Lithium ferrite (Li_0.5Fe_2.5O₄) nanoparticles as anodes for lithium ion batteries

Abstract

Li_0.5Fe_2.5O₄ nanoparticles of about 80 nm were synthesized through a hydrothermal method, followed by a solid state reaction between LiOH·H₂O and Fe₂O₃. The Li_0.5Fe_2.5O₄ nanoparticles exhibit a remarkable high capacity (up to 1124 mA h g⁻¹), a good cycle stability (650 mA h g⁻¹ after 50 cycles) and excellent coulombic efficiency.

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Introduction

The lithium-ion battery has been one of the most important and widely used rechargeable power sources because of its high energy density, long lifespan, and environmentally friendliness.^1,2 Graphite is currently used as the commercial anode material. However, graphite-based anodes have a low theoretical capacity of around 372 mA h g⁻¹, which cannot satisfy the increasing demand for high energy density lithium-ion batteries.³ Therefore, it is essential to develop new anodes made from low cost and non-toxic electrode materials of higher energy density and better cycling stability.

Lithium ferrite (Li_0.5Fe_2.5O₄) is an important transition metal spinel oxide with the advantage of low cost, environmental friendliness, and easy fabrication.^4,5 It has been extensively studied for various technological applications such as the components of microwave devices and potential cathode materials in lithium batteries.^6–10 Iron-based compounds including LiFeO₂ and Fe_xO_y have high theoretical capacities and are non-toxic, environmentally friendly, and low costs.^11–13 Li_0.5Fe_2.5O₄ materials have not been used as the anode for the Li-ion battery. In this paper, we report the synthesis of Li_0.5Fe_2.5O₄ nanoparticles through hydrothermal and a solid state reaction processes. The Li_0.5Fe_2.5O₄ nanoparticles exhibit good cyclability and high reversible lithium storage capacity.

Experimental

Materials synthesis

The LiFeO₄ nanoparticles were produced by a two-step process involving hydrothermal and subsequent heating treatments. The commercial FeCl₃ (purity 98%), and NH₄H₂PO₄ (purity 98%) were dispersed into water under vigorous magnetic stirring (300 rpm), and then transferred into a Teflon-lined stainless steel autoclave. The autoclave was heated at 220 °C for 1 h. After cooling down to room temperature naturally, the precipitate was collected by centrifugation and washed with distilled water first and then absolute ethyl ethanol for several times. The obtained samples were mixed with LiOH·H₂O (purity 98%) in ethanol to form a homogenous mixture, and dried at the room temperature. Finally, the dried sample was heated at different temperatures for 10 h under the ambient atmosphere.

Materials characterization

The morphology and structure of the powders were determined using a field-emission scanning electron microscope (SEM, FEI NOVA-450) and transmission electron microscope (TEM, PHILIPS TECNAI F30). The crystalline structure of the materials were measured using X-ray diffraction (XRD, BRUKER D8) using Cu K_α radiation at 40 kV and 40 mA with a step size of 0.02° and step time of 5 seconds. Sample surface area was measured using the Brunauer–Emmett–Teller (BET) method with a Quanta Autosorb-iQ2-MP-ANG-VP instrument.

Electrochemical measurements

The electrodes were prepared using a mixture of active material, carbon black (AB), and polyvinylidene fluoride (PVDF) at a weight ratio of 70 : 20 : 10, dispersed in N-methylpyrrolidone (NMP) to form homogenous slurry. The slurry was spread onto a copper foil and dried at 110 °C for 20 h in a vacuum oven. After drying, the electrode foils were pressed and then punched into a circular shape. Electrochemical experiments were carried out using CR2025-type coin cells assembled in an argon-filled glove box. Li metal foil was used as a counter electrode and 1 M solution of LiPF₆ in ethylene carbonate (EC) and dimethyl carbonate (DMC) (1 : 1, v/v) was selected as the electrolyte. The galvanoscope charge–discharge tests were conducted at a current density of 100 mA g⁻¹ and cut-off voltages of 0.01 and 3 V at room temperature using an automatic Land battery instrument. Cyclic voltammetry (CV) was also conducted on the VMP3 electrochemical workstation (BIO-LOGIC SA France) between voltages 0.01 and 3.0 V at a scan rate of 0.1 mV s⁻¹.

Results and discussion

Fig. 1a shows the typical SEM and inserted TEM images of the sample after annealing at 600 °C for 10 h. It can be seen that the powder contains spherical particles and the inserted TEM image confirms nanocrystalline structure of the particles. The size distribution in Fig. 1b shows a diameter around 80 nm, which is consistent with a large surface area of the sample at 19.5 m² g⁻¹.

Fig. 1 SEM and TEM images (a), and particle size distribution (b) of the sample obtained from Fe₂O₃ and LiOH·H₂O after heating at 600 °C.

The XRD pattern of the sample is shown in Fig. 2. The diffraction peaks can be indexed to a crystalline structure of spinel Li_0.5Fe_2.5O₄ (PDF no. 74-1911), which belongs to the space group P4₁32, and no other impurity peaks can be observed.

Fig. 2 XRD pattern of the samples obtained at 600 °C from Fe₂O₃ and LiOH·H₂O.

Fig. 3a shows the CV curves of the Li_0.5Fe_2.5O₄ anode. During the cathodic polarization in the first cycle, a spiky peak was observed at 0.97 V corresponding to the complete reduction of iron from Fe³⁺ to Fe⁰⁺:^14,15

2Li_0.5Fe_2.5O₄ + 15Li⁺ + 15e⁻ → 5Fe⁰ + 8Li₂O (1)

Fig. 3 (a) Cyclic voltammetry (CV) curves, (b) discharge–charge curves, and (c) cycling performance of Li_0.5Fe_2.5O₄ nanoparticles.

On the other hand, in the anodic polarization process, a broad peak was recorded at about 1.05 V corresponding to the oxidation of Fe⁰ to Fe²⁺ and further oxidization to Fe³⁺:^16,17

2Fe⁰ + 2Li₂O ↔ 2Fe^IIO + 4Li⁺ + 4e⁻ (2)

After the second cycle, the CV curves are very stable for the Li_0.5Fe_2.5O₄ nanoparticles electrode indicating enhanced stability during the lithiation and delithiation processes.

Fig. 3b shows the corresponding discharge–charge voltage profiles of Li_0.5Fe_2.5O₄ nanoparticles. During the first discharge, a long plateau appears at about 0.8 V versus Li/Li⁺, indicating a complete reduction of iron from Fe³⁺ to Fe⁰⁺. For the charge curve, a wide slope located at 1.5–2.0 V is observed, corresponding to the oxidation of Fe⁰ to Fe²⁺, with part of the Fe further oxidized to Fe³⁺. The electrochemical behavior is consistent with the results of the CV measurement. The first and second discharge capacity of Li_0.5Fe_2.5O₄ electrode are 1124 mA h g⁻¹ and 746 mA h g⁻¹, respectively. The initial capacity loss of the Li_0.5Fe_2.5O₄ nanoparticle electrode is 33.7% and could result from the formation of a solid electrolyte interface (SEI) on the iron oxide surface during the first lithium insertion process.¹⁸ Fig. 3c shows the cycling performance of the Li_0.5Fe_2.5O₄ anode, indicating a reversible capacity of about 650 mA h g⁻¹ after 50 cycles. The coulombic efficiency of Li_0.5Fe_2.5O₄ nanoparticles is above 96% after 10 cycles.

The typical first discharge–charge curves of the Li_0.5Fe_2.5O₄ nanoparticles electrode at different rates are shown in Fig. 4. The discharge capacity at different current rates, e.g. 100 mA g⁻¹, 200 mA g⁻¹, 500 mA g⁻¹ and 1000 mA g⁻¹, are, 991, 824, 749 and 707 mA h g⁻¹, respectively, indicating a good high-rate performance. The excellent capacity retention may be related to cross linked nanoparticle structure of the materials, which can accommodate the volume change of the Li⁺ insertion/extraction during the charge–discharge processes, and offer a small diameter to enhance lithium diffusion, and yet still provide a limited surface area to prevent excessive side reactions.¹⁹

Fig. 4 Discharge and charge voltage profiles of the Li_0.5Fe_2.5O₄ nanoparticles electrode at different current densities.

Recently, cheap and non-toxic iron-based compounds,^20–22 such as Fe₂O₃, Fe₃O₄ and LiFeO₂, have been used as anodes for lithium ion batteries, showing excellent electrochemical performances in term of specific capacity, cycle performance and rate capability. The capacity of lithium storage is mainly achieved through the reversible conversion reaction between Li⁺ and Li_xFe_yO_z, forming Fe nanocrystals dispersed in Li₂O matrix. For example, Fe₂O₃ nanotubes showed a highly reversible discharge capacity of 950 mA h g⁻¹ in the second cycle and maintained 929 mA h g⁻¹ even after 30 cycles, and excellent rate performance (the 918 mA h g⁻¹ discharge capacities at 0.5 A g⁻¹ current density and 882 mA h g⁻¹ discharge capacities at 1 A g⁻¹ current density).²⁰ The initial discharge capacity of the carbon nanotubes-66.7 wt% Fe₃O₄ nanocomposite electrode is 988 mA h g⁻¹, and its recharge capacity retention after 145 cycles remains 645 mA h g⁻¹.²¹ α-LiFeO₂–C nanocomposite electrode delivered a good reversible capacity and cycle stability (540 mA h g⁻¹ at 848 mA g⁻¹ current density after 200 cycles).¹¹ The obtained results show that Li_0.5Fe_2.5O₄ nanoparticle anode shows excellent electrochemical properties, including a high capacity in the initial discharge (up to 1124 mA h g⁻¹), a high reversible capacity and good cycle stability (650 mA h g⁻¹ after 50 cycles at a current density of 100 mA g⁻¹) and high-rate performance (991 mA h g⁻¹ at a current density of 100 mA g⁻¹, 707 mA h g⁻¹ at a current density of 1 A g⁻¹).

The morphology and electrochemical performance of the lithium ion oxide nanoparticles are sensible to the synthesis conditions. As the SEM images in Fig. 5 show the lithium ion oxide nanocomposites produced at different annealing temperatures have the same particle shape but different sizes. The powder produced at 500 °C has a smaller particle size of around 70 nm in diameter and a large particle size of 100–200 nm for the sample produced at a high temperature of 700 °C.

Fig. 5 SEM images of lithium ion oxide nanocomposites produced at (a) 500 °C; (b) 700 °C.

Fig. 6 shows that the sample produced at 700 °C has the first discharge capacity of 1092 mA h g⁻¹ and a stable capacity of 500 mA h g⁻¹. The nanocomposites obtained at 500 °C demonstrates the first discharge capacity of 1215 mA h g⁻¹ and a low stable capacity of 200 mA h g⁻¹. Therefore, the nanocomposite produced at 600 °C has the best electrochemical performance.

Fig. 6 Cycling performance of lithium ion oxide nanoparticles obtained at 500 °C and 700 °C.

Conclusions

Combined hydrothermal and solid state reaction approach has been used to fabricate novel Li_0.5Fe_2.5O₄ nanoparticles with size of 80 nm at 600 °C. Electrochemical measurements show that Li_0.5Fe_2.5O₄ nanoparticles as anode exhibit a remarkable high capacity in the initial discharge (up to 1124 mA h g⁻¹), a high reversible capacity and good cycle stability (650 mA h g⁻¹ after 50 cycles at a current density of 100 mA g⁻¹) and excellent coulombic efficiency.

Acknowledgements

Financial support from the National Natural Science Foundation of China (Grant no. 51201037), the Beijing Natural Science Foundation (Grant no. 2122020), the National High Technology Research and Development Program (863 Program)(Grant no. 2013AA032002), China Iron & steel Research Institute Group Foundation (Grant no. SHI11AT0540A), and Advance Technology & Materials Co., Ltd Innovation Foundation (Grant no. 2011JA01GYF, Grant no. 2011JA02GYF and Grant no. 2013JA02PYF) is acknowledged.

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