• PDF / 114,410 Bytes
• 6 Pages / 612 x 792 pts (letter) Page_size
• 76 Downloads / 272 Views

DOWNLOAD

REPORT

---

0988-QQ08-08

Electrochemical Insertion of Li into Sr2MO2Cu2S2 (M = Mn, Co, Ni) Sylvio Indris1, Dongli Zeng1, Jordi Cabana1, Catherine F. Smura2, Oliver J. Rutt2, Simon J. Clarke2, and Clare P. Grey1 1 Chemistry Department, State University of New York at Stony Brook, 100 Nichols Rd., Stony Brook, NY, 11794 2 Department of Chemistry, University of Oxford, South Parks Road, Oxford, OX1 3QR, United Kingdom

ABSTRACT The layered oxysulfides Sr2MO2Cu2S2 (M = Mn, Co, Ni) consist of alternating perovskite-type Sr2MO2 layers and copper sulfide layers. We studied the electrochemical insertion of Li into these three samples. By this we were able to study the influence of the nature of the transition metal on the Li insertion process which appears to be at least partially reversible. While the Mn compound clearly shows a Cu-Li exchange reaction, the electrochemical process for the two other compounds is more complex. The lithiated materials were studied by 7Li MAS NMR. INTRODUCTION Li-ion batteries are used as power sources in portable electronic devices like notebooks and cell phones. Increasing technological demands result in a need of new electrode and electrolyte materials. The layered oxysulfides Sr2MO2Cu2S2 (M = Mn, Co, Ni) consist of alternating perovskite-like Sr2MO2 layers and copper sulfide layers which resemble fragments of the Cu2S anti-fluorite structure (Figure 1) [1]. The divalent transition metal cations M 2+ are located inside planar oxygen squares. Two sulfur ions on top and at the bottom of these squares complete a distorted octahedron and provide the linkage of the M 2+ to the Cu+ ions with bond angles of about 125°. It was shown in a previous study [2] for the Mn compound that the copper can be replaced electrochemically and reversibly by lithium. Furthermore, the influence of the thickness of the copper sulfide layers on the electrochemical behavior was investigated [2]. In this study we present investigations of the influence of the transition metal ion on the electrochemical properties by substitution of Mn for Co and Ni.

Figure 1. The structure of Sr2MO2Cu2S2 (M = Mn, Co, Ni).

EXPERIMENT Electrochemical lithium insertion/extrusion tests were performed in two-electrode cells using lithium foil as counter electrode. The working electrode consisted of a powder mixture of the sample of interest with 10-20% acetylene black. The electrolyte was 1M LiPF6 in ethylene carbonate/dimethyl carbonate (EC:DMC) (1:1 ratio). The cells were tested using an Arbin potentiostat in galvanostatic mode at C/20 rate (i.e. insertion of two Li per formula unit within 20 hours). The cycling performance of Sr2MnO2Cu1.5S2 was previously shown to be satisfactory in the voltage range from 2.7 to 1.1 V vs Li+|Li, but the capacity values (50 mAh/g) are small [2]. When expanding the window from 3 V to 0.02 V, a higher capacity of 250 mAh/g can be achieved in the first discharge at C/20 (Fig. 2, left) in a battery with 17% carbon (acetylene black). This would correspond to the reaction of almost 4 Li per formula unit with the compou