Monodispersed LiFePO₄@C core–shell nanostructures for a high power Li-ion battery cathode

Highlights

• Monodispersed LiFePO₄ nanopillows were solvothermally synthesized with ethylene glycol.

• LiFePO₄@C core–shell nanostructures were prepared by a facile method in high yield.

• LiFePO4@C core–shell nanostructures show nice capacity retention and high rate capacity.

• A facile method was proposed simply to prepared fully carbon-coated LiFePO₄ cathode.

Abstract

In this paper, monodispersed LiFePO₄ nanopillows have been successfully synthesized via solvothermal route with ethylene glycol (EG) as reaction medium. Subsequently, with the basis of the solvothermally synthesized monodispersed LiFePO₄ nanopillows, monodispersed LiFePO₄@C core–shell nanostructures are facilely prepared in high yield. Based on the experimental results, the formation mechanism of the monodispersed LiFePO₄ nanopillows has been discussed simply. The monodispersed LiFePO₄@C core–shell nanostructures exhibit nice capacity retention and high rate capacity due to the full carbon-coating and the well-crystallized nanosized particles.

Introduction

Over the past decade, tremendous efforts have been performed to find alternatives to the toxic and expensive cathodes currently employed in commercialized lithium-ion batteries, such as cobalt-oxide-based materials. As an alternative, the ordered olivine lithium iron phosphate, LiFePO₄, has been intensively investigated since the pioneering work of Padhi et al. [1]. Moreover, LiFePO₄ is regarded as the most promising cathode materials for power batteries used in electric vehicles (EVs) and hybrid electric vehicles (HEVs), due to the moderate flat voltage plateau (3.4 V versus Li⁺/Li), high theoretical specific capacity (170 mAh g⁻¹), intrinsic thermal safety, environmental compatibility, and abundance of iron resources in nature [2], [3]. However, the rate performance of the original LiFePO₄ was significantly restricted by sluggish kinetics of electron and lithium-ion transport [4], [5], [6], [7]. Thus, enormous attempts have been carried out to improve the rate performance by enhance the conductivity, including surface coating or admixing with carbon and other electronically conductive materials [8], [9], [10], [11], reducing particle size and controlling morphology [10], [12], and doping with isovalent or supervalent cations [5], [6], [13], [14].

In recent years, some works on the modification of LiFePO₄ particles represent significant progresses in the improvement of the rate performance. Wang et al. reported that a core–shell LiFePO₄ nanocomposite prepared by an in situ polymerization restriction method could present a capacity of 90 mAh g⁻¹ at rate of 60 C [10]. Wu et al. suggested that a nanocomposite with highly dispersed LiFePO₄ nanoparticles in a nanoporous carbon matrix couple discharge at a rate up to 230 C [15]. Zhou et al. also illustrated that a nanocomposite, in which the LiFePO₄ primary nanoparticles are wrapped homogeneously and loosely with a graphene 3D network, could deliver a capacity of 70 mAh g⁻¹ at 60 C discharge rate [9]. All the above-mentioned reports demonstrated that full coating with carbon [10] is essential to attain high electrochemical performance. However, we know that the approaches based on the thermal decomposition of carbon-containing precursors, which have been widely used for the preparation of carbon-coated LiFePO₄ particles [8], [9], [10], [16], [17], [18], [19], [20], generally involve a high temperature treatment for forming carbon-coating. Due to the agglomeration of the LiFePO₄ nanoparticles and the crystal further growth occurred during the high temperature treatment, only partial surface of the LiFePO₄ particles is coated with the conductive carbon [10], [20]. Whereas the full carbon-coating facilitates the high power batteries, it is essential to pursue the preparation of the full carbon-coated LiFePO₄ nanoparticles in high yield.

The popular strategy to synthesize LiFePO₄ is the solid state reaction route performed at high temperature under inert atmosphere. Frequently, a few impurities, such as Li₃Fe₂(PO₄)₃, Fe₂O₃, and LiPO₄, are involved in the final products [21], [22]. Moreover, due to the calcinations at high temperature it is also very difficult to obtain fine and homogeneous particles. In order to obtain LiFePO₄ with fine particles for high rate performance and high capacity, it is essential to take advantage of solution synthetic route, such as precipitation [23], sol–gel [24], [25], polyol [26], [27], and hydrothermal reaction route [8], [11], [13], [28 ], [29], [30], [31]. Among these methods, hydrothermal reaction receives particular attention due to the mild operation temperature, well crystallization, simple process and low cost. Otherwise, hydrothermal reaction can be easily tailored to produce nanostructures by the modulation of reaction temperature, concentration of precursors, reaction medium solvent, and the addition of organic compounds [32], [33]. In a pioneering attempt, Whittingham et al. have successfully realized the synthesis of LiFePO₄ via hydrothermal reaction route [28 ], [29]. Due to severe displacement of Fe in Li site, the products hydrothermally synthesized at 120 °C show low electrochemical activity. With the hydrothermal treatment temperature increasing above 180 °C, the displacement of Fe in Li site can be remarkably suppressed. However, probably due to the particle size enlargement or the presence of impurities brought about from the oxidation of Fe(II) salt the electrochemical activity improves slightly [29], [30]. Subsequent works have shown that the impurities can be avoided by introducing some organic reagents, such as ascorbic acid, polyacrylic acid, and citric acid, to prevent the oxidation of Fe(II) to Fe(III) [8], [13], [16], [31].

Recently, a novel hydrothermal system, in which the ethylene glycol (EG) was used with water or other organic reagents as the reaction medium, has been developed to prepare size- and morphology-controlled LiFePO₄ nanostructures for improving the electrochemical properties [12], [34], [35], [36], [37]. Rangappa et al. [34] reported that LiFePO₄ nanorods and hierarchical flower-like microstructures have been synthesized via solvothermal reaction route by employing EG with hexane or oleic acid as reaction medium solvent. Teng et al. [35] solvothermally synthesized the LiFePO₄ nanodendrites by employing EG/water mixture solvent as reaction medium assisted with the surfactant of dodecyl benzene sulphonic acid sodium (SDBS). As anodized alumina oxide (AAO) is used as template in the hydrothermal reaction system, the preparation of LiFePO₄ nanorod arrays is also realized via hydrothermal method with EG and water mixture solvent as reaction medium [36]. More recently, Goodenough et al. [12] develop a novel solvothermal approach, in which EG and ethylenediamine mixture solvent was used as reaction medium, combined with high temperature calcinations to synthesize LiFePO₄ microspheres with an open three-dimensional (3D) porous microstructure in a large scale. These porous LiFePO₄ microspheres show excellent rate capability and cycle stability. Obviously, the EG plays an important role in the morphology-controlled synthesis of LiFePO₄ particle for improving the electrochemical properties mentioned above. During the synthesis processes, EG not only as a stabilizer suppresses the particle growth and the agglomeration but also as a reducer suppresses the oxidation of Fe(II) [12].

Herein we report a facile process for the preparation of the monodispersed LiFePO₄@C core–shell nanostructures in high yields, in which carbon-coating fully covers the highly crystalline monodispersed LiFePO₄ nanopillows with a size of ca. 100 nm in diameter and ca. 100–200 nm in length. As shown in Scheme 1, our strategy includes two steps: one is the synthesis of the monodispersed LiFePO₄ nanopillows via a simple solvothermal reaction route with ethylene glycol as reaction medium solvent under the effect of ascorbic acid as reducer, and the second is the full carbon-coating realized by dispersing the obtained monodispersed LiFePO₄ nanopillows in ascorbic acid solution and following high temperature decomposition after drying. Due to the high perfect crystallization and the full carbon-coating, the monodispersed LiFePO₄@C core–shell nanostructures exhibit nice capacity retention and high rate capacity.