36
Ionics 11 (2005)
Progress in the Lithium Insertion Mechanism in
Cu3e
B. M a u v e r n a y , M.-P. Bichat, F. Favier, L. M o n c o n d u i t * , M. Morcrette • and M.-L. Doublet ~ Laboratoire des Agr6gats Mol6culaires et Mat6riaux Inorganiques YLaboratoire de Structure et Dynamique des Syst~mes Moldculaires et Solides • Laboratoire de R6activit6 et Chimie des Solides Universit6 Montpellier II, Place E. Bataillon - F34095 Montpellier Cedex 5 - FRANCE Universit6 de Picardie Jules Verne, 33, rue St Leu - F80039 Amiens Cedex - FRANCE *
[email protected]
Abstract. Copper phosphide, Cu3P has been synthesized using a ceramic route, and its electrochemical behaviour versus lithium has been studied studied galvanostatic and potentiodynamic measurements and in situ X-ray diffraction analysis. The insertion/extraction mechanism proceeds with the formation of at least three different LixCu 3 •
(x = 1, 2, 3) phases. The electrochemical
behaviour of Cu3P samples obtained from ceramic and solvothermal syntheses are compared to further understanding of the complex redox mechanism occurring during insertion/extraction. Firstprinciple electronic structure calculations show that discharge probably begins with the formation of a solid solution LixCu3_yP(X < 0.5).
1. Introduction The search for electrode materials alternative to graphitic
strong hybridisation of the metal d-orbitals and the phosphorus p-orbitals and hence to the high stability of the
carbon in rechargeable lithium-ion batteries is a major
MP4 entities that constitute the framework. The major
topic in the area of energy storage, and interest in study of new families of negative electrode materials for secondary
drawback at present for these ternary phases is related to the high lithium content of the starting materials. With
batteries. In this field, transition metal pnictides, and
the MnP 4 system [5], the electrode undergoes a reversible
more specifically transition metal phosphides have very attractive performance, exhibiting large volumetric and
solid state (crystalline) transformation upon cycling MnP4 + 7Li ---- Li7MnP4. Unfortunately, the first cycle capacity dramatically fades in the following cycles. In the inter-
specific capacities [1-6]. These phosphides are characterized by facile structural rearrangements that are correlated to easy lithium insertion and sometimes to rever-
metallic Li~InSb systems [9,10] combining a posttransition metal and a heavier pnictogen, the less covalent
sible crystalline-amorphous transition. In the Li• series (M = V, Ti) [7,8], close structural relationship
character of the In-Sb bonds, as well as the lower metal
exists between the lithiated and the delithiated materials.
those of systems built on early transition metals. In this case, the reaction of lithium with InSb occurs by pro-
Theoretical electronic calculations have shown that lithium extraction/insertion accompanies slight contraction/elongation of the M-P bonds, leading to small elec-
oxidation state, induce different redox properties from
gressive substitution of M by lithium to give the binary
trode volume expansion [1,3]. This unusual redox induced
Li3Sb phase. The reaction is not however highly reversible, and the extruded metal is usually highly divided and
mechanism has been directly correlated to the strong co-
not detected by X-ray diffraction (XRD). The Cu3P
valent character of the metal-phosphorus bond, i.e. to
system, combines a late transition metal and a pnictogen
Ionics 11 (2005) and may be regarded as an intermediate case. This phase has recently been reported as being promising negative electrode material [11]. It was first synthesized as a thick film on a copper foil, using a very simple solid-state reaction at low temperature. Other synthetic routes were then investigated to correlate the electrochemical performance of Cu3P to the powder morphology [12], showing that nano-structured materials favour high initial capacities but that micro-sized powders show better capacity retention than nano-structured and bulk materials, all these results being uncorrelated to degree of crystallinity. We also investigated the electrochemical properties of the hexagonal Cu3P phase towards lithium, and more particularly the insertion/extraction mechanism, using galvanostatic and potentiodynamic measurements, electron microscopy and in situ XRD [13]. For powders obtained by solvothermal synthetic routes, the results led us to propose that discharge proceeds in four different steps : (a)
Cu3P + x Li --* LixCu3P (x ~ 0.3) (b) LixCu3P + (l-x) Li ~ LiCu2P + Cu ~ (c) LiCu2P + Li ---" Li2CuP + Cu ~ (d) Li2CuP + Li ----"Li3P (or Li2+~Cul_~P + o~Cu~ (ct ~ 1)) + Cu ~ As lithium reacts electrochemically with LixCu3P, LiCu2P and Li2CuP, respectively, Cu ~ is reduced to Cu ~ and extruded from the active phase. The five phases involved in the mechanism being structurally closely related, a high degree of reversibility is achieved for each reaction. During charge, lithium is progressively extracted from a Li-rich amorphous phase and copper is progressively re-oxidized. At the end of charge, crystalline Cu3P is recovered. These interesting and promising properties are however not fully elucidated, since 5 Li § are exchanged with the solvothermal Cu3P sample (Cu3P-ST) instead of the 3 Li + expected from the reactions (a) to (d). To explain this "extra" capacity we have investigated the electrochemical behaviour of high temperature samples (Cu3P-HT). Our goal was to compare Li § electrochemical insertion mechanisms in copper phosphides obtained from different synthetic routes, and to investigate their structural behaviour upon cycling. Special attention was paid to the mechanism operating at the end of discharge, as the formation of Li3P has not been clearly characterized in the Cu3P-ST sample. First-principle electronic structure calculations and full structural relaxations have also been performed to follow the structural changes upon lithiation.
37 Experimental and computational details are given section 1. Galvanostatic and potentiodynamic measurements and in situ X-ray diffraction results are presented section 2 for the high temperature ceramic sample Cu3PHT. Section 3 is devoted to the electrochemical mechanism upon cycling and to the comparison between Cu3P-ST and -HT samples.
2. Experimental and Computational Details Cu3P was synthesized at high temperature with 2.8/1 ratio of copper metal and red phosphorus as powders in a sealed silica tube. The temperature was increased to 600 ~ using a ramp of 50 ~ and held for 48 h. The samples were air quenched. Powder purity and crystallinity were studied by powder XRD for each Cu3P sample. Sampling was done in Lindeman capillaries and XRD patterns were recorded on a Philips X-pert diffractometer operating with a CuK~I radiation in the DebyeScherrer geometry. Electrochemical lithium insertion/extraction experiments were performed in SwagelokTM-type cells assembled in an argon-filled dry box, with oxygen and water contents below 5 ppm. These cells consist of a composite electrode containing 10-12 mg of active material mixed with 15 wt.% of acetylene black as positive electrode, a lithium metal disk as negative electrode, and a Whattman GF/D borosilicate glass microfiber separator saturated with 1 M LiPF 6 (EC:DMC, 1:1) electrolyte solution (Merck S.A) placed inbetween. Electrochemical insertion was monitored using a Mac Pile (Biologic SA) in the range 2.00-3.00 V to 0.01V operating in galvanostatic or potentiodynamic modes. Various C/n scan rates were used, C/n representing a full charge or discharge in n hours. In situ XRD were recorded using a D8 X-ray monochromateddiffractometer (CoKc~I), at various states of charge and discharge of the Li/Cu3P system using a specifically designed electrochemical cell. The electrode material was sampled underneath a beryllium window used as current collector. The complete device was then placed in the diffractometer and connected to the Mac-Pile system. These experiments were performed in potentiodynamic mode at a rate of C/35 (Li/10 h). Electronic structure calculations were performed using the density functional theory (DFT) code VASP [14]. Full structural relaxations were carried out for various lithium contents in order to evaluate their relative stability. Projector augmented wave pseudo-potentials (PAW) [15] were used for the basis set, as they are
38
Ionics 11 (2005)
Fig. 1. X-ray diffraction patterns (Cu Kcq) of Cu3P prepared by ceramic route. Insert corresponds to the SEM image of Cu3P powder as obtained.
expected to give a better description of alkali metal atoms than ultra-soft pseudo-potentials (US-PP) [ 16]. 3. Results High temperature synthesis leads to particles with average size varying from 10 to 100 g m according to SEM measurements. The XRD of Cu3P-HT powder is shown in Fig. 1. The diffraction peaks were indexed as a single
Table 1. Unit cell parameters obtained for the relaxed Cu3P, Li2CuP, LixCu
