Li_0.44MnO₂: an intercalation electrode with a tunnel structure and excellent cyclability

Li0.44MnO2 possesses a complex tunnel structure composed of sheets of edge-sharing MnO, octahedra and columns of edge-sharing MnO, 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 LixMnO2; 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+(1M)/Li, at 0.5 mA cm(-2) corresponding to a capacity of 85-90 mA h g(-1).