Open-Circuit Voltage Study of Graphite-Coated Copper Foil Electrodes in Lithium-Ion Battery Electrolytes Mingchuan Zhao, a, * Mingming Xu, a, * Howard D. Dewald, a, ** ,z and Robert J. Staniewicz b a Department of Chemistry and Biochemistry, Clippinger Laboratories, Ohio University, Athens, Ohio 45701, USA b Saft Research and Development Center, Cockeysville, Maryland 21030, USA The open-circuit voltage OCV of graphite-coated copper foil electrodes in Li-ion battery electrolytes was found to vary over time. A detailed study showed that the OCV first rapidly decreased until reaching a minimum, and then gradually increased until reaching a steady state. These results were compared with OCV studies of graphite-coated aluminum foil and copper foil without graphite coating. The influence of hydrofluoric acid and thermal treatment of the graphite coating was also studied. Combined with copper dissolution studies using atomic absorption spectroscopy, it was found that the interaction of the graphite coating with electrolyte solution was the main causative factor that resulted in the OCV variation. © 2002 The Electrochemical Society. DOI: 10.1149/1.1527050 All rights reserved. Manuscript submitted October 4, 2001; revised manuscript received July 15, 2002. Available electronically December 5, 2002. Ambient temperature rechargeable Li-ion batteries have been un- der extensive study since their introduction into the market by Sony in 1991. 1 Such batteries have many outstanding characteristics, such as high energy density, high cell voltage, and fewer safety concerns, compared to conventional techniques. 2 The active material of the negative electrode in a Li-ion cell is carbon based. Li can be inter- calated into or deintercalated from the carbon during the charge or discharge process. The positive electrode active material is a lithi- ated transition metal oxide, such as LiNiO 2 , LiCoO 2 , or LiMn 2 O 4 . Copper foil is used as the negative electrode current collector and aluminum foil is used as the positive electrode current collector in all Li-ion cells currently in production. The electrolyte can be either a solid polymer electrolyte or a nonaqueous liquid electrolyte. A liquid electrolyte is usually comprised of a Li salt and various sol- vents from the ester, ether, or carbonate families. Application of small portable Li-ion cells in electronic devices, such as cellular telephones and notebook computers, has achieved great commercial success. The sale value of Li-ion cells exceeded that of NiMH and NiCd in the total portable cells market in 1999. 3 More recently, large Li-ion cells have been investigated for applica- tions requiring large power density and energy density, such as elec- trical vehicles and satellites. 2,3 In these applications, long-term sta- bility of each component in the cell is required. The electrochemical stability of the anode materials, graphite-coated copper foil Cu-C, is one of the concerns. In order to understand the intrinsic stability of the anode materi- als in Li-ion battery electrolytes, we have studied previously the electrochemical behavior of Cu foil electrodes and Cu-C foil elec- trodes in different nonaqueous organic carbonate Li-ion battery elec- trolyte solutions in half-cell reactions. 4,5 Recently, open-circuit volt- age OCV studies on Cu foil electrodes without graphite coating have been performed. 6 In that study, the OCV variation over time of Cu foil electrodes was observed and subsequently studied in detail. Combined with our previous findings of Cu dissolution, it was sug- gested that impurities, such as HF, could oxidize copper foils in nonaqueous electrolyte solutions, which resulted in the OCV varia- tion of Cu foil electrodes over time. 6 The OCV studies provided some insight into the intrinsic stability of uncoated Cu foil in the Li-ion electrolyte solutions. In the work reported here, similar OCV studies on Cu-C foil electrodes were performed. The OCV of graphite-coated aluminum foil Al-C was studied for comparison. The effect of some pertinent cell factors, such as aging of the refer- ence electrode, HF addition, and thermal treatment of the Cu-C foil electrodes was also studied. The results are compared with the find- ings on uncoated Cu foil electrodes and summarized. Experimental OCV study of Cu-C foil electrodes was performed using the same homemade three-electrode cell as in the OCV study of Cu foil electrode. 4 The working electrode WE was Cu-C foil, which was cut into a 1  1 cm flag. The flag was then connected to a 22-gauge Ni wire by pressing the tip of the Ni wire onto the flag pole. The graphite coating was scraped from tip part of the flag pole. In the comparison study using an Al-C foil, the electrode was prepared in the same manner. The reference electrode RE was made by rolling and pressing a 1  1 cm Li foil onto the tip of a Ni wire and was assembled in a dry box in electrolyte solution in a glass tube con- taining a 6 mm diam porous Vycor tip Bioanalytical Systems, BAS, MF-2042. The auxiliary electrode AE was a 0.5 mm diam Pt wire coil 23 cm length, BAS, MW-1033. Two electrolytes were used in the OCV studies: i 1 M LiPF 6 in ternary mixtures of ethylene carbonate EC-dimethyl carbonate DMC-methyl ethyl carbonate MEC, 1:1:1 vol., and ii 1M LiPF 6 in ternary mixtures of propylene carbonate PC-EC-DMC 1:1:3 vol.. The electrolyte solutions were obtained from EM Industries/Merck K. G. a. A. and were prepared from 99.98% purity solvents 20 ppm H 2 O, as determined by a Karl Fischer titration and Stella LiPF 6 . The electrolytes were guaranteed at 80 ppm HF and were analyzed at Saft as 50 ppm using an acid-base titration. The electrolytes were shipped to Ohio University and stored in a dry box. Electrolyte was frozen before degassing for 30 min and then thawed. The procedure was repeated three times before it was moved into the dry box. The Li metallic foil, the battery-grade Cu-C foil, and Al-C foil were all supplied by Saft. The Li foil was obtained from Cyprus Foote Mineral Company. The Cu foil 12 m thick grade LP1/LP3, was obtained from Fukuda Metal Foil and Powder Co. The elec- trodeposited foil one matte side and one shiny side had a purity of 99.9% with the major trace element being Cr at 130 ppm. The coating consisted of a blend of 50 wt % mesocarbon microbeads MCMB, 10-28 and 50 wt % Timcal SFG-44 graphites using poly- vinylidene fluoride PVDF as a binder. The carbon loading per electrode face is 13 or 26 mg/cm 2 double side coated. The Al foil 20 m thick grade H18 Temper, was obtained from Nippon Foil Mfg. Co. The foil both sides shiny had a purity of 99.85% with the major trace element being Cu at 0.03%. The coating was applied with a reverse comma bar coater and consisted of a blend of 47.7 wt % mesocarbon microbeads MCMB 10-28, 47.7 wt % Timcal SFG-44 graphites using 4.5 wt % PVDF as a binder. The carbon loading per electrode face is 4.6 mg/cm 2 . * Electrochemical Society Student Member. ** Electrochemical Society Active Member. z E-mail: dewald@ohio.edu Journal of The Electrochemical Society, 150 1 A117-A120 2003 0013-4651/2002/1501/A117/4/$7.00 © The Electrochemical Society, Inc. A117 ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 54.198.146.19 Downloaded on 2016-10-30 to IP