Aqueous cold-sintering of Na0.7CoO2: A benign, facile method for the densification of Na-ion compounds

The properties of the hexagonal P2 sodium cobaltate, Na0.7CoO2 (NCO) phase densified using the conventional and cold sintering methods were investigated, and we demonstrated that the cold sintering method yields NCO with a relative density of over 98 % (compared to 90 % attained after conventional sintering) while maintaining high room temperature conductivity (10−2 Scm−1). In X-ray diffraction, the original P2-phase framework is retained regardless of the sintering process, and the magnetic properties of NCO strongly depend on the densification route. Cold-sintered samples showed a classic paramagnetic response down to 2 K, while conventionally sintered samples developed a spin-glass behaviour below 6 K. The emergence of the spin-glass state on conventionally sintered ceramics was attributed to the enhanced disorder of Co3+ and Co4+ ions. A multitude of complex superstructures associated with Na ordering was identified for both cold and conventionally sintered NCO in electron diffraction.