Langmuir 1992,8, 1861-1867
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Li+ Extraction/Insertion with Spinel-Type Lithium Manganese Oxides. Characterization of Redox-Type and Ion-Exchange-TypeSites Qi Feng,* Yoshitaka Miyai, Hirofumi Kanoh, and Kenta Ooi' Government Industrial Research Institute, Shikoku, 2-3-3 Hananomiya-cho, Takamatsu 761, Japan Received October 22, 1991. I n Final Form: April 1, 1992 Spinel-type lithium manganese oxides which can be expressed by a general formula LinMnz-,04(16 n 6 1.33,O 6 x 6 0.33, n 6 1+ x ) were prepared by reacting MnC03 with Li2C03at 400 OC and 800 "C. The extractionlinsertion reactions of lithium ions with the spinel samples were investigated by chemical, X-ray, and thermal analyses, FT-IR spectroscopy,and pH titration. Lithium ions were extractedlinserted by two different mechanisms: redox type and ion exchange type. Therefore, the Li+ extraction/insertion sites could be classified into redox type and ion exchange type. Li+ ions were preferentially extracted1 inserted fromlinto the ion-exchange sites. The spinel-type lithium manganese oxides could be divided into two parts: those with trivalent Mn ions and those with only tetravalent Mn ions (with Mn defects in 16d octahedral sites). The numbers of redox-type and ion-exchange-typesites correlated well with the amounts of trivalent Mn ions and the Mn defects, respectively. The proportions of the two types of sites varied depending on preparation conditions (heat treatment temperature and Li/Mn mole ratio of starting material) of the lithium manganese oxide spinels.
Introduction Spinel-type lithium manganese oxides show excellent selectivity for the adsorption of Li+ after topotactic extraction of Li+ with an acid.l-1° The adsorptive properties make the material practicable as a Li+ a cathode material for lithium batteries,11-13 and an electrode for selective electroinsertion of Li+.14 LiMnz04 with lithium at the 8a tetrahedral sites and manganese(111)and manganese(1V) at the 16d octahedral sites of a cubic closed-packed oxygen framework15is a typical spineltype lithium manganese oxide. An acid treatment of LiMnzO4 causes the removal of nearly all of the lithium while maintaining the spinel structure. Hunter16 has proposed a redox mechanism
-
4(Li)[Mn"'Mn'V10, + 8H+ 3(n)[Mn'Vz]04+ 4Li+ + 2Mn2++ 4Hz0 (1) for the topotactic extraction of lithium and defined the spinel-type manganese oxide obtained as X-MnOZ, where 0 , [I, and 0 are 8a tetrahedral sites, 16d octahedral sites, and vacant sites, respectively. Thackeray et al.l1JZhave (1) Vol'khin, V. V.; Leont'eva, G . V.; Onolin, S. A. Neorg. Mater. 1973, 6, 1041. (2) Leont'eva, G . V.; Chirkova, L. G. Zh.Prikl. Khim. 1988,61,734. (3) Shen, X. M.; Clearfield, A. J. Solid State Chem. 1986,64, 270. (4) Ooi, K.; Miyai, Y.; Katoh, S. Sep. Sci. Technol. 1986,21, 755. (5) Ooi, K.; Miyai, Y.; Katoh, S. Sep. Sci. Technol. 1987,22, 1779. (6) Ooi, K.; Miyai, Y.; Katoh, S. Solvent Extr. ZonErch. 1987,5,561. (7) Ooi, K.; Miyai, Y.; Katoh, S.;Maeda, H.; Abe, M. Bull. Chem. SOC. Jpn. 1988, 61, 407. (8) Ooi, K.; Miyai, Y.; Katoh, S.;Maeda, H.; Abe, M. Chem. Lett. 1988, 989. (9) Ooi, K.; Miyai, Y.; Katoh, S.;Maeda, H.; Abe, M. Langmuir 1989, 5, 150. (IO)Ooi, K.; Miyai, Y.; Katoh, S.;Maeda, H.; Abe, M. Langmuir 1990, 6, 289. (11) Thackeray, M. M.; David, W. I. F.; Bruce, P. G.; Goodenough, J. B. Mater. Res. Bull. 1983, 18, 461. (12) Thackeray, M. M.; Johnson, P. J.; Picciotto, L. A. De.; David, W. I. F.; Bruce, P. G.; Goodenough, J. B. Mater. Res. Bull. 1984, 19, 179. (13) +esouw, M. H.; Kock, A. De.; Picciotto, L. A. De.; Thackeray, M. M.; Davld, W. I. F.; Ibberson, R. M. Mater. Res. Bull. 1990,25, 173. (14) Kanoh, H.; Ooi, K.; Miyai, Y.; Katoh, S. Langmuir 1991,7,1841. (15) Wickham, D. G.; Croft, W. J . Phys. Chem. Solid 1958, 7 , 351. (16) Hunter, J. C. J. Solid State Chem. 1981, 39, 142.
investigated the structures of the Li+ extracted and inserted manganese oxide in detail by X-ray and neutron diffraction analyses. We have studied the Li+ adsorptive properties of XMnOz in the aqueous phase. It shows a specific selectivity for Li+ among alkali, alkaline-earth, and transition-metal A stoichiometric investigation in the (X-MnOZ + LiOH) system has shown that the insertion of Li+ involves an evolution of about a 90% equivalent amount of oxygen gas, as well as a reduction of Mn(1V) to Mn(II1) in the solid phase.g But such reactions do not occur in the (XMnOz + KOH) system. We have therefore proposed a redox mechanism for the Li+ insertion
-
(0)[Mn'~~]0, + nLiOH (Linnl-,,) [Mn"',Mnwz-n104
+ (n/2)Hz0+ (n/4)0,
(2)
After further study of the Li+ insertion reactions for the spinel-type lithium manganese oxides prepared under different conditions (Li/Mn mole ratio and temperature), we have found that Li+-specific ion-exchange and nonspecific ion-exchange reactions also occur during lithium insertion, in addition to the redox reaction of eq 2.17 We have classified the insertion sites into three groups: redoxtype sites, Li+-specificion-exchange sites, and nonspecific ion-exchange sites. The proportion of each group varied depending on the preparation conditions of the spineltype lithium manganese oxide precursors. The present paper describes the origin of the two kinds of sites (redox-type and ion-exchange-type) from the chemical and structuralstandpoints. Lithium manganese oxide spinels have distinct characteristics compared with other oxide-type spinels, e.g. an easy conversion between Mn(II1) and Mn(1V) owing to the movement of electrons, and an easy migration of Li+ in the oxygen framework. These characteristics enable the lithium manganese oxide to have various kinds of spinels with different oxidation states of Mn and with different Li/Mn mole ratios. For example, Li1.33Mn1.6704or (Li)[Lio.&'inw1.671 0 4 is a typical (17) Ooi, K.; Miyai, Y.; Sakakihara, J. Langmuir 1991, 7, 1167.
0143-746319212408-1861$03.00/0 0 1992 American Chemical Society
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1862 Langmuir, Vol. 8, No. 7, 1992 spinel with Li/Mn = 0.8 and tetravalent Mn alone.18J9We propose in the present paper that the spinel-typeprecursor having trivalent Mn gives a redox-type site on the basis of eq 1 while the precursor having only tetravalent Mn gives an ion-exchange-type site by Li+/H+ion exchange.
Experimental Section Materials. Lithium manganese oxides were prepared by heattreating mixtures of MnC03 and LizC03 with different Li/Mn mole ratios for 4 h at 400 or 800 "C in air. Li+ Extraction/Insertion Reactions. Lit extraction/ insertion reactions were investigated at room temperature for three kinds of lithium manganese oxides prepared under different conditions (Li/Mn mole ratio, heating temperature): 0.5-800 (0.5,800 "C); 0.5-400 (05,400 "C); 0.8-400 (0.8,400 "C). In the extraction study, lithium manganese oxide was immersed in a 0.2 M (1M = 1mol dm-3) HC1 solution of varying volumes with stirring for 2 days to obtain various kinds of lithium-extracted samples. The samples were filtered, washed with water, and air-dried at 70 OC. In the insertion study, 2 g of acid-treated sample (0.5-800(H), 0.5-0.400(H), or 0.8-400(H) (Table 11))was immersed in a LiOH solution (500 cm3) of varying concentration with stirring for 4 days to obtain various kinds of lithium-inserted samples. The inserted samples were filtered, washed with water, and air-dried at 70 "C. The samples obtained from the highest concentration (0.1 M) of LiOH were designated as 0.5-800(Li), 0.5-400(Li), and 0.8-400(Li), respectively. Chemical Properties. The available oxygen of each sample was determined by the standard oxalic acid method.20 The mean oxidation number (.&) of manganese was evaluated from the value of available oxygen. The lithium and manganese contents were determined after dissolvingthe sample with a mixed solution of HzSO4 and HzO2. Lithium concentration was determined by atomic absorption spectrometry and manganese by absorption spectrometry at 523 nm after oxidizing Mn to Mn(VI1) with (NH4)2S20sa pH Titration. A 0.1-g portion of each acid-treated sample was immersed in a mixed solution (10 mL) of MCl + MOH (M Li, Na) in varying ratio with intermittent shaking at 25 "C.After the sample was shaken for 7 days," the pH of the supernatant solution was determined with a Horiba Model M8s pH meter. Physical Analyses. An X-ray analysis was carried out using a Rigaku type RAD-I1X-ray diffractometer with graphite monochromator. A mechanical deviation of diffraction angles was corrected by scanning the whole range of angle with silicon powder. Infrared spectra were obtained by the KBr method on a JEOL infrared spectrometer, Model JTR-RFX3001. Differential thermal analysis and thermal gravimetric (DTA-TG) curves were obtained on a MAC Science thermal analyzer (System 001, TG-DTA 2000) at a heating rate of 10 "C/min.
I
I
10
20
I
I
I
30 40 281degree
50
Figure 1. X-ray diffraction patterns of lithium manganeseoxides prepared at 400 "C with different Li/Mn mole ratios: 0, peaks corresponding to spinel phase; A, a-Mn203; 0 , LizMnOs. LilMn =
Results and Discussion Preparation and Characterization of Spinel-Type Lithium Manganese Oxides. X-ray diffractionpatterns of lithium manganese oxides prepared at 400 and 800 "C with different Li/Mn mole ratios are shown in Figures 1 and 2, respectively. Pure spinel-type lithium manganese oxides were obtained in a range of Li/Mn mole ratio from 0.5 to 0.8 at 400 "C,but only at 0.5 in the case of 800 "C heating. A mixture of spinel-typelithium manganese oxide and monoclinic LizMnO3 was formed in the region Li/Mn > 0.8 at 400 "Cand Li/Mn > 0.5 at 800 "C;the proportion of monoclinic LipMnOa increased with an increase in the Li/Mn. In the region Li/Mn < 0.5, a-MnzO3 phase (400 "C heating) and yMnz03 phase (800 "C heating) were formed in addition to the spinel-type phase. An analysis of available oxygen showed that in the case of 400 "C heating, the mean oxidation number of manganese ( 2 ~ ~ (18)Blasse, G.J. Znorg. Nucl. Chem. 1963,25, 743. (19) B h , G.Philips Res. Rep., Suppl. 1964, No. 3, 1. (20)Japan Industrial Standard (JIS)1969, M8233.
10
20
30 40 28ldegree
50
I
0
Figure 2. X-ray diffraction patterns of lithium manganeseoxides prepared at 800 "C with different Li/Mn mole ratios: 0 , peaks corresponding to spinel phase; A, yMnzO3; 0 , LizMnOs.
of the lithium manganese oxide increases with the increase of Li/Mn and reached 4 (tetravalence) at Li/Mn = 1,after which it remained constant (Figure 3). The ZM,value is lower for the sample 0.5-800 than for the sample 0.5-400. Structure of Lithium Manganese Oxide Spinel. The differences in the Li/Mn mole ratio and ZM,for these spinels can be explained in terms of the distributions of )cations and vacant sites in the spinel. The spinel structure has to satisfy the following conditions:
c N i + Nu = 3 or E N i d 3 1
1
(3)
Langmuir, Vol. 8, No. 7, 1992 1863
Li+ Extractionlznsertion
Table XI. Cation Distributions and Values of z for Spinel-Type Lithium Manganese Oxides sample
0.5-800 0.5-400 0.65-400 0.8-400
z
0.78 0.57 0.33 0.18
LiMnzO4 1 Li1.33Mn1.6704 0 0
c 3.00
0.5
ld
f
1.0
1.5
the spinels can be written as
2.0
LilMn mole ratio
Figure 3. Relation between mean oxidation number of Mn and Li/Mn mole ratio for the lithium manganese oxides prepared at 400 O C (0)and 800 O C (A).
Table I. Compositional and Structural Parameters of Spinel-Type Lithium Manganese Oxides Li/
sample
0.5-860 0.5-400 0.65-400 0.8-400
ZM,, Mn 3.60 3.70 3.81 3.89
0.480 0.485 0.628 0.754
ao,A 8.23 8.21 8.18 8.14
x
0.04 0.09 0.20 0.28
n 0.94 0.93 1.13 1.30
formula Lio.~oo.loMn1,se04 Lio.9300.1sMn1.9104 Lil,p.&.wMnl,~O4 Lil.aMnl.7204
LiMnz04 Lil,&inl,&
3.5 0.5 8.25O 0 1 LiMnzO4 4 0.8 8.19* 0.33 1.33 Lil.33Mnl.6704 a After ref 15. After ref 19.
For the condition of electroneutrality, we have C N i Z i= N d o = 8
(4)
1
and for the spinel with tri- and tetravalent manganese (5)
where Ni is the number of metal ion i, N , the number of vacant sites of 8a and 16d, and NO (=4) the number of oxygen anions in one structural unit. Zi and 20(=2) are valences of the metal ion i and oxygen anion, respectively. The present spinels can be expressed by a general formula Li,Mnz-,Or (1d n d 1.33,O d x d 0.33, n d x + 1)or Lin[31-,+,Mn~-,04 using eqs 3,4, and 5, by considering the conditions 3.5 d Z M n d 4(0 d N&(III)d 1)and 0.5 d Li/Mn d 0.8. The values of x and n can be evaluated from Z M n and lithium-manganese mole ratio (Li/Mn) as follows:
ZMn= (8 - n)/(2- x ) = 8/(2 - X ) - Li/Mn x = 2 - 8/(Li/Mn
(6)
+ &),
n/(Li/Mn) = 2 - x
(7)
n = Li/Mn(2 - x ) The compositional and structural parameters for the spinel-type lithium manganese oxides are summarized in Table I. The lithium manganese oxide spinel prepared here have a structure with Mn not at 8a tetrahedral sites but at 16d octahedral sites, since the (220) and (422) reflections are not observed in the X-ray powder patterns (Figure 1and 2).16 Therefore, spinels have a framework (o)[OZMd1I2MnIv~-x-z104 (0Q z d 1)with Mn defects in 16d octahedral sites; here x is the number of the Mn defect and z the number of tirvalent Mn. Li+ can be distributed to both the 8a tetrahedral sites and the defecta in 16d octahedral sites of the framework. Thus the cation distribution of
(Li,-ynl-,,+y) [Liy~xyMn1r12MnIvZ-x-21 0,
(8)
where y is number of Li+ distributed to 16d octahedral sites 01 d x ) and z can be derived from z'n-4~ (9) Sincelithium is a very weak scatterer of X-rays, the position of Li+ (the value of y) cannot be determined from the X-ray diffraction pattern. A neutron diffraction method may be the only method to determine the exact Li+ distribution. The neutron diffraction study on LizMQOD spinel by Kock et aLZ1has indicated that ita cation distribution is (Lio.~&~~.ll) [00.zzMn1.7~104with all Li+ in 8a tetrahedral sites. The other study by David et alez2has indicated that Li+ occupy both 8a (tetrahedral) and 16c (octahedral) sites in LizMn204 and only 8a sites in Lio.2Mnz04. The cation distributions and values of z for the present spinels are given in Table 11. The proportions of Mn(III), Mn(IV), and Mn defects vary with the preparation conditions, in spite of the fact that they all have a spinel phase. The number of Mn(II1) increases with the increase of heat treatment temperature as well as with the decrease of Li/Mn ratio (Table 11). The number of Mn defects varies in reverse order (Table I). The sample 0.5-800 is close to the normal spinel LiMnzOr and 0.8-400 to the spinel Li1.33Mn1.6704 in structure and composition, respectively. TG curves for the samples 0.5-400 and 0.8-400 show a weight loss between 500 and 700 "C; this corresponds to the transformation from the Mn defect spinal to LiMnzO4 and LizMnOa, accompanied by an evolution of oxygen gas. This indicates that the Mn defect spinel is metastable and transforms to the stable state at temperatures higher than 500 OC. Li+ Extraction Reaction. Three kinds of lithium manganese oxide spinels (0.5-800,0.5-400, and 0.8-400) were used as original samples to investigate the mechanism of Li+ extraction/insertion reactions. The Li+ extraction was carried out using an HC1 solution. The &, vs Li/Mn curves showed different variation patterns with each lithium manganese oxide system (Figure4). The Z M n value in the 0.5-800 system linearly increased with a decrease of Li/Mn ratio, while that in the 0.8-400system was almost constant, except for a slight increase in the region Li/Mn < 0.2. The 0.5-400 system showed a twostage process; &, is almost constant from starting Li/Mn (0.485) to about 0.25, and then linearly increases with a decrease of Li/Mn with the same slope as that of the 0.5800 system. The increases of ZM,were attended by the dissolution of Mn2+ ions in the extraction reactions. (21) Kock, A. De.;Roseouw, M. H.;Picciotto, L. A.; Thackeray, M. M.; David, W. I. F.; Ibberson, R. M. Mater. Res. Bull. 1990,26,657. (22) David, W. I. F.; Thackeray, M. M.; Picciotto, L. A. De.; Goodennough, J. B. J. Solid State Chem. 1987,67, 316.
1864 Langmuir, Vol. 8, No. 7, 1992 s
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I
I
1
l i
? l l , , , l , l , l -
1
3.40
I
0.2
0.4
0.6
0.8
LilMn mole ratio
Figure 4. Variations of mean oxidation number of Mn with the extraction (solid marks) and insertion (open marks) of lithium ions: (m, 0 ) 0.5-800 system; ( 0 , O ) 0.5-400 system; (A,A) 0.8400 system. Table 111. Compositions of Li+-Extracted and -Inserted Samples sample H/Mn Li/Mn &,, formula h Li+ extracted 0.5-800(H) 0.02 0.05 4.03 Lio.loHo.~oo.slMnl.s504 0.5-400(H) 0.27 0.03 3.95 Lio.osHo.sooo.sshl.& -I 0.8-400(H) 0.61 0.02 3.97 Lio.o~Hl.07oo.lsMnl.7404 I 4 10 20 30 40 50 60 Li+ inserted 0.5-800(Li) 0.02 0.42 3.63 L~~.ssHo.MOO.I~M~I.WO~ 28 /degree 0.5-400(Li) 0.07 0.38 3.77 Lio.73Ho.l3o0,uMnl.~04 0.8-400(Li) 0.22 0.46 3.87 Lio.elHo.seoo.orMnl.7~04 Figure 5. X-ray diffraction patterns of the Li+-extracted (top) and Li+-inserted (bottom) samples: (top) (AI)0.5-800(H); (B1) These results indicate the presence of two types of 0.5-400(H); ((21) 0.8-400(H); (bottom) (Ad0.5-800(Li); (B2)0.5extraction reaction; one is the redox-type reaction which 400(Li);(Cz)0.8-400(Li). The compositionsof the samples are is described as eq 1,and the other the Li+/H+ion-exchangegiven in Table 111. I
type reaction. The increase of &,, corresponds to the oxidation of manganese from Mn(II1) to Mn(1V) due to a disproportionation reaction. The unchanged ZM,can be explained by a Li+/H+ion exchange reaction. The Li+ extraction with 0.5-800 is characterized by the redox-type reaction, indicating the presence of a large number of redox-type sites. The reaction with 0.8-400 can be characterized as an ion-exchange-typereaction, indicating the presence of a large number of ion-exchange-typesites. The reaction with 0.5-400 contains both the redox- and ionexchange-type reactions. The ZM,VS Li/Mn curve for 0.5400 suggests that Li+ions are preferentially extracted from the ion-exchange sites. X-ray,IR, and DTA-TG Analyses. X-ray diffraction analyses were carried out for the Li+-extracted samples (0.5-800(H), 0.5-400(H), and 0.8-400(H)); their Li/Mn ratio was less than 0.05 as shown in Table 111. The diffraction peaks of the spinel structure remained after Li+ extraction, but the peaks shifted to slightly higher 219 values with the extraction of Li+ (Figure 5). This indicates that the Li+ extraction reactions proceed topotactically, involving a decrease of lattice constant (ao) of the spinel structure. The plots of a0 against Li/Mn for the Li+ extraction reaction are shown in Figure 6. By comparing Figure 6 with Figure 4, we can point out that the redoxtype extraction brings a larger decrease of a0 value than the ion-exchange-type extraction. The larger decrease of a0 in the redox-type reaction is caused by the decrease of the effective ionic radius of Mn due to the oxidation from Mn(II1) to Mn(1V). Two spinel phases were observed in the case of the 0.5-800 system in the region Li/Mn < 0.25. These correspond to a A-Mn02 phase having an a0 value of 0.803 nm and to a Lb.aMn2Or phase having an a0 value of 0.813 nm.9t22923Such a two-phase separation was not found in the 0.8-400 and 0.5-400 systems. (23)Goodenough, J. B.; Thackeray,M. M.; David, W. I. F.; Bruce, P. G. Rev. Chim. Miner. 1984,21,435.
I
8.25 &
8.20
-28.15
-J
'
8.10
a'
8.05 I
I
0.2 0.4 LilMn mole ratio
I
0.6
C
a
Figure 6. Plots of the lattice constant (ao) of a spinel structure against the Li/Mn mole ratio for the Li+extraction (solid marks) and insertion (open marks): (m, 0)0.5-800 system; ( 0 , O ) 0.5400 system; (A,A), 0.8-400 system.
IR spectroscopic analysis was carried out for the original samples (0.5-800,0.5-400, and 0.8-400) and Li+-extracted samples (0.5-800(H), 0.5-400(H), and 0.8-400(H)), as shown in Figure 7. In the spectra of the original samples, the absorption band at 3400 cm-l and bands at 1530 and 1640 cm-l can be assigned to stretching and bending vibrations of absorbed water, respectively. The bands in the region from 400 to 700 cm-1 can be assigned to Mn-O stretching vibrati0ns.l~~A small band at 870 cm-l shifts to 920 cm-l and a new band at 3430 cm-1 appears following the extraction of Li+ in the cases of 0.8-400 and 0.5-400 systems. But these bands are not found in the spectra of 0.5-800 and 0.5-800(H). The band at 3430 cm-l can be assigned to stretching vibration of the lattice -OH group and the band at 920 cm-l to lattice coupling vibration of the H+-form spinel.' These results correlated well with the fact that the Li+/H+reaction occurs in the 0.8-400(H) and 0.5-400(H) systems but rarely occurs in the 0.6-800
Li+ Extractionlhertion
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200
I
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400
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600
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800
Temperature I 'C
Figure 8. DTA (top) and TG (bottom) curves for Lit-extracted samples. Symbols are the same as those in Figure 5 (top).
I
3600
I
I
I
2 000
I 1200
400
Wavenumber/cm-' Figure 7. IR spectra of original (top) and Lit-extracted (bottom) samples: (top) (A,,) 0.5-800;(BO)0.5-400;(Co)0.8-400;(bottom) symbols are the same as those in Figure 5 (top).
system. The bands at 870 and 920 cm-l may be useful for the evaluation of the number of ion-exchange sites. The DTA-TG curves for samples 0.5-400(H) and 0.8400(H) showed an endothermic peak at 70 "Cwith a weight loss (Figure 8); this corresponds to the evaporation of absorbed water. An exothermic peak at about 270 "Ccorresponds to a transformation from A-Mn02 to 8-Mn02.9 A weight loss around 220 "Ccorresponds to the dissipation of water by the condensation of the lattice-OH group of the spinel structure, accompanied by a transformation from H+-form spinel to P-MIIO~.~~ The lattice proton contents can be evaluated from the weight loss between 150 and 300 OC. Composition of Li+-ExtractedSamples. A general formula for the Li+-extracted samples can be written as follows: (10) The parameters n, m, and x can be evaluated from the following equations using experimental values of protonmanganese mole ratio (H/Mn), lithium-manganese mole ratio (Li/Mn), and &,, with the condition of eletroneutrality. The compositions for Li+-extracted samples are LinHm01+x-m-nMn2-x04
n + m + (2-x)Z,,=8 n/(2 - x ) = H/Mn m/(2 - x ) = Li/Mn
most of the Li+ were extracted by the redox-type reaction of eq 1for this sample. It is noteworthy that most of Mn are tetravalent in the Li+-extracted samples. Shen et al. have proposed the formation of (H)[Mn1I*MnNIO4spinel by the Li+/H+ion exchange of (Li)[Mn111MnN104.3 The present result, however, does not show such an ion exchange reaction for (Li)[Mn1I1MnIV1 0 4 spinel. We think (H) [Mn*I1MnN104 easily decomposes to A-Mn02 and Mn(I1) in an acid solution, owingto a disproportionation reaction of Mn(II1) to Mn(I1) and Mn(1V). Model of Li+ Extraction. On the basis of the facts that the redox-type extraction of one Li+ is attended by adisproportion of one Mn(II1) ion by the reaction Mn(II1) l/ZMn(IV) + '/ZMn(II) and most of the Mn are tetravalent in the Li+-extracted samples, we can assume that one Mn(II1) ion corresponds to one redox-type site, and one Mn defect in an octahedral site corresponds to four ion-exchange-type sites. Thus in the general formula 8, the number of redox-type sites and ion-exchange sites can be evaluated to be z and 4x, respectively. Formula 8 can be divided into two parts. One is the redox-type part
-
(Li,) [Mn111,Mn",104, (12) where z is the number of redox-type sites in one spinel structural unit, corresponding to the number of trivalent Mn. The other is the ion-exchange type part
(L~,,o
[ L ~ ~ ~ , , M ~ " ~ ,04(1-,) -~I (13) where 4x is the number of ion-exchange-type sites, corresponding to 4 times the number of Mn defects. The extraction of Li+ on the redox sites can be described by reaction 14,and that on the ion-exchangesites by reaction 15
(11)
shown in Table 111. The increasing order (0.5-800(H) < 0.5-400(H) < 0.&400(H)) of lattice proton content correlated well with the increasing order of the number of ion-exchange-type sites. The presence of only trace amounts of lattice proton in 0.5-800(H) indicates that
1-2-4x+y)
-
4(Li,)[Mn111,Mn1v,104, + 8zH' 3z(0)[Mn"~lO, + 4zLi+ + 2zMn2++ 4zH20 (14) (Li4,,~,-24x+y)[Liyo,-yMn'V2,~,l 04+,)+ 4xH+ * (H4x-y~1-2-4x+y) ~~y~,~yMn1V2~,~,104~1~z, + 4xLi' (15) A proton-free spinel manganese oxide (n)MnzOr, which
1866 Langmuir, Vol. 8, No. 7, 1992
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Table IV. Contents and Proportions of the Redox-Type Site and Ion-Exchange-Tyw Site SamDk
0.5-800 0.5-400 0.65-400 0.8-400 LiMnzO4 Lil.ssMnl.a~O4
redox site (/Mn) calcd exptl 0.40 0.44 0.30 0.27 0.19 0.10 0.07 0.5 0
ion-exchange site (/Mn) calcd exptl 0.02 0.08 0.27 0.19 0.44 0.61 0.65 0 0.8
corresponds to A-Mn02 as termed by Hunter,16is obtained by extracting Li+ from redox-type sites. A proton-form spinel manganese oxide is formed by extracting Li+ from ion-exchange-type sites. Since we cannot distinguish between the redox-type and the ion-exchange-type sites from the X-ray diffraction pattern, it is reasonable to think that the redox-type and ion-exchange-type sites are mixed with each other, sharing the Mn-0 framework of a spinel, to form a one-phase solid solution system.24 In such a case, electrons can move almost freely over the Mn atoms of the framework. Therefore, the extraction reaction of Li+ can be written schematically as in Figure 9a. The lithium ions migrate to the surface of the spinel powder and are transported to the solution phase. The dissolution of Mn according to eq 14 also proceeds a t the surface of the powder owing to the free movement of electrons. Therefore, the dissolution may proceed for the Mn atoms on the surface regardless of whether it comprises a redox site or an ionexchange type site. Evaluation of Site. The validity of the above model regarding the formation of two kinds of sites can be evaluated by comparing the calculated values of site numbers with the experimental ones. According to the model described above, the contents and proportions of both the redox-type and ion-exchange-type sites do not change followingLi+extraction although the total number of sites per gram of inititial lithium manganese oxide spinel decreases owing to the dissolution of Mn2+. Therefore, the proportions of the sites can be calculated as follows
=d n (16) fi,, = 4xJn (17) where frdfrom fion are proportions of the redox-type site and ion-exchange-type site, respectively. These values can be estimated from the chemical compositions of the original lithium manganese oxide spinel (Table IV). The experimental value for the number of ion-exchangetype sites can be directly determined from a lattice proton content (Table 111) of the Li+-extracted sample. On the other hand, the number of redox-type sites cannot be directly determined from the chemical composition of the Li+-extracted sample. But it can be evaluated on the basis of eq 14 by using the amount (nm) of Mn dissolved, the amount ( n ~ iof) Li+ extracted, and the difference ( a i l Mn) of Li/Mn ratio from the original lithium manganese oxide fred
redox site content (/Mn) = 2(nM,/nLi)(ALi/Mn) (18) The experimental values of the contents and proportions of the two kinds of site are given in Table IV. The agreement between the calculated and experimental values is comparatively good for both the redox-type and ion-exchange-type sites. This agreement justifies our model (Figure 9a) for the formation of different kinds of sites. This indicates that the contents and proportions of (24) Kozawa, A,; Powers, R. A. Electrochem. Technol. 1967, 5 635.
fred,
?6
fiom
calcd 83 61 30 14 100 0
%
calcd 17 39 70 86 0 100
eXDt1
96 50 10
eXDt1
4 50 90
0 M# 0 MI? 0 Mnb 0 Mr?
0
0
0
0
0
0
0 Mnk 0 Mr? 0 Mni 0 Mnk
0
O B 0
0
0
0
'10
o m A I 0
o m 0
0
0
(a)
0 Mn' I
Io n-exchange
Redox
Solution chase
/Mnb
(b)
0 Mn4* 0 Mnh 0 Mr? 0 Mr? 0 M '$
l1 o roo MI? 0
yr? 0
MI-? 0 Mn4*0 O o 0 Mnb 0 Mnb O /
o
y
0
Mn 0
MI?
0
0
0
0
0
0
0 MI? 0 Mr? 0 Mr?
Solid phase Ion-exchange
Redox
Solution phase
Figure 9. Schematic representations of (a) Li+ extraction reactions and (b) Li+ insertion reactions: 0,redox-typesites; O , ion-exchange-type sites. the redox-type and ion-exchange-type sites can be predicted by using Li/Mn and ZM,values of the lithium manganese oxide spinels, without a direct study on the extraction/insertion reactions. The calculation for the model materials suggests that all sites in the normal spinel LiMn204 are redox-type sites and in the Li1.33Mn1.6704 spinel are ion-exchange sites. The content and proportion of each site depend on the preparation conditions of the original lithium manganese samples. The number of ion-exchange-type sites increases with an increase in the starting Li/Mn mole ratio at 400 "C. The ion-exchange-type sites (Mn defecta in the octahedral site) are unstable above 500 O C ; they tend to transform to redox-type sites (LiMnzO4) and virtually only
Langmuir, VoE. 8, No. 7,1992 1867
Li+ Extructionlhertion
redox-type sites are formed at 800 "C. This tendency has been generally observed. The redox-type mechanism eqs 1 and 2 has been proposed for the samples prepared between 800 and 850 "C.933 Leont'eva et al,1*2 and Shen et al.3 have proposed an ion-exchange mechanism for the samples which are prepared below 520 "C, where ionexchange sites form easily. Li+ Insertion Reaction. The Li+ insertion reaction was studied using the Li+-extracted samples (0.5-800(H), 0.5-400(H), and 0.&400(H). Rsdox-type and ion-exchange type reactions also take place in a LiOH solution. A linear relationship between ZM,and Li/Mn is observed with the 0.5-800 system (Figure 4). On the other hand, a twostage process is observed with the 0.5-400 and 0.8-400 systems. The ZM,values are constant in the first stage of Li+ insertion, corresponding to the ion-exchange-type Li+insertion,while they decrease in the second stage owing to the redox-type reaction. These ZM,vs Li/Mn curves indicate that Li+ ions are preferentially inserted in the ion-exchange-type sites. The chemical compositions of Li+-insertedsamples (0.5800(Li),0.5-400(Li), and 0.8-400(Li)) from a 0.1 M LiOH solution are given in Table 111. The decrease of the lattice proton content with the progress of Li+insertion indicates the presence of the ion-exchange-type insertion reaction in the 0.8-400 and 0.5-400 systems. However, the compositional data indicate that the lattice protons in 0.8400(Li) and 0.5-400(Li) cannot be fully exchanged by Li+ even in a 0.1 M LiOH solution. This suggests the presence of two types of ion-exchange sites with different acidities. According to the model described as eq 15, there are two types of lattice protons in the acid-treated samples: the protons at 8a tetrahedral sites and those at Mn defects in 16d octahedral sites. These two kinds of lattice protons may have different acidities and may correspond to two types of ion-exchange sites in the Li+-extracted sample. The amount of redox-type insertion, (Li/Mn),d, can be evaluated by subtracting the amount of ion-exchange-type insertion from the total amount of inserted Li+ as follows, (Li/Mn)d = ((Li/Mn)II - (Li/Mn)I) - ((H/Mn)I - (H/ Mn)II), where subscripta I and I1 refer to the mole ratio before and after the Li+ insertion, respectively. The (Li/ Mn)dvalues are given as 0.37,0.15, and 0.05 for the system of 0.5-800, 0.5-400, and 0.8-400, respectively, using the data in Table 111. Comparing these values with the numbers of redox sites in Table IV, we can conclude that most of the redox type sites are filled with Li+ by the treatment with a 0.1 M LiOH solution. It also indicates that a further lithium insertion into LiMnzO4 is difficult in aqueous phase. The X-ray diffraction patterns of the Li+-inserted samples showed that the spinel structures remained after Li+insertion with a shift of the diffraction peaks to slightly lower 28 values (Figure 5(bottom)). This indicates that the Li+insertion reactions proceed topotactically,involving an increase in the lattice constant of the spinel structure. The a0 vs Li/Mn curve for the insertion reaction is in agreement with that for the extraction reaction in the 0.5800 system (Figure 6). On the other hand, the a0 vs Li/Mn curve for the insertion reaction shows an upward deviation from that for the extraction reaction in the case of the 0.8-400 system. The upward deviation may be caused by the reduction of Mn(1V) to Mn(II1) in the region Li/Mn > 0.3. The Li+ insertion reaction can also be regarded as a
lL
t
12
10 8
La 6 4 2
OH- added I m?q.g-'
Figure 10. pH titration curves for Li+-extracted samples: sample, 0.100 g; soh, 0.1 M MC1+ MOH (M= Li (solid marks) or Na (open marks)); total volume of solution, 10 mL; temperature, 20 "C;,.( 13 0.5-800(H), ,.( 0)0.5-400(H),(A,a)0.8400(H);(- - -) blank titration, (- - -) Li+-specificsites.
one-phase solid solution reaction which is schematically represented as in Figure 9b. The redox-type insertion reaction attends a reduction of Mn(1V) to Mn(II1) at solid surface. The diffusion of Li+from the surface to theredoxtype site in the bulk attends a diffusion of electrons over Mn atoms. pHTitration. The pH titration curves of Li+-extracted samples in a (0.1 M LiCl + LiOH) and a (0.1 M NaCl NaOH) solution are shown in Figure 10. The apparent capacities for Li+ were remarkably larger than those for Na+ over the pH range studied, indicating that all the samples showed lithium-ion-sieve properties. The high selectivityfor Li+ is due to the steric effect of the insertion site. The Na+ ion uptake can be regarded as the number of nonspecific ion-exchange sites on the surfaces of the powder.17 The net titration curves (dotted lines in Figure 10)for the Li+-specificsites can be obtained by subtracting Na+ uptake from total Li+ uptake, assuming that the number of nonspecific sites is equal to Na+ uptake." Li+specific sites of the ion-exchange-type sample 0.&400(H) show astronger aciditythan those of the redox-type sample 0.5-800(H). This indicates that Li+-specificion-exchange sites have a stronger acidity than those of redox-type sites. Sample 0.5-400(H), which is classified as a mixed type, shows a dibasic acid behavior. A stronger acidic site, dissociating below pH 4, can be ascribed to Li+-specific ion-exchange sites, and weaker acidic sites, dissociating above pH 5, can be ascribed to redox-type sites.
+
Conclusions The spinel-type lithium manganese oxides can be described by the general formula (Lin-ynl-n+y) [LiyOryMn111,Mn1V2-r-,104(1d n d 1.33,O d x d 0.33,O d z d 1, y d x , n d 1 + x). These spinels have two types of sites for Li+extraction/insertion reactions: redox-typeand the ion-exchange-typesites. The proportions of each site can be predicted from the values of Li/Mn mole ratio and ZM, of the spinels. The ion-exchange-typesites predominantly form below 500 "C while the redox-type sites form at higher temperatures. The Li+ extraction/insertion reactions occur preferentially with the ion-exchange-type sites.
