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Anatase TiO as an Anode Material for Rechargeable Aqueous Aluminium-Ion Batteries: Remarkable Graphene Induced Aluminium Ion Storage Phenomenon Homen Lahan, Ratan Boruah, Anil Hazarika, and Shyamal Kumar Das J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09494 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 7, 2017

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The Journal of Physical Chemistry

Anatase TiO2 as an Anode Material for Rechargeable Aqueous Aluminium-ion Batteries: Remarkable Graphene Induced Aluminium ion Storage Phenomenon Homen Lahan,† Ratan Boruah, †Anil Hazarika, $ Shyamal K. Das*† †

Department of Physics, Tezpur University, Assam 784028, India

$

Sophisticated Analytical Instrumentation Centre, Tezpur University, Assam 784028, India

ACS Paragon Plus Environment

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ABSTRACT: The electrochemical intercalation and extraction of Al3+ ion in anatase TiO2 is illustrated in various aqueous electrolytes in an attempt to demonstrate the viability of TiO2 as an anode material for rechargeable aqueous aluminium-ion batteries. It is well understood that the primary barrier for diffusion of Al3+ ion in TiO2 is the poor electronic conductivity of TiO2. It is revealed that a small fraction of graphene (< 2 wt%) could induce ultrafast diffusion of Al3+ ion in TiO2. Estimation shows that graphene remarkably enhances the Al3+ ion diffusion coefficient in TiO2 by 672 times. Discharge capacities in the range of 33-50 mAhg-1 are obtained at high current rate of 6.25 Ag-1 for graphene incorporated TiO2. It is also seen that the nature of electrolytes critically influences the Al3+ ion insertion phenomenon. The possibility of reversible crystal phase transition of TiO2 to aluminium titanate due to Al3+ ion intercalation-extraction is also demonstrated for the first time.


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INTRODUCTION The quest for “beyond Li-based batteries” has stimulated extremely active global research efforts in alternative and sustainable energy storage and generation devices based on materials with greater earth-abundance than lithium over the past decade.1, 2 Therefore, there is a sudden but conspicuous resurgence of research interest in rechargeable Na- and Al-based battery chemistries.2, 3 Research activities on these two systems were once pursued concurrently with Liion batteries during 1970s.4, 5 However, the phenomenal success of Li-ion batteries significantly dwarfs the interest on these chemistries. The renewed interest in rechargeable Al-based battery chemistry is due to several attractive features.3 First, Al is inert and easy to handle in ambient atmosphere. Second, Al has the ability of exchanging three redox electrons per cation and, thereby, it offers the promise of enhanced energy storage than monovalent cations. Third, cost of Al minerals or Al metal is relatively reasonable. Hence, the economics of deliverable energy (i.e. cost per kWh) from Al-batteries is expected to be appealing for consumers. Rechargeable Al-ion batteries (AIBs) now can be classified into two categories namely non-aqueous AIB and aqueous AIB depending on the respective use of non-aqueous and aqueous electrolytes in these systems. Additional notable distinction is the direct use of Al metal as an anode in non-aqueous AIB. In recent times, Archer et al. first profoundly demonstrated the feasibility of room temperature non-aqueous AIB with V2O5 nanowires as cathode and ionic liquid/AlCl3 based electrolyte.6 More recently, other cathode materials such as VO2, graphitic foam, Mo6S8, Ni3S2, polythiophene, copper hexacyanoferrate are also showed promising results in non-aqueous Al-ion cells.7-11 These investigations certainly spotlight the possible greater promise of AIBs. However, similar to the suffering of other metal-batteries, non-aqueous AIBs also show the possibility of severe dendrite formation on the Al metal anode.12-15 Reed et al. and


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Chen et al. in two separate studies very clearly provided the microscopic evidence of Al dendrite growth in Al-ion cells.14, 15 On the other hand, one very detrimental feature of the ionic liquid based Al3+ ion conducting electrolytes is the strong ability for corrosion.3, 14 Coupled to high flammability of non-aqueous electrolytes, it might make AIBs vulnerable to thermal runaway. One apparent approach to resolve these two concerns is to employ alternative low voltage Al3+ ion intercalating or alloying anode and aqueous electrolytes. This inevitably results in rechargeable aqueous AIB similar to aqueous Li/Na-ion batteries. This type of aqueous battery could be a replacement for non-aqueous systems where safety is of prime importance. There is very sparse number of articles in aqueous AIBs in the literature.16-20 Liu et al. for the first time proposed a prototype of aqueous AIB with copper hexacyanoferrate as cathode, TiO2 nanotube arrays as anode and aqueous solution of Al2(SO4)3 as electrolyte in the year 2015.16 The observed discharge capacity is 21 mAhg-1 at an average discharge voltage of 1.6 V. The same group also identified for the first time the Al3+ ion intercalation/extraction phenomenon in anatase TiO2 nanotube arrays in aqueous solution of AlCl3.17 As an anode material, TiO2 is an attractive candidate because of advantages like relative ease in processing, high chemical stability, nontoxicity and low production cost. The most important feature in anatase TiO2 is the ability to intercalate Al3+ ions at very low voltage (-1.2 V to -1.4 V vs Standard Calomel Electrode) - an essential criterion to achieve high energy density batteries.17 There are very few articles where Al3+ ion electrochemistry in TiO2 is investigated.1719, 21, 22

Liu et al. observed a maximum discharge capacity of 75 mAhg-1 at current density of 4

mAcm-2.17 He et al. could significantly improve the Al3+ ion storage capacity in black mesoporous anatase TiO2 nanoleaves.18 The observed capacities are in the range of 250-280 mAhg-1 at a current density of 50 mAg-1. Kazazi et al. also reported capacity of 180 mAhg-1 at a


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current density of 50 mAg-1 in TiO2 nanospheres.19 On the other hand, Liu et al. proposed that presence of Cl− ions in the aqueous electrolyte is necessary for facile intercalation-extraction of Al3+ ions in TiO2 nanotube arrays.21 Sang et al. again reveal that acidity (pH) of the aqueous electrolytes also influences this process.22 These preliminary investigations certainly emphasize the existence of several unknown parameters which need to be unearthed to fundamentally understand and improve the Al3+ ion storage in TiO2 in aqueous electrolytes. Herein, we report the very complex Al3+ ion storage behavior in anatase TiO2. It is highlighted that electronic conductive agent such as graphene is an indispensable component in improving the Al3+ ion intercalation-extraction process in TiO2. Graphene, carbon nanotube and Ag nanoparticles are used as model conductive agents in the study. It is estimated that graphene remarkably enhances the Al3+ ion diffusion coefficient in TiO2 by 672 times. It is seen that the Al3+ ion insertion process is dependent on the nature of electrolytes. It is also shown for the first time that reversible crystal phase transition of TiO2 to Al2TiO5 due to Al3+ ion intercalationextraction is possible. EXPERIMENTAL SECTION Materials Titanium tetraisopropoxide (TTIP, Sigma Aldrich) was used as the titania precursor. Graphene and multi-walled carbon nanotube (CNT) were commercially available from Sisco Research Laboratories Pvt. Ltd. India (product no. 55093) and Sigma (product no. 724769) respectively. The graphene nanopowder and CNT were treated with concentrated HNO3 for 24 hr and washed thoroughly by distilled water several times. The acid treated graphene and CNT were used for the synthesis. Synthesis


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Synthesis of pristine TiO2/graphene-TiO2/CNT-TiO2: The materials were synthesized by a solvothermal method as reported earlier by our group.23 For a typical synthesis, 30 mg of acid treated graphene/CNT was dispersed in 26 ml water-ethanol (50% v/v) mixture by ultrasonication for 30 min. Then, 2.6 g of sucrose was dissolved and 2.73 ml concentrated HCl was added. After being stirred for 15 min, 3.9 ml of titanium tetraisopropoxide was added slowly under stirring condition. The solution was stirred for 30 min. The solution was then transferred to a Teflon-lined stainless steel autoclave of capacity 50 ml and heated at 160 oC for 6 h and then cooled to room temperature. The resultant black product was recovered by centrifugation and washed with deionized water and ethanol several times and dried at 110 oC for 24 h. It is noted here that no special procedure was adopted to remove any free graphene or CNT. The dried product was calcined at 450 oC for 3 h in air at a heating rate of 2 oC min-1. The pristine TiO2 was synthesized by the same procedure without addition of either CNT or graphene. The pristineTiO2, graphene-TiO2 and CNT-TiO2 are designated as TiO2, G-TiO2, and CNT-TiO2 respectively. It is worth to mention here that optimized amount of graphene is necessary to achieve the best possible electrochemical activity. Our selection of 30 mg graphene in the synthesis is based on our prior investigation on sodium-ion electrochemistry.23 It was observed that the sodium storage capacities were lower whenever 10 mg or 50 mg of graphene was used in the synthesis. To be consistent, identical amount of CNT is also used for the synthesis of CNTTiO2. Synthesis of Ag-TiO2: For the synthesis of Ag-TiO2, initially a solution containing Ag ions is prepared following a reported procedure.24 In a typical synthesis, 0.25 mM AgNO3 and 0.25 mM trisodium citrate containing 100 ml solution was prepared in water. Thereafter, 100 mg of pristine TiO2 were dispersed by sonication in the above solution. While stirring the sol


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vigorously, 2.4 mL of 10 mM NaBH4 was added all at once. The stirring was continued for 2 h. The resultant grey product was recovered by centrifugation and washed with deionized water and ethanol several times and dried at 110 oC for 24 h. The dried product was not calcined further. The Ag-TiO2 is designated as Ag-TiO2. Synthesis of TiO2 nanoparticle: 2.75 ml of TTIP is dissolved in 100 ml of ethanol. This transparent solution was slowly added to 100 ml of deionized water and stirred continuously. The reaction was continued for 1 h. The resultant white product was recovered by centrifugation and washed with deionized water and ethanol several times and dried at 110 oC for 24 h. The dried product was further calcined at 450 oC for 4 h under air at a heating rate of 2 oCmin-1. Characterization The crystallographic phase identification was performed using powder X-ray diffraction (BRUKER AXS D8 FOCUS; Cu-K radiation, λ = 1.5406 Å). The morphology was observed by scanning electron microscopy (SEM, JEOL JSM 6390LV) and transmission electron microscopy (TEM, FEI Tecnai G2 20). Specific surface area (BET) was obtained from nitrogen adsorptiondesorption isotherms (Quantachrome). Thermogravimetric analysis (TGA, SHIMADZU, TGA50) was performed under oxygen at a heating rate of 5 oC/min. Two probe dc electrical conductivity measurements of the materials were performed using a Keithley 2400 SourceMeter. The materials were palletized with identical dimension for the measurement. Electrochemical Analysis Electrode slurries were made from active materials (TiO2, G-TiO2, CNT-TiO2, Ag-TiO2), polyvinylidene fluoride (PVDF) and N-methyl-2-pyrrolidone (NMP). The weight ratio is active material:PVDF = 80:20. The slurry was cast on stainless steel plate and dried at 120 OC for 12 h. The cyclic voltammetry and galvanostatic cycling experiments were performed in a conventional


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three-electrode electrochemical cell using Biologic SP300 electrochemical work station. Pt electrode and aqueous Ag/AgCl electrode were used as the counter and reference electrodes respectively. Ionic conductivity of electrolytes was measured from ac-impedance spectra obtained by scanning in the frequency range of 1-106 Hz at signal amplitude of 0.05 V. The utilized electrolytes and their properties are mentioned in Table S1. Extreme care must be taken while mixing AlCl3 in H2O because the reaction is exothermic and reacts very violently particularly at higher molar concentrations. Electrochemical impedance spectra (EIS) of the cells before discharge and after discharge were measured over the frequency range of 1 mHz-200 kHz at signal amplitude of 0.1 V. All the electrochemical measurements were conducted at room temperature (25 oC). For ex-situ SEM and XRD measurements, the electrodes were harvested after required discharge/charge cycle and dried at 110 oC for 24 h. RESULTS AND DISCUSSION

Figure 1. SEM micrographs of (a) TiO2, (b) G-TiO2, (c) CNT-TiO2 and (d) Ag-TiO2, TEM micrographs of (e, f) G-TiO2 and (g, h) Ag-TiO2, (i) XRD patterns, (j) N2 adsorption/desorption isotherms, (k) Thermogravimetric analysis.


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The SEM and TEM images reveal the spherical morphology of all the materials (figure 1). The dimensions of the TiO2 microspheres are very broad (Figure S1). It ranges from 0.5 µm to 5 µm. The spherical morphology is attained due to the formation of amorphous carbon spheres during the hydrothermal treatment of the sucrose.25 Simultaneously, TiO2 nanocrystals nucleate in these carbon spheres by in situ hydrolyzation of titanium tetraisopropoxide resulting in amorphous carbon-TiO2 microspheres. The amorphous carbon burns out from the carbon-TiO2 microspheres upon calcination at 450 oC in air. The TGA analysis (Figure 1k), which was performed under O2 atmosphere, clearly shows that there is no weight loss for the calcined pristine TiO2 microspheres indicating complete combustion of amorphous carbon. A thin layer of graphene covering the TiO2 nanocrystals could be seen in G-TiO2 (Figure 1f). Similarly, CNT entangled over TiO2 microspheres is observed in CNT-TiO2 (Figure S4). A comparison of lower magnification SEM images (Figure S5) indicates absence of free or residual graphene/CNT in the final product which signifies that graphene or CNT is completely intertwined inside the TiO2 microspheres. In the case of Ag-TiO2, nanoparticles of Ag are uniformly spread over the surface of TiO2 microspheres (Figure 1g, h). The approximate size of Ag nanoparticles is in the range of 5-20 nm (Figure S6). The XRD patterns (Figure 1i) identify that all the materials crystallized in anatase phase (JCPDS No. 21-1272). The crystallite size is estimated using the Scherrer equation. From the full width at half maximum (FWHM) at the (101) peak (2 = 25.50o), the crystallite size is determined to be approximately 10-12 nm for all the materials which signifies that the microspheres are composed of TiO2 nanocrystals. The main characteristic diffraction peak of metallic Ag at 2θ = 38.12o (JCPDS-87-01717) is expected to be overlapped with the (004) peak of anatase TiO2. One of the important features of the TiO2 microspheres is the relatively high packing density (Figure S7). The packing density of TiO2 microspheres is


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estimated to be 0.32 g cm-3, whereas it is 0.14 g cm-3 for sol-gel synthesized TiO2 nanoparticles. It is noted here that high packing density of electrode materials is a prerequisite for achieving high volumetric energy density energy storage devices. The combustion of amorphous carbon during calcination results in mesopores in the TiO2 microspheres and it is evident from the obtained type-IV N2 adsorption/desorption isotherms (Figure 1j).The BET surface areas of TiO2, G-TiO2, CNT-TiO2 and Ag-TiO2 are 80 m2g-1, 35 m2g-1, 48 m2g-1 and 71 m2g-1 respectively. Thermogravimetric analysis confirms the presence of 1.62 wt-% and 1.40 wt-% of graphene and CNT in G-TiO2 and CNT-TiO2 respectively (Figure 1k).

Figure 2. CV curves of (a) G-TiO2 and (b) TiO2 in 1 M AlCl3 electrolyte at a scan rate of 5 mVs1

, (c) CV curves of G-TiO2 at different scan rates and (d) variation of redox peak currents versus

scan rates according to equation I  k 0.5 (see text for detail). Anodic peak is considered here.


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The insertion/extraction electrochemistry of Al3+ ions in TiO2 was evaluated in conventional three electrode electrochemical cell. The electrolytes were aqueous solutions of AlCl3 (0.01-1 M), Al2(SO4)3 (0.5 M) and Al(NO3)3 (1 M). The ionic conductivities of these electrolytes are mentioned in Table S1. The typical conductivities of 1M AlCl3, 0.5 M Al2(SO4)3 and 1M Al(NO3)3 aqueous solutions are 1.7 x 10-1 Ω-1cm-1, 8.5 x 10-2 Ω-1cm-1 and 2.1 x 10-1 Ω1

cm-1 respectively. Cyclic voltammetry (CV) was performed to observe the Al3+ ion insertion in TiO2

initially in 1M AlCl3 electrolyte. Figure 2a shows the CV profiles obtained from G-TiO2 at a scan rate of 5 mVs-1. A pair of cathodic and anodic redox peaks at -1.27 V and -0.99 V (vs. Ag/AgCl) respectively is detected for G-TiO2. Interestingly, the electrochemical activity in pristine TiO2 is merely negligible except a very broad peak at -1.22 V in the cathodic sweep (Figure 2b and S8). To understand the Al3+ ion insertion mechanism, CV was performed at different scan rates as shown in figure 2c. The current response (I) at peak potentials could be related to scan rates (  ) according to the following equation: I  k1  k2 0.5 , k1 and k2 are constants.26, 27 The first term corresponds to surface mediated ion storage (or capacitive) behavior and second term is due to diffusion-controlled ion insertion process. A straight line functional dependence of I

 0.5

versus

 0.5 signifies the equal contributions from both capacitive and diffusion controlled processes. As shown in figure S9, there is no such straight line dependence for G-TiO2. However, a linear dependence is clearly observed when the peak current response is plotted against scan rates according to the equation I  k 0.5 (figure 2d). It signifies the diffusion-controlled Al3+ ion insertion process in G-TiO2. Therefore, the cathodic and anodic peaks could be respectively related to the Al3+ ion intercalation and extraction processes in G-TiO2.


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Figure 3. CV curves of G-TiO2 at a scan rate of 10 mVs-1 in (a) 1 M AlCl3 and (b) 0.25 M AlCl3 electrolytes, CV curves of (c) CNT-TiO2 and (d) Ag-TiO2 at various scan rates in 0.25 M AlCl3 electrolyte. To check the stability of G-TiO2, CV was performed at a scan rate of 10 mVs-1 for several cycles (Figure 3a) in 1M AlCl3 electrolyte. It is seen that the electrochemical activity decreases considerably after 10th cycle. Careful observation reveals that the electrolyte color changes from transparent to deep orange while undergoing the experiment (Figure S10). After cycling, the active material was removed from the stainless steel current collector and the collector was examined thoroughly. It is observed that the stainless steel current collector was corroded considerably during the course of experiment (Figure S11). It is well known that AlCl3 is very corrosive and, therefore, it is expected to cause corrosion of stainless steel. It suggests, therefore, that 1M concentration of AlCl3 may not be a favorable electrolyte composition for Al-


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ion cells. The previously published reports show no rationale of using 1 M AlCl3 aqueous electrolyte.17-19, 21, 22 In order to verify it, CV experiments were performed at different molar concentrations of AlCl3. It is clearly observed that Al3+ ion could be inserted even at a low concentration of 0.01 M-AlCl3 (Figure S12a). However, the cathodic and anodic peaks are prominently visible only above 0.25 M-AlCl3 (Figure S12c). Interestingly, the cycling stability also improved significantly at this particular concentration (figure 3b). Moreover, the stainless steel current collector shows negligible corrosion in 0.25 M-AlCl3 after cycling (Figure S11b). The Al3+ ion insertion/extraction process is also found to be dominated by diffusion in 0.25 MAlCl3 electrolyte (Figure S13). It is important to understand the underlying mechanism behind the electrochemical activity of G-TiO2. The pristine TiO2 show little such activity as mentioned earlier. The CV profiles of pristine graphene also indicate that graphene is completely electrochemically inactive for Al3+ ions in the measured voltage range (figure S14). Therefore, the role of graphene is quite intriguing. The outcome of electrical conductivity measurements on G-TiO2 and TiO2 suggests the possible beneficial role of graphene. It is clear so far that Al3+ ion diffuses in the crystal structure of TiO2. However, the electrochemical faradic reaction that undergoes in TiO2 is a coupled reaction of the charged carriers: Al3+ ions and e-. Hence, facile diffusion of Al3+ ions in TiO2 is possible only with adequate transport of electrons in TiO2. The electrical conductivity (σ) data support this proposition. G-TiO2 exhibits three order higher electrical conductivity than pristine TiO2 (Figure S15 and Table S2). To further corroborate, experiments were performed with carbon nanotube and Ag nanoparticle modified TiO2. As shown in CV profiles (figure 3c), CNT-TiO2 shows electrochemical activity for Al3+ ions. On the other hand, the behavior of AgTiO2 is similar to pristine TiO2 (figure 3d). A comparison shows that the electrochemical activity


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of Al3+ ions in G-TiO2 is much higher than CNT-TiO2 (figure S16). Interestingly, the electrical conductivities of G-TiO2, CNT-TiO2, Ag-TiO2 and TiO2 vary according to the following order: σ (G-TiO2) > σ (CNT-TiO2) > σ (Ag-TiO2, TiO2). These comparative studies immensely signify the importance of enhanced electrical conductivity of TiO2 in supporting better electrochemical activity for Al3+ ions. Enhancement in electrical conductivity is again manifested in the Al3+ ion diffusion coefficient. According to the Randles-Sevick equation, the diffusion coefficient of any migrating ion is linearly proportional to the square of the slope of I  k 0.5 curve.26, 27 The evaluation of Al3+ ion diffusion coefficients using this relationship shows that the diffusion coefficient in G-TiO2 is remarkably higher than the diffusion coefficient in CNT-TiO2 and TiO2 (Figure S17). It is 672 times and 5.4 times higher than TiO2 and CNT-TiO2 respectively. Therefore, it can be commented that graphene forms an excellent percolation network in the TiO2 nanocrystals for facile transport of electrons and it results in improved Al3+ ion diffusion in G-TiO2.

Figure 4. Electrochemical impedance spectra of (a) TiO2, (b) G-TiO2, (c) CNT-TiO2 and (d) AgTiO2 before discharge, after 1st discharge and after 10th discharge.


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In order to gain further understanding of the differences in the electrochemical activities of G-TiO2, CNT-TiO2, Ag-TiO2 and TiO2, the materials were analyzed by electrochemical impedance spectroscopy (EIS) in the frequency range of 200 kHz to 1 mHz. Each electrode material shows a semicircle in the measured frequency range before discharge process as shown in figure 4. In general, appearance of two semicircles in the measured frequency range correspond to solid electrolyte interphase (SEI) formation at high frequency region and chargetransfer reaction at medium-to-low frequency region.28, 29 The emergence of single semicircle rules out the formation of SEI. Further evidence for absence of any SEI is derived from the exsitu SEM images obtained from the discharge products of G-TiO2 and TiO2 after 1st discharge cycle (Figure S18). The images clearly show that there is no SEI related decomposition layer on the discharged products. Therefore, the semicircle (the diameter represents the charge-transfer resistance) could be fairly attributed to the charge-transfer reaction at the electrode-electrolyte interface.28 It is observed that G-TiO2 has the smallest diameter of the semicircle. It once again signifies that graphene minimizes the resistance for charge carriers (i.e. Al3+ and e-) in TiO2 crystals. Interestingly, the diameters of the semicircles considerably decrease after 1st discharge in all cases. The order of decrease in diameter (d) is as follows: d (G-TiO2) < d (CNT-TiO2) < d (TiO2, Ag-TiO2). This decrease suggests that the charge-transfer resistance decreases after Al3+ ion intercalation in TiO2. Reduction in resistance of TiO2 nanotubes up to three order of magnitude after Al3+ ion doping is also reported by Zhong et al.30 Such decrease in chargetransfer resistance was not observed in previous reports.17-19, 21, 22 The diameter slightly increases after 10th discharge cycles in certain cases. It is primarily due to the disintegration of the electrode materials from the current collector.


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Figure 5. CV curves of (a) G-TiO2 in 0.5 M Al2(SO4)3, (b) TiO2 in 0.5 M Al2(SO4)3, (c) G-TiO2 in 1 M Al(NO3)3 and (d) TiO2 in 1 M Al(NO3)3 electrolytes at various scan rates. The electrochemical activities of G-TiO2 and TiO2 are also explored in 0.5 M Al2(SO4)3 and 1 M Al(NO3)3 aqueous electrolytes. CV profiles of G-TiO2 show considerable activity in Al2(SO4)3 electrolyte (figure 5a). Although the cathodic peak is not prominently visible, the anodic peak is detected at -0.98 V (vs. Ag/AgCl) at a scan rate of 5 mVs-1. The trend is quite similar for TiO2 as could be seen in AlCl3 electrolytes (figure 5b). Surprisingly, both G-TiO2 and TiO2 are electrochemically inactive in Al(NO3)3 electrolyte (figure 5c,d). The ionic conductivities of 1 M AlCl3 and 1 M Al(NO3)3 electrolytes are almost identical (table S1). In fact, the ionic conductivity of 0.5 M Al2(SO4)3 electrolyte is one order magnitude lower than 1 M Al(NO3)3 electrolyte. Hence, the deviations in electrochemical activities of TiO2 in different electrolytes due to ion transport in electrolytes could be ruled out. Again, the pH values of all these electrolytes are also almost identical (table S1). Therefore, it appears that the intercalation


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process of Al3+ ion in TiO2 could be influenced by the surrounding anions.21 However, further studies are required for comprehensive understanding of this complex process. It is noted in all CV profiles that there is a sudden rise of current in the cathodic sweep (figure 2, S13). To understand the origin of it, CV was performed on pristine stainless steel current collector in AlCl3 electrolytes. As shown in figure S19, it is observed that there is large cathodic current below -1 V (vs. Ag/AgCl) in both 0.25 M and 1 M AlCl3 electrolytes. It is primarily attributed to hydrogen evolution at such low potentials.31 The hydrogen evolution in the form of small air bubbles near the current collector was visible by naked eyes while performing the experiments. Clearly in our case, the hydrogen evolution process interfered with the Al3+ ion intercalation process in TiO2 because of the identical potential range (< -1 V). This could be probably mitigated by use of alternative current collectors. Earlier literature reports mentioned the use of titanium, platinum and nickel as current collector while investigating the Al3+ ion electrochemistry in TiO2.17-19 A comparison of CV profiles at 0.25 M and 1 M AlCl3 electrolytes shows that the rise in cathodic current is very large in 1 M AlCl3 than 0.25 M AlCl3 electrolyte (Figure S19c). It signifies that hydrogen evolution could be controlled by using appropriate composition of electrolyte.


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Figure 6. (a) Galvanostatic discharge/charge curves of G-TiO2 at a current density of 6.25 Ag-1 in 0.25 M AlCl3 electrolyte at 25 oC and (b) respective variation of charge/discharge capacities with cycle number. Ex-situ XRD patterns before discharge, after 1st discharge and after 1st charge for (c) G-TiO2 and (d) TiO2. Galvanostatic charge-discharge experiments were performed to further evaluate the electrochemical performance of G-TiO2 and TiO2. Figure 6a and S20a represent the respective charge-discharge curves obtained from G-TiO2 in 0.25 M AlCl3 and 1M AlCl3 electrolytes in the voltage range of -0.5 V to -1.075 V (vs. Ag/AgCl) at a current density of 6.25 Ag-1. The initial discharge capacities of G-TiO2 are 33 mAhg-1 and 50 mAhg-1 respectively for 1 M AlCl3 and 0.25 M AlCl3 electrolytes. The capacities appear to be lower, however these are obtained at a very high current rate of 6.25 Ag-1. Contrarily, pristine TiO2 did not show any charge-discharge characteristics with noticeable capacities (Figure S21). The charge-discharge experiments unravel various complex issues. First, the experiments could not be performed at deep discharge


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(up to –1.5 V vs. Ag/AgCl). A typical discharge curve obtained from G-TiO2 when discharged to -1.5 V at a current rate of 6.25 Ag-1 is shown in Figure S22a. The discharge plateau continues at 1.24 V. Similar behavior was also observed when the discharged voltage was restricted to -1.2 V (Figure S22b). Second, the charge-discharge experiments were performed at low current densities (< 6.25 Ag-1). The typical discharge curves are shown in figure S23. It is seen that the discharge could not be completed over the measured time. Moreover, the discharge plateau shifts to more positive potential with decreasing current density. To understand this unusual behavior, charge-discharge experiments were performed with pristine stainless steel current collector. It is observed that there is a long discharge plateau at -1 V in the discharge curve (figure S24). It could be again attributed to the hydrogen evolution reaction. Similar behavior was also observed in the CV curves of stainless steel as discussed previously. Hence, it could be commented that there is a possibility of hydrogen evolution reaction taking part in the discharge process. Since the hydrogen evolution is an irreversible process, the charge capacity is always lower than the discharge capacity. Therefore, it was quite challenging to perform the charge-discharge experiments. Based on repeated experimental outcomes, the charge-discharge experiments were performed in the voltage range of -0.5 V to -1.075 V (vs. Ag/AgCl) at a current density of 6.25 Ag-1. Nonetheless, the charge-discharge curves together with CV analysis signify that reversible Al3+ ion intercalation-extraction is possible in TiO2. Further evidence for the Al3+ ion intercalation process can be derived from the ex-situ XRD patterns as shown in figure 6 (c, d). The XRD pattern of the 1st discharged state G-TiO2 electrode shows two additional small diffraction peaks at 2θ = 18.2o and 29.3o in addition to the prominent anatase TiO2 phase. It was difficult to index such small intensity peaks. However, careful analysis shows that the diffraction peaks at 2θ = 18.2o and 29.3o could be indexed to Al2TiO5 (JCPDS-70-1435) and Al2Ti7O15


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(JCPDS-84-1641) phases respectively. The existence of anatase phase in the discharged product may signify that not all TiO2 nanoparticles are transformed to aluminium titanate phases. It is most likely that Al3+ ions could not diffuse to the entire anatase TiO2 crystal during discharge at high current rate (6.25 Ag-1). After 1st charge, these two additional peaks almost disappeared. It underscores that Al3+ ion intercalation process in TiO2 involves complex crystal phase transition steps. On the other hand, these peaks are not prominently visible in pristine TiO2 electrodes. Such crystal phase transition of TiO2 from anatase to lithium titanate is also observed in electrochemically lithiated TiO2.32 We highlight here that prominent Al2TiO5 phase could be observed with smaller sized TiO2 crystals upon electrochemical Al3+ ion intercalation (Figure S25). CONCLUSION In summary, we illustrated the Al3+ ion intercalation-extraction electrochemistry in anatase TiO2 utilizing various forms of conductive agents and aqueous electrolytes. At first, it is convincingly argued that enhanced electronic conductivity is very vital for facile Al3+ ion diffusion in TiO2. In this context, graphene played an inevitable role. It is estimated that graphene remarkably enhances the Al3+ ion diffusion coefficient in TiO2 by 672 times. The influence of electrolyte composition on Al3+ ion diffusion is also elaborated. Additionally, the very complex nature of Al3+ ion intercalation process is also discussed from galvanostatic cycling. The possibility of crystal phase transition of TiO2 to aluminium titanate phases due to Al3+ ion intercalation is also shown. Therefore, we believe the present work will be beneficial in identifying suitable electrodes, electrolytes and current collectors for rechargeable aqueous AIBs. ASSOCIATED CONTENT


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Supporting Information. Tables, SEM images, TEM images, photographs of current collectors and electrolytes, CV curves, conductivity measurements, charge-discharge curves. AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENT This work is supported by Science and Engineering Research Board, Department of Science and Technology, Government of India (Grant No.: YSS/2015/000765). REFERENCES (1) Muldoon, J.; Bucur, C. B.; Gregory, T. Quest for Nonaqueous Multivalent Secondary Batteries: Magnesium and Beyond. Chem. Rev. 2014, 114, 11683-11720. (2) Yabuuchi, N.; Kubota, K.; Dahbi, M.; Komaba, S. Research Development on Sodium-ion Batteries. Chem. Rev. 2014, 114, 11636-11682. (3) Das, S. K.; Mahapatra, S.; Lahan, H. Aluminium-ion Batteries: Developments and Challenges. J. Mater. Chem. A 2017, 5, 6347-6367. (4) Holleck, G. L. The Reduction of Chlorine on Carbon in AlCl3‐KCl‐NaCl Melts. J. Electrochem. Soc. 1972, 119, 1158-1161. (5) Whittingham, M. S. Chemistry of Intercalation Compounds: Metal Guests in Chalcogenide Hosts. Prog. Solid State Chem. 1978, 12, 41-99.


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