Improving the high-temperature performance of LiMn2O4 spinel electrodes by coating the active mass with MgO via a sonochemical method

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Abstract

LiMn2O4 spinel coated sonochemically with MgO was studied as an active mass in composite cathodes in standard Li electrolyte solutions for Li-ion batteries at 60 °C. Solutions comprising 1 M LiPF6 in a mixture of ethylene, dimethyl, and diethyl carbonates (EC–DMC–DEC; 2:2:1) were used. It was possible to obtain LiMn2O4 particles fully covered by porous magnesia films that allow a free transport of Li-ions. Electrodes comprising LiMn2O4, modified by MgO showed a higher capacity retention compared to the electrodes comprising an uncoated active mass, specially at elevated temperatures. We suggest that the presence of an MgO film on the surface of the LiMn2O4 particles, reduces the detrimental effect of HF contamination present in LiPF6 solutions, reduces the electrodes’ impedance and improves their kinetics.

Introduction

LiMn2O4 spinel has been realized as a promising cathode material for lithium-ion batteries because of promising properties such as low cost, abundant precursors, non-toxicity and environmental benigness [1], [2]. However, LiMn2O4 electrodes in the 4 V (vs. Li/Li+) region suffer from capacity fading, especially at elevated temperatures, mainly due to detrimental surface reactions with acidic contaminants in the electrolyte solutions, and dissolution of manganese that leads to detrimental structural changes of the active mass [3], [4]. LiPF6 itself always brings with it HF contamination, which is detrimental to the performance of both negative and positive electrodes [5]. We discovered that LiMO2 cathode materials (M=Co, Ni, Mn, etc.) may react spontaneously with solution species, resulting in the formation of surface films [6]. Hence, it appears that Li-ion battery cathodes are also SEI-type electrodes [7] namely, their electrochemical behavior may be controlled by surface films through which Li-ion transport takes place. Major possible surface reactions of the cathodes may be driven by chemical properties of both their active mass and solution species. For instance, HF reacts with LiMO2 cathode materials to form surface LiF, and some of the cathode materials, e.g., LiNiO2, are nucleophilic and attack the electrolytic alkyl carbonate molecules, thus forming –OCO2Li surface groups and/or inducing formation of polymeric species such as polycarbonates [6], [7]. It should be noted that the capacity fading found for both Li–C and LiMO2 electrodes in Li-ion batteries is largely due to surface phenomena related to the above reactions. Too intensive surface films formation may increase the electrodes’ impedance and even isolate electrically part of the active mass [8]. At elevated temperatures, the above-described surface phenomena occur even more intensively; higher electrode impedance may be developed, and pronounced surface-related capacity fading of electrodes in Li-ion batteries can be observed. Several methods for improving the stability and cycleability of LiMn2O4 spinel cathodes have been investigated. Among them, partial substitution of Mn ions (doping) by other metal ions and surface modification of the active material may improve the cycling performance [9], [10], [11]. Coating of the surface of LiMn2O4 particles by surface species such as LiCoO2 [12], V2O5 [13], Al2O3 [14], SiO2 [15], and MgO [16], in order to improve the cycling performance of LiMn2O4 electrodes was reported. Most of these coating were performed through chemical processes, solution-based techniques such as sol–gel transformation, solution precipitation, and micro-emulsion [9], [10], [11], [12], [13], [14], [15], [16]. Sonochemical methods are relatively new, simple, and operate at ambient conditions [17]. They are widely used for the synthesis of nanoparticles [18], [19], [20]. Sonochemical methods can also be used for the uniform coating of materials’ surfaces [21], [22], [23]. This method is based on the phenomenon of acoustic cavitation, which involves the formation, growth, and collapse of bubbles in liquid, in which extreme conditions, such as very high transients temperatures, which may exceed 5000 °C [24] in localized hot spots, and an ultrafast cooling rate, >1000 °C/s can be obtained. Ultrasound radiation also induces very efficient agitation of reaction mixtures [25]. We report herein on a sonochemical method of surface modification of LiMn2O4 particles by coating them with a thin layer of MgO. This coating seems to improve their performance as a cathode material for Li-ion batteries at elevated temperatures.

Section snippets

Experimental

The LiMn2O4 spinel powder (particle size of 5–10 mM) was obtained from Merck KGaA (highly pure, Li battery grade) and was used as received. Ammonium hydroxide (99.99%) and (CH3CO2)2Mg · 4H2O (99.99%) were obtained from Aldrich Chemical Co. and were used as received. For the preparation of coated materials, 500 mg of LiMn2O4 were first sonicated for 10 min in order to disperse all particles in 100 ml of doubly distilled water. The required amount of precursors MgSO4 · 4H2O or (CH3CO2)2Mg · 4H2O

Results and discussion

Using 2, 5, 10, and 15 mol% of magnesium acetate vs. LiMn2O4 in the solution mixtures provided an active solid mass containing MgO at 1.3, 4.2, 5.5, and 5.6 mol% (respectively) of the LiMn2O4, as was analyzed by atomic absorption. FTIR studies showed that the sonochemical products are LiMn2O4 containing Mg(OH)2 (a typical υOH peak at 3647–3697 cm−1). Upon heating, the Mg(OH)2 is converted to MgO (FTIR spectra of the samples heated to 450 °C do not have this υOH peak). Fig. 1 shows typical TGA

Conclusion

In this communication, we present preliminary results that demonstrate a relatively easy and efficient procedure to produce LiMn2O4 particles coated by nanosized magnesia. Sonication of slurries comprising LiMn2O4 particles and magnesium sources such as MgSO4 or Mg acetate, produces LiMn2O4 coated by a Mg(OH)2 film. Further heating of this product to 450 °C transforms the Mg(OH)2 to MgO, thus producing magnesia-coated LiMn2O4. When the amount of the Mg source in the slurries is around 10%

Acknowledgments

Partial support for this work was obtained from the BMBF, the German Ministry of Science, in the framework of the DIP program for Collaboration Between Israeli and German Scientists, and by the EC in the framework of the fifth program (the NanoBatt project).

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