High-Energy Density Redox Flow Lithium Battery with Unprecedented

Subscriber access provided by GAZI UNIV

Article

High Energy Density Redox Flow Lithium Battery with Unprecedented Voltage Efficiency Feng Pan, Qizhao Huang, Hui Huang, and Qing Wang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b04558 • Publication Date (Web): 08 Mar 2016 Downloaded from http://pubs.acs.org on March 14, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Chemistry of Materials is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 8

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chemistry of Materials

High Energy Density Redox Flow Lithium Battery with Unprecedented Voltage Efficiency Feng Pan1, Qizhao Huang1, Hui Huang2 and Qing Wang*1 1

Department of Materials Science and Engineering, Faculty of Engineering, NUSNNI-NanoCore, National University of Singapore, Singapore, 117575, E-mail: [email protected] 2

Surface Technology Group, Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore

KEYWORDS: redox flow, lithium battery, voltage efficiency

ABSTRACT: Redox flow lithium battery (RFLB) has decoupled energy storage and power generation units as the conventional redox flow battery, while it stores energy in solid materials by virtue of the unique redox targeting concept. Hence it presents superior energy density and represents a promising approach for large-scale energy storage. In RFLB, the potential difference between the redox shuttle molecules used in the same electrolyte normally brings about an intrinsic voltage hysteresis, resulting in a compromised voltage efficiency of the battery. Here we report a novel redox shuttle molecule pair to minimize the voltage hysteresis: anatase TiO2 is reduced by bis(pentamethylcyclopentadienyl)chromium (CrCp*2), while LixTiO2 is oxidized by cobaltocene (CoCp2). The potential difference between CoCp2 and CrCp*2 is only 0.15 V. A redox flow lithium battery is successfully demonstrated with an unprecedented voltage efficiency of 84%. The RFLB shows good cycling stability and over 90% Coulombic efficiency was demonstrated in the first 50 cycles.

Introduction Redox flow battery (RFB) has been an important large-scale electrochemical energy storage technology for power grid and sustainable energies (i.e. wind and solar),1 because of its unique feature of decoupled energy storage and power generation. However, the energy density of conventional RFB has been restricted by the limited solubility of active species in both the catholyte and anolyte. For instance, a maximum concentration of 2.5 mol/L was reported for the most developed all-vanadium flow battery (VRB), which is however one order of magnitude less than that in the commonly used Li+-storage materials for lithium-ion batteries.2 Low volumetric capacity and consequently low energy density result in large footprint and high capital cost for installation and maintenance. To enhance the energy density, various approaches have been proposed.3-6 Among them, the redox flow lithium battery (RFLB) has shown great promise.4-7 In RFLB, solid Li+-storage materials are immobile and kept in the storage tank, while the charge transport is conducted by the redox shuttle molecules circulating between the tank and cell. Reversible chemical lithiation and delithiation of the solid material, or “redox targeting”8 process by which the material is lithiated (reduced) by a molecule with lower redox potential, while delithiated (oxidized) by another molecule with higher redox potential, represents the key process dictating the

operation of RFLB. The potential difference between these two molecules brings about an intrinsic voltage hysteresis. For example, in the previous study we reported RFLB cathodic half-cell with LiFePO4 as the solid material. LiFePO4 (3.45 V vs. Li/Li+) was oxidized by FcBr2+ and reduced by Fc. The potential difference between FcBr2 and Fc is about 0.30 V,5 which leads to an inevitable voltage loss during charge/discharge. In addition, we have used cobaltocene (CoCp2) and bis(pentamethylcyclopentadienyl)cobalt (CoCp*2) as the redox mediators for the chemical lithiation/delithiation of anatase TiO2 in a RFLB anodic half-cell.6 Anatase TiO2 is an important anode material for lithium-ion batteries due to its excellent Li+ uptake capability and structural stability.9 The electrochemical lithium insertion and extraction process of TiO2 is described below: + + ↔

(1)

The x value of inserted lithium (mole fraction) is close to 0.5 for anatase TiO2 at maximum accommodation,10, 11 corresponding to a [Li+] of 22.5 mol/L (~10 times as high as vanadium concentration in VRB). The average redox potential for TiO2 lithiation/delithiation reaction is around 1.80 V vs. Li/Li+. The intrinsic voltage hysteresis, resulted from the potential difference between CoCp2 (1.90 V vs. Li/Li+) and CoCp*2 (1.36 V vs. Li/Li+), was about 0.54 V. In particular, the redox potential of CoCp*2 is unnecessarily low for the reduction of TiO2. The voltage efficiency of a flow battery is defined as the ratio of cell voltage between discharge and charge.12

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Evidently, the voltage hysteresis caused by the potential difference between the two redox molecules inevitably deteriorates the voltage efficiency of RFLB. To minimize the voltage loss, it is desirable to utilize redox molecules with potential close to that of TiO2. In the present study, we report on a novel redox targeting system of anatase TiO2 for RFLB application. Herein bis(pentamethylcyclopenta-dienyl)chromium (CrCp*2) is employed as the reducing agent for the lithiation of TiO2, while CoCp2 is used for the delithiation of LixTiO2. The potential difference between CrCp*2 and CoCp2 is only 0.15 V. The reversible chemical lithiation and delithiation of TiO2 was demonstrated in a RFLB half-cell, and an unprecedented voltage efficiency of 84% was attained.

Experimental Section Materials and Electrode Preparation. The following chemicals were used as received: bis(pentamethylcyclopentadienyl)chromium (CrCp*2) and cobaltocene (CoCp2), lithium perchlorate (LiClO4) and dehydrated propylene carbonate (PC). Nanocrystalline Al2O3 (40-50 nm particle size) was purchased from Alfa Aesar and anatase TiO2 paste (DSL 90-T, ~20 nm particle size) was obtained from Dyesol. Fluorine-doped tin oxide (FTO) glass (TEC-15) was purchased from Pilkington NSG Group. FTO-Al2O3-TiO2 electrodes were prepared by screen-printing, with a thin Al2O3 mesoporous layer being firstly printed on FTO glass and sintered at 125 oC for 2 min, which then follows the printing of TiO2 layer on top of the Al2O3 layer. Thereafter, the electrodes were annealed at 500 oC for 15 min prior to use. Electrochemical Test. The cyclic voltammetry (CV) measurements were carried out with a multichannel potentiostat (Metrohm Autolab, PGSTAT302N). A three-electrode cell was used with glassy carbon as the working electrode, lithium foils as the counter and reference electrodes. The concentration of CoCp2 and CrCp*2 were both 5 mM. The supporting electrolyte was 1 M LiClO4 in PC. All the electrolyte preparations and CV measurements were conducted in an argon-filled glove box with H2O and O2 level less than 1 ppm. Spectroelectrochemical Test. An airtight 3-electrode optical cell was employed for the spectroelectrochemical measurement. FTO-Al2O3-TiO2, Pt and Ag wires were used as the working, counter and quasi-reference electrodes, respectively. The potential of the quasi-reference electrode was calibrated with ferrocene (Fc/Fc+), and calculated against Li/Li+. The electrolyte was 1 M LiClO4 in PC containing 5 mM redox molecules. A constant current density of 0.177 mA cm-2 was applied by an Ivium Electrochemical Analyzer during the spectroelectrochemical measurement. Absorption spectra were collected using a Shimadzu UV-Vis-NIR spectrophotometer (Solidspec-3700).

Page 2 of 8

Raman and XPS Measurements. FTO-Al2O3-TiO2 film was assembled in an electrochemical cell as the working electrode, while lithium foils were used as the counter and reference electrodes. Lithiation and delithiation processes was conducted in 1 M LiClO4 in PC containing 5 mM CrCp*2 and CoCp2. A constant voltage of 1.60 V was applied by a multichannel potentiostat for the lithiation process, while 2.10 V for the delithiation process. The lithiated and delithiated TiO2 electrodes were washed thoroughly with PC in the glove box. After vacuum drying at room temperature, the electrodes were sandwiched with a piece of glass slide and sealed with epoxy glue for Raman measurements. The Raman spectra were collected at room temperature with a Renishaw inVia Raman microscope, using a laser with wavelength of 785 nm as the excitation source. For the X-ray photoelectron spectroscopy (XPS) analysis, the samples were sealed in an airtight box and transferred from the glove box into the XPS chamber. The XPS analysis was conducted with a Kratos Analytical Axis Ultra DLD Spectrometer. Monochromated Al K was the radiation source and all the measurements were done in vacuum. Carbon, C (1s), was used as the reference for all the samples. The XPS results were fit with XPSPeak software. Redox Flow Lithium Battery Test. The battery performance was evaluated with both static glass cell and flow stack cell. The assembly of the static glass cell has been reported previously.6 The glass cell contained 1 ml electrolyte, and the concentration of both redox molecules was 5 mM. About 4 mg TiO2 powder was dispersed in the electrolyte. The flow stack cell was assembled with two stainless steel plates with an active size of 2 cm × 2 cm, along with a piece of lithiated Nafion membrane,13 lithium foil as the anode, and nickel foam as the cathodic current collector. 3 mL of electrolyte was employed. Porous TiO2 pellets (12 mg, equivalently 50 mM) were immersed in the electrolyte in a glass vial. The electrolyte was circulated between the stack cell and glass vial by a peristaltic pump. The flow cell was cycled in galvanostatic mode in the voltage range of 1.40 to 2.80 V. The current density is 0.025 mA cm-2. Lithiated Nafion membranes were used as separator for both static glass cell and flow stack cell. Galvanostatic Intermittent Titration Technique (GITT) Measurement. GITT measurement was carried out in a flow stack cell with the above-mentioned multichannel potentiostat. The flow rate is about 0.5 ml/s. Prior to the test, the cell was fully discharged. The GITT measurement was conducted in the voltage range of 1.40-2.80 V. The cell was charged at a constant current density of 0.025 mA cm-2 for 600 s, followed by a relaxation at open circuit state for 600 s. The procedure was continued until the voltage reached the threshold values.

Results and Discussion


Page 3 of 8

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. a) CV curves of bis(pentamethylcyclopentadienyl)chromium (CrCp*2) and bis(cyclopentadienyl) cobalt (CoCp2). -1 The supporting electrolyte is 1 M LiClO4 in PC and the scan rate is 100 mV s . The potentials of CrCp*2 and CoCp2 closely 4

straddle that of TiO2.

b) Illustration of redox targeting reactions of TiO2 lithiation and delithiation.

Electrochemical Analysis. Cyclic voltammetry (CV) was conducted on CrCp*2 and CoCp2. Single electron transfer reactions were observed for both molecules.14 As shown in Figure 1a, CrCp*2 molecule reveals well-defined oxidation and reduction peaks at 1.82 V and 1.72 V vs. Li/Li+, respectively. The half-wave potential of CrCp*2 is determined to be 1.77 V using E1/2 = (Epc + Epa)/2. In addition, CoCp2 also shows clear-cut oxidation and reduction peaks at 1.97 V and 1.88 V vs. Li/Li+, respectively, yielding an E1/2 of 1.92 V (Figure 1a). Since the lithiation/delithiation potential of TiO2 is ~1.80 V, thermodynamically TiO2 would be reduced (lithiated) by CrCp*2 and oxidized (delithiated) by CoCp2+ (Figure 1b). It is worth emphasizing that the potential difference between CrCp*2 and CoCp2 is only 0.15 V, which is considerably smaller than the previous reported value between CoCp*2 and CoCp2 (0.54 V) and that between Fc and FcBr2 (0.30 V). The good reversibility of CrCp*2 and CoCp2 was retained even after 1000 cycles, indicating good stability of both molecules (Figure S1). To gain further insights into the redox kinetics of both molecules, various scan rates were applied in the CV measurement. Both CrCp*2 and CoCp2 present good reversibility as the cathodic and anodic peaks remain symmetry, with only slight shifts with increasing scan rate. In the scan rate range of 10~320 mV s-1, the peak currents (ip) of both molecules show linear relationship with the square root of scan rate (v1/2), which is an indication of the diffusion-controlled reversible reactions (Figure S2). The diffusion coefficients could be estimated based on the Randle-Sevcik equation: i 2.69 10 / /

/

(2)

where n is the number of electrons transferred, A is the electrode area, D is the diffusion coefficient, and C is the bulk concentration of CrCp*2 and CoCp2. The obtained diffusion coefficients are summarized in Table 1. The

Table 1. Redox potential and diffusion coefficients of CrCp*2 and CoCp2. 2

Molecule

E1/2 / V

D / cm s

CrCp*2

1.77

1.54 ×10

CoCp2

1.92

2.52 ×10

-1

-7 -6

diffusion coefficient of CoCp2 is one order of magnitude higher than CrCp*2. This difference is plausibly due to the volume effect of the 10 methyl groups on the cyclopentadienyl rings of CrCp*2. The diffusion coefficients of CrCp*2 and CoCp2 are comparable with the previously reported active species in non-aqueous redox flow battery.15, 16 Redox flow lithium battery performance. The chemical lithiation and delithiation of TiO2 by CrCp*2 and CoCp2 were studied by a RFLB static glass cell (inset of Figure 2). Anatase TiO2 particles were added in the PC/LiClO4 electrolyte in one compartment of the glass cell. For demonstrating purpose, lithium foil was used as counter electrode in the other compartment. A lithiated Nafion membrane separated the two compartments. The glass cell delivered zero capacity in the absence of redox molecules, since there was no electrical contact between the TiO2 particles and the Ni foam electrode. In comparison, CrCp*2 and CoCp2 yielded two pairs of voltage plateaus in the absence of TiO2, corresponding to the sequential oxidation and reduction of the two redox molecules (Figure 2a). When adding 5 mM CrCp*2 and CoCp2 as well as 4 mg TiO2 particles in the same compartment, the cell showed much extended charge/ discharge curves (Figure 2b). The electrochemical reactions on electrode and the chemical reactions in tank during discharging and charging can be written as: Discharging:



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2. Galvanostatic charge/discharge curves of the inset electrochemical cell. a) 5 mM CoCp*2 and CoCp2 redox molecules dissolved in the catholyte; b) 4 mg TiO2 powder dispersed in the catholyte of (a). The current density is 0.1 -2 mA cm .

On electrode:

%"∗

+ e → %"

∗

(4)

In tank: + + %"∗ → + %"∗

(5)

Charging: On electrode: %"∗ → %"∗ + e

!" → !" + e

(6) (7)

In tank: + !" → + + !"

Figure 3 a) Schematic of redox flow lithium battery stack cell. b) Charge/discharge curve of redox flow lithium battery. Inset: dt/dV vs. V curves; c) GITT test of redox flow lithiumbattery. (pulse time: 600s, relaxation time: 600s); d) battery cycle performance.

Li+ per unit TiO2 (or Li0.27TiO2) was attained. This result indicates that CrCp*2 holds excellent reactivity to reduce TiO2, despite its potential is rather close to TiO2.

(3)

!" + e → !"

Page 4 of 8

(8)

In the discharging process, a small plateau appeared at 1.78 V (step a), which was due to the reduction of CoCp2+ (reaction 3). The capacity of this plateau indicates that almost 100% of CoCp2+ is reduced. Subsequently, a much-extended plateau was found at 1.57 V (step b). This plateau is attributed to the reduction of CrCp*2+ into CrCp*2 (reaction 4), followed by the redox targeting reaction of CrCp*2 with TiO2 particles, regenerating CrCp*2+ (reaction 5). In the charging process, a small plateau was first shown at 1.92 V (step c) as a result of the oxidation of CrCp*2 (reaction 6). Then a much-extended plateau was observed at 2.11 V (step d). This plateau is an indication of the oxidation of CoCp2 into CoCp2+ (reaction 7), followed by the redox targeting reaction of CoCp2+ with the LixTiO2, regenerating CoCp2 (reaction 8). In the charge/discharge cycle, TiO2 was lithiated (reduced) by CrCp*2, and the LixTiO2 was delithiated (oxidized) by the CoCp2+. From the capacity, it could be estimated that 0.27

The voltage hysteresis of the glass cell is about 0.54 V, which results in a voltage efficiency of 74.4%. Compared with the previous report, in which CoCp*2 and CoCp2 was demonstrated in a glass cell, the overpotential has been significantly reduced by nearly 0.50 V.6 However, the voltage loss is still unendurably high due to the large internal resistance, which is determined by impedance spectroscopy to be as high as 1-2 kΩ (Figure S3). As shown in the inset of Figure 2, since the two electrodes are separated far apart in the two compartments, large resistance may arise from the membrane and electrolyte, as well as the membrane-electrolyte interface. So in order to minimize the internal resistance, an optimized stack cell configuration with the two electrodes tightly pressed on the membrane was employed. Figure 3a illustrates the architecture of the stack cell. Porous TiO2 pellets were placed in the storage tank, while the electrolyte was circulated between the storage tank and stack cell. Impedance measurement indicates that the internal resistance of the stack cell is reduced to 200-300 Ω (Figure S3). Two voltage plateaus appear at 1.82 V and 2.02 V during the charging process, while at 1.90 V and 1.73 V during the discharging process, respectively. (Figure 3b) The overall voltage hysteresis is about 0.32 V. Compared with the glass cell configuration, the voltage efficiency of the stack cell was increased to 84%. To further understand the electrochemical behavior of RFLB, galvanostatic intermittent titration technique (GITT) measurement was performed. The voltage values recorded at open circuit condition (OCV) instantaneously exclude the overpotentials arisen from the IR drop in the


Page 5 of 8

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. a) XPS results of Ti2p and Ti3s spectra for original, lithiated and delithiated TiO2 samples; b) Raman results of -1 original, lithiated and delithiated TiO2 samples in the range of 100-1000 cm .

electrolyte and membrane, and that from the interfacial electron transfer on the electrodes.17 As depicted in Figure 3c, the OCV values fit the potentials of redox molecules reasonably well. In the charging process, an overpotential of 60-100 mV was found at different states of charge, in which about 40 mV stemmed from the IR drop. Similarly, in the discharging process, an overpotential of 120-160 mV was observed, which is obviously larger than that in the charging process. This is plausibly a result that [CrCp*2]/[CrCp*2+] > [CoCp2+]/[CoCp2], caused by the relatively sluggish kinetics of the redox targeting reaction of TiO2 by CrCp*2 in the lithiation process as compared to that by CoCp2+ in the delithiation process. The relatively small diffusion coefficient of CrCp*2 might also contribute to the overpotential, but this effect should be less significant considering the fast flow rate. From the GITT test, it is concluded that a substantial improvement of ionic conductivity of membrane would be critical to further reduce the overpotential since the IR drop was mostly from the membrane. Meanwhile, increasing the concentration of redox molecules will also facilitate the kinetics of the redox targeting reactions, thus to a greater extent alleviate the voltage hysteresis. The cycling performance of the stack cell is displayed in Figure 3d. It shows an initial capacity of 1.2 mAh, equivalent to Li0.25TiO2. The porous structure of the TiO2 pellet is expected to provide necessary contact area for the redox targeting reactions between the redox molecules and material. However, as the pellets were randomly placed in the storage tank and perfect “filtering” of the redox electrolyte through TiO2 was yet achieved, some of the material may gradually lose reactivity with

trapped “dead” electrolyte and cause capacity loss. This may explain the relatively poor capacity retention of 62.5% after 50 cycles. Crossover of redox species through the membrane may also result in such capacity fading. The Coulombic efficiency of the cell is 90%-95%. In addition, it should be noted that only 4 mg TiO2 was used in the present study. For demonstration purpose, the solid material only makes up a small volume of the electrolyte in the tank. However, upon practical application, the storage tank of RFLB should be fully filled by TiO2 granules with embedded channels and interconnected pores for the flow of electrolytes. Assuming the tank is filled by TiO2 granules with a porosity of 50%, the reachable equivalent volumetric capacity will be 301.6 Ah/L. The extraordinary volumetric capacity is due to the high Li+ concentration in Li0.5TiO2 (22.5 M). XPS and Raman Analysis. The structural transformations of TiO2 induced by reversible chemical lithiation and delithiation was examined by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The chemical lithiation and delithiation of TiO2 lead to significant changes in the XPS spectra. Prior to lithiation, the Ti was in +4 valance state, for which the Ti (3s) peak was recorded at 62.1 eV, and the Ti (2p3/2) and (2p1/2) peaks at 458.6 eV and 464.3 eV (Figure 4a), respectively. The reduction reaction caused the shift of Ti 2p peaks by ~2.0 eV, resulting in two well-resolved peaks at lower binding energy. This shift is a strong indication of trapped electrons in localized states reducing Ti4+ to Ti3+. Meanwhile, Li (1s) peak appeared at 55.1 eV after reduction. The lithiation ratio of TiO2 is estimated to be



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5. UV-vis spectra of lithiation process of TiO2 under redox targeting reaction. Inset: Absorption kinetics of TiO2 recorded at 430 nm under galvanostatic mode.

around 19%, corresponding to Li0.19TiO2. This result broadly agrees with our previous report based on CoCp*2, implying the same reaction mechanism. Despite that the redox potential of CrCp*2 (1.77 V vs. Li/Li+) is fairly close to the lithiation potential of TiO2, the low concentration CoCp*2 still presents excellent reactivity for the insertion of Li+ into the TiO2 lattice. The lithiated TiO2 was further oxidized by CoCp2+. As shown in Figure 4a, excellent reversibility was observed with all Ti3+ returning back to their original Ti4+ state, and Li (1s) peak vanishing entirely. The Raman spectrum of anatase TiO2 showed 5 fingerprint peaks, among which 3 peaks at 143, 196, and 396 cm-1 are due to the bending of O-Ti-O bonds, and 2 peaks at 516 and 637 cm-1 are corresponding to the stretching of Ti-O bonds.18 Upon reaction with CrCp*2, the lithiated sample (LixTiO2) showed a typical orthorhombic structure, which is different from the tetragonal structure of anatase TiO2.19 9 active bands are assigned for the orthorhombic LixTiO2 spectrum, as shown in Figure 4b.18 The LixTiO2 was then delithiated by CoCp2+. The delithiated sample revealed identical Raman spectrum to the pristine anatase TiO2, substantiating the good reversibility of the chemical lithiation and delithiation by the pair of redox molecules. Spectroelectrochemical Analysis. Spectroelectrochemical analysis has been employed to investigate the electrochemical lithiation and delithiation process of TiO2 in several reports.20 Here in situ UV-Vis measurement was conducted to monitor the absorbance changes of TiO2 upon redox targeting reactions. A 3-electrode spectroelectrochemical cell was employed for the in situ measurement, in which a porous TiO2 film screen-printed on an Al2O3-coated FTO was used as the working electrode.6 Under a constant current, the CrCp*2+

Page 6 of 8

will firstly be reduced on FTO, and then diffuse through the Al2O3 spacer, and react with TiO2. Figure 5 shows the UV-Vis spectra during the lithiation process, in which the “capacitive regime” and “insertion regime” are clearly seen. The beginning of the reaction could be counted as “capacitive regime”, and the absorption increased monotonously with wavelength. The absorption is attributed to the accumulation of delocalized electrons in the absence of Li+ insertion. After 200 s, a well-developed absorption peak was observed at 430 nm, and a broad hump emerged at around 750 nm. Meanwhile, a striking color change was rapidly built up on the electrode. This period could be counted as “insertion regime”, during which electron injection accompanies Li+ insertion. The reversible chemical lithiation/delithiation process was further investigated under kinetics mode. Absorption at 430 nm was continuously monitored upon applying a constant current (inset of Figure 5). Consistent with that in the “capacitive regime”, the absorbance of TiO2 had almost no change in the first 100 s of reaction, while the voltage dropped steeply to trigger the reduction of CrCp*2+ on FTO. The absorbance then rose steadily with the lithiation of TiO2 by CrCp*2, when the electrode entered the “insertion regime”. The process was reversed by transferring the lithiated electrode to a CoCp2 solution, and a reverse constant current was applied. Absorbance of the electrode decreased gradually due to the delithiation of LixTiO2 by CoCp2+, and became zero at the end of charging, corroborating the good reversibility of the redox targeting reactions.

Conclusions With the redox targeting reactions of CrCp*2 and CoCp2, reversible lithiation/delithiation of anatase TiO2 was successfully demonstrated. CrCp*2 showed excellent reactivity toward the lithiation of TiO2 despite relatively small driving force. So as a result of the narrowed redox potential gap between CrCp*2 (1.77 V vs. Li/Li+) and CoCp2 (1.92 V vs. Li/Li+), improved voltage efficiency of 84% was attained in redox flow lithium battery half-cell. Moreover, the redox targeting reactions showed good reversibility and stability, and the flow cell delivered a Coulombic efficiency over 90%. We believe the results represent a significant progress in the development of redox flow lithium battery and attest the viability of redox targeting concept for high density large-scale energy storage.

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. The stability test of bis(pentamethylcyclopenta-dienyl)chromium (CrCp*2) and cobaltocene; Cyclic voltammetry test of a) CrCp*2 and b) CoCp2 under various scan rates; impedance plot of stack cell and glass cell

AUTHOR INFORMATION Corresponding Author


Page 7 of 8


*Qing Wang, E-mail: [email protected]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We acknowledge financial support by the National Research Foundation, Prime Minister’s Office, Singapore under its Competitive Research Program (CRP Award No. NRF-CRP8-2011-04). We thank Dr. Chuankun Jia and Mr. Yunguang Zhu for fruitful discussions.

REFERENCES 1. Díaz-González, F.; Sumper, A.; Gomis-Bellmunt, O.; Villafáfila-Robles, R., A review of energy storage technologies for wind power applications. Renewable Sustainable Energy Rev. 2012, 16, 2154-2171. 2. Skyllas-Kazacos, M.; Kazacos, G.; Poon, G.; Verseema, H., Recent advances with UNSW vanadium-based redox flow batteries. Int. J. Energy Res. 2010, 34, 182-189. 3. Duduta, M.; Ho, B.; Wood, V. C.; Limthongkul, P.; Brunini, V. E.; Carter, W. C.; Chiang, Y.-M., Semi-Solid Lithium Rechargeable Flow Battery. Adv. Energy Mater. 2011, 1, 511-516. 4. Wei, X.; Xu, W.; Vijayakumar, M.; Cosimbescu, L.; Liu, T.; Sprenkle, V.; Wang, W., TEMPO-Based Catholyte for High-Energy Density Nonaqueous Redox Flow Batteries. Adv. Mater 2014, 26, 7649-53. 5. Huang, Q.; Li, H.; Gratzel, M.; Wang, Q., Reversible chemical delithiation/lithiation of LiFePO4: towards a redox flow lithium-ion battery. Phys. Chem. Chem. Phys. 2013, 15, 1793-7. 6. Pan, F.; Yang, J.; Huang, Q.; Wang, X.; Huang, H.; Wang, Q., Redox Targeting of Anatase TiO2for Redox Flow Lithium-Ion Batteries. Adv. Energy Mater. 2014, 4, 1400567. 7. Jia, C.; Pan, F.; Zhu, Y. G.; Huang, Q.; Lu, L.; Wang, Q., High–energy density nonaqueous all redox flow lithium battery enabled with a polymeric membrane. Sci. Adv. 2015, 1, e1500886. 8. Wang, Q.; Zakeeruddin, S. M.; Wang, D.; Exnar, I.; Gratzel, M., Redox targeting of insulating electrode materials: a new approach to high-energy-density batteries. Angew Chem Int Ed Engl 2006, 45, 8197-200. 9. Deng, D.; Kim, M. G.; Lee, J. Y.; Cho, J., Green energy storage materials: Nanostructured TiO2 and Sn-based anodes

for lithium-ion batteries. Energy Environ. Sci. 2009, 2, 818-837. 10. Kavan, L.; Kalbáč, M.; Zukalová, M.; Exnar, I.; Lorenzen, V.; Nesper, R.; Graetzel, M., Lithium Storage in Nanostructured TiO2 Made by Hydrothermal Growth. Chem. Mater. 2004, 16, 477-485. 11. Lindström, H.; Södergren, S.; Solbrand, A.; Rensmo, H.; Hjelm, J.; Hagfeldt, A.; Lindquist, S.-E., Li+ ion insertion in TiO2 (anatase). 2. Voltammetry on nanoporous films. J. Phys. Chem. B 1997, 101, 7717-7722. 12. Ponce de León, C.; Frías-Ferrer, A.; González-García, J.; Szánto, D. A.; Walsh, F. C., Redox flow cells for energy conversion. J. Power Sources 2006, 160, 716-732. 13. Liang, H.; Qiu, X.; Zhang, S.; Zhu, W.; Chen, L., Study of lithiated Nafion ionomer for lithium batteries. J. Appl. Electrochem. 2004, 34, 1211-1214. 14. Astruc, D., Electron-reservoir complexes and other redox-robust reagents: functions and applications. New J. Chem. 2009, 33, 1191-1206. 15. Zhao, Y.; Ding, Y.; Song, J.; Li, G.; Dong, G.; Goodenough, J. B.; Yu, G., Sustainable Electrical Energy Storage through the Ferrocene/Ferrocenium Redox Reaction in Aprotic Electrolyte. Angew Chem Int Ed Engl 2014, 53, 11036–11040. 16. Wei, X.; Cosimbescu, L.; Xu, W.; Hu, J. Z.; Vijayakumar, M.; Feng, J.; Hu, M. Y.; Deng, X.; Xiao, J.; Liu, J.; Sprenkle, V.; Wang, W., Towards High-Performance Nonaqueous Redox Flow Electrolyte via Ionic Modification of Active Species. Adv. Energy Mater. 2015, 5, 1400678. 17. Ventosa, E.; Skoumal, M.; Vázquez, F. J.; Flox, C.; Morante, J. R., Operando studies of all-vanadium flow batteries: Easy-to-make reference electrode based on silver– silver sulfate. J. Power Sources 2014, 271, 556-560. 18. Hardwick, L. J.; Holzapfel, M.; Novák, P.; Dupont, L.; Baudrin, E., Electrochemical lithium insertion into anatase-type TiO2: An in situ Raman microscopy investigation. Electrochim. Acta 2007, 52, 5357-5367. 19. Koudriachova, M.; de Leeuw, S.; Harrison, N., Orthorhombic distortion on Li intercalation in anatase. Phys. Rev. B 2004, 69, 054106. 20. van de Krol, R.; Goossens, A.; Meulenkamp, E. A., Electrical and optical properties of TiO in accumulation and of lithium titanate Li0.5TiO2. J. Appl. Phys. 2001, 90, 2235-2242.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 8

SYNOPSIS TOC One of the biggest challenges of redox flow battery is the low energy density. To address the issue, a novel redox flow lithium battery with greatly enhanced energy denisty is being developed. The present work reports a redox molecule pair which significantly improves the voltage efficiency of redox flow lithium battery anodic half cell, and holds great promise for high density large-scale energy storage.

High Energy Density Redox Flow Lithium Battery with Unprecedented Voltage Efficiency ToC graphic


8

High-Energy Density Redox Flow Lithium Battery with Unprecedented

Recommend Documents