Search

Cold sintering is a recently introduced densification method that is of interest due to the lower energy used in the process, the ability to densify metastable materials, the ability to integrate materials to develop unique composites, and the ability to synthesis materials that can be more easily recycled. Collectively, these advantages make cold sintering a promising approach for processing of battery components, and co-sintering into all solid-state batteries. To obtain high electrochemical performance with cold sintering, detailed control of the surface chemistry of powders is needed, to avoid or minimize the impact of carbonate formation in grain boundaries, and to limit concentrations of secondary phases that are a result of incongruent dissolution processes. In this paper, we outline the importance of surface chemical reactions of powders in cold sintering, and the mitigating processes that can be adopted to control these reactions and obtain high performance and unique opportunities for Li and Na-oxide secondary batteries. We focus on a series of examples that demonstrate how the control of low temperature densification (cold sintering) can address the environmental sensitivity of many battery materials, and highlight the issues faced and open questions. These examples include cold sintering of air-sensitive solid electrolytes, re-processing of crushed solid electrolytes, surface passivation of air-sensitive active materials for processing in air, and fabrication of solid-solid macroscopic interfaces for solid-state batteries.

Although zinc oxide has a high melting point of 1975 °C, the recent development of the cold sintering process has shown that highly dense polycrystalline ceramics can be obtained at 150 °C or below. As such unusual densification does not yet have fully elucidated underlying mechanisms, this study aims to provide a comprehensive investigation of transient morphological and chemical interfaces during isothermal dwell. The chemical “transient” interface involves the formation of a zinc carboxylate complex by reacting the host zinc oxide powder and acetic acid solution, which is later consumed for the recrystallization on the lattice of an adjacent grain with a decomposition of the complex. Hence, the final outcome can be a residual-free zinc oxide phase. Regarding the morphological evolution, gas physisorption studies were conducted to quantify the area of solid–vapor interfaces and the Schlaffer model was used to analyze the activation energy for specific surface area reduction. Moreover, transmission electron micrographs revealed that the recrystallization occurs at the interface between crystalline grain and carbon-rich amorphous phase. After 30 minutes of isothermal dwell, the area of the amorphous phase was significantly decreased while grain size was increased, potentially indicating the recrystallization-induced grain growth and pore closure. The chemical transition at the zinc oxide surface was further investigated using ReaxFF molecular dynamics simulation, observing that acetate molecule acts as a bridging complex to connect zinc ions to the surface. Overall, understanding the evolution of the transient interface is a crucial subject to obtaining a residual-free cold-sintered sample with desired grain size and properties for application developments.

Surface charge and surface adsorption on an alpha-alumina powder were investigated in a 99.5/0.5 wt% ethanol/water suspension. A model is proposed in which ethanol molecules are dissociatively adsorbed as ethoxide ions and protons to Lewis acid and base sites on the surface. These ions can then desorb separately from the surface. The surface, therefore, acts as a catalyst for the autoprotolysis of the solvent and creates its own ionic atmosphere which cannot be predicted directly from chemistry in the bulk of the solvent. 3.5μmol/m2of HCl is reversibly adsorbed to the surface by replacement of ethoxide ions adsorbed to surface acid sites by chloride ions. 2.4μmol/m2of KOH is adsorbed to the surface by replacement of surface adsorbed protons with potassium ions. More rapid desorption of negative ions from the surface leaves the particles with a net positive surface charge except when the concentration of negative ions in solution is sufficient to supress this desorption and net surface charge goes to zero. Surface charge is found to be a function only of the activity of the negative ions in solution at the surface. No significant negative surface charge was measured under any conditions here. Equilibrium constants for surface adsorption and charge density as a function of surface activity of ethoxide and chloride ions are calculated. No effect of adsorbed potassium ions on the surface potential was found.