Amorphous Vanadium Oxide Coating on Graphene by Atomic Layer Deposition for Stable High Energy Cite This: Chem

ChemComm View Article Online COMMUNICATION View Journal | View Issue Amorphous vanadium oxide coating on graphene by atomic layer deposition for stable high energy Cite this: Chem. Commun., 2014, 50, 10703 lithium ion anodes† Received 16th June 2014, a b c a a a Accepted 23rd July 2014 Xiang Sun, Changgong Zhou, Ming Xie,* Tao Hu, Hongtao Sun, Guoqing Xin, Gongkai Wang,a Steven M. Georgec and Jie Lian*a DOI: 10.1039/c4cc04580g www.rsc.org/chemcomm Uniform amorphous vanadium oxide films were coated on graphene above 1.5 V (vs. Li/Li+). It is well known that intercalation via atomic layer deposition and the nano-composite displays an below 2 V may cause the structure instability7 and vanadium exceptional capacity of B900 mA h gÀ1 at 200 mAgÀ1 with an dissolution.8 Further lowering the potential below 1.5 V will excellent capacity retention at 1 A gÀ1 after 200 cycles. The capacity introduce decomposition and conversion reactions.6 The dramatic contribution (1161 mA h gÀ1) from vanadium oxide only almost volume change during charge–discharge will also cause severe reaches its theoretical value. pulverization and particle agglomeration. The structural instability and vanadium dissolution eventually lead to significant capacity Vanadium pentoxide (V2O5) is a well-studied material for failure of V2O5 as anodes. Vanadium dissolution is also a major lithium cathode application due to its high specific capacity, drawback for cathode application. 1,2 natural abundance, and relatively low cost. V2O5 possesses In this study, we deposited vanadium oxide on graphene multiple redox states similar to other transition metal oxides. If nanosheets by atom layer deposition (ALD). The vanadium oxide- 5+ 0 a full reduction of V to V is possible, V2O5 has a theoretical based graphene composite anode shows a reversible capacity capacity of 1472 mA h gÀ1, the highest capacity among all of the close to 900 mA h gÀ1 at 200 mA gÀ1 and is stable for at least À1 metal oxides. Therefore, V2O5 canalsobeanidealmaterialfor 200 cycles at a greater current density of 1 Ag ,suggestingno high energy anodes. However, limited data are available for V2O5 significant capacity loss due to vanadium dissolution. The capacity anodes in which the potential high capacity cannot be realized contribution from V2O5 only almost approaches its theoretical limit. with a stable cycling performance.3–6 For example, Liu et al. We have previously demonstrated that ALD is a viable tool to form Published on 24 July 2014. Downloaded by University of Colorado at Boulder 27/09/2014 15:50:56. reported double-shelled V2O5–SnO2 nano-capsules which exhibit metaloxidesoncarbonaceoussubstratesforelectrochemicalappli- a reversible capacity of 600 mA h gÀ1 at 250 mA gÀ1 after 50 cycles.3 cations.9,10 The strong chemical linkage between carbonaceous Poizot reported crystalline V2O5 with an initial discharge of substrates and active materials ensures the long cycling stability. À1 4 675 mA h g which fades completely after 40 cycles. V2O5 In addition, ultrathin films or nano-sized particles formed by ALD nanorods displayed 450 mA h gÀ1 at 30 mA gÀ1 after only 20 cycles.5 will greatly alleviate volume changes compared to their bulk counter- A recent study on vanadium oxide aerogels shows a high capacity of parts. Nano-size can further shorten the electron transport and 1000 mA h gÀ1 at a rate of 118 mA gÀ1 for only 30 cycles.6 lithium diﬀusion paths, leading to an enhanced power density. The storage of lithium ions in V2O5 starts with a reversible This result further highlights the significance of using ALD to intercalation–deintercalation reaction at a potential usually synthesize high energy density electrode materials. The V2O5 ALD was carried out in a static rotary reactor 10 as reported previously. V2O5 ALD used vanadyl oxytriisoprop- a Department of Mechanical, Aerospace & Nuclear Engineering, 1 oxide with H2O at 150 1C with a growth rate of B1 Å per cycle. Rensselaer Polytechnic Institute, 110 8th Street, Troy, NY 12180, USA. The higher growth rate in a rotary reactor than in a typical ALD E-mail: [email protected]; Fax: +1-518-276-6025; Tel: +1-518-276-6081 b Natural Science Department, Lawrence Technological University, Southfield, process is due to the insuﬃcient purge of H2O on high surface 11 MI 48075, USA area powders, resulting in additional CVD growth. After ALD c Department of Chemistry and Biochemistry and Department of Chemical and deposition, the density of the V2O5–G nano-composite is much Biological Engineering, University of Colorado at Boulder, Boulder, higher than that of the pristine graphene powder (Fig. S1, ESI†). Colorado 80309, USA. E-mail: [email protected] A better volumetric energy density is expected for the V2O5–G † Electronic supplementary information (ESI) available: Detailed experimental section, optical pictures, TGA data, discharge capacity of graphene, detailed nanocomposites due to the enhanced density. Thermogravi- calculation of V2O5 contribution, and the SEM image of V2O5–G after cycling. metric analysis (TGA) indicates that V2O5–G contains 59.2 wt% See DOI: 10.1039/c4cc04580g V2O5 with as few as 50 cycle V2O5 ALD (Fig. S2, ESI†). This is This journal is © The Royal Society of Chemistry 2014 Chem. Commun., 2014, 50, 10703--10706 | 10703 View Article Online Communication ChemComm Fig. 1 (a) XRD patterns of as-synthesized and annealed V2O5–G; and SEM images of (b) V2O5–G, (c) V2O5–G annealed at 200 1C, (d) V2O5–G annealed at 330 1C. À1 Fig. 2 (a) Cyclic voltammetry of V2O5–G at a scan rate of 0.5 mV s ; À1 (b) voltage profile of V2O5–G at a current density of 200 mA g ; (c) charge and discharge capacity of V2O5–G and annealed V2O5–G versus cycle because the rotary reactor allows conformal coating along the À1 number at a current density of 200 mA g ; (d) rate performance of V2O5–G. 3-D structure of the graphene matrix and high precursor utilization compared to viscous flow-type ALD, which usually requires hundreds of cycles to achieve similar active material two pairs of redox peaks at 0.49/0.82 V and 0.93/1.3 V started to mass loadings.1 From the derivative weight change, the weight emerge. These peaks were eventually stabilized and became the loss occurred at 80, 300, and 430 1C, which correspond to major contributors to the charge storage. A similar behaviour the loss of physisorbed water, hydrated water, and burning can be observed from the voltage profile (Fig. 2b), where the of graphene, respectively. In order to study the effect of crystal- voltage plateaus at above 1.5 V gradually disappeared. It is well linity and morphology on electrochemical performance, we known that the lithium storage occurs through intercalation annealed V2O5–G in air at different temperatures. As observed above 1.5 V while decomposition and conversion reactions from X-ray diffraction (XRD) patterns in Fig. 1(a), the un-annealed dominate at below 1.5 V. A recent X-ray photoelectron spectro- 6 50 ALD cycle V2O5–G shows only a broad graphene diffraction scopy (XPS) study has revealed that the capacity storage feature with 2y at 24 degree.10 No diffraction peaks from crystal- in vanadium oxide aerogel anodes involves decomposition line V2O5 are observed, indicating the amorphous nature of followed by conversion. Therefore, we conclude that the V2O5 in the as-deposited V2O5. After annealing at 200 1C for 1 h, extra our study first undergoes the decomposition reaction forming the broadened diffraction peaks can be observed. Upon further VOx phase, and then follows the conversion reaction to V2O increasing the annealing temperature to 330 1C, the XRD pattern and Li2O. These two reactions were found to be highly reversible. Published on 24 July 2014. Downloaded by University of Colorado at Boulder 27/09/2014 15:50:56. shows intense peaks which can be assigned to the orthorhombic The formation of V2O at the end of the discharge cycle is due to the 1+ 0 V2O5 (JCPDS No. 01-0539). improbability of the V to V ,whichwasalsoconfirmedbythe 6 The pristine graphene shows a wrinkled honeycomb micro- recent XPS study. Thus, the practical theoretical capacity of V2O5 is structure consisting of thin layered nano-platelets (Fig. S3, 1178 mA h gÀ1, corresponding to an 8-electron redox reaction. ESI†). No agglomerated particles can be seen after deposition, The significant irreversible capacity loss during the first indicating that the V2O5 thin film is conformably coated on the charge–discharge cycle can be attributed to the formation of SEI. surface of graphene. This is a unique advantage of ALD over It is also interesting to note that the discharge capacity remains wet chemistry-based synthesis by which particle agglomeration almost constant or becomes greater after the first cycle, although is usually observed. Post-annealing at 200 1C does not lead to the capacity contribution eventually changes. The vanadium obvious morphological changes (Fig. 1c). involved in the intercalation reactions at the beginning may The electrochemical properties of the V2O5–G anodes gradually evolve into the decomposition and conversion reac- were first examined by cyclic voltammetry at a sweep rate of tions, and the capacity stabilizes after B10 cycles. The long- À1 0.5 mV s and a voltage range from 0.01 to 3 V. As shown in term cycle stability of the amorphous and annealed V2O5–G is Fig. 2a, in the first discharge cycle, four reduction peaks at 2.78, compared in Fig. 2c. The as-synthesized V2O5–G showed 2.48, 2.1, and 1.54 V can be related to the multi-step lithium slightly decreased capacity in the first five cycles and then the intercalation of V2O5, corresponding to the formation of different capacity kept increasing for the rest of the cycles.

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