Li1.01Mn1.97O4 surface modification by poly(3,4-ethylenedioxythiophene)
Introduction
The spinel LiMn2O4, which has been defined as a “green” material for its non-toxicity, abundance and low-cost, has been recognized as a suitable alternative cathode to LiCoO2 in lithium-ion batteries and has been the focus of growing research activities to investigate and overcome its drawback of high capacity fading, particularly at high temperature, in ethylene carbonate-dimethyl carbonate-LiPF6, electrolyte, which is widely utilized in lithium-ion batteries [1], [2], [3], [4].
Our research is focused on the development of new composite electrodes based on LiMn2O4 spinels and thiophene-based electronically conducting polymers operating at 4 V in which the polymer serves both as conducting agent and as binder and, given its electroactivity in the potential range of LiMn2O4, contributes to the composite capacity. We selected two types of LiMn2O4, one from Honeywell and the other home-made by sol–gel procedure (both can be written as Li1.01Mn1.97O4). Non-stoichiometric spinels ensure cell parameters that prevent the Jahn-Teller effect. Several factors are recognized as responsible for the capacity fading of LiMn2O4, such as the Jahn-Teller distortion, the dissolving of manganese Mn2+ (Mn2+ arise from the disproportionation reaction of Mn3+ that is induced by HF present in LiPF6-based electrolytes) and electrolyte decomposition. Different strategies to minimize the spinel capacity loss have been pursued, such as partial substitution of manganese cations with others and of oxygen with fluorine and by altering the surface chemistry of the spinel particles by inorganic materials [2], [5], [6]. The in situ polymerization of conducting polymer on LiMn2O4 is another feasible strategy because of the oxidative properties of lithium manganese oxides in an acidic medium. The growth of polypyrrole on LiMn2O4 operating at 4 V [7], [8] and of pEDOT into V2O5 [9] has already been investigated. By pursuing this strategy, we produced and characterized a composite based on a non-stoichiometric spinel and poly(3,4-ethylenedioxythiophene) (pEDOT) operating at 4 V [10] for the first time. We found that this was the best procedure among those tested for composite electrode preparation with pEDOT because it assures good electric contact between the inorganic particulate and the organic conductor, which enables the electronic charge transfer to lithium manganese oxide.
The present paper deals with non-stoichiometric spinels covered by pEDOT chemically grown on the oxide particle surface to prevent direct contact of manganese oxide with the electrolyte. The results of galvanostatic charge/discharge cycles and impedance spectroscopy measurements on electrodes based on pEDOT-covered commercial and home-made Li1.01Mn1.97O4 spinels, the latter prepared via the sol–gel route, are reported and compared to those of electrodes based on spinels without polymer covering to investigate the efficacy of a polymer barrier to electrolyte.
Section snippets
Experimental
The Li1.01Mn1.97O4 was prepared by sol–gel procedure after [11] starting from lithium and manganese acetates and glycolic acid as a chelating agent. The product was calcined at 750 °C for 10 h; the commercial Li1.01Mn1.97O4, with 3.54 as the average Mn oxidation number, was provided by Honeywell GmbH. The particle size of both spinels, SG and HW, evaluated by SEM, was ca. 0.5 μm. The in situ preparation of the polymer Li1.01Mn1.97O4/pEDOT composite was carried out as described in [10] by oxidation
Results and discussion
The electrochemical characterization of composite electrodes started with cyclic voltammetries (CVs) in order to select the cut-off potentials of the charge/discharge galvanostatic cycles on the basis of the redox potentials of the electroactive materials. Fig. 1 shows the CVs of a polymer SG/P-c electrode at 0.1 mV s−1 and of the pEDOT/C electrode (pEDOT 80%, carbon 16.5%, binder 3.5%) at 1 mV s−1. The CV of SG/P-c still shows the two oxidation peaks and the corresponding reduction peaks typical
Conclusions
This work investigates the barrier effect of pEDOT on LiMn2O4 in polymer composite electrodes based on commercial and home-made non-stoichiometric spinels. The capacity fade of these polymer composite electrodes (22–24% over 100 cycles at 1C and at 32 °C) is slightly lower than that of corresponding conventional composite electrodes with nanometric carbon as conducting additive. Although the pEDOT covering has a beneficial effect on the oxide particles, its barrier effect is still insufficient
Acknowledgements
The authors would like to thank MURST-ENEA legge 95/95 and MURST Cofin 2000 for financial support, and Honeywell GmbH for kindly providing Li1.02Mn2O4.05.
References (14)
- et al.
Electrochim. Acta
(1999) - et al.
Electrochim. Acta
(1999) - et al.
Electrochem. Commun.
(2002)et al.Electrochem. Commun.
(2003) Solid State Ionics
(1997)- et al.
Solid State Ionics
(2000) Prog. Solid State Chem.
(1997)- et al.
J. Electrochem. Soc.
(1990)
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