Polypyrrole-coated α-LiFeO₂ nanocomposite with enhanced electrochemical properties for lithium-ion batteries

Abstract

A conducting α-LiFeO₂-polypyrrole (α-LiFeO₂-PPy) nanocomposite material was prepared by the chemical polymerization method as a cathode material for lithium-ion batteries. The porous α-LiFeO₂ was prepared via the microwave hydrothermal method and a post-annealing. The X-ray diffraction, Fourier transform infrared spectroscopy, and field emission scanning electron microscopy measurements showed that the α-LiFeO₂ nanoparticles were coated with PPy. The polypyrrole coating improves the reversible capacity and cycling stability (104 mAh g⁻¹ at 0.1C after 100 cycles) for lithium-ion batteries. Even at the high rate of 10C, the electrode showed more than 40% of the capacity at low rate (0.1C).

Introduction

Lithium ferrite (LiFeO₂), a layered cathode material, has attracted much attention because it is non-toxic, environmentally friendly, and low-cost [1], [2]. As is well known, α-NaFeO₂-type LiCoO₂ cathode materials are in widespread use in commercial lithium-ion batteries. Layered LiCoO₂ has a rock-salt structure, where alternate layers of Li and Co occupy the octahedral sites of a cubic close packed oxygen array [3]. LiCoO₂ is more toxic and more expensive than oxides of other transition metals (Mn, Ni, Fe, etc.) [4], [5], [6]. LiFeO₂ has different forms, including the α-, β-, γ-forms, etc. α-LiFeO₂ has a disordered-cation cubic rock-salt structure with space group Fm3m. β-LiFeO₂ with space group C2/c is formed as an intermediate phase during the ordering process. γ-LiFeO₂ with space group I4₁/amd is obtained by reducing the symmetry from cubic to tetragonal by ordering the Li⁺ and Fe³⁺ ions at octahedral sites [1], [2], [3], [6].

α-LiFeO₂ has many advantages as a cathode material for the lithium-ion battery as a substitute for LiCoO₂ in terms of lower price and environmental friendliness. The charging reaction can be written as:

LiFe^IIIO₂ → xLi⁺ + xe⁻ + Li_1−xFe_1−x^IIIFe_x^IVO₂

with x = 1, this reaction provides a theoretical capacity of 282 mAh g⁻¹. Kanno et al. [3], however, reported a maximum value of x = 0.1 for the α-NaFeO₂-type structure. The charged electrodes should contain iron in a mixed oxidation state (III and IV). The first charge voltage plateau is above 4 V, corresponding to the Fe³⁺/Fe⁴⁺ couple reaction, however, large voltage hysteresis is observed during the discharge step. Sakurai et al. reported that unusual Fe⁴⁺ ions generated during charging may play an important role in the occurrence of voltage hysteresis [7]. Kanno et al. also pointed out that the conversion proceeds from the corrugated layered structure LiFeO₂ to an amorphous phase during the first charge, and the charge-discharge process after the second cycle proceeds in the amorphous phase [3]. According to the structural change in α-LiFeO₂ in the charge/discharge process, as determined by X-ray diffraction (XRD) and X-ray diffraction near-edge structure (XANES) spectroscopy, Morales et al. confirmed that Fe²⁺ may exist after the first discharge, and the strong exothermic peak close to 398 K in the differential scanning calorimetry (DSC) curve may result from the reaction of Fe⁴⁺ with electrolyte [8].

The electrical conductivity is extremely low, however, because the iron ions on lithium sites block the lithium diffusion pathways. There are mainly two ways to increase the electrical conductivity: one way is to fabricate nanosized α-LiFeO₂. Nanosized materials have short pathway lengths for lithium ion transport and a large contact area between the electrode and electrolyte for improving the reaction rate at the interface [9]. The other way is to coat a conductive material onto the surface of the α-LiFeO₂ [8]. Using the hydrothermal method, nanosized FeOOH can be prepared in different crystal phases and morphologies [10], [11], [12], [13], [14]. Later, FeOOH can be converted into α-LiFeO₂ via a solid-state reaction. Polypyrrole (PPy) is a popular conducting polymer due to its ability to act as a binder and store electric charge (C_ppy = 72 mAh g⁻¹) [15], [16], [17]. Our group has successfully used PPy to improve the performance of cathode and anode materials in lithium-ion batteries, in such composites as S-PPy, SnO₂-PPy, and LiV₃O₈-PPy [18], [19], [20]. The theoretical capacity of polypyrrole-coated M (M = S, SnO₂, and LiV₃O₈) can be calculated as following according to the theoretical capacity of M and the theoretical capacity of PPy: C_M × wt.% of α-LiFeO₂ + C_PPy × wt.% of PPy. However, the synthesis of polypyrrole-coated α-LiFeO₂ composite for use in lithium-ion batteries has not been explored yet.

In this study, α-LiFeO₂-PPy nanocomposite was synthesized using a chemical polymerization method. The structural characterization and electrochemical performance of the α-LiFeO₂-PPy composite are discussed and compared with the performance of bare α-LiFeO₂ cathode material.