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Surface Tailored Acetylene Black for High Voltage Lib Application
In recent years, high voltage Lithium-ion secondary batteries (LiB) has been attractive energy storage system especially for electric vehicles due to its high energy density compared to commercialized LiB. In high voltage LiB system, although there are still a lot of obstacles for practical use, most serious problem is that gas generation during charging. [1] This gas generation is induced from decomposition of electrolyte on the surface of composed materials such as positive electrode active materials, binder, and also conductive additive, as we call it conductive agent which could improve cell performance as well as other components do. The decomposition of electrolyte on the surface of conductive agent is tend to depend on its specific surface area (SSA). Furthermore, surface functional groups, crystalline and crystal defects of conductive agent could related to the decomposition of electrolyte in previous research by X-ray Photoelectron Spectroscopy, Raman and Electron Spin Resonance (ESR) [2,3]. Therefore, the surface condition of conductive agent is important to suppress the gas generation for high voltage LiB application.
In this research, we modified the surface of conductive agent to reduce surface functional groups or defects that are the factors related to decomposition of electrolyte. Then we evaluated the amount of generated gas and cell performances of the electrode consisted of surface tailored conductive agent at high voltage.
(Experimental & Results)
Acetylene Black (AB) was prepared as conductive agent (Particle size: 48 nm and SSA: 39 m2/g) and Tailored-AB, which is coated AB with inactive chemical materials such as SiO2 by sol-gel method [4,5]. The fundamental electrochemical performance was evaluated with simple half cell (2032type coin cell) by cyclic voltammetry (CV), and the amount of generated gas at high voltage (about 5.0 V) was evaluated with pouch cell by high-temperature storage test. The electrodes for electrochemical performance were made from AB and PVdF and the mass ratio was set to AB:PVdF=1:1.The electrode for high-temperature storage test was made from LiNi0.5Mn1.5O4 (NMS), AB and PVdF and the mass ratio was set to NMS:AB:PVdF= 90:5:5. This fabricated positive electrode and negative electrode which is mainly composed of natural graphite were paired in a pouch cell. At all test, 1 mol / dm3 LiPF6 in ECFDEC1F2 [vol %] was selected as electrolyte. The amount of generated gas was evaluated by measurement of cell volume after storage for 72 hours at 60oC under full charged state.
Crystal defects were measured by Electron Spin Resonance (ESR). Crystal defects detected by ESR are assigned as conductive electron spin (CES) and localized electron spin (LES). LES are corresponding to crystal defects on the surface of AB, because almost all spins of carbon are present on the surface of carbon blacks [6,7,8].
Figure shows SEM images (BSE, Back Scattered Electron) of (a)pristine AB and (b)Tailored-AB. In this figure, the contrast of SiO2 was brighter than that of carbon. It was found that Tailored-AB was partially coated by SiO2. According to our results, Tailored-AB could have much potential to suppress the decomposition of electrolyte and gas generation compared with pristine AB. The results of ESR measurement which is a representative index of defects might indicate that defects of Tailored-AB decreased by chemical modification. In this presentation, further discussions about more detail results, other chemical modification and the analysis method will be presented
(Reference)
(1) H. Wang, et al., Anal. Chem., 86, 6197, (2014).
(2) M. Nishikawa, et al., 55th battery discussion, 2B04, (2014).
(3) T. Sonoda, et al., 41st carbon material Society, 1A08, (2014).
(4) W. L. Zhang, et al., American Chemical Society, 28, 7055, (2012).
(5) T. Sonoda, et al.,7th Lithium Battery Discussions, to be presented at the end of June in 2015
(6) S. Morozowski, et al., Carbon, 9, 97 (1971).
(7) S. Ishii, et al., Physica E, 19, 149 (2003).
(8) K. Fujimoto, et al., J. S. Rubber Industry, 46, 3 (1973).