Enhanced electrochemical performance of carbon-coated Li2MnSiO4 nanoparticles synthesized by tartaric acid-assisted sol–gel process

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Abstract

A tartaric acid-assisted sol–gel process was used to synthesize Li2MnSiO4 (SG-LMS) nanopowders with orthorhombic structures (Pmn21 space group). The Li-active SG-LMS nanoparticles were fully surrounded by a conducting amorphous carbon layer/matrix that was formed by carbonization of the tartaric acid during post-annealing of the dry gel. The SG-LMS electrode exhibited higher specific capacity and superior cycle retention as compared to the LMS electrode prepared by a conventional solid-state reaction. Such high electrochemical performance originated from the presence of a high-purity phase, a large surface area, and an efficient electron transport path facilitated by the conductive carbon coating of the SG-LMS electrode.

Introduction

Rechargeable Li-ion batteries have become increasingly popular for various applications in portable electronics and electric vehicles [1], however, conventional cathode materials such as LiCoO2, LiMn2O4, and LiFePO4 need to be improved with respect to energy density, rate capability, structural stability, etc [2], [3], [4]. Thus, one of the current areas of interest in Li-ion battery research is the search for new cathode materials that have superior electrochemical properties and structural stability. Among the types of cathode materials available for Li-ion batteries, Li orthosilicates (i.e. Li2MSiO4, M=Fe, Mn, etc.) have recently attracted widespread attention owing to their various advantages such as high theoretical capacity (>300 mA h g−1), which originates from extracting two Li ions per formula unit based on the reaction Li2M2+SiO4↔M4+SiO4+2Li++2e, high thermal stability through a strong Si–O covalent bond, environmental benignancy, and low cost. In particular, Li2MnSiO4 (LMS) is more promising than Li2FeSiO4 because the Mn2+/Mn3+ and Mn3+/Mn4+ redox couples provide higher cell voltage than Fe2+/Fe3+, leading to high energy density [5], [6]. However, the use of orthosilicates as a Li-ion battery cathode has a critical limitation caused by their low electronic conductivity (~10−14 S cm−1 for the LMS) as compared to other cathodes such as LiCoO2 (~10−4 S cm−1) and LiMn2O4 (~10−6 S cm−1) at room temperature, thereby limiting the possibility of using these orthosilicates in practical cells. Although conductive carbon coatings have been generally used to overcome this problem, the poor electrochemical properties, which are mainly attributed to incomplete coating and crystal growth at high temperature, remain to be solved [7], [8], [9]. Therefore, optimization of an effective carbon coating method is very important for orthosilicate materials.

Recently, Lee et al. reported synthesis of in-situ carbon-coated LMS nanoparticles by employing an adipic acid-assisted sol–gel process, resulting in superior electrochemical performance [10]. They suggested that the use of an appropriate carboxylic acid as a chelating agent and carbon source in the synthesis of LMS was critical for achieving effective carbon coating, phase purity, and cyclability of the Li-ion battery cathode. The sol–gel method has been known as a convenient process for preparing nanomaterials with good stoichiometry and morphology as compared to the conventional solid-state approach. Nanostructured active materials for Li-ion batteries afford multiple advantages such as higher electrode/electrolyte contact areas and shorter path lengths for both electron and ion transport.

Herein, we report the formation of LMS powders through a tartaric acid-assisted sol–gel (SG) process, and we compare the process with conventional solid-state (SS) methods, investigating the effect of the synthetic method on the phase purity of the samples obtained (SG- and SS-LMS). In view of literature survey results, the use of tartaric acid as a chelating agent for the synthesis of LMS has not been reported so far. Furthermore, we demonstrate enhanced specific capacity and cycle retention through control of carbon coating, particle size, and phase purity of the LMS powders used as a Li-ion battery cathode.

Section snippets

Synthesis of Li2MnSiO4 powder

SG method: Carbon-coated Li2MnSiO4 nanoparticles were synthesized by a tartaric acid assisted sol–gel method, in which Mn(NO3)·4H2O, tetraethyl orthosilicate (TEOS), LiNO3, and tartaric acid (TA) were used as source materials. In a typical synthesis procedure, TEOS and TA with a weight ratio of 1:1 were dispersed in ethanol, and then a deionized water solution containing a stoichiometric amount of Mn(NO3)4H2O and LiNO3 was added to the prepared solution under constant stirring. The solution

Results and discussion

Fig. 1a shows the XRD patterns of the SG-LMS and SS-LMS powders. Nearly single-phase LMS powders were readily obtained via the tartaric acid-assisted sol–gel process and post-annealing at 600 °C for 8 h. Their diffraction peaks could be well indexed on the basis of orthorhombic LMS with a Pmn21 space group (Fig. 1b), which was proposed to be isostructural with low-temperature Li3PO4 [11], [12]. Mn2+ ions are located in the 2a tetrahedral sites within an [SiO4] anionic silicate network that

Conclusions

In summary, we have successfully synthesized nearly pure-phase LMS nanopowders by a sol–gel process in the presence of tartaric acid. The specific discharge capacity and cycle retention of the SG-LMS electrode (~113 and ~105 mA h g−1 after the first and thirtieth cycles, respectively, at a rate of C/20) were measured to be superior to those of the SS-LMS electrode (43 and 20 mA h g−1 for the first and fourth cycle, respectively, at the same rate). This superior electrochemical performance was

Acknowledgements

This work was supported by a grant from the Fundamental R&D Program for Technology of World Premier Materials funded by the Ministry of Trade, Industry & Energy (MOTIE), Republic of Korea.

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