An electrochemical model was developed to describe alternating current ͑ac͒ impedance experimental studies conducted on lithium-ion positive electrodes. The model includes differential mass and current balances for the positive electrode's composite structure, as well as details of the oxide-electrolyte interface. A number of specialized experiments were conducted to help define the parameter set for the model. The electrochemical ac impedance model was used to examine aging effects associated with the positive electrode.

An electrochemical model was developed to examine hybrid pulsed power characterization ͑HPPC͒ tests on the positive electrode of lithium-ion cells. By utilizing the same fundamental equations as in previous electrochemical impedance spectroscopy studies, this investigation serves as an extension of the earlier work and a comparison of the two techniques. The electrochemical model was used to examine performance characteristics and limitations for the positive electrode during HPPC tests. Parametric studies using the electrochemical model and focusing on the positive electrode thickness were employed to examine methods of slowing electrode aging and improving performance.A wide variety of analytical and electrochemical diagnostic tools 1 are being used to extensively study the lithium-ion battery technology for hybrid electric vehicle applications under the Department of Energy's Advanced Technology Development Program. 2 Hybrid pulsed power characterization ͑HPPC͒ tests 3 and electrochemical impedance spectroscopy ͑EIS͒ studies are regularly conducted on cells during aging to ascertain their performance. In the HPPC tests, cell voltage changes are monitored while short-duration ͑Ͻ20 s͒ high-current discharge and charge pulses are applied to the cell at various states of charge ͑SOC͒.This modeling work is directed toward extending earlier EIS modeling studies 4,5 to the HPPC testing effort. Although both electrodes in the lithium-ion cell have been studied using a similar electrochemical model, the discussion here will be focused on the modeling of the positive electrode because of its importance to the cell's overall impedance rise. 6,7 Both the EIS and HPPC models are based on the same fundamental equations and utilize the same parameter set. Ideally, a series of well-characterized experiments can be conducted to obtain all the parameters independently. Simulations using the models can then be compared to the EIS and HPPC test results with no parameter adjustments to gauge their self-consistency, as well as the accuracy of the individual models. At the present time, the interfacial parameters can only be determined from fitting the EIS model to the experimental results. Comparing the HPPC tests to the simulations then becomes the primary indicator of the model accuracy.The positive electrode under study has a composite structure made of a layered nickel oxide ͑LiNi 0.8 Co 0.15 Al 0.05 O 2 ͒ active material, carbon black and graphite additives for distributing current, and a poly͑vinylidene fluoride͒ ͑PVDF͒ binder all on an aluminum current collector. The active material is made of spherical secondary oxide particles, 10 m diam, which is composed of faceted approximately 0.2-0.5 m primary particles. The electrolyte is 1.2 M LiPF 6 dissolved in a mixture of ethylene carbonate ͑EC͒ and ethyl methyl carbonate ͑EMC͒ and a Celgard microporous membrane is used as the separator. The negative electrode has a composite coating of graphite and PVDF binder all on a copper current collector. A detailed description of the cel...

No abstract

No abstract