1388-2481/00/$ - see front matter q2000 Elsevier Science S.A. All rights reserved. PII S1388-2481 ( 99 ) 00151-4 Tuesday Jan 11 02:26 PM StyleTag -- Journal: ELECOM (Electrochemistry Communications) Article: 159 www.elsevier.nl/locate/elecom Electrochemistry Communications 2 (2000) 104–107 Thermal investigations of transitional metal oxide cathodes in Li-ion cells Rajeev Venkatachalapathy, Chang Woo Lee, Wenquan Lu, Jai Prakash * Center for Electrochemical Science and Engineering, Department of Chemical and Environmental Engineering, Illinois Institute of Technology, 10 W 33rd Street, Chicago, IL 60616, USA Received 11 November 1999; accepted 19 November 1999 Abstract Accelerating rate and differential scanning calorimeters were used to investigate the thermal behavior of LiCoO 2 and LiNi 0.8 Co 0.2 O 2 cathodes in fully charged Li-ion cells. The heat flow for LiNi 0.8 Co 0.2 O 2 was observed to be three times higher than that for LiCoO 2 . The net heat transferred out of the cell fabricated with a LiNi 0 . 8 Co 0.2 O 2 cathode was found to be 51% higher than the net heat transferred out of the cell fabricated with LiCoO 2 . Although LiNi 0 . 8 Co 0.2 O 2 was found to be thermally less stable than LiCoO 2 , the electrochemical performance of the LiNi 0 . 8 Co 0.2 O 2 cathode under high power discharge was found superior to LiCoO 2 . q2000 Elsevier Science S.A. All rights reserved. Keywords: LiCoO 2 ; LiNi 0.8 Co 0.2 O 2 ; Accelerating rate calorimeter; Differential scanning calorimeter; Thermal runaway 1. Introduction Much concern over the thermal safety of lithium batteries has stimulated worldwide interest in the development of new materials that would improve the thermal stability of these batteries. Different cathodes and anodes have been studied and new theories have been proposed by various authors to understand the difference in their thermal behavior [1–6]. This paper presents the electrochemical performance and thermal investigations on LiCoO 2 and LiNi 0.8 Co 0.2 O 2 cathode materials using an accelerating rate calorimeter (ARC) and a differential scanning calorimeter (DSC). 2. Experimental 2.1. ARC studies Two types of cell chemistry were investigated in this study. The first group of cells had LiCoO 2 as the positive electrode while the second group of cells had LiNi 0.8 Co 0.2 O 2 . Both the cells had a graphite anode, polypropylene separator and 1 M LiPF 6 in EC/DMC electrolyte. A combination of an accel- erating rate calorimeter (ARC, model 2000, Arthur D. Little Co., Cambridge, MA) and Arbin cycler (ABTS 4.0) was * Corresponding author. Tel: q1-312-567-3639; fax: q1-312 567-8874; e-mail: prakash@iit.edu used to monitor the thermal runaway profiles of these 18 650 cells of different chemistry. Details of the experimental setup are given elsewhere [7–9]. The electrochemical performance of these cells was studied using the Arbin cycler at various power discharge rates. 2.2. DSC studies A differential scanning calorimeter (DSC 7, Perkin- Elmer) was used on laboratory scale cells to investigate the thermal stability of the fully charged LiNi 0.8 Co 0.2 O 2 and LiCoO 2 cathode materials. The fully charged state was cho- sen as this state poses greater thermal hazards than any other discharged state. The charged cells were opened in a dry glove box under argon atmosphere and the materials were recov- ered. The recovered cathode materials including electrolyte were sealed in a standard aluminum DSC pan. DSC scans were carried out at a heating rate of 10 8C min y1 from 30 to 300 8C under nitrogen purging. 3. Results and discussion Fig. 1 shows the thermal runaway profile of the full-size 18 650 Li-ion cell fabricated with a LiCoO 2 cathode under fully charged conditions. The onset of self-heating of the cell was observed to be at 98 8C. At 167 8C, however, the cell voltage dropped down to zero indicating the onset of thermal