A Carbon/Titanium Vanadium Nitride Composite for Lithium Storage

DOI: 10.1002/cphc.201000537

Guanglei Cui,*[a, b] Lin Gu,[c, d] Arne Thomas,[e] Lijun Fu,[a] Peter A. van Aken,[c] Markus Antonietti,[e] and Joachim Maier*[b]

In recent years, substantial efforts have been made with regard high power and high energy requirements. It was furthermore to the explorations of novel anode materials for lithium batter- reported that incorporation of vanadium in titanium nitride ies with superior recharging-ability and minimal capacity fade. would stabilize the latter one as to a range of applications Among a variety of options, carbonaceous materials are prom- such as electrodes for supercapacitors.[7] To the best of our ising candidates.[1] Graphite is widely used nowadays as knowledge, nearly no literature exists concerning the exploita- anodes in commercial cells,[2] nevertheless its capacity is re- tion of titanium vanadium nitrides as anode materials for lithi- À1 stricted to 372 mAhg (maximum stage by forming LiC6). um batteries. Since nanostructured materials have been report- Therefore, extensive research is currently focusing on the elec- ed to facilitate diffusion of electrolyte and improve mass trans- trochemical performance of carbon-based composites which port properties of lithium ions,[8] nanostructuring together with promise advantages for lithium-storage applications owing to structural and compositional control of metal nitrides becomes their intrinsic characteristics possessing a vast number of crucial to influencing the overall electrochemical performance. active sites and offering presuppositions for a fast electronic Herein, we demonstrate nanostructured composites consisting and ionic transporting network.[3] On the other hand, low cost, of titanium vanadium nitride and carbon to be novel anode high molar density and superior chemical resistance of the materials with excellent performances. transition metal nitrides render them desirable candidates for A facile strategy for preparation of nanostructured metal ni- the next-generation of lithium batteries and supercapacitors.[4] trides/carbon composite has been developed by Markus Anto- Among them, TiN was reported to possess a good electronic nietti’s group.[9] In this work, composites of titanium vanadium conductivity but with minor capacity and hence usually used nitride and carbon were synthesized by a high-temperature as current collector, whereas VN was reported to exhibit a route using mesoporous carbon nitrides as a reactive tem- larger capacity for thin film lithium battery and supercapacitor plate.[10] In order to optimize the performance of the anode, a applications despite poor electronic conductivity.[5] Therefore a few nanostructured composites were synthesized with the composite of titanium–vanadium nitride and carbon (TiVN/C) possibility to tune continuously the ratio of the metallic com- can be expected to deliver the ingredients for efficient trans- ponents. Four ethanolic precursor solutions with different con- port and storage. The concept of establishing an effectively centrations of Ti(OC2H5)4 and VO(OC2H5)3 were prepared re- mixed conducting network formed by three phase composites spectively. For a fair comparison, pure Ti(OC2H5)4 and

(storage material + electronically conducting phase + ionic VO(OC2H5)3 were also treated by the same procedure. In the conductor phase) even on a nanoscale is best demonstrated second reaction phase, as-prepared nanostructured carbon ni- by the hierarchical structure in ref. [6], which matches both tride was used as a reactive template, which was dried and infiltrated into the above mentioned solution. An inherently formed amorphous oxide product was then obtained by ex- [a] Dr. G. Cui, L. Fu Qingdao Institute of Bioenergy and Bioprocess Technology posing the composites to air, and subsequent heating up to Chinese Academy of Sciences 8008C under nitrogen atmosphere. During decomposition of Songling Road 189, Laoshan District , 266101 Qingdao (P. R China) carbon nitride, the metal oxide confined in the porous struc- Fax: (+ 86)532-80662744 ture was converted into nitrides. The as-prepared composites E-mail: [email protected] are denoted hereafter as TiN/C, TiVN/C-1, TiVN/C-2, TIVN/C-3, [b] Dr. G. Cui, Prof. Dr. J. Maier Max Planck Institute for Solid State Research TiVN/C-4 and VN/C, respectively. Combined measurements Heisenbergstasse 1, D-70569 Stuttgart (Germany) using elemental analysis (EA) and inductively coupled plasma Fax: (+ 49)711-6891722 (ICP) was applied to acquire information on elemental compo- E-mail: [email protected] sition of the resulting materials. Composition of different TiVN/ [c] Dr. L. Gu, Dr. P. A. van Aken C compounds determined by ICP and EA are shown in Table 1, Max Planck Institute for Metals Research Heisenbergstraße 3, D-70569 Stuttgart (Germany) where a fair amount of carbon was detected. The average ef- [d] Dr. L. Gu fective electronic d.c. conductivities of these composite were 1 WPI Advanced Institute for Materials Research measured to be 4.12, 0.81, 0. 33, 0.17, 0.15 and 0.03 mScmÀ , Tohoku University for TiN/C, TiVN/C-1, TiVN/C-2, TiVN/C-3, TiVN/C-4 and VN/C, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577 (Japan) respectively. [e] Dr. A. Thomas, Prof. Dr. M. Antonietti X-ray diffraction (XRD) was used to characterize the as-pre- Max-Planck-Institut fr Kolloid- und Grenzflchenforschung Wissenschaftspark Golm, D-14424 Potsdam (Germany) pared composites (Figure 1). The XRD patterns indicate that Supporting information for this article is available on the WWW under the as-prepared products consist of crystalline metal nitrides http://dx.doi.org/10.1002/cphc.201000537. and show absence of crystalline titanium oxide/vanadium

ChemPhysChem 2010, 11, 3219 – 3223 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 3219 shown in Figures 2e and f. The Table 1. Composition of different TiVN/C composites determined by ICP and EA analysis (N,C). For comparison, the initial molar ratios of the metals in the precursor solution are also shown. lattice spacing difference of 1.8% is significant and offers an Sample V/Ti mol ratio V/Ti mol ratio Elemental composition of V/Ti powder by ICP evidence for the formation of a (Solution) (Powder) solid solution (TiÀVÀN com- Ti V N Carbon [11] [wt%] [wt%] [wt%] [wt%] pound). Lithium storage properties of TiN/C pure – 46.23 0 13.53 31.23 TiVN/C-1 1:10 0.09 41.26 4.01 13.17 32.41 TiVN/C-1, TiVN/C-2, TiVN/C-3 and TiVN/C-2 1:3 0.27 30.41 12.01 12.19 32.03 TiVN/C-4 electrodes were investi- TiVN/C-3 1:1 0.98 22.10 23.01 12.79 33.49 gated by galvanostatic dis- TiVN/C-4 3:1 3.03 10.52 33.92 12.39 34.01 charge/charge experiments be- VN/C pure – 0 45.14 12.41 33.45 tween the voltage windows of

Figure 1. XRD patterns using Cu Ka radiation of the TiVN/C composites, TiVN/C-1, TiVN/C-2, TIVN/C-3, TiVN/C-4.

oxide in the composite consistent with ref. [10]. All the composites exhibit diffraction patterns with cubic symmetry. It is worthy to note that all observed peaks do not fit to the stan- dard JCPDS peak positions of VN and TiN. A shift to higher angles in the diffraction patterns was observed with increasing vanadium content. This is indicative of formation of a homoge- neous TiÀVÀN solid solution. In addition, the grain size de- creases consistently with a progressive broadening of the diffraction peaks. The final grain size is estimated to be approxi- mately 5 nm as calculated from the Scherer equation applied to the (111) and (200) peaks of the TiÀVÀN. Figure 2. Transmission electron micrographs revealed the formation of a

Energy-filtered transmission electron microscopy (EFTEM) carbon/TixV1ÀxN solid solution composite. a) A zero-loss-filtered BF electron confirms a similar chemical profile of Ti and V. A zero-loss fil- micrograph with the corresponding elemental distribution shown in (b). tered bright-field (BF) electron micrograph is shown in Fig- c) HRTEM micrograph of the TiVN/C-2 with the corresponding diffractogram shown in (d) reveals the half {111}-plane spacing of 8.32 nmÀ1. e) HRTEM ure 2a. The color-coded image shown in Figure 2b demon- micrograph of the pure TiN with the corresponding diffractogram shown in strates the elemental distribution of carbon (blue), Ti (red) and (f) reveals the half {111}-plane spacing of 8.17 nmÀ1. V (green). Since the overlapping of red and green color gives a yellow contrast, no isolated Ti or V was detected indicating the m formation of a TiÀVÀN solid solution. The solid solution forma- 0.01 V to 3 V in EC/DMC solution containing 1 LiPF6.As tion has been confirmed by comparison of the lattice spacings shown in Figure 3b, the cells were cycled at a rate of between the TiVN/C (taking TiVN-2 for instance) and TiN which 74.4 mAgÀ1 for 20 cycles. Then the current densities were in- was carried out using high-resolution transmission electron mi- creased stepwise to 22.32 AgÀ1. The reversible capacity is croscopy (HRTEM). An HRTEM micrograph and the correspond- around 453, 596, 631 and 678 mAhgÀ1 after a few cycles at ing diffractogram of TiVN/C-2 are shown in Figures 2c and d. 74.4 mAgÀ1, corresponding to TiVN/C-1, TiVN/C-2, TiVN/C-3 The reciprocal half d spacing between {111} planes was mea- and TiVN/C-4 electrode respectively. Highly stable capacities of sured to be 8.32 nmÀ1, smaller than that of pure TiN reciprocal around 47 mAhgÀ1 (TiVN/C-1), 95 mAhgÀ1 (TiVN/C-2) and half {111} spacing of 8.17 nmÀ1. For comparison, the HRTEM mi- 54 mAhgÀ1 (TiVN/C-3) could still be obtained at the highest crograph and the corresponding diffractogram of TiN are current densities of 22.32 A gÀ1, respectively. TiVN/C-4 exhibits

3220 www.chemphyschem.org 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemPhysChem 2010, 11, 3219 – 3223 Figure 3. a) Galvostatic charge and discharge profiles of TiVN/C-2 electrodes at a current density of 74.4 mAgÀ1 between the voltage limits of 0.01 V to m 3 V in EC/DMC solution containing 1 LiPF6. b) Cycling and rate performance of TiVN/C-1, TiVN/C-2, TiVN/C-3 and TiVN/C-4 electrodes cycled in EC/ m DMC solution containing 1 LiPF6. a lithium discharge capacity better than the others at a low rate of 74.4 mAgÀ1. It is quite interesting to note that TiVN/C-2 Figure 4. Multi-cyclic voltamogramm of a) TiN/C, b) TiVN/C-2 and c) VN/C shows an improved rate capacity performance at higher cur- electrodes at a scan rate of 1 mV sÀ1 in a potential window from 0.01–3 V in rent densities. These results indicate that TiN content in the EC/D. composite plays a significant role in improving the electrode rate performance. Therefore, it is reasonable that more efficient electron transportation in the composite is necessary for im- spond to Li extraction from and insertion into the nitride, proving the rate performance. By examining its galvanostatic common for transition-metal-nitride (MN) electrodes as previ- discharge (Li insertion)/charge (Li extraction) curves and multi- ously reported.[14] It is reasonable to assume a decomposition cyclic voltammograms (CVs), it is possible to find evidences for of carbon/metal nitride solid solutions composites into metallic a proper explanation. M particles dispersed into the Li3N matrix, which is analogous Figure 3a shows the respective galvanostatic discharge (Li to the report by Poizot et al. on lithium reacting with transition insertion)/charge (Li extraction) curves of the TiVN/C-2. During metal oxides.[15] In our case, the metallic M particles must be of the first discharge (intercalation), a voltage plateau is observed nanosized dimension, since the size of the original nitride is at about 0.5 V which can be attributed to the decomposition merely about 5 nm, which is a prerequisite of this kind of of the electrolyte and the formation of a solid electrolyte inter- mechanism. Note that the peaks at about 1 V seem to be face (SEI).[12] CV measurements reveal further details of the more reversible than the previously reported nitride. It is electrochemical performance of these composite electrodes. highly possible that carbon coating can be advantageous for CVs of samples TiN/C, VN/C and TiVN/C-2 are displayed in the nitride reaction with Li. This is consistent with the forma- Figure 4. During the first cycle, two reduction peaks at voltages tion of a stable interface for carbon-coated inorganic nano- of about 0.5 V and 0.1 V and one oxidation peaks at a voltage structures as preciously reported.[16] of about 0.1 V was observed in common for the three samples. The redox peaks at about 1.0 V of the TiN/C electrode is ob- The couple of peaks at about 0.1 and 0.2 V corresponds to Li served to exhibit a less ohm resistance than VN/C owing to a intercalation/deintercalation into carbon layers.[13] After the better electronic conductivity. For the composite sample of first cycle, a couple of peaks at around 1.0 V were observed for TiVN/C-2, the peaks at 1.0 V seem to be more indistinct than TiN/C and VN/C samples. These peaks at around 1 V may corre- that of the TiN/C and VN/Cs’, which can be ascribed to the for-

ChemPhysChem 2010, 11, 3219 – 3223 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemphyschem.org 3221 mation of a solid solution. It is shown that the TiVN/C compo- losses of carbon K edges at 280 eV, Ti and V L2,3 edges at 460 eV site has a significant contribution from the double-layer capaci- and 515 eV with the energy selecting window of 8 eV. tance besides Faradaic redox reactions. The better rate perfor- For electrochemical experiments, two-electrode Swagelok-type mance may arise from the electrical double-layer capacitor cells were used. The working electrodes were prepared by mixing contribution formed from non-specific sites of the surfaces ex- the samples with poly(vinyldifluoride) (PVDF) and pasted on pure posed to the electrolyte and also from the interfaces between Cu foil (99.6%, Goodfellow). Glass fiber (GF/D, Whatman) was em- carbon and nitrides. The latter might correspond to an interfa- ployed as a separator and pure lithium foils served as the counter m cial Li storage process similar to that reported for nano-sized electrode. The electrolyte consists of a solution of 1 LiPF6 in eth- ylene carbonate (EC)/dimethyl carbonate (DMC) (1:1 in volume) so- transition-metal oxides at low potential by J. Maier et al.[17] The lution obtained from Ube Industries Ltd. The cells were assembled exact contribution of each mechanism is, however, difficult to in an argon-filled glove box. The electrochemical performance was be quantitatively determined, especially for the nanostructured tested at different current densities in the voltage range of 0.01– material with complex fractal geometry of the nano-sized ag- 3 V on an Arbin MSTAT battery test system. Multi-cyclic voltammo- glomerates, porosity and heterogeneous crystallographic sites. gram measurements were performed with a VoltaLab 80 electro- Correlating the nanostructure and electronic conductivities chemical workstation at a scan rate of 1 mVsÀ1. of the composite, our experimental results are explained as the follows: the produced solid solution, which can be deduced from the HRTEM images, possesses favorable effectively mixed Acknowledements conducting properties owing to rapid lithium diffusion (see Figure 2S in the Supporting Information). Carbon coating in The authors are indebted to the Max Planck Society and ac- the composite is also desirable for reversible lithiation and deli- knowledge support in the framework of EnerChem project of the thiation of nitrides. Consequently, the good rate performance Max Planck Society, Germany. The authors appreciate helpful dis- of the TiVN/C-2 electrode is enhanced as a result from the sig- cussion with Prof. Li-quan Chen and nitride reference samples of- nificantly improved efficiency rooting in a mixed transportation fered by Dr. Haibo Wang of the Chinese Academy of Sciences. G. network, which is partly corroborated by a better electronic Cui acknowledge support by “100 Talents” program of the Chi- conductivity. nese Academy of Sciences and National Natural Science Founda- In summary, our results clearly demonstrate the possibility tion of China (Grant Nos., 20971077) for an improved performance of lithium storage using TiVN/C composite as anode material for lithium batteries. These mate- Keywords: electronic conductivities · lithium batteries · rials are characterized by electronic conductivity, which also nanostructured materials · titanium–vanadium nitride provide favorable lithium diffusion pathways. 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3222 www.chemphyschem.org 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemPhysChem 2010, 11, 3219 – 3223 [14] a) Z.-W. Fu, Y. Wang, X.-L. Yue, S.-L. Zhao, Q.-Z. Qin, J. Phys. Chem. B [17] J. Maier, Nat. Mater. 2005, 4, 805; P. Balaya, H. Li, L. Kienie, J. Maier, Adv. 2004, 108, 2236; b) J. Rowsell, V. Pralong, L. F. Nazar, J. Am. Chem. Soc. Mater. 2006, 18, 1421; J. Jamnik, J. Maier, Phys. Chem. Chem. Phys. 2003, 2001, 123, 8598; c) Y. Takeda, M. Nishijima, M. Yamahata, K. Takeda, N. 5, 5215. Imanishi, O. Yamamoto, Solid State Ionics 2000, 130, 61; d) H. Li, P. Balaya, J. Maier, J. Electrochem. Soc. 2004, 151, A1878. [15] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J. M. Tarascon, Nature 2000, 407, 496. [16] a) W.-M. Zhang, X.-L. Wu, J.-S. Hu, Y.-G. Guo, L.-J. Wan, Adv. Funct. Mater. 2008, 18, 3941; b) L. Zhi, Y. Hu, B. Hamaoui, X. Wang, I. Lieberwirth, U. Received: July 5, 2010 Kolb, J. Maier, K. Mllen, Adv. Mater. 2008, 20, 1727. Published online on September 30, 2010

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