Passivation effect for current collectors enables high-voltage aqueous sodium ion batteries

Materials Today Energy 14 (2019) 100337

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Passivation effect for current collectors enables high-voltage aqueous sodium ion batteries Zhiguo Hou a, Xueqian Zhang a, Huaisheng Ao b, Mengke Liu b, Yongchun Zhu b, *, Yitai Qian b, ** a

Department of Applied Chemistry, Jiangsu University of Technology, 1801 ZhongWu Road, 213001, Changzhou, China Hefei National Laboratory for Physical Science at Microscale, Department of Applied Chemistry, University of Science and Technology of China, 96 JinZhai Road, 230026, Hefei, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 April 2019 Received in revised form 14 June 2019 Accepted 29 June 2019

The narrow electrochemical stability window (1.23 V) of electrolyte has hindered the development of aqueous sodium ion batteries greatly, especially the choice of electrode materials. Here, through theoretical calculation and experimental proof, it's found that the deposition of oxide films on both Al cathode and Ti anode current collectors exhibits strong passivation effect. When the thickness of Al2O3 film on Al is about 3 nm and the thickness of TiO2 film on Ti is about 5 nm, the electrochemical stability window of aqueous electrolyte can be expanded to 3.5 V. Therefore, TiS2 with low working potential (1.5 V versus Naþ/Na) can be used as anode material in aqueous sodium-ion battery for the first time. We assembled a full cell coupling TiS2 anode with Prussian blue cathode in 15 M NaClO4 aqueous electrolyte. The full cell of 2.6 V demonstrates cycle life up to 1000 times with high energy density (100 Wh kg1) and high rate capability (30 C). © 2019 Elsevier Ltd. All rights reserved.

Keywords: Aqueous sodium ion batteries Electrochemical stability window Current collectors TiS2 anode Electrode-electrolyte interphase

1. Introduction Aqueous sodium ion batteries (ASIBs) have attracted increasing attention for large-scale energy storage applications due to their several intrinsic advantages: (i) non-flammable electrolyte and no need of rigorous manufacturing facility, (ii) abundant sodium resources and low-cost electrolyte solvent and salts [1e4]. However, the practical application of ASIBs remains remote because of the low energy density resulting from their inherently narrow electrochemical stable window [5]. The narrow electrochemical stability window limits the operating voltage and especially the selection of suitable working electrodes. A series of materials with high specific capacity whose working potential is located beyond the H2 evolution potential (or O2 evolution potential) cannot function in aqueous battery systems [6e8]. As a result, up to now, few available electrode materials have been reported for ASIBs and the energy density is still no more than 60 Wh kg1 [2,3,9e14].

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Zhu), [email protected] (Y. Qian). https://doi.org/10.1016/j.mtener.2019.06.012 2468-6069/© 2019 Elsevier Ltd. All rights reserved.

Recently, the electrochemical stability window of aqueous electrolyte has been demonstrated to be expanded in “water-insalt” lithium ion electrolyte [5,15e17]. When the electrolyte concentration reached 21 m (mol/kg), the abundance of free water clusters was significantly diminished and thus the electrochemical activity of water was suppressed. Using a LiMn2O4 cathode and a Mo6S8 anode, the battery exhibited high energy density (100 Wh kg1), long cycle stability and high coulombic efficiency with working voltage of 2.3 V [5]. A room temperature hydrate-melt electrolyte with concentration of 27 m enabled a reversible reaction of commercial Li4Ti5O12 negative electrode at a very low reaction potential [18]. Coupled with LiCoO2 cathode, the full cell delivered an energy density of 130 Wh kg1 with operation voltage of 2.3e3.1 V. Using high concentration NaClO4 aqueous electrolyte (17 m) can widen the electrochemical window to 2.8 V, in which the full battery using Na2MnFe(CN)6 cathode and NaTi2(PO4)3 anode operates at 2.0 V [19]. Most of works are focused on electrolyte, while little attention is paid on current collectors. In practice, the water electrolysis (hydrogen evolution or oxygen evolution) mainly occurs at the surface of current collector during charge/discharge cycles [20]. Ti is the mostly used current collector in aqueous battery. But Ti exhibits high-activity on hydrogen evolution and suffers from

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hydrogen-induced cracking [21e23]. Very recently, the Al with spontaneous formed aluminum oxide film was used as anode current collector in “water-in-salt” aqueous lithium ion battery, which exhibits large overpotential on hydrogen evolution [18,24]. In contrast to hydrogen or oxygen evolution of water splitting, where catalysts were used to reduce the overpotential of hydrogen or oxygen formation [25,26], it is expected a passivation effect that the overpotential of hydrogen or oxygen formation is large enough to inhibit water electrolysis on current collector in aqueous battery. When passivated oxide film covered on the surface of metal current collectors, the energy barrier of water electrolysis will be increased [27,28], which can expand the stable window of aqueous electrolyte. In addition, according to previous anti-corrosive research, the oxide film on current collector surface can also inhibit its corrosion [29e31]. The coating on current collectors by the conductive materials can improve the electrochemical properties of batteries [32]. But the oxide film on the surface of current collector will result in conductive loss [33]. Therefore, it is important to find a balance between passivation effect and conductivity. In this work, through theoretical calculation and experimental proof, we found that the deposition of Al2O3 film on Al or TiO2 film on Ti used can increase the energy barrier of water splitting. The thickness of oxides film on the metallic surface can be well controlled by atomic layer deposition (ALD) or heat treatment in oxygen. When the thickness of Al2O3 film on Al is about 3 nm and the thickness of TiO2 film on Ti is about 5 nm, these oxide film covered metallic current collectors provide an expanded electrochemical stability window of ~3.5 V TiS2 with low reaction potential of 1.5 V versus Naþ/Na was used as anode material in ASIBs for first time. Moreover, in concentrated aqueous electrolyte of 15 M NaClO4, there formed a dense electrode-electrolyte interphase on TiS2 surfaces effectively protecting TiS2 from electrolyte corrosion. Coupled with manganese Prussian blue cathode, the resultant ASIBs demonstrated a high working voltage of 2.6 V and exhibited an energy density of 100 Wh kg1 with long cycle stability up to 1000 cycles.

2. Method and materials 2.1. Material syntheses The sodium perchlorate monohydrate, MnSO4, trisodium citrate, Na4Fe(CN)6, NaNO3, sulfur and titanium were all analytical reagent and purchased from Aladdin Industrial Corporation. The electrolyte solutions were prepared by dissolving the Na salt in ultrapure water. The cathode material sodium manganese hexacyanofferrate was synthesized through a co-precipitation reaction. Firstly, analytical pure 0.845 g MnSO4 and 1.47 g trisodium citrate was dissolved in 100 mL deionized water (solution A) and 2.112 g Na4Fe(CN)6 was dissolved in 100 mL water (solution B). Then solution A and B were co-injected into 200 mL water in which dissolved 60 g NaNO3 with a speed of 0.5 mL per minute under magnetic stirring. After aged 12 h, the white precipitation was filtered washed with water and ethanol three time. And then the product was dried under vacuum at room temperature for 10 h. The anode material was prepared through solid reaction. Analytical pure sulfur powder and titanium powder were mixed by stoichiometric ratio. Then the mixed powder was transferred to a quartz tube. After treated by vacuum for 5 min, the tube was heat-sealed and annealed under 660  C for 7 days. After heated treatment, the tube was cut using diamond glass cutter and the product was dumped out carefully and stored in 10 mL centrifuge tube sealed by plastic wrap to avoid direct contacting with air.

2.2. Material characterizations X-ray diffraction patterns of products were carried out on a Philips X'pert X-ray diffractometer with Cu Ka radiation (l ¼ 1.54182 Å) at a scan rate of 0.008842 s1. The microstructure of products was examined on a field-emitting scanning electron microscope (FESEM, JEOL-JSM-6700F). Raman spectra were recorded on an Invia Raman spectrometer, with an excitation laser wavelength of 514.5 nm. The atomic layer deposition was carried out on a Picosun, Sunale R-200 Advanced device. The deposition temperature of Al2O3 is 250  C. The deposition temperature of TiO2 is 450  C. The thickness of TiO2 and Al2O3 film was evaluated on ellipsometer SOPRA, GES5E instrument. 2.3. Electrochemical measurements The working cathode electrodes were prepared by mixing the active material, super P and polyvinylidene fluoride (PVDF 5 wt%) binder in a weight ratio of 80:10:10 to form a viscous slurry easy to be smeared onto Ti sheet. The anode electrodes were prepared by mixing active material, super P and PVDF in a weight ratio of 80:10:10 and then pasted onto Al foil current collector, respectively. Unless stated, the active loading of both anode and cathode electrodes was about 15 mg cm2. The as-prepared electrodes were dried in vacuum oven at 60  C and then pressed at a pressure of 5 MPa for 2 min using a manual hydraulic press. The electrochemical stability window of electrolyte was measured via linear sweep voltammetry by using three-electrode cell with a current collector working electrode, Pt counter electrode and Ag/AgCl (in saturated KCl aqueous solution, 2.92 V versus Naþ/Na) reference electrode at a scan speed of 0.1 mV s1 on a CHI 600E electrochemical work station. The current collector such as Ti and Al is shining without oxidation film purchased from Aladdin Industrial Corporation and washed with water and ethanol before using. The heat treatment temperature of Al is 250  C in oxygen condition with variety time from 10 s to 30 min. And the heat treatment temperature of Ti is 600  C in oxygen condition with treat time from 10 s to 10 min. The CV results of cathode and anode electrodes were measured using three-electrode system with a cathode (or anode electrode with active loading of about 15 mg and active area about 1 cm2) working electrode, Pt counter electrode and Ag/AgCl reference electrode at a scan rate of 0.1 mV s1 on CHI 600E electrochemical work station. The full ASIBs cells were assembled in CR2016-type coin cell using a MFCN cathode, TiS2 anode and glass fiber as separator in 15 m NaClO4 aqueous electrolyte and cycled galvanostatically on a Land cycler (Wuhan Kingnuo Electronic Co., China) at room-temperature. All electrolyte solutions were purged with N2 for half an hour to drive away oxygen. And all batteries asassembled were treated with 12 h standing before tested. 3. Results and discussion 3.1. Al foil and Ti sheet current collectors Fig. 1 shows redox potentials of some electrode materials that can possibly be used for aqueous sodium ion batteries (ASIBs). Materials whose working potential should be located between the H2 evolution potential and O2 evolution potential are suitable for ASIBs. However, the thermal stable window of aqueous electrolyte is approximately 1.23 V, which is narrow than that of organic electrolyte (>3 V). Here, NaClO4 is chosen as solute due to its abundance and low-cost. The electrolyte concentration is an important factor that affects Naþ conductivity and electrolyte viscosity. As shown in Supplementary Fig. 1a, the Naþ conductivity increases first, and then decreases with the rising of electrolyte

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Fig. 1. Electrochemical stability range of water and redox potentials of possible electrode materials for aqueous sodium ion batteries.

concentration. The viscosity increase rapidly with the increase of the concentration. The electrochemical stability window also has correlationship with electrolyte concentration. As shown in Supplementary Fig. 1b, the overall stability window expands as the concentration increases, with both oxygen and hydrogen evolution potentials pushed well beyond the thermodynamic stability limits of water. The 15 M electrolyte exhibits moderate Naþ conductivity (70 mS cm1) and expanded electrochemical stability window (3.5 V). The higher concentration (17 M) electrolyte exhibits low Naþ conductivity (30 mS cm1) and high viscosity 100 (mPa s), which is harmful to Naþ intercalation/extraction. Considering above factors, the 15 M electrolyte should be the best one. The electrochemical stability window of 15 M NaClO4 aqueous electrolyte on fresh Al and Ti current collectors are evaluated by means of linear sweep voltammetry (LSV) using a low scan rate (0.1 mV s1). Fresh Al foil is not stable in this concentrated aqueous electrolyte. There can be observed bubble on the surface of Al foil after soaked in aqueous electrolyte for a little while. The onset oxidation of fresh Ti sheet occurs at 3.9 V, which is prior to oxygen evolution process on Pt plate. The stability window measured on Ti sheet is about 1.85 V. Although the room temperature sodium ion batteries in nonaqueous electrolyte have obtained great progress at very recently [34e38], only very limited materials can be used in ASIBs. According to previous aqueous electrocatalytic research, the oxides exhibit high energy barrier on water electrolysis which can effectively suppress the hydrogen or oxygen evolution [39,40]. Theoretical computation and analysis in Fig. 2a shows that the energy barrier of hydrogen evolution reaction is increased after covered with Al2O3 film on the surface of Al current collector. Similarly, after coated with TiO2 film on the surface of Ti, the energy barrier (DGO* - DGHO*) of oxygen evolution reaction is also increased, which indicates the overpotential of oxygen evolution on TiO2 film is higher than that on Ti. The calculation results suggest that the stable window of aqueous electrolyte can be widened with the help of oxide film passivation effect. However, the conductivity of the current collector demonstrates a negative correlation with increasing the thickness of oxide film. Therefore, it is important to find out a balance between passivation effect and conductivity maintenance of the current collector covered with oxide film. By means of atomic layer deposition (ALD) method, the Al2O3 film with controllable thickness is deposited on Al current collector and the TiO2 film with controllable thickness is deposited on Ti current collector. In the case of Al2O3 film deposited on Al current collectors, the thicknesses of Al2O3 film are distributed from 0.5, 1,

1.5, 2, 3 to 5 nm. As for TiO2 film deposited on Ti current collectors, the thicknesses of TiO2 film are designed to 1, 2, 3, 5 and 10 nm, respectively. The influence of oxide film on both aqueous electrolyte electrochemical window and the current collector conductivity were evaluated. As shown in Fig. 2b, when the thicknesses of deposited Al2O3 film are 0.5, 1, 1.5, 2 and 3 nm, the hydrogen evolution potential on Al current collectors are 1.8, 1.6, 1.5, 1.3 and 1.2 V versus Naþ/Na in 15 m NaClO4 aqueous electrolyte, respectively. As the thickness of aluminum oxide reaches 5 nm, there is still no obvious current of hydrogen evolution detected until 1 V versus Naþ/Na. Those mean that after deposited Al2O3 on the surface of Al foil anode current collector, the hydrogen evolution potential is pushed much lower than that using Ti as current collector. The conductivity of current collector plays an important role in battery electrochemical performances. The resistivity of Al current collectors were measured to find out the influence of deposited Al2O3 film with different thickness on conductivity. As shown in Fig. 2d, when the thicknesses of deposited Al2O3 film are 0.5, 1, 1.5, 2, 3 and 5 nm, the resistivity of current collectors are 0.9, 1.8, 2, 2.5, 3 and 8.9 U cm2, respectively. Considering hydrogen evolution and conductivity, Al current collector deposited with 3 nm Al2O3 film may be a balance point. As for Ti cathode current collector, when the thicknesses of deposited TiO2 reached to 5 nm, the oxygen evolution potential is about 4.8 V versus Naþ/Na (shown in Fig. 2c). The resistivity of Ti current collectors deposited with TiO2 film was also measured. As shown in Fig. 2e, when the deposited TiO2 film thicknesses are 1, 2, 3, 5 and 10 nm, the resistivity of current collectors are 2.5, 5, 6, 7 and 12 U cm2, respectively. Therefore, the Ti current collector coated with 5 nm TiO2 film should be the critical point, after weighting up between oxygen evolution and conductivity. As experimental and theoretical demonstrated above, the Al2O3 or TiO2 film deposited on metals can increase the energy barrier of water electrolysis. When the thickness of Al2O3 on Al is 3 nm and the thickness of TiO2 on Ti is 5 nm, the stable window of electrolyte is expanded to 3.5 V and the conductivity is remained which enables the battery function well. To facial and large-area form oxide film on current collector, chemical oxidation method is adopted to form uniform and dense oxide film on metallic current collector. Through groping for oxidation temperature to control the film growth, the optimum treatment temperatures for Al and Ti foil are 250  C and 600  C, respectively. The heating rates during the fabrication of Al2O3 layers on Al current collector and TiO2 layers on Ti current collector are 20  C min1 and 15  C min1, respectively. After heat treatment in oxygen at 250  C for 10 s, 30 s, 1 min, 5 min, 10 min and 30 min, the thicknesses of

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Fig. 2. Stability of Al and Ti current collectors in 15 m NaClO4 aqueous electrolyte. (a) Adsorption energy of O* and HO* on Ti or TiO2 and adsorption energy of H2O* and H* on Al or Al2O3 surfaces, respectively. (b) The potential of hydrogen evolution on Al, Pt and fresh Ti and the thickness of Al2O3 coated on Al is 0.5 nm, 1 nm, 1.5 nm, 2 nm, 3 nm and 5 nm, respectively. (c) The potential of oxygen evolution on Ti and Pt electrodes and the thickness of TiO2 coated on Ti is 1 nm, 2 nm, 3 nm, 5 nm and 10 nm, respectively. (d) Resistivity and hydrogen evolution potential of Al current collectors coated with different thickness of Al2O3. (e) Resistivity and oxygen evolution potential of Ti current collectors coated with different thickness of TiO2.

oxide coated on the Al foil are 0.5 nm, 0.9 nm, 1.4 nm, 2.3 nm, 3.1 nm and 4.4 nm, respectively (as shown in Fig. 3a), which are measured using variable angle spectroscopic ellipsometry. Here, the six Al foil after heat treatment are labeled as A-0.5, A-1, A-1.5, A-2, A-3 and A-5. As shown in Supplementary Fig. 2, X-ray photoelectron spectroscopy (XPS) was carried out of Al foil after heat treatment. There are two peaks located at 72.8 and 74.9 eV, which can be attributed to the Al metal 2p and signals of Al3þ [41]. It indicates that the oxide covered on the surface of Al foil is Al2O3 after heat treatment. The XRD patterns of Al foil after different heat treatment time were also presented. There are no other peaks except for cubic Al even after 30 min heat treatment as shown in Supplementary Fig. 3. It suggests that the Al2O3 film covered on Al foil is amorphous. The Ti sheet was also subjected to heat treatment at 600  C in oxygen with different time. The thicknesses of oxide on Ti are 1.2,

2.3, 3.1, 5.0 and 10.2 nm for heat treat time of 10 s, 30 s, 1 min, 5 min and 10 min, respectively (shown in Fig. 3d). Here, the five Ti sheet after heat-treated are labeled as T-1, T-2, T-3, T-5 and T-10 for convenience. As shown in Supplementary Fig. 4, the XPS results of Ti sheet for T-1 show that there exist three distinct peaks located at 454, 458 and 464 eV, respectively, which can be attributed to the 2p of Ti metal and Ti4þ [42e44]. It suggests that the oxide covered on surface of Ti sheet is TiO2. The XRD patterns were carried out to investigate the structure and crystalline form of the TiO2. There are no other peaks except for Ti until heat-treated 1 min (shown in Supplementary Fig. 5a). For T-3, there arises a peak at about 27.6 , which should be ascribed to the rutile. The Raman spectra indicate that there exists rutile on Ti sheet for T-3 (as shown in Supplementary Fig. 5b). As shown in Fig. 3c, the resistivity of current collectors are 0.5, 1.0, 1.5, 2.2, 2.5 and 8.5 U cm2 for A-0.5, A-1,

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Fig. 3. Composition and structure of Al foil and Ti sheet after heat treatment in air. (a) Thickness of aluminum oxide on Al foil surface after different heat treatment time measured on ellipsometer. (b) The potential of hydrogen evolution on heat-treated Al foil in 15 m NaClO4 aqueous electrolyte. (c) Balance point of resistivity and hydrogen evolution potential of Al current collectors coated with different thickness of Al2O3. (d) Thickness of titanium oxide on Ti sheet surface after different heat treatment time measured on ellipsometer. (e) The potential of oxygen evolution on heat-treated Ti sheet in 15 m NaClO4 aqueous electrolyte at a scan rate of 0.1 mV s1. (f) Balance point of resistivity and oxygen evolution potential of Ti current collectors coated with different thickness of TiO2.

A-1.5, A-2, A-3 and A-5, respectively. As for Ti current collector, the resistivity of current collectors are 1.5, 3, 4.5, 5.2 and 10.5 U cm2 for T-1, T-2, T-3, T-5 and T-10, respectively (as shown in Fig. 3f). The electrochemical stability of Al and Ti foil after heat treatment was also evaluated by means of LSV at a scan rate of 0.1 mV s1 in 15 M NaClO4 aqueous electrolyte. After heat treatment, the Al2O3 formed on the surface of Al foil effectively inhibit the oxidation of Al, ensuring its stable in aqueous electrolyte. The hydrogen evolution potentials are 1.8, 1.7, 1.5, 1.4, 1.2 and 1 V versus Naþ/Na for A-0.5, A-1, A-1.5, A-2, A-3 and A-5, respectively (Fig. 3b). The hydrogen evolution potential is far beyond the thermodynamic stability of water which is similar to the results of ALD. The oxygen evolution on Ti sheet is effectively suppressed after heat treatment (as shown in Fig. 3e). For T-10, the oxygen evolution is completely suppressed until 5 V versus Naþ/Na. The LSV results demonstrate that the oxide film formed on surface of Al foil and Ti sheet by heat treatment can effectively protect them from water erosion and suppress the hydrogen (or oxygen) evolution. Combined the results of ALD and heat-treatment, the optimal thickness for Al2O3 and TiO2 is 3 nm and 5 nm, respectively, those are sample A-3 and T-5. 3.2. Naþ intercalation electrodes As demonstrated above, when the thickness of Al2O3 on Al is 3 nm and the thickness of TiO2 on Ti is 5 nm, the stable window of electrolyte can be expanded to 3.5 V and the conductivity is remained which enable the battery function well. Thereafter, a series of materials with high specific capacity whose working potential is located beyond the H2 evolution potential (or O2 evolution potential) can function well in aqueous battery systems. 3.2.1. TiS2 anode Layered TiS2 has been used for negative electrodes in nonaqueous sodium ion batteries [45,46]. But it has never been reported in an aqueous electrolyte, previously, because of the low

function potential that will trigger intensive hydrogen evolution on electrode surface. In this work, TiS2 was selected as anode material for ASIBs which is synthesized by solid-state reactions (see Method section). The XRD pattern shows that the as-prepared TiS2 powders are hexagonal structure, without other detected impurities (Supplementary Fig. 6a). The scaning electron microscopy images show that the as-prepared TiS2 is hexagonal nanoplate with thickness of about 100 nm (Supplementary Fig. 6b). The electrochemical properties of TiS2 as anode material for ASIBs are evaluated using fresh Al, A-3 and A-5 as current collector, respectively. As shown in Fig. 4a, using fresh Al foil as anode current collector, there is a redox couple located at 2.3 V versus Naþ/Na which are attributed to the Naþ intercalation process of TiS2 [45]. The cathodic current was increased quickly blow 1.8 V versus Naþ/Na, which suggests that the anode electrode is suffered from severe hydrogen evolution and the second intercalation peak is drown in the hydrogen evolution reaction. When using A-3 as current collector, two reversible redox couples emerged at 1.7 V and 2.3 V, respectively. And the second intercalation peak is also discernible at about 1.5 V which corresponds to further Naþ intercalation into TiS2 interlayer [46]. When using A-5 as current collector, the electrode suffers from severe polarization. The electrode impedance was evaluated by using electrochemical impedance spectroscopy (EIS). The impedance of electrode using A-5 as current collector is higher than that of using A-3 as current collector by one order of magnitude (as shown in Supplementary Fig. 7). So the large polarization of TiS2 using A-5 as current collector should be triggered by the increased electrode impedance. The charge/discharge performance of TiS2 anode using different current collectors was also measured. As shown in Fig. 4b, the initial charge capacity of electrode using fresh Al foil as current collector is about 220 mAh g1, exceeding the theoretical specific capacity of TiS2 [46]. It should be caused by the hydrogen evolution reaction at low working potential. The electrode using A-3 as current collector exhibits the capacity of 140 mAh g1 with stable cycling. The electrode using A-5 as current

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Fig. 4. Electrochemical properties of TiS2 anode and MFCN cathode in 15 m NaClO4 aqueous electrolyte. (a) CV profiles of TiS2 anode electrodes using different time heat-treated Al foil as current collector. (b) Charge capacity and (c) long cycle performance of anode electrodes at a current density of 150 mA g1. (d) CV profiles of MFCN cathode electrodes using different time heat-treated Ti sheet as current collector. (e) Charge capacity and (f) long cycle performance of cathode electrodes at a current density of 150 mA g1.

collector exhibits capacity of 100 mAh g1, which is lower than that using A-3 as current collector. As shown in Supplementary Fig. 8a, the Voltage-Capacity profiles of anode at 1 C rate show that two voltage plateaus are located at about 1.7 V and 2.3 V (versus Naþ/ Na). The capacity drops from 140 to 90 mAh g1 when the rate varies from 1 to 30 C (Supplementary Fig. 8b). Moreover, the anode undergoes 200 cycles at 1C (150 mA g1) with about 95% initial capacity retention (shown in Fig. 4c), indicating the high reversibility of TiS2 anode and favorable suppression of electrolyte decomposition on the anode surface. As shown in Supplementary Fig. 13, the initial coulombic efficiency using fresh Al foil current collector is only 37%. After a few cycles the coulombic efficiency increases, but it is still lower than 90%. When using A-3 as current collector, the initial coulombic efficiency of the electrode is 80% and quickly increases over 97% after 5 cycles. However, the coulombic efficiency of electrode using A-5 as current collector is no higher than 95%. Based on the EIS and charge/discharge cycling results, the electrode using A-3 as current collector delivers the best performance.

3.2.2. Prussian blue cathode Prussian blue analogues sodium manganese hexacyanofferrate (MFCN) with low-cost and high specific capacity of 150 mAh g1 is a promising cathode material for non-aqueous sodium ion batteries [47,48]. Our previous work has reported that MFCN exhibits two Naþ intercalation/extraction with a specific capacity of 150 mAh g1 during discharging/charge process in aqueous sodium ion battery [49]. The morphology and phase of the MFCN are shown in Supplementary Fig. 14. The electrochemical property of MFCN as cathode material using different Ti sheet current collector for ASIBs was evaluated. As shown in Fig. 4d, an anodic current was observed up 4.2 V using fresh Ti sheet as current collector which should be resulted from oxygen evolution. When using T-5 as current collector, the anodic current was effectively suppressed. The electrode impedance using T-10 as current collector is much larger than that using T-5 as current collector (EIS results as shown in Supplementary Fig. 11). Fig. 4e display the charge performance of cathode electrode. The initial charge capacity of electrode using

fresh Ti sheet as current collector is about 360 mAh g1, which should trigger serious oxygen evolution at high working potential. As shown in Supplementary Fig. 12a, the Voltage-Capacity profiles of anode at 1 C rate show that two voltage plateaus are located at about 3.6 V and 4.2 V (versus Naþ/Na). The specific capacity of the MFCN cathode drops from 135 to 60 mAh g1 when the rate varies from 1 to 30 C (Supplementary Fig. 12b). The cathode undergoes 200 cycles at 1C (150 mA g1) with over 97% Coulomnic efficiency, indicating the high reversibility of cathode and effective suppression of electrolyte decomposition on the cathode surface (as shown in Fig. 4f). The electrode using T-5 current collector exhibits initial coulombic efficiency of the electrode is 80%, and it quickly increases over 97% after 5 cycles (shown in Supplementary Fig. 13). The capacity of electrode using T-10 current collector is about 105 mAh g1 and faded quickly. Based on the EIS and charge/discharge cycling results, the electrode using T-5 as cathode current collector delivers the best performance.

3.2.3. Full cell Therefore, the full cells using A-3 and T-5 as current collector, TiS2 and MFCN for anode and cathode materials were assembled. The cathode/anode mass ratio was controlled to 1:1 (15 mg active material load). The CV test of full cell for the first five cycles was carried out at a scan rate of 0.1 mV s1 (Supplementary Fig. 14). There are two cathodic peaks located at 1.75 V and 2.5 V, respectively. These anodic peaks were emerged at 2.1 V, 1.7 V and 1.55 V, respectively. After the initial cycle, the left four cycles were almost overlapped with each other suggesting high reversible of the full battery. The galvanostatic profiles of full cell show that two voltage plateaus are located at about 1.75 V and 2.45 V. The initial charge and discharge capacity is about 65 and 52 mAh g1 at a rate of 1C (based on total weight of both cathode and anode materials), respectively. After 150 cycles, the discharge capacity remained 52 mAh g1, the capacity retention is as high as 90% (shown in Fig. 5b). The coulombic efficiency is above 97% during cycling except the first five cycles. The low coulombic efficiency (<98%) at 1C rate is the effect of residual hydrogen and oxygen evolution during charging process. Although there are trace parasitic water

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Fig. 5. Full cell electrochemical characterization. (a) The full cell voltage profiles at 1 C rate (1C ¼ 150 mA g1). (b) Cycle performance at 1 C. (c) Rate-capability and Ragone plot of full cell. (d) Long cycle performance at 5 C. (e) Schematic of power a red LED. (f) Performance of ASIBs based on various electrochemical couples.

splitting reactions, the XRD results of cathode and anode materials after long cycle performance show that the structure of electrode material keep stable after long cycle performance (as shown in Supplementary Fig. 15). Therefore, the full cell can exhibit good cycle stability considering low coulombic efficiency. Fig. 5c displays rate capability of the full cell. Even at a high rate of 30 C, the discharge capacity still remains 30 mAh g1. Ragone plots of discharged energy density and power density are shown in insert of Fig. 5c. A specific energy density of 100 Wh kg1 based on total weight of active electrode materials was obtained, when the power density is 200 W kg1. The long cycle performance of the full cell at a high rate of 5 C was demonstrated (as shown in Fig. 5d). After 1000 cycles, about 92% capacity retention and near 100% Coulombic efficiency are obtained. The full cell not only exhibits high energy density, but also demonstrates high power density, which is comparable to that of supercapacitor. For instance, the full battery can deliver a specific energy density of 40 Wh kg1 at a high power density of 5000 W kg1. Due to the 2.3 V high operation voltage, one cell can be capable of illuminating a red LED as shown in Fig. 5e. A full cell using Mo6S8 as anode material and MFCN as cathode material also cycles reversibly at 2.0 V and exhibits capacity of 80 mAh g1 based on the mass of anode material. Similarly, a symmetric cell using Na3V2(PO4)3 as both cathode and anode materials cycles reversibly at 2.3 V and delivers capacity of 30 mAh g1 based on the total mass of electrode materials (Supplementary Fig. 16). After 200 cycles, the LSV results of the electrochemical stable window on A-3 current collector is still 1.4 V, which suggests that the Al2O3 layer keep stable. Similarly, the electrochemical stable window on T-5 current collector is still 4.5 V after 200 cycles, which suggests that the TiO2 layer is stable (Supplementary Fig. 17). The high operating voltage and energy density of the full cell are the highest record among reported ASIBs and are comparable to that of aqueous lithium ion batteries using “hydrate-melt” electrolyte and “water in salt” electrolyte (Fig. 5f). Compared with “water in salt” aqueous electrolyte, own to passivation effect of current collector, much more electrode materials can be used in aqueous electrolyte exhibiting high energy

density at a moderate concentration with low-cost (as shown in Table S1). 3.3. Discussion In addition, it is interesting to find that such stable cycling performance and high coulombic efficiency are only observed in the 15 M NaClO4 electrolyte. As for 1 M NaClO4 electrolyte, the TiS2 anode did not work at all (shown in Supplementary Fig. 18). The ClO 4 is a typical “structure breaking” anion, high concentration ClO 4 can destroy the strong hydrogen bonds of the water molecules decreasing the amount of free water clusters thus reducing its electrochemical activity. The high concentration electrolyte enabled reversible reaction of TiS2 anode at a low reaction potential. We suppose that there may form a dense electrode-electrolyte interphase effectively isolating TiS2 from electrolyte, which not only suppresses hydrogen evolution, but also protects anode from side reaction with water. To reveal the existence of interphase, transmission electron microscopy (TEM) images and Raman spectrums and of the TiS2 after 100 cycles at 1 C were conducted. The TEM images (Fig. 6a and b) of the cycled TiS2 material show that the surface of hexagonal-TiS2 is covered by the amorphous regions. The Raman spectra of the cycled TiS2 were also carried out. As shown in Fig. 6c, the Raman lines at 241 and 343 cm1 can be assigned to Eg and A1g modes of TiS2, respectively [50]. On the basis of Raman results, we believe that in the 15 M electrolytes, there is an interphase serves as an electron barrier preventing the reduction of water while allowing Naþ migration. Evidence also comes from TEM images of the cycled TiS2, whose surface, when compared with the pristine state, is found to be covered with a crystalline phase 4e18 nm thick (Fig. 6b). The irregular interplanar spacing of this crystalline phase identifies it as imperfect crystalline TiO2. After 5 cycles in 1 M NaClO4 electrolyte, the specific Eg and A1g peaks of TiS2 are totally disappeared and there arise three distinct peaks at 150, 405 and 630 cm1, which should be ascribed to Eg and B1g modes of titanium oxide, respectively [51]. It suggested that after 5 cycles in 1 M NaClO4 electrolyte, the TiS2 is totally

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Z. Hou et al. / Materials Today Energy 14 (2019) 100337

Fig. 6. (a) TEM image of as-synthesized TiS2 with a scale bar of 5 nm. (b) TEM image of TiS2 after 100 cycles at a rate of 1 C. (c) Raman spectra of TiS2.

transformed into titanium oxide. As contrary, after 100 cycles in 15 M NaClO4 electrolyte, the specific Eg and A1g peaks of TiS2 are coexisted with titanium oxide, which indicates that there is only partial TiS2 are oxidized into titanium oxide. Based on the results of Raman spectra and TEM images, we can conclude that the TiS2 was coated with amorphous titanium oxide. The amorphous oxide acted as a dense interphase that can protect TiS2 anode material from side reaction with electrolyte. 4. Conclusions In summery, we report ASIBs with high working voltage and high energy density. In such ASIBs, the stability window of aqueous electrolyte is successfully expanded to 3.5 V as a result of passivation effect of oxide film on current collectors. Moreover, in concentrated electrolyte of 15 M NaClO4 electrolyte, a dense electrode-electrolyte interphase formed on TiS2 surfaces. Therefore, TiS2 with low reaction potential was demonstrated as anode materials in aqueous battery for first time. The resultant ASIBs using TiS2 as anode and MFCN as cathode delivered an energy density of 100 Wh kg1 with an open circuit voltage (OCV) up to 2.6 V and was cycled at nearly 100% coulombic efficiency for up to 1000 cycles. This work opens new avenues to aqueous sodium ion battery. The capability to expand the water electrochemical stability window to ~3.5 V through controllable modifying the metallic current collector could offer new opportunities to solve the narrow operating stable window of aqueous electrolyte and define an advanced step towards more selection of suitable electrode materials to achieve high energy density for aqueous battery systems. Acknowledgements This work was financially supported by National Key Research and Development Program of China (2016YFB0901503), National Natural Science Foundation of China (NO. 21875238, 21521001). We would like to give special thanks of the support from teacher Guangming Liu for the measurements of the ion conductivity and vapour pressures of the electrolyte solutions. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.mtener.2019.06.012. References [1] C.D. Wessells, R.A. Huggins, Y. Cui, Nat. Commun. 2 (2011) 550e554. [2] J. Whitacre, T. Wiley, S. Shanbhag, Y. Wenzhuo, A. Mohamed, S. Chun, E. Weber, D. Blackwood, E. Lynch-Bell, J. Gulakowski, J. Power Sources 213 (2012) 255e264.

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