Combined Neutron Diffraction, NMR, and Electrochemical Investigation of the Layered-to-Spinel Transformation in LiMnO2

The conversion that occurs from layered LiMnO2 to spinel on electrochemical cycling has been studied by neutron diffraction and NMR. Neutron diffraction results indicate that the tetrahedral sites in the Li layers that share faces with octahedral sites in the transition metal layers are occupied even following the first charge to 4.6 V. NMR results are consistent with the conversion from the monoclinic, Jahn−Teller distorted, to the rhombohedral, layered phase on charging. On subsequent discharging, clear evidence for the monoclinic phase is seen by NMR indicating the presence of Jahn−Teller distorted domains in the material at 3.5 V and below. The fraction of monoclinic phase decreases gradually as a function of cycle number and disappears by 35 charge−discharge cycles. Diffraction patterns obtained as a function of cycle number were refined with a structural model that included both a layered phase (with octahedral and tetrahedral site occupancy) and a spinel phase, with the fraction of spinel increasing from 0.12 (5 cycles) to 0.93 following 92 cycles. Both diffraction and NMR results indicate that the spinel phase that nucleates from the layered material is stoichiometric and does not contain Li occupancy on the Mn sites. An additional site is seen by NMR which reaches a maximum in cycles 25−50, which is assigned to tetrahedrally coordinated Li in a local environment intermediate between that of the spinel and the layered phase. The observation of this site by NMR is associated with two characteristic peaks in the incremental capacity plot at 3.75 and 3.9 V on charge and discharge. The data indicate that the mechanism for Li insertion and removal into these local environments is complex and involves simultaneous structural rearrangements. The intermediate environment decreases in concentration on subsequent cycling as the concentration of the spinel phase continues to grow.