Mg2Nb34O87 Porous Microspheres for Use in High-Energy, Safe

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

Mg2Nb34O87 Porous Microspheres for Use in High-Energy, Safe, FastCharging, and Stable Lithium-Ion Batteries Xiangzhen Zhu,†,‡,§ Qingfeng Fu,†,‡,§ Lingfei Tang,†,‡,§ Chunfu Lin,*,†,‡,§ Jian Xu,†,‡,§ Guisheng Liang,†,‡,§ Renjie Li,†,‡,§ Lijie Luo,‡,§ and Yongjun Chen‡,§ †

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Key Laboratory of Advanced Materials of Tropical Island Resources (Hainan University), Ministry of Education, Haikou 570228, China ‡ State Key Laboratory of Marine Resource Utilization in South China Sea and §College of Materials and Chemical Engineering, Hainan University, Haikou 570228, China S Supporting Information *

ABSTRACT: M−Nb−O compounds are advanced anode materials for lithium-ion batteries (LIBs) due to their high specific capacities, safe operating potentials, and high cycling stability. Nevertheless, the found M−Nb−O anode materials are very limited. Here, Mg2Nb34O87 is developed as a new M− Nb−O material. Mg 2 Nb 34 O 87 porous microspheres (Mg2Nb34O87-P) with primary-particle sizes of 30−100 nm are fabricated based on a solvothermal method. Mg2Nb34O87 has an open 3 × 4 × ∞ Wadsley−Roth shear structure and a large unit-cell volume, leading to its largest Li+ diffusion coefficients among all the developed M−Nb−O anode materials. In situ X-ray diffraction analyses reveal its high structural stability and intercalating characteristic. These architectural, conductivity, and structural advantages in Mg2Nb34O87-P lead to its most significant intercalation pseudocapacitive contribution (87.7% at 1.1 mV s−1) among the existing M−Nb−O anode materials and prominent rate capability (high reversible capacities of 338 mAh g−1 at 0.1C and 230 mAh g−1 at 10C). Additionally, this new material exhibits a safe operating potential (∼1.68 V), an ultrahigh initial Coulombic efficiency (94.8%), and an outstanding cycling stability (only 6.9% capacity loss at 10C over 500 cycles). All of these evidences indicate that Mg2Nb34O87-P is an ideal anode material for high-energy, safe, fast-charging, and stable LIBs. KEYWORDS: Mg2Nb34O87, anode, electrochemical performance, porous microsphere, lithium-ion battery

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INTRODUCTION Lithium-ion batteries (LIBs) with long cycle lives and high energy densities are expected to be widely used in large-scale applications, such as smart grids and electric vehicles.1−5 According to the discharge−charge mechanism, typical anode materials are classified into three types: alloying type (Ge, Sn, Si, etc.),6−8 conversion reaction type (Fe2O3, Co3O4, SnO2, SnS2, etc.),9−12 and intercalation type (graphite, Ti-, Nb-based oxides, etc.).13−16 Intercalation-type anode materials may be the most promising for LIBs due to their higher initial Coulombic efficiencies and cycling stability compared with the conversion-reaction-type and alloying-type materials. Among the explored intercalation-type anode materials, graphite and Li4Ti5O12 are extremely popular. Graphite exhibits a high theoretical capacity of 372 mAh g−1, but safety concerns arise from its extremely low operating potential of ∼0.1 V. This potential is close to that of lithium plating, leading to the generation and growth of lithium dendrites.17−19 In contrast, Li4Ti5O12 shows superior safety. The generation of lithium dendrites can be restrained by its relatively high operating potential of ∼1.55 V.20 Nevertheless, its extensive application © XXXX American Chemical Society

in LIBs is limited by its low theoretical capacity of 175 mAh g−1. Thus, to meet the large-scale applications of LIBs, it is of great importance to explore new anode materials with comprehensively good properties, including high safety, capacities, rate capabilities, and cycling stability. In recent years, intercalation-type M−Nb−O anode materials, such as TiNb 2 O 7 , 21−32 Ti 2 Nb 10 O 29 , 33−43 TiNb24O62,44 FeNb11O29,45−47 GaNb11O29,48 Cr0.5Nb24.5O62,49 and ZrNb24O62,50 have gained extensive attention due to their high specific capacities, safe operating potentials, high cycling stability, and significant intercalation pseudocapacitive behavior. The M−Nb−O materials contain rich redox chemistry of Nb4+/Nb5+ and Nb3+/Nb4+ (Ti3+/Ti4+, Nb4+/Nb5+, and Nb3+/Nb4+ for Ti−Nb−O), resulting in their high theoretical capacities of 379−403 mAh g−1,21,44,45,48−50 which are at least twice that of Li4Ti5O12 and even surpass that of graphite. These redox couples are active in safe potentials of Received: March 10, 2018 Accepted: June 22, 2018

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DOI: 10.1021/acsami.8b03997 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Schematic fabrication process of Mg2Nb34O87-P.

>0.8 V,21,44,45,48−50 inhibiting the generation of lithium dendrites and solid-electrolyte interface (SEI) layers.18,19,21,23 The M−Nb−O materials show Wadsley−Roth shear structures, which are built by corner- and/or edge-sharing octahedra together with a small ratio (0−4%) of tetrahedra.21,44,45,48−50 The edge-sharing octahedra stabilize the crystal structure. The tetrahedra anchor the octahedra, contributing to the stable structure.44,49,50 These superior structural characteristics result in the high cycling stability of M−Nb−O. In addition, because the Wadsley−Roth shear structures are quite open, M−Nb−O materials have intrinsic intercalation pseudocapacitive behavior,51,52 which is beneficial for their specific capacities, cycling stability, and rate capabilities in electrochemical energy storage.28,34,37,46−50 Despite of the above advantages, the developed M−Nb−O materials generally suffer from insufficient Li+ and/or electronic conductivities, thereby restricting their rate capabilities. According to a previous study from YetMing Chiang’s group,53 effects to improve the electronic conductivity may have little impact on the rate capability due to the fact that the electronic conductivity significantly increases once the lithiation process starts; and the Li+ conductivity actually works as the rate-limiting factor after the initial lithiation process. Therefore, further development of new M−Nb−O compounds with high Li+ conductivities is highly anticipated so as to fulfill the aforementioned four key requirements: high safety, capacities, rate capabilities, and cycling stability. Here, we develop Mg2Nb34O87 as a new and highly Li+ conductive M−Nb−O anode material. Mg2Nb34O87 microsized particles (Mg2Nb34O87-M) are prepared by a traditional solid-state reaction at a high sintering temperature of 1200 °C. The Mg2+ ion has a large ionic radius of 0.720 Å in the octahedral site,54 which is larger than those of Ti4+ (0.605 Å), Ga3+ (0.620 Å), Cr3+ (0.615 Å), and Fe3+ (0.645 Å).55 Thus, Mg2Nb34O87 can exhibit a large unit-cell volume, inferring its large Li+ diffusion coefficient and high Li+ conductivity (note that the Li+ conductivity is proportional to the Li+ diffusion coefficient according to the Nernst−Einstein relationship).30,39,49,50 Besides the large Li+ diffusion coefficient, the selection of Mg2+ further has two more benefits. First, the light atomic weight of Mg2+ together with the large Nb5+ content (68.4 wt %) in Mg2Nb34O87 leads to its high theoretical capacity of 396 mAh g−1. Second, the high bond energy per Mg−O bond (3.95 eV) benefits the structural stability of Mg2Nb34O87.56 As a result, Mg2Nb34O87 is expected to deliver excellent cycling stability. To further enhance the electrochemical performance (especially the rate capability) of Mg2Nb34O87, Mg2Nb34O87 porous microspheres (Mg2Nb34O87-P) are innovatively synthesized through a simple solvothermal route and subsequent low-temperature sintering at 800 °C. This intriguing nanostructure can provide short Li+/ electron transportation distances within the nanosized primary

particles, a large electrolyte/electrode interface area, a high tap density, easy electrolyte penetration, and good structural stability.23,26,34,37,38 Consequently, Mg2Nb34O87-P owns comprehensively good electrochemical performance with an ultrahigh intercalation pseudocapacitive contribution (87.7% at 1.1 mV s−1), safe operating potential (∼1.68 V), ultrahigh initial Coulombic efficiency (94.8%), ultrahigh charge capacity (338 mAh g−1 at 0.1C), excellent cycling stability (93.1% capacity retention at 10C over 500 cycles), and superior rate capability (230 mAh g−1 at 10C), perfectly fulfilling the aforementioned four key requirements.

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MATERIALS AND METHODS

Material Preparation. Mg2Nb34O87-M with microsized particles was synthesized by a traditional solid-state reaction method. Stoichiometric amounts of MgO (2 mmol, 99.9%, Aldrich) and Nb2O5 (17 mmol, 99.9%, Aldrich) powers were mixed in 10 mL ethyl alcohol and then ball-milled by a high-energy ball-milling machine with zirconia balls (SPEX 8000M) for 60 min. The obtained slurry was dried in an oven at 80 °C and then sintered at 1200 °C for 4 h by using a tube furnace. Mg2Nb34O87-P with porous microspheres was synthesized by a facile solvothermal process and subsequent low-temperature sintering (Figure 1). A tetrabutylammonium hydroxide solution (TBA, 10 wt % in H2O, Aladdin) was used as the surfactant. Mg(NO3)2·6H2O (99.9%, Aladdin) and NbCl5 (99.9%, Aladdin) were employed as Mg and Nb precursors, respectively. A 1.18 mmol Mg(NO3)2·6H2O, 20 mmol NbCl5, and 60 mL isopropanol (IPA) were mixed and stirred for 1 h, forming a uniform solution. Then, 0.5 mL TBA was dropped into the mixed solution, which was stirred for another 1 h. The resultant solution was devolved to a Teflon-lined stainless steel autoclave whose inner volume was 100 mL. The autoclave was maintained in an oven at 200 °C for 24 h. The obtained precipitates were centrifuged, washed with ethyl alcohol and deionized water for three times, and then dried at 80 °C in an oven overnight. The fully dried powders were sintered at 800 °C for 2 h. Sample Characterization. The X-ray diffraction (XRD) experiments were implemented on an X-ray diffractometer (Brucker D8) to characterize the crystal structures of Mg 2 Nb 34 O 87 -M and Mg2Nb34O87-P. A Rietveld refinement of the XRD spectrum of Mg2Nb34O87-M was conducted with the general structure analysis system suite of programs.57−59 An X-ray photoelectron spectroscopy (XPS) equipment (Thermo Escalab 250Xi) was adopted to define the chemical valences of the Mg and Nb elements. An inductively coupled plasma atomic emission spectroscopy (ICP-AES) equipment (Agilent 7500ce) was employed to determine the Mg/Nb ratios in Mg2Nb34O87-M and Mg2Nb34O87-P. High-resolution transmission electron microscopy (HRTEM, FEI Tecnai, G2F20) and fieldemission scanning electron microscopy (FESEM, Hitachi S-4800) equipments were used to reveal the morphology differences between Mg2Nb34O87-M and Mg2Nb34O87-P. Nitrogen physisorption was performed on a surface area analysis equipment (Micromeritics ASAP 2020) to examine the specific surface areas of the two Mg2Nb34O87 samples and the pore-size distribution of Mg2Nb34O87-P. Electrochemical Test. The electrochemical performance of Mg2Nb34O87-M and Mg2Nb34O87-P was analyzed by means of B


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Figure 2. (a) XRD patterns of Mg2Nb34O87-M and Mg2Nb34O87-P (not all peaks were indexed owing to too many peaks). (b) Crystal structure of Mg2Nb34O87.

Mg2Nb34O87 lattice, thereby benefiting the Li+ transportation.30 To further enhance the electrochemical performance of Mg2Nb34O87, Mg2Nb34O87-P is innovatively fabricated by a surfactant-assisted solvothermal reaction and subsequent lowtemperature sintering at 800 °C. TBA served as the surfactant during the solvothermal process, modifying the surface interaction between the Mg2Nb34O87-precursor particles and the IPA solvent.34,60 Therefore, the surface free energy of the Mg2Nb34O87-precursor particles could be increased as the liquid−solid phase separation proceeded. The increased surface free energy could favor large curvatures at the particle surfaces and thus the formation of Mg2Nb34O87-precursor secondary particles with a spherical shape. Compared with Mg2Nb34O87-M, Mg2Nb34O87-P exhibits obviously broader diffraction peaks, revealing its significantly smaller grain sizes. This decrease in grain size is undoubtedly related to the lowtemperature sintering at 800 °C. It is noteworthy that 800 °C is enough for the crystallization of the Mg2Nb34O87 nanomaterial because the nanomaterial shows a large driving force and short ion transportation distances during the sintering. Figure S1 displays the very similar high-resolution XPS spectra of the cations in both Mg2Nb34O87 samples. The binding energy at 1303.7 eV (Figure S1a) corresponds to the Mg 1s XPS peak of Mg2+ and those at 206.8/209.5 eV (Figure S1b) correspond to Nb 3d5/2/Nb 3d3/2 of Nb5+. Therefore, the chemical valences of Mg and Nb in the two Mg2Nb34O87 samples are Mg2+ and Nb5+, respectively. The ICP-AES tests show that the molar ratios of Mg to Nb in Mg2Nb34O87-M and Mg2Nb34O87-P are 2.04:34 and 1.89:34, respectively, which are consistent with the expected value (2:34). The morphological features of Mg 2 Nb 34 O 87 -M and Mg2Nb34O87-P were examined using FESEM and TEM. The low-magnification FESEM image of Mg2Nb34O87-P (Figure 3a) exhibits uniform porous microspheres. The average diameter of these microspheres is ∼500 nm. The corresponding high-magnification FESEM image (Figure 3b) reveals that

CR2016-type coin cells containing lithium foils, microporous polypropylene films (Celgard 2325), electrolyte, and working electrodes. The electrolyte was 1 M LiPF6 (DAN VEC) in a mixed solvent containing ethylene carbonated, dimethyl carbonate, and diethylene carbonate with a volume ratio of 1:1:1. Each working electrode was constructed by a dry film containing Mg2Nb34O87-M/ Mg2Nb34O87-P (65 wt %), poly(vinylidene fluoride) binder (10 wt %), and Super P carbon (25 wt %) on a Cu plate. An eight-channel battery-testing system (CT-3008, Neware) was used to evaluate the galvanostatic discharge−charge properties (1C = 396 mA g−1). An electrochemical workstation (Zahner Zennium, Kronach) was employed to record the cyclic voltammetry (CV) curves. The in situ XRD tests were performed on an in situ cell with an X-ray-transparent Be window, and the resultant patterns were collected between 17 and 42° during the first two discharge−charge cycle at 0.5C.

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RESULTS AND DISCUSSION Figure 2a presents the XRD spectra of Mg2Nb34O87-M and Mg2Nb34O87-P. All of the diffraction peaks of Mg2Nb34O87-M can be designated as a perfect Wadsley−Roth shear structure (A2/m space group), similar to Ti2Nb10O29.33 The detailed crystal structure of Mg2Nb34O87 is clarified by a Rietveld refinement (Supporting Information). The resultant structure is illustrated in Figure 2b, and the refined lattice constants are available in Tables S1 and S2. Mg2Nb34O87 has a crystal structure described as 3 × 4 × ∞ Wadsley−Roth shear structure consisting of M′O6 (M′ = Mg and Nb) octahedra. These M′O6 octahedra spread on different shear ac-planes and share corners and/or edges, guaranteeing a high structural stability and generating open-tunnel-like interstitial spaces for Li+ accommodation, which respectively contribute to high cycling stability and theoretical/practical capacities. Mg2Nb34O87 shows lattice constants with a = 15.60459(13) Å, b = 3.83071(2) Å, c = 20.64403(13) Å, β = 113.096(6)°, and V = 1135.119(161) Å3, which are obviously larger than those for Ti2Nb10O29 previously reported.39 The enlarged unitcell volume infers wider Li+ transportation paths in the C


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Figure 3. (a) Low- and (b) high-magnification FESEM images of Mg2Nb34O87-P. (c) Low- and (d) high-magnification TEM images of Mg2Nb34O87-P. (e) Nitrogen adsorption−desorption isotherms of Mg2Nb34O87-M and Mg2Nb34O87-P (the inset plot displays Barrett−Joyner− Halenda (BJH) desorption pore-size distribution of Mg2Nb34O87-P). (f) HRTEM image of Mg2Nb34O87-P (the inset presents its selected area electron diffraction (SAED) pattern). (g) TEM−energy-dispersive X-ray (EDX) images of Mg2Nb34O87-P.

allows liquid electrolyte to easily diffuse into the pores and facilitates the solvated-Li+ transportation into Mg2Nb34O87, also benefiting the electrochemical kinetics.23,26,34,37,44 To further understand the structural features of Mg2Nb34O87-M and Mg2Nb34O87-P, HRTEM and SAED characterizations were carried out. The HRTEM images of Mg2Nb34O87-M (Figure S3a) and Mg2Nb34O87-P (Figure 3f) display interplanar distances of 0.386 and 0.313 nm, respectively, matching well the (402̅) and (066) crystallographic planes of the Mg2Nb34O87 phase. Meanwhile, the SAED patterns (Figure S3b and the inset of Figure 3f) perfectly match those of the monoclinic Wadsley−Roth shear structure (A2/m space group). These HRTEM and SAED outcomes match with the XRD analyses. From the EDX elemental mapping images (Figures S4 and 3g), uniform distributions of Mg, Nb, and O can be seen, which verifies the formation of pure Mg2Nb34O87 products. CV experiments were implemented to investigate the redox kinetic properties of Mg2Nb34O87 at 0.2 mV s−1 within 3.0−0.8 V. The lower potential limit of 0.8 V is above the reduction potential of most organic electrolytes. Consequently, little SEI layers generated on the surface of the Mg2Nb34O87 particles and thus minor irreversible capacity can be observed during the electrochemical reaction. As can be seen from Figure 4a,

each microsphere is composed of numerous primary particles with diameters of 30−100 nm. Rough exteriors and a large number of pores appear in the microspheres, which arise from the generation of H2O and CO2 during the decomposition of the dense Mg2Nb34O87 precursor (Figure S2a) at 800 °C. The typical TEM images of Mg2Nb34O87-P (Figure 3c,d) confirm its porous-microspherical morphology. In sharp contrast, Mg2Nb34O87-M consists of huge particles with sizes ranging from ∼1 to ∼8 μm (Figure S2b). To measure the specific surface areas and pores-size distributions of the as-prepared products, N2 adsorption− desorption experiments were carried out and the results are given in Figure 3e. Mg2Nb34O87-P exhibits a Branauer− Emmett−Teller (BET) specific surface area of 10.68 m2 g−1, 1 order of magnitude larger than that of Mg2Nb34O87-M (1.02 m2 g−1). Because Mg2Nb34O87-P has a larger electrolyte/ electrode interface area, its electrochemical kinetics are undoubtedly better. The pore-size distribution of Mg2Nb34O87-P (the inset of Figure 3e), calculated from the desorption data using the Barrett−Joyner−Halenda (BJH) model, reveals hierarchical pore sizes centered at ∼45 and ∼12 nm, which are in correspondence to the intersphere and interparticle pores, respectively. This hierarchical porosity provides a good electrolyte/electrode interface contact that D


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Figure 4. (a) CV curves of Mg2Nb34O87-M and Mg2Nb34O87-P at 0.2 mV s−1. (b) (i) First-cycle discharge−charge curves at 0.1C and second-cycle discharge−charge curves at (ii) 0.1C, (iii) 0.5C, (iv) 1C, (v) 2C, (vi) 5C, and (vii) 10C of Mg2Nb34O87-M and Mg2Nb34O87-P. (c, d) Rate capabilities of Mg2Nb34O87-M and Mg2Nb34O87-P. (e) Cycling stability of Mg2Nb34O87-M and Mg2Nb34O87-P at 10C and Coulombic efficiency of Mg2Nb34O87-P.

each CV curve depicts a couple of spike peaks at ∼1.7/∼1.6 V and a couple of wide protrusions in 1.5−0.8 V, rooted in the oxidation/reduction reactions of the Nb4+/Nb5+ and Nb3+/ Nb4+ redox couples,50 respectively. The intermediate potential for the spike peak-couple corresponds to the average operating potential of M−Nb−O.30,42,44,47−50 Consequently, the average operating potential of Mg2Nb34O87 was determined to be ∼1.68 V, similar to those of M−Nb−O anode materials previously reported (∼1.64 V for TiNb2O7,30 ∼1.70 V for Ti2Nb10O29,42 ∼1.66 V for TiNb24O62,44 ∼1.65 V for FeNb11O29,47 ∼1.69 V for GaNb11O29,48 ∼1.65 V for Cr0.5Nb24.5O62,49 and ∼1.67 V for ZrNb24O62).50 This operating potential is far above the lithium-plating potential, making the Mg2 Nb34 O 87-based LIBs intrinsically safe. Compared with Mg2Nb34O87-M, Mg2Nb34O87-P shows smaller potentials in both the CV peak couples (Table S3), indicative of its smaller polarization. Moreover, the intensities of its CV peaks are greater. Hence, Mg2Nb34O87-P presents better electrochemical kinetics, undoubtedly resulting from its porous-microspherical morphology. The electrochemical performance of the Mg2Nb34O87-M and Mg2Nb34O87-P electrodes was evaluated by a whole train of galvanostatic discharge−charge measurements, as shown in Figure 4b. Both the electrodes were successively tested at 0.1, 0.5, 1, 2, 5, and 10C. Each discharge (charge) curve consists of (i) a sloping region from 3.0 V (or the open circuit voltage for the first cycle) to ∼1.7 V, (ii) a short plateau at ∼1.7 V, and (iii) another sloping region from ∼1.7 to 0.8 V. The first and last regions can correspond to two solid-solution reactions. In contrast, the second region can be rooted in a double-phase transformation reaction.23,33 The shape of the discharge

(charge) curve is highly consistent with that of the cathodic (anodic) CV curve (Figure 4a). At a small current rate of 0.1C, Mg2Nb34O87-M displays a high Coulombic efficiency of 89.3% and a high reversible capacity of 290 mAh g−1 in the first cycle, whereas Mg2Nb34O87-P exhibits huge values of 94.8% and 338 mAh g−1, surpassing those of the investigated graphite and M−Nb− O anode materials.21−50 The reason for the high initial Coulombic efficiencies can be that little SEI layers form during the electrochemical reaction of Mg2Nb34O87. With increasing current rate, Mg2Nb34O87-M retains high capacities of 253, 228, 205, 179, and 149 mAh g−1, respectively, at 0.5, 1, 2, 5, and 10C (Figure 4b−d). Clearly, Mg2Nb34O87-M exhibits a significantly higher rate capability than the previous M−Nb−O anode materials having the same (similar) crystal structures and particle sizes (only 80, 57, and 121 mAh g−1 at 10C, respectively, for Ti2Nb10O29, FeNb11O29, and GaNb11O29 microsized particles42,47,48). For Mg2Nb34O87-P, the capacities are further increased to 323, 299, 277, 254, and 230 mAh g−1 at the corresponding current rates (Figure 4b−d). This even higher rate capability of Mg2Nb34O87-P is consistent with the electrochemical impedance spectroscopy result (Figure S7). It should be emphasized that the superior rate capability of Mg2Nb34O87-P is one of the best results in the research community of the M−Nb−O anode materials (Table S4).21−50 Additionally, at 10C, Mg2Nb34O87-P presents excellent cycling stability, with up to 93.1% capacity retention after 500 cycles. This percentage is very close to that of Mg2Nb34O87-M (93.5%) (Figure 4e). To clarify the reasons for the prominent electrochemical performance of Mg2Nb34O87-P, more characterizations have been performed. E


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Figure 5. CV curves of (a) Mg2Nb34O87-M and (b) Mg2Nb34O87-P at different sweeping rates. Percentages of intercalation pseudocapacitive contributions of (c) Mg2Nb34O87-M and (d) Mg2Nb34O87-P at various sweeping rates. CV curves with specific intercalation pseudocapacitive contributions of (e) Mg2Nb34O87-M and (f) Mg2Nb34O87-P at 1.1 mV s−1.

Li+ diffusion coefficient by 1−3 orders of magnitude.62 Compared with Ti2Nb10O29, Mg2Nb34O87 has a ∼1.5% larger unit-cell volume (Table S1), indicating its ∼1.5% expanded O2− framework. For instance, the distance between the O2− layers along the b-direction is increased by ∼0.5% (note that the b value of Mg2Nb34O87 is ∼0.5% larger). Therefore, the approximately 2 orders of magnitude larger Li+ diffusion coefficients for Mg2Nb34O87 have been achieved. It is noteworthy that lithiation is the rate-limiting process in the electrochemical reaction of Mg2Nb34O87 because the Li+ diffusion coefficient for lithiation is smaller than that for delithiation. During lithiation, external Li+ ions enter the outer part of the Mg2Nb34O87 particles and then transport to the inner part. Thus, the Li+ concentration in the outer part is larger than that in the inner part. Because more Li+ ions are located at the Li+ transportation paths in the outer part, the Li+ diffusion coefficient for lithiation is smaller. It is well known that niobium-based compounds are the typical materials with intercalation pseudocapacitive behavior.28,34,37,46−50 In LIBs, the possible contributions to electrochemical Li+ storage involve a diffusion-controlled process and a surface-controlled process (pseudocapacitive behavior). The latter process is much faster than the former one. According to a previous study,51,52 the pseudocapacitive behavior can be examined through the correlation between the peak current (I) and the sweep rate (v) as follows eq 2

The CV experiments were further implemented at larger sweep rates. Figure 5a,b record the resultant CV curves of Mg2Nb34O87-M and Mg2Nb34O87-P, respectively. These CV curves display similar peak shapes during both anodic and cathodic processes, where the redox peak currents increase with the sweep rate. Mg2Nb34O87-P always exhibits a smaller polarization than Mg2Nb34O87-M. From these CV data, the Li+ diffusivity of Mg2Nb34O87 can be evaluated. A linear relationship between the redox peak current (Ip) and the sweep rate’s square root (v0.5) can be found in Figure S5. Based on this founding, the apparent Li+ diffusion coefficients D of Mg2Nb34O87 for lithiation and delithiation can be calculated from the slopes in Figure S5 by empolying the classical Randles−Sevcik equation61 Ip = 2.69 × 105 × n1.5CSD0.5v 0.5

(1)

where n, C, and S are the charge transfer number (eq 1), the allowed Li+ molar concentration in the Mg2Nb34O87 crystals (66.32 mol L−1), and the real surface area of Mg2Nb34O87-M (2.39 × 10−4 m2) calculated based on its BET specific surface area, respectively. Mg2Nb34O87 owns excellent Li+ diffusion coefficients of 3.23 × 10−13 cm2 s−1 for lithiation and 6.36 × 10−13 cm2 s−1 for delithiation, which are the largest values for the developed M−Nb−O compounds,42,46,47 and approximately 2 orders of magnitude larger than those of Ti2Nb10O29 (5.43 × 10−15 cm2 s−1 for lithiation and 6.52 × 10−15 cm2 s−1 for delithiation) gained by the same CV method.42 It is well known that the Li+ diffusion coefficient is extremely sensitive to the lattice constants and unit-cell volume.2,62,63 The Li+ diffusion in the metal oxide (such as M−Nb−O and Li4Ti5O12) lattice is sensitively determined by the interstitial spacing of the O2− framework (for a metal oxide, the size of the O2− framework is equal to the unit-cell volume).62 If the O2− framework is expanded, the activation energy for the Li+ diffusion will be decreased and thus the Li+ diffusion coefficient will be increased. A previous report revealed that only 1% increase in the unit-cell volume could significantly increase the

I = avb

(2)

in which a and b are changeable factors. A b value of 0.5 shows that the Li+ storage is completely regulated by the diffusioncontrolled process, whereas a b value of 1 means that the surface-controlled process completely occupies a dominant position. When the b value is located in 0.5−1, the electrochemical reaction includes both processes. A larger bvalue means a larger proportion of the surface-controlled process. Figure S6 displays the log(I)−log(v) plots of the two Mg 2 Nb 34 O 8 7 products. The calculated b-values of F


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Figure 6. (a) Pristine in situ XRD patterns of Mg2Nb34O87 within 3.0−0.8 V at 0.5C (two cycles). (b) In situ XRD patterns and corresponding discharge−charge curves of Mg2Nb34O87 within 3.0−0.8 V at 0.5C (two cycles). Detailed shifts of (c) (011), (400), (d) (215̅), (411̅), and (e) (317̅) XRD peaks of Mg2Nb34O87 (one cycle).

where c and d are adjustable parameters. Figure 5c,d respectively exhibit the significant pseudocapacitive contributions of Mg2Nb34O87-M and Mg2Nb34O87-P at all the sweep rates. The pseudocapacitive contribution increases with the increasing sweep rate, revealing that the pseudocapacitive behavior has a distinct advantage for the overall high-rate capacity in Mg2Nb34O87. Similar to the situation of the bvalues, Mg2Nb34O87-P always provides larger pseudocapacitive contributions than Mg2Nb34O87-M. At 1.1 mV s−1, the pseudocapacitive contribution in Mg2Nb34O87-P is remarkable, up to 87.7% (Figure 5f), the maximum among the developed M−Nb−O materials at the same sweep rate.24,33−37 This most

Mg2Nb34O87-M for the cathodic and anodic processes are as large as 0.69 and 0.85, respectively (Figure S6a). The corresponding values for Mg2Nb34O87-P are even increased to be 0.74 and 0.86 (Figure S6b). These large b-values demonstrate dominant pseudocapacitive behavior in the Li+ storage of Mg2Nb34O87. To further reveal the hybrid electrochemical kinetics in Mg2Nb34O87-M and Mg2Nb34O87-P, the detailed contributions of the diffusion-controlled process (cv1/2) and pseudocapacitive behavior (dv) can be calculated according to eq 352 I(V ) = cv1/2 + dv

(3) G


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electrolyte/electrode interface area, small electron/Li+ transportation distances in the primary particles, and easy electrolyte penetration into the inner particles, respectively. Additionally, Mg2Nb34O87-P presents significant intercalation pseudocapacitive behavior with the largest pseudocapacitive contribution of 87.7% among the developed M−Nb−O materials at 1.1 mV s−1. As an anode material, Mg2Nb34O87P delivers an operating potential of ∼1.68 V and a reversible capacity of 338 mAh g−1 at 0.1C, which demonstrate its ultrahigh safety and capacity, respectively. Besides, its rate capability and cycling stability are also outstanding, which can be inferred by its high capacity of 230 mAh g−1 at 10C with only 6.9% capacity loss after 500 cycles. Clearly, Mg2Nb34O87P perfectly fulfills the four key requirements of the high safety, capacity, rate capability, and cycling stability, thereby becoming very promising in the high-energy, safe, fastcharging, and stable LIBs aimed for large-scale applications. This work provides a useful guide for the future electrodematerial design in the research community of high-performance LIBs.

significant pseudocapacitive behavior of Mg2Nb34O87-P can be due to two factors. First, Mg2Nb34O87 has a 3 × 4 × ∞ Wadsley−Roth shear structure and a large unit-cell volume. This open crystal structure enables fast Li+ transportation in the Mg2Nb34O87 lattice.51,52 Second, Mg2Nb34O87-P has a relatively large specific surface area. Based on the above structural, morphological, and electrochemical characterizations, the outstanding rate capability of Mg2Nb34O87-P could be elucidated as follows. (i) Among the existing M−Nb−O materials, no one can compare with Mg2Nb34O87 with respect to the Li+ diffusion coefficient. Its Li+ diffusion coefficients are approximately 2 orders of magnitude larger than those of Ti2Nb10O29. (ii) The porousmicrospherical morphology of Mg2Nb34O87-P enables short Li+/electron transportation lengths in the nanosized primary particles and a large electrolyte/electrode interface area. (iii) Among the developed M−Nb−O materials, Mg2Nb34O87 shows the most significant intercalation pseudocapacitive behavior. The largest pseudocapacitive contribution of 87.7% is achieved at 1.1 mV s−1. The combined effect of these three factors can greatly improve the rate capability. To study the lithiation−delithiation mechanism of Mg2Nb34O87, in situ XRD technique was employed. The in situ XRD tests of the Mg2Nb34O87-M/Li in situ cell were performed during the galvanostatic discharge−charge processes at 0.5C. Figure 6a,b illustrates the in situ XRD patterns undergoing the first two cycles accompanied by the corresponding discharge−charge curves. During the firstcycle discharge process, the (011), (215̅), (411̅), and (317̅) patterns, respectively, centered at 23.8, 32.2, 33.2, and 38.9°, gradually shift toward lower 2θ angles. In contrast, the (300) and (400) peaks at 18.5 and 24.7° experience a process of moving to higher 2θ angles first and then returning back. In the subsequent charge process, all these peaks gradually shift back to their original positions. The detailed shifts of the (011), (400), (215̅), (411̅), and (317̅) peaks are depicted through Figure 6c−e. In addition, no impurity peaks can be detected during the repeated discharge−charge processes, apart from the unchanged weak peak caused by BeO at 41.3°.64 According to the above analyses, the fact that Mg2Nb34O87 is an intercalation-type anode material with a superior structural reversibility can be determined. The eq 4 explains its lithiation−delithiation mechanism in a reasonable way

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b03997. Detailed process of Rietveld refinement; XPS spectra of cations in Mg2Nb34O87-M and Mg2Nb34O87-P (Figure S1); FESEM images of Mg2Nb34O87-P precursor and Mg2Nb34O87-M (Figure S2); HRTEM image and SAED pattern of Mg2Nb34O87-M (Figure S3); EDX mapping image and spectrum of Mg2Nb34O87-M (Figure S4); relationship between redox peak current (Ip) and sweep rate’s square root (v0.5) for Mg2Nb34O87-M (Figure S5); determination of b values using relationship between peak current to sweep rate of Mg2Nb34O87-M and Mg 2 Nb 34 O 87 -P (Figure S6); Nyquist curves of Mg2Nb34O87-M and Mg2Nb34O87-P (Figure S7); results of crystal analyses in Mg2Nb34O87 and Ti2Nb10O29 (Table S1); atomic parameters of Mg2Nb34O87 (Table S2); polarization of Mg 2 Nb 3 4 O 8 7 -M/Li and Mg2Nb34O87-P/Li cells at 0.2 mV s−1 (fourth cycle) (Table S3); comparisons of electrochemical performance of Mg2Nb34O87-M/Mg2Nb34O87-P with previously reported M−Nb−O anode materials (Table S4) (PDF)

Mg 2Nb34 O87 + x e− + x Li+ ↔ LixMg 2Nb34 O87 (0 ≤ x ≤ 68)

ASSOCIATED CONTENT

S Supporting Information *

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(4)

This intercalating characteristic undoubtedly results in the excellent cycling stability of the Mg2Nb34O87 products (Figure 4e).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

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ORCID

Chunfu Lin: 0000-0003-0251-7938 Yongjun Chen: 0000-0001-6587-7581

CONCLUSIONS We explore highly Li+ conductive Mg2Nb34O87 as a new and superior M−Nb−O anode material for high-performance LIBs. Mg2Nb34O87-P with a porous-microspherical morphology is synthesized through a novel solvothermal process and a postannealing treatment at only 800 °C. Mg2Nb34O87 shows a Ti2Nb10O29-type structure whose unit-cell volume is 1.5% larger, resulting in its approximately 2 orders of magnitude larger Li+ diffusion coefficients, which surpass those of the M− Nb−O materials previously reported. Mg2Nb34O87-P exhibits a relatively large specific surface area, very small primary particles, and hierarchical pores, thus offering a large

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation of China (51762014 and 51502064). REFERENCES

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