Synthesis of LiFePO₄/C using ionic liquid as carbon source for lithium ion batteries

Highlights

• LiFePO₄/C materials were synthesized by a hydrothermal method using ionic liquid [VEIm]NTf₂ as carbon source.

• Charge/discharge capacities of LiFePO₄/C were 141.3/136.4 mAhg^− 1at 0.1 C and 110.7/109.9 mAhg^− 1 at 1 C, respectively.

• [VEIm]NTf₂ is a promising carbon source to improve the electrochemical properties of LFP cathode.

Abstract

LiFePO₄/C (LFP/C) materials are synthesized by a hydrothermal method with ionic liquid 1-vinyl-3-ethylimidazolium bis(trifluoromethylsulfony)imide ([VEIm]NTf₂) as carbon source. Carbon films of 5–10 nm are successfully coated on the surface of LiFePO₄ (LFP) particles and serve as the protective layers of LFP particles during cycling. The carbon materials also fill the gap between LFP particles, which creates electron transfer paths. Due to the integrated carbon materials, the LFP/C exhibits significantly improved reversibility, cycle stability, rate performance, and charge and discharge capacity. These results demonstrate a simple and scalable application of ionic liquid [VEIm]NTf₂ as carbon source toward electrochemical energy storage.

Introduction

With the increasing demands for commercialization of rechargeable batteries, lithium ion batteries (LIBs) gradually become the first choice of energy storage in many fields, such as laptops, mobile phones, smart grid and electric vehicle [1], [2], [3]. Since discovered by Padhi [4], olivine lithium iron phosphate LiFePO₄ has attracted enormous attentions as an ideal cathode material for LIBs due to its many beneficial properties, such as low cost, low toxicity, high-voltage plateau, environmental friendliness, excellent thermal stability, and excellent electrochemical performances [5]. However, the low diffusivity of Li⁺ ions in LFP and low electrical conductivity hindered the wide applications of LFP electrodes in industry [6]. To address these intrinsic drawbacks of LFP cathode, many methods have been investigated, including doping with other metal ions [7], [8], [9], [10], [11], reducing particle size [12], [13], [14], and coating conductive agents (carbon, conductive polymer, etc.) [3], [8], [12], [15], [16], [17], [18], [19]. Among these methods, carbon coating technology is gaining growing interests since the integration of carbon coating not only prevents the ion dissolution and migration, but also alleviates the electrode polarization [2], [20]. According to their chemical structures, the commonly used carbon sources can be divided into polymeric and nonpolymeric sources [21], [22]. Polypropylene [23], [24], [25], [26], polypyrrole [27], [28], [29], [30], [31], polyvinyl alcohol [32], [33] and polythiophene [34] are the widely used polymeric sources. The nonpolymeric carbon sources include the glucose [35], [36], [37], citric acid [38], [39], and lauric acid [40]. Ionic liquids (ILs) have been studied to a lesser extend as carbon source but are gaining increased interests recently due to its negligible vapor pressure and designable structure [41], [42], [43], [44], [45]. The negligible vapor of ILs leads to a mitigated evaporation of ILs during the carbonizations process and sequentially an easy processing and shaping process. The designable structure enables a controllable ratio of cation and anion components in ILs, which can tune the doping contents of heteroatoms in the carbon materials. ILs also possess the advantages of low viscosity, excellent liquidity, high thermal stability and excellent wettability, when compare with the conventional carbon sources [46], [47], [48]. Therefore, ILs are preferred to be used as the carbon sources for the electrode materials of LIBs. Zhao and coworkers obtained porous Li₄Ti₅O₁₂ with uniform nitrogen-doped (N-doped) carbon coating by using ILs as carbon sources, which showed improved rate capability and cycling performance [47]. Shen and coworkers prepared Si@N-doped carbon nanoparticles with silicon nanoparticle as the core and N-doped carbon as the shell, by using ionic liquid (3-cyanopyridine/H₂SO₄) as both the N and C sources. The obtained Si@N-doped carbon presented a high reversible capacity of 725 mAh/g after 100 discharge/charge cycles at a current density of 420 mA/g [49]. The ionic liquid 1-butyl-3-methylimidazolium tetrachlorocobalt ([BMIm]₂[CoCl₄]) is used to produce the N-doped mesoporous carbon supported CoO@Co nanoparticles which were used as electrode materials and improved the performance of Li-O₂ batteries [50]. In our previous work, we prepared LFP particles coated with N-doped carbon membrane by using the microwave pyrolysis of ionic liquid 1-butyl-3-methylimidazolium dicyanamide ([BMIM]-N(CN)₂). The obtained N-doped LFP/C showed excellent discharge capacity and cyclic performance [51].

In this work, we have produced LFP/C by using [VEIm]NTf₂ as carbon source. The LFP/C is first characterized by a variety of techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy to determine the quality of the electrode materials. The electrode materials are then assembled into a half-cell to measure the electrochemical performance. Because of the excellent wettability of [VEIm]NTf₂, the derived carbon films tightly coated on the surface of LFP, which creates electron paths between LFP particles, leading to an increased electrical conductivity. The strongly bonded carbon films limit the growth of LFP particles, therefore, reduce the insertion and deinsertion path of Li⁺ ions. It is found that the LFP/C has significantly improved reversibility, cycle stability, rate performance, and charge and discharge capacity. The improved performance of the LFP/C electrode can be attributed to the decreased resistance and the reduced Li⁺ path created by the carbon coatings on the LFP particles. These results indicate that [VEIm]NTf₂ is a promising carbon source for electrode materials in LIBs, which would be suitable for widespread applications.