UCLA

This dissertation reviews and clarifies the fundamental thermodynamic relationships relevant to the interpretation of potentiometric entropy measurements on lithium-ion batteries (LIBs) to gain insight into the physicochemical phenomena occurring during cycling. First, contributions from configurational, vibrational, and electronic excitations to the entropy ofan ideal intercalation compound used as a cathode in a battery system were analyzed. The results of this analysis were used to provide an interpretative guide of open circuit voltage Uocv(x, T) and entropic potential ∂Uocv(x, T)/∂T measurements to identify different mechanisms of intercalation, including (i) lithium intercalation as a homogeneous solid solution, (ii) ion ordering reactions from a homogeneous solid solution, (iii) first-order phase transitions involving a two-phase coexistence, and/or (iv) first-order phase transitions passing through a stable intermediate phase. These interpretations were illustrated with experimental data for different battery electrode materials including TiS2, LiCoO2, Li4/3Ti5/3O4, LiFePO4, and graphite electrodes with metallic lithium as the counter electrode. The systematic interpretation of Uocv(x, T) and ∂Uocv(x, T)/∂T can enhance other structural analysis techniques such as X-ray diffraction, electron energy-loss spectroscopy, and Raman spectroscopy.

Thermal signatures associated with electrochemical and transport phenomena occurring in LIB systems were investigated by performing potentiometric entropy measurement and isothermal operando calorimetry on LIB systems. Here, LIB system consisting of electrodes made of TiNb2O7 and PNb9O25 were investigated. The potentiometric entropy measurements of TiNb2O7 and PNb9O25 featured signatures of intralayer ion ordering upon lithiation that could not be observed with in situ X-ray diffraction. Furthermore, entropy measurements also confirmed the semiconductor-to-metal transition taking place at PNb9O25 upon lithiation. Furthermore, isothermal operando calorimetry measurements indicated that the nature of heat generation was dominated by Joule heating, which sensitively changed as the conductivity of the electrode increased with increasing lithiation. The heat generation rate decreased at the TiNb2O7 and PNb9O25 electrode upon lithiation due to the decrease in electrical resistivity caused by the semiconductor-to-metal transition also observed in potentiometric entropy measurements. In addition, the time-averaged irreversible heat generation rate indicated that the electrical resistance of the lithium metal electrode was constant and independent of the state of charge while the electrical resistance of the PNb9O25 changedsignificantly with the state of charge. Moreover, calorimetry measurements have shown that the electrical energy losses were dissipated entirely in the form of heat. Furthermore, the enthalpy of mixing, obtained from operando calorimetry, is found to be small across the different degrees of lithiation, pointing to the high rate of lithium-ion diffusion at the origin of rapid rate performance of TiNb2O7 and PNb9O25.

Moreover, the effect of particle size on the electrochemical performance and heat generation in LIB systems were investigated using two LIBs consisting of electrodes made of either(W0.2V0.8)3O7 nanoparticles synthesized by sol-gel method combined with freeze-drying or (W0.2V0.8)3O7 microparticles synthesized by solid-state method. Galvanostatic cycling confirmed that the electrode made of (W0.2V0.8)3O7 nanoparticles featured larger capacity and better retention at high C-rates than that made of the (W0.2V0.8)3O7 microparticles. Entropic potential measurements performed at slow C-rate indicated that both nanoparticles and microparticles underwent a semiconductor to metal transition, and nanoparticles underwent a two-phase coexistence region over a narrower range of composition. Operando calorimetry measurements at high C-rate established that the heat generation rate increased at the (W0.2V0.8)3O7 electrode upon lithiation due to an increase in charge transfer resistance regardless of particle size. Moreover, the time-averaged irreversible heat generation rate was slightly but systematically smaller at the electrode made of nanoparticles. Furthermore, the specific dissipated energy and the contribution from enthalpy of mixing caused by lithium concentration gradient was notably smaller for (W0.2V0.8)3O7 nanoparticles. These observations were attributed to the fact that nanoparticles were less electrically resistive and able to accommodate more lithium while lithium ion intercalation therein was more kinetically favorable.