Langmuir 1991, 7, 1841-1842
1841
Selective Electroinsertion of Lithium Ions into a Pt/X-MnOz Electrode in the Aqueous Phase Hirofumi Kanoh,' Kenta Ooi, Yoshitaka Miyai, and Shunsaku Katoh Government Industrial Research Institute, Shikoku, 2-3-3 Hananomiya-cho, Takamatsu 761, Japan Received April 1, 1991.I n Final Form: June 17, 1991 The electrochemical insertion of alkali-metal ions in the aqueous phase was investigated by using a Pt/X-Mn02 electrode which was obtained by the electrochemical extraction of Li+ from a Pt/LiMnzOd electrode. The cyclic voltammogram of a 0.01 mol/dm3 LiCl solution showed a definite electric current, corresponding to the Li+ insertion/extraction reaction with a X-MnOZ electrode involving the redox of manganese. The same electrodes in a 0.01 mol dm3 NaCl or KC1 solution showed little electric current in the region between 0.2 and 1.0 V vs calomel e ectrode. Only Li+ ions were taken up by the Pt/X-Mn02 electrode from a mixed solution of alkali-metal chlorides.
I
Spinel-type manganese oxide, A-Mn02, can be obtained by the topotactic extraction of Li+ from LiMnzOr with acid.112 Our previous studies have shown that it has a remarkably high selectivity for Li+ among alkali-metal, alkaline earth metal, and transition-metal ion^.^^^ We have proposed a new type of redox mechanism. Instead of an ion-exchange mechanism for Li+ insertion in the (A-Mn02 + LiOH) system, the mechanism proceeds as follows: MnO2 + xLiOH -Li,MnOz + (x/2)H20 + (x/4)02, where x = Li/Mnratio.s16 Since a mixed Mn site valence produces a good electronic conductor, the Li+-insertion process can be divided into two steps: (1) insertion of Li+ into the tetrahedral vacant sites of A-Mn02 accompanying a reduction of Mn(1V) to Mn(II1) and (2) migration of excess positive charge (-xe) to the surface of A-Mn02 followed by the oxidation of OH- in the aqueous solution. This paper describes such selective electroinsertion of Li+ into a Pt/A-MnOz electrode in an aqueous solution of metal chlorides. A Pt/A-MnOz electrode was produced by the electrochemical extraction of Li+ from a Pt/ LiMnzOr electrode which was prepared from a platinum electrode (10 X 10 X 0.3 mm) by a thermal decomposition method. A small amount of a mixed solution of LiN03 and Mn(N03)2 (2 mol/dmS, Li/Mn = 0.5) was brushed on a Pt plate forming a thin layer of the solution. After drying at room temperature, the plate was heated to 1103 K for a few minutes and then cooled to room temperature. This brushing-heating treatment was repeated 20 times. Finally, the plate was annealed at 1103 K for 5 min to form the Pt/LiMnzOr electrode. X-ray diffraction analysis showed the formation of a uniform spinel phase of LiMnzOr (ASTM No. 18-736) with a lattice constant of 0.825 nm. The Mn and Li content of the electrode was determined by atomic absorption spectrometry after dissolving the oxide layer with a mixed solution of HC1 and H202. The Li/Mn ratio of the electrode was found to be 0.47. The electrochemical extraction of Li+ was carried out by applying a constant potential (1V vs calomel electrode (2 mol/dm3 KCl)) to the Pt/LiMnzOr electrode in a 0.01 mol/dm3 LiCl solution at 298 K. After the electric current
* To whom correspondence should be addressed.
(1) Hunter, J. C. J. Solid State Chem. 1981,39, 142.
( 2 ) Goodenough, J. B.; Thackeray, M. M.; David, W. I. F.; Bruce, P. G. Reu. Chim. Miner. 1984,21,435. (3) Shen, X. M.; Clearfield, A. J. Solid State Chem. 1986,64, 270. (4) Ooi,K.; Miyai, Y.; Katoh, S. Soluent Extr. Zon Exch. 1987,5,561. (5) Ooi,K.;Miyai,Y.;Katoh,S.;Maeda,H.;Abe,M.Chem.Lett. 1988, 989. (6) Ooi, K.; Miyai, Y.; Katoh, S.; Maeda, H.; Abe, M. Langmuir 1989, 5, 150.
was decreased from 1.5 mA to less than 5 PA, the Pt plate was washed with water and air-dried. The X-ray analysis of the Pt/h-MnOz electrode showed that a topotactic Li+ extraction had occurred with a slight decrease of the lattice constant to 0.805 nm. The Li/Mn ratio of the electrode was found to be 0.01, indicating that more than 97% of Li+ was extracted from the Pt/LiMnzOr electrode during the electrochemical treatment. The average thickness of the X-Mn02 layer was calculated as 0.75 pm from the Mn content (3.9pmol) using4.5g/cm3asthe densityof A-MnO2 as described in the literature.' The insertion/extraction reactions of alkali-metal ions with Pt/X-MnOz were investigated by cyclic voltammetry in 20 cm3 of alkali-metal chloride solutions at a scan rate of 0.1 mV/s at 298 K. Calomel and Pt-wire electrodes were used as reference and counter electrodes, respectively. The cyclic voltammogram in 0.01 mol/dm3 NaCl or KC1 solutions showed little electric current during the potential sweep between 0.2 and 1 V (Figure 1). However, in the case of 0.01 mol/dm3 LiCl solution, a definite cathodic current was observed during the forward sweep (potential decrease) with two peaks at 0.69 and 0.52 V, and nearly the same degree of anodic current was observed during the reverse sweep (potential increase) with peaks a t 0.67 and 0.82 V. The cathodic current corresponds to the Li+ insertion involving the reduction of manganese(1V) of A-Mn02phase. Similarly, the anodic current corresponds to the Li+ extraction involving the oxidation of the manganese. These results indicate that the electrochemical insertion/extraction reaction proceeds only for Li+ among these three alkali-metal ions. The voltammogram in the LiCl solution varied depending on the sweep conditions. The increase in the scan rate caused an increase in the peak current, accompanying a shift of both the anodic and cathodic current peaks in less noble and noble directions, respectively. The two distinct peaks observed for the forward sweep changed to a single peak at a scan rate above 0.5 mV/s. These results suggest that the insertion of Li+ is a relatively slow process compared with the usual electrochemicalreaction involving electron transfer. The stability of the Pt/X-Mn02 electrode was examined over 10 electrochemical insertion/extraction cycles in a 0.01 mol/dm3 LiC solution with the same conditions described above. The voltammograms obtained for each cycle resembled one another, all having the peaks corresponding toLi+insertion and extraction. The peak current decreased to 85% of the initial value by the fourth cycle of insertion/extraction, but remained steady after four
0743-7463/91/2407-1841$02.50/0 0 - 1991 American Chemical Societv
1842 Langmuir, Vol. 7,No. 9,1993
Letters
Table I. Values of Lattice Constant (a01 and Li/Mn Mole Ratio in Li+-Inserted Electrodes electrolyte
ao/nm
LiCl + NaCl (0.01 mol/dm3 each)LiCl + KCl (0.01 mol/dm3 each) LiCl + RbCl (0.01 mol/dm3 each) LiCl + CsCl (0.01 mol/dm3 each) LiCl + NaCl KCl RbCl CsCl (0.01 mol/dm3 each)
0.823 0.823 0.825 0.823 0.823 (0.804)O
+
+
+
Li/Mn 0.45 0.40 0.41 0.43 0.32
Na/Mn n.d.6
K/Mn
Rb/Mn
Cs/Mn
n.d. n.d. n.d.
n.d.
n.d.
n.d. n.d.
than 0.002). A minor peak was observed. b Not detected (alkali meb ‘Mn ratio was -a 10-
4
E
z 1
Li+ insertion is different both below and above the midinsertion point (Li/Mn = 0.25), probably owing to the formation of a stable species of Lb.sMn204 in the course of the insertion reaction.6 In the present study, the formation of Li,).&n204 species was examined by applying a constant potential (0.65 V,a slightly less noble potential than that of the first peak in the forward sweep) to the Pt/A-MnOZ electrode in a 0.01 mol/dm3 solution. After the electric current was decreased to less than 5 PA, the X-ray and chemical analyses of the electrode were carried out. The lattice constant and the Li/Mn ratio of the electrode were0.813nm and 0.26, respectively;these values were close to those in the one-half insertion phase reported earlier? These results suggest the formation of Li0.&~04 species in the trough between the two peaks. The uptakes of alkali-metal ions by the Pt/h-MnOz electrode were evaluated after the forward sweep from 1 to 0.2 V in various mixed solutions of alkali metal chlorides (Table I). A remarkable lithium ion sieve property was observed. Sodium and cesium ions scarcely influence the Li+ insertion; the Li/Mnratios after the sweep were nearly equal to that of the starting Pt/LiMnzOr electrode in the case of the [LiCl + MC1 (M = Na or Cs)] solution and the uptakes of other alkali-metal ions were negligible. Potassium and rubidium ions seem to influence the Li+ insertion slightly, but K+ and Rb+ uptakes were not detected by atomic absorption spectrometry. The specific selectivity for Li+ can easily be explained by the steric effect of the tetrahedral sites of A-Mn02: the sites may be too small for metal ions with large ionic radii to enter.
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-
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