Insertion of Calcium Ion into Prussian Blue Analogue in Nonaqueous

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Insertion of Calcium Ion into Prussian blue Analogue in Non-Aqueous Solutions and Its Application to a Rechargeable Battery with Dual Carriers Tohru Shiga, Hiroki Kondo, Yuichi Kato, and Masae Inoue J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b10245 • Publication Date (Web): 23 Nov 2015 Downloaded from http://pubs.acs.org on November 28, 2015

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The Journal of Physical Chemistry

Insertion of Calcium Ion into Prussian blue Analogue in Non-Aqueous Solutions and Its Application to a Rechargeable Battery with Dual Carriers Tohru Shiga*, Hiroki Kondo, Yuichi Kato, and Masae Inoue Toyota Central Research & Development Laboratories Inc. Nagakute-city, Aichi-ken, 480-1192 Japan

ABSTRACT. We observed the first electrochemical insertion of Ca2+ into Prussian blue analogue, MnFe(CN)6, in non-aqueous solutions of Ca(CF3SO3)2 and various solvents including ionic liquid at 60 °C. The kinetics for the Ca2+ insertion reaction were studied by cyclic voltammetry, and were compared those of Na+ intercalation. By coupling this phenomenon with metallic anodes two energy storage devices were made. Ca anode produced a primary cell which operated voltage around 2.0V. When Mg plate was used as an anode, the negative active material associated with CF3SO3- which we have already reported was newly formed at the surface of Mg plate. By combining the negative active material we have fabricated a novel rechargeable battery using dual ion transport species of Ca 2+ for the cathode and CF3SO3- for the anode, and demonstrated that the battery showed repeated discharge/charge performance.

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1. Introduction Rechargeable batteries with polyvalent ion carriers have been extensively studied as post Li-ion batteries for the past three decades. For magnesium (Mg) divalent-ion systems, Aurbach et al. first succeeded in designing a prototype of a secondary Mg-ion battery using a MgxMo3S4 cathode and a Grignard-type electrolyte for electrochemical deposition and dissolution of magnesium.1 This epoch-making discovery encouraged further research into Mg.2-4 In recent advances, new cathode materials such as WSe 2 and MgFeSiO4,

and boron clusters as highly stable electrolyte have been developed.5-7

Calcium (Ca) is the fifth most abundant element in the earth with an attractive reduction potential (-2.87V vs SHE). However, it has received far less attention due to the big barrier to Ca electrochemistry; there are few, if any, electrolytes that achieve reversible deposition and dissolution of Ca, and therefore, non-aqueous Ca-O2 and Ca-S batteries are primary cells.8,9 No progress on cathode materials except for MnO2 and V2O5 has been made.10 The objective of this paper is to obtain a foothold that opens the way for future Ca-ion batteries. In our previous paper11 we investigate Mg deposition and dissolution in some ionic liquids at 60℃ and found a new redox couple when using Mg(CF 3SO3)2-N-methyl-Npropylpiperidinium bis(trifluoromethanesulfonyl)

amide,

PP13+TFSA-, electrolyte,

separated from Mg deposition and dissolution. The redox couple was based on film formation and release associated with CF3SO3-, Mg2+ and TFSA-, indicating a negative active material of CF3SO3- as the ion transport species. According to our model of the film formation, the cations of PP13 + and Mg2+ in the electrolyte do not take part in the film formation. This suggests that we can use some other cathodes. If the cathode is based

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upon the intercalation and de-intercalation of calcium ion, we will have designed a novel rechargeable battery by combining the cathode with a Ca2+-carrier and the negative active material having the anion as the ion transport species. Prussian blue materials (AxMM’(CN)6.nH2O A:alkali metal, M,M’: transition metal, Abbreviation: PB) with an open framework on the order of 5 Å have attracted much attention as active materials in battery systems. Electrochemical insertion of monovalent ions such as Li+, Na+, and K+ has been made in aqueous and non-aqueous solutions.12-17 Cui et al. reported the highly reversible insertion of divalent ions such as Mg 2+, Ca2+, Sr2+, and Ba2+ into nanoscale PB in aqueous solutions.18 Okubo et al. studied the interfacial charge transfer of a PB thin film with hydrated ions (Li+, Na+ and Mg2+). They used LiClO4, NaClO4 and Mg(ClO4)2 as supporting salts and observed insertion of the three cations in aqueous solutions and insertion of Li + and Na+ in non-aqueous propylene carbonate solution. However, the insertion of Mg2+ in non-aqueous propylene carbonate has not been successful.19 The non-insertion of Mg2+ was explained by a large desolvation energy due to the strong Lewis acidity and slow diffusion caused by the strong Coulombic repulsion.20,21 Recently, Menke et al reported

the insertion of Al 3+ into

CuH(CN)6 in Al(CF3SO3)3-diglyme electrolytes. However, the reversible capacity due to the Al3+ insertion was very small (about 10 mAh/g).22 This paper researches PB analogues that can accommodate intercalation and deintercalation of divalent Ca ions in non-aqueous solutions. The key issue for the discovery of the negative material mentioned above was to carry out the deposition test at 60 °C. The high temperature condition and the surrounding ionic solvent may accelerate quick diffusion and the decrease in de-solvation energy. Therefore, the insertion of Ca2+ into PB



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analogues in some organic solvents including ionic liquids at 60 °C was examined first. Next, we fabricated a non-aqueous rechargeable battery having dual ion transport species of Ca2+ for the cathode and CF3SO3- for the anode. We then demonstrate that the novel dual-carrier battery shows evidence of being rechargeable. For reference, we also investigated the NaCF3SO3 system in the same manner. 2. Experimetal Section Materials. Ca(CF3SO3)2 (Aldrich) and NaCF3SO3 (Kishida Chemicals, battery grade) were used as supporting salts. The salts were dried under vacuum at 150 °C for 6 hours before mixing with organic solvents. Trimethyl phosphate (TMP, battery grade), propylene carbonate (PC), and dimethyl sulfoxide (DMSO) were available from Kishida Chemicals. The ionic liquids was N,Ndiethyl-N-methyl-N-(2-methoxyethyl)

ammonium

bis(trifluoro-methanesulfonyl)

amide

(DEME+TFSA-, Kanto Chemicals). Mg film (Nilako Corporation, thickness=0.2mm) and platinum (Pt) film (Tanaka Holdings, thickness=0.1mm) were cut off and polished by sand paper (#400) in an argon-filled glove box before use in the electrochemical cell. Synthesis of PB. The PB material was prepared as follows. Mn(NO3)2-6H2O and K3Fe(CN)63H2O (Aldrich) were separately dissolved in distilled water under N2 bubbling. Both aqueous solutions were mixed and stirred at room temperature, and after a short time a griseous compound precipitated. The precipitates were collected by centrifugation, and washed by distilled water several times. Finally, they were dried in vacuum at 120 °C for 10 hours.23 Our PB material was yellowish brown (Figure S1 in the supporting information). According to elemental analysis and thermogravimetry-differential thermal analysis, the chemical composition of our PB material was K0.1Mn1Fe1.1(CN)6-4.2H2O.


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Cyclic voltammetry. Cyclic voltammetry (CV) for some organic solvent electrolytes was first carried out at 60°C to understand the insertion of Ca2+ or Na+ into the PB active material. A beaker cell was fabricated with PB working electrode, Pt counter electrode, and an Ag+/Ag reference electrode (BAS, RE-7. Electrolyte solution: an acetonitrile solution containing 0.01M AgNO3 and 0.1M (C4H9)4NClO4) (Figure S2). The PB carbon sheet was prepared by dry-mixing K0.1Mn1Fe1.1(CN)6 powder (57% by weight), carbon black (Tokai Carbon, TB5500, 33%) and Teflon powder as a binder (Daikin, F-104, 10%). The PB-carbon sheet (1.3mg) was compressed with Pt mesh grid (Nilako, 80 mesh) to obtain the PB working electrode. The electrolytes were prepared by dissolving Ca(CF3SO3)2 or NaCF3SO3 into organic solvents. The organic solvents were trimethyl phosphate (TMP), dimethyl sulfoxide (DMSO), and propylene carbonate (PC, Kishida Chemical, battery grade). The concentrations of the salts were between 0.1mol/L (mole of salt per solvent volume) and 0.3mol/L. Ionic liquid electrolytes were DEME+TFSA--based solutions. For the Ca(CF3SO3)2 system, 0.5 mL of TMP was added to 5ml of DEME+TFSAelectrolyte to assist in the dissolution of Ca(CF3SO3)2. The concentrations of Ca(CF3SO3)2 and NaCF3SO3 were 0.05mol/L and 0.2mol/L, respectively. Before the CV test, the cell was stored in a 60 °C chamber for 10 hours. The CV tests were carried out by using a potentiometer (IVIUM Technologies, IVIUMSTST XR). The sweep rate varied from 0.1mV/sec to 10mV/sec. The CV measurements for the anode were conducted as follows. A beaker cell was fabricated with Mg plate (Nilako, thickness = 0.2 mm) as a working electrode, Pt counter electrode, and an Ag+/Ag reference electrode (BAS, RE-7). The storage of the cell at 60 °C before the CV test was carried out in the same way. Three-electrode test. A three-electrode cell was fabricated using a PB-carbon sheet (1.2mg) compressed with Pt mesh grid as cathode, Mg anode, and an Ag+/Ag reference electrode (Figure



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S2). Prior to the test, the cells were stored in a thermostat-controlled chamber at 60 °C for 8h to prepare the solid electrolyte interface (SEI). Measurement of cell performance. Electrochemical coin cells (Figure S3) were fabricated with a carbon cathode incorporating PB powder sandwiched between two SUS mesh grids, along with a Mg anode (16 mm in diameter, 0.2 mm thick), the ionic liquid electrolytes, and three 25-µm-thick polyethylene separators (Tonen Chemical Corp.). The finished carbon cathode (total weight 10 mg, surface area 1.5386 cm2, thickness 0.12 mm) was then sandwiched between two 80-mesh SUS grids (Nilako Corp., SUS304). The electrolyte of 0.3 mL was transferred into the cell. The measurements of the electrochemical cell performance were made at 60 °C. Before the electrochemical test, the cell was stored in a 60 °C chamber for 20 hours. The discharge and charge cycle was repeated between 1.1 and 2.8V for Ca. The Na cell drove the charge voltage up to 2.5V. The discharging-charging performance of the cells was followed using a Hokuto Denko charge/discharge instrument (HJ1001SM8A) by applying a 0.005 mA (0.87 mA/g per weight of PB powder in cathode) discharge current at 60 °C. When the discharge voltage reached 1.1 V, the current was reversed. The charging was continued up to a voltage of design for a new rechargeable battery using dual ion transport species. Surface analysis. Raman spectra were acquired with a JASCO laser Raman spectrophotometer (NRS 3300) to study the formation of the film on the Mg anode before and after discharging. The samples were set in an argon-filled quartz cell. The wavelength of the excitation laser was 532 nm and the spot was 30 µm in diameter. Time-of-flight secondary ion mass spectrometry (TOFSIMS) analysis of the Mg anode and Pt cathode before and after discharging, and after charging,


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was also performed using a TOFSIMS-5 spectrometer (ION-TOF GmbH) over an area of 0.5 mm × 0.5 mm. 3. Results and Discussion CV profiles for organic solvents. We selected organic solvents with high relatively dielectric constants ℇr, TMP (ℇr = 21), DMSO (ℇr = 47), and PC (ℇr = 65), to promote the dissolution of Ca(CF3SO3)2 .24 TMP could dissolve Ca(CF3SO3)2 up to 0.5 mol/L (mole of salt per solvent volume). Figure 1a shows the CV curves for Ca(CF3SO3)2 electrolytes using various solvents at 60℃. The origin position of Ag+/Ag reference electrode corresponded to +3.168 V vs. Li+/Li. The concentration of Ca(CF3SO3)2 was 0.1mol/L. In Fig. 1a, the green line represents the CV profile for TMP at the scan rate of 1 mV/sec. A broad shoulder and an increase in the cathodic current, attributed to Ca2+ insertion, were detected near -0.02 V and below -0.5 V vs. Ag+/Ag at 60 °C, respectively. In the anodic scan, the two sharp peaks, located at around +0.02 V and +0.39 V, correspond to the de-intercalation of Ca2+. These results were matched with two de-insertion reactions at different potentials with Ca2+ in aqueous solutions as reported by Cui et al.12 The different steps probably relate to the insertion of Ca2+ into different crystallographic sites, but the precise mechanism of insertion is presently unknown. The retention of intercalation to deintercalation was 88.9%. A small amount of Ca2+ could not be removed from the PB open frame. The CV profile for the PC electrolyte is described by the blue line. It exhibited a shoulder near 0.10 V and an increase in the cathodic current below -0.27 V vs. Ag+/Ag. The anodic peaks were located at -0.07 V and +0.24 V. These cathodic and anodic signals were similar to those observed in the TMP. The DMSO electrolyte (black) had two peaks at -0.41 V and -0.07 V in the cathodic scan, but the anodic signal corresponded to the former peak was ambiguous. The anodic peak at +0.21 V corresponds to the cathodic response at -0.07 V. The CV curves for NaCF3SO3



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electrolytes are displayed in Figure 1b. The concentration of NaCF3SO3 was 0.2 mol/L. The TMP electrolyte showed a redox couple with high current at -0.23 V/+0.16 V, and a redox couple with low current at +0.38 V/+0.68 V. The two redox couples were characteristic, and also detected in the PC electrolyte. Concerning redox couples of PB compounds, Goodenough et al. investigated KMFe(CN)6-Na+ systems (M:Mn, Fe, Co, Ni, Cu, Zn).25 Comparing their results, it seems that the large peaks at -0.23 V/+0.16 V for TMP in this study corresponds to oxidationreduction of the high-spin Fe(III)-Fe(II) couple bonding to N, and the small signals at +0.38 V/+0.68 V to the low-spin Fe(III)-Fe(II) couple. In the DMSO electrolyte, only one redox couple was observed at -0.38V/+0.26V. Since the Coulombic efficiencies except for DMSO electrolyte were about 98%, the insertion and extraction of Ca2+ was reversible. After the CV test using the DMSO electrolytes, light brown precipitates were observed on the Pt counter electrode. TOF-SIMS and Raman spectroscopic measurements were carried out to investigate these precipitates. Figure 2 shows a TOF-SIMS spectrum for positive ions. The signals m/z =54.938 and 55.854 are due to Mn + and Fe+, respectively. When the latter signal was enlarged, it divided into two peaks due to Fe+ and MnH+ as seen in the inserted figure. The Raman spectrum of the precipitates was displayed in Figure S4. Two signals due to the vibration of CN- were observed at 2107 cm-1 and 2145 cm-1. The TOF-SIMS and Raman results indicated that the precipitates were composed of Fe, Mn and CN, and it appeared that the PB analogue, MnFe(CN)6 , dissolved into the DMSO electrolytes at 60℃.


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0.4

0.04

.

0.06 TMP

0.02 0

Current (A/g)

.

0.08

Current (A/g)

DMSO

-0.02 -0.04 PC

-0.06

0.3 0.2

TMP

DMSO

0.1 0 -0.1 -0.2

PC

-0.3

-0.08 -1

-0.5

0

0.5

1

-0.4 -0.75

1.5

-0.25

Potential (V vs Ag)

0.25

0.75

1.25

Potential (V vs Ag)

Figure 1. CV profiles for the electrolytes dissolving Ca(CF3SO3)2 (a) and NaCF3SO3 (b) in various solvents at 60 °C ; trimethyl phosphate (TMP, green), propylene carbonate (PC, blue), and dimethyl sulfoxide (DMSO, red). The sweep rate was 1 mV/sec.

Intensity (counts)

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3 x10x10

3

606.0

F e+

MnH +

5.0

404.0 3.0

202.0

Intensity (counts)

Intensity (counts)

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44 x10 x10

Mn+

1.0 55.90

55.90

1.5 1.5

55.95

56.00

55.95

Mass (u) 56.00

1.0 1.0

F e+

0.5 0.5

F eH +

0 55.0 55.0

55.5 55.5

56.0

56.0

56.5

56.5

57.0 57.0 Mas s (u) Mass (u)

Figure 2. TOF-SIMS spectrum of positive ions for the precipitates on platinum (Pt) plate in Ca(CF3SO3)2DMSO solution. The mass range was between 54 and 58. The inserted figure shows the spectra between m/z= 55.9 and 56.0.

The intensities of cathodic and anodic peaks and the peak separation between them depend on the diffusion coefficient of the carrier in bulk PB material and the rate constant at the interface between active materials and electrolyte, and they are strongly influenced by temperature.26 The



CV curves for TMP electrolytes were measured at 25, 45 and 60℃. For the Ca(CF3SO3)2 -TMP system, two anodic peaks for the de-intercalation of

Ca2+ were observed (Fig.1a). The

NaCF3SO3 -TMP system showed one anodic peak at +0.16 V. These anodic peak currents of a logarithmic scale were plotted as a function of reciprocal of absolute temperature (Figure 3). The unique slopes of all plots were described. The slope of the NaCF3SO3 -TMP system was about one half of that of the Ca(CF3SO3)2 -TMP system. The Arrhenius plot indicates indirectly that the electrochemical de-intercalation of Na+ occurred smoothly compared to Ca2+. This is influenced by slower diffusion of Ca2+. Anodic peak current (A/g)　　　。

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1

0.1

0.01

0.001 2.9

3

3.1

3.2

3.3

3.4

3.5

-3

1/T　( x10 )

Figure 3. Temperature dependence on anodic peak current observed in CV profiles; (○) at +0.02 V and (△) at +0.39 V for Ca(CF3SO3)2-TMP, and (◇) at +0.16 V for NaCF3SO3-TMP.

In addition, we investigated the mechanism for the charge storage observed in the Prussian blue analogue. An examination of the voltammetric sweep rate dependence allows us to quantitatively distinguish the capacitive contribution to the current response. The current response at a fixed potential can be generally expressed as

I(V) = k1ν + k2ν (1/2)

(1)


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where I is the peak current, ν is the sweep rate, and k1 and k2 are constants.27-30 The first term in Eq. 1 involves the faradaic contribution from the charge transfer process with surface atoms (referred to as pseudocapacitance), and the nonfaradaic contribution from the effect of electrical double layer-distortion. The two components cannot be separated. The second term reflects the faradaic contribution from the carrier insertion process, and it is assumed the current obeys a power-law relationship with the square root of sweep rate. To elucidate the charge storage of PB with Ca2+ or Na+, the CV test was performed using various sweep rates. Figure 4 shows the relationship between the anodic peak current and the square root of sweep rate in the CV test for the Ca(CF3SO3)2-TMP. The sweep rate varied from 0.2 mV/sec to 10 mV/sec. The current of the anodic peak at around +0.39 V (△) varies linearly with ν

(1/2)

up to 3 mV/sec, suggesting that the

extraction process from the PB analogue depends on diffusion of Ca 2+. The peak current deviated from ν (1/2) beyond 3 mV/sec. It seems that Ca2+ insertion and extraction cannot be achieved easily under high sweep rates. Since the signal intensity at around -0.07 V (○) was kept constant, the energy storage was saturated in an instant (Figure S5). The electrochemical behavior for NaCF3SO3-TMP was also examined by the CV technique which provides a variation in scan rates between 0.5 mV and 10 mV/sec. Figure 5 shows peak current vs ν (1/2) plots. As the cathodic and anodic peak currents at -0.23 V and +0.16 V were clear, they were plotted as the square root of sweep rate. The observed currents were one order of magnitude larger than in the Ca 2+ system. Both peak currents were proportional to ν

(1/2)

, indicating that the intercalation and de-intercalation reactions

depend on diffusion of Na+.



.

100

Peak current (μA)

80 60 40 20 0 0

0.5

1

1.5

2

2.5

3

3.5

Square root of sweep rate

Figure 4. Relationship between anodic peak current observed in CV profiles and square of sweep rate; (○) at -0.07 V and (△) at +0.39 V for Ca(CF3SO3)2-TMP. The concentration of Ca(CF3SO3)2 was 0.15 mol/L.

.

1.4 1.2

Peak current (A/g)

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1 0.8 0.6 0.4 0.2 0 0

1

2

3

4

Square root of scan rate

Figure 5. Relationship between anodic and cathodic peak currents observed in CV profiles and sweep rate; (○) at -0.23 V and (◇) at +0.16 V for NaCF3SO3-TMP. The concentration of NaCF3SO3 was 0.30 mol/L.

CV profiles for DEME+TFSA- . Figure 6a and 6b show CV profiles for DEME+TFSAelectrolytes dissolving Ca(CF3SO3)2 and NaCF3SO3 at 25 °C and 60 °C, respectively. Since the Ca(CF3SO3)2-DEME+TFSA- electrolyte was a highly viscous liquid and the concentration of the salt was 0.05 mol/L, the sweep rate in the CV test was set to 0.2 mV/sec. The CV curves during the 1st and 5th cycles are shown in Figure S5. Only a shoulder in the cathodic current attributed to Ca2+ insertion was detected near -0.27 V vs. Ag+/Ag at 60 °C (blue line, in Fig. 6a). In the anodic


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scan, two broad peaks, located at around +0.16 V and +0.33 V, correspond to the de-intercalation of Ca2+. However, the two CV peaks in the anodic scan were ambiguous at 25 °C, and very weak (red line). The ionic liquid, DEME+TFSA-, could dissolve NaCF3SO3 up to 0.5 mol/L. The CV test was carried out by using a 0.2 mol/L solution. The NaCF3SO3 -DEME+TFSA- electrolyte showed one distinct and broad peak due to Na+ insertion at +0.03 V at 60℃ (Fig.6b, blue). A small signal was also recognized at +0.58 V. These signals for MnFe(CN)6 in a mixed solvent of ethylene carbonate and diethyl carbonate were reported by Goodenough et al.24 The anodic peak corresponding to the cathodic signal at +0.03 V was detected at +0.43V. The cathodic and anodic peaks were dissymmetric, and the latter had a large current and a narrow half width. These signals were apparent at 25 °C at +0.15 V and +0.55 V, respectively. The peak separation between the anodic and cathodic peaks depends on the diffusion coefficient of the carrier in bulk PB material and the rate constant at the interface between PB and electrolyte. It was found that the peak positions in the NaCF3SO3-DEME+TFSA- electrolyte reflecting the intercalation/deintercalation reaction shifted to the lower temperature-side at 60 °C, and the peak separation was unchanged. 0.02

0.4

60℃

0.01

0.3

Current (A/g) .

Current (A/g) .

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0

25℃ -0.01

60℃ -0.02

0.2

25℃

0.1 0 -0.1

-0.03 -1

-0.5

0

0.5

Potential (V vs Ag)

1

-0.2 -1

-0.5

0

0.5

1

Potential (V vs Ag)

Figure 6. CV profiles for DEME+TFSA- electrolytes dissolving (a) Ca(CF3SO3)2 and (b) NaCF3SO3 at 25 °C (red) and 60 °C (blue). The sweep rate was 0.2mV/sec.



The activation energies in the ionic liquid systems were also calculated from the temperature dependence of the observed signals for de-intercalation reactions. The CV curves were measured at 25, 45 and 60 °C. The anodic peak currents are plotted as a function of reciprocal of temperature (Figure 7). For the Ca(CF3SO3)2-DEME+TFSA- system, the relationship between peak current and reciprocal temperature is described for the de-intercalation reactions at +0.33 V. The NaCF3SO3–DEME+TFSA- system showed one definite redox couple at +0.03 V/+0.43 V. The unique slope of the latter was about a half of that of the former. Although the peak current was low in the ionic liquids because of high viscosity, the values of the slope were the same as those in organic solvents. It was confirmed that the electrochemical de-intercalation of Na+ occurred smoothly compared to Ca2+ even in viscous electrolytes.

Anodic peak current (A/g)　　　。

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1

0.1

0.01

0.001 2.9

3

3.1

3.2

3.3

3.4

3.5

-3

1/T　( x10 )

Figure 7. Temperature dependence on anodic peak current observed in CV profiles; (○) at +0.33 V for Ca(CF3SO3)2-DEME+TFSA-, and (◇) at +0.43 V for NaCF3SO3 -DEME+TFSA-.

To elucidate the charge storage of PB with Ca 2+ or Na+ in an ionic liquid, the CV test was performed at 60 °C with using various sweep rates. Figure 8 shows the relationship between the anodic peak current and the square root of sweep rate in the CV test for the


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DEME+TFSA- electrolytes. For the Ca(CF3SO3)2 system, the sweep rates were between 0.2 mV/sec and 2 mV/sec. The CV profiles were monochromatic under high scan rates beyond 3 mV/sec, indicating that the insertion and extraction of Ca2+ would not happen. The current of the anodic peak at around +0.33 V (◇) is roughly proportional to ν (1/2), suggesting that the extraction process from the PB analogue depends on diffusion of Ca 2+ in the ionic liquid. The electrochemical behavior for NaCF 3SO3-DEME+TFSA- was also examined by using both anodic and cathodic scans. The CV technique provided a variation in sweep rates between 0.2 mV/sec and 3 mV/sec. Since the anodic peak current at +0.43 V (△) was clear, they were plotted as a function of square root of sweep rate. The peak current varied linearly with ν(1/2). It was clear that the intercalation and de-

.

intercalation reactions in the ionic liquid depend on diffusion of Na +.

Peak current(A/g)

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


1 0.8 0.6 0.4 0.2 0 0

0.5

1

1.5

2

2.5

Square root of sweep rate

Figure 8. Relationship between anodic peak current observed in CV profiles and square of sweep rate; (◇) at +0.33 V for Ca(CF3SO3)2-DEME+TFSA-, and ( △ ) at +0.43 V for NaCF3SO3-DEME+TFSA-. The concentrations of Ca(CF3SO3)2 and NaCF3SO3 were 0.05 mol/L and 0.2 mol/L, respectively.

Cell performance.

The electrochemical Ca2+ intercalation into the PB analogue in non-

aqueous electrolytes showed its potential as a cathode material. On the other hand, Ca is an attractive anode material having the reduction potential of +0.17 V vs Li+/Li. Although



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the deposition and stripping of Ca was difficult in non-aqueous solutions, we challenged ourselves to combine both active materials. We first examined discharge/charge curves of the electrochemical cells using Ca(CF3SO3)2-DEME+TFSA- and Ca(CF3SO3)2-TMP. We fabricated beaker cells composed of PB cathode, Ca anode, and the electrolytes. Figure 9 shows the 1st and 2nd discharge-charge curves for the batteries at 60 °C. For the Ca(CF3SO3)2-DEME+TFSA-, The operating voltage was about 2 V, which was much lower than the theoretical value (3.2V). This was caused by IR drop of the SEI at the surface of the Ca anode. The 1st discharge capacity of the battery using Ca(CF 3SO3)2DEME+TFSA- electrolyte was 55.9 mAh/g (capacity per weight of PB), which was determined from the amount of PB in the cathode. The electrochemical reaction of MnFe(CN)6 with Ca2+ non-aqueous electrolyte can be expressed as xCa2+ +2xe- + MnFe(CN)6 ⇔ CaxMnFe(CN)6

(2)

Therefore, the capacity of 55.9 mAh/g corresponds to x = 0.28. The operating voltage was about 2 V, which was much lower than the theoretical value (3.2V). This was caused by IR drop of the SEI at the surface of the Ca anode. When the charge process started, the cathode potential and the cell voltage increased gradually, indicating that some of Ca 2+ was extracted from the PB analogue. The 1st charge capacity was 12.3 mAh/g. The cell with Ca anode looked like a rechargeable battery. According to our three-electrode-test, the anode potential was kept constant at -2.1 V vs. Ag+/Ag on discharge, and at -2.7 V on charge (Figure S7). Since the anode potential on charge was higher than the reduction potential of Ca2+, the deposition of Ca2+ could not occur at Ca anode. In the second discharge-charge cycle, the discharge and charge capacities were 8.2 mAh/g and 4.5 mAh/g, respectively. The cell using the Ca(CF3SO3)2-TMP electrolyte showed 68.1


Page 17 of 25

mAh/g of discharge capacity at the 1st cycle, and the charging capacity up to 3.5 V was much small (5.4 mAh/g). Thus, the two kinds of the cells with Ca anode were primary cells.

.

4

Cell voltage (V)

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


Charge

3.5 3 2.5

Discharge

2 1.5 2nd

1

1st

0.5 0 0

20

40

60

80

Capacity (mAh/g)

Figure 9. The 1st and 2nd discharge/charge curves of electrochemical cells using Ca anode and Ca(CF3SO3)2-DEME+TFSA- (red) and Ca(CF3SO3)2-TMP (black) at 60 °C.

In our previous paper,11 we observed a redox couple based on film formation and release associated with CF3SO3- on the Mg electrode in an ionic liquid electrolyte (Mg(CF3SO3)2/ PP13+TFSA-), indicating that the film worked as an anode material with anion carrier. Before fabrication of a coin cell, we conducted a three-electrode test using PB cathode, Mg anode, and Ca(CF3SO3)2-DEME+TFSA- electrolyte to obtain a rechargeable battery, and monitored time changes in cathode and anode potentials for discharge and charge processes. The reference electrode was a non-aqueous Ag+/Ag electrode from Nilako. The test for the NaCF3SO3-DEME+TFSA- electrolyte was also carried out in the same manner. Figure 10a shows time changes in cathode and anode potentials, and cell voltage using Ca(CF3SO3)2-DEME+TFSA- electrolyte. As shown in Fig. 10a, the cell using Mg anode had a discharge plateau near 1.5V (black). When the current polarity was changed, the cell drove two-step voltages at 2.1 V and 2.7 V. The profile for the cathode potential



(blue) was similar to that of the cell voltage. The cathode potential had a discharge plateau near 0 V vs. Ag+/Ag, and two-step charges at 0 V and 0.3 V were observed. On the other hand, the profile for the anode potential was characteristic (red), and the potential was constant -1.6 V vs. Ag+/Ag for discharge, and exhibited a two-down profile at -2.0 V and at -2.3 V. Thus, the time change in cell voltage was influenced by the cathode potential rather than the anode potential. Figure 10b shows time changes in cathode and anode potentials, and cell voltage on discharge and charge processes for the NaCF3SO3 system. The cell voltage and the cathode potential had a two-step discharge, which is reflected in the CV curve in Fig. 6b. The anode potential was kept constant at 1.6 V vs. Ag+/Ag on discharge, and at -1.95 V on charge. The profile for the anode potential was similar to that observed in the Mg(CF 3SO3)2-PP13+TFSA- electrolyte during the discharge/charge process.11 Therefore, the time change in cell voltage was influenced by cathode potential rather than anode potential.

.

.

.

3 2

Cell voltage

Potential (V vs Ag)

Potential (V vs Ag)

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 18 of 25

1 0

Cathode

-1

Anode

-2

Discharge 20

40

2

Cell voltage

1 0

Cathode

-1 -2

Discharge

-3

-3 0

3

60

80

100

120

140

0

Capacity (mAh/g)

50

Anode 100

150

Capacity (mAh/g)

Figure 10. Potential profiles in a three-electrode test using Mg anode and DEME+TFSA- electrolytes dissolving (a) Ca(CF3SO3)2 and (b) NaCF3SO3 at 60℃. The blue and red lines represent cathode and anode potentials, respectively.


Page 19 of 25

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Finally, we fabricated a coin cell composed of PB cathode, Mg anode, and Ca(CF3SO3)2DEME+TFSA- electrolyte to obtain a rechargeable battery. Figure 11a shows dischargecharge curves during the 1st -20th cycles for the battery at 60 °C. The battery using the Ca(CF3SO3)2-DEME+TFSA- showed one step in the cell voltage during the 1st discharge process. The discharge plateau was observed near 1.55 V. The 1st discharge capacity of the anion battery was 62.1mAh/g (capacity per weight of PB), which was determined from the amount of PB in the cathode. The electrochemical reaction of MnFe(CN) 6 with Ca2+ non-aqueous electrolyte can be expressed by equation (2). Therefore, the capacity of 62.1 mAh/g corresponds to x = 0.31. When the polarity of the current was reversed during the charge process, the cell voltage increased to approximately 2.1 V, with a gradual increase in voltage. The total charge capacity was 67.3 mAh/g. The Coulombic efficiency (ratio of charge to discharge capacities) was 108%. Since the capacity due to the extraction of K+ from the pristine PB was 10.1 mAh/g, it seems that the efficiency of more than 100% was caused by the extraction of K+ from the pristine PB. If the difference (56.2 = 67.3 - 10.1 mAh/g) is really due to Ca2+ de-intercalation, the efficiency was 90.5%. Thus, the Ca2+ intercalation and de-intercalation reaction is not fully reversible. It seems that the irreversible reaction had a great effect on capacity fading on cycle performance. For the cell using NaCF3SO3-DEME+TFSA- electrolyte (Figure 11b), a two-stage discharge process was observed. The two steps in the cell voltage during the 1 st discharge correspond to two processes for the intercalation of Na+ into PB, corresponding to the signals at 3.77 V and 3.16 V in Fig. 2b. The total discharge capacity at the 1 st cycle was 68.3 mAh/g.31,32 The cell voltage increased slowly with time during charging. The value of x calculated from equation 3 is given by x = 0.67.



xNa+ + xe- + MnFe(CN)6 ⇔ NaxMnFe(CN)6

(3)

The charge capacity at the 1st cycle was 76.4 mAh/g, and then the Coulombic efficiency due to Na+ intercalation was calculated to be 96.9% (= (76.4-10.1)/68.3 mAh/g). The insertion/de-insertion of Na+ occurs smoothly compared to that by Ca2+ carrier. Figure S8 shows cycle performance for both cells at 60 °C. The open and solid symbols represent discharge and charge capacities, respectively. The battery using Ca 2+ showed larger capacity fade against the Na+ cell, which was caused by the degradation of the insertion reaction at the cathode and partially reversible film formation at the Mg anode.

3

Cell voltage (V) .

3.5

Cell voltage (V) .

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 20 of 25

3 2.5

Charge

2 1.5

Discharge

1

20th

10th 5th

2nd 1st

2.5 Charge

2

Discharge

1.5 1

20th 10th 5th 2nd 1st

0.5

0.5 0

0

0

20

40

60

80

0

Capacity (mAh/g)

20

40

60

80

Capacity (mAh/g)

Figure 11. Discharge-charge curves for our batteries using Mg anode and DEME+TFSA- electrolytes dissolving Ca(CF3SO3)2 (a) and NaCF3SO3 (b) at 60 °C. Cycle number: 1st (black), 2nd (pink), 5th (orange), 10th (blue), and 20th (green).

Surface analysis. To elucidate the discharge behavior in the Ca(CF3SO3)2 and NaCF3SO3DEME+TFSA- systems, the Mg electrodes were analyzed before and after the 1st discharge using Raman technique. Photographs of Mg electrodes before and after discharge, and after charge were shown in Figure S9. The blackish thin film was observed at the surface of Mg electrode


Page 21 of 25

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after discharge. It mostly disappeared during charging. Figure 12 shows Raman spectra for the Mg electrodes, a, and b, and for reagents of NaCF3SO3 and DEME+TFSA-, c, and d, between 200 cm-1 and 1400 cm-1. The Raman spectrum a for the specimen in NaCF3SO3DEME+TFSA- provided information on the film formed on the Mg metal electrode, and was in accordance with Raman spectrum c of the NaCF3SO3 reagent. Five peaks at 372, 591, 787, 1104, and 1247 cm-1 were detected, which were characteristic of the vibrations in CF3SO3 anions: SO3 rocking, SO3 bending, symmetric C-S stretching, asymmetric SO3 stretching, and symmetric CF3 stretching, respectively.33, 34 The signal at 1060cm-1 seems to reflect asymmetric SO3 stretching in the isolated SO3 unit. Therefore, the surface of the Mg plate after discharge in NaCF3SO3-DEME+TFSA- was covered with a film mainly associated with CF3SO3- anions. The Raman spectrum b which reflects the discharge product on Mg anode in Ca(CF3SO3)2-DEME+TFSA- was inconsistent with Raman spectra a and c.

For the

Ca(CF3SO3)2-DEME+TFSA- system, the Raman signal at 692 cm-1 was assigned to symmetric SN-S stretching in the TFSA ions.35, 36 A broad signal at 1041 cm-1 was due to the asymmetric stretching of CF3 units. The peaks at 410 cm-1 reflected the rocking and stretching modes of SO2 in TFSA. The weak signal near 578 cm-1 was caused by the SO3 bending. Based on the Raman data, the interface in Ca(CF3SO3)2-DEME+TFSA- was formed mainly by reaction between CF3SO3 and TFSA anions .



.

1250

Intensity (a.u.)

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 22 of 25

1000

a

750 500

b

250

c

0 200

d 400

600

800

Raman shift

1000 1200 (cm -1 )

1400

Figure 12. Raman spectra of the Mg electrodes after discharge in the cells using NaCF3SO3DEME+TFSA- (a), and Ca(CF3SO3)2-DEME+TFSA- (b), and reagents of NaCF 3SO3 (c), and DEME+TFSA- (d).

4. Conclusion To date, Ca electrochemistry has attracted little attention due to problems with cathode materials and electrolytes. We first reported on the electrochemical insertion of Ca 2+ into a PB analogue in some organic solvents including an ionic liquid at 60 °C, indicating a new cathode material for a Ca ion battery. The kinetics for Ca 2+ insertion behavior were investigated and compared to that for Na+ intercalation. In addition, we

challenged

ourselves to design a rechargeable battery based on Ca 2+ insertion into PB analogue. Since the Ca deposition and dissolution in organic electrolytes were very difficult, we utilized an anode material associated with film formation by CF 3SO3- in a particular ionic liquid. By combining the Ca2+ insertion and the anode material, we have fabricated a novel rechargeable battery with dual ion transport species. The battery showed repeatedly discharge/charge performance. Since it provides a solution to the issue of metal crystal growth at an anode, a prototype of our secondary battery demonstrates the potential for an extremely safe battery.


Page 23 of 25

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■ ASSOCIATE CONTENT

■ REFERENCES

Supporting Information

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This material is available free of charge via the Internet at http://pubs.acs.org. Photograph of MnFe(CN)6, Test apparatus, Typical CV profiles, Electrode potential profiles, Cycle performance, Photographs of Mg plates before and after discharge, and after charge. ■ AUTHOR INFORMATION Corresponding Author E-mail: [email protected] Tel：+81-561-71-7607 ■ NOTES The authors declare no competing financial interest. ■ Author Contributions T.S.

conceived and

carried out

the

experiments, analyzed the data and wrote the paper, H.K prepared Prussian blue material and had an effective discussion for the anode material, Y.K and M.I made Raman and TOF-SIMS analysis in this work.



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TOC Graphic R

e-

e-

X Mg

Mg2+ Ca2+

CF3SO3Film formation & release by CF3SO3-


Insertion of Calcium Ion into Prussian Blue Analogue in Nonaqueous

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