Enhanced Electrochemical Performance of Maghemite/Graphene Nanosheets Composite as Electrode in Half and Full Li–Ion Cells

Highlights

• First report on the ability of γ-Fe₂O₃/graphene composite as anode in full LIBs.

• The composite delivers a higher capacity than that of pure iron oxide.

• The composite exhibits a good rate capability.

• Beneficial effect of prelithiation to mitigate the inherent irreversible capacity.

Abstract

A composite consisting of iron oxide/graphene nanosheets (GNS) is synthesized by hydrothermal treatment of a mixture of graphitic oxide and FeCl₂ in the presence of N₂H₄ as reducing agent. Special attention is given to the characterization of the iron oxide owing to the disparity of criteria found in literature to distinguish between maghemite (γ-Fe₂O₃) and magnetite (Fe₃O₄). Here, the oxide is accurately characterized as γ-Fe₂O₃. The beneficial effect of the simultaneous formation of GNS and γ-Fe₂O₃ is reflected in the capacity of the cell, which is much higher than those of the individual components. This may be the result of a synergistic effect of the good conducting properties of graphene, its buffering action on the volume changes undergone by γ-Fe₂O₃ on reacting with Li and the high capacity of this oxide. The composite also has good rate capability and can recover its capacity at low current intensities after prolonged cycling at high rates. The composite performs very well in a full cell configuration with LiFePO₄ as cathode, where it exhibits an average discharge capacity of 122 mAh g⁻¹ and excellent capacity retention over 50 cycles. In addition, the cell has good rate capability at current densities as high as 5C.

Introduction

Although most commercial Li-ion batteries use graphite as anode, a variety of materials have been investigated in order to increase the cell capacity, and therefore its specific energy. Some of these materials are transition metal oxides, which are able to store more Li per gram than graphite, thus giving rise to higher specific capacities. Special attention in this respect has been given to Fe, Co and Ni based oxides [1], [2], [3], [4]. Ni and especially Co are expensive and toxic, so the practical interest of the former element is questionable while iron oxides are not subject to these problems, [5] so they may be effective alternatives. An important drawback of their cycling behavior is the volume changes undergone upon discharging and charging that results in the disintegration and disconnection of the active material from the current collector, which is the origin of the capacity fading [6], [7].

This shortcoming is very often circumvented by using carbon-based composites. In addition to buffering expansions and shrinkages, carbon has conductive properties that enhance the mobility of the charge carriers. In the last few years, graphene has become one of the most widely studied materials for Li-ion battery electrodes. Interest in this material has been aroused by two outstanding properties, namely: its high specific capacity (744 mAh g¹, which is twice that of graphite) and its high electronic conductivity, up to 10² S m⁻¹ for GNS, depending on the preparation method used [8]. In fact, graphene has been the subject of much research intended to improve the performance of these electrochemical devices [9]. GNS have also been used as additives to improve the performance of other electrochemically active materials (particularly tin, nickel and cobalt oxides) [10].

Magnetite (Fe₃O₄) is the most widely iron oxide studied in composites with GNS as anode for Li-ion batteries. Unfortunately, the electrochemical response of Fe₃O₄/GNS composites differs widely as regards to performance in Li cells. For example, while Behera [11] obtained specific capacities, after 100 cycles, as high as 1200 mAh g⁻¹ at a rate of 2000 mA g⁻¹–really very high for this type electrode–, Lian et al. [6] with current density of 50 mA g⁻¹, and Huang et al. [12] at a rate of 500 mA g⁻¹, obtained capacity values of 50% of that at the 40^th cycle. Hematite (α-Fe₂O₃) has also been studied but at a lower extent and with similar performances as magnetite [13], [14], [15]. There is little information on the electrochemical performance of maghemite (γ-Fe₂O₃) as anode material, and even less in GNS composites. We only know a single article on this composite recently published [16]. This contrasts with the abundant literature available for magnetite [6], [7], [11], [12], [17], [18], [19], [20], [21], [22], [23], [24] and hematite [13], [14], [15] based composites. The confusion between magnetite and maghemite might be one of the reasons of this circumstance.

Besides the absence of studies on maghemite/GNS composites, there is the problem of the lack of an accurate knowledge of the actual nature of the iron oxide. So far, most of reports on iron oxide/GNS composites have assumed the magnetite to be the iron oxide phase present in the composite, occasionally on the grounds of an arguable interpretation of the characterization results. Magnetite (Fe₃O₄) and maghemite (γ-Fe₂O₃) are two structurally related phases that are difficult to differentiate by X-ray diffraction and Raman spectroscopy, which are commonly used to characterize these composites [7], [12], [17], [18], [19], [20], [21], [22], [23], [24]. We accurately identified the phase formed as γ-Fe₂O₃ rather than Fe₃O₄ in spite of the experimental conditions used in the composite synthesis.

As stated above, most studies on the electrochemical properties of graphene based composites in Li batteries have been carried out in half cells (versus a Li metal electrode). In fact, very few papers have addressed their use in full Li-ion batteries [22], [25]. In this work, we examined Li storage properties in a full cell configuration using LiFePO₄ (LFPO) as cathode. The outstanding performance of this composite in both half and full cells opens promising prospects for a new generation of Li-ion batteries exploiting the synergistic effects of the electrochemical properties of the two components, which lead to improved reversible Li storage.