New Anode Material for Lithium-Ion Batteries: Aluminum Niobate

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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New Anode Material for Lithium-Ion Batteries: Aluminum Niobate (AlNb11O29) Xiaoming Lou,†,‡,∥ Renjie Li,†,‡,∥ Xiangzhen Zhu,‡ Lijie Luo,‡ Yongjun Chen,‡ Chunfu Lin,*,†,‡ Hongliang Li,† and X. S. Zhao*,†,§ †

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Institute of Materials for Energy and Environment, School of Materials Science and Engineering, Qingdao University, Qingdao 266071, China ‡ State Key Laboratory of Marine Resource Utilization in South China Sea, College of Materials and Chemical Engineering, Hainan University, Haikou 570228, China § School of Chemical Engineering, The University of Queensland, St Lucia, Brisbane, Queensland 4072, Australia S Supporting Information *

ABSTRACT: This paper describes the syntheses and electrochemical properties of a new niobate compound, aluminum niobate (AlNb11O29), for Li+ storage. AlNb11O29-microsized particles and nanowires were synthesized based on the solidstate reaction and solvothermal methods, respectively. In situ X-ray diffraction results confirmed the intercalating mechanism of Li+ in AlNb11O29 and revealed its high structural stability against cycling. The AlNb11O29 nanowires with a novel bamboo-like morphology afforded a large interfacial area and short charge transport pathways, thus leading to the observed excellent electrochemical properties, including high reversible Li+-storage capacity (266 mA h g−1), safe operating potential (around 1.68 V), and high initial Coulombic efficiency (93.3%) at 0.1 C. At a very high rate (10 C), the AlNb11O29 nanowires still exhibited a capacity as high as 192 mA h g−1, indicating their good rate capability. In addition, at 10 C, 96.3% capacity was retained over 500 cycles, indicating superior cycling stability. A full cell fabricated with AlNb11O29 nanowires as the anode and LiNi0.5Mn1.5O4 microparticles as the cathode delivered a high energy density of 390 W h kg−1 at 0.1 C. This work suggests that the AlNb11O29 nanowires hold a great promise for the development of high-performance lithium-ion batteries for large-scale energy-storage applications. KEYWORDS: niobates, aluminum niobate, AlNb11O29 nanowires, lithium-ion batteries

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Nb4+/Nb3+ range from 0.8 to 2.0 V, thus ensuring safe operations of LIBs. In addition, Ti2Nb10O29 has a shear ReO3type crystal structure built by blocks consisting of 3 × 4 × ∞ corner- and edge-sharing octahedra. The edge-sharing between octahedra greatly stabilizes the structure against charge/ discharge. Furthermore, Ti2Nb10O29 stores charges mainly via an intercalation-pseudocapacitive mechanism, significantly enhancing its charge-storage capacity.53,54 However, pristine Ti2Nb10O29 suffers from poor rate capability owing to its low-charge conductivity. Hence, it is important to develop new niobate compounds having improved electrochemical properties. Here, we designed and synthesized a new niobate anode material, namely, aluminum niobate (AlNb11O29). AlNb11O29 possesses a Ti2Nb10O29-type crystal structure but a significantly higher Li+ conductivity. Additionally, AlNb11O29 can have an improved cycling stability because of the ultrahigh Al−O bond energy (511 kJ mol−1).57

INTRODUCTION Nowadays, lithium-ion batteries (LIBs) are used for large-scale energy-storage applications, such as solar cells and electric vehicles.1−8 Graphite with a theoretical capacity of 372 mA h g−1 is the common anode material for LIBs. However, its low operating potential (around 0.1 V) unavoidably leads to the formation of lithium dendrites during charge/discharge, bringing in safety concerns, particularly for high-power applications.9−13 Recently, niobates have been shown to be promising anode materials because they possess not only high theoretical lithium-ion-storage capacity (in the range between 374 and 403 mA h g−1) but also a safe operating potential (>0.8 V).14−30 In the niobate family, Ti−Nb−O oxides, such as Ti2Nb10O29 and TiNb2O7, have been shown to exhibit interesting electrochemical properties.31−56 For example, Ti2Nb10O29 possesses a theoretical lithium-ion storage capacity of 396 mA h g−1, involving charge-transfer mechanisms of both one-electron transfer (Ti4+ ↔ Ti3+) per titanium and twoelectron transfer (Nb5+ ↔ Nb3+) per niobium. The operating potentials of the redox couples of Ti4+/Ti3+, Nb5+/Nb4+, and © XXXX American Chemical Society

Received: November 17, 2018 Accepted: January 14, 2019

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DOI: 10.1021/acsami.8b20246 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

temperature. Samples were degassed at 200 °C for 2 h before the adsorption measurements. Half-Cell Measurement. Electrochemical properties of the samples were studied using CR2016-type coin cells. The electrolyte was 1 M hexafluorophosphate (LiPF6, DAN VEC) in a mixture of diethylene carbonate, dimethyl carbonate, and ethylene carbonate (volume ratio of 1:1:1). Celgard 2325 microporous polypropylene membranes were used as the separator. Lithium plates were employed as the counter/reference electrode. The working electrode was prepared by mixing 65 wt % active material (AlNb11O29-M or AlNb11O29-N), 25 wt % carbon black, and 10 wt % polyvinylidene fluoride in N-methylpyrrolidone to form a slurry. Subsequently, the slurry was pasted on a copper foil. The mass loading of AlNb11O29 was ∼1.4 mg cm−2. After drying in a vacuum oven at 110 °C for 10 h, the electrode was pressed using a roller. Galvanostatic discharging/charging measurements were performed on a Neware battery tester in the voltage range between 3.0 and 0.8 V at 25 °C. The current rate of 1 C was set to be 390 mA g−1 because the theoretical capacity of AlNb11O29 is 390 mA h g−1. Cyclic voltammetry (CV) measurements were conducted on CHI660E electrochemical workstation at scan rates ranging from 0.2 to 1.1 mV s−1. Full Cell Measurement. Full cells were assembled using CR2016type coin cells and employed LiNi0.5Mn1.5O4 microparticles (XNN5, Shenzhen Kejing Star Technology Company) as the cathode and AlNb11O29-N as the anode. The LiNi0.5Mn1.5O4/AlNb11O29-N weight ratio in full cells was fixed to be 2.8:1. The same electrolyte and separator that were used for the half-cell fabrication were used for the LiNi0.5Mn1.5O4/AlNb11O29-N full-cell fabrication.

AlNb11O29-microsized particles and nanowires were successfully synthesized based on solid-state reaction and solvothermal methods, respectively. Their structural characteristics, electrochemical properties, and working mechanisms were systematically studied through various characterizations. It turned out that the AlNb11O29 nanowires with a bamboolike morphology exhibited outstanding electrochemical properties for Li+ storage, including high initial Coulombic efficiency and reversible capacity, good rate capability, and excellent cycling stability.

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MATERIALS AND METHODS

Sample Preparation. Two methods were employed in this work to prepare AlNb11O29 samples, as schematically illustrated in Figure 1.

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RESULTS AND DISCUSSION Figure 2 shows the XRD spectra of AlNb11O29-M and AlNb11O29-N samples. All of the XRD peaks are characteristic

Figure 1. Preparation processes of AlNb11O29-M and AlNb11O29-N. The solid-state reaction method yielded AlNb11O29 particles of the micrometer size, hereinafter designated as AlNb11O29-M. Al2O3 (Aladdin, 99.0%) and Nb2O5 powers (Sinopharm, 99.5%) were mixed in a zirconia pot at a molar ratio 1:11. Then, 10 mL of ethanol was added as a dispersing agent. The mixture was ball-milled in a SPEX 8000M mixer for 1 h. After drying, the solids were sintered at 1300 °C for 4 h to obtain sample AlNb11O29-M. The electrospinning method, on the other hand, produced AlNb11O29 nanowires with a bamboo-like morphology, hereinafter designated as AlNb11O29-N. First, 1 mmol C15H21O6Al (Macklin, 99.99%) and 11 mmol C10H25NbO5 (Alfa Aesar, 99.999%) were dispersed in a mixed solvent containing 4 mL of acetic acid and 10 mL of ethanol under stirring to form solution A. Then, 1 g of polyvinylpyrrolidone (PVP, Mw: 1 300 000, Macklin) was dispersed in a mixed solvent of 1 mL of acetic acid and 20 mL of ethanol to form solution B. Second, solution A was added to solution B drop by drop under vigorous mixing for 10 h to obtain a homogeneous solution, which was transferred to a 25 mL plastic syringe. Third, a TL-01 electrospinning machine was used to prepare nanowires on a piece of aluminum foil (20 cm × 20 cm) under a voltage of 16 kV at an injecting speed of 0.4 mm−1 with a distance of 15 cm between the syringe needle and the aluminum foil collector. After drying at 80 °C, followed by sintering at 850 °C for 4 h, sample AlNb11O29-N was obtained. Characterization. The crystal structure and phase purity were characterized by using the X-ray diffraction (XRD) technique on a Bruker D8 X-ray diffractometer. To analyze the crystal structure, a Rietveld refinement was conducted using the GSAS program.58,59 To investigate Li+-storage mechanism, in situ XRD measurements were conducted on a homemade in situ cell with a beryllium window. Particle sizes, morphologies, and microstructures were characterized on a Hitachi S-4800 field-emission scanning electron microscope and an FEI Tecnai G2 F20 S-TWIN high-resolution transmission electron microscope. The elemental oxidation states and compositions were investigated by using the X-ray photoelectron spectroscopy (XPS) technique on a Thermo ESCALAB 250Xi X-ray photoelectron spectroscope. Nitrogen adsorption experiments were performed on a Micromeritics ASAP 2020 surface-area analyzer at the liquid-nitrogen

Figure 2. (a) XRD spectra of AlNb11O29-M and AlNb11O29-N, and Rietveld refinement results of AlNb11O29-M. (b) Crystal structure of AlNb11O29.

of the standard XRD spectrum of Ti2Nb10O29 (JCPDS card no. 72-159), indicating that both samples prepared in this work exhibited a Ti2Nb10O29-type crystal structure. No extra XRD peaks because of impurities such as Al2O3 or Nb2O5 can be seen, suggesting a pure AlNb11O29 phase. The Rietveld refinement of the XRD spectrum of AlNb11O29-M is shown in Figure 2a, whereas the lattice parameters including the refined unit cell and other sectional atomic data are given in Tables S1 and S2. It can be concluded that AlNb11O29 belongs to a monoclinic crystal structure with a shear ReO3 type and A2/m space group, which is constructed by blocks consisting 3 × 4 × ∞ octahedra. Al3+ and Nb5+ ions are disorderly located at all of the octahedra according to an Al/Nb atomic ratio of 1:11 (Figure 2b). These octahedra are connected through corner- and edge-sharing, not only ensuring the good structural stability but also forming open-tunnel-like interstitial sites. AlNb11O29 exhibits lattice constants a = 1.555789(85) nm, b = B


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Figure 3. (a) FESEM image of AlNb11O29-M. (b) FESEM image of the as-electrospun nanowires. (c) FESEM image of AlNb11O29-N. (d) TEM image of AlNb11O29-N. (e) HRTEM image and SAED pattern (inset) of AlNb11O29-M. (f) HRTEM image and SAED pattern (inset) of AlNb11O29-N. (g) EDX images of AlNb11O29-N.

Figure 4. CVs of (a) AlNb11O29-M and (b) AlNb11O29-N at 0.2 mV s−1. CVs, pseudocapacitive contribution ratios at different scan speeds, and pseudocapacitive contributions at 1.1 mV s−1 for (c,f,h) AlNb11O29-M and (d,g,i) AlNb11O29-N. (e) Relationship between peak current of cathodic/anodic reaction and scan speed for AlNb11O29-M.

0.381126(16) nm, c = 2.053599(94) nm, β = 113.303(4)°, and V = 1.118354(112) nm3. It is noteworthy that this b value (i.e., double of the interlayer spacing in Figure 2b) is larger than that of Ti2Nb10O29.60 Because the sizes of the interstitial sites are determined by the b value in the crystal structure,61 AlNb11O29 has larger interstitial sites, undoubtedly benefiting Li+ diffusivity. Figure S1 reveals the similar Al 2p and Nb 3d XPS spectra of the two AlNb11O29 samples. The binding

energies of Al 2p (74.6 eV), Nb 3d3/2 (209.9 eV), and Nb 3d5/2 (207.2 eV) correspond to Al3+ and Nb5+, respectively.62,63 The FESEM image in Figure 3a shows the morphology and particle size of AlNb11O29-M. Its prismatic particles are about 1−5 μm in length and 0.2−2 μm in diameter, reflecting its monoclinic crystal structure. Figure 3b displays the morphology of the as-electrospun sample with smooth nanowire C


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Figure 5. (a) Discharging/charging curves of AlNb11O29-M and AlNb11O29-N at 0.1−10 C. (b) Rate capabilities of AlNb11O29-M and AlNb11O29N. (c) Percentage capacity of AlNb11O29-M and AlNb11O29-N at 0.1−10 C. (d) Cycling stability of AlNb11O29-M and AlNb11O29-N at 10 C.

1.68 V based on a previous method,17−24,37 which is favorable for safe operation of LIBs. It can also be found that the CV curves of the subsequent three cycles almost overlapped, suggesting good reversibility of AlNb11O29-M for Li+ storage. Figure 4b depicts the CV curves of AlNb11O29-N under the same test conditions, which are similar to those of AlNb11O29M, indicating the same redox kinetic properties in both AlNb11O29 materials. Figure 4c,d presents the four CV cycles at 0.2, 0.4, 0.7, and 1.1 mV s−1 for AlNb11O29-M and AlNb11O29-N, respectively. It can be seen that polarization increased with the increase in scan speed. However, AlNb11O29-N exhibited weaker polarization and stronger peak intensity than AlNb11O29-M, suggesting that the former possessed better electrochemical kinetics than the latter. From these CV curves, the Li+ diffusion coefficients, D, of AlNb11O29 were calculated from the classical Randles−Sevcik equation (eq 1)64

surfaces and a small average diameter of around 200 nm. However, the nanowire surfaces of AlNb11O29-N, which were sintered at 850 °C, became rougher, resulting in a unique bamboo-like nanostructure (Figure 3c). In addition, the average diameter of AlNb11O29-N went down to around 160 nm, which resulted from the crystallization of AlNb11O29 and the pyrolysis of PVP during the heat treatment. The significantly smaller particle size in AlNb11O29-N was verified by its large Brunauer−Emmett−Teller (BET) specific surface area of 9.74 m2 g−1 (Figure S2), up to 15.5 times larger than that of AlNb11O29-M (only 0.59 m2 g−1). Besides the above benefits, the electrospinning method was also propitious to prevent AlNb11O29-N from agglomerating. The low-magnification TEM image in Figure 3d confirmed the bamboo-like nanostructure of AlNb11O29-N. The HRTEM images of AlNb11O29-M in Figure 3e and AlNb11O29-N in Figure 3f show lattice spacings of 0.266 and 0.303 nm, respectively, corresponding to the (315̅) and (406̅) crystallographic planes of AlNb11O29. The selected-area electron diffraction (SAED) patterns of the two AlNb11O29 samples (the insets of Figure 3e,f) confirmed their monoclinic crystal structure of the 3 × 4 × ∞ shear ReO3 type and A2/m space group, in good coincidence with the XRD and HRTEM results. The energy-dispersive X-ray spectroscopy (EDX) elemental mapping images in Figures 3g and S3 revealed that all elements were distributed homogeneously, indicating high purity of samples. CV tests at different scan speeds in the potential window of 3.0−0.8 V were conducted to understand the electrochemical mechanisms of AlNb11O29. Figure 4a shows the first four CV cycles of AlNb11O29-M at 0.2 mV s−1. In the first cathodic scan, the strong reduction peak at 1.60 V could be attributed to the reduction of Nb5+ to Nb4+, and the two broad peaks ranging from 1.01 to 1.32 V could be assigned to the reduction of Nb4+ to Nb3+.19 In the first anodic scan, the strong oxidation peak centered at 1.76 V could be attributed to the oxidation of Nb4+ to Nb5+, and the two broad peaks within 1.19−1.46 V could be assigned to the oxidation of Nb3+ to Nb4+. The average operating potential of AlNb11O29 was estimated to be around

Ip = 2.69 × 105 × n1.5CSD0.5v 0.5

(1)

where Ip, n, C, S, and v are the redox peak current, the number of charge transfer, the Li+ molar concentration in AlNb11O29M, the real surface area of AlNb11O29-M, and the scan speed, respectively. A linear relationship between Ip and v0.5 was found from Figure 4e for both Li+ extraction and insertion processes. After calculation, AlNb11O29 displayed large Li+ diffusion coefficients of 1.12 × 10−13 and 2.39 × 10−13 cm2 s−1 for Li+ insertion and extraction, respectively, more than one order of magnitude larger than those of Ti2Nb10O29 (5.43 × 10−15 and 6.52 × 10−15 cm2 s−1 for Li+ insertion and extraction, respectively).45 The significantly faster Li+ diffusivity in AlNb11O29 was undoubtedly rooted in its larger interstitial sites in its crystal structure (Table S1). It has been reported that niobium-based oxides are typical intercalation pseudocapacitive anode materials for LIBs because of their open-crystal structures.17−24,39,53,65,66 According to a well-established method,21−25 the pseudocapacitive behavior at four different scan speeds was analyzed using eq 2 I(V) = kv + mv 0.5 D

(2) DOI: 10.1021/acsami.8b20246 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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previously reported niobate microparticles (Table S3).17,19,21,22,25,37,48,60 For AlNb11O29-N, its capacity retention was further increased to 96.3%, probably rooted in its stable nanoarchitecture with restricted self-aggregation. To confirm the good stability of AlNb11O29-N, the ex situ FESEM images of AlNb11O29-N after 500 cycles at 10 C are recorded in Figure S5, which revealed that the nanowire morphology was kept after the cycling test. It is noteworthy that the capacity loss of AlNb11O29-N at 10 C (0.0074% per cycle) is smaller than that of the W3Nb14O44 nanowires prepared by the same electrospinning method at 1000 mA g−1 (0.036% per cycle).27 The better cycling stability of AlNb11O29-N can be because of the stronger Al−O bond than the W−O bond. On the basis of the above discussion, it can be concluded that AlNb11O29 is a better anode material than Ti2Nb10O29 because of the former’s three advantages, including a higher initial Coulombic efficiency, higher rate capability, and better cycling stability. To understand the interaction mechanism of Li+ with AlNb11O29 during discharging/charging, in situ XRD measurements were conducted on the AlNb11O29-M/Li in situ cell at 0.5 C between 0.8 and 3.0 V. Figure 6a shows the in situ XRD

This equation describes a relationship between current (I) and scan speed (v) at a given potential, in which k and m are adjustable factors. Both the pseudocapacitive contribution (kv) and diffusion-controlled contribution (mv0.5) were calculated. AlNb11O29-M showed large pseudocapacitive contribution ratios of 40.1, 46.1, 53.2, and 63.3% at 0.2, 0.4, 0.7, and 1.1 mV s−1, respectively (Figure 4f), suggesting the remarkable pseudocapacitive performance at all of the scan speeds. The pseudocapacitive contribution ratios of AlNb11O29-N reached 59.2, 67.9, 74.9, and 79.9% (Figure 4g) at 0.2, 0.4, 0.7, and 1.1 mV s−1, respectively. Figure 4h,i depicts the detailed pseudocapacitive contributions at 1.1 mV s−1 for the two electrode materials. It can be clearly seen that AlNb11O29-N exhibited more remarkable pseudocapacitive behavior than AlNb11O29-M, which undoubtedly came from its significantly larger specific surface area. Figure 5a shows the discharging/charging curves of the two AlNb11O29 samples within 3.0−0.8 V at various current rates. These curves have similar shapes and match well with the shapes of the CV curves in Figure 4a,b. At 0.1 C, the twoelectrode materials exhibited similar reversible capacities (266 mA h g−1) and initial Coulombic efficiencies (about 94%). It should be noted that the very high initial Coulombic efficiency, resulting from little electrolyte decomposition and solidelectrolyte interphase formation at the potential of >0.8 V, is beneficial for practical applications of AlNb11O29.17−20,22,31−56 With the increase in current rate, all of the discharging curves dropped and all of the charging curves rose because of the electrode polarization. However, AlNb11O29-N consistently displayed much less electrode polarization, verifying its improved electrochemical kinetics. Meanwhile, AlNb11O29-M delivered the reversible capacities of 252, 225, 199, 165, and 131 mA h g−1 at 0.5, 1, 2, 5, and 10 C, respectively, (Figure 5b). This rate capacity of AlNb11O29-M is obviously higher than that of the previously investigated Ti2Nb10O29 microparticles (Ti2Nb10O29-M) with similar particle sizes (only 80 mA h g−1 at 10 C).60 In the case of AlNb11O29-N, its capacities were further increased to 256, 246, 234, 214, and 192 mA h g−1 at their corresponding current rates, suggesting its higher rate capability (Figure 5b). To get a better observation, the relative capacity percentages of two AlNb11O 29 samples were calculated, taking their corresponding capacities at 0.1 C as the standard. As shown in Figure 5c, the relative capacity percentage of AlNb11O29-N at 10 C is as large as 72.5%, which even surpasses that of AlNb11O29-M at 5 C (61.2%) and approaches that of AlNb11O29-M at 2 C (74.1%). This higher rate capability of AlNb11O29-N was further confirmed through the electrochemical impedance spectroscopy tests (Figure S4). Four benefits of the bamboo-like AlNb11O29 nanowires led to the outstanding rate capability of AlNb11O29-N. First, the large Li+ diffusion coefficients of AlNb11O29 facilitated the Li+ transport. Second, the large specific surface area of AlNb11O29-N enabled a large reaction area between electrolyte and AlNb11O29-N. Third, the small AlNb11O29 primary particles decreased the charge transport distances. Finally, AlNb11O29-N exhibited a very significant pseudocapacitive behavior. These benefits synergistically worked to promote the rate capability. Figure 5d compares the cycling stability of the two AlNb11O29 samples. The capacity of AlNb11O29-M at 10 C is 200 mA h g−1 in the initial cycle and finally stabilized at around 186 mA h g−1 after 500 cycles, showing a high capacity retention of 93.2%. This cycling stability surpasses those of

Figure 6. (a) Pristine in situ XRD spectra of AlNb11O29/Li in situ cell during initial discharging/charging process at 0.5 C. (b) Evolution of intensity vs Bragg position and discharging/charging curves of AlNb11O29/Li in situ cell during initial discharging/charging process at 0.5 C.

spectra, which revealed the Li+ insertion and extraction processes in the AlNb11O29-M electrode. In the Li+ insertion process, the main XRD peaks of AlNb11O29 centered at 23.7, 32.1, and 33.1°, respectively, corresponding to the (011), (215̅), and (411̅) crystallographic planes, slightly shifted toward lower angles (Figure 6b). In contrast, the main peak centered at 24.7°, corresponding to the (400) crystallographic plane, moved toward higher angles and then returned back. These peak shifts indicated the successful insertion of Li+ into the AlNb11O29 lattice, generating LixAlNb11O29. This process was well-reversed in the subsequent charge process, suggesting the almost complete extraction of Li+ from the LixAlNb11O29 lattice and verifying the high initial Coulombic efficiency of AlNb11O29. Additionally, the crystal structures of AlNb11O29-N at different lithiated/delithiated states were checked by ex situ HRTEM and SAED tests (Figure S6). All of the HRTEM images and SAED patterns corresponded to the monoclinic E


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Figure 7. (a) Charging/discharging curves of LiNi0.5Mn1.5O4/AlNb11O29-N full cell at 0.1 C. (b) Rate capability of LiNi0.5Mn1.5O4/AlNb11O29-N full cell. (c) Cycling stability of LiNi0.5Mn1.5O4/AlNb11O29-N full cell at 1 C. (d) Photograph of an LED lit up by LiNi0.5Mn1.5O4/AlNb11O29-N full cell.

3 × 4 × ∞ shear ReO3 type and A2/m space group) possesses a larger interlayer spacing than Ti2Nb10O29, leading to faster Li+ diffusion in the former than in the latter. In situ XRD results revealed an intercalating charge storage feature of AlNb11O29 and an excellent structural reversibility for Li+ storage. The nanowire morphology of AlNb11O29-N is favorable for charge transport and electrochemical reactions at the electrode/electrolyte interface. As a result of these merits, AlNb11O29-N delivered comprehensively good electrochemical properties, including a high reversible capacity (266 mA h g−1 at 0.1 C), safe operating potential (around 1.68 V at 0.1 C), high initial Coulombic efficiency (93.3% at 0.1 C), high rate capability (192 mA h g−1 at 10 C), and prominent cycling stability (capacity retention of 96.3% at 10 C after 500 cycles).

crystal structure of AlNb11O29 with the 3 × 4 × ∞ shear ReO3type and A2/m space group. The lattice spacing of the (400) plane slightly increased at 0.8 V but was restored to the origin value at 3.0 V. Therefore, AlNb11O29 was confirmed to be an intercalating anode material with excellent structural stability and reversibility. This desirable feature together with strong Al−O bonds undoubtedly led to the excellent cycling stability of AlNb11O29. The Li+ insertion−extraction mechanism can reasonably be expressed as eq 3 AlNb11O29 + x e− + x Li+ ↔ LixAlNb11O29 (0 ≤ x ≤ 22)

(3)

A LiNi0.5Mn1.5O4/AlNb11O29-N full cell was assembled and tested. The results are shown in Figure 7. As can be seen from Figure 7a, at 0.1 C, the full cell delivered initial charging and discharging capacities of 238 and 195 mA h g−1, respectively. The initial Coulombic efficiency was about 82%. After the initial cycle, the Coulombic efficiency was maintained around 100%, demonstrating highly reversible reactions in the full cell. The full cell discharged 168, 146, 127, 99, and 64 mA h g−1 at 0.5, 1, 2, 5, and 10 C, respectively (Figure 7b). In particular, it delivered a high energy density of 390 W h kg−1 at 0.1 C, significantly higher than those of other reported full cells, such as LiMn2O4/Li4Ti5O12 (200 W h kg−1)67 and LiNi0.5Mn1.5O4/ TiNb2O7 (280 W h kg−1).35 Additionally, it exhibited a reversible capacity of 136 mA h g−1 and high capacity retention of 93.2% at 1 C after 100 cycles (Figure 7c). This prominent stability behavior was caused by the intercalating nature of both LiNi 0.5 Mn 1.5 O 4 and AlNb 11 O 29 . This interesting LiNi0.5Mn1.5O4/AlNb11O29-N full cell was slighting up a light-emitting diode (LED), as seen from Figure 7d, indicating its practicability.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b20246. XPS spectra of Al 2p and Nb 3d in AlNb11O29-M and AlNb11O29-N; linear fitting curves for BET specific surface areas of AlNb11O29-M and AlNb11O29-N; EDX mapping images of AlNb11O29-M; Nyquist plots of AlNb11O29-M and AlNb11O29-N; ex situ FESEM image of AlNb11O29-N after 500 cycles at 10 C; ex situ HRTEM images and SAED patterns of AlNb11O29-N at different lithiated/delithiated states: pristine state, first discharged to 0.8 V, first charged to 3.0 V, and charged to 3.0 V after 100th cycles; results of crystal analyses by Rietveld refinements in AlNb11O29 and Ti2Nb10O29; fractional atomic parameters of AlNb 11 O 29 ; and comparisons of cycling stability of AlNb11O29-M with those of niobate microparticles previously reported (PDF)

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CONCLUSIONS A new niobate compound, namely, aluminum niobate (AlNb11O29), has been successfully designed and synthesized. This new material, as anode in LIBs, exhibited significantly improved electrochemical properties than any other niobate compounds (such as the popular Ti2Nb10O29) having been reported so far. The AlNb11O29 compound with a Ti2Nb10O29type crystal structure (the monoclinic crystal structure with the

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (C.L.). *E-mail: [email protected] (X.S.Z.). ORCID

Chunfu Lin: 0000-0003-0251-7938 F


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X. S. Zhao: 0000-0002-1276-5858 Author Contributions ∥

X.L. and R.L. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51762014), Taishan Scholar Program of Shandong Province, World-Class Discipline Program and Taishan Scholar’s Advantageous and Distinctive Discipline Program of Shandong Province, and Australian Research Council (ARC FL 170100101). We are grateful for Prof. Shi Xue Dou of University of Wollongong for his comments on the manuscript.

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