MULTILAYER COMPOSITE SOLID ELECTROLYTES FOR LITHIUM ION BATTERIES (2016)

Lithium ion batteries (LIBs) are becoming the standard energy storage option for an increasingly diverse range of applications from mobile phones to cars. The conventional liquid electrolytes based LIBs are prone to failure in conditions such as high operating temperature, solvent leakage, lithium dendrites formation and thermal runaway, etc. All-solid-state lithium ion batteries (ASSLIBs) provide a promising power strategy to overcome the drawbacks of liquid electrolyte by substituting the highly flammable organic liquid electrolyte with solid electrolytes (SEs). However, up to the present time, the SEs fabrication for practical ASSLIB construction is still a significant challenge. The existing problems include 1) lower ionic conductivity compared to liquid electrolyte, 2) poor solid-solid contact interface between electrode and electrolyte, 3) volume change of the electrode and 4) the unstable interface of lithium metal/polymer electrolytes causes further capacity fading. With the aim of fabricating SEs which possess optimal properties, a novel SE was developed by forming a multilayer structure. The multilayer SE was fabricated using polymeric and ceramic electrolytes, which can integrate the merits from different layers and materials and optimize its overall performance. In order to choose an ideal ceramic material for the multilayer electrolyte fabrication, three different types of ceramic electrolyte material were synthesized, characterized and evaluated, including Li1.3Ti1.7Al0.3(PO4)3 (LATP), Li7La3Zr2O12 (LLZO) and Li0.5La0.5TiO3 (LLT). Their mechanical strength, ionic conductivity, ease of fabrication and synthesis, and economic expenses of synthesis were evaluated experimentally. The influence of sintering temperature, synthesis route, working temperature and pressure to the overall conductivity were evaluated. From experimental observation and analysis, it was concluded that LATP was an ideal candidate for multilayer electrolyte fabrication for its high conductivity, ease of fabrication and synthesis, etc. The electrochemical properties of polymer electrolyte PEO10-LiN(CF3SO2)2, which was fabricated through hot pressing and solvent casting methods respectively, and also gel-polymer electrolyte PVdF-HFP-LiN(CF3SO2)2 were characterized. The lithium ion transference number, ionic conductivity and thermo-stability were evaluated and discussed. Based on the characterized ceramic and polymer electrolytes, the multilayer electrolyte was fabricated through various lamination protocols, which include hot pressing, dip coating and spray coating methods. It was found that negligible interfacial resistance exist at LATP/LLT and SPE material. Also, an enhanced ionic conductivity was found for the bilayer of LATP/solvent casted SPE. This phenomenon was attributed to the formation of a composition region at the polymer/ceramic electrolyte interface. It was suggested that the boundary of polymer body and ceramic grains may induce a pathway for enhanced ionic transportation. The porous LATP was fabricated using PMMA/PVA/PVB as the pore maker. The influencing factors of sintering temperature, material selection of ceramic and pore makers and fabrication methods deserve further investigation. All-solid-state lithium ion coin cell was successfully fabricated and characterized using the as-prepared multilayer electrolyte and lithium metal anode. The coin cell demonstrated satisfactory charge/discharge capability and cyclability at an elevated temperature of 70 °C. The thickness of SE, operating temperature, material types were important factors in the overall resistance of the multilayer solid electrolyte. The unstable lithium/polymer electrolyte interface at high temperature and high potential is the critical problem for developing ASSLIBs with better cyclability in practice. In the end, future work was proposed and discussed based on the existing work, including 1) multilayer fabrication using glass-ceramic material; 2) optimization of porous ceramic electrolyte; 3) multilayer composite electrolyte using ceramic stabilizer at the lithium/electrolyte interface.