Li-rich cathodes are one of the promising high capacity energy storage materials for future applications. While layered Li-rich cathodes have been studied the most, they are susceptible to phase transformation when Li is highly depleted. To address this problem, beta-Li2MO3 (M = transition metal) has been considered since its three-dimensionally connected structure can prevent phase transformation. So far, only beta-Li2IrO3 has been reported. Here, we systematically investigate the trend of beta-Li2MO3 (M = 3d, 4d, and 5d transition metals) as cyclable Li-rich cathodes using electronic structure calculations. We propose and demonstrate that the charge transfer energy computed from density of states is a physical descriptor that can predict the activity and reversibility of oxygen redox and voltage. Although 3d transition metal substitutions to beta-Li2MO3 are desired cost-wise, it promotes O-O dimer formation, indicating irreversible phase degradation and voltage hysteresis, and for this, higher 4d and 5d elements are required. We expect that the proposed charge transfer energy descriptor can be extended to investigate various other Li-rich cathodes.