The discharge specific capacity reaches 139.9m Ah/g,and the capacity retention is 91.95 % after 100 cycles and 77.9 % after 200 cycles.Besides,the specific capacity of the highly oriented cathode with 75 mg/cm² mass loading are 56.5 % and 134.4 % higher than the traditional cathode under 0.5C and 1C respectively.Combining with the composite solid electrolyte,the improved rate capability can still be effectively realized under 4.3 V charging voltage for the highly oriented structure,with the suppression effect for the lithium dendrite growth.After successfully fabricating the solid-state battery with high mass loading of the active cathode material with the oriented structure and lithium metal anode,its areal specific capacity is 28.8 % higher than the one with the highly oriented LiCoO₂ but lower mass loading after 200 cycles.The cycling stability,cycle life,and rate capabilities are all better than the low mass loading battery.Verifying the effectiveness of the highly oriented structure,the second section of this thesis is to extend the structure to the solid electrolyte field.The ice-templating method is used to fabricate the LAGP/PEO composite solid electrolyte with highly oriented structure of LAGP ceramics for high ionic conduction.The polymer PEO is added for flexibility.The conductivity of this composite electrolyte is 1.67 × 10-4 S/cm,which 7 times higher than the randomly distributed LAGP in the PEO matrix.Moreover,the safety and stability are both better than the randomly distributed LAGP in the PEO matrix and pure polymer PEO.Under 0.3 C,the composite solid electrolyte with highly oriented structure can well accommodate the Li Fe PO4 cathode,showing the capacity retention 93.3% and Coulombic efficiency over 99 % over 300 cycles.Under a larger current 0.6 C,this battery has excellent capacity retention of 87.4 % over 400 cycles.The results indicate that composite solid electrolyte with highly oriented structure can be successfully applied in the high performance solid-state batteries,and the ice-templating method is a promising way to fabricate the solid electrolyte with high conductivity,which should be further explored in the research and real applications in the solid-state battery field.Based on the previous sections,it is reasonable to apply the ice-templating method in other directions in the battery system.The third section is about the designing and application of the ice-templating in Li-S battery.The highly oriented reduced graphene oxide（r GO）layered structure with low density,high conductivity,large specific surface area,and high porosity is fabricated as the cathode matrix in Li-S battery.Through dip-dry method,TiO₂ nanoparticles are uniformly coated on the surface of the layered r GO as the highly oriented TiO₂/r GO cathode matrix,which is combined with the eutectic catholyte discussed later in the thesis for a highly oriented r GO based Li-S battery.The TiO₂/r GO cathode matrix with highly oriented structure effectively improves the rate capabilities and cycling stability of the Li-S battery with the eutectic catholyte.Compared with the regular r GO without any specific structure and the carbon paper cathode matrices,the specific capacity of the highly oriented r GO cathode matrix is 48.7% and 126.7% higher respectively under 1 C condition.The specific capacity remains at 698 m Ah/g over 300 cycles under 0.2 C,indicating that the rate capability and cycling stability can be effectively enhanced by the design of the highly oriented cathode matrix in Li-S batteries.