Mn(II) dissolution from lithium manganese oxide (LiMn2O4) positive electrode and Mn plating on graphite negative electrode are the well-known failure mode in lithium-ion chemistry [1]. The Mn plating degrades the electrochemical performances of graphite electrodes in two ways. One is the self-discharge of graphite, which is accompanied by Mn plating. The other is the degradation of solid electrolyte interphase (SEI), which leads to forthcoming electrolyte decomposition and film deposition. As a result, cell polarization becomes severe due to an increment in film resistance, which eventually leads to a cell failure [2]. To overcome the issues raised by Mn plating on graphite electrodes, several countermeasures have been made. For examples, the scavengers for hydrofluoric acid (HF), which attacks the positive electrodes to leach out Mn ions, have been used as the electrolyte additive. Also Mn(II) scavengers have been developed [3,4]. In this study, 1,4,8,11-tetraazacyclotetradenate (cyclam) and 1,4,7,10-tetraazacyclododecane (cyclen) is tested as a Mn(II) scavenger. Those tetradentate azamacrocyclic compounds are applied as an electrolyte additve or attached on the chains of polyvinylalcohol (PVA) binder. The Mn plating on graphite electrodes seems to be suppressed due to a strong compex formation with Mn(II) ions. Namely, the Mn plating is thermodynamically unfavored due to the complex formation since the reduction potential of Mn(II) moves to the more negative direction than that for the solvent-solvated Mn(II) ions. Moreover, the Mn plating is kinetically hindered due to higher overpotential that is caused by a strong chelate complex formation.
Experimental
A beaker-type three-electrode cells were fabricated with a graphite electrode (DAG 87 : Super P : PVdF binder = 85:5:10 in wt. ratio), a Li counter electrode and a Li reference electrode. 10 mM of Mn(ClO4)2 was dissolved into an electrolyte solution (1.0 M LiPF6 in EC/DEC, 1/1 = v/v) to simulate the Mn(II)-dissolved electrolyte. To examine the additive effect, 1.0 wt. % of cyclam was added into the Mn(II)-containing electrolyte solution. The cyclen-attached PVA was synthesized by mixing 8 wt. % PVA/water solution and 50 mg of cyclen that carries a carboxylic acid functional group. After casting the mixed solution on Cu foil, esterification was performed at 150 0C for 12 h under vaccum. To examine the Mn(II) scavenging action, the cyclen-attached PVA/Cu foil was immersed in the Mn(II)-containing electrolyte solution for 48 h. Finally, the Mn(II) ions entrapped in the cyclen-attached PVA were leached out by using the mixture of nitric, hydrofluoric and hydrochloric acid. Mn(II) concentration was analyzed by using inductively-coupled plasma atomic emission spectroscopy (ICP-AES).
Results and discussion
The differential capacity (dQ/dV) plots obtained in the first lithiation period show that the reduction current responsible for Mn plating, which appears in the wide potential range (0.0 ~ 0.8 V vs. Li/Li+) in the Mn(II)-containing electrolyte, disappears when cyclam is added (Fig. 1a). Clearly, cyclam plays the Mn(II) scavenging role. The Mn(II) concentration entrapped in the cyclen-attached PVA is compared with that entrapped in the pure PVA. The comparison shown Fig. 1b ascertains the strong compex formation betweeen Mn(II) ions and cyclen being attached in PVA. The cyclen-attached PVA can be used as a binder for graphite electrode fabrication, which has Mn(II) scavanging function.
References
[1] D.H. Jang, Y.J. Shin, S.M. Oh, J. Electrochem. Soc., 1996, 143, 2204.
[2] S. Komaba, N. Kumagai, Y. Kataoka, Electrochim. Acta, 2002, 47, 1229.
[3] S. Komaba, T. Itabashi, T. Ohtsuka, H. Groult, N. Kumagai, B. Kaplan, H. Yashiro, J. Electrochem. Soc., 2005, 152, A937.
[4] M. Koltypin, D. Aurbach, L. Nazar, B. Ellis, Electrochem. Solid-State Lett., 2007, 10, A40.
Figure 1. (a) The first lithiation differential capacity plots obtained from the Li/graphite cell. The used electrolyte solution is indicated in the inset. The galvanostatic charge/discharge cycling was performed at a specific current density of 70 mA g-1. (b) the concentration of Mn(II) ions that were leached from the PVA/Cu foil and cyclen-attached PVA/Cu foil.