Transport and electrochemical properties of the LiyCrxMn2−xO4 (0 < x < 0.5) cathode material
Introduction
For several years now, the LiMn2O4 system has been attracting enormous interest as being a potential cathode material for 4 V reversible lithium batteries. Its reversible capacity equals 100–130 mA h g−1 and is comparable to that of the presently used LiCoO2. However, manganese spinel is cheaper and more environmentally friendly. Manganese spinel undergoes a phase transition at 290 K, leading from a high-temperature cubic to a low-temperature orthorhombic phase. This transition takes place for the critical concentration of Mn3+ ions (Jahn–Teller ions). Most researchers attribute an insufficient number of work cycles in manganese spinel based batteries to the presence of that phase transition [1], [2]. In order to extend battery life, many laboratories in the whole world have been investigating electrochemical properties of doped spinels LiyMxMn2−xO4 (M = Al, Mg, Ti, V, Cr, Fe, Co, Ni, Cu, Zn) [3], [4], [5], [6], [7], [8]. These studies, however, are limited to determining lattice parameters and electrochemical characteristics. For many dopants, the charging curve changes its shape significantly, thus suggesting a modification in the electronic structure of the manganese spinel. There is a complete lack of research devoted to transport properties of doped manganese spinel. It is those properties which, to a great extent, determine the usability of a cathode material.
Electronic transport in manganese spinel LiMn2O4 at room temperature (i.e. at the working temperature of Li-ion batteries), is very difficult. A low value of conductivity, on the order of 10−4 S cm−1, is related to the small polarons mechanism. Their migration activation energy is high, and equals 0.2–0.3 eV [9].
Previous results obtained by the authors revealed that the polaron mechanism of charge transport, in the manganese spinel, is very stable. It is not influenced by the change in oxygen non-stoichiometry, excess lithium in the manganese sublattice, hydrostatic pressure, or chemical intercalation [10], [11], [12].
The aim of the present paper is the determination of transport and electrochemical properties of the chromium substituted manganese spinel LiCrxMn2−xO4 in the composition range of x = 0–0.5. Studies of electrical conductivity and thermoelectric power, performed as a function of temperature (800–1100 K) and oxygen pressure (1–10−4 atm), at equilibrium conditions, allowed the determination of structures of ionic and electronic defects, related to the non-stoichiometry of the oxygen sublattice. The charge transport mechanism could also be determined in a wide temperature range. The studies of structure, electrical conductivity and thermoelectric power, performed at low temperatures, together with the DSC results for doped manganese spinel, LiCrxMn2−xO4, allowed the determination of charge transport mechanism in the composition range of x (0.1–0.5), and at temperatures 200–320 K, i.e. at temperatures close to the battery working temperature. Another aim of the present paper is to resolve in the electronic diagram the relative positions of manganese and chromium levels in the chromium doped manganese spinel (based on studies of electrical and electrochemical properties) in order to correlate the changes in the Fermi level position with the changes in the Li+/LiyCrxMn2−xO4 cathode potential.
Section snippets
Preparation of samples
Spinel compounds with a chemical composition LiCrxMn2−xO4 (0 < x < 0.5) were prepared by solid state reaction at 800 °C. Stoichiometric amounts of Li2CO3, MnCO3 and Cr2O3 were mixed in an agate mortar and synthesized in air for 24 h at 800 °C. They were grounded again in the mortar and annealed in air at 800 °C. Such obtained materials were pressed under the pressure of 10 Ton cm−2 to obtain pellets 1 mm thick, with a diameter of 10 mm. The pellets were annealed in air for 48 h at 800 °C and cooled rapidly
Electrical and thermal properties of LiCrxMn2−xO4 at low temperatures
Fig. 1 shows the temperature dependence of electrical conductivity (Fig. 1a), thermoelectric power (Fig. 1b) and the DSC results (Fig. 1c, [14]) for LiCrxMn2−xO4 with the chromium content of 0.1, 0.2, 0.3, 0.4 and 0.5. For comparison's sake, the characteristics for stoichiometric spinel are also shown. In the case of stoichiometric spinel, anomalous electrical and thermal effects accompany the phase transition observed at 290 K (between a cubic and orthorhombic phase) [9]. The obtained results (
Conclusions
The substitution of Mn3+ ions with the Cr3+ ones, results in the suppression of phase transition in the manganese spinel. At low temperatures, chromium does not participate in charge transport. A close correlation between the amount of substituted chromium xCr and battery charging curves is related to the electronic structure of cathode material. A 4 V potential plateau corresponds to the oxidation of Mn3+ into Mn4+, while a 5 V plateau is related to the oxidation of Cr3+ into Cr4+. A jump in the
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