Microwave-assisted hydrothermal synthesis of nanocrystalline lithium-ion sieve from biogenic manganese oxide, its characterization and lithium sorption studies

- Biogenic birnessite was used to synthesize nano-sized lithium-ion sieves.
- Microwave-assisted hydrothermal method facilitates LMO formation with a short time.
- Sorption capacity of Li on nanoparticles was four times higher than microparticles.
- Effects of crystal size on properties of HMOs were studied.

Biogenic birnessite (BB) is a stable form of manganese oxide. It is widely distributed in the natural environment and originates from microbial oxidation. It has potential applications in functional material fabrication because of its unique morphology. Using a microwave-assisted hydrothermal method, nano-sized lithium-ion sieves were prepared from BB with a short reaction time. A combination of sorption experiments and structural characterization was used to compare Li uptake by nanoparticles with that by microparticles. X-ray diffraction (XRD) patterns showed that the nano- and microparticles had similar fundamental structures, but the lattice parameter of nanoparticles is smaller than microparticles. Mn K-edge X-ray absorption fine structure (XAFS) spectroscopy showed that the oxidation state of Mn increased from 3.50 to 3.69 with decreasing crystal size, and the Mn-Mn atomic distance decreased from 2.92 to 2.89 Å. Li extraction resulted in significant cleavage of the microparticle surfaces. The oxidation state of Mn increased to 4.0, and the Mn-Mn atomic distance decreased to 2.86 Å. XRD showed that dissolution of the polycrystalline phase of the nanoparticles occurred during acid washing. However, the EXAFS spectrum was similar to that of the original material before acid washing. The specific surface areas and Li-sorption capacities of the nano-sized lithium-ion sieves prepared from manganese carbonate were significantly higher than those of a similarly prepared micro-sized lithium-ion sieve. The results obtained in this work suggest that BB is a promising starting material for the energy-saving fabrication of functional materials for highly efficient Li recovery.