Practical uses of lithium-ion batteries(LIB) are rapidly growing especially in large batteries for full electric vehicle(EV), plug-in hybrid electric vehicle(PHEV) and so on, but improvements of their characteristics(“high-energy density”, “high power”, “long cycle-life”, “safety” and “lower cost”) are still strongly demanded.
SDX® is a surface coated aluminum current collector(AL) with carbon black(CB) and organic binder, and the thickness of the coated layer is about 1μm, and SDX®makes a cell internal resistance lower and adhesion between cathode active materials and AL stronger, so as to improve battery performance dramatically[1,2]. The control of the internal resistance of LIB by the clarification of the mechanism is one of the biggest key items to improve them. So the mechanisms of the interface resistance between cathode active material layer and AL have been investigated and discussed.
In general, cell internal resistance is separated simply into electronic resistance and ionic resistance. We successfully separated the electronic resistance into material resistance and interface resistance between cathode active material layer and AL. And then we independently measured the material resistance and the interface resistance by Electrode Resistance Meter(HIOKI E.E. CORPORATION) which can separate electrode resistance into material and interface resistance.
In the battery which used LFP as cathode active material, the quantitative investigation of the contribution of SDX® and conducting additives to the electronic resistance reduction was provided by the result of measurement using Electrode Resistance Meter(Fig.1). The interface resistance of the cathode with AL was higher than the material resistance of that by almost one order of magnitude. The interface resistance of the cathode with SDX® was lower than that with AL by almost one order of magnitude. It is not only presumed that the interface resistance is dominant in the electronic resistance but confirmed that the interface resistance is maintained at lower level by SDX®. Because the interface resistance was reduced by SDX®, it has been reported that conducting additives could be reduced much in cathode.
By application of SDX®, it was confirmed that the interface resistance of LFP could be decreased largely irrespective of a distribution state of conducting additives. As a result, cell internal resistance and discharge capacity retention of LFP was constant irrespective of coating speed.
In this paper, more detailed examination will be carried out, and resistance reduction effect by SDX®in LIB will be tried to be clarified.
Firstly, the preparation of electrode slurry was carried out by dispersing LFP as a cathode active material, CB as a conducting additive and polyvinylidene difluoride(PVdF) as binder into N-methylpyrrolidone(NMP). The compounding ratio of LFP, CB and PVdF was set to 90:5:5 as a solid mass ratio. Secondly, cathode electrodes were prepared by coating the slurry onto AL or SDX®and dried, before they were pressed at several conditions(e.g. 1.6, 2.0 or 2.5g/cc). By laminating one sheet of anode and one sheet of cathode, pouch type cell was assembled.
3. Results and discussion
The material resistance and the interface resistance of the cathode were measured by Electrode Resistance Meter(Fig.2). The interface resistance of the cathode with AL increased as the electrode density lowered and finally the interface resistance of the cathode of electrode density of 1.6g/cc with AL could not be measured because of high resistance. On the other hand, it was confirmed that the interface resistance of the cathode with SDX®was constant irrespective of the electrode density.
Cell internal resistance(DCR) were measured(Fig.3). The cell DCR with AL increased when the electrode density lowered, and the cell of the cathode of electrode density of 1.6g/cc with AL did not work because of high resistance. It was confirmed that the cell DCR with SDX® was constant irrespective of the electrode density showing similar behavior to the interface resistance of the cathode with SDX®.
Cell discharge performance was also evaluated(Fig.4). It confirmed that the cell discharge performance of the cathode of electrode density of 2.5g/cc with AL was almost same as that with SDX®. But when the electrode density with SDX® lowered, especially at high rate(10, 20C), the cell capacity retention with SDX®was improved.
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2. M. Ohmori et al., Electrochemistry, 79, 165, (2012)
3. M. Ohmori et al., Prime2012
4. A. Takeda, H. Yokouchi, H. Tomozawa, Electrochemistry, 2C25, (2015)