A new phase of Li-intercalated rutile-structured titania, Li_xTiO₂ for x=0.75, is predicted on the basis of first principles calculations. The existence of this phase has been noted in X-ray diffraction measurements but its structure has not previously been determined. The new phase has important consequences for the performance of Li_xTiO₂ as a battery cathode material. On the basis of the computed energetics of phases at x=0.75, 0.5 and 0.25 phases, and a careful comparison of the computed and measured diffraction data, a two phase model of intercalation for 0.25<x<0.75 is proposed. It is suggested that nucleation of the new x=0.75 phases results from kinetic restrictions under operating conditions. The structural features of the new phase and their manifestation in X-ray diffraction are discussed. The transformation of the new phase to an hexagonal phase at x=0.8 is analysed and its role in cathode degradation discussed.