Enhanced lithium storage in a VO₂(B)-multiwall carbon nanotube microsheet composite prepared via an in situ hydrothermal process

Abstract

A novel VO₂(B)-multiwall carbon nanotube (MWCNT) composite with a sheet-like morphology was synthesized by a simple in situ hydrothermal process. The morphology and structural properties of the samples were investigated by X-ray diffraction (XRD), thermogravimetric analysis (TGA), field emission scanning electron microscopy (FE-SEM), and transmission electron microscopy (TEM). FE-SEM observations demonstrated that the nanosheets are frequently grown together in the form of bundles composed of numerous nanosheets, each with a smooth surface and a typical length of 300–500 nm, width of 50–150 nm, and thickness of 10–50 nm. Electrochemical measurements were carried out using different discharge cut-off voltages. Electrochemical tests show that the VO₂(B)–MWCNT composite cathode features long-term cycling stability and high discharge capacity (177 mAh g⁻¹) in the voltage range of 2.0–3.25 V at 1 C with a capacity retention of 92% after 100 cycles. The electrochemical impedance spectra (EIS) indicate that the VO₂(B)–MWCNT composite electrode has very low charge-transfer resistance compared with pure VO₂(B), indicating the enhanced ionic conductivity of the VO₂(B)–MWCNT composite. The enhanced cycling stability is attributed to the fact that the VO₂(B)–MWCNT composite can prevent the aggregation of active materials, accommodate the large volume variation, and maintain good electronic contact. We strongly believe that the VO₂(B)–MWCNT composite can be considered as a potential cathode material for lithium-ion batteries.

Introduction

Cathodes are very indispensable and a key part of lithium-ion batteries (LIBs), and great research efforts have been devoted to cathode materials in order to decrease costs and to address safety issues [1]. Among all the cathode materials, the most widely studied systems include compounds with three-dimensional (LiMn₂O₄, LiFePO₄) or layered (LiMO₂ with M = Co, Ni) structures which exhibit good topotactic insertion/deinsertion properties [2]. LiCoO₂ is the most popular cathode materials, owing to the convenience and simplicity of preparation [3]. The Li_xCoO₂ system has been studied extensively [4], [5] and exhibits excellent cyclability at room temperature for 1 > x > 0.5. The specific capacity of the material is limited to the range of 137–140 mAh g⁻¹, although the theoretical capacity of LiCoO₂ is 273 mAh g⁻¹ [6]. Li_xCoO₂ is also very expensive and highly toxic, which is unfortunate, considering its good electrochemical properties and easy synthesis. In an attempt to develop alternative cathodes, layered LiNiO₂, spinel LiMn₂O₄, and olivine LiFePO₄ have become attractive, since Ni, Mn, and Fe are less expensive and less toxic than Co. However, LiNiO₂ suffers from structural changes, thermal runaway, and difficulties in synthesizing it as an ordered material with all Ni³⁺ [7], [8], [9], [10]. Furthermore, LiMn₂O₄ suffers from severe capacity fade at elevated temperatures, and several factors, such as manganese dissolution, Jahn–Teller distortion, loss of crystallinity, and development of microstrain during cycling have been reported to be responsible for the capacity fade [11], [12], [13]. Also, LiMn₂O₄ has a limited capacity of <120 mAh g⁻¹, which is less than that of LiCoO₂ (140 mAh g⁻¹). Olivine-like LiFePO₄ appears to be an interesting positive electrode material for Li-ion batteries because of its low toxicity, low cost, long cycle life, and cell safety. However, a major limitation of this material, which prevents it from being used in large-scale applications, is its poor high-rate performance, owing to its low electronic conductivity and low ionic diffusion coefficient [14], [15]. For Li-ion intercalation applications, vanadium oxides are an attractive alternative, as vanadium is known to exist in a wide range of oxidation states from +2, as in VO, to +5, as in V₂O₅, and the vanadium oxides have the potential to offer much higher capacities along with the essential advantages of low cost, abundant source material, and easy synthesis. Over the decades, vanadium oxides have attracted special interest because of their outstanding structural flexibility combined with their interesting chemical and physical properties for catalytic and electrochemical applications [16]. Among the various known vanadium oxides, metastable oxides such as VO₂(B), H₂V₃O₈, V₂O₅-δ, V₂O₅, and LiV₃O₈ have been found to show interesting cathode properties in lithium cells [17], [18], [19], [20]. VO₂ exhibits four different polymorphic structures, including the most stable VO₂(R) with rutile structure, the monoclinic VO₂(R) with a slightly distorted rutile structure, the tetragonal structure of VO₂(A), and the metastable VO₂(B) with monoclinic structure [21]. VO₂(B), in particular, with its metastable monoclinic structure, is a promising cathode material for both organic and aqueous lithium-ion batteries [22], [23], [24]. The crystal structure of VO₂(B) consists of sheets of edge sharing VO₆ octahedra linked by corner sharing to adjacent sheets along the c-direction of the unit cell [25]. This sharing structure is related to the structural stability and the consequent resistance to lattice shearing during cycling in the lithium-ion battery [26]. Recently, considerable efforts have been made toward the preparation of VO₂(B) nanocrystals. Several nanostructured VO₂(B) materials, including nanowires, nanobelts, nanorods, nanoneedles, nanoribbons, and urchin-like morphologies have been obtained [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38]. However, its poor cycling lifetime (usually less than 20 cycles) greatly limits practical applications. The capacity fading of nanoscale VO₂(B) is probably due to vanadium dissolution as in other polymorphs below a discharge voltage of 2 V [39], [40] and also the nanosized forms of VO₂(B), which have large specific surface areas and high surface energies, making it easier to form the agglomerated state during the electrochemical cycling, thus increasing the charge transfer resistance [41], [42].

Recently, material scientists have shown that three-dimensional (3D), hierarchical, micro-/nano-structures, such as VO₂(B) microflowers [43] and VO₂(B) hollow microspheres [42], exhibit improved electrochemical properties because they not only inherit the properties of the nano-units, but also have collective properties from the self-assembly of nano-units into microscale structures. From this point of view, microscale structures (composed of nano-units) could be the best choice of material, rather than single nano-units. In addition, coating the surface of the electrode material with carbon has also been proven to be an effective way to increase the cycling stability [44], [45], [46]. The detailed mechanism responsible for this improvement remains unclear; one possible explanation is the enhanced electronic conductivity resulting from the carbon layer [47].

Inspired by this concept, we have synthesized a novel VO₂(B)–multiwall carbon nanotube (MWCNT) microsheet composite. The controlled synthesis of VO₂(B) is relatively difficult because vanadium is known to exist in a wide range of oxidation states from +2 to +5, and VO₂(B) tends to be transformed (>300 °C) to thermodynamically more stable rutile VO₂ [26], which is not usable as a cathode material for the lithium-ion battery. Electrode using carboxymethyl cellulose (CMC) binder is also being reported for the first time for this material. The composite microstructure exhibits high lithium storage properties and provides good electronic contact owing to the good mechanical properties and high conductivity provided by MWCNTs. Furthermore, the electrochemical measurements demonstrate that the VO₂(B)–MWCNT microsheet composite can be used as an alternative cathode material in lithium-ion batteries with high capacity, good cycling stability, and high-rate capability.