Air stable copper phosphide (Cu 3 P): a possible negative electrode material for lithium batteries Heriberto Pfeiffer a , Franck Tancret a , Marie-Pierre Bichat b , Laure Monconduit b , Fr  ed eric Favier b , Thierry Brousse a, * a Laboratoire de G enie des Mat eriaux, Ecole Polytechnique de l’Universit e de Nantes, BP 50609, 44306 Nantes Cedex 3, France b Laboratoire des Agr egats Mol eculaires et Mat eriaux Inorganiques, UMR-CNRS 5072, Universit e Montpellier II, cc015, 34095 Montpellier Cedex 05, France Received 24 November 2003; received in revised form 23 December 2003; accepted 23 December 2003 Published online: 20 January 2004 Abstract Air stable copper phosphide was synthesized as a thick film over a copper foil by a very simple solid-state reaction at low temperature. X-ray diffraction confirms that the layer is pure Cu 3 P. Scanning electron microscopy reveals a porous microstructure consisting of agglomerated particles with 10 lm of diameter. Electrochemical reaction of Cu 3 P with lithium leads to an amor- phisation of the thick film during the first discharge, and Cu 3 P does not seem to recover upon subsequent charge. Structural and microstructural analyses coupled with electrochemical tests emphasize a complex behavior of the copper phosphide material. An initial capacity of 415 mAh/g has been measured with a stable reversible capacity close to 200 mAh/g on subsequent cycles without the help of binder and/or conductive additives. Although the gravimetric capacity values obtained with Cu 3 P are slightly below the graphite capacity, the volumetric capacity of Cu 3 P (1473 Ah/L) is 80% higher than those of graphite (800 Ah/L). Ó 2003 Elsevier B.V. All rights reserved. Keywords: Air stable phosphides; Copper phosphide; Lithium ion batteries; Solid-state reaction 1. Introduction Since the 1980s different compounds have been studied as negative electrodes for rechargeable lithium batteries [1]. Although carbon–based compounds are the most commonly studied materials, oxides [2–5] or in- termetallics [6–8] have also been investigated as possible anodes in lithium batteries. Surprisingly, most of these alternative materials were originally designed for other purposes. For example, SnO 2 was used as gas sensor before it was envisioned as anode material in Li-ion batteries [9], and NiSn coating has been used for years for its anti-corrosion properties [10]. Phosphides are also of technological interest in the fields of corrosion, me- chanical engineering and semiconductors [11–13]. In- terestingly, several phosphides and transition metal pnictides have been recently tested as anode candidates for Li-ion batteries (e.g., CoP 3 [14,15], MnP 4 [16], Li x TiP 4 [17–20], FeP 2 [21], and CuP 2 [22]). Different reaction mechanisms have been proposed depending on the transition metal M, as well as the M/P and Li/P ratios in the starting compound. For some materials, such as Li 9 TiP 4 [17–20], or MnP 4 [16], the pristine structure can be reversibly modified and reconstructed upon electrochemical cycling. In other cases, the initial phosphide is decomposed and lithium reversibly reacts with phosphorous to form a binary compound, such as Li 3 P. The main interest in these phosphides, compared to oxides or intermetallics, is their high reversible ca- pacities and, in some cases, their good cyclability due to small volume changes upon charge/discharge process. Despite these improved electrochemical properties compared to other alternative anode compounds, the * Corresponding author. Tel.: +33-240-683-173; fax: +33-240-683- 199. E-mail address: thierry.brousse@polytech.univ-nantes.fr (T. Brousse). 1388-2481/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2003.12.012 Electrochemistry Communications 6 (2004) 263–267 www.elsevier.com/locate/elecom