Li 2 MnO 3 loses oxygen, reversibly, on heating above ∼600 °C. By 1100 °C, 1% of the oxygen has been lost, giving a stoichiometry of Li 2 MnO 2.97 . Depending on the synthesis conditions, materials with a range of oxygen contents may be produced. Associated with the variable oxygen content, partial reduction of Mn 4+ to Mn 3+ occurs, as demonstrated by a direct correlation between electrochemical capacity, when used as a cathode in a rechargeable Li test cell, and oxygen content.

To find the origin of a large initial irreversible capacity and capacity fading with cycling for Fe-substituted LiCoO 2 (LiCo 1Ϫy Fe y O 2 ), the LiCo 0.8 Fe 0.2 O 2 positive electrode was selected for study by ex situ X-ray diffraction, Co and Fe K-edge X-ray absorption, and 57 Fe Mössbauer spectroscopies. A disordering of Fe ions from 3b͑0, 0, 1/2͒ to 6c͑0, 0, 3/8͒ sites was detected for initial charged samples through X-ray Rietveld analysis and Co and Fe K-edge X-ray absorption near-edge structures and extended X-ray fine structures spectra. The valence state of Fe ions in the 6c site was determined to be a 3ϩ/4ϩ mixed valence state from the isomer shift values obtained by 57 Fe Mössbauer spectra and mean M-O distance values. 50% of the iron ions become disordered after the initial charge process and more than 20% remain in 6c sites after the first and tenth discharge processes. The existence of Fe 3ϩ␦ (0 Ͻ ␦ Ͻ 1) ions on the interstitial 6c site can block fast Li conduction in the Li layer of the layered rock-salt structure (R3m). This leads to a lack of reversibility in Fe-substituted LiCoO 2 positive electrode materials.

Two mechanisms of doping Li(3)NbO(4), which has an ordered, rock salt superstructure, have been established. In the "stoichiometric mechanism", the overall cation-to-anion ratio is maintained at 1:1 by means of the substitution 3Li(+) + Nb(5+) --> 4Ni(2+). In the "vacancy mechanism", Li(+) ion vacancies are created by means of the substitution 2Li(+) --> Ni(2+). Solid solution ranges have been determined for both mechanisms and a partial phase diagram constructed for the stoichiometric join. On the vacancy join, the substitution mechanism has been confirmed by powder neutron diffraction; associated with lithium vacancy creation, a dramatic increase in Li(+) ion conductivity occurs with increasing Ni content, reaching a value of 5 x 10(-4) Omega(-1) cm(-1) at 300 degrees C for composition x= 0.1 in the formula Li(3-2x)Ni(x)NbO(4). This is the first example of high Li(+) ion conductivity in complex oxides with rock salt-related structures.