Effect of Carbon Source as Additives in LiFePO 4 as Positive Electrode for Lithium-Ion Batteries K. Zaghib, a, * ,z J. Shim, b A. Guerfi, a P Charest, a and K. A. Striebel b, * a Institut de Recherche d’Hydro-Québec, Varennes QC, J3X 1S1, Canada b Lawrence Berkeley National Laboratory, Environmental Energy Technologies Division, Berkeley, California 94720, USA The electrochemical properties of LiFePO 4 cathodes with different carbon contents were studied to determine the role of carbon as conductive additive. LiFePO 4 cathodes containing from 0 to 12% of conductive additive carbon black or mixture of carbon black and graphite were cycled at different C rates. The capacity of the LiFePO 4 cathode increased as conductive additive content increased. Carbon increased the utilization of active material and the electrical conductivity of electrode, but decreased volumetric capacity of electrode. This composition LiFePO 4 with 3 wt % of carbon and 3 wt % of Graphite is suitable for HEV application. © 2005 The Electrochemical Society. DOI: 10.1149/1.1865652 All rights reserved. Manuscript submitted November 5, 2004; revised manuscript received December 1, 2004. Available electronically March 1, 2005. LiFePO 4 has been studied as the cathode-active material in Li rechargeable batteries because of their low cost, low toxicity, and relatively high theoretical specific capacity of 170 mAh/g. 1,2 How- ever, one major problem preventing the commercialization of LiFePO 4 is poor rate capability because of low electronic and ionic conductivity. Many researchers have tried several methods to im- prove the performance of LiFePO 4 . 3,4 Recently, Huang et al. re- ported that nanocomposites of LiFePO 4 and conductive carbon pre- pared by coating a carbon gel could approach theoretical capacity at room temperature and high rate. 5 However, as Chen and Dahn have already reported, 6 small particle size needs more carbon coating. Therefore, the energy density of the material decreases with decreas- ing grain size because of inactive and bulky carbon. However, they did not report the variation for volumetric and gravimetric energy densities of LiFePO 4 electrode by the amount of conductive additive which is added during electrode preparation. It is important to find the optimum content of conductive additive which affects the utili- zation of active material and the energy density of electrode. In this paper, we investigate the gravimetric and volumetric capacities, rate capability and cyclability of LiFePO 4 electrodes with different car- bon contents. Experimental LiFePO 4 cathodes consist of active material LiFePO 4 coated with carbon from Phostech, 16 carbon black CB, graphite G from Hydro Québec, PVdF and carbon-coated Al current collector. LiFePO 4 powder was coated by carbon around 1% during synthesis. All electrodes were supplied from Hydro Quebec HQ. Table I shows the electrode composition in detail. Before electrochemical test, electrodes were dried at 120°C under vacuum and then trans- ferred to an Ar-filled glove box. These electrodes were tested in a Swagelok type half-cell with Li reference and counter electrodes and Celgard separator. The electrolyte was 1 M LiBF 4 in EC/DEC  1/1 solvent. All cells were assembled in an Ar-filled glove box and tested at room temperature. The cutoff voltage of these cells was 2.5 to 4.0 V. To measure the rate capability, the cell was charged to 4.0 V at C/2 and then discharge to 2.5 V at specific rates from C/5 to 5C. Constant cycling was carried out 80 times at C/2 to evaluate capac- ity fade rate. Results and Discussion Figure 1 shows the performance of LiFePO 4 electrodes with dif- ferent conductive additive content for constant C/2 cycling. Most electrodes show excellent cyclability until 80 cycles. This figure shows that LiFePO 4 is very reversible for lithium intercalation. However, specific capacities of LiFePO 4 electrodes are very differ- ent according to carbon content. As the content of conductive addi- tive increases, specific capacity increases continuously. For the elec- trode with 6% CB and 6% G, the utilization at C/2 rate was over 82%, assuming that the theoretical capacity of LiFePO 4 is 170 mAh/g. Figure 2 shows the voltage profiles of LiFePO 4 electrodes con- taining 10% CB for various C rates. Discharging rates were changed from C/5 to 5C while charging rate was C/2 for all cycles. The IR drops during high-rate discharging in the flat plateau region depend strongly on discharge current. The ohmic resistance of electrode mainly causes this polarization. These imply that this electrode is well conductive but the utilization is low. Figure 3 shows the depen- dence of the specific capacity mAh/g of active material of LiFePO 4 electrodes on conductive additive content at different C rates. Specific capacity increases with increasing carbon content and does not show a maximum value until 12% of carbon content. The lines in the figure were fit with the data from the electrodes contain- ing only carbon black. Note that the slopes of all fitting lines are similar regardless of C rate. In Fig. 2, the potential of 10% CB containing electrode showed flat plateau during discharge and then, decreased sharply to cutoff voltage at the end of discharge, even at 5C rate because of lithium diffusion limitation through particle. If the electrode was quite conductive, the rate capability of electrode would strongly depend on lithium diffusion through active particle. As shown in Fig. 3, the variation of rate capability on carbon content is similar because our electrode is as conductive since the rate ca- pability is not affected. Figure 4 shows the variation of gravimetric capacity mAh/g of electrode of LiFePO 4 electrode through carbon content. Gravimetric capacity also increased as carbon content in- creased, even with the addition of carbon in electrode decreases active material loading. The lines in the figure, which were calcu- lated from the equations in Fig. 3, match well with actual values. To calculate the variation of volumetric capacity, apparent electrode * Electrochemical Society Active Member. z E-mail: zaghib.karim@ireq.ca Table I. Electrode composition LiFePO 4 % Carbon black CB % Graphite G % PVdF % Active loading  mg/cm 2  Thickness μm Electrode density g/cm 3  88 0 0 12 4.8 39 1.41 85 3 0 12 4.9 32 1.71 82 6 0 12 3.9 32 1.49 78 10 0 12 3.0 30 1.28 82 0 6 12 4.9 34 1.45 79 3 6 12 4.6 38 1.20 76 6 6 12 4.3 31 1.07 Electrochemical and Solid-State Letters, 8 4 A207-A210 2005 1099-0062/2005/84/A207/4/$7.00 © The Electrochemical Society, Inc. A207