Spinels of the general formula Li2–δM2O4are an essential class of cathode materials
for Li-ion batteries, and their optimization in terms of electrode
potential, accessible capacity, and charge/discharge kinetics relies
on an accurate understanding of the underlying solid-state mass and
charge transport processes. In this work, we report a comprehensive
impedance study of sputter-deposited epitaxial Li2–δMn2O4thin films as a function of state-of-charge
for almost the entire tetrahedral-site regime (1 ≤ δ
≤ 1.9) and provide a complete set of electrochemical properties,
consisting of the charge-transfer resistance, ionic conductivity,
volume-specific chemical capacitance, and chemical diffusivity. The
obtained properties vary by up to three orders of magnitude and provide
essential insights into the point defect concentration dependences
of the overall electrode potential. We introduce a defect chemical
model based on simple concentration dependences of the Li chemical
potential, considering the tetrahedral and octahedral lattice site
restrictions defined by the spinel crystal structure. The proposed
model is in excellent qualitative and quantitative agreement with
the experimental data, excluding the two-phase regime around 4.15
V. It can easily be adapted for other transition metal stoichiometries
and doping states and is thus applicable to the defect chemical analysis
of all spinel-type cathode materials.