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insertion reactions in spinel-type manganese oxides
Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c 2001 Elsevier Science B.V. All rights reserved.
917
Magnetic effects on Li^ extraction/insertion reactions in spinel-type manganese oxides Y. Kawachi,' I. Mogi.^ H. Kanoh.' K. Ooi' and S. Ozeki' ^Department of Chemistry, Faculty of Science, Shinshu University, 3-1-1 Asahi, Matsumoto, Nagano 390-8621, Japan Institute for Materials Research, Tohoku University. 2-1-1 Katahira-cho, Aobaku, Sendai, Miyagi 980-0812, Japan ^ Government Industrial Research Institute. Shikoku, 2217-14 Hayashicho, Takamatsu, Kagawa 761-0395. Japan Magnetic effects on Li+ extraction/insertion reactions in manganese oxides >u-Mn02 were investigated with cyclic voltammetry (CV) and potential step chronoamperometry (PSCA) under magnetic field less than 30 T. The peak currents of CV were increased by magnetic fields, especially in the reduction side. Since the chemical diffusion coefficient of Li^ and the number of electrons involved in a rate-determining step were independent of magnetic fields, magnetic fields may promote the redox reaction and/or increase Li^ concentration at manganese oxide surfaces contact with a solution. The latter means Li^-insertion (adsorption). 1. INTRODUCTION LiMn204 has a spinel structure with Li"^ at the tetrahedral sites and Mn^" and Mn"*^ at the octahedral sites of a cubic closed-packed oxygen framework [K 2]. By acid treatment of LiMn204, Li* are almost removed to become A.-Mn02 while maintaining the spinel structure. /L-Mn02 has a high selectivity for Li" among alkaline metal, alkaline earth metal, and transition metal ions in aqueous solutions. The extraction and insertion of Li^, which is a topotactic process, is accompanied by a redox reaction [1,2]:
918 4(Li)[Mn(III)Mn(IV)]04 + 8H^ — 3( )[Mn(IV)2]04 + 4Li" + 2Mn^" + 4H2O
(1)
()[Mn(IV)2]04 + LiOH — (Li)[Mn(III)Mn(IV)]04 + (1/2)H20 + (1/4)02
(2)
Magnetic fields may affect various processes, such as spin-coupling, magnetizationchanging processes, electric flow. etc. Therefore, the Li-insertion/extraction must be controlled by a steady magnetic field, presumably, through magnetic effects on diffusion of Li"^ ions and electrons, a redox reaction, and magnetic energy changes (magnetic susceptibility changes). In this study, magnetic effects on Li^ extraction/insertion reactions in >.-Mn02 were examined by means of cyclic voltammetry (CV) and potential step chronoamperometry (PSCA). 2. EXPERIMENTAL SECTION Pt/LiMn204 electrodes were prepared by thermal decomposition of a mixed solution of LiNOa and Mn(N03)2 (2 mol dm ^ Li/Mn mole ratio=0.5) containing sodium dodecyl sulfate which was brushed onto a Pt plate (10 X 10 x 0.3 mm^) to form a thin layer. After drying the brushed plates at room temperature, the plates were heated at 1093 K for 2 minutes in air cooled down to room temperature.
This treatment process was repeated 6 times. The
electrochemical extraction of Li^ was carried out by anodic treatment, applying the potential of 0.2 to 1.2 V vs. Ag-AgCl to the Px/UUmO^ electrode in a 0.1 mol dm'^ LiCl/0.05 mol dm"^ borate buffer solution (pH 7.5) at scan rate 1.0 mV/s and 298 K. After that, the LiMn204 electrode was washed with water. CV and PSCA were measured by a potentiogalvanostat HAB-151 (Hokuto Denko Co.. Ltd.) at 298 K under magnetic fields (7/^30 T). 3. RESULTS AND DISCUSSION The CV data at various scan rates were analyzed by the following equations [3]: \E^-Eo\ = RTIan^F {0.780 + In (Du"^ Ik'') + In (an^FvIRT) ^'^] /p = QA957>nFAACDu^'W\an^FvlRT)"^
^^"^
919 where £p is the peak potential [mV], £o the formal potential [mV]. a the transfer coefficient, «a the number of electrons involved in a rate-determining step, k^^ the standard rate constant [cm/s], V the scan rate [V/s], /p the peak current [A], A the electrode area [cm^], AC the concentration difference, Cs - C'b (C's; concentration of Li^ within the manganese oxide at the solution interface [mol/cm^], CV; initial bulk concentration of Li" within the manganese oxide [mol/cm ]), Z)|, the chemical diffusion coefficient of Li" ions, n the number of electrons per molecule oxidized or reduced.
The chemical diffusion coefficient (DLI) and the standard rate
constant (ii^) can be evaluated from the plot of |£p-£ol vs. log v and Zp vs. v'^^ when A AC value is obtained from PSCA. The PSCA data were analyzed using the following equation [3]: k=v.
i(n =
inFAACDj^^)i;r^'^t^'^)J^(-]fexp{-k-l^/D^,i)
(5)
where / is the grain boundary distance (/= 0.50 fim). In this model, Cottrell behavior is observed until Du t= 0.25/1 The plots of/ (/) vs. /"'^^ from eq. (5) fit the experimental curve obtained by PSCA. The slope for the Cottrell behavior of the plot was used to evaluate A AC. The time corresponding to /= 0.25/VDI , ^ /^ gave the chemical diffusion coefficient of Li* in the manganese oxide.
-0.2
0
0.2
0.4
0.6
O.X
I
1.2
1.4
£/V VS. Ag-AgCI
Fig. 1. Changes of in-situ cyclic voltammograms with magnetic fields at scan rate 5.0 mV/s and 298 K.
920 10 i '
- I —
•
•
«
1
•
r-
-^ peak-a -^ peak-b ^ peak-c -^ peak-d
.0 6
\
'
'
1
-
4
f ^^
2h
1^^^^^i|~^^^^^ll^
^
1
27
0 H /T
.
.
.
1
.
. .
20
Fig. 2. Variations of the chemical diffusion coefficient of lithium ions with magnetic field (in-situ) applied in the order of left to right side of the abscissa.
H/T
Fig. 3. 3. Variations of the number of electrons involved in the ratedetermining step with magnetic field.
The peak currents of CV, especially the reduction current (peaks a and b), increased with increasing magnetic field (Fig. 1). Fig. 2 shows variations of the chemical diffusion coefficient of lithium ions Z)|,, with magnetic field. Du decreased by first application of a 27 T magnetic field. After the exposure to a magnetic field, Du became independent of magnetic fields. Variations of the number of electrons involved in the rate-determining step, n-aa, with magnetic fields are shown in Fig. 3. n^a values were almost unchanged even under magnetic field. Since Du and n^a were unchanged, as demonstrated by the experiments, and A should be constant, nAC should increase with magnetic field on basis of eq. 4, as suggested by the increase in /p with magnetic field: Magnetic fields may promote the redox reaction and/or the concentration of Li^ at manganese oxide surfaces contact with the solution. The latter demonstrates to Li^-insertion (adsorption). 4. CONCLUSIONS Magnetic fields may promote the redox reaction and/or Li^-insertion reaction without changing Di.j and n^a. REFERENCES 1. K. Goi, Hyomen, 33 (1995) 563. 2. H. Kanoh, Q. Feng, Y. Miyai and K. Ooi, J. Electrochem. Soc, 140 (1993) 3162. 3. H. Kanoh, Y. Miyai and K. Ooi, J. Electrochem. Soc. 142 (1995) 702.
917
Magnetic effects on Li^ extraction/insertion reactions in spinel-type manganese oxides Y. Kawachi,' I. Mogi.^ H. Kanoh.' K. Ooi' and S. Ozeki' ^Department of Chemistry, Faculty of Science, Shinshu University, 3-1-1 Asahi, Matsumoto, Nagano 390-8621, Japan Institute for Materials Research, Tohoku University. 2-1-1 Katahira-cho, Aobaku, Sendai, Miyagi 980-0812, Japan ^ Government Industrial Research Institute. Shikoku, 2217-14 Hayashicho, Takamatsu, Kagawa 761-0395. Japan Magnetic effects on Li+ extraction/insertion reactions in manganese oxides >u-Mn02 were investigated with cyclic voltammetry (CV) and potential step chronoamperometry (PSCA) under magnetic field less than 30 T. The peak currents of CV were increased by magnetic fields, especially in the reduction side. Since the chemical diffusion coefficient of Li^ and the number of electrons involved in a rate-determining step were independent of magnetic fields, magnetic fields may promote the redox reaction and/or increase Li^ concentration at manganese oxide surfaces contact with a solution. The latter means Li^-insertion (adsorption). 1. INTRODUCTION LiMn204 has a spinel structure with Li"^ at the tetrahedral sites and Mn^" and Mn"*^ at the octahedral sites of a cubic closed-packed oxygen framework [K 2]. By acid treatment of LiMn204, Li* are almost removed to become A.-Mn02 while maintaining the spinel structure. /L-Mn02 has a high selectivity for Li" among alkaline metal, alkaline earth metal, and transition metal ions in aqueous solutions. The extraction and insertion of Li^, which is a topotactic process, is accompanied by a redox reaction [1,2]:
918 4(Li)[Mn(III)Mn(IV)]04 + 8H^ — 3( )[Mn(IV)2]04 + 4Li" + 2Mn^" + 4H2O
(1)
()[Mn(IV)2]04 + LiOH — (Li)[Mn(III)Mn(IV)]04 + (1/2)H20 + (1/4)02
(2)
Magnetic fields may affect various processes, such as spin-coupling, magnetizationchanging processes, electric flow. etc. Therefore, the Li-insertion/extraction must be controlled by a steady magnetic field, presumably, through magnetic effects on diffusion of Li"^ ions and electrons, a redox reaction, and magnetic energy changes (magnetic susceptibility changes). In this study, magnetic effects on Li^ extraction/insertion reactions in >.-Mn02 were examined by means of cyclic voltammetry (CV) and potential step chronoamperometry (PSCA). 2. EXPERIMENTAL SECTION Pt/LiMn204 electrodes were prepared by thermal decomposition of a mixed solution of LiNOa and Mn(N03)2 (2 mol dm ^ Li/Mn mole ratio=0.5) containing sodium dodecyl sulfate which was brushed onto a Pt plate (10 X 10 x 0.3 mm^) to form a thin layer. After drying the brushed plates at room temperature, the plates were heated at 1093 K for 2 minutes in air cooled down to room temperature.
This treatment process was repeated 6 times. The
electrochemical extraction of Li^ was carried out by anodic treatment, applying the potential of 0.2 to 1.2 V vs. Ag-AgCl to the Px/UUmO^ electrode in a 0.1 mol dm'^ LiCl/0.05 mol dm"^ borate buffer solution (pH 7.5) at scan rate 1.0 mV/s and 298 K. After that, the LiMn204 electrode was washed with water. CV and PSCA were measured by a potentiogalvanostat HAB-151 (Hokuto Denko Co.. Ltd.) at 298 K under magnetic fields (7/^30 T). 3. RESULTS AND DISCUSSION The CV data at various scan rates were analyzed by the following equations [3]: \E^-Eo\ = RTIan^F {0.780 + In (Du"^ Ik'') + In (an^FvIRT) ^'^] /p = QA957>nFAACDu^'W\an^FvlRT)"^
^^"^
919 where £p is the peak potential [mV], £o the formal potential [mV]. a the transfer coefficient, «a the number of electrons involved in a rate-determining step, k^^ the standard rate constant [cm/s], V the scan rate [V/s], /p the peak current [A], A the electrode area [cm^], AC the concentration difference, Cs - C'b (C's; concentration of Li^ within the manganese oxide at the solution interface [mol/cm^], CV; initial bulk concentration of Li" within the manganese oxide [mol/cm ]), Z)|, the chemical diffusion coefficient of Li" ions, n the number of electrons per molecule oxidized or reduced.
The chemical diffusion coefficient (DLI) and the standard rate
constant (ii^) can be evaluated from the plot of |£p-£ol vs. log v and Zp vs. v'^^ when A AC value is obtained from PSCA. The PSCA data were analyzed using the following equation [3]: k=v.
i(n =
inFAACDj^^)i;r^'^t^'^)J^(-]fexp{-k-l^/D^,i)
(5)
where / is the grain boundary distance (/= 0.50 fim). In this model, Cottrell behavior is observed until Du t= 0.25/1 The plots of/ (/) vs. /"'^^ from eq. (5) fit the experimental curve obtained by PSCA. The slope for the Cottrell behavior of the plot was used to evaluate A AC. The time corresponding to /= 0.25/VDI , ^ /^ gave the chemical diffusion coefficient of Li* in the manganese oxide.
-0.2
0
0.2
0.4
0.6
O.X
I
1.2
1.4
£/V VS. Ag-AgCI
Fig. 1. Changes of in-situ cyclic voltammograms with magnetic fields at scan rate 5.0 mV/s and 298 K.
920 10 i '
- I —
•
•
«
1
•
r-
-^ peak-a -^ peak-b ^ peak-c -^ peak-d
.0 6
\
'
'
1
-
4
f ^^
2h
1^^^^^i|~^^^^^ll^
^
1
27
0 H /T
.
.
.
1
.
. .
20
Fig. 2. Variations of the chemical diffusion coefficient of lithium ions with magnetic field (in-situ) applied in the order of left to right side of the abscissa.
H/T
Fig. 3. 3. Variations of the number of electrons involved in the ratedetermining step with magnetic field.
The peak currents of CV, especially the reduction current (peaks a and b), increased with increasing magnetic field (Fig. 1). Fig. 2 shows variations of the chemical diffusion coefficient of lithium ions Z)|,, with magnetic field. Du decreased by first application of a 27 T magnetic field. After the exposure to a magnetic field, Du became independent of magnetic fields. Variations of the number of electrons involved in the rate-determining step, n-aa, with magnetic fields are shown in Fig. 3. n^a values were almost unchanged even under magnetic field. Since Du and n^a were unchanged, as demonstrated by the experiments, and A should be constant, nAC should increase with magnetic field on basis of eq. 4, as suggested by the increase in /p with magnetic field: Magnetic fields may promote the redox reaction and/or the concentration of Li^ at manganese oxide surfaces contact with the solution. The latter demonstrates to Li^-insertion (adsorption). 4. CONCLUSIONS Magnetic fields may promote the redox reaction and/or Li^-insertion reaction without changing Di.j and n^a. REFERENCES 1. K. Goi, Hyomen, 33 (1995) 563. 2. H. Kanoh, Q. Feng, Y. Miyai and K. Ooi, J. Electrochem. Soc, 140 (1993) 3162. 3. H. Kanoh, Y. Miyai and K. Ooi, J. Electrochem. Soc. 142 (1995) 702.