Graphene-doped carbon/Fe₃O₄ porous nanofibers with hierarchical band construction as high-performance anodes for lithium-ion batteries

Highlights

• GN@C/Fe₃O₄ are synthesized via in-situ electrospinning and thermal treatment.

• GN@C/Fe₃O₄ show unique dark/light banding with a hierarchical porous structure.

• Doped graphene induces a uniform distribution of smaller size Fe₃O₄ nanoparticles.

• Doped graphene provides more active sites and accommodate the volume change.

• GN@C/Fe₃O₄ electrode displays a reversible capacity of 872 mAh/g after 100 cycles.

Abstract

Porous graphene-doped carbon/Fe₃O₄ (GN@C/Fe₃O₄) nanofibers are synthesized via in-situ electrospinning and subsequent thermal treatment for use as lithium-ion battery anode materials. A polyacrylonitrile (PAN)/polymethyl methacrylate (PMMA) solution containing ferric acetylacetone and graphene oxide nanosheets is used as the electrospinning precursor solution. The resulting porous GN@C/Fe₃O₄ nanofibers show unique dark/light banding and a hierarchical porous structure. These nanofibers have a Brunauer–Emmett–Teller (BET) specific surface area of 323.0 m²/g with a total pore volume of 0.337 cm³/g, which is significantly greater than that of a sample without graphene and C/Fe₃O₄ nanofibers. The GN@C/Fe₃O₄ nanofiber electrode displays a reversible capacity of 872 mAh/g at a current density of 100 mA/g after 100 cycles, excellent cycling stability, and superior rate capability (455 mA/g at 5 A/g). The excellent performance of porous GN@C/Fe₃O₄ is attributed to the material’s unique structure, including its striped topography, hierarchical porous structure, and inlaid flexible graphene, which not only provides more accessible active sites for lithium-ion insertion and high-efficiency transport pathways for ions and electrons, but also accommodates the volume change associated with lithium insertion/extraction. Moreover, the zero-valent iron and graphene in the porous nanofibers enhance the conductivity of the electrodes.

Introduction

Lithium-ion batteries have been extensively used for intermittent use of renewable energies and in green energy storage devices, such as electric vehicles and hybrid electric vehicles due to their high energy and power density, good cycle stability, minimal memory effect, and environmental friendliness [1], [2], [3], [4], [5]. Conventional graphite-based anode materials cannot meet the ever-growing demands of high energy density batteries owing to their limited specific capacity (372 mAh/g) [6]. Therefore, the development of new high performance anode materials is highly desirable. Among alternative anode materials, transition metal oxides, such as FeO_x, CoO_x, and SnO_x [7], [8], [9], [10], which possess high theoretical specific capacities and natural abundances, have attracted considerable attention. Magnetite (Fe₃O₄) has been widely investigated owing to its large specific capacity (924 mAh/g), low cost, high abundance in nature, and non-toxicity [11], [12], [13]. However, bare Fe₃O₄ suffers from poor electronic conductivity, poor ion transport kinetics, and severe structural instability owing to the large volume expansion and constriction (93%) that occurs in Fe₃O₄ during discharge/charge cycles, which results in rapid fading of the capacity [14], [15]. In addition, the low conductivity of Fe₃O₄ also inhibits the achievement of high capacity and stable cycle performance at high current densities [16]. Moreover, several reports have indicated that solid-electrolyte interphase (SEI) films may form gradually on the surface of MO_x materials, leading to constant consumption of the active materials [17], [18].

To date, several strategies have been proposed to overcome the aforementioned issues, such as nanostructuring, carbon coating, and nanocompositing [19], [20], [21]. Various nanostructured C/Fe₃O₄ composites, including nanofibers [22], nanotubes [23], nanospheres [24], nanoparticles [25], and nanorods [26], have been designed as anodes to improve the electrochemical performance of lithium-ion batteries. Among these materials, one-dimensional (1D) carbon nanofibers exhibit kinetic properties that are far better than those of particle-like carbon matrixes owing to their orientated electronic and ionic transport paths [27]. Gu et al. prepared C/Fe₃O₄ composite nanofibers by electrospinning and subsequent heat treatment [28]. When used as an LIB anode material, the C/Fe₃O₄ composite nanofiber electrodes exhibited a reversible capacity of 508 mAh/g at a current density of 100 mA/g after 100 cycles. Recently, Qin et al. prepared porous C/Fe₃O₄ nanofibers by electrospinning a solution of polyacrylonitrile (PAN) and polystyrene (PS) containing Fe₃O₄ nanoparticles [29]. The resulting C/Fe₃O₄ electrode displayed an initial reversible capacity of 1015 mAh/g at a current density of 200 mA/g and acceptable rate capabilities (1092, 982, 796, 677, and 523 mAh/g at 100, 200, 500, 1000, and 2000 mA/g, respectively). However, as the agglomeration of Fe₃O₄ nanoparticles in the carbon matrix could not be effectively prevented in these composites, there is insufficient space around the Fe₃O₄ nanoparticles to buffer any changes in volume. Owing to the release of mechanical stress during the lithiation/delithiation process, continuous maintenance of the structural integrity of electrode materials with Fe₃O₄ nanoparticles embedded in irregular compact or porous carbon matrices remains challenging.

Graphene, a monolayer or few layers of sp²-bonded carbon atoms densely packed in a two-dimensional (2D) honeycomb lattice [30], has attracted considerable scientific interest as a potential anode material for lithium-ion batteries owing to its unique chemical and physical properties, such as ultra-thin structure, high specific area, excellent electrical conductivity, and mechanical flexibility [31]. Choi et al. prepared Fe₃O₄-decorated graphene balls for use as anode materials via spray pyrolysis [32]. The Fe₃O₄-decorated graphene balls displayed high initial discharge and charge capacities of 1374 and 974 mAh/g, respectively, at a current density of 2 A/g, and the discharge capacity was 690 mAh/g after 1000 cycles. Dong et al. synthesized structure-tuned Fe₃O₄/graphene composites using a hydrothermal method [33]. The resulting Fe₃O₄/graphene composites delivered a high reversible capacity of 1070 mAh/g after 160 cycles at a current density of 200 mA/g, these composites also exhibited good rate capability and cyclic stability. Crumpled graphene showed high strength and good flexibility, this material effectively buffered structural stress caused by iron oxide volume changes, and also prevented the aggregation of Fe₃O₄ nanoparticles during lithiation/delithiation cycles [34], [35].

In this study, a porous graphene-doped C/Fe₃O₄ nanofiber electrode with a novel structure was designed to improve the electrochemical properties of lithium-ion batteries. The material was easily prepared by electrospinning followed by a thermal treatment. The resulting porous GN@C/Fe₃O₄ nanofibers exhibited a hierarchical structure with well-defined bands formed from graphene sheet-wrapped Fe₃O₄ nanoparticles uniformly embedded in the porous carbon matrix. The fractal porous structure, which consisted of channels between bands and micro/mesopores inside bands, and the flexible graphene provided shorter pathways for lithium-ion transport and effectively accommodated Fe₃O₄ volume changes that occurred during the charge/discharge process. In addition, the electrical conductivity of the electrode materials was significantly improved due to the presence of zero-valent iron and graphene in the matrix of the porous nanofibers. Consequently, the porous graphene-doped C/Fe₃O₄ nanofiber electrode material presented high reversible capacity, excellent cycle stability, and good rate performance, making this material a promising anode for lithium-ion batteries.