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Nov 23, 2015 - An electrochemical energy storage system with high energy density, stringent safety, and reliability is highly desirable for next-gener...
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Aqueous Rechargeable Alkaline CoxNi2−xS2/ TiO2 Battery

Jilei Liu,†,⊥,‡,§ Jin Wang,†,#,§ Zhiliang Ku,† Huanhuan Wang,† Shi Chen,† Lili Zhang,*,‡ Jianyi Lin,⊥ and Ze Xiang Shen*,†,# †

Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore ⊥ Energy Research Institute@NTU (ERI@N), Nanyang Technological University, Singapore 639798, Singapore ‡ Heterogeneous Catalysis, Institute of Chemical Engineering and Sciences, A*Star, 1 Pesek Road, Jurong Island 627833, Singapore # Energy Research Institute (ERI@N), Interdisciplinary Graduate School, Nanyang Technological University, Singapore 637553, Singapore S Supporting Information *

ABSTRACT: An electrochemical energy storage system with high energy density, stringent safety, and reliability is highly desirable for next-generation energy storage devices. Here an aqueous rechargeable alkaline CoxNi2−xS2 // TiO2 battery system is designed by integrating two reversible electrode processes associated with OH− insertion/ extraction in the cathode part and Li ion insertion/extraction in the anode part, respectively. The prototype CoxNi2−xS2 // TiO2 battery is able to deliver high energy/power densities of 83.7 Wh/kg at 609 W/ kg (based on the total mass of active materials) and good cycling stabilities (capacity retention 75.2% after 1000 charge/discharge cycles). A maximum volumetric energy density of 21 Wh/l (based on the whole packaged cell) has been achieved, which is comparable to that of a thin-film battery and better than that of typical commercial supercapacitors, benefiting from the unique battery and hierarchical electrode design. This hybrid system would enrich the existing aqueous rechargeable LIB chemistry and be a promising battery technology for large-scale energy storage. KEYWORDS: aqueous, rechargeable, alkaline, CoxNi2−xS2/TiO2 energy efficiency and good cycle stability.10 However, they fall short in the safety issues arising from the flammability of the organic electrolyte and the reactivity of the electrode materials with the organic electrolytes in the case of overcharging or short-circuiting.11,12 Another challenge regarding LIBs is the limited rate capability and specific power that are restricted by the limited ionic conductivities of the organic electrolyte. In addition, the cost of LIBs is comparatively high, resulting from the special cell designing, the high requirements of manufacturing processes, and the high cost of organic Li salts and organic electrolytes.9,10,12 Shifting the rechargeable battery systems from an organic electrolyte to an aqueous electrolyte is intrinsically attractive, benefiting from several merits: (i) the high ionic conductivities of the aqueous electrolyte, which is generally on the order of 1 S/m, 2 orders of magnitude higher

T

he growing demands for electric automotive and regenerative energy storage applications and the increasing concern for climate change and pollution drive the search of high-performance electrochemical power sources that are also safe to operate, economically viable, and environmental friendly.1−4 Although rechargeable battery systems such as lead−acid, nickel−metal (i.e., cadmium, iron, zinc, or cobalt), Ni−MH, and Li ion batteries are already widely used in numerous technical applications, the intrinsic drawbacks of these systems impede their applications for large-scale energy storage.5 For example, the lead−acid and nickel−metal batteries are limited by their low specific energy density and the employing of environmentally threatened electrode materials. The nickel−iron battery is challenged by the self-discharge as a result of the corrosion and poisoning of the iron electrode.6,7 The Ni−MH possesses higher energy density, but delivers limited high-rate capability, large self-discharge, and poor lowtemperature capability.8,9 Lithium ion batteries (LIBs) are regarded as the most promising power sources for portable or electric vehicle applications because of high energy density and © XXXX American Chemical Society

Received: October 6, 2015 Accepted: November 22, 2015

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DOI: 10.1021/acsnano.5b06275 ACS Nano XXXX, XXX, XXX−XXX

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Figure 1. Schematics of the CoxNi2−xS2 // TiO2 battery. (a) Electrodes’ structure based on GF/CNTs hybrid films and CoxNi2−xS2 // TiO2 cell structure. (b) Working mechanism of the CoxNi2−xS2 // TiO2 battery and involved electrochemical reactions.

than those of organic electrolytes.9 (ii) the intrinsic safety, and (iii) the low cost.8,13−15 The electrochemical behavior of an aqueous rechargeable battery (ARB) is similar to that of organic systems, with Li+ being intercalated into or extracted from the active materials during electrochemical redox reaction processes. The first aqueous rechargeable LIB utilized a LiMn2O4 cathode and a VO2(B) anode in 1994.16 Since then, numerous electrochemical redox couples have been explored for aqueous rechargeable LIBs, including LiMn2O4 // LixV2O5,17 LiNi1/3Mn1/3Co1/3O2 // LixV2O5,18 LiCoO2 // LiV3O8,19,20 LiMn2O4 // LiTi2(PO4)3,12 LiMn0.05Ni0.05Fe0.9PO4 // LiTi2(PO4)3,21 and LiFePO4/C // LiV3O8.22 Although attractive, the aqueous rechargeable LIB systems suffer from limited energy density and severe capacity degradation of cathode materials upon battery cycling.10,23 These resulted from (i) H+ co-insertion into the electrode structure accompanied by Li+/ H+ exchange during charge/discharge processes, (ii) penetration of water into the structure, and (iii) dissolution of active materials in the aqueous electrolytes.16−19,23−25 Several approaches, such as cathode modification using dopants or additives, electrode/electrolyte interface control using a coating, and electrolyte composition adjusting, have been proposed to address these issues.26−28 Despite significant improvements made, aqueous rechargeable LIBs are still not satisfying for practical applications in terms of specific capacity and cycling stability. In this work, a type of aqueous rechargeable alkaline CoxNi2−xS2 // TiO2 battery system is designed by integrating two reversible electrode processes associated with OH− insertion/extraction in the CoxNi2−xS2 cathode part and Li+ ion insertion/extraction in the TiO2 anode, respectively. CoxNi2−xS2 nanosheets and an anatase TiO2 thin film supported on lightweight graphene foam (GF)/carbon nanotube (CNT) hybrid films were chosen as the cathode and the

anode (Figure 1a). A mixed aqueous alkaline solution of 2 M LiOH/4 M KOH was used as the electrolyte. The coupling of an alkaline battery active cathode with an aqueous rechargeable LIB anode together would promote OH− insertion/extraction in the cathode part and Li+ insertion/extraction in the anode simultaneously and overcome the capacity limitation and cycling degradation of traditional cathode materials (i.e., lithium transition metal oxide compounds) in aqueous LIBs (Figure 1b). The CoxNi2−xS2 // TiO2 battery delivers superior electrochemical performance in terms of the energy/power densities and cycling stability, benefiting from the unique battery and hierarchical electrode design.

RESULTS AND DISCUSSION Structural Characterization of GF/CNTs/CoxNi2−xS2 and GF/CNTs/TiO2 Hybrid Films. CoxNi2−xS2 was grown on the GF/CNTs hybrid films directly via an in situ electrochemical deposition method (see Experimental Section).29 Typical field-emission scanning electron microscopy (FESEM) images of the resulting GF/CNTs/CoxNi2−xS2 hybrid electrode are shown in Figure 2a. Clearly, flower-like CoxNi2−xS2 nanosheets surround the CNTs uniformly with outer diameters ranging from 200 to 500 nm, depending on the mass loading amount (Figure S1). The areal mass loading of CoxNi2−xS2 could be tuned from 0.7 to 2.0 mg/cm2 easily by adjusting the deposition time (Figure S1). TEM images (Figure 2b) verify the high crystalline properties of CoxNi2−xS2 and the intact contact between it and CNTs. This is favorable for effective charge transfer, giving rise to enhanced rate performance. The fast Fourier transformation (FFT) electron diffraction pattern (inset in Figure 2b) reveals the (100), (002), and (110) planes of CoxNi2−xS2 (space group: P63/ mmc). X-ray diffraction (XRD) analysis further confirms the high crystalline properties of CoxNi2−xS2 (Figure 2c). All the peaks could be indexed to Co0.12Ni1.88S2 (JCPDS #70-2849), B

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Figure 2. Characterization of the GF/CNTs/CoxNi2−xS2 hybrid film. (a) Typical FESEM image (inset is the magnified SEM image) and (b) TEM image (inset is the fast Fourier transformation (FFT) pattern of the rectangle area). (c) XRD pattern and (d) corresponding element mappings of S, Ni, and Co. XPS spectra of S 2p (e), Ni 2p (f), and Co 2p (g).

is relatively weak. These features indicate the small amount of Co element, in good agreement with XRD results. Deterministic control over the thickness or mass loading of the TiO2 active materials is achieved via atomic layer deposition (ALD), which is a key technology for uniform and conformal layer deposition with atomic-scale thickness control, based on alternating self-limiting surface reactions.33,34 Representative SEM images in Figure 3a reveal that the TiO2 film coated conformally on the GF/CNTs electrode support is composed of uniformly distributed nanosized particles, forming a welldefined interconnected network structure. The mesoporous structure of the TiO2 films with a pore size of 2−20 nm can be further confirmed by TEM image (Figure 3b) and N 2 adsorption/desorption test (Figures S2c,d). The porous structure has a specific surface area (SSA) of 78 m2/g. The mesoporous structures are favorable for intense and fast charge storage/release in the GF/CNTs/TiO2 electrode. The fast Fourier transformation electron diffraction pattern (inset in Figure 3b) reveals the (101) and (112) planes of anatase TiO2 (space group: I41/amd). The well-defined lattice fringe with a distance of 0.35 nm corresponds to the d-spacing of the (101) plane. Figure 3c plots the X-ray diffraction pattern of the GF/ CNTs/TiO2 electrode. In addition to the graphite band at 2θ = 26.4° due to the carbon substrate, all the characteristic peaks

except for the crystalline peak at 26.4°, which results from the GF/CNTs substrate, and the peak at 46.5°, which corresponds to the small ratio of Co4S3 (JCPDS #02-1458). The N2 adsorption/desorption test reveals the mesoporous structure of the GF/CNTs/CoxNi2−xS2 electrode with a pore size distribution from 2 to 10 nm (Figures S2a,b). The specific surface area is as high as 133 m2/g. The porous structure could facilitate the electrolyte ion diffusion and thus maximize ionic conductivity. The corresponding element mappings in Figure 2d illustrate the uniform distribution of S, Ni, and Co. More details about the composition and oxidization states of CoxNi2−xS2 were collected via X-ray photoelectron spectroscopy (XPS) analysis (Figure 2e−g). Two typical peaks centered at 163.8 and 168.7 eV were detected (Figure 2e), corresponding to metal−sulfur (Ni−S or Co−S) bonding and a sulfate impurity, respectively.30,31 The deconvolution of the Ni 2p delivers a spin−orbit doublet characteristic of Ni2+ and two shakeup satellites (Figure 2f).30−32 The peaks centered at 853.5 and 871.5 eV are assigned to Ni 2p3/2 and Ni 2p1/2, respectively. The Co 2p spectrum is displayed in Figure 2g.30−32 In addition to the typical Co 2p3/2 and Co 2p1/2 spin−orbit doublets, Ni LMM is also observed at the lower binding energy. Furthermore, the intensity of the Co 2p peaks C

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Figure 3. Characterization of the GF/CNTs/TiO2 hybrid film. (a) Typical FESEM image (inset is enlarged view) and (b) TEM image (inset is the fast Fourier transformation (FFT) pattern of the rectangle area). (c) XRD pattern and (d) Raman spectra. XPS spectra of Ti 2p (e) and O 1s (f).

Cox Ni 2 − xS2 + OH− ↔ Cox Ni 2 − xS2OH + e−

can be well indexed as anatase TiO2 (JCPDS 71-1167).35,36 The Raman spectrum (Figure 3d) exhibits characteristic peaks of anatase TiO2 at 144, 398, 516, and 640 cm−1, agreeing well with XRD results.36,37 The typical peaks of the graphene, D, G, and 2D bands, are also observed. The relatively low intensity of the disorder-induced D band at 1350 cm−1 corroborates the high quality of the graphene foam support. The XPS Ti 2p spectrum of the TiO2 electrode with two peaks centered at 458.7 and 464.4 eV is characteristic of TiO2, corresponding to Ti 2p3/2 and Ti 2p1/2, respectively.38 The absence of Ti 2p3/ 2 at lower binding energy, e.g., 457.6 for Ti3+ and 456.4 eV for Ti2+, further verifies the pure TiO2 phase (Figure 3e).39,40 The O 1s spectrum (Figure 3f) can be resolved into two peaks centered at 530.1 and 532.1 eV, corresponding to the crystal lattice oxygen (O2−) bonded with Ti4+ ions in TiO2 and the oxygen in the hydroxyl group (−OH) or carboxyl group (O− CO), respectively.39,41 Electrochemical Properties of GF/CNTs/CoxNi2−xS2 and GF/CNTs/TiO2 Electrodes. The electrochemical properties of an individual GF/CNTs/CoxNi2−xS2 cathode and GF/ CNTs/TiO2 anode were investigated using a three-electrode system with 2 M LiOH/4 M KOH aqueous electrolyte. Figure 4a and b exhibit typical cyclic voltammetry (CV) curves of GF/ CNTs/CoxNi2−xS2-1.1 and GF/CNTs/TiO2-2.0, respectively, at various sweep rates. The two electrodes are selected because they are charge-matching electrodes (Figure S8) (1.1 and 2.0 denote the areal mass densities of the corresponding active materials in units of mg/cm2, while the areal mass density of the substrate, i.e., GF/CNTs hybrid films, is 0.75 mg/cm2) (more details about the calculation of active material areal density can be found in SI Figure S3). As shown in Figure 4a a pair of welldefined redox peaks in the range 0−0.5 V was characterized explicitly, which corresponds to the reversible redox reaction with CoxNi2−xS2:29,42

(1)

The GF/CNTs/TiO2 electrode (Figure 4b) displays a redox couple in the range of −1.1 V to −0.8 V, corresponding to the reversible electrochemical reduction/oxidation of the anatase titanium oxide in a Li+-containing aqueous electrolyte.43 The lithium ion insertion/deinsertion reaction within anatase TiO2 is illustrated as follows: TiO2 + x Li+ + x e− ↔ LixTiO2

(2)

The redox peaks in both Figure 4a and b were found to shift depending on the sweep rate. The anodic peaks (Ea) shift to a more positive potential, while the cathodic peaks (Ec) shift to a more negative potential position with increasing sweep rate, giving rise to the increase in the peak potential separation (ΔEa,c) (Figures S4f and S7a).44,45 The increase in the redox peak separation with increasing sweep rate implies the loss of reversibility, which is most probably due to charge diffusion polarization within the electrode material.46 The intensity of the redox peaks increases with increasing sweep rate. The Ipa vs ν plot displays a nonlinear behavior, while the plot of Ipa vs ν1/2 is linear (Figure 4c). Likewise the intensity of the corresponding cathodic peak is also linear with respect to ν1/2, indicating the oxidization/reduction of CoxNi2−xS2 is diffusion limited.47−50 A similar trend is also noted for anatase TiO2 (Figure 4d). These agree well with previous reports about Ni(OH)251 and LiFePO452 and corroborate their intrinsic battery-type electrochemical characteristic. By fitting the slope of the Ip vs ν1/2 plot, the lithium diffusion coefficients in GF/ CNTs/TiO2 during cathodic and anodic processes could be estimated (Figure S7b).53−55 The lithium diffusion coefficients for Li+ insertion and extraction were calculated to be 4.9 × 10−15 and 5.5 × 10−15, respectively. These values are quite comparable with previous reports for nanostructured titanium oxide films in aqueous electrolyte43 and higher than that for D

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Figure 4. Electrochemical characterizations of the hybrid electrodes. Left column: GF/CNTs/CoxNi2−xS2-1.1 cathode. Right column: GF/ CNTs/TiO2-2.0 anode. (a, b) Cyclic voltammetry curves at various sweep rates (ν). (c, d) Variation of anodic peak current (Ipa) with ν and ν1/2. (e, f) Galvanostatic discharge curves at various current densities.

surfactant-templated anantase TiO2 in aprotic electrolyte.55 Note that extraction is faster than insertion, suggesting a much faster lithium ion diffusion in anatase TiO2 than in LixTiO2.54 The accelerated lithium ion diffusion is attributed to the unique 3D electrode design, which provides fast charge transfer and shorter ion diffusion length. Typical discharge curves were plotted for the GF/CNTs/ CoxNi2−xS2-1.1 electrode (Figure 4e) and GF/CNTs/TiO2-2.0 (Figure 4f) with various discharge current densities. Distinct discharge plateaus were observed for both the CoxNi2−xS2 electrode and TiO2 electrode, matching well with corresponding redox peaks in the CV curves. The gravimetric capacities of CoxNi2−xS2-1.1 and GF/CNTs/TiO2-2.0 at different current densities (Figure 4e and f) were calculated from the corresponding galvanostatic discharge curves following the equation C* = IΔt/m, where I is the discharging current, Δt is discharging time, and m is the mass of the individual electrode. The GF/CNTs/CoxNi2−xS2-1.1 electrode delivers a discharge capacity of 201.2 mAh/g at a current density of 1 A/g. At a high scan rate of 4 A/g, the gravimetric capacity is retained at 108.4 mAh/g. The specific capacity of the GF/CNTs/TiO2-2.0 electrode is 104.2 mAh/g at 1 A/g, corresponding to the

reversible insertion of lithium ions forming LixTiO2 with x as high as 0.26. This insertion ratio is higher than that of TiO2 nanotube arrays56 and comparable with surfactant-templated anatase TiO257 in aqueous electrolyte. The GF/CNTs/TiO22.0 electrode delivers a specific capacity of 56.3 mAh/g even at 4 A/g, with the capacity retention of 54%. The good rate capabilities for both electrodes are attributed to the unique hybrid electrode design with the nanostructured active materials grown directly around the high-conductive carbon nanotubes. Each particle of active materials has its “own” current collector and, thus, provides a highly conductive pathway for electrons, a fast mass transport channel in the aqueous electrolyte, and a short electrolyte ion/proton diffusion length. Electrochemical impedance spectroscopy (EIS) measurements were carried out to gain additional kinetic characteristics of hybrid electrodes. The equivalent series resistances are estimated to be 3.9 Ω for GF/CNTs/ CoxNi2−xS2-1.1 and 8.1 Ω for GF/CNTs/TiO2-2.0, respectively (Figures S5 and S7c). These relatively small serial resistances ensure fast charge transfer between electrolytes and electrodes. Additionally, these hybrid electrodes exhibit good cycling performance benefiting from the intact mechanical contact E

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Figure 5. Electrochemical performance of the CoxNi2−xS2 // TiO2 battery. (a) Cyclic voltammetry curves and (b) peak potentials vs sweep rate (ν). Inset is the peak potential separation, ΔEa,c, vs sweep rate (ν). (c) Variation of anodic peak current (Ipa) with ν and ν1/2. (d) Galvanostatic discharge curves at various current densities. (e) Ragone plot. (f) Cycling performance of the CoxNi2−xS2 // TiO2 battery. Inset shows the CV curves (10 mV/s) for the 1st and 1000th cycles.

detail, the GF/CNTs/CoxNi2−xS2-0.7 illustrates the best specific capacity (∼241.3 mA h/g at 1 A/g) and capacity retention (∼57%) when the current density increases from 1 A/g to 4 A/g (Figure S4c). The capacity drops to 178.3 mA h/ g at 1 A/g for GF/CNTs/CoxNi2−xS2-2.0, with a capacity retention of ∼47% (Figure S4d). This is attributed to the decrease in electrical conductivity and the increase in electrode polarization (Figures S4f and S5) resulting from the increase in the thickness. The series equivalent resistance increases from 3.6 Ω for GF/CNTs/CoxNi2−xS2-0.7 to 5.1 Ω for GF/CNTs/ CoxNi2−xS2-2.0 (Figure S5). In addition, the redox peak potential separation, ΔEa,c, increases when more active materials are deposited, indicating the sacrifice of redox reaction reversibility. This is magnified by the sharp increase in ΔEa,c under a higher sweep rate (Figure S4f). These features highlight the importance of electrode design to achieve optimized electrochemical performance. Electrochemical Performance of the CoxNi2−xS2 // TiO2 Cell. Charge balance by adjusting the mass loadings of cathode/anode was carried out based on the above results (Figure S8). The mass ratio of the CoxNi2−xS2 cathode and

between active materials and the current collector. The capacity retentions of 92.9% and 85.4% have been delivered for GF/ CNTs/CoxNi2−xS2-1.1 (Figure S6) and GF/CNTs/TiO2-2.0 (Figure S7d), respectively, after 1000 cycles at a sweep rate of 10 mV/s. The good rate and cycling performance of hybrid electrodes are attributed to (i) a unique hierarchical electrode design. Note that conformal coaxial tubular structures enabled by in situ electrochemical deposition for CoxNi2−xS2 and ALD for TiO2 have been identified in this work (Figures 2a and 3a). It has been reported that the structure with intact mechanical contact in active materials/CNTs interface provides reliable mechanical integrity during cycling58−60 and, more importantly, reduces inhomogeneous charging at the irregular shape edges of the electrode.61−63 (ii) Chemical bonding exists at the active materials/carbon support interface (Figure 3f). (iii) The intrinsic electrochemical stability of both NiS64,65 and TiO243,57,66,67 in alkaline electrolyte also contributes to the good rate and cycling performance of hybrid electrodes. The electrochemical performance in terms of gravimetric capacity and rate performance of hybrid electrodes is found to be active material mass loading dependent (Figure S4). In F

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account. This high energy density is 2-fold that of typical commercial supercapacitors and comparable with that of metal oxide-based asymmetric supercapacitors. The maximum specific power density is 1.1 kW/kg, which is comparable to that of commercial supercapacitors. The volumetric energy and power densities were also estimated based on the whole cell configuration (Table S1). The maximum volumetric energy density of 21.0 Wh/l is 2-fold higher than the reported MnO2based72 and Ni(OH)2-based asymmetric supercapacitors46 and is higher that that of a carbon-supported Ni/Fe cell,51 benefiting from the large packaged cell density. The maximum volumetric power density is 0.77 kW/l. Good cycling performance is also demonstrated by the CoxNi2−xS2 // TiO2 battery, with 75.2% capacity retention after 1000 charge/ discharge cycles (Figure 5f). This is attributed to the unique electrode design that provides good mechanical and electrical integrity. During the long charge/discharge process, the overall GF/CNTs-supported electrode structures were well maintained, although the surface of the electrode becomes rougher and forms some aggregations (Figure S11). The practical application of the CoxNi2−xS2 // TiO2 battery was further demonstrated by powering light-emitting diodes (LEDs) via connecting in series. Benefiting from the high power/energy density, the cell could power nine orange LEDs (1.8 V, 30 mA, 5 mm diameter) simultaneously over 1 min after fast charging (Figure S12). The above preliminary results demonstrated the feasibility of the aqueous LiOH-mediated CoxNi2−xS2 // TiO2 alkaline battery. In principle, the coupling of an alkaline battery active cathode with an aqueous rechargeable anode together could promote OH− insertion/extraction in the cathode part and Li+ insertion/extraction in the anode part simultaneously. This feature may lead to the following merits: (i) Decent energy capacity due to breaking the capacity limitation of the cathode in traditional aqueous rechargeable LIBs; (ii) good rate performance because of the larger proton diffusion coefficient (on the order of 10−12 to 10−8)48,73 than that of Li ions in a traditional cathode for aqueous rechargeable LIBs; (iii) good cycling stability benefits from the intrinsic electrochemical stability of both NiS64,65 and TiO243,57,66,67 in alkaline electrolyte. Structurally, the hybrid electrode design with mesporous active materials grown directly on a highly conductive GF/CNTs electrode support ensures: (1) favorable electrochemical kinetics benefiting from the hierarchical porous structure of the active materials; (2) good mechanical integrity, accounting for the high cycling stability; (3) large mass loading of the active materials and high density of the packaged cell, accounting for the high gravimetric and volumetric energy densities. This also eliminates the additives (e.g., carbon black and binder). The unique design in both battery and electrode structure makes the CoxNi2−xS2 // TiO2 battery a promising energy storage device. The CoxNi2−xS2 // TiO2 couple by no means represents the optimum electrode pair for the aqueous rechargeable alkaline battery. Many types of metal oxides/ sulfates are known to be able to be intercalated with Li in the right chemical potential range. More effects are under way to optimize this kind of aqueous rechargeable alkaline battery for a potential market.

TiO2 anode was determined to be 1.1:2. CoxNi2−xS2 // TiO2 cells were therefore fabricated by using GF/CNTs/CoxNi2−xS21.1 and GF/CNTs/TiO2-2.0 hybrid electrodes. The total active material weight is 12.4 mg with a cell size of 4 cm2. The aqueous alkaline battery delivers good reversibility with welldefined redox couples and small ΔEa,c values (Figure 5a,b). The peak potential separation, ΔEa,c, is much smaller than that of the individual electrode, indicating the good reversibility. The overall electrochemical reaction could be described as follows:29,31,42,56 Cox Ni 2 − xS2 + TiO2 + y LiOH ↔ Cox Ni 2 − xS2OHy + Li yTiO2

(3)

During charge, CoxNi2−xS2 is oxidized to CoxNi2−xS2OHy (eq 1), while the TiO2 electrode is converted to LiyTiO2 (eq 2), accompanied by the OH− insertion and Li+ ion insertion processes at the cathode and anode, respectively. The discharge process involves the reverse reactions. The linear Ipa vs ν1/2 and Ipc vs ν1/2 plots of the CoxNi2−xS2 // TiO2 battery corroborate that the redox reactions are diffusion-limited (Figure 5c).47−49,51,52 The typical galvanostatic discharge curves of the CoxNi2−xS2 // TiO2 cell exhibit distinct plateaus in the range 1.1−1.2 V depending on the current density, agreeing well with the CV results (Figure 5d). The measured operating voltage is lower than that of the LiMn2O4 // LiTi2(PO4)3 (∼1.5 V)12 and Ni(OH)2/TiO2 cells (∼1.74 V).56 However, it is comparable to or higher than those of Ni-MH, Ni−Cd, or Ni−Fe batteries (∼1.2 V), LiMn2O4 // polypyrrole-decorated MoO3 (∼1.22 V),68 aqueous LiCoO2 // LiV3O8 (∼0.9 V),20 and LiMn0.05Ni0.05Fe0.9PO4 // LiTi2(PO4)3 (∼0.92 V),21 suggesting its feasibility for energy storage applications. The specific capacities obtained at high current densities, e.g., 73.4 mAh/g at 2.6 A/g and 60.7 mAh/g at 3.9 A/g (Figure 5d and Figure S9), indicate its good rate performance. A specific capacity of 86 mA h/g at 0.6 A/g can be retained even after deep cycling at 3.9 A/g, corroborating the good cycling performance (Figure S9). These are attributed to the unique hierarchical porous structures and good charge conductivity of the active materials (i.e., NiS ≈ 10−5 Ω-cm69 and anatase TiO2 ≈ 10−1 Ω-cm70) and hence relatively small serial resistances (R = 10.1 Ω) of the packaged CoxNi2−xS2 // TiO2 cell (Rs = 4.7 Ω; Rct = 5.4 Ω) (Figure S10). Note that the discharge (Figure 5d) completes in less than 10 min, demonstrating fast charge/ discharge capability. Ragone plots are illustrated in Figure 5e. The CoxNi2−xS2 // TiO2 battery delivers an energy density of 83.7 Wh/kg at 609 W/kg, based on the total mass of the active materials. Even at a high power density of 3.1 kW/kg, an energy density of 48.5 Wh/kg can still be achieved. Such a good performance is superior to that of aqueous LIBs in the literature, such as LiMn2O4 // LiTi2(PO4)312 and LiMn2O4 // polypyrroledecorated MoO3,68 and comparable to that of a Ni−Fe battery,51,71 as depicted in the same plot for comparison. Benefiting from the lightweight feature of a GF/CNTs hybrid film current collector, the loading of active materials is high, accounting for ∼53.5% weight percentage of the total packaged battery (Figure S3 and Table S1). This is also higher than that of commercial Sony LIBs (∼50%). The packaged cell is therefore expected to achieve a LIB-level energy density while delivering a supercapacitor-level power density. For instance, at a power density of 1.1 kW/kg, a gravimetric energy density of 17.5 Wh/kg is retained when the whole cell mass is taken into

CONCLUSIONS In summary, we propose an aqueous rechargeable alkaline battery system by integrating two reversible electrode processes associated with OH− insertion/extraction in the cathode and Li G

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The charge balance between the positive electrode and negative electrode is obtained by the equation C−m− = C+m+, where C− and C+ are the capacity of the anode and cathode, respectively, and m− and m+ are the mass of the anode and cathode, respectively. The energy density (E) and power density (P) were calculated according to the equations

ion insertion/extraction in the anode, respectively. The prototype CoxNi2−xS2 // TiO2 battery delivers high volumetric energy/power densities and good cycling stability. This hybrid system would enrich the existing aqueous rechargeable LIB chemistry and be a promising battery technology for large-scale energy storage.

E=

EXPERIMENTAL SECTION Material Synthesis. GF/CNTs Hybrid Films. These were synthesized by a previously reported method.74 The GF/CNTs hybrid films were wetted and cleaned with concentrated HNO3 prior to the electrochemical deposition. Electrochemical Deposition of CoxNi2−xS2 Nanosheets. The CoxNi2−xS2 nanosheets were electrochemically deposited on a GF/ CNTs hybrid film directly in a mixture of 7.5 mM Ni(NO3)2·6H2O, 5 mM Co(NO3)2·6H2O, and 0.5 M thiourea (CS(NH2)2) aqueous solution at a sweep rate of 10 mV/s with working windows at −1.2− 0.2 V (vs SCE). The mass loading of CoxNi2−xS2 was controlled via adjusting the number of cyclic voltammmetries from 10 to 80. The resulting GF/CNTs/CoxNi2−xS2 hybrid film was then washed thoroughly, dried, and then used as the cathode. The samples are denoted as GF/CNTs/CoxNi2−xS2-n, where n is the areal mass density of CoxNi2−xS2 (in unit of mg/cm2). Atomic Layer Deposition of the TiO2 Film. The TiO2 deposition was done by a Beneq TFS 200 system using titanium tetrachloride (TiCl4, 99.99%, Sigma-Aldrich) and H2O as the Ti and O precursors, respectively. The thickness was controlled by the cycles. The GF/ CNTs/TiO2 was then obtained after annealing at 500 °C for 4 h in air. The samples are denoted as GF/CNTs/TiO2-n, where n is the mass loading amount of TiO2 (mg/cm2). Assembly of Aqueous Rechargeable GF/CNTs/CoxNi2−xS2 // GF/ CNTs/TiO2 Battery. CoxNi2−xS2 // TiO2 batteries with high performance were assembled using a piece of GF/CNTs/CoxNi2−xS2 (1 × 4 cm2) and a piece of GF/CNTs/TiO2 (1 × 4 cm2), with an electrolytesoaked (2 M LiOH + 4 M KOH) separator in between. Material Characterizations. The morphology and element information were characterized by transmission electron microscopy (TEM, JEM-2010, 200 kV), field-emission scanning electron microscopy (JEM-7600F, 10.0 kV), Raman spectroscopy (Renishaw, 532 nm excitation laser), X-ray diffraction (Bruker D-8 Avance), and X-ray photoelectron spectroscopy on a VG ESCALAB 250 spectrometer (Thermo Electron, Altrincham, U.K) (Al Kα X-ray source (1486 eV)). Nitrogen adsorption/desorption measurements were carried out on an ASAP 2020 instrument based on Brunauer− Emmett−Teller theory. Thermogravimetric analysis (TGA) was conducted on a TGA Q 500 (Thermal Analysis Instruments, Burlingotn, MA, USA) in air. Electrochemical Measurements. Electrochemical measurements including galvanostatic charge/discharge curves, cyclic voltammetry curves, and electrochemical impedance spectroscopy (100 kHz to 0.01 Hz) were conducted on an electrochemical workstation (CHI 760D). A three-electrode system was adopted for the evaluation of the electrochemical performance of individual electrodes (Pt as the counter electrode, saturated calomel electrode (SCE) as the reference electrode) prior to the fabrication of the battery. The software Z-View was used for EIS fitting. Calculations. The specific capacity (C*, mAh/g) of individual electrodes was calculated from the corresponding charge/discharge curves according to the equation C* =

∫0

Δt

IV dt

P = E /Δt where I is the discharging current, V is the discharging voltage, Δt is the discharge time, dt is the time differential, and m is the total mass of the active materials. The lithium diffusion coefficient in the nanostructured anatase TiO2 was calculated by the classical Randles−Sevick equation at 25 °C:43,54,75 Ip = (2.69 × 105)n3/2AD01/2C0v1/2 where Ip (A g−1) is the peak current density, n is the electron transfer number, A (cm2 g−1) is the apparent surface area, D0 is the diffusion coefficient of the rate-limiting species (Li+), C0 (mol cm−3) is the maximum lithium concentration (C0 = 0.024 mol cm−3 for x = 0.5),54 and ν (V/s) is the sweep rate.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.5b06275. Additional SEM images of GF/CNTs/CoxNi2−xS2 hybrid electrode (Figure S1) with different loading amounts; N2 adsorption/desorption isotherms and corresponding pore-size distributions (Figure S2); TGA curves (Figure S3); CV, charge/discharge curves, specific capacity vs current density plots, and ΔEa,c vs sweep rates plots (Figure S4); EIS spectra (Figure S5) and cycling performance (Figure S6) of the GF/CNTs/CoxNi2−xS2 hybrid electrode; peak potentials vs sweep rate plots, peak current (Ip) with ν1/2 plot, EIS spectra and cycling performance of GF/CNTs/TiO2 hybrid electrode (Figure S7); charge balance (Figure S8); rate performance of CoxNi2−xS2 // TiO2 cell (Figure S9); EIS spectra (Figure S10), and time-dependent optical images of LED applications driven by two CoxNi2−xS2 // TiO2 cells connected in series (Figure S12); SEM images before and after cycling test (Figure S11); electrochemical parameters of a CoxNi2−xS2 // TiO2 battery (Table S1) (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected] (L. Zhang). *E-mail: [email protected] (Z. X. Shen).

I Δt m

Author Contributions §

J. L. Liu and J. Wang contributed equally to this work.

Notes

The specific capacity (C*, mAh/g) of CoxNi2−xS2 // TiO2 in twoelectrode configuration was calculated according to the equation

The authors declare no competing financial interest.

I C* = Δt M

ACKNOWLEDGMENTS The authors acknowledge support from the Energy Research Institute@NTU (ERI@N).

where I is the discharging current, M is the total mass of the two electrodes, and Δt is the discharge time. H

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J

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