Increasing the energy density of cathode materials is a central goal of the ongoing research in Li-ion batteries. This implies improving simultaneously the capacity and the potential of the materials used at the positive electrode of the electrochemical device. While the electrochemical potential is well controlled through an appropriate choice of the redox centre involved in the Li-driven electrochemical reactions, the increase of capacity appears more difficult to achieve without penalizing the material structural stability.[1-3] So far, materials showing the highest energy densities are the Li-rich layered transition metal oxides (LLOs) for which a cumulative cationic and anionic redox activity has been demonstrated. Ever since, numerous materials have been synthesized with various combinations of 3d, 4d, 5d or non transition metals [4-6] and different mechanisms have been proposed to rationalize their electrochemical activity vs. Li or Na with no real consensus about the directions to follow to overcome the structural instability that often comes along with the anionic process. In this paper, we review the different interpretations proposed in the literature to address the fundamental questions of increasing both the potential and the capacity of current positive electrodes with the hope that a common language will help in clarifying the relationship between the material electronic structure, the potential, the (extra)-capacity and the consequences of high-energy-density on the material structural stability. This unified picture highlights that a trade-off needs to be found for the development of high-energy-density materials for alkali-ion batteries.

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