UCLA

This dissertation aims to investigate the thermal behavior of materials and electrodes made from novel synthesis and fabrication methods in lithium-ion batteries and electrochemical capacitors. This dissertation also aims to develop novel characterization techniques with rigorous design and validation. A combination of experimental measurements, numerical simulations, and theoretical analysis are presented.

First, this dissertation compares the thermodynamics behavior and the operando heat generation in lithium-ion battery electrodes made of Ti2Nb2O9 microparticles or nanoparticles synthesized by solid-state or sol-gel methods. Electrochemical testing showed that electrodes made of Ti2Nb2O9 nanoparticles exhibited larger specific capacity, smaller polarization, and better capacity retention at large currents. Potentiometric entropy measurements revealed that both types of electrodes showed similar thermodynamics behavior governed by lithium intercalation in solid solution. However, electrodes made of Ti2Nb2O9 nanoparticles featured smaller overpotential and faster lithium ion transport. In fact, operando isothermal calorimetry during galvanostatic cycling revealed smaller instantaneous and time-averaged irreversible heat generation rates at electrodes made of Ti2Nb2O9 nanoparticles, highlighting their smaller resistive losses and larger electrical conductivity.

Similarly, this dissertation compares NMC622 lithium-ion battery electrodes fabricated using a novel 3D printing process or the conventional 2D tape casting process. Potentiometric entropy measurements revealed that their thermodynamics behavior were identical and consisted of lithium deintercalation in solid solution. However, operando isothermal calorimetry indicated that the 3D printed electrodes featured larger specific capacity and better rate performance, attributed to their larger electrode/electrolyte interfacial surface area and electrical conductivity as well as their faster lithium ion transport. Therefore, the instantaneous heat generation rates were smaller in 3D printed electrodes, reducing the overall specific electrical energy and thermal energy dissipation per unit charge stored.

Furthermore, this dissertation proposes a novel and fast microcalorimetry electrothermal impedance spectroscopy (ETIS) method based on heat generation rate measurements at each electrode of a lithium-ion battery cell. This new method is capable of retrieving the open-circuit voltage, the entropic potential, and the partial entropy changes at each electrode from measurements at a single temperature. It also shortens the measurement duration to a few hours compared to several days using the galvanostatic intermittent titration technique (GITT). This novel microcalorimetry ETIS method was first validated with numerical simulations and then experimentally demonstrated on PNb9O25 or TiNb2O7 battery cells.

Finally, this dissertation validates the step potential electrochemical spectroscopy (SPECS) method and refines the associated analysis capable of differentiating the contributions of electrical double layer formation and Faradaic reactions to the total charge storage in threedimensional porous pseudocapacitive electrodes. The modified Poisson-Nernst-Planck model coupled with the Frumkin-Butler-Volmer theory were used to numerically reproduce experimental data obtained from the SPECS method accounting for interfacial, transport, and electrochemical phenomena. The fitting analysis of the SPECS method was modified for the Faradaic current. The new model can accurately predict the individual contributions of EDL formation and Faradaic reactions to the total current. Moreover, the contributions of EDL formation at the electrode surface or at the electrode/electrolyte interface within the porous electrode can be identified. Similarly, the Faradaic reactions due to surface-controlled or diffusion-controlled mechanisms can be distinguished.