Mass and Charge Transport in Li_1−δCoO₂ Thin Films─A Complete Set of Properties and Its Defect Chemical Interpretation

Abstract

Lithium insertion materials are an essential class of mixed ionic and electronic conductors, and their electrochemical properties depend on the resistive and capacitive interplay of ions and electrons. However, complete sets of the corresponding elementary material parameters, that is, composition-dependent ionic and electronic conductivity, chemical capacitance, and charge-transfer resistance, are rarely reported for lithium-ion battery electrode materials. Moreover, the interpretation of these properties from a defect chemical point of view is not very common. In this work, the impedance of sputtered Li1−δCoO2 thin films is analyzed to extract the fundamental electrochemical properties as a function of state-of-charge (SOC). Within the accessible SOC range, the charge transfer resistance and ionic conductivity vary by more than 1 order of magnitude. The chemical capacitance determined from impedance spectra agrees excellently with the differential capacitance from charge/discharge curves, and, in the dilute regime, even matches the absolute values predicted by defect thermodynamics. The evolution of lithium diffusivity along the charge curve is deconvoluted into the separate contributions of ionic conductivity and chemical capacitance. Finally, we apply the principles of defect chemistry to evaluate the observed trends in terms of lithium activity and point defect concentrations and provide a tentative defect model that is consistent with our results. The consistency of impedance measurements, cycling data, and thermodynamic theory highlights the key role of the chemical capacitance as a powerful material descriptor and emphasizes the relevance of defect chemical concepts for all lithium insertion electrode materials.

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

“…The good agreement of our dilute defect model with the experimental data over a rather wide stoichiometry range is somewhat surprising, given the high carrier concentrations involved. In a similar electrochemical study on Li 1‑δ CoO 2 , substantial deviations from the simple model without a defect interaction already appeared at about 10% Li vacancies . In general, defect interactions (or other changes of the materials with varying defect concentrations) seem to be less relevant for the spinel-type electrode compared to layered oxides; this is already visible in the steeper slopes and irregularities of the plateau regions for layered cathodes.…”

“…For the forward scan, starting at 3.70 V, R ct decreases exponentially from an initial value of about 7600 Ω cm 2 (1/ R ct ≈ 1.3 × 10 –4 S/cm 2 ) down to only 29 Ω cm 2 at 4.15 V. Above 4.15 V, R ct starts to increase again, reaching a nearly constant value of 70 Ω cm 2 at 4.40 V. The obtained values are in good agreement with literature. , In the backward scan, starting at 4.39 V, the values of R ct are slightly higher, with a maximum deviation of about +29% around 3.85 V, but otherwise closely match those from the forward scan. Since R ct does not only depend on the electrode’s surface concentration of ionic charge carriers, but furthermore varies with the concentration-dependence of the corresponding Galvani potential step across the LMO/electrolyte interface, , its variation with Li activity can be highly complex and a mechanistic discussion is beyond the scope of this work. At this point, it is sufficient to state that the variation of R ct qualitatively reflects the Li concentration in the material, transitioning from a vacancy-controlled insertion reaction with very few tetrahedral Li vacancies at low potentials to a Li + -controlled (high potential) insertion reaction.…”

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

“…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.…”

Nickel-Doped Titanium Oxide with Layered Rock-Salt Structure for Advanced Li-Storage Materials

“…The plots were studied by using the fitting analyses using an equivalent circuit (Figure b). In general, the diffusion coefficient value can be estimated from the Warburg impedance profile in electrochemical impedance spectroscopy results. , An inclined line with 45° appears by semi-infinite planar ion diffusion on the electrode. The impurity-doped TiO 2 electrodes showed the inclined straight lines.…”