ИСТИНА

The layered Li(M,Li)O2 oxides containing 3d transition metals are one of the best cathode material families for rechargeable Li-ion batteries. For the further improvement of their performance (energy density) the cathode materials should operate at higher voltage. However, it results in the irreversible oxidation of the oxygen sublattice with the oxygen release, which hampers their application. The use of 4d transition metal oxides can overcome this obstacle. Since 4d metals have higher electronegativity in comparison with 3d ones, their M-O bonds are more covalent. It allows to reversibly oxidize oxo- O2- to peroxo-like species O2n- (2<n<4) without O2 release [1]. We studied the behaviour of LiRhO2 at high operating voltages. Up to 3.85 V (vs. Li+/Li) 0.55 Li was deintercalated without changes in the structure motif. However, the further delithiation resulted in the formation of a new phase with utterly different structure, as it was observed by in-situ synchrotron powder diffraction. To derive it, we used quantitative electron diffraction tomography (EDT), which is an ab-initio crystal structure determination method of nano-sized (~200 nm) single crystals. The technique is based on scanning of reciprocal space by tilting a crystal with 1 degree step and taking electron diffraction (ED) patterns. Since the patterns are acquired mainly out of zone, dynamical effects are reduced and such ED can be considered as quasi-kinematical [2]. The delithiated phase has a 3D framework structure with 1×1 rutile channels and bigger 2×1 ramsdellite ones. The space group is C2/m and cell parameters are a=14.188(2) Å, b=3.0740(2) Å, c=4.5050(7) Å, β=92.087(8)°. Upon lithiation (discharge) the 3D phase only partially transforms back to the layered one and the Li are introduced into bigger ramsdellite channels. At the further discharge up to 1.2 V (vs. Li+/Li), Li occupies even rutile channels, which result in extra 20% Li in comparison with stoichiometry of pristine phase [3]. The authors acknowledge support from FWO under grant G040116N.