DEVELOPMENT OF ADVANCED ELECTROLYTES FOR ALL-SOLID-STATE Li-ION/Li METAL BATTERIES

Abstract

Accelerated development in areas of portable electronics, electric transport and renewable energy requires safe and reliable rechargeable batteries. Currently available secondary batteries cannot provide necessary performance and suffer from rapid degradation and loss of stability. One of the most promising systems 'close' to this niche are the all-solid-state batteries (ASSBs). Besides thermal, mechanical, chemical and electrochemical stability, introducing solid electrolytes into battery provides other advantages such as prolonged cycle life and, most importantly, the use of Li metal as anode to achieve the highest energy density. In this research, NASICON-structured Li1.3Al0.3Ti1.7(PO4)3 (LATP) ceramic and ultra-thin (PEI/PAA)30 gel polymer electrolyte (GPE) have been studied as solid lithium ion conducting materials. The correlation between morphology and ionic conductivity in LATP ceramics was revealed by fabricating through various synthesis routes and optimization of their procedures. Formation of reported impurity phases has been resolved through alternating precursors and adding extra lithium salts to prevent its loss during the calcination. The most optimal calcination temperature was identified for high pure LATP ceramic for various fabrication methods. Despite different synthesis routes, the crystal cell volume of all materials was similar; however the shape and size of the particles significantly differed. Well-defined cubic shaped LATP with larger grains tend to have higher ionic conductivity due to a great densification degree. The main problem of LATP, reduction by Li metal, was resolved by applying protective layers. Interlayers made of lithium ion conducting polymer materials are the most promising due to mechanical stability and ability to prevent dendrite growth and simplicity of preparation. Coating LATP pellets with ultra-thin artificial layer with composition of (PEO/PAA)30 was found to be very effective. The interlayer deposition by layer-by-layer (lbl) technique prevented the side reaction and decreased interfacial impedance at the electrode and electrolyte boundary. Symmetric cell with Li metal and polymer coated LATP electrolyte performed over 1 500 hours at 0.5 mA/cm-2 demonstrating an outstanding performance. The lbl technique was also utilized to form thin gel polymer electrolyte on electrodeposited Ni-Sn alloy anode on a Ni foam to design a 3D full cell. Mechanically strong ultra-thin electrolyte evenly coated the surface of 3D electrode providing high lithium ion conduction, and the full cell demonstrated stable performance of 100 galvanostatic charge-discharge cycles with a capacity retention of 90% at 0.1 mA cm-2.