Harshad Tataria

Aging Mechanisms of LiFePO4 Batteries Deduced by Electrochemical and Structural Analyses

The performance loss of lithium-ion batteries with lithium iron phosphate positive chemistry was analyzed using electrochemical characterization techniques such as galvanostatic charge-discharge at different rates, ac impedance, and hybrid pulse power characterization measurements. Differentiation analysis of the discharge profiles as well as in situ reference electrode measurement revealed loss of lithium as well as degradation of the carbon negative; the cell capacity, however, was limited by the amount of active lithium. Destructive physical analyses and ex situ electrochemical analyses were performed at test completion on selected cells. While no change in positive morphology and performance was detected, significant cracking and delamination of the carbon negative was observed. In addition, X-ray diffraction analysis confirmed the changes in the crystal structure of the graphite during cycling. The degradation of the carbon negative is consistent with the observations from the electrochemical analysis. Ex situ electrochemical analysis confirmed that active lithium controlled cell capacity and its loss with cycling directly correlated with cell degradation. The relationship between carbon negative degradation and loss of active lithium is discussed in the context of a consistent overall mechanism.

Energy storage system for GM Volt — lifetime benefits

The rapid expansion of hybrid and alternative fuel vehicles into the marketplace will make a significant impact on greenhouse gas emissions and reduction in dependence on volatile world oil market. However, there are several obstacles preventing mass adoption of these automotive improvements. These obstacles include alternative energy costs, fluctuation in oil prices, and supplies of alternative energy. This paper will analyze the Chevy Volt Extended Range Electric Vehicle (EREV) gas savings, CO2 reduction, and reduction in oil consumption based on a real world model derived from actual vehicle and driver profiles. The benefits of the Volt and other alternative energy vehicles are dependent upon the number of vehicles used by the overall population. Government regulations and the price of oil will greatly influence the rapid adoption of these vehicles.

USABC development of 12 volt battery for start-stop application

Global automakers are accelerating the development of fuel efficient vehicles, as a part of meeting regional regulatory CO2 emissions requirements. The micro hybrid vehicles with auto start-stop functionality are considered economical solutions for the stringent European regulations. Flooded lead acid batteries were initially considered the most economical solution for idle-stop systems. However, the dynamic charge acceptance (DCA) at lower state-of-charge (SOC) was limiting the life of the batteries. While improved lead-acid batteries with AGM and VRLA features have improved battery longevity, they do not last the life of the vehicle. The United States Advanced Battery Consortium (or USABC, a consortium of GM, Ford, and Chrysler) analyzed energy storage needs for a micro hybrid automobile with start-stop capability, and with a single power source. USABC has analyzed the start-stop behaviors of many drivers and has developed the requirements for the start-stop batteries (Table 3). The testing procedures to validate the performance and longevity were standardized and published. The guideline for the cost estimates calculations have also been provided, in order to determine the value of the newly developed modules. The analysis effort resulted in a set of requirements which will help the battery manufacturers to develop a module to meet the automotive Original Equipment Manufacturers (OEM) micro hybrid vehicle requirements. Battery developers were invited to submit development proposals and two proposals were selected for 50% cost share with USABC/DOE.