Structural study of the Li_(0.5)Na_(0.5)MnFe _2(PO_4)_3and Li_(0.75)Na _(0.25)MnFe_2(PO_4)_3alluaudite phases and their electrochemical properties as positive electrodes in lithium batteries

摘要

The alluaudite lithiated phases Li_(0.5)Na_(0.5)MnFe _2(PO_4)_3and Li_(0.75)Na _(0.25)MnFe_2(PO_4)_3were prepared via a sol-gel synthesis, leading to powders with spongy characteristics. The Rietveld refinement of the X-ray and neutron diffraction data coupled with ab initio calculations allowed us for the first time to accurately localize the lithium ions in the alluaudite structure. Actually, the lithium ions are localized in the A(1) and A(1)′ sites of the tunnel. M?ssbauer measurements showed the presence of some Fe~(2+)that decreased with increasing Li content. Neutron diffraction revealed the presence of a partial Mn/Fe exchange between the two transition metal sites that shows clearly that the oxidation state of the element is fixed by the type of occupied site. The electrochemical properties of the two phases were studied as positive electrodes in lithium batteries in the 4.5-1.5 V potential window, but they exhibit smaller electrochemical reversible capacity compared with the non-lithiated NaMnFe _2(PO_4)_3. The possibility of Na ~+/Li~+ion deintercalation from (Na,Li)MnFe _2(PO_4)_3was also investigated by DFT+U calculations.