Elsevier

Journal of Power Sources

Volume 76, Issue 1, 1 November 1998, Pages 91-97
Journal of Power Sources

Long cycle life Li–Mn–O defective spinel electrodes

https://doi.org/10.1016/S0378-7753(98)00144-XGet rights and content

Abstract

A Li–Mn–O defective spinel phase material has been synthesized through repeated grinding and heating (T<400°C) steps from a mixture of LiOH and MnO2. The sample has been characterized by X-ray diffraction, thermogravimetric analysis and elemental and oxidation state analyses. At a C/12 charge/discharge rate, the material showed a first-cycle capacity of almost 200 mAh/g that decreased upon cycling to a stable value of 120 mA h/g after the 20th cycle. At a higher rate (C/3.7), the material showed a long cycle life upon lithium insertion/deinsertion with more than 50% of the initial capacity delivered at the 750th cycle. The work was developed within the ALPE (Advanced Lithium Polymer Electrolyte) battery project, an Italian project devoted to the realization of lithium polymer batteries for electric vehicle applications.

Introduction

The insertion of lithium in Li–Mn–O compounds was first investigated by Hunter [1], Tackeray et al. 2, 3, Goodenough et al. [4], and Tackeray [5]. Since the discovery, several papers and patents have been seen in the past decade 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12. Such a wide academic and industrial interest in lithiated manganese oxides for use as cathodes in lithium batteries originates from the major advantages shown by these materials. In fact, although their reversible specific capacities are lower than the theoretical specific capacities of the LiMO2 compounds (M=Co, Ni), they potentially are the lowest cost available cathodic materials for rechargeable lithium batteries. Further, the battery-related industry is well acquainted with the use and manipulation of manganese oxides, i.e., there is a wide range of experience on the handling of these materials. Finally, and perhaps most importantly, manganese oxides are more environmentally safe than other commercially used cathodic materials.

Much of the work performed on manganese oxides is concentrated on the spinel phase (LiMn2O4) although more recently a large interest has been devoted to the synthesis of layered or amorphous compounds 10, 11, 12. The spinel material can intercalate or deintercalate approximately 1 equivalent of lithium per mol (LixMn2O4, 0<x<2). These processes are accompanied by two distinct structural changes; a cubic–cubic phase transition associated with lithium extraction (0<x<1) and a cubic–tetragonal phase transition accompanying lithium insertion (1<x<2). Both processes can be electrochemically driven in the same material by placing it in an electrochemical cell and selecting the appropriate voltage difference with respect to a lithium anode. The first process (Li-extraction) is driven when the cell voltage is slightly above 4 V (4-V plateau) while the lithium insertion process spontaneously proceeds with a cell voltage close to 2.8 V (3-V plateau). In theory, the capacity of both plateaus could be accessed by discharging and charging the cell between 2.5 V (vs. Li) and 4.5 V (vs. Li) with a resulting total capacity of 300 mA h/g of active material. In practice, the reversibility and maximum amount of lithium involved in each of the two processes strongly depend on the synthesis conditions used. Highly crystalline materials synthesized at temperatures above 700°C [5]showed a good reversibility over the 4-V process. On the other hand, the same materials showed very poor cycle performance over the 3-V process. This was interpreted as due to the irreversible cubic–tetragonal phase transition, consistent with the onset of the Jahn–Teller effect, when the Mn3+/Mn4+ ratio in the material becomes larger than unity 2, 3, 13. Spinel phase manganese oxides synthesized at lower temperatures (350–450°C) showed high capacities, good cycling behavior and good rate capabilities in the 3-V plateau. The improvements were ascribed to the defective structure and a reduced crystallinity of the low temperature spinel phase and to the high specific surface area 14, 15.

Defective spinels like Li2Mn4O9 or Li4Mn5O12 have been proposed as excellent materials for lithium insertion process over the 3-V plateau [16]on the basis of structural and electrochemical data. Li2Mn4O9 was shown to intercalate up to 1.7 equivalents of lithium per mol with only a marginal change in the unit cell volume [14]. Further lithiation induces a tetragonal distortion consistently with the onset of the Jahn–Teller effect, the extent of which is lower than in the pure spinel phase LiMn2O4. A theoretical capacity of 213 mAh/g and a practical capacity of 160 mAh/g for cycles in the voltage range from 3.3 V to 2.3 V have been, respectively, predicted and demonstrated for Li2Mn4O9 but the cycle performance have been demonstrated for only a few cycles [16].

The present work started from this background. The investigations were devoted to the synthesis and the electrochemical characterization of Li–Mn–O defective spinels. The work was initially focused on the selection of the starting materials as well as the optimization of the synthesis conditions. As it will be shown in the following, the work was successful and it enhanced the electrochemical properties of the material in terms of cycle life.

Section snippets

Experimental

Li–Mn–O spinel powders were synthesized from LiOH (Carlo Erba) and MnO2 (Carlo Erba) as starting materials. A stoichiometric mixture of LiOH and MnO2 (Li:Mn=1:2) was poured into a tungsten carbide (WC) jar internally coated with polyethylene and mixed by a planetary mill for 6 h with WC balls. Ethanol was used as dispersing medium.

The slurry was dried at 70°C and the resulting powder was heated at 380°C in air for 96 h with intermediate grinding steps. Both the heating and cooling rates were

Results and discussion

Fig. 1 shows a high magnification SEM image (10 000×) of the surface of a single grain of the synthesized material. The image clearly shows the mushroom-like morphology of the material surface that explains the relatively high surface area (29 m2/g) measured by single point BET.

Fig. 2 illustrates the diffractogram of the material taken after the thermal treatment (96 h). The main peaks belong to the Li–Mn–O spinel phase but additional small peaks are present, marked by asterisks, most likely

Conclusions

A Li–Mn–O material has been synthesized through repeated grinding and heating (T<400°C) steps from a mixture of LiOH and MnO2. The material, characterized by X-ray diffraction, thermogravimetric analysis and elemental and oxidation state analyses, was identified as a defective spinel phase with a general formula Li1−δMn2−2δO4 (δ=0.05). As a result of the low temperature synthesis (380°C), the material had poor crystallinity and contained a small fraction of a γ/β MnO2 phase with included

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