The Mechanism of Decreasing Resistance by SDX™ in Lithium Ion Battery

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1. Introduction Practical uses of lithium-ion batteries are rapidly growing especially in large batteries for electric vehicles (EV), energy storage systems (ESS) and so on, but improvements of their characteristics still are strongly demanded. Large improvements of battery characteristics of LFP (LiFePO4) and NMC (LiNi1/3 Mn1/3 Co1/3 O2) as cathode active material, by using ‘SDXTM’ which is a conducting carbon coated aluminum current collector, have been reported [1,2]. The control of internal resistance of lithium ion batteries by the clarification of mechanism of internal resistance is one of the biggest key items to improve them. So the mechanisms of interface resistance between aluminum current collector and cathode active material layer have been investigated and discussed [3]. In the battery which used LFP as cathode active material, recently the quantitative investigation of contribution to electronic resistance reduction of SDXTMand conducting additives was provided by the result of measurement using ELECTRODE RESISTANCE METER (Under development by HIOKI E.E. CORPORATION) which could divide electrode resistance into material resistance and interface resistance (Fig.1). The interface resistance of the cathde with aluminum current collectors was higher than the material resistance of that by almost one order of magnitude. The interface resistance of the cathode with SDXTMwas lower than that with aluminum current collectors by almost one order of magnitude. Because the interface resistance was reduced by SDXTM, it was reported that conducting additives were able to be reduced much in cathode [4]. In this paper, more detailed examination will be carried out, and the effect of SDXTMin lithium ion battery will be tried to be clarified. 2. Experiment The preparation of electrode slurry was carried out by dispersing LFP as cathode active material, conducting additives and polyvinylidene difluoride (PVdF) into N-methylpyrrolidone (NMP). Cathode electrodes were prepared by coating the slurry onto SDXTM or aluminum current collectors in several coating speeds (e.g. 300, 400 or 500mm/min), by drying and by pressing them. Artificial graphite (SCMGTM) was used for anode active material, and similarly anode electrodes were prepared by coating its slurry (aqueous solution) onto copper current collectors, by drying and by pressing them. 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). In each coating speed, the interface resistance of the cathode with aluminum current collectors was higher than the material resistance of that. The interface resistance of the cathode with SDXTM was lower than that with aluminum current collectors, and it was almost same as the material resistance of the cathode with SDXTM. The interface resistance of the cathode with aluminum current collectors increased when the coating speed increased, but the interface resistance of the cathode with SDXTMwas constant without depending on coating speed. Cell DCRs were measured (Fig.3). The cell DCR with aluminum current collectors increased when the coating speed increased, but the cell DCR with SDXTMwas constant without depending on coating speed. It was confirmed that the cell DCR with aluminum current collector showed similar behavior to the interface resistance of the cathode with aluminum current collector. The visualization results of a dispersion state of conducting additive in an electrode will be presented.