Effect of Cell Impedance on Lithium Transport
Lithium transport through intercalation compounds, including transition metal oxides and carbonaceous materials, has been known for a long time to be limited by solid-state lithium diffusion and most studies on the cell reaction kinetics have been accordi
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Effect of Cell Impedance on Lithium Transport
Lithium transport through intercalation compounds, including transition metal oxides and carbonaceous materials, has been known for a long time to be limited by solid-state lithium diffusion and most studies on the cell reaction kinetics have been accordingly focused on lithium transport through the active materials [1–10]. However, a number of anomalous transport behaviors, which have never been explained on the basis of the “diffusion-controlled” concept, have been reported for various materials, demanding a new model to explain the lithium transport behavior [11–23]. The understanding of these anomalous phenomena entered a new phase when the real mechanism of lithium transport was revealed to be a “cell-impedance-controlled” process, not a “diffusion-controlled” one [24–35]. This chapter deals with the typical anomalous lithium transport behaviors and explains the importance of the cell impedance as the main factor affecting lithium transport.
8.1 8.1.1
Anomalous Features of Lithium Transport Non-Cottrell Behavior at the Initial Stage of Lithium Transport
In chronoamperometry, it is expected that the current transient would show a linear relation between the logarithmic current and the logarithmic time with a slope of 0.5 in the initial stage of the diffusion process (Cottrell behavior, see Chap. 2 for the details), insofar as the solid-state lithium diffusion governs the whole insertion process. However, this is not the case for most lithium insertion materials. Shown in Fig. 8.1 are the logarithmic current transients, obtained from Li1dNiO2 and graphite.
S.-I. Pyun et al., Electrochemistry of Insertion Materials for Hydrogen and Lithium, Monographs in Electrochemistry, DOI 10.1007/978-3-642-29464-8_8, # Springer-Verlag Berlin Heidelberg 2012
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8 Effect of Cell Impedance on Lithium Transport
Fig. 8.1 Experimental current transients obtained from (a) lithium nickel dioxide (Li1dNiO2) and (b) graphite (Reprinted from Lee et al. [28], Copyright #2001 with permission from Elsevier Science; Shin and Pyun [32], Copyright #2003 with permission from Kluwer Academic/Plenum Publishers)
Whether the lithium ion diffusion into them follows the Cottrell behavior or not can be more effectively judged by the I(t)· t1/2 versus ln t plot: In the Cottrell region, the value of I(t)· t1/2 remains constant, irrespective of the diffusion time. Shown in Fig. 8.2 is the I(t) t1/2 versus ln t plot reproduced from Fig. 8.1. There is a local maximum in the plots and no region with the Cottrell character is found. Some researchers claimed that the local maximum corresponded to the semiinfinite planar diffusion region, representing the Cottrell behavior [36–39]. However, it is quite unlikely that this is the Cottrell region, in that it has no time length. That is,
8.1 Anomalous Features of Lithium Transport
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Fig. 8.2 I(t)·t1/2 versus ln t plot reproduced from (a) Fig. 8.1a and (b) Fig. 8.1b (Reprinted from Shin and Pyun [32], Copyright #2003 with permission from Kluwer
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