Cold-sintering of Li6.25La3Al0.25Zr2O12 electrolytes and cathode materials for Lithium-ion battery applications

This thesis aims to investigate the cold sintering of LLZO under ambient air conditions for battery applications. In this thesis, LLZO was milled to reduce particle size and enhance packing during pressure application. The as-received LLZO powder’s size and chemistry were analysed and compared with the milled and heat-treated powders. Milling reduced the average particle size (D50) from 127 µm to between 2.3 µm and 15.2 µm. Additionally, the milled powder exhibited the formation of La2Zr2O7 and Li2CO3, visible in the Raman spectra, which were subsequently removed upon heat treatment. The removal of La2Zr2O7 is attributed to the reintegration of Li from the decomposition of Li2CO3 on the particle surface to form LLZO.
The properties of Li6.25La3Al0.25Zr2O12 (LLZO) cold-sintered using water, 5M LiCl, and formic acid were compared before and after heat treatment. cold sintering LLZO under ambient air conditions using water and formic acid resulted in ionic conductivities of 1.99x10-8 and 1.33x10-9 S/cm, respectively. Whereas, cold sintering utilising 5M LiCl resulted in one of the highest ionic conductivities of 2.74x10-5 S/cm found in literature, with only LLZO cold sintered using non-aqueous solvents such as dimethyl formaldehyde (DMF), or the aqueous solvent 5M LiCl plus heat treatment, which achieved higher conductivities of ~x10-4 S/cm and 3x10-5 S/cm, respectively. The main downside of using 5M LiCl was the mechanical instability of the sintered LLZO due to the moisture sensitivity of the LiCl salt, which remained within the cold-sintered pellet. The LiCl was removed upon heat treatment of the pellet.
LiCoO2 (LCO) was cold-sintered to develop an anode layer to co-sinter a bilayer with LLZO. To cold-sinter the LCO and achieve mechanical integrity, two options were employed. First, a mixture of 20 wt% LLZO with 80 wt% LCO powder (LCO80) was used. Achieving a density of 80.2%. Secondly, formic acid is utilised as a solvent, achieving a relative density of 84.8%. This is one of the few examples of cold sintering of LCO in the literature, and it yields a density only slightly lower than that of conventionally sintered LCO (87.4%).
Attempts to conventionally sinter LCO/LLZO bilayers at both 1000 oC and 1200 oC failed due to thermal shrinkage and melting of the LCO, respectively. Whereas an LCO80/LLZO bilayer was successfully achieved utilising cold sintering. Secondary electron microscopy (SEM) revealed the LCO80/LLZO interface to be fused/well-bonded, but during the heat treatment process, layers began to separate due to interfacial stress. Raman spectroscopy also revealed the formation of LaCoO3 within the LLZO layer after the heat treatment, but no changes were noted within the LCO80 layer. To mitigate the layer separation, a buffer layer of 30 wt% of LCO and 70 wt% LLZO (LCO30) was inserted between the LLZO and LCO80 layers. This resulted in no delamination between either of the two interfaces, illustrating a clear reduction in the stresses across the interfaces.
Overall, cold sintering of LLZO can be a viable method to fabricate multilayer structures for battery applications, but requires further research to improve properties and density to make it a more viable alternative to currently used processes.