Graphite for Energy Applications

Abstract

INTRODUCTION Graphite is widely used as active material for anodes in lithium ion batteries (LIB), and by selection and processing, graphitic materials can be optimized for energy and power applications, respectively. We have looked at contributions of particle size and materials processing to capacity and discharge rate capabilities of natural and artificial graphite. This work continues research reported earlier4. METHODOLOGY First, we compared coated spherical natural graphite of regular and fine size at discharge rates from C/20 to 30C. Our interest was to assess diffusion as limiting factor1,2, We then looked at two kinds of artificial graphite of similar size which were derived from different processes. The investigation was done on half cells (G/EC-DMC-LiPF6/Li). The electrode composition was the same throughout: 91 % graphite, 7% binder PVDF 9305 from Kureha and 2 % carbon black Super P. Thin electrodes (≈30mm, 1 g/cc) were prepared for high and thick electrodes (≈60mm, 1.5 g/cc) for lower rates. Due to its mechanical properties, the desired electrode density of 1.5 g/cc proved impossible for artificial graphite #2. The two first cycles with the thick electrodes were done at constant current (CC) at C/20, and then the intercalation was done at C/5 followed by a step at constant voltage (CV) to reach full intercalation. For thin electrodes the intercalation was at done at C/5 with CV step throughout. RESULTS AND DISCUSSION Physical and chemical characterization: Table (1) presents physical and chemical characteristics of the materials. Figure (1) exhibits differences in texture with artificial graphite #2 having higher rugosity. Test Analysis Natural #1 Natural #2 Artificial #1 Artificial #2 Type of Graphite Coated NG Coated NG Synthetic Synthetic Ash (%) 0.02 0.024 < 0.01 < 0.01 True Density (g/cc) 2.18 2.2 2.25 2.13 Tap Density (g/cc) 1.05 1.14 1.21 0.34 Surface Area (m2/g) 2.06 1.09 1.15 6.45 Particle size (Microtrac, mm) D10 5.51 10.62 5.38 4.22 D50 8.34 16.9 13.22 10.46 D90 13.47 27.09 28.59 20.71 Table (1): Characteristics of two kinds each of coated natural and of artificial graphite Figure (1): SEM of artificial graphite #1 (left) and #2 (right). Electrochemical Characterization: Artificial Graphite Artificial graphite #1 and #2 have comparable discharge capacity of 330 mAh/g at C rate, but differ significantly at higher discharge rates (table 2). Although at same level of graphitization3 (86-87%), the differences in processing show in surface area, tap density (table 1) and surface rugosity (figure 1). The evolution of the discharge capacities as a function of discharge rate are presented in figure (2). Figure (2): Charge and discharge of artificial graphite #1 and #2 in half-cell with thin electrode. Artificial #1 Artificial #2 Rates (C) 1/2 discharge voltage (V) Capacity (mAh/g at 2 V) Capacity (% of capacity at C) 1/2 discharge voltage (V) Capacity (mAh/g at 2 Volts) Capacity (% of capacity at C) 1 0.184 332 100 0.170 331 100 5 0.362 330 99 0.305 331 100 10 0.579 323 97 0.467 329 99 20 0.946 282 85 0.796 318 96 30 1.135 228 69 0.960 289 87 Table (2): Gravimetric capacity and voltage vs. Li/Li+ at half discharge for artificial graphite #1 and #2 Spherical coated natural graphite Electrochemical results from spherical graphite #1 and #2 at high rate discharge are presented in figure (3) showing similar behaviors up to 10C discharge. At 20C and 30C we see a higher impedance for the finer spherical graphite #1. Figure 3: High rate discharges of coarse (right) and fine (left) spherical graphite CONCLUSION We have seen that differences in processing are reflected both in physical characteristics and in electrochemical behavior. The nature of graphite affects both the intrinsic capacity and the ability to sustain high discharge rates. The discharge capacity at 30C of artificial graphite reached 280 mAh/g, 85% of the capacity at C rate. The lower packing density could be an issue for energy applications, but may not be acceptable for power applications. REFERENCES 1) Persson, K. et al. The journal of physical chemistry letters 1(8) (2010) 1176-1180. 2) Yu, P. et al. Journal of The Electrochemical Society, 146(1) (1999)8-14. 3) J. Maire, J. Mering. Chemistry and physics of carbon 6 (1970): 125-190. 4) Henry FX. et al. (2006) “New developments in the advanced graphite for lithium-ion batteries”. In: NATO Science Series II: Mathematics, Physics and Chemistry, vol 229. Springer, Dordrecht Figure 1