Additive Manufacturing of Ceramic Li7La3Zr2O12 Solid Electrolytes for Solid-State Battery Applications

Abstract

Solid-state lithium-ion batteries (SSLIB) are increasingly attracting attention due to their advantageous properties, including enhanced safety, non-flammability, higher energy density, durability, and lightweight design, which are achieved by eliminating the toxic and flammable electrolyte material present in conventional lithium-ion batteries. However, a significant obstacle to the commercialization of SSLIB lies in the processing methods. While electrodes are typically processed similarly for both conventional and solid-state batteries, the processing of thin solid electrolytes presents unique challenges. Inorganic ceramic solid electrolytes, particularly garnet-based Lithium lanthanum zirconium oxide (LLZO) ceramic solid electrolytes, are gaining interest due to their high ionic conductivity and thermal stability.

Additive manufacturing (AM) is a studied fabrication method for processing various materials including ceramics. Direct ink writing (DIW), an extrusion-based AM method, is known for its material versatility, ease of operation, and capability for multi-material printing making it suitable for battery applications. The advantages of using AM for manufacturing batteries, notably ceramic electrolytes, are high freedom of design, increased areal energy density, and volumetric density.

In this thesis, a novel sintering set-up was developed to densify ~150-250 µm thin Li6.4La3Zr1.4Ta0.6O12 (Ta-doped LLZO, LLZTO) ceramic electrolytes prepared via DIW fabrication. The challenges encountered in ink synthesis, DIW printing, and sintering are outlined along with effective solutions. This research aims to investigate these effects on LLZO ceramic solid electrolytes for applications in all-SSLIB.

The ink synthesis and optimization greatly impact the drying properties of the ceramic and processability. The properties of the ink such as viscosity, flowability, and its homogeneity need to be controlled to ensure optimal shear thinning behaviour. A volumetric flow rate model is developed using four different suitable dispensing tips to analyze the flow rate with respect to pressure applied for extrusion. This work uncovers the optimized ink composition consists of a solid loading of 30-35 wt%, 0.35 wt% dispersant (equivalent to 1 wt% concerning the solid loading), a 3:7 solvent ratio, and a 1:1 binder-plasticizer ratio (both at 6.45 wt%) suitable for DIW.

Subsequently, the solid electrolytes undergo sintering at temperatures exceeding 1100℃ to promote particle bonding and form a solid, conductive electrolyte. The properties of the final LLZO ceramic, including density, grain size, and ionic conductivity, are highly dependent on the sintering process. The phase, relative density, and ionic conductivity properties of sintered LLZTO are evaluated using techniques such as powder x-ray diffraction (XRD), scanning electron microscopy (SEM), and electrochemical impedance spectroscopy (EIS).

Preliminary findings in this thesis highlight the need for refining the processing techniques for fabrication and sintering of ceramic electrolytes to overcome challenges associated with lithium loss at temperatures exceeding 1000℃, sintering conditions, and poor reproducibility. This research thesis identified optimized sintering conditions, and set-up required to address the common sintering challenges to reproduce flat, high density thin solid electrolyte with the desired crystal cubic phase. The sintering results show optimal conditions are de-binding at 700℃ for 12 hours and sintering at 1100℃ for 1.5 hours with 10 wt% excess LiOH. The thin solid electrolytes are sandwiched between graphite and MgO plates within the MgO crucible. The EIS results show a high ionic conductivity comparable to literature findings and commercially available LLZTO ceramic electrolytes.