Current understanding and applications of the cold sintering process

In traditional ceramic processing techniques, high sintering temperature is necessary to achieve fully dense microstructures. But it can cause various problems including warpage, overfiring, element evaporation, and polymorphic transformation. To overcome these drawbacks, a novel processing technique called “cold sintering process (CSP)” has been explored by Randall et al. CSP enables densification of ceramics at ultra-low temperature (≤ 300 °C) with the assistance of transient aqueous solution and applied pressure. In CSP, the processing conditions including aqueous solution, pressure, temperature, and sintering duration play critical roles in the densification and properties of ceramics, which will be reviewed. The review will also include the applications of CSP in solid-state rechargeable batteries. Finally, the perspectives about CSP is proposed

Microstructure and electrical properties of conductive ceramics produced by cold sintering process

Electronic and ionic conductive ceramics with excellent electrical properties, including indium tin oxide (ITO), alumina-doped zinc oxide (AZO), gallium-doped garnet-type Li7–3xGaxLa3Zr2O12 (xGa-LLZO) and NASICON-type Li1.3Al0.3Ti1.7(PO4)3 (LATP), are attractive for the applications in optoelectronic devices and rechargeable batteries. In this project, a new developed cold sintering process (CSP) was applied to produce the conductive ceramics with significantly reduced sintering temperatures. AZO semiconducting ceramic with relative density of 98.3% was produced by cold sintering at optimum CSP conditions of 250 ºC, 300 MPa, 2M of acetic acid, and 60 min. The as-CSPed AZO samples presented dense microstructure with small average grain size of 0.55 μm and high phase purity. The CSPed AZO ceramics attained low resistivity of 4.20 × 10-3 Ω·cm after post-annealed at 900 ºC for 3 h in argon attributing to the increased oxygen vacancies and the facilitated crystallisation. Comparing with conventional sintered AZO ceramics, the CSPed AZO ceramics showed much higher sinterability and lower resistivity at the same processing temperature. ITO ceramics with relative density of 98.6% and average grain size of 2.0 μm were fabricated through cold sintering at 250 °C for 1 h under 300 MPa with subsequent post-annealing at 1400 °C for 4 h. The cold-sintered ITO samples consisted of nanocrystalline phase and amorphous phase. The subsequent post-annealing process not only facilitates crystallisation of the amorphous phase and further densification, but also promotes the development of uniform microstructure and electronic transport properties, resulting in low resistivity of 3.63 × 10-4 Ω·cm. 0.5Ga-LLZO solid-state electrolyte with relative density of 79.2% and ionic conductivity of 8.88 × 10-5 S cm-1 was produced by cold sintering at 250 ºC and 300 MPa for 1 h with Li-solution (20M LiNO3 and 7M LiOH) as transient solution. Utilising the Li-solution with high concentrated Li+ and pH value significantly restrained proton exchange and improved the ionic conductivity. After post-annealed at 1100 ºC for 4 h, high relative density of 92.2% and ionic conductivity of 5.08 × 10-4 S cm-1 were achieved, due to the reduction of grain boundary resistance. LATP solid-state electrolyte with relative density of 86.9% and ionic conductivity of 1.83 × 10-6 S cm-1 was produced by cold sintering at 250 ºC and 300 MPa for 1 h with DI water as transient solution. Post-annealing at 900 ºC for 1 h induced further densification and grain growth, resulting in the increase in density to 88.0% and ionic conductivity to 1.32 × 10-3 S cm-1. Microcracks were observed in post-annealed ceramics at 1000 ºC and above because of the high thermal expansion anisotropy.</p

Microstructure and ionic conductivities of NASICON-type Li₁.₃A1₀.₃Ti₁.₇(PO₄)₃ solid electrolytes produced by cold sintering assisted process

Li1.3Al0.3Ti1.7(PO4)3 (LATP) is a promising solid electrolyte for lithium-ion batteries. However, it is challenging to densify LATP ceramics at reduced sintering temperature while preserving their electrical properties. Herein, LATP ceramics were pre-densified via cold sintering process (CSP) at 250 °C for 1 h and exhibited room temperature ionic conductivity of 2.01 × 10−6 S/cm. Subsequent post-annealing at as low as 900 °C for 1 h resulted in two orders of magnitude improvement in both grain boundary conductivity and total conductivity, compared to those of as-CSPed LATP. The optimal total conductivity (4.29 × 10−4 S/cm) obtained from post-annealed material is among the best reported values so far. It is also 5 times greater than the conductivity (8.51 × 10−5 S/cm) of the conventionally sintered LATP. We propose that post-annealing effectively eliminates amorphous insulating phases generated during CSP and promotes crack-free microstructure with moderate grain growth, which collectively contributes to dramatically enhanced conductivity. This work unambiguously demonstrates that CSP-assisted process can avoid the detrimental effects of high temperature associated with conventional sintering on microstructure and conductivity, and thus is a cost-effective processing route for fabrication of solid-state electrolytes for battery applications