The layered Li₂MnO₃ is a key component in the Li/Mn-rich cathode materials, and provides a representative end-member for the spectrum of compositions reported in the literature. While exhibiting excellent initial capacity, the Li excess materials suffer from voltage fade, high impedance, and poor rate capability, particularly in the Mn-rich, high Li excess concentration regime.  In this talk we present insights about the Li electrochemical reactivity of Li₂MnO₃, from first-principles calculations. In particular, we elucidate the bulk lithiation/delithiation mechanism in pristine bulk Li_xMnO₃ for 1< x < 2, involving coherent Li migration from the two Li-containing layers. Surprisingly, and contrary to expectations from available experimental results, we find that the pristine material exhibits excellent Li mobility enabling a Li rate capability on par with the well-known layered LiCoO₂. For lower Li contents (x < 1), we have previously shown that Li₂MnO₃ becomes increasingly unstable towards manganese migration into the Li layer as well as oxygen loss at the surface. We systematically investigate the influence of these factors on the lithiation/delithiation process, which provides a hypothesis for the experimentally observed slow, and increasingly poor, electrochemical response of this material.