Ž . Journal of Power Sources 91 2000 107–117 www.elsevier.comrlocaterjpowsour Palladium-microencapsulated graphite as the negative electrode in Li-ion cells Ping Yu, Bala S. Haran, James A. Ritter, Ralph E. White, Branko N. Popov ) Department of Chemical Engineering, Center for Electrochemical Engineering, UniÕersity of South Carolina, Columbia, SC 29208, USA Received 19 October 1999; accepted 16 February 2000 Abstract A Pd-encapsulated graphite electrode was used as the negative electrode in Li-ion cells. Through dispersion of ultrafine nanoparticles of palladium on the surface of graphite, the interfacial properties of the carbon surface were modified. The presence of the palladium Ž . dramatically reduces the initial irreversible capacity of the graphite in propylene carbonate PC -based electrolyte. Palladium suppresses the solvated lithium ion intercalation and improves the charge–discharge performance and initial coulombic efficiency of graphite. For example, 10-wt.% of Pd-nanoparticles dispersed on the surface of graphite increases the initial charge–discharge coulombic efficiency Ž . from 59% to 80.3%. Electrochemical impedance spectroscopy EIS indicates that palladium dispersed on graphite increases the ohmic conductivity and also improves the Li insertion rate into graphite. However, an excess amount of palladium on graphite leads to a decrease in the charge–discharge efficiency due to the consumption of lithium by the formation of Li PdO . q 2000 Elsevier Science 2 2 S.A. All rights reserved. Keywords: Nanoparticles; Dispersed palladium; Irreversible capacity; Graphite; Lithium intercalation; Li-ion battery 1. Introduction Various carbon-based materials have been widely inves- w x tigated as the negative electrode in Li-ion batteries 1–5 . Of these materials, graphite is favored as the negative electrode because it is low-cost and exhibits a high specific capacity, the most desirable discharge potential profile and superior cycling life. However, a major problem associated with the use of graphite concerns the irreversible reactions Ž that take place during the first charge lithium intercala- . tion . These reactions consume a significant amount of Ž . active material Li which leads to a loss in capacity that cannot be recovered during subsequent cycling. In com- mercial Li-ion cells, the loss of lithium due to the irre- versible reactions is normally compensated for with the w x use of excess cathode material 6,7 . But this leads to a decrease in the specific energy density and thus an in- crease in the cell cost. Moreover, these irreversible reac- tions can cause gas evolution, which may result in safety ) Corresponding author. Tel.: q 1-803-777-7314; fax: q 1-803-777- 8265. Ž . E-mail address: popov@engr.sc.edu B.N. Popov . problems such as cell can buckling, cell venting, elec- w x trolyte spillage, and even fire 6–8 . These irreversible reactions are also much worse in Ž . propylene carbonate PC -based electrolyte than in ethy- Ž . w x lene carbonate EC -based electrolyte 9–11 and PC alone can cause severe degradation of the graphite structure w x wx 4,9–11 by a process called AexfoliationB 2 . PC decom- poses without forming a stable passive layer on the edge w x surface of graphite 4,9–11 ; and hence, it continues to solvate with lithium ions, which then cointercalate and undergo a reduction reaction inside the graphene layers. These sequential events give rise to the large irreversible capacity associated with the use of PC-based electrolytes w x 8,11 . One common approach taken to improve the capacity of carbon in PC-based electrolytes is to modify existing carbons with dopants such as P, B and N. However, both the parasitic irreversible reaction and the Li intercalation reaction take place at the interface between the carbon and the electrolyte. Hence, no significant improvement in the properties of the graphite can be achieved unless the interfacial properties are improved. Recently, some efforts have focussed on modifying the surface of graphite to reduce electrolyte decomposition, especially that associ- 0378-7753r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. Ž . PII: S0378-7753 00 00466-3