Defect Chemistry of Spinel Cathode Materials─A Case Study of Epitaxial LiMn₂O₄ Thin Films

Abstract

Spinels of the general formula Li 2−δ M 2 O 4 are 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 Li 2−δ Mn 2 O 4 thin films as a function of state-ofcharge 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.

Chemical capacitance measurements reveal the impact of oxygen vacancies on the charge curve of LiNi0.5Mn1.5O4−δ thin films

“…The remaining portion of the transmission line is identical to an ideal open Warburg element W o , which describes the impedance transition from semi-infinite to finite-space diffusion with characteristic phase angles of 45° and 90°, respectively. In our previous studies on Li 1− δ CoO 2 and Li 2− δ Mn 2 O 4 , 1,2 we have shown that the diffusional impedance of real intercalation electrodes can be fitted with an anomalous finite-space diffusion element embedded in the impedance analysing software EC-Lab (Biologic, France), which is closely related to the anomalous open Warburg element proposed by Bisquert. 41 The corresponding impedance expression readswith a nonideality parameter 0 ≤ α ≤ 1.…”

“…For further investigations, we therefore resort to impedance measurements, as established in our previous studies on Li 1− δ CoO 2 and Li 2− δ Mn 2 O 4 . 1,2 This virtually eliminates the effect of background currents and allows us to extract not only the volume-specific chemical capacitance C V chem ( i.e. , differential capacity), but also the charge transfer resistance R ct , the ionic conductivity σ ion and the Li chemical diffusion coefficient D̃ as a function of SOC.…”

“…Interestingly, defect chemical models based on dilute-solution thermodynamics can offer valuable insights into the SOC-dependent transport properties of Li storage materials over a surprisingly wide range of charge carrier concentrations. 1–3 Nonetheless, defect interactions inevitably come into play when exploiting the full charge/discharge capacity of a given Li storage material.…”

Electrochemical (PITT, EIS) and Analytical (SIMS) Determination of Li Diffusivities at the Onset of Charging LiNi0.33Mn0.33Co0.33O2 Electrodes

“…Basically, in ionic solids chemical diffusion is ambipolar. , In the present case, Li vacancies and holes have to be considered as relevant defects . In general, both species contribute to the chemical diffusivity.…”

“…Thus, the conductivity of electronic charge carriers is orders of magnitude higher than that of ionic charge carriers. In case the concentration of electronic charge carriers is much higher than the vacancy concentration, we can write in approximation , D chem = D V = D tracer / f V where D V is the diffusivity of the vacancies and f V is the mole fraction of the vacancies. That means chemical diffusivities are higher than tracer diffusivities divided by the mole fraction of vacancies.…”