(518d) Analysis of Electrode Materials for Use in Lithium Ion Cells for Automotive Applications

Abstract:

            Understanding the individual electrode materials used in lithium ion cells is of prime importance for their long term use in hybrid electric vehicles. Several analytical techniques have been useful in this regard. The In-Situ Raman spectroscopy technique has been employed before[1]^,[2] to understand the behavior of negative and positive electrode particles, individually in real time. Efforts also can be seen in the literature that discuss space and time-resolved Raman spectro-microtopography techniques[3]  applied to a working battery with LiCoO₂/graphite electrode materials. As mentioned in Ref.3, this technique was qualitative at that stage and these authors have suggested that this technique could be used to generate dynamic images of lithium transport within the electrodes by mapping the state of charge as a function of time and space. The authors also identified the rate of spectral acquisition as the rate limiting step. If in-situ Raman techniques can be successfully employed in dual insertion lithium ion cells to understand solid-phase lithium concentration gradients in both electrodes in real time during a charge-discharge process, one can come up with better engineering solutions to overcome problems at the cell level that pose limitations to their performance at the system level. This presentation will attempt to discuss efforts along those lines that have been carried out at Ford Motor Company with electrode materials other than LiCoO₂, which was used in Ref.3. In the future, experimental results can be compared with modeling predictions as well.

            To begin with, the open-circuit potential data of the positive electrode material (proprietary) is needed. This information can be obtained from slow-rate charge-discharge data (in our case C/27 rate) as shown in Figure 1 and then converting the specific capacity to intercalation coefficient of Li in the positive electrode material. These data were obtained using the cell configuration shown in Figure 2, which is termed the ?hockey- puck? cell. Similar experiments were performed to characterize the negative electrode (carbon) material as well. These electrodes were cycled a few times both in half-cell as well as in full-cell configurations to make sure that they have good cyclability before performing any in-situ Raman experiments. In-situ Raman experiments were then carried out in an ?edge cell? configuration (figure not shown here) in which, as the name suggests, one can observe the progress of experiments across a cell sandwich. Some of the preliminary results and methods to obtain state of charge estimations from Raman data will be discussed at the meeting.

Figure1. Slow rate charge-discharge data for the positive lithium intercalation electrode

Figure 2 Hockey-puck cell used for half cell and non-optical experiments

Acknowledgements:

Ford Motor Company is acknowledged for financial support of this research work and Dr.Dawn Bernardi for helpful discussions. Dr. Thomas F Fuller, Professor, Georgia Tech is acknowledged for his support.

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