The intermediate-temperature, monoclinic β-phase of Li ₂TiO₃ shows a stoichiometry range from 47 to 51.5 mol % TiO₂. This broad stoichiometric range may be exploited for industrial applications, such as breeder material in a fusion reactor or a microwave dielectric. Here, density functional theory is employed to calculate formation energies for the intrinsic defect species, allowing the identification of the mechanisms responsible for accommodating both excess Li₂O and TiO₂ across a wide range of temperatures and oxygen partial pressures. The results predict that while the exact mode of accommodating non-stoichiometry depends on factors such as the temperature and oxygen partial pressure, cation disorder plays a major role in the incorporation of non-stoichiometry and that oxygen defects are of relatively minor importance.