Lithium-ion batteries using transition-metal oxides as positive electrodes are currently .... tests for both citrate and succinate LiMn1.8Co0.2O4 show the better ...
Structural, Magnetic and Electrochemical Properties of the Spinel LiMn2-yCoyO4 Nanosized Powders N. Amdouni, F. Gendron, A. Mauger, C. M. Julien PCMS, Université El-Manar II, Tunis, Tunisia Institut des Nano-Sciences de Paris, Université Pierre & Marie Curie, Paris, France Department MIPPU, CNRS, Paris, France
in Mobile Energy edited by A. Nathan, G. Amaratunga, M. Nookala, L.G. Scanlon, E. Morinobu (Mater. Res. Soc. Symp. Proc. 973E, Warrendale, PA, 2007), BB4-4
© Mater. Res. Soc., Warrendale, PA, 2007
Structural, Magnetic and Electrochemical Properties of the Spinel LiMn2-yCoyO4 Nanosized Powders N. Amdouni1, F. Gendron2, A. Mauger3, C. M. Julien2 1
PCMS, Université El-Manar II, Tunis, Tunisia Institut des Nano-Sciences de Paris, UMR 7588, Université Pierre & Marie Curie, 140 rue de Lourmel, 75015Paris, France 3 Department MIPPU, CNRS, campus Boucicaut, 75015 Paris, France 2
ABSTRACT We present the synthesis, structure, magnetic properties and electrode behaviour of LiMn2-yCoyO4 (0≤y≤0) spinel oxides prepared by the wet-chemitry via the carboxylic acid route. LiMn2-yCoyO4 samples crystallise with the cubic spinel-like structure (Fd3m S.G.). Optical spectra indicate that the vibrational mode frequencies and relative intensities of the bands are sensitive to the covalency of the (Co,Mn)-O bonds. Magnetic susceptibility and electron spin resonance measurements show the compositional dependence of the magnetic parameters, i.e. Curie temperature, Curie-Weiss constant and Néel temperature, when Mn is substituted by Co. The overall electrochemical capacity of LiMn2-yCoyO4 oxides have been reduced due to the 3d6 metal substitution, however, a more stable charge-discharge cycling performances have been observed when electrodes are charged up to 4.3 V as compared to the performance of the native oxide. INTRODUCTION Lithium-ion batteries using transition-metal oxides as positive electrodes are currently dominating the portable power sources industry for laptop computers and mobile telephones. The great success of Li-ion type cells has stimulated searches on high-performance for alternative positive electrodes such as LiMn2O4, but the electrochemical properties of spinel LiMn2O4 are highly dependent on the synthesis route, composition, and thermal history of the samples [1]. The partial substitution of metal cations for the Mn forming the spinel derivatives LiMn2-yMyO4 (M=Ni, Co, Al, Cr, Fe, etc.) solid solutions is a strategy to improve significantly the electrochemical cycling of LiMn2O4 materials, but at the expense of a decrease in initial capacity within the useful voltage window, i.e. below 4.4 V [2-7].. Such a successful result is due to replacement of the Mn3+ Jahn-Teller (JT) ions, which provoke a tetragonal phase transition in the 3-V region [8-9]. Despite the decreasing first discharge capacity upon substitution, cobalt-doped spinels seem to be the most promising substituted materials investigated so far [9-12]. Two main effects appear with the presence of Co in LiMn2O4: (1) an enhancement of the exchange current density and (2) an easy charge-transfer reaction of the active material [9]. In this work, we present the characterization and electrode behaviour of LiMn2-yCoyO4 (0≤y≤1) prepared by wet-chemistry via the citrate precursor routes. The phase evolution is studied as a function of the cobalt substitution and the modification on the intercalation and deintercalation of Li ions. EXPERIMENT Undoped LiMn2O4 and LiMn2-yCoyO4 (0≤y≤1) powders were prepared with the same technique, i.e. the carboxylic acid-assisted wet chemistry, from metal acetates via inorganic
polymerization reactions in solution according to the method reported by Julien et al. [13]. In this work, citric acid was used as polymeric agent that promotes the crystallization. Stoichiometric amounts of acetate hydrates of Li, Mn and Co (Fluka, purum p.a. grade) were dissolved in triple distilled water and mixed well with an aqueous solution of either citric or succinic acid (Fluka, MicroSelect grade). The powder X-ray diffraction (XRD) patterns were obtained on a Philips X'Pert PRO MRD (PW3050) diffractometer equipped with a Cu anticathode (CuKα radiation λ = 1.54056 Å) at room temperature. The XRD data were analysed by the Rietveld profile refinements using the Fullprof program. The particle morphology of the Li(Mn,Co)2O4 powders was examined with a scanning electron microscope (SEM, Philips XL30). Magnetic measurements were performed in a Quantum Design MPMS (SQUID) magnetometer. Zerofield-cooled (ZFC) and field-cooled (FC) magnetic susceptibility data were obtained in a field at 10 kOe in the temperature range 4≤T≤300 K. Electrochemical measurements were carried out at room temperature following the experimental procedure previously described using Teflon laboratory-cell hardware and Mac-Pile system [14]. Quasi open-circuit voltage curves were recorded using current pulse of 0.14 mA/cm2 applied for 1 h followed by a relaxation period of 0.5-1.0 h, which allows to record the transient voltage for the determination of chemical diffusion coefficients of Li+ ions in the LiMn2-yCoyO4 lattice. RESULTS AND DISCUSSION Single-phase LiMn2-yCoyO4 powders were synthesized after further heating of the fluffy brownish black precursors to 400-700 °C in air for 48 h. Citric acid acted as fuels due to the presence of carboxylic COOH groups in this complexion agent. It provided decomposition of the metal complexes at low temperature, and yielded the impurity-free LiMn2-yCoyO4 powders when heated above 400 °C. XRD patterns of LiMn2-yCoyO4 (0≤y≤1) oxides synthesized by sol-gel method are shown in Fig. 1. 800
400
(551)
(533)/(622)
(444)
(440)
(531)
(511)
(331)
y=1.0
(400)
(311)
(111)
600
(citrates)
(222)
XRD intensity (arb. units)
LiMn2-yCoyO4
y=0.8 y=0.6
200
y=0.4 y=0.2
0
y=0.0
20
40 60 Two-theta (degree/CuKα)
80
FIG. 1. XRD patterns from LiMn2-yCoyO4 for 0≤y≤1 powders synthesised by the sol-gel method from citrate precursors. Powders were fired at 700°C in air for 48 h. Miller indices of the Bragg peaks are those of the cubic Fd3m space group.
Samples crystallized with the cubic spinel-like structure (Fd3m S.G.) with the decreasing lattice parameters upon substitution of Co for Mn due to the smaller ionic radius of CoIII (74 pm) cations. As shown in Fig. 2, the variation in the cubic parameter follows an almost Vegard's law. The good crystallinity is confirmed by SEM investigations (Fig. 3). The powders observed by SEM were found to be submicron-sized particles, i.e. the grain size average is about 200 nm for LiMnCoO4. 8.25
(a)
a parameter (Å)
8.20 8.15 8.10
LiMn2-yCoyO4 (citrate)
8.05 8.00
0.0
0.2
0.4
0.6
y(Co) in LiMn2-yCoyO4
0.8
1.0
FIG. 2. The lattice parameter of the cubic structure, Fd3m space group, as a function of Co substitution for LiMn2-yCoyO4 (0≤y≤1) powders.
b
a
FIG. 3. Scanning electron micrographs of LiMn2-yCoyO4 powders synthesized by the wetchemical citrate route for (a) y=0.0, and (b) y=0.6. Submicron-sized particles are obtained. The temperature dependence of the magnetic susceptibility of LiMn2-yCoyO4 (0≤y≤1) samples (not shown here) shows a drastic change upon substitution of Co for Mn [15]. In particular, the strong anomaly at T=30 K disappears progressively with the introduction of Co3+ in the spinel lattice. This is a consequence of the disappearance of the Mn3+ ions in the [Mn2]O4 sublattice. For LiCoMnO4, Mn4+ are the only paramagnetic species if we assume that low-spin Co3+ ions are diamagnetic species as confirmed below. In the entire temperature range (5-300 K), the sample y=1.0 follows a paramagnetic behavior. The curves χm-1(T) can be represented by a straight line obeying the Curie-Weiss law, χ m−1 = C p /(T − θ p ) , that allows the determination of the magnetic constants θp and Cp. The composition dependence of the Curie constant and Weiss temperature for LiMn2-yCoyO4 compounds are reported in Fig. 4. The effective magnetic moment number of LiMn2-yCoyO4 spinels is obtained from 2 p eff = p2
Mn 4 +
+ (1 − y ) p 2
Mn3+
+ yp 2
Co3+
,
(1)
where, p
Mn 4 +
, p
and p
Mn3+
Co 3+
are the spin-only paramagnetic moment numbers of Mn4+,
Mn3+ and Co3+, respectively. Spin-only numbers are calculated in the case of quenched angular momentum using p=2[S(S+1)]1/2, where S is the spin quantum number. The theoretical values of the Curie constant were calculated using the spin-only values of µeff=3.87 µB (S=3/2) and 4.90 µB (S=2) for magnetic moments of Mn4+ and Mn3+, respectively. B
B
5
LiMn1-yCoyO4 (θp
