Journal of Power Sources 189 (2009) 303–308 Contents lists available at ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour Li-ion electrolyte modeling: The impact of adding supportive salts D. Danilov ∗ , P.H.L. Notten Eindhoven University of Technology, Den Dolech 2, 5600 MB Eindhoven, The Netherlands article info Article history: Received 31 July 2008 Accepted 10 September 2008 Available online 24 September 2008 Keywords: Li-ion battery Electrolyte modeling abstract In recent work the ionic transportation properties of organic electrolyte in Li-ion batteries has been described in detail by the present authors, taking into account ionic diffusion and migration processes. Advanced battery electrolytes may, however, be composed of various salts. Therefore the ionic transport properties of such complex electrolytes have been investigated from a theoretical point of view. Detailed information about transient and steady-state behavior of the electrolyte has been simulated, including potential gradients and the diffusion and migration fluxes for all ions. It was found that supportive elec- trolytes are an effective way to reduce the electric field and, consequently, the migration overpotential. Simultaneously, the diffusion overpotential, in general, increases. Nethertheless, supportive salts reduce the total overpotential across the electrolyte, especially when high currents are applied for short periods of time. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Improving the properties of advanced Li-ion batteries is chal- lenging and a requirement for high power applications. It has been reported that the electrolyte of Li-ion batteries is respon- sible for substantial energy losses, especially under high current (dis)charging conditions [1]. The reason for these losses is the limited ionic conductivity of the non-aqueous Li-salt containing electrolytes. Mathematical modeling is traditionally used to investigate the performance of rechargeable batteries [2–4]. In the case of Li-ion batteries the ionic transportation properties of the organic elec- trolyte has been described in detail, taking into account diffusion and migration processes [4]. In recent work the present authors have provided a detailed analysis of the various overpotential con- tributions based on single salt electrolytes [5]. It is, however, known that advanced electrolytes are often composed of various salt com- ponents. The theoretical investigation of this specific and more complex case is described in the present work. A mathematical model describing multi-component salt con- taining Li-ion electrolytes is proposed. The model consists of four coupled parabolic-type Partial Differential Equations (PDE’s). A practically important question of how to improve the ionic conduc- tivity properties of Li-based electrolytes is considered. It is found that adding supportive salts is an efficient way to reduce the electric field and, consequently, the overpotential. ∗ Corresponding author. Tel.: +31 40 2478105; fax: +31 40 2478190. E-mail address: danilov@eurandom.tue.nl (D. Danilov). 2. Theoretical set-up The elementary processes, occurring inside a conventional Li-ion battery, are schematically shown in Fig. 1. The main elec- trochemical storage reactions at the LiCoO 2 electrode can be represented by LiCoO 2 charge ⇄ discharge Li 1-x CoO 2 + xLi + + xe - , (0 ≤ x ≤ 0.5). (1) describing the extraction of Li-ions from the positive electrode during charging and the insertion of Li + ions during discharging. The corresponding reactions at the negative electrode can be described by C 6 + zLi + + ze - charge ⇄ discharge Li z C 6 , (0 ≤ z ≤ 1) (2) As a result of these electrochemical charge transfer reactions, Li-ions must cross the electrolyte under current flowing conditions (see Fig. 1). The electrolyte in Li-ion batteries is based on a disso- ciated Li-salt, e.g. LiPF 6 or LiClO 4 , which cannot be considered as a well ionic-conductive medium. The ions in the electrolyte are transported by both diffusion and migration, this latter process being induced by the electric field between the electrodes across the electrolyte. Consider that a conventional electrolyte is composed of a sin- gle salt, e.g. LiPF 6 . An interesting question is whether its properties can be improved by adding a “neutral salt” which does not affect the main electrochemical storage reactions. Consider the situation that another 1 M neutral salt, such as KClO 4 , is added to the standard 0378-7753/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jpowsour.2008.09.050