Morphological control of CaxMn1−xNb2O6 columbites for use as lithium hosts in batteries

Abstract

Ternary niobium-based oxides have shown great potential as negative electrodes for Li-ion batteries due to their structural stability and high-rate capabilities. A hydrothermal reaction at 240 °C is used to synthesise MnNb₂O₆ (MNO) from different alkaline MOH solutions with M = Li, Na, K, yielding MNO-Li, MNO-Na, and MNO-K and a new phase, Ca_0.25Mn_0.75Nb₂O₆ (CMNO). The MNO materials were characterised structurally using powder X-ray diffraction revealing that they have disordered columbite structures, with Mn and Nb randomly distributed over octahedral sites, which become ordered upon annealing at 1000 °C. In contrast, CMNO has an ordered structure, which may be due to the larger ionic radius of Ca²⁺. Only 25% of Mn can be replaced by Ca. The crystalline morphology of the samples is dependent on the synthesis medium, with intricately shaped particles of 10s of nm using SEM. Spectroscopic studies (XPS and XAFS) confirm the Mn²⁺ oxidation state in all materials. MNO-Na has higher cyclability towards Li uptake with capacity retention of 64% and specific capacity of 182 mA h g⁻¹ after 200 cycles. CMNO showed the highest capacity after 50 cycles, 142 mA h g⁻¹, which indicates that structure expansion controls the quantity of Li inserted. Ex situ PXRD studies revealed that all materials undergo a conversion reaction during the first lithiation although MNO-Na and MNO show better structural stability regaining their crystallinity after the first delithiation. Electrochemical impedance spectroscopy reveals a complex Li lithiation/delithiation mechanism at different voltages.