Elsevier

Electrochimica Acta

Volume 180, 20 October 2015, Pages 756-762
Electrochimica Acta

Li2MnSiO4 nanorods-embedded carbon nanofibers for lithium-ion battery electrodes

https://doi.org/10.1016/j.electacta.2015.08.161Get rights and content

Highlights

  • Phase-pure and rod-shaped nano Li2MnSiO4 were synthesized by solvothermal process.

  • Li2MnSiO4 nanorods grew along the [010] direction that facilitated Li-ion transport.

  • Li2MnSiO4 nanorods-embedded carbon nanofibers were prepared by electrospinning.

  • Li2MnSiO4/carbon nanofibers exhibited excellent cycling stability with 200th cycle.

  • Li2MnSiO4 is well-suited for use as a long-term cycling electrode in LIBs.

Abstract

Minute Li2MnSiO4 nanorods embedded in carbon nanofibers (LMS/CNFs) are prepared via an ethanol-based solvothermal process, and then by electrospinning and subsequent carbonization processes. The LMS nanorods (NRs) have lengths and widths of 15–20 nm and 5 nm, respectively, and grow lengthwise, i.e., along the [010] direction, which facilitates Li-ion transport in the LMS during charging/discharging. In addition, these LMS NRs are well incorporated in the electrospun LMS/CNFs after carbonization. The LMS/CNFs exhibit an excellent cycling stability with a capacity retention of 96% for up to 150 cycles at voltages of 1.5–4.75 V vs. Li/Li+ and a current rate of 33.3 mA g−1. The cycling stability of the LMS/CNFs results from the nano-architecture formed between the LMS NRs and the CNFs.

Introduction

Since being first commercialized in the 1990s, rechargeable lithium-ion batteries (LIBs) have constituted the most promising energy storage and conversion technology for portable devices (mobile electronics, laptops) and large-scale devices (electric vehicles, hybrid electric vehicles); this promise stems from their eco-friendliness, high energy and power density as well as stable cycle performance [1], [2], [3], [4]. Commercial cathode materials such as layered LiCoO2, Li[Ni,Co,Ni]O2, and spinel oxide LiMn2O4 are expensive, toxic, and may have low structural stability owing to the release of oxygen from the lattice [5]. As such, attempts to develop LiFePO4 have focused on non-toxic polyanionic compounds as alternatives to the conventional cathode materials [5], [6]. Various polyanionic compounds such as phosphates [7], silicates [8], fluorophosphates [9], and borates [10] have attracted interest for use as possible cathode materials in LIBs.

More recently, lithium transition metal orthosilicates Li2MSiO4 (M = Mn and Fe) have been suggested as especially promising candidate cathode materials. Both lithium ions in the structure may reversibly de-/intercalate per transition metal ion (sequential M2+/M+3 and M3+/M4+ redox couples); this will lead to a higher theoretical capacity, 333 mA h g−1, than that of the phosphates (LiFePO4; 170 mA h g−1). Furthermore, Li2MSiO4 exhibits high thermal and structural stability owing to strong Si-O covalent bonding, and consists of naturally abundant and environmentally friendly elements [11], [12], [13], [14]. The Mn2+/Mn3+ and Mn3+/Mn4+ redox couples also deliver higher cell voltages than Fe2+/Fe3+, and Mn4+ is more stable than Fe4+ when lithium ions undergo the redox process; therefore, Li2MnSiO4 (LMS) is considered a more promising cathode than Li2FeSiO4 [3], [11], [15]. However, LMS has a large irreversible capacity and poor cycle stability stemming from the extremely low electrical conductivity (∼10−14 S cm−1 at room temperature); these factors combined with the instability of the structure, resulting from the Jahn-Teller distortion associated with Mn3+ during cycling, have limited the use of the LMS in practical applications [12], [16].

These shortcomings have been overcome by using various techniques to synthesize the nanoparticle or nanostructure LMS/carbon composites [17], [18]. For example, Li2MnSiO4/carbon nanofiber (LMS/CNFs) composites have been fabricated via electrospinning. Electrospinning is a facile method that can be used to prepare various inorganic/CNFs. Electrospun CNFs offer many lithium insertion sites resulting in decrease of charge transfer resistance between the active materials and electrolyte. Also, they exhibit a shorter diffusion path and faster intercalation kinetics resulting from their high ratio of area to mass [19], [20], [21], [22]. However, the LMS/CNFs described in previous studies still exhibited only short-term cycle stability.

Therefore, in this study, we describe the synthesis of minute LMS nanorods-embedded carbon nanofibers and their electrochemical properties. The overall synthetic process of the LMS/CNFs is illustrated in Scheme 1. The LMS/CNFs were prepared via a two-step process; i.e., (I) phase-pure and rod-shaped LMS nanorods (NRs) were synthesized from an ethanol-based solvent by using a facile solvothermal method and (II) the resulting LMS NRs were incorporated in polymer nanofibers via electrospinning and (III) subsequent carbonization processes. Electrochemical measurements revealed a stable cyclability over 150 cycles and relatively stable cycling retention at room temperature.

Section snippets

Synthesis of LMS NRs

LMS NRs were synthesized via the solvothermal method using ethanol as the solvent. A portion (8 mmol) of LiOH (98%, Sigma–Aldrich) were dissolved in 100 mL of ethanol; 2 mmol of MnCl2∙4H2O (99%, Sigma–Aldrich) or Mn(NO3)2∙4H2O (97%, Sigma–Aldrich) and Si(OCOCH3)4 (99%, Sigma–Aldrich) were dissolved in another 100 mL of ethanol. These two solutions were then mixed for several hours, under magnetic stirring, in order to form a precursor. The precursor solution was then rapidly transferred to a

Results and discussion

We used the solvothermal method to synthesize LMS particles by varying the concentration of the source, reaction temperature, and time. The concentration of the LiOH source was varied first; Fig. 1a shows the XRD patterns of products formed at various concentrations of LiOH. When LiOH was dissolved stoichiometrically in ethanol, LMS particles were not synthesized and Li-deficient Mn2SiO4 (i.e., LiOH 2 in Fig. 1a) formed instead. However, the reflection peaks in the pattern exhibited close

Conclusions

We synthesized phase-pure LMS NRs using an ethanol-based solvothermal process. LMS NRs grew along the [010] direction that facilitated Li-ion transport in the LMS hosts. Furthermore, LMS NR-embedded CNFs were prepared via an electrospinning process; these NRs were uniformly distributed in the CNFs. The architecture of the CNFs resulted in improved electrical conductivity of the NRs and prevented Mn from dissolving in the electrolyte. As such, the LMS/CNFs exhibited excellent cycling stability

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Ministry of Science, ICT, and Future Planning (2011-0030300) and by Korea Small and Medium Business Administration (S2230272). This work was also supported by the R&D Center for Valuable Recycling(Global-Top R&BD Program) of the Ministry of Environment (2014001170002).

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