High-rate capability of three-dimensionally ordered macroporous T-Nb2O5 through Li+ intercalation pseudocapacitance
Graphical abstract
Introduction
Lithium-ion batteries (LIBs) are now being extensively applied and developed with the rapid advances of portable electronic devices. Beyond that, LIBs offer a more fascinating option for large-scale applications in the electric vehicles (EVs) and hybrid electrical vehicles (HEVs), due to the advantages of high energy & power density and long cycle life [1], [2], [3]. However, the performance of commercial LIB is seriously limited by the graphite anode. Specifically, low Li+ diffusion coefficient of graphite greatly reduces its high-rate performance, and the low operating potential (approximately 0.1 V vs. Li+/Li) probably leads to lithium dendrites formation, causing serious internal short circuit and safety concerns [4], [5], [6]. To address these above problems, a feasible strategy is to develop alternative anode material with rapid Li+ insertion/extraction and safe operating potentials.
Over the past decade, transition metal oxides, regarded as promising anode materials, have been widely studied due to their various and high oxidation states, such as MnO2, Co3O4, Nb2O5 and Li4Ti5O12 [7], [8], [9]. Among these oxides, Li4Ti5O12 and Nb2O5 have attracted tremendous research attention because of their excellent rate capability and cycle stability [8], [10]. Beyond that, the considerable safety advantages, deriving from that their high redox potentials (›1.0 V) well match to the LUMO of the liquid-carbonate organic electrolyte, ensure their promising safe application in the EVs and HEVs [11], [12]. Compared with Li4Ti5O12, which generally presents specific capacity of 160–170 mA h g−1 with operating potential of 1.55 V vs. Li+/Li, Nb2O5 exhibits a higher theoretical capacity of 200 mA h g−1 and lower working potential mainly ranging from 1.0 to 1.5 V [13], and therefore supplies higher energy density for the full battery systems. More importantly, characteristic of voltage-time profiles of Nb2O5 is more beneficial to the state of charge (SOC) estimation, in good favour of the survey of open circuit voltage of full batteries when combined with a cathode [14]. The advantage can be attributed to the slow slope in charge/discharge curves of Nb2O5, other than flat voltage followed by an abrupt rise/drop of Li4Ti5O12. Hence, preferential application of Nb2O5 as high-power anode material is highly anticipated.
Generally, Nb2O5 exists in several crystal structures determined by annealing temperature, including pseudo-hexagonal (TT-Nb2O5, 500 °C), orthorhombic (T-Nb2O5, 600–800 °C), tetragonal (M-Nb2O5, 1000 °C) and monoclinic (H-Nb2O5, 1100 °C) [15]. Previous studies demonstrated that the T-Nb2O5 exhibited high capacity and excellent rate performance among these phases due to the advantageous structure [16], [17], [18]. In particular, layer structure of T-Nb2O5 offered open channels between interconnected sheets composed of NbOx polyhedral [19], [20], and Li+ intercalation occurred along the preferred pathways of (001) plane, causing decreased energy barriers and enhanced charge transfer [13], [21]. More recently, Dunn's group found a prominent pseudocapacitive Li+ intercalation mechanism in T-Nb2O5 [22], [23], [24], which played very important role in the rapid lithium diffusion and storage. Therefore, further understanding on the pseudocapacitive lithium storage in T-Nb2O5 and its application as a high-efficiency host material are desired.
Pseudocapacitive charge storage, based on faradaic charge-transfer reactions, typically occurs at the surface or near-surface of active materials [25]. Hence, design and construction of porous materials with high surface area is a good strategy to full play the advantages of pseudocapacitive effects in T-Nb2O5. 3DOM architecture is an ideal configuring to effectively facilitate pseudocapacitive Li+ intercalation process, due to the high surface-area-to-volume ratio with nanoscale walls and high active surface area [26]. Besides, the rough and hierarchical 3D porous structure can supply large amount interstitial sites for Li+ intercalation, significantly improving the lithium storage capacity.
In this work, we firstly report the facile synthesis of 3DOM T-Nb2O5 anode material. Superior electrochemical performance with high rate capability of 106 and 77 mA h g−1 at the rate of 20 C and 50 C were achieved. Even more, significant pseudocapacitive effects were found and corresponding contribution were quantitatively calculated. This work demonstrated a dominant contribution of pseudocapacitive effect in the high charge/discharge rates, which resulted to excellent rate capability and cycle stability, indicating that T-Nb2O5 could be a promising candidate for anode material of high-power LIBs.
Section snippets
Experimental
Preparation of 3DOM T-Nb2O5: Monodispersed polystyrene (PS) dispersions were obtained employing a typical emulsifier-free emulsion polymerization technology. The PS microspheres were close-packed into colloidal crystals templates after natural drying process at 50 °C for 24 h. Precursor solutions were obtained by dissolving a desired amount NbCl5 into an absolute ethyl alcohol solvent at room temperature. Subsequently, the PS colloidal crystals templates were immersed in the precursor solution
Results and discussion
The synthesis schematic of 3DOM T-Nb2O5 (Fig. 1a) can be briefly described in three steps. Firstly, the PS microspheres formed close-packed templates via a self-assembled process (Fig. S1). Then, the obtained templates were immersed in NbCl5/ethanol solution, where the precursor solution will permeate into the interstice of the self-assembly PS microspheres templates spontaneously via a capillary attraction. After a standing process of 12 h, excessive NbCl5/ethanol solution was removed via
Conclusions
In summary, 3DOM T-Nb2O5 with hierarchical mesoporous structure were prepared by a facile approach employing PS microspheres as templates. Depending on the unique and rational structure, the 3DOM T-Nb2O5 anode presented a high reversible capacity of 210 mA h g−1, and even 106 and 77 mA h g−1 were achieved at the charge/discharge rates of 20 C and 50 C, respectively. Furthermore, it maintained a high capacity retention of 89.9% at a rate of 10 C after 100 cycles. The outstanding electrochemical
Acknowledgements
This work was supported by the National Natural Science Foundation of China (No. 51472065). We also thank the financial support by Ph. D Student Foreign Visiting Research Project Funding of Harbin Institute of Technology (HIT). G. Y. and S. L. would like to thank Prof. Xueliang (Andy) Sun for guidance and discussion, and the academic support by the Nanomaterials and Energy Group at University of Western Ontario.
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These authors contributed equally to this work.