Lithium-excess layered cathode materials for lithium-ion batteries of the form Li₂MnO₃-LiMO₂ (M=transition metal(s)) show high capacities (>200 mAhg^-1) due to the activation of the Li₂MnO₃ component.¹ This activation process occurs during the first cycle at >4.4V, where it is widely accepted that oxygen is removed from the Li₂MnO₃ component. Thus, Li_2-XMnO_3-δ is formed upon cycling, creating a structure with both oxygen vacancies (V_O) and lithium vacancies (V_Li). It has been shown previously that the V_Li and V_O interact and that it is energetically favorable for V_Li to form near V_O.² V_O concentration in the Li_2-XMnO_3-δ is believed to be dilute, although the exact amount of V_O(δ) is unclear.

To estimate the V_O concentrations, calculations of the chemical expansion coefficient for vacancies in Li₂MnO₃ are combined with experimental stress measurements of a lithium-excess layered cathode material. The coupled V_O and V_Li have previously been shown to exhibit correlated anisotropic chemical expansion in Li_2-XMnO_3-δ at dilute concentrations.³ Thus, the chemical expansion coefficients for V_O and V_Li in the system are broken down into a dilute component for coupled V_O and V_Li and a component to capture the rest of the V_Li which are far away from V_O  and contribute to the high capacity of these materials. The computational results are used to interpret stress measurements from a multibeam optical stress sensor (MOSS) on Li_1.2Mn_0.55Ni_0.125Co_0.125O₂.

1. Thackeray, M. M.; et al., Journal of Materials Chemistry 2007, 17(30), 3112-3125.

2. James, C.; et al., Solid State Ionics 2016, 289, 87-94.

3. James, C.; et al., MRS Advances 2016, 1(15), 1037-1042. doi:10.1557/adv.2016.48.