Influence of the Electrode Morphology on Lithium-Ion Battery Performance

In the future the application of lithium ion batteries as an energy carrier for electric and hybrid electric vehicles will become important field in automotive industry. To achieve this aim, the power density, energy density and long term stability of a battery have to be increased. Sophisticated mixing processes as well as the calendaring process (electrode compression) are known to significantly affect the above mentioned criteria, in particular the electrochemical cell performance [1-3]. However, the underlying cause-effect relationships are to a large extend still unclear.

The presented work investigates the influence of the electrode morphology on the electrochemical performance of the lithium ion battery, aiming at a model which correlates the intrinsic electrode design with characteristics, influence and dependencies of the electrode conductive paths. The performed experiments focused on the observation of changes in the electrode morphology upon external compression force using a novel analytical method, “quasi in-situ” compression scanning electron microscopy. For instance, direct insight into changes of particle-particle distances or pore volumes and their distribution may be obtained. Apart from electrode morphology analysis the electronic conductivity at the same compression pressure is investigated. Exemplary, figure 1a illustrates the changes of the electronic conductivity of the electrode as a function of pressure, as measured with the setup shown in figure 1b. While the porosity of the coating is decreased from initially 45% to 40%, the electronic conductivity increases significantly. Additional determination of the electrode compound performance in an electrochemical half cell (discharge rate test, average discharge potential and lifetime) for different compression forces is made. From the entire data set, a model is derived which correlates the morphological changes e.g., conductive additive network change with the resulting electrochemical performance.

Fig.1: (a) Measurement of electrode conductivity for varying force and a schematic illustration of compressed and uncompressed electrode (b)Experimental setup of conductivity measurements for varying force F; a small current (10 mA) is applied to the system, the response in voltage is recorded.

References

[1]  H. Y. Tran, G. Greco, C. Täubert, M. Wohlfahrt-Mehrens, W. Haselrieder, A. Kwade, Journal of Power Sources 210 (2012), 276

[2]  Y. H. Chen, C. W. Wang, X. Zhang, A. M. Sastry, Journal of Power Sources 195 (2010), 2851

[3]  H. Zheng, J. Li,  X. Song, G. Liu, V.S. Battaglia, Electrochimica Acta 71 (2012), 258