Capacity fade of Sony 18650 cells cycled at elevated temperatures Part I. Cycling performance P. Ramadass, Bala Haran, Ralph White, Branko N. Popov * Department of Chemical Engineering, University of South Carolina, Columbia SC 29208, USA Received 2 August 2002; accepted 30 August 2002 Abstract The capacity fade of Sony 18650 Li-ion cells increases with increase in temperature. After 800 cycles, the cells cycled at RT and 45 8C showed a capacity fade of 30 and 36%, respectively. The cell cycled at 55 8C showed a capacity loss of about 70% after 490 cycles. The rate capability of the cells continues to decrease with cycling. Impedance measurements showed an overall increase in the cell resistance with cycling and temperature. Impedance studies of the electrode materials showed an increased positive electrode resistance when compared to that of the negative electrode for cells cycled at RT and 45 8C. However, cells cycled at 50 and 55 8C exhibit higher negative electrode resistance. The increased capacity fade for the cells cycled at high temperatures can be explained by taking into account the repeated film formation over the surface of anode, which results in increased rate of lithium loss and also in a drastic increase in the negative electrode resistance with cycling. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Impedance; Electrode resistance; Electrochemical impedance spectroscopy 1. Introduction The capacity fade of Li-ion batteries can be attributed to the unwanted side reactions that occur during overcharge which causes electrolyte decomposition, structural changes, passive film formation and active material dissolution [1]. Several other capacity fading mechanism namely complete exfoliation of the electrode material due to solvent co- intercalation [2] and electronic isolation of active mass [3] has also been clearly addressed. Many recent advances in Li-ion batteries have made them good performing cells at temperature range between 30 and þ40 8C [4]. Zhang et al. [5] using electrochemical impedance spectro- scopy (EIS) studied the capacity fade of commercially available LiCoO 2 based Li-ion cells at room temperature. The results indicated that the positive electrode contributes more to the capacity fade with cycling. The capacity loss was attributed to higher impedance of LiCoO 2 electrode with cycling due to continuous electrolyte oxidation under overcharging conditions. Ramadass et al. [6] studied the capacity fade of commer- cially available spinel based Li-ion cells. They showed that the capacity fade of the cell depends upon the charging rate and on the cut-off potential used to charge the cell and they attributed capacity fade to structural degradation at the cathode and loss of active materials at both electrodes due to electrolyte oxidation. According to Aurbach et al. [7], the possible reasons for capacity fading in Li-ion batteries upon continuous cycling were, degradation of secondary active material (positive and negative electrode), degradation of the electrolyte solution due to reactions with reactive components in the Li-ion batteries and surface reactions on both electrode surfaces that increase the impedance of the electrodes. For safe use of Li-ion cells in portable electronics, it is critical to study their performance at temperatures higher than 40 8C. Elevated temperature accelerates the degrada- tion of the battery materials, which causes a decline in capacity and premature cell death. Also, raising the tem- perature one might cause the onset of thermal runaway to occur, where the cell temperature increases dramatically as a result of reactions at the electrode interface that are exother- mic in nature. Thus our study focuses on estimating the capacity fade of commercial Li-ion cells cycled at high temperatures. We choose Sony 18650 cells with LiCoO 2 as the positive electrode material. The objective was to compare the cell Journal of Power Sources 112 (2002) 606–613 * Corresponding author. Tel.: þ1-803-777-7314; fax: þ1-803-777-8265. E-mail address: popov@engr.sc.edu (B.N. Popov). 0378-7753/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0378-7753(02)00474-3