Stepwise Reduction of Electrochemically Lithiated CoreâShell

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Stepwise Reduction of Electrochemically Lithiated Core-Shell Heterostructures Based on the Prussian Blue Analogue Coordination Polymers K0.1Cu[Fe(CN)6]0.7•3.5H2O and K0.1Ni[Fe(CN)6]0.7•4.4H2O Carissa H. Li, Marcus K Peprah, Daisuke Asakura, Mark W Meisel, Masashi Okubo, and Daniel R. Talham Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm503639a • Publication Date (Web): 08 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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Chemistry of Materials

Stepwise Reduction of Electrochemically Lithiated Core-Shell Heterostructures Based on the Prussian Blue Analogue Coordination Polymers K0.1Cu[Fe(CN)6]0.7·3.5H2O and K0.1Ni[Fe(CN)6]0.7·4.4H2O

Carissa H. Li1, Marcus K. Peprah2, Mark W. Meisel2,*, Daniel R. Talham1,* 1 2

Department of Chemistry, University of Florida, Gainesville, FL 32611-7200, USA

Department of Physics and NHMFL, University of Florida, Gainesville, FL 32611-8440, USA

Daisuke Asakura3, Masashi Okubo4,* 3

Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8568, Japan 4

Department of Chemical System Engineering, School of Engineering, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo, 113-8656, Japan

* To whom correspondence should be addressed.

Email: [email protected] (D.R.T.);

[email protected] (M.W.M.); [email protected] (M.O.)

ACS Paragon Plus Environment


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Abstract The magnetic properties of a series of core-shell particles based on the Prussian blue analogues

K0.1Cu[Fe(CN)6]0.7⋅3.5H2O

and

K0.1Ni[Fe(CN)6]0.7⋅4.4H2O

(CuFe-PBA@NiFe-PBA) are investigated as a function of electrochemical titration and lithium ion insertion. The particles, with average size 305 ± 50 nm, are reduced using the galvanostatic

intermittent

titration

technique

to

prepare

10

samples

of

Lix(CuFe-PBA@NiFe-PBA) with 0 ≤ x ≤ 1.0. Magnetization as a function of temperature for each member of the series reveals the ferromagnetic ordering of the individual components, NiFe-PBA (Tc ~ 24 K) and CuFe-PBA (Tc ~ 18 K). The magnetic ordering of each component is suppressed upon reduction and Li+ incorporation, but in a stepwise fashion with the CuFe-PBA core reduced before the NiFe-PBA shell. The separate reductions of the core and shell are also seen in magnetization vs. field measurements at low temperature. By introducing a lattice-gas model, the enthalpy changes (∆Hi) associated with each redox couple after Li-ion insertion were calculated and applied to the mean field approximation to reproduce the magnetic transition temperatures. The results are significant as the CuFe-PBA@NiFe-PBA particles had previously been shown to exhibit superior performance over the individual components as cathode materials for lithium ion batteries, although the stepwise reduction had not previously been discerned. Furthermore the report is the first showing the systematic control of magnetism in core-shell coordination polymers


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heterostructures by electrochemical guest ion insertion. Keywords: Stepwise reduction, Prussian blue analogue, core-shell heterostructure, electrochemical Li-ion insertion, lattice-gas model, mean field approximation



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Introduction As with other classes of solid-state materials, coordination polymer heterostructures provide the opportunity to add function beyond the properties of the individual components. A recent example is in the area of alkali ion storage for battery cathode materials.1,2 The cyanometallate coordination polymers, or Prussian blue analogues (PBAs), are investigated to be a new class of potential electrode materials exploiting their stable redox properties and large and uniform porosity.3-10 Among those analogues, potassium copperhexacyanoferrate (CuFe-PBA) has a high capacity (~ 120 mAh/g) for lithium ion storage as both the copper and iron sites in the network are redox switchable. However, the capacity degrades rapidly on account of a cubic to tetragonal solid-state phase transition that accompanies the copper site reduction. On the other hand, the PBA potassium nickelhexacyanoferrate (NiFe-PBA) is highly stable to redox cycling, even if it has only one redox active site and therefore lower capacity (60 mAh/g).11 When the CuFe-PBA is coated with the NiFe-PBA in a core-shell particle, the cubic-to-tetragonal phase transition of the core is suppressed and the result is that the heterostructure delivers enhanced performance over the sum of the individual components. Prussian blue analogues also enjoy significant interest because of varied magnetic behavior including light-switchable magnetism that is tuneable with relatively simple synthetic modifications.12-17 The cyanide bridge is an effective mediator of magnetic


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exchange, and by changing the metal ions and their oxidation states in the general formula of AjMk[M′(CN)6]l·□m⋅nH2O (A: counter cation; M and M′: transition metal ions; □: [M′(CN)6]3- vacancies, hereafter represented as MM′-PBA) the properties can be tuned.18-27 Okubo and coworkers recently showed that the magnetism in some PBA’s can be transformed through electrochemical guest ion insertion.28-30 In particular, the disappearance of the ferromagnetic transition near 20 K in the CuFe-PBA was observed at its fully discharged state, where the spin configuration of Fe ions were decreased to S = 0 by its reduction from Fe3+ to Fe2+.28 In addition, by taking advantage of quantitative electrochemical titration, precise control between ferromagnetism and paramagnetism in a NiFe-PBA (Tc ~ 24 K) was further achieved.29 The magnetic ordering temperature gradually decreases as more Li-ions are incorporated into the open framework structure, accompanied by a reduction in the overall magnetization. The

present

study

investigates

the

magnetic

behaviour

of

a

series

of

Lix(CuFe-PBA@NiFe-PBA) core-shell particle samples upon electrochemical reduction. As with the individual materials, the magnetic response can be tuned, between ferromagnetism and paramagnetism, by carefully selected electrochemical titration. Importantly, the two components of the core@shell heterostructues are shown to be switched individually, resulting in stepwise changes in the magnetization. The result provides insight into the mechanism of electrochemical reduction associated with lithium ion storage. Somewhat



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counter intuitively, the magnetic response shows that the CuFe-PBA core is reduced before the NiFe-PBA shell, a result that is not discernible from the electrochemical response alone, because the Fe2+/3+ redox couples of the core and shell closely overlap. Experimental Section All reagents were purchased from Sigma-Aldrich, Fisher Scientific, or Acros Organics and used without further purification. Deionized water used in synthetic procedures was obtained from a Barnstead NANOpure purification system. The filters used during the synthesis were Fast PES Bottle Top Filters with 0.45 µm pore size (Nalgene). K0.1Cu[Fe(CN)6]0.7⋅3.5H2O (CuFe-PBA). The CuFe-PBA particles were synthesized by a co-precipitation method, using aqueous solutions of CuCl2·2H2O and K3Fe(CN)6 as the precursor solutions. Details of the synthetic procedures and precursor concentrations are described in the supporting information. The product solution was kept under vigorous stirring for 18 h after complete addition. To remove the excess precursors and collect the as-synthesized particles, they were filtered and washed with nanopure water. For collection and analysis, the particles were redispersed in a 50:50 solvent mixture of water and acetone and dried at room temperature. K0.1Cu[Fe(CN)6]0.7·3.5H2O. 218 ± 54 nm. Yellow powder. IR (KBr): 2103 cm-1 (s, νCN, CuII-NC-FeIII). EDX (Cu/Fe) 58.6: 41.4. Anal. Calcd for C4.2H7.0N4.2O3.5K0.1Cu1.0Fe0.7: C, 18.08; H, 2.51; N, 21.09. Found: C, 17.97; H, 2.04; N, 20.01.


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K0.1Cu[Fe(CN)6]0.7⋅[email protected][Fe(CN)6]0.7⋅4.4H2O (CuFe-PBA@NiFe-PBA). The previously prepared pure phase CuFe-PBA particles were redispersed in deionized water and used as the cores for growing the core-shell particles. Aqueous solutions of NiCl2·6H2O and K3Fe(CN)6 were added to the core dispersion using a peristaltic pump at a rate of 10 mL/h. Details are described in the supporting information. The as-prepared core-shell particles were filtered and washed with nanopure water before being dried at room temperature. K0.1Cu[Fe(CN)6]0.7·[email protected][Fe(CN)6]0.7·4.4H2O. 305 ± 50 nm. Yellow powder. IR (KBr): 2168 cm-1 (s, νCN, NiII-NC-FeIII); 2103 cm-1 (s, νCN, CuII-NC-FeIII). EDX (Cu/Ni/Fe) 30.9: 27.5: 41.6. Anal. Calcd for C4.2H7.9N4.2O4.0K0.1Cu0.5Ni0.5Fe0.7: C, 17.72; H, 2.78; N, 20.67. Found: C, 17.65; H, 2.27; N, 19.64. Instrumentation. Infrared spectra were recorded on a Nicolet 6700 Thermo Scientific Spectrophotometer. Typically 16 scans are taken between 2300 cm-1 and 1900 cm-1 with a precision of 0.482 cm-1. Powder samples were mixed with KBr and pressed into a pellet using 3000 PSI (20.6 MPa). A scan of pure KBr is taken as a background reference. Combustion analysis to determine carbon, hydrogen, and nitrogen (CHN) percentages was performed by the University of Florida Spectroscopic Services Laboratory. Transmission electron microscopy (TEM) was performed on a JEOL-2010F HRTEM at 200 kV. The TEM grids (ultrathin carbon film on holey carbon support film, supported by 300 mesh gold, from Ted-Pella, Inc) were prepared by dropping, onto the grid, 40µL of a solution containing 5 mg



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of sample dispersed by sonication in 1 mL of water for 30 minutes. Energy dispersive X-ray spectroscopy (EDX) was performed with an Oxford Instruments EDS X-ray Microanalysis System coupled to the HRTEM microscope. A total of 3 scans were performed on different parts of the sample and then averaged to give relative atomic percentages for copper, iron, and nickel. In order to see the core@shell structure of the particles, EDX linescans were performed on copper, iron, and nickel. The chemical formulas were based on metal compositions from EDX, adding counter cation (potassium) as determined by the number of hexacyanoferrate vacancies to ensure electroneutrality. Powder X-ray diffraction (XRD) measurements were performed on a X’Pert Powder diffractometer using Cu Kα radiation in steps of 0.008° over the 2θ range of 10-70°. Typically ~ 10 mg of particles were mounted with double-sided tape on a glass slide. Electrochemical experiments. Electrochemical intercalation and de-intercalation of Li-ions were performed by using a three-electrode glass cell, in which lithium metal was employed as counter and reference electrodes. Each sample (50 mg) was ground with acetylene black (13.3 mg) and polytetrafluoroethylene (PTFE, 3.3 mg) into a paste and used as the working electrode. The mass loading is ca. 1 mg/cm2. For the electrolyte, 1 M LiClO4 ethylene carbonate (EC) – diethyl carbonate (DEC) solution (1:1 v/v %) was used. Quantitative Li-ion insertion were carried out by the galvanostatic intermittent titration technique (GITT), in


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which a low-density current was repeatedly applied for 10 min followed by an interruption of 30 min. Magnetic Measurements. The samples, each with typical mass of ~ 6 mg, were measured with a commercial magnetometer, Quantum Design MPMS XL-7. Each sample was cooled from 300 K to 2 K in zero applied field for the zero-field cooled (ZFC) or in 100 G for field cooled (FC) measurements. Before zero-field cooling, a degaussing sequence was performed at room temperature to remove any pinned flux in the superconducting magnet.31 Samples were then cooled at 10 K/min from 300 K to 10 K and 2 K/min from 10 K to 2 K. At 2 K for the ZFC procedure, a field of 100 G was applied, the sample was recentered, and then data were collected over a range of set temperatures up to 40 K while the warming rate between points was a maximum of 5 K/min. The sample was then warmed to 300 K, and the FC protocol repeated the cooling steps taken for the ZFC sequence. At 2 K for the FC procedure, data were collected up to 40 K. Next, the sample was FC in 100 G to 2 K, where the hysteresis loops were acquired by sweeping the field from 100 G to 70 kG to -70 kG and then back to 70 kG. Results and Discussion The CuFe-PBA@NiFe-PBA core-shell particles with the average formula unit of K0.1Cu0.5Ni0.5[Fe(CN)6]0.7⋅3.95H2O ({K0.1Cu[Fe(CN)6]0.7⋅3.5H2O}0.5@{K0.1Ni[Fe(CN)6]0.7⋅4.4H2O}0.5) were prepared by growing



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the NiFe-PBA shell on top of previously prepared CuFe-PBA core particles, as reported earlier.1,2 An STEM image and EDX linescans of the cubic core-shell particle are shown in Figure 1. Room temperature powder X-ray diffraction yields unit cell parameters of 10.12 Å and 10.17 Å for the CuFe-PBA and NiFe-PBA, respectively (Figure1c). The FTIR spectrum, Figure S1a, shows well-defined peaks at both the CuFe-PBA and NiFe-PBA cyanide stretching frequencies, confirming the presence of both materials in the lattice. The particle size distribution with an average size of 305 ± 50 nm was determined from TEM images, as shown in Figure S1b. (b)

(a)

(c)

Figure 1. a) STEM image and b) EDX linescans of a CuFe-PBA@NiFe-PBA particle prior to Li-ion insertion. Data are counts for copper (blue), nickel (green), and iron (red) detected against the position of the electron beam along the line in the STEM image; c) Room temperature X-ray diffractogram of the CuFe-PBA@NiFe-PBA core-shell particles, prior to Li-ion insertion, from 2θ = 10 to 70 degrees.


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Electrochemical

Li-ion

insertion

was

performed

to

obtain

a

series

of

Lix(CuFe-PBA@NiFe-PBA) core-shell particles at several different levels of lithiation using a galvanostatic intermittent titration technique (GITT) that repeatedly applies a low constant specific current of 18 mA/g for 10 min following by 30 min interruption. The electrochemical reaction can be described as

(CuFe-PBA)0.5@(NiFe-PBA)0.5 + x Li+ + x e- ↔ Lix{(CuFe-PBA)0.5@(NiFe-PBA)0.5}.

(1)

For 0 < x < 0.7 a theoretical capacity of 66 mAh/g is achieved and corresponds to reduction of the Fe2+ ions in both the core and the shell based on the composition given previously. In Figure 2, the open circuit voltages (OCVs) for Lix(CuFe-PBA@NiFe-PBA) show a maximum of 1.0 Li-ion can be inserted or extracted per unit formula of the core-shell particle indicating that Li+ insertion involves both the Fe3+/2+ and Cu2+/+ couples, as previously reported.1 The magnetic properties can be significantly altered by Li-ion insertion, as the initial paramagnetic Fe3+ centers are reduced to diamagnetic Fe2+ ions.28,29 Figure 3a represents the field cooled magnetization measured at 100 G for the Lix(CuFe-PBA@NiFe-PBA) series. For

x = 0, two separate transitions are observed, seen more clearly as peaks in the derivative plots of Figure 3b. The peak at 18 K corresponds to the ferromagnetic ordering of the CuFe-PBA and the other at 24 K corresponds to the NiFe-PBA ordering, consistent with the reported



values of the pure phase materials,28,29 indicating the core and shell order independently with no significant magnetic interaction between the two components in the core-shell geometry. For 0 < x ≤ 0.32 the magnetic transition originally at 18 K gradually decreases as the number of inserted Li+, x, increases, whereas the NiFe-PBA peak remains basically at the same position and similar magnitude. Upon further reduction 0.32 < x ≤ 0.64, the NiFe-PBA peak on the derivative plot gradually shifts from the original 24 K to lower temperatures, providing strong evidence that the core and shell are reduced separately and that the CuFe-PBA core reduction takes place before the NiFe-PBA shell (Scheme 1).

5.0 4.5

+

Potential (V vs. Li/Li )

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4.0 3.5 3.0 2.5 2.0 1.5 0.0

0.2

0.4

0.6

0.8

1.0

x in Lix(CuFe-PBA@NiFe-PBA)

Figure 2. The open circuit voltages (blue circles) as a function of the Li-ion content x in Lix(CuFe-PBA@NiFe-PBA). The solid line is the calculated result based on the lattice gas model with ∆H1 = −3.37 eV, ∆H2 = −3.27 eV and ∆H3 = −3.06 eV.32


(a)

3.65

Lix(CuFe@NiFe)-PBA

2.92

µ0H =100 G

2.19 1.46 0.73

0.0 0.2

Li

x

0.4 0.6 0.8 1.0 5

10

15

20

25

30

35

40

T (K)

(c)

(b) 0.0

0.0 dM/dT (×10 emu⋅G /mol⋅K)

-0.1

-0.1

-0.2

µ0H = 100 G

-0.3

-0.4

Figure

x = 0.00 x = 0.11 x = 0.21 x = 0.32

0

5

3.

10

a)

15

20 T (K)

25

Temperature

30

35

x = 0.42 x = 0.53 x = 0.74 0.64 x = 0.74 x = 0.85 x = 1.00

3

3

dM/dT (×10 emu⋅G /mol⋅K)

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


M (×103 emu⋅G/mol)

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-0.2

40

dependences

-0.3

µ0H = 100 G

0

of

5

the

10

15

FC

20 T (K)

25

30

35

magnetizations

40

for

Lix(CuFe-PBA@NiFe-PBA) at each concentration of inserted Li-ion, x; b) and c) The derivatives of the FC magnetization curves showing the evolution of the Curie temperatures,

Tc, of the CuFe-PBA (~ 18 K at x = 0) and NiFe-PBA (~ 24 K at x = 0) upon Li-ion insertion. The Tc value is taken to be the minimum of the derivatives, indicating the insertion of the Li-ion reduces the paramagnetic Fe3+ to diamagnetic Fe2+ in the CuFe-PBA core before the NiFe-PBA shell.



Scheme 1. Schematic illustration of the stepwise reduction of Fe centers in the CuFe-PBA@NiFe-PBA core-shell heterostructure during the electrochemical Li-ion insertion process. The yellow part of the cube indicates the oxidized (charged) states and the blue part indicates the reduced (discharged) states, while the corresponding x value is also displayed.

obs. calc.

30

core CuFe-PBA shell NiFe-PBA

25 20

TC (K)

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15 10 5 0 0.0

0.2

0.4

0.6

0.8

1.0


Figure 4. Variation of the Curie temperature, Tc, for the CuFe-PBA core and NiFe-PBA shell during lithiation. The symbols represent measured points, determined from the peak values in derivatives in Figure 3b and 3c, while the broken lines are calculated.


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Figure 4 summarizes the shifts of Curie temperatures of the CuFe-PBA and NiFe-PBA as a function of x. The number of inserted Li-ions inserted upon reduction of the core or shell matches well with the available Fe3+ centers presented in each component. According to elemental analysis, the original core-shell particle sample is composed of 50% of the CuFe-PBA

core

material

and

50%

of

the

({K0.1Cu[Fe(CN)6]0.7⋅3.5H2O}0.5@{K0.1Ni[Fe(CN)6]0.7⋅4.4H2O}0.5),

NiFe-PBA corresponding

shell to

approximately 0.35 Fe3+ centers in the core and 0.35 Fe3+ centers in the shell per unit formula. For 0.7 < x ≤ 1.0, the insertion reaction can be ascribed to the reduction of Cu2+ to Cu+,1,28,33 whereas the Ni2+ remains redox-inactive under the experimental potential range.11 Therefore, the electrochemical reaction of Lix(CuFe-PBA@NiFe-PBA) core-shell particles can be separated into three consecutive steps, with the first and second step involving the Fe3+ → Fe2+ reduction in the CuFe-PBA core and NiFe-PBA shell, respectively, as given by

(CuFe-PBA)0.5@(NiFe-PBA)0.5 + x Li+ + x e- ↔ {Lix(CuFe-PBA)0.5}@(NiFe-PBA)0.5, for 0 < x ≤ 0.35,

and then

{Li0.35(CuFe-PBA)0.5}@(NiFe-PBA)0.5 + (x - 0.35) Li+ + (x - 0.35) e- ↔


(2)


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{Li0.35(CuFe-PBA)0.5}@{Li(x-0.35)(NiFe-PBA)0.5}, for 0.35 < x ≤ 0.7.

Page 16 of 27

(3)

The third step is the Cu2+ → Cu+ reduction in the CuFe-PBA, namely

{Li0.35(CuFe-PBA)0.5}@{Li0.35(NiFe-PBA)0.5} + (x - 0.7) Li+ + (x - 0.7) e- ↔ {Li(x-0.35)(CuFe-PBA)0.5}@{Li0.35(NiFe-PBA)0.5}, for 0.7 < x ≤ 1.0.

(4)

The reduction behavior shows that the core is reduced before the shell, and during the first reduction step, the NiFe-PBA shell behaves as an efficient electron and ion transport layer. This order of reduction is in agreement with the slightly higher redox potential of the Fe3+/2+ redox couple for CuFe-PBA relative to NiFe-PBA,34,35 even though the CuFe-PBA is encased in a shell. Furthermore, after all the Fe centers in the core and shell are reduced, the Li-ions can still transport through the fully reduced shell, allowing Cu2+ reduction to occur.


Page 17 of 27

(a) x = 0.00 x = 0.11 x = 0.21 x = 0.32 x = 0.42 x = 0.53 x = 0.64 x = 0.74 x = 0.85 x = 1.00

5 0

3


10

T=2K

-5

-10 -60

-40

-20

0

20

40

60

µ0H (kG)

(b)

(c)

4

4 3

1


x = 0.00 x = 0.11 x = 0.21 x = 0.32

2

0

2 1

x = 0.42 x = 0.53 x = 0.64 x = 0.74 x = 0.85 x = 1.00

0

3

3


3

-1

-1

T=2K

-2

-2

-3 -2

-1

0 µ0H (kG)

1

2

x = 0.00 x = 0.11 x = 0.21 x = 0.32

6

4

2

-4

-3

-2

-1

-2

-1

0

1

2

3

µ0H (kG)

(e)

8

0 -5

-4 -3

3

8

dM/dH (emu/mol)

(d)

T=2K

-3

-4 -3

dM/dH (emu/mol)

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


0 1 µ0H (kG)

2

3

4

5

x = 0.42 x = 0.53 x = 0.64 x = 0.74 x = 0.85 x = 1.00

6

4

2

0 -5

-4

-3

-2

-1

0 1 µ0H (kG)

2

3

4

5

Figure 5. a) Field dependences of magnetizations for Lix(CuFe-PBA@NiFe-PBA) at 2 K and for different concentrations of inserted Li-ion; b) and c) Expanded views between -3 kG and 3 kG showing the hysteresis loop for each x; d) and e) Derivatives of the field dependence plots showing the reduction in the coercive field (Hc) and remnant magnetization indicative of the conversion from a ferromagnet to a paramagnet upon Li-ion insertion.



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Separate responses of the core and shell can also be seen in magnetization vs. field plots. Magnetic hysteresis loops between -0.3 T and 0.3 T during Li-ion insertion are displayed in Figure 5b and 5c, indicating a ferromagnetic interaction between the metal ions in the lattice. Upon Li-ion insertion, the overall coercive field (Hc) slightly increases at low insertion levels (0 ≤ x ≤ 0.32), following by continuous decrease from 2200 G (x = 0.32) to 230 G (x = 0.64). Changes in the Hc can be more clearly seen in the derivative plots (Figure 5d and 5e). The increase in overall Hc during the reduction of CuFe-PBA is likely due to the slight increase in the domain size of NiFe-PBA arising from the release of the interface strain upon core reduction, in which the lattice shrinks slightly.1 After the core is reduced, subsequent reduction of the shell for 0.35 < x ≤ 0.7 leads to a loss of any ferromagnetic order and Hc decreases. The material behaves as a paramagnet after all of the Fe3+ is reduced, for x > 0.7. The core-shell system can be described by lattice-gas model when Li-ions are distributed in the host particle as guest ions,36 for example, as they are in our case, by electrochemical Li-ion insertion. Consider there are three states, associated with three different redox couples (Fe3+/2+ in CuFe-PBA, Fe3+/2+ in NiFe-PBA, and Cu2+/+ in CuFe-PBA), in the CuFe-PBA@NiFe-PBA core-shell particles in which Li-ions can exist. According to the Fermi-Dirac distribution, the average number of Li-ions in each state per formula unit (xi/xi0, where xi0 is the maximum number of moles of Li-ion of state i) can be described as


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=

1 (5) + +1

where Ei is the site energy of state i and E is the redox potential of the material. Note that here Ei is defined as the enthalpy change associated with state i after insertion of one Li-ion.36 The Li-ion distribution can be expressed by

1 (6) ∆ + +1

() =

where ∆H1, ∆H2, and ∆H3 are the enthalpy changes associated with the redox reaction of Fe3+/2+ in CuFe-PBA, Fe3+/2+ in NiFe-PBA, and Cu2+/+ in CuFe-PBA, respectively, after insertion of one Li-ion. Since the maximum number of inserted Li-ions in each state is directly related to the number of available redox centers in the core and shell, the redox potential, E, can be determined. The solid line in Figure 2 is the calculated result with ∆H1 = −3.37 eV, ∆H2 = −3.27 eV, and ∆H3 = −3.06 eV, deriving from the OCVs (blue circles in Figure 2). The theoretical calculation fits well with the experimental results, which further verifies the obtained ∆Hi values. Figure 6 shows xi, the Li-ion content of each state, as a function of x in Lix(CuFe-PBA@NiFe-PBA). During the lithiation processes, the number of Li-ions in each



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state increases sequentially, with x1 increasing first followed by x2 and finally x3. This sequence suggests the preferential reduction of Fe3+ sites over Cu2+ sites with the Fe3+ in the core more easily reduced than those in the shell, which clearly reproduces the sequence of reduction that has been observed in the magnetometer. Having obtained ∆Hi and xi, they can be applied to the magnetism mean field approximation.37 Based on the above distribution, the Curie temperature of the lithiated CuFe-PBA core and NiFe-PBA shell can be estimated as29

1− = () ( ) !

1 (7) ∆ +

+1

and

$ = $ 1− () ( ) !

1 (8) ∆% +

+1

The broken lines in Figure 4 shows the calculated results of the change in the Curie temperature of CuFe-PBA and NiFe-PBA as a function of x in Lix(CuFe-PBA@NiFe-PBA). Similar results were observed in the experimental data (open squares and triangles).


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0.8

0.6

xi

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x1

0.4

x2

x3

0.2

0.0 0.0

0.2

0.4

0.6

0.8

1.0


Figure 6. xi as a function of x in Lix(CuFe-PBA@NiFe-PBA).

Conclusions A key result is that magnetometry provides a sensitive analysis to observe subtleties of the core-shell electrochemical titration that were not evident from the electrochemical analysis nor X-ray diffraction studies, providing insights into how core-shell coordination polymer particles behave in lithium ion battery cathodes. The fact that the CuFe-PBA core is reduced before the NiFe-PBA shell was not evident in charge-discharge cycles, but is clearly indicated in magnetometry. To reduce the core, the NiFe-PBA shell acts as an efficient electron and ion transport layer in both the Fe3+ oxidation state, when the CuFe-PBA Fe2+/3+ couple is being reduced, but also in the Fe2+ state, when the core Cu+/2+ couple is reduced. Furthermore, controlling the location of redox active centers in parallel with the guest ion distribution within the three dimensional open framework structures allows facile switching between ferromagnetism and paramagnetism of the materials, for the first time showing the systematic



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control of magnetism in coordination polymer heterostructures by electrochemical guest ion insertion.

Supporting Information Details of materials synthesis, IR characterization, particle size dispersion analysis, field-cooled and zero-field cooled magnetization plots are included as supporting information. This information is available free of charge via the Internet at http://pubs.acs.org/.

Acknowledgements This work was supported by the National Science Foundation via DMR-1005581 and DMR-1405439 (DRT), DMR-1202033 (MWM), and DMR-1157490 (NHMFL) and the state of Florida. M.O. was supported by a KAKENHI on Innovative Areas (“Coordination Programming” Area 2107) from MEXT, and the Industrial Technology Research Grant Program in 2010 from the New Energy and Industrial Development Organization (NEDO), Japan.


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TOC Figure.


Stepwise Reduction of Electrochemically Lithiated CoreâShell

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