Li_0.44MnO₂ possesses a complex tunnel structure composed of sheets of edge-sharing MnO₆ octahedra and columns of edge-sharing MnO₅ square pyramids both lying parallel to the c axis of the orthorhombic unit cell (space group Pbam). The sheets and columns corner share thus defining two types of tunnels. The smaller of these contains one Li⁺ ion site and the larger two pairs of Li⁺ sites. There are no less than five crystallographically distinct Mn sites. The lithium content of the host has been varied by chemical and electrochemical intercalation/deintercalation over the range Li_xMnO₂ ; 0.25<x<0.63. The associated change in the lithium site occupancies has been determined by a combination of powder neutron diffraction and electrochemical measurements. The structure demonstrates a remarkable ability to cycle lithium with no perceptible loss of capacity when cycled over the range 2.8–3.6 V vs. Li⁺(1 m)/Li, at 0.5 mA cm^–2 corresponding to a capacity of 85–90 mA h g^–1 .