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.

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.

Pronounced Li+/H+ exchange of doped Li7La3Zr2O12 (LLZO) takes place in hot water. LIBS and ICP-OES analysis reveal the importance of grain boundaries in this ion exchange process.

Cubic Li7La3Zr2O12 (LLZO) garnets are among the most promising solid electrolytes for solid-state batteries with the potential to exceed conventional battery concepts in terms of energy density and safety. The...

Lithium lanthanum titanate Li0.29+δLa0.57TiO3 (LLTO) is a promising material in Li ion battery application, due to its ambient stability and high ionic conductivity. When it is subjected to a high Li chemical potential, additional Li ions intercalate into vacant A sites, which is balanced by the reduction of Ti4+ ions to Ti3+. At this point, LLTO becomes a mixed ion and electron conductor, which means that it undergoes a transition from an electrolyte to a high rate capable electrode material in the potential range below ca 1.7 V vs Li metal. However, the exact voltage of the transition from electrolyte to the electrode, as well as the electronic conductivity of reduced LLTO were still unknown. Here, we investigate the thermodynamics of lithium insertion as well as ion and electron conductivity of reduced LLTO by employing a galvanostatic intermittent titration technique and electrochemical impedance spectroscopy. We can show that LLTO gradually changes from electrolyte material to a mixed conductor, with an ion transference number that depends on the Li chemical potential. Lastly, we present a defect chemical model that fits excellent to the U(δ) curves and the conductivity data.