Short-range ordering in a battery electrode, the ‘cation-disordered’ rocksalt Li_1.25Nb_0.25Mn_0.5O₂

Powder diffraction data, left, X-ray crystal structure, centre, and battery performance, right, the the more ordered and less ordered forms of LiNbMnO.

This work was carried out in collaboration with the groups of Prof Clare Grey and Dr Siân Dutton at the University of Cambridge.

Summary by Mike Jones

Li_1.25Nb_0.25Mn_0.5O₂ is a promising new cathode material for Li-ion batteries. It has a reversible capacity to rival that of current state-of-the-art materials, which is thought to be achieved in part through the reversible reduction and oxidation (‘redox’) of oxygen. This O redox is an important but poorly understood phenomenon at present. It is thought to occur in addition to traditional transition metal (TM) redox, which is the way in which charge is stored (and released) in current battery materials. The greater-than 1:1 Li:TM ratio means that more Li can be accommodated per unit volume than commercial battery materials such as LiCoO₂ and Li[NiMnCo]O₂, and the O redox provides an extra redox couple through which to turn this extra Li storage into extra energy density.

The majority of crystalline materials consist of an ordered array of different elements or ions, each occurring at regular intervals. Current commercial Li-ion batteries contain ordered lithium transition metal oxides—for example LiCoO₂ consists of alternating layers of LiO₆ and CoO₆ octahedra. However, some materials form disordered arrays, with a supposedly completely random distribution of ions throughout the crystal lattice. Cation-disordered materials, where the metals have a disordered distribution, have been for many years ignored as potential cathode materials—as they were assumed to have poor Li diffusion throughout the structure. It has recently been found that this cation-disorder can in fact enable easier motion of the Li-ions, as well as improved O redox and a greater structural stability compared to ordered, layered materials. They have therefore recently garnered considerable interest, and are now the focus of intense research as promising new energy materials. However, while these materials lack long-range cation order, the short-range structure of these materials—i.e. whether the cations are truly randomly distributed across the crystal lattice—is still poorly understood, leaving the structural roots of their interesting redox chemistry obscure.

Using a combination of long-range and local structural probes, including X-ray diffraction, neutron pair distribution function analysis, magnetic susceptibility and NMR spectroscopy, we have uncovered short-range cation order in nominally cation-disordered Li_1.25Nb_0.25Mn_0.5O₂. This local ordering is related to the ordered rocksalt γ-LiFeO₂, where Li and Fe order onto two separate interpenetrating metal sub-lattices. It can be rationalised by considering the principle of electroneutrality—i.e. by considering a preference for the local ionic charges to balance throughout the material. We demonstrate a correlation length of approximately 125 Å, which is an order of magnitude greater than any previously reported for similar materials. This degree of ordering has significant implications for the electrochemical performance of this (and related) phases, and is highly sensitive to the synthesis conditions.