Electrochemical Mechanism And Mathematical Modeling Of Lithium Iron Phosphate Nanoparticles

Battery technology is one of the key technologies that hinder the large scaleapplication of electric vehicles. In recent years, lithium iron phosphate （LiFePO₄） hasbeen intensively investegated because of its chemical safety, cylcing stabililty, low-costand nontoxicity. However, in both the enineering and scientific research of LiFePO₄battery, there were many scientific puzzles which cannot be satisfactorily solved bytraditional theories. Based on the understanding of the results from first-principlecalculations and microscopic experiments, a novel mathematical model forinterfacial lithium intercalation and bulk phase transformation dynamics in asingle LiFePO₄nanoparticle has been proposed, which couples aButler-Volmer-type charge transfer reaction rate model with the Cahn-Hilliardphase filed model in a thermodynamically self-consistent way. This is the firsttheoretical tool for lithium intercalation and phase transformation dynamics atnanoscale.Via the thermodynamic analysis of this electrochemically-driven phasetransformation process, a necessary condition for phase separation has beenidentified: only if the equilibrium equation has two stable solutions, the systemwould phase separate. Under the assumptions made in this dissertation, once theapplied voltage exceeds40mV, there exists only one solution for the equilibriumequation, therefore, the system will not phase separate.For systems working under constant current conditions, an integralconstraint should be introduced, which will help to connect the microscopicdynamics with the macroscopic responses. Under this integral constraint, anon-equilibrium linear stability analysis has been performed, which shows that,the time constant of filling the particle will decrease as the current increases; aslong as it is smaller than the time constant of instability growth, phaseseparation will be suppressed, and the system will behave as a “quasi-solidsolution”. The governing equation has been solved numerically by finitedifference method. The simulation results are consistent with the theoreticalanalysis, suggesting that, for LiFePO₄nanoparticles, phase separation will be suppressed under high current operations. Macroscopic voltage plateau and exsitu observation of biphase particles masked the in situ dynamics.Using a solvothermal method, LiFePO₄particles with size100nm hasbeen obtained. This material exhibits stable cycling performance and candischarge more than140mAh/g at0.1C-rate,90mAh/g at10C-rate. Based on thecontrolled phase transformation dynamics in single nanoparticle, a stochasticmathematical model has been established for predicting the behavior of phasetransformation in porous electrode. Potentiostatic Intermittent TitrationTechnique （PITT） has been applied to porous electrodes made of as-obtainednano LiFePO₄. For step sizes of5mV,10mV and20mV, the current responses ofcharge and discharge are asymmetrical, but all of them exhibit phase separationbehavior; for step sizes of40mV,100mV and150mV, the asymmetry isdiminished. After compared with the most recent publications, a concept of“pseudo-phase separation” has been proposed to explain the discrete phaseseparation behavior of porous electrode.