Porous Flower-like α-Fe2O3 As a High Performance Anode for Lithium-Ion Batteries

Since commercializing of the first lithium-ion battery by Sony Corporation in 1991, graphitic carbons have been used as anode materials. Graphitic carbons owe to their favorable characteristics such as eco-friendly, stability upon cycling, low operating potential versus Li/Li⁺, etc. however, carbon materials also poses several challenges that include limited specific capacity  (~ 372 mAh g^-1), formation of a solid-electrolyte interphase (SEI), lithium plating or formation of a surface film at high current rates, etc. To overcome these difficulties, research has been focused on transition metal oxides as alternate anode materials. Various metal oxides (to mention a few Co₃O₄, MnO₂, Fe₃O₄ and Fe₂O₃) have been reported to undergo conversion reactions while exhibiting lower voltages with high reversible capacities than the traditional graphitic carbon counterparts. This has motivated many researchers to study the conversion reaction of oxide materials.¹ Among the transition metal oxides studied, hematite α-Fe₂O₃ has been intensively studied due to its low cost, excellent stability, environmental friendly properties and also providing high theoretical capacity 1007 mAh g^-1 based on 6 Li⁺per formula unit.

One of the challenging issues in conversion reaction of a bulk electrode material is to maintain structural integrity during electrochemical cycling with associated issues such as volume changes, pulverization, etc., which affect the performance.²Congruently, through nanostructured materials such as nanoparticles, nanocubes, nanorods, nanotubes and nanoflowers, we may facilitate an increase in the electrochemical performance due to short diffusion length for Li-ion and electronic transport. Among these, porous materials have been studied widely because of the easy percolation of electrolyte into core of particles which result in high rate capability. Porous materials can also sustain mechanical stress generated by volume expansion/contraction during the electrochemical cycling.

In the present work, we report synthesis of iron alkoxide precursors using  4.4 mmol FeCl₃.6H₂0, 90 mmol urea and 124 mmol tetra butyl ammonium bromide are added to 180 ml of ethylene glycol in 250 ml round bottomed flask, stirred for 10 min to get homogeneous solution and refluxed at 195 ^oC for 30 min. The obtained green precipitate of iron alkoxide is washed with ethanol and dried at 60 ^oC for 12 h. The resultant dried powder is calcined at a range of temperatures from 300 to 500 ^oC for 3 h in air.³ A porous flower-like nanostructured α-Fe₂O₃ is obtained by subsequent calcination of iron alkoxide precursor. The phase purity and morphology of the final product (α-Fe₂O₃) are studied by XRD, SEM and TEM analysis. The microscopic images show (Fig. 1) a flower-like morphology. The BET surface area and its pore size of the α-Fe₂O₃ are measured and they are 64 m² g^-1 and 6 nm, respectively. The α-Fe₂O₃ powder is subjected to galvanostatic charge/discharge experiments between 0.01 and 3.0 V versus Li/Li⁺. The charge-discharge profile is shown in Fig. 2. The discharge capacities obtained for 1^st, 10^th and 20^th cycle are of 1058, 1044 and 917 mAh g^-1 respectively for the sample prepared at 300 ^oC. These values are greater than the values reported so far on α-Fe₂O₃. Porous nature of α-Fe₂O₃ prepared in the present study is responsible for high discharge capacity and also high rate capability. Results of these studies including both physical and electrochemical investigations will be presented.

 

Acknowledgements: SS and TRP acknowledge University Grants Commission (UGC), Government of India for Dr. D. S. Kothari fellowship and Senior Research Fellowship, respectively.

 

References:

1. M. V. Reddy et al,  Adv. Funct. Mater., 2007 17,2792.

2. P. G. Bruce et al, Angew. Chem. Int. Ed., 2008, 47, 2930.

3. S. Shivakumara et al, Electrochem. Lett., 2013, 2, A60.