Na1.68H0.32Ti2O3SiO4·1.76H2O as a Low-Potential Anode Material

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Na1.68H0.32Ti2O3SiO4·1.76H2O as a Low-Potential Anode Material for Sodium-Ion Battery Yao Liu,† Jingyuan Liu,† Yifan Wu,‡ Duan Bin,† Shou-Hang Bo,*,‡ Yonggang Wang,† and Yongyao Xia*,† †

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Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Institute of New Energy, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), Fudan University, Shanghai 200433 China ‡ University of MichiganShanghai Jiao Tong University Joint Institute, Shanghai Jiao Tong University, 800 Dong Chuan Road, Minhang District, Shanghai 200240 China S Supporting Information *

ABSTRACT: Sodium-ion batteries (SIBs) have been considered as a promising candidate for large-scale energy storage applications, because of the low cost of sodium element and a broad choice of cathode materials which do not contain expensive raw materials. However, a lack of promising anode materials still hinders the development of SIBs technology. Herein, we for the first time report a new onedimensional tunnel-structure anode material, Na1.68H0.32Ti2O3SiO4·1.76H2O, for SIBs. This material can deliver a reversible capacity of 110 mAh g−1 at a current density of 20 mA g−1 and with an average working voltage of 0.4 V vs Na+/Na. The structure changes of this material during discharge/charge processes were investigated by using in situ laboratory X-ray diffraction. The results indicated that sodium insertion proceeds via a topotactic intercalation pathway. We also identified predehydration as an effective avenue to further improve the capacity of Na1.68H0.32Ti2O3SiO4·1.76H2O anode (reversible capacity of 131 mAh g−1 at a current density of 20 mA g−1 after the predehydration process). KEYWORDS: titanosilicate sitinakite, anode material, sodium-ion battery, intercalation, hydrothermal synthesis, in situ XRD

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hazard. Alloy-based materials, such as Si, Sn, Sb, and Bi, are also anode candidates. However, these materials usually suffer from large volume change and low capacity retention during electrochemical cycling.8 Phosphide materials have been recently exploited as Na-ion battery anode materials.9 However, the high reactivity of these materials against the electrolyte and therefore complex electrode/electrolyte interface chemistry are difficult to alleviate. Oxide materials, in particular those utilizing the Ti4+/Ti3+ redox couple with low electrochemical potential, are another promising class of anode materials. These titanium-based oxide materials are usually air-stable, easy to synthesize, and nontoxic.10 In addition, most titanium-based materials (such as, NaTiO2 (∼1.1 V vs Na/Na+), TiO2 (∼0.7 V), Na2Ti6O13 (∼0.9 V), Na2Ti3O7 (∼0.6 V), and Li4Ti5O12 (∼0.8 V)) operate at relatively safe potentials, which are far from the Na metal plating voltage (0 V vs Na+/Na). However, the increased safety of Ti-based oxides is achieved at the expense of reduced energy density. It is, therefore, of uttermost importance to place the electrochemical potentials of Ti-based oxides at optimized voltages through compositional tuning.

ith emphasis on environmental protection and reducing fossil fuel consumption, rechargeable batteries are becoming increasingly important to sustain the development of our society.1 Lithium-ion batteries (LIBs) are ubiquitous in applications in our daily life for their excellent electrochemical performance, such as in portable devices, electric vehicles (EVs), or plug-in hybrid electrical vehicles (PHEVs).2 For large-scale applications, however, such as grid-scale energy storage, the low-cost and earth-abundant sodium-ion batteries (SIBs) technology is a strong competitor of the lithium counterpart. In recent years, we have witnessed a revival of research interest in SIBs.3 To date, a variety of sodium-ion battery cathode materials have been discovered, while the choices of anode materials are still limited. Unlike lithium and potassium,4,5 graphite cannot be used as an intercalation anode for sodium-ion battery. This constitutes one of the greatest challenges for sodium-ion battery research and motivates a search for other anode alternatives. Disordered carbon materials have been intensively studied, which have shown a large degree of sodium reversible intercalation.6,7 However, the densities of these disordered carbon materials are low, leading to low volumetric energy density. The slow sodium intercalation kinetics also limits the rate capability of these materials. Moreover, the low intercalation potential poses potential risks for sodium metal plating and thus a safety © XXXX American Chemical Society

Received: August 24, 2018 Accepted: September 25, 2018 Published: September 25, 2018 A

DOI: 10.1021/acsaem.8b01412 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Table 1. Electrochemical Performance of Various Titanium-Based Anode Materials for Sodium-Ion Battery Based on Previous Reports anode materials P2-Na0.66[Li0.22Ti0.78]O2 P2-Na0.6[Cr0.6Ti0.4]O2 P2-Na0.66Ni0.17Co0.17Ti0.66O2 Na0.8Ni0.4Ti0.6O2 O3-type Na0.66Mg0.34Ti0.66O2 Na2Ti6O13 Na2Ti3O7 NaTiO2 Na1.68H0.32Ti2O3SiO4·1.76H2O

reversible capacity (mAh g−1) current density (mA g−1) or Rete voltage window (av V vs Na/Na+) cycle no. 105 110 100 105 100 70 102 150 131

10.6 11.2 C/5 C/5 C/10 C/5 35.6 C/10 20

0.4−2.5 (∼0.7 V) 0.5−2.5 (∼0.8 V) 0.15−2.5 (∼0.6 V) 0.01−2.5 (∼0.8) 0.4−2.5 (∼0.8) 0.5−2.5 (∼0.9 V) 0.01−2.5 (∼0.6 V) 0.6−1.6 (∼1.1 V) 0−1.5 (∼0.4 V)

200 200 300 250 128 200 90 60 200

ref 21 22 23 24 25 26 27 28 this work

Figure 1. (a) Rietveld refinement of the powder synchrotron XRD pattern of the pristine NTSO. The experimental data are shown in black dots, the calculated pattern is shown in red, and the difference curve is shown in blue. (b) Crystal refined structure. (c) SEM images of NTSO. (d−f) TEM and HR-TEM images of NTSO and the corresponding selected area electron diffraction pattern (insert).

oxygen can be adjusted, therefore modulating the energy (i.e., voltage) required to extract electrons from the transition metal orbital during the charge process.11 This inductive effect has been elegantly demonstrated in Fe-based electrode materials.

Inductive effect has long been regarded as a commonly accepted strategy in tuning the voltages of battery materials. By introducing polyanion (such as PO43−) into a transiton metal oxide framework, the bond strength and covalency of metal− B

DOI: 10.1021/acsaem.8b01412 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials The Fe3+/Fe2+ redox couple lies at 1.1 V (vs Li/Li+) in typical oxides12 and is shifted to 2.6 V in LiFeBO3,13 3.4 V in LiFePO4,14 3.8 V in Li2FeP2O7,15 and 4.0 V in LiFeSO4F,16 following the trend of increasing inductive effect or electron withdrawing capability of these polyanions. Conversely, one can use a similar strategy to tune the energy (i.e., voltage) required to insert ions (Li+ or Na+) during the discharge process. Very recently, Xia’s group17 reported a new anode material Li2TiSiO5 for lithium-ion battery application, in which the Ti4+/Ti3+ redox couple is reduced to below 1.0 V (vs Li/ Li+) from 1.5 V as is typically observed in Ti-only lithium oxides (e.g., Li2Ti3O7 and Li4Ti5O12). Herein, as discussed below, we explore whether a similar effect of potential reduction exists in the closely related sodium titanosilicate materials. A series of sodium titanosilicate materials with an ideal formula of Na2Ti2O3SiO4·2H2O have been investigated by Clearfield and co-workers in the 1990s,18 because these materials can ion exchange with a wide array of alkali and alkaline-earth cations (e.g., Li+, Na+, K+, Cs+, Ca2+, and Sr2+), finding application in purification of waste solutions.19 The ion-exchange property is closely correlated with the structural characteristics. Na2Ti2O3SiO4·2H2O is crystallized in the space group of P42/mcm (space group No. 132) with onedimensional tunnel structure (along the c lattice direction, ∼5.5 Å × 5.5 Å in size). In this specific structure, while part of the Na atoms and crystal water molecules are contained in the tunnel space and are labile, the rigid structural framework is formed through connected Na−O, Ti−O, and Si−O polyhedra together with the remaining Na atoms and crystal water molecules. In reality, such materials with an ideal formula (i.e., Na2Ti2O3SiO4·2H2O) have never been reported before. Instead, because of insufficient tunnel space, the Na and O tunnel sites are usually occupied by less than 50%, leading to a series of reported phases with Na and/or H2O content less than 2 in the formula unit (e.g., NaHTi2O3SiO4·2H2O, Na1.64H0.36Ti2O3SiO4·1.8H2O20). In these phases, the much smaller protons (H+) are introduced to charge compensate the loss of Na cations through protonation of the crystal water which already exists in the structure, forming H3O+. It was postulated that these protons preferentially attach to tunnel water molecules, although the exact positions of the protons have not been determined experimentally. In this work, we have prepared Na1.68H0.32Ti2O3SiO4· 1.76H2O (NTSO) through hydrothermal synthesis and, for the first time, employed this material as sodium-ion battery anode. The formula of this material was determined based on Rietveld refinement results and charge neutrality considerations. This material can deliver a reversible capacity of 110 mAh g−1 at a current density of 20 mA g−1, with a safe average working voltage of 0.4 V vs Na+/Na, showing the lowest average working potential among different types of Ti-based anodes. The discharge specific capacity is also comparable with most Ti-based anodes and exhibits a high cycle stability (Table 1). Figure 1a shows Rietveld refinement of the powder synchrotron X-ray diffraction (SXRD) pattern of the pristine material. The diffraction data show excellent counting statistics even at a d-spacing of approximately 0.9 Å, which allows accurate structural determination of the as-prepared material. All diffraction lines can be indexed with a tetragonal lattice with the space group of P42/mcm (space group No. 132). The refined crystal structure is presented in Figure 1b. The

refinement and structural parameters are tabulated in Table 2 and Tables S1 and S2 of the Supporting Information. We Table 2. Crystallographic data for Na1.68H0.32Ti2O3SiO4· 1.76H2O based on the Rietveld refinement of the synchrotron X-ray data Radiation

Synchrotron X-ray (11BM, APS)

Crystal system Space group Lattice parameters Cell volume Density (calculated) λ Rwp Rp χ2

tetragonal P42/mcm (#132) a = b=7.80414(8) Å, c = 11.97029(5) Å, α=β=γ=90° 729.05(2) Å3 2.756(2) g cm−3 0.412732 Å 13.793% 9.952% 1.563

identified the 8o sites which occupancy of only 0.34 Na per formula. The total sodium content was calculated as 1.68 Na per formula. The hydrogen atoms are invisible to X-rays. Therefore, based on charge neutrality consideration, the formula of the as-prepared materials is calculated to be Na1.68H0.32Ti2O3SiO4·1.76H2O. The water content was also determined via thermogravimetric analysis (Figure S1), which was estimated to be approximately 1.9 H2O per formula unit, similar to the refinement results. The structure can be better understood when the formula was rewritten with “f″ and “t″ superscripts denoting atoms or atomic groups that are situated within the framework or tunnel space, Na1fNa0.68tTi2O3SiO4· [H2O]1f[H2O]0.44t[H3O+]0.32t, which shows that 0.68 Na, 0.44 H2O, and 0.32 H3O+ coexist in the tunnel (Table S1). The morphology of the as-prepared NTSO material was characterized by field emission scanning electron microscopy (FE-SEM) and high-resolution transmission electron microscopy (HR-TEM). The SEM image is displayed in Figure 1c. The sample consists of micrometer-sized cuboids (approximately 0.1 μm × 0.2 μm × 0.4 μm in dimension). These cuboids are single crystal in nature, as evidenced by selected area electron diffraction patterns (SAED; Figure 1e, insert). When the selected cuboid was viewed along the [100] zone axis, the observed diffraction pattern matched well with the simulated pattern based on the crystal structure as determined from SXRD patterns. Energy dispersive X-ray mapping (EDX mapping) data are shown in Figure S2, indicating homogeneous distribution of all elements contained in NTSO. As-prepared NTSO was employed as anode in SIBs. We first present the half-cell characterization data, followed by full cell test utilizing Na3V2(PO4)2O2F (NVPF) as positive electrode. The half-cell charge/discharge curves and cycle performance of NTSO are shown in Figure 2. Current density of the galvanostatic measurements was 20 mA g−1. The initial discharge specific capacity was 291 mAh g−1 (∼3.6 Na per formula unit), and the charge specific capacity was 110 mAh g−1 (∼1.4 Na per formula unit) (Figure 2a), showing large irreversible capacity. The capacity of subsequent cycles is stabilized at approximately 109 mAh g−1. The rate capability of NTSO (second cycle) is shown in Figure 2b. The discharge specific capacities were 96, 85, 73, and 62 mAh g−1 at current densities of 50, 100, 200, and 500 mA g−1, respectively. The cycle stability was evaluated under current density of 500 mA g−1. A discharge specific capacity of 40 mA g−1 can be achieved even after 1000 cycles (Figure 2c), corresponding to 66% of C

DOI: 10.1021/acsaem.8b01412 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Figure 2. (a) Charge−discharge curves of NTSO under current density of 20 mA g−1 in the voltage window of 0.01−1.5 V. (b) Rate capability under current densities of 20, 50, 100, 200, and 500 mA g−1, respectively. (c) Cycle performance of the anode material in sodium-ion battery under current density of 500 mA g−1.

Figure 3. Electrochemical performance of full cell with NTSO as anode and NVPF as cathode: (a, b) Charge/discharge curves and cycle performance of the full cell (before cycle measurements, the full cells were formed under a small current).

Figures S5 and S6). In the discharge process, the DNa+ values are in the range from 0.5 × 10−11 to 4.6 × 10−11 cm2 s−1. In the charge process, the DNa+ values are in the range from 0.6 × 10−11 to 3.2 × 10−11 cm2 s−1. This is considerably higher than the sodium-ion diffusion coefficient for the Na2Ti3O7 anode material in SIBs (3.48× 10−12 cm2 s−1), accounting for the improved rate capability of the NTSO anode.27 The full cell performance utilizing Na3V2(PO4)2O2F as cathode was subsequently investigated. NVPF was synthesized according to a previous report.29 The XRD pattern (Figure S7) of as-prepared NVPF suggests that the sample is single phase without impurities. Before full cell testing, NVPF was first investigated in half-cell configuration, and the cycling curves and cyclibility test are shown in Figures S8 and S9, consistent with reported electrochemistry data.29 Before cycle measurements, the full cells were subjected to a formation cycle under

the capacity retention (the cycle stability under current density of 20 mA g−1 is presented in Figure S3). To determine if the reaction is of capacitive or intercalation in nature, electrochemical reaction kinetics were investigated using cyclic voltammogram (CV) measurements with cavity microelectrode. The results are displayed in Figure S4. For sweep rate ranging from 1 to 50 mV s−1, the b-value for cathodic peaks is 0.533, indicating that the electrochemical reaction kinetics are linearly diffusion controlled. When the sweep rates are increased to 50 mV s−1, a change in slope corresponds to a decrease in b-value to 0.521 for the cathodic current. These results indicate that the reaction is of intercalation nature and the reaction kinetics are controlled by bulk diffusion. We further evaluated the diffusion coefficient of Na+ (DNa+) based on galvanostatic intermittent titration technique (GITT; D

DOI: 10.1021/acsaem.8b01412 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Figure 4. TEM images of NTSO anode discharged to 0.2 V (a, b) and 0.01 V (c, d) under low current density (10 mA g−1) and the corresponding SAED patterns (insert).

small current (current density is 10 mA g−1). The electrochemical performance of NTSO//NVPF full cell is presented in Figure 3. The initial discharge specific capacity was 113 mAh g−1 and was maintained at 77 mAh g−1 after 50 cycles, with an average voltage of approximately 3.3 V. The current density and specific capacity are calculated based on the negative electrode (i.e., NTSO) only. The energy density of the full cell is calculated to be 119 Wh kg−1 based on the total mass loading. Indeed, low initial Coulombic efficiency is currently a problem for the electrode material. The optimization is expected with follow-up research. Several approaches have been proposed to address the capacity inefficiency in the first cycle in current lithium-ion battery, such as high-concentration lithium salt, stabilized lithium metal powder, and lithium-rich cathode, which might be potential approaches to solve this problem. If the Achilles’ heel of low initial Coulomb efficiency of NTSO can be solved, the electrochemical performance of the full cell would be competitive with current commercial sodium-ion battery.30 The structural changes during Na+ insertion/extraction were investigated by using HR-TEM coupled with in situ laboratory XRD. Figure 4a shows the TEM image of NTSO anode discharged to 0.2 V, and the corresponding SAED pattern is shown in Figure 4b. The morphology of NTSO after discharging to 0.2 V remained almost unchanged compared with that of the pristine compound. The SAED pattern shows regular scattering spots which can still be indexed by the tetragonal lattice of NTSO, suggesting that these particles are still single crystal in nature and that the structural framework of NTSO was not altered during the deep discharge to 0.2 V vs Na+/Na. Even after being discharged to 0.01 V (Figure 4b,c), the morphology and structure still remain similar as in the pristine sample. These results demonstrated the structure and

morphology of NTSO were maintained during the discharge process, which are consistent with the results of in situ laboratory XRD shown below. The EDX mapping of NTSO after full discharge to 0.01 V is shown in Figure S10. All elements contained in NTSO are distributed homogeneously suggesting no phase separation. Furthermore, the amount of sodium element in the fully discharged sample appears to increase compared with the pristine sample, consistent with a Na intercalation process during discharge. The oxidation state of Ti at pristine and full discharge states were investigated by using X-ray photoelectron spectroscopy (XPS, Figure S11), which indicated the main redox reaction is focusing on the Ti4+ and Ti3+ during the insertion of sodium ions. In addition, the characteristic peaks for metallic Ti0 (Ti 2p1/2 at 459.2 eV and Ti 2p3/2 at 454.2 eV) appeared, which might form part of the metallic Ti0 on the surface of the electrode. In situ XRD data during C/30 (about 0.1 mA g−1) rate cycling is shown in Figure 5, with the electrochemistry data shown on the top left of Figure 5. The voltage window is 0.01− 1.5 V. The mass loading is about 10 mg. The XRD patterns have been selected between 20 and 40°. Diffraction patterns were collected every 30 min, resulting in a total of 80 scans (60 scans for discharge process and 20 scans for charge process). To better visualize, Figure 5 presents 24 selected scans. No new diffraction peaks indicative of first-order phase transition were observed in the diffraction patterns at different stages of charge (or discharge), suggesting that Na+ insertion/extraction in NTSO does not involve conversion reactions. While diffraction peaks stay almost at the same locations characteristic of “zero-strain” material, relative peak intensity change is present suggesting Na intercalation. The change in peak intensity is optimally visualized in the magnified view of 2θ ranges from 27 to 28°, from 33.5 to 35°, and from 35.5 to 37° E

DOI: 10.1021/acsaem.8b01412 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Figure 6. Charge−discharge curves of NTSO sintered under 450 °C under current density of 20 mA g−1 in the potential range from 0.01 to 1.5 V vs Na/Na+.

predehydration (The charge/discharge curves of NTSO sintered under 150, 250, 650, and 850 °C are shown in Figure S14). In summary, tetragonal NTSO was first characterized as anode material in SIBs. NTSO anode delivered a reversible capacity of 110 mAh g−1 and great cycle stability, with an average working voltage of 0.4 V vs Na+/Na. The full cell performance coupling with NVPF cathode was also investigated, which delivered a reversible discharge specific capacity of 113 mAh g−1 with an average working voltage of 3.3 V, yielding a high full-cell energy density of 119 Wh kg−1, comparable with currently commercialized Na-ion batteries. The reversible capacity of the full cell can still be maintained at 77 mAh g−1 after 50 cycles. The crystal structure, dehydration, and morphology of NTSO were also systemically investigated. It was observed that sodium-ion insertion proceeds via a topotactic intercalation pathway as evidenced by in situ XRD results. Finally, we identified predehydration is an effective avenue to further increase the reversible capacity of NTSO. The present work may provide a new approach to explore high-performance anode materials for SIBs, which may bring broad insterests from both acedemia and industry.

Figure 5. In situ XRD patterns collected during the first charge/ discharge of the NTSO electrode cycled between 0 and 1.5 V under a current rate of C/30. The labeled diffraction peaks correspond to the graphite and current collector (Al). The rest of the peaks are from the material.

(Figure 5 bottom). These results suggest that the structural framework of NTSO during the cycling process has not been altered substantially and that the Na insertion/extraction processes proceed through topotactic (de)intercalation. While the framework water is an essential structural building unit, tunnel water in NTSO can be possibly removed leading to additional Na intercalation sites (i.e., capacity). We have therefore performed thermogravimetric (TG) and SXRD measurements to investigate whether tunnel crystal water can be removed without collapse of the structural framework (Figure S1 and Figure S12). The TG measurement shows the sequence of water loss at different temperatures. When the sample was heated to 650 °C, that is the stage to which we assigned the loss of framework water, substantial change in SXRD was observed compared with the pristine sample, suggesting that the tunnel-structured framework is destroyed. When the sample was heated to lower temperature (e.g., 450 °C), the diffraction pattern is the same as the pristine sample, suggesting that the water removal process is in fact reversible. Through these measurements, it was concluded that part of the water molecules in NTSO can be removed without destroying the tunnel structural framework and that the maximum dehydration temperature is close to 450 °C. The results of the Raman spectrum of Na1.68H0.32Ti2O3SiO4·1.76H2O sintered under other temperatures at 150, 250, 650, and 850 °C further prove the structure changes process (Figure S13). The charge/discharge curves of NTSO sintered under 450 °C are shown in Figure 6, which can deliver a reversible capacity of 131 mAh g−1 at a current density of 20 mA g−1, representing 19% increase in capacity compared with samples without

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EXPERIMENTAL METHODS

See the Supporting Information for details on the experimental procedures.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.8b01412. Experimental details; (Tables S1 and S2) atomic site information and bond distances; (Figures S1−S14) Rietveld refinement results, characterization of materials (TG, EDX mapping, TEM, and XRD, etc.), and electrochemical test (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.-H.B.). *E-mail [email protected] (Y.X). F

DOI: 10.1021/acsaem.8b01412 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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(16) Ati, M.; Melot, B. C.; Chotard, J. N.; Rousse, G.; Reynaud, M.; Tarascon, J. M. Synthesis and electrochemical properties of pure LiFeSO4F in the triplite structure. Electrochem. Commun. 2011, 13, 1280−1283. (17) Liu, J.; Pang, W. K.; Zhou, T.; Chen, L.; Wang, Y.; Peterson, V. K.; Yang, Z.; Guo, Z.; Xia, Y. Li2TiSiO5: a low potential and large capacity Ti-based anode material for Li-ion batteries. Energy Environ. Sci. 2017, 10, 1456−1464. (18) Poojary, D. M.; Bortun, A. I.; Bortun, L. N.; Clearfield, A. Structural Studies on the Ion-Exchanged Phases of a Porous Titanosilicate, Na2Ti2O3SiO4·2H2O. Inorg. Chem. 1996, 35, 6131− 6139. (19) Khainakov, S. A.; Bortun, A. I.; Bortun, L. N.; Clearfield, A.; Trobajo, C.; Garcia, J. R. Hydrothermal synthesis and characterization of alkali metal titanium silicates. J. Mater. Chem. 1999, 9, 269−272. (20) Milne, N. A.; Griffith, C. S.; Hanna, J. V.; Skyllas-Kazacos, M.; Luca, V. Lithium Intercalation into the Titanosilicate Sitinakite. Chem. Mater. 2006, 18, 3192−3202. (21) Wang, Y.; Yu, X.; Xu, S.; Bai, J.; Xiao, R.; Hu, Y. S.; Li, H.; Yang, X. Q.; Chen, L.; Huang, X. A zero-strain layered metal oxide as the negative electrode for long-life sodium-ion batteries. Nat. Commun. 2013, 4, 2365. (22) Wang, Y.; Xiao, R.; Hu, Y. S.; Avdeev, M.; Chen, L. P2Na0.6[Cr0.6Ti0.4]O2 cation-disordered electrode for high-rate symmetric rechargeable sodium-ion batteries. Nat. Commun. 2015, 6, 6954. (23) Guo, S.; Liu, P.; Sun, Y.; Zhu, K.; Yi, J.; Chen, M.; Ishida, M.; Zhou, H. A High-Voltage and Ultralong-Life Sodium Full Cell for Stationary Energy Storage. Angew. Chem., Int. Ed. 2015, 54, 11701− 11705. (24) Guo, S.; Yu, H.; Liu, P.; Ren, Y.; Zhang, T.; Chen, M.; Ishida, M.; Zhou, H. High-performance symmetric sodium-ion batteries using a new, bipolar O3-type material, Na0.8Ni0.4Ti0.6O2. Energy Environ. Sci. 2015, 8, 1237−1244. (25) Zhao, C.; Avdeev, M.; Chen, L.; Hu, Y.-S. An O3-type Oxide with Low Sodium Content as the PhaseTransition-Free Anode for Sodium-Ion Batteries. Angew. Chem., Int. Ed. 2018, 57, 7056−7060. (26) Rudola, A.; Saravanan, K.; Devaraj, S.; Gong, H.; Balaya, P. Na2Ti6O13: a potential anode for grid-storage sodium-ion batteries. Chem. Commun. 2013, 49, 7451−7453. (27) Rudola, A.; Saravanan, K.; Mason, C. W.; Balaya, P. Na2Ti3O7: an intercalation based anode for sodium-ion battery applications. J. Mater. Chem. A 2013, 1, 2653−2662. (28) Wu, D.; Li, X.; Xu, B.; Twu, N.; Liu, L.; Ceder, G. NaTiO2: a layered anode material for sodium-ion batteries. Energy Environ. Sci. 2015, 8, 195−202. (29) Guo, J. Z.; Wang, P. F.; Wu, X. L.; Zhang, X. H.; Yan, Q.; Chen, H.; Zhang, J. P.; Guo, Y. G. High-Energy/Power and LowTemperature Cathode for Sodium-Ion Batteries: In Situ XRD Study and Superior Full-Cell Performance. Adv. Mater. 2017, 29, 1701968. (30) Bauer, A.; Song, J.; Vail, S.; Pan, W.; Barker, J.; Lu, Y. The Scale-up and Commercialization of Nonaqueous Na-Ion Battery Technologies. Adv. Energy Mater. 2018, 8, 1702869.

Yao Liu: 0000-0001-5514-1917 Duan Bin: 0000-0002-4142-9052 Yonggang Wang: 0000-0002-2447-4679 Yongyao Xia: 0000-0001-6379-9655 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Use of the Advanced Photon Source (11-BM) at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC0206CH11357. This work was supported by the National Key Research and Development Program of China (Grant No. 2016YFB0901500) and the National Natural Science Foundation of China with Grant No. 21333002.

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DOI: 10.1021/acsaem.8b01412 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX