1167
Langmuir 1991, 7, 1167-1171
Mechanism of Li+ Insertion in Spinel-Type Manganese Oxide. Redox and Ion-Exchange Reactions Kenta Ooi,' Yoshitaka Miyai, and Jitsuo Sakakihara Government Industrial Research Institute, Shikoku, 2-3-3Hananomiya-cho, Takamatsu 761,Japan Received June 25,1990.I n Final Form: December 3,1990 Three kinds of spinel-type manganese oxide were prepared by heating a mixture of MnOOH and Li2CO3 at different conditions (varyingthe starting Li/Mn ratio and temperature) followed by an acid treatment with a HC1solution. The insertion reactions of lithium ions in these samples were investigatedby chemical, X-ray, DTA-TG analyses, and pH titration. The insertion sites could be classified into three groups, e.g. a redox-type site, a Li+-specificion-exchange site, and a nonspecific ion-exchange site. The proportion of each group varied depending on the preparation conditions of manganese oxide. The pH titration study showed that the Li+-specificion-exchangesite had a stronger acidity than the redox-type site. The origin of these sites is discussed in terms of the physicochemical properties of the heat-treated precursors. The variation of the oxidation state of manganese in the precursor relates to the formation of different kinds of site.
Introduction Spinel-type manganese oxide is an interesting material for its remarkably high selectivity for lithium ions in the aqueous phase.l-l3 It can be obtained by a thermal crystallization of Li+-introducedmanganese oxide followed by the extraction of lithium ions with an acid. Its Li+selective property is closely related to the formation of such a material as LiMn204 during thermal crystallization while undergoing preparation. LiMnzOr has a spinal structure with lithium a t the tetrahedral sites and manganese(II1) and manganese(1V) at the octahedral sites of a cubic closed-packed oxygen f r a m e ~ 0 r k . lTreatment ~ of LiMnzOr with acid causes the removal of nearly all the lithium from the tetrahedral sites while maintaining a spinel-type structure.l6 We have studied the lithium-ion sieve properties of the spinel-type manganese oxide in the aqueous phase. The selectivity sequences at pH 4 approximately are Na+ < K+ < Rb+ < Cs+ 0
2 20 VI
d
10
Time/ h
Figure 3. Time courses of oxygen evolution during the insertion of lithium ions: suspension, 2 g of sample in 180 cm3 of water; solution, 1 M LiCl 0.5 M LiOH, 20 cm3; temperature, 25 "C; 0, M-0.5-400; A, M-0.5-850; 0,M-0.2-850.
+
DTA-TG curves for M-0.2-850 showed an endothermic peak below 100 "C with a considerable weight loss; this corresponds to the evaporation of absorbed water. The water content, calculated from the weight loss at 300 "C, increased in the order Mn-0.5-850 -C Mn-0.5-400 C Mn0.2-850. Li+ Insertion Reaction. The insertion reaction of lithium ions was attended by the evolution of oxygen gas in the case of the (0.1 M LiCl + 0.050 M LiOH) system (Figure 3). The evolution of oxygen gas is caused by the presence of the redox-type reaction, which is described as eq 1. The evolved amount of gas increased in the order M-0.5-400 C M-0.2-850 C M-0.5-850. The X-ray diffraction patterns and DTA-TG curves of the Li+-inserted samples are given in Figures 4 and 5. The diffraction peaks corresponding to a spinel structure remained after insertion, but the peaks shifted to slightly lower 28 values. The lattice constants after Li+ insertion were 0.818,0.820, and 0.822 nm for M-0.5-400, M-0.5-850, and M-0.2-850, respectively. This indicates that the insertion of lithium ion proceeds topotactically with a slight increase in the lattice constant. The exothermic peak around 280 "C disappeared and the endothermic peaks around 570 "C were greatly reduced by the Li+ insertion. The weight loss due to the evaporation of water was also reduced.
Langmuir, Vol. 7, No. 6,1991 1169
Li+ Insertion in Manganese Oxide
I
$1 I
10
20
30 40 28 / degree
50
Figure 4. X-ray diffraction patterns of spinel-type manganese oxides after the Li+insertion. Symbols are the same as those in
Figure 1.
i
P1
I
I
----_/
200
600
400
Temperature /
"C
Figure 5. DTA (top) and TG (bottom) curves of Li+-inserted samples. Symbols are the same as those in Figure 1.
The chemical compositions of the manganese oxides before and after Li+ insertion are given in Table I. The capacity for lithium ions increased in the order M-0.5-400 < M-0.2-850 < M-0.5-850. The Li+ insertion resulted in a decrease of available oxygen; this corresponds to the reduction of manganese from Mn(1V) to Mn(II1). The decrease of available oxygen indicates the presence of the redox-type insertion reaction of eq 1. The decreased amount of available oxygen correlated with the evolved amount of oxygen gas during the insertion reaction. The Li+ insertion also resulted in a decrease in water content, probably owing to the H+/Li+ion-exchange reaction or to the steric exclusion of water molecules. This suggests the presence of the ion-exchange-type insertion reaction. Li+ Insertion in a LiCl Solution. The insertion isotherms in a LiCl solution (Figure 6) could be represented by the following Langmuir equation:16 (CIA) = (1/A,)C + (K/A,) (5) Here C is the concentration of lithium ions, A is the amount of insertedlithium ions, A, is the maximum of the insertion sites for lithium ions, and K is a constant related to the energy of insertion. The A , value can be evaluated from the slope of a CIA vs C plot. The calculated values of A, were 0.087,0.027, and 0.078 mole per mole of manganese for M-0.5-400, M-0.5-850, and M-0.2-850, respectively. Classification of Sites. The above results show that there are different types of insertion sites in the spinel(16) Rubin, A. J.; Mercer, D. L. Adsorption of Znorganics at SolidLiquidInterface; Anderson, M. A., Rubin, A. J., Eds.; Ann Arbor Science Publishers, Inc.: Ann Arbor, MI, 1981; p 263.
type manganese oxide. The insertion reaction in a (0.1 M LiCl + 0.050 M LiOH) solution indicates the presence of two types of sites: redox-type and ion-exchange sites. The previous study with pH titration has suggested the presence of two groups of ion-exchange sites: Li+ specific and nonspecific sites.1° Therefore, we can classify them into three principal groups, redox-type, Li+-specific ionexchange, and nonspecific ion-exchange sites. The proportion of each group can be evaluated by using the chemical analysis data described above. The total amounts of Li+-insertion sites are given in Table I. The number of redox-type sites can be evaluated from the amount of evolved oxygen in Figure 3 on the basis of eq 1. The Li+specific ion-exchange sites can be evaluated from the A, value, which is obtained from the insertion isotherm in a LiCl solution. The A, value is responsible for the number of H+/Li+ion-exchangesites, since the redox-type reaction does not proceed in a LiCl solution because of a low concentration of hydroxide ions. In addition, the pH titration study with respect to sodium ions shows that the nonspecific ion-exchange reaction proceeds slightly in the pH range below 5, as will be shown in the following section. The residual sites other than redox-type and Li+-specific ion-exchange sites can be regarded as nonspecific. The results of site classification show that the proportion of each site varies depending on the preparation conditions (Figure 7). Sample M-0.5-850is characteristic in that more than 90% of the sites is comprised of the redox-type site. In the other two samples, the redox-type site comprises about half of the total insertion sites. Samples M-0.5-400 and M-0.2-850 have a larger number of Li+-specific ionexchange sites than M-0.5-850. The Li+-specific ionexchange site comprises about 30% of the total sites in the former samples. The amount of nonspecific ionexchange site increased in the order M-0.5-850 < M-0.5400 < M-0.2-850. It comprises 24% of the sites of M-0.2850. Acid-Base Property of Insertion Site. The pH titration curves in (0.1 M LiCl + LiOH) and (0.1 M NaCl NaOH) solutions are summarized in Figure 8. The apparent capacities for lithium ions were remarkably larger than those for sodium ions over the pH range studied, indicating that all the samples show a lithium ion sieve property. The pH titration curves toward lithium ions differed slightly with the preparation conditions. The apparent capacity for lithium ions increased in the order M-0.5-850 < M-0.2-850 < M-0.5-400 at pH 4, while the increasing order changed inversely at pH 10. This indicates that sample M-0.5-400 has a larger number of strong acid sites than the other two samples. M-0.5-400 is characterized by having two types of insertion sites: an acidic site capable of dissociating below pH 6 and a weaker acidic site capable of dissociating above pH 10. Comparing the results of site classification in Figure 7 with the pH titration data in Figure 8, we can point out some acid-base characteristics of each site. The pH titration curve of M-0.5-850 shows little lithium ion uptake in the pH range below 4. This suggests an apparently weak acid character of the redox-type site, because the sites in M-0.5-850 are comprised mainly of the redox type. The residual part, the titration curves below pH 4, can be ascribed to those on Li+-specific ion-exchange sites, since the apparent lithium ion capacities at pH 4 agreed comparatively well with the amounts of Li+ specific ionexchange sites calculated from the A, values. This suggests that the Li+-specific ion-exchange sites have a stronger acidity than the redox-type sites. The apparent capacity for sodium ions was negligibly
+
Ooi et al.
1170 Langmuir, Vol. 7, No. 6, 1991
sample M-0.5-400 M-0.5-850 M-0.2-850
Li/Mn 0.045 0.056 0.014
Table I. Chemical Analysis of Manganese Oxides. before Li+ insertion after Li+ insertion O,*/Mn H20/Mn Li/Mn O,/Mn H20/Mn 0.16 0.296 0.90 0.96 0.12 0.05 0.430 1.04 0.86 0.03 0.43 0.327 0.92 1.00 0.34
ALi/Mn 0.251 0.374 0.313
change in composition AO,/Mn AH20/Mn -0.06 -0.04 -0.18 -0.02 -0.09 -0.08
a The contents of Li, HaO, and available oxygen are expressed as the mole ratios per mole of Mn. * Available oxygen determined by oxalic acid method.
O
d
i
2
3
z
.
5
OH- a d d e d I meq.9-I
,
I
I
2 C L i I 1 moldm-3
1
1
3
Figure 6. Insertion isotherms of lithium ions in a LiCl solution (bottom) and Langmuir plots (top): sample, 1.0 g; volume of M-0.5-400, (A)M-0.5solution, 100 cm9;temperature; 25 O C ; (0) 850, ( 0 )M-0.2-850.
0.4
Figure 8. pH titration curves with respect to lithium and sodium A, ions. Sample, 0.10 g; soln, 0.1 M MC1+ MOH (M = Li (0, 0)or Na (e,A, E));total volume of solution, 10 cms; temperature, 25 OC; (0, e) M-0.5-400, (&A)M-0.5-850, (0,B)M-0.2850. The dotted line describes blank titration.
Origin of Site. The insertion reaction is closely related to the extraction reaction of lithium ions from the heattreated precursor. Hunter has proposed the redox-type reaction in the case of LiMnzO~spinel, which was obtained by a solid-phase reaction with MnzO3 and Li2CO3ls 4LiMn204 + 8H+
A
B
C
Sample
Figure 7. Classification of sites in spinel-type manganese oxide: m, redox-type site;B, Li+-specificion-exchange si% C],nonspecific ion-exchange site. Symbols are the same as those in Figure 1.
small in M-0.5-400 and M-0.5-850 over the pH range studied. Only M-0.2-850 showed an apparent capacity of 0.8 mequiv/g at pH 10. The apparent capacities for sodium ions agreed well with the numbers of nonspecific sites evaluated from the chemical analysis data in Figure 7. This agreement justifies our method of site classification, since the pH titration curve with sodium ions is closely related to the acid-base property of the nonspecific ionexchange site. The curves show the nonspecific site to have a weak acid character. The acidity is about the same as those on other manganese oxides (Y-MnOz and 6-Mn02);17J8they behave as a weak acid ion-exchanger in an aqueous solution owing to the dissociation of the surface hydroxyl group. (17) Tamura, H.; Katayama, N.; Nagayama, M.; Furuichi,R. Nippon Kagaku Kaiahi 1987, 1524. (18)Balistriere,L. S.;Murray, J. W. Ceochim. Cosmochim.Acta 1982, 46, 1041.
-
+
4Li'
2Mn2+ + 4H20
+
30Mn20,
(3)
The LiMnzO4 has a structure with lithium ions a t the tetrahedral sites of a ccp oxygen f r a m e ~ 0 r k . l The ~ topotactic extraction of lithium ions causes a formation of vacant sites a t the tetrahedral position. The formation of vacant sites can be achieved by two properties:lBJg (1)a solid-state diffusion of lithium ions in the [Mn]zO4 framework, and (2) a surface disproportionation reaction: 2Mn3+ Mn2+ + Mn4+. Shen et al. have proposed an ion-exchange mechanism for the topotactic extraction of lithium ions from the LiMn204 spinel, which was obtained with y-MnOz and LiOH6
-
a
~
+
-
H+ n
O~ ~ n 2 0~4+ Li+ ~
(4)
Leont'eva et al. have prepared precursors by the reaction of manganese hydroxide with LiOH followed by heat They have observed such spinel-type lithium manganates as Li[Lio.%Mn1.77]0 4 and Lio.ssMnl.7sOr in addition to LiMn204.1*4 The former two samples are characterized by having manganese of tetravalent state alone. The acid treatment of these may give manganese oxides with ion-exchange sites; these sites may, however, have a different chemical reactivity from HMnzOr. The formation processes of the spinel-type manganese oxides were investigated in terms of the differences in the (19) Goodenough,J. B.; Thackeray, M. M.; David, W. I. F.; Bruce, P. G. Reu. Chim. Miner. 1984,21, 435.
Langmuir, Vol. 7, No. 6, 1991 1171
Lit Insertion in Manganese Oxide
Table 11. Chemical Properties of Heat-TreatedPrecursor starting Li/Mn ratio
heating temp, OC
Li/Mn
0.5 0.5 0.2
400 850 850
0.51 0.49 0.21
chemical composition OdMn HsO/Mn 0.90 0.78 0.61
0.024 0.009 0.005
oxidation state of Mna
Mu/LLb
3.80 3.56 3.22
0.24 0.52 2.0
Oxidation stateof Mn = 2( 1+ Olv/Mn). Mole ratio of dissolved manganeseto extracted lithium during acid treatment. The acid treatment was carried out by adding 10 g of precursor to 5 dm3 of HCl solution (0.25 M).
physicochemical properties of the heat-treated precursors. The chemical analyses of the precursors show that the oxidation state of manganese varies depending on the preparation conditions (Table 11); it has a tendency to decrease with an increase in the heating temperature and with a decrease in the starting Li/Mn ratio. The Li and Mn concentrations of the supernatant solutions during the acid treatment were determined by atomic absorption spectrometry. The mole ratios (Mndi/ LieJ of dissolved manganese to extracted lithium were evaluated from these concentrations (Table 11). The precursor obtained at Li/Mn = 0.5 and 850 "C shows a chemical composition close to a LiMnzO4 spinel. Since the manganese oxide (M-0.5-850) obtained has a redox-type site mainly, we can conclude that the acid treatment of LiMnzO4 largely results in the formation of the vacant tetrahedral sites according to eq 3. The Mndb/ Lie, value (0.52) also supports the process of the reaction eq 3. The alternative reaction, the formation of protonated tetrahedral sites according to eq 4, hardly proceeded at all. Since H M n ~ 0 4contains a trivalent manganese, it may be susceptible to disproportionation in an acidic media. The precursor (Li/Mn = 0.5, 400 "C)contains manganese oxide with high oxidation state. This suggests the presence of lithium manganates (Li1.33Mnl.6704 or Li2MnOa) with tetravalent manganese in the precursor. Li1.BMn1.6704 has a spinel structure with part of the octahedral site being occupied by lithium as Li[Lio.~Mnl.67] Li2MnO3 has a monoclinic structureFl but the diffraction pattern resembles that of LiMnzO4 spinel. The precursor shows the smallest Mnd,/Li,, value. It shows a tendency to form a large amount of Li+-specificion-exchange sites, compared with the other two samples. Lithium manganates with tetravalent manganese can form stable protonated sites according to the topotactic Li+/H+ ionexchange reaction without a disproportionation reaction. Such protonated sites may correspond to the Li+-specific ion-exchange sites. We have investigated the Mg2+extraction reaction from MgzMn04, which also consists of a tetravalent manga-
nese.22 The acid treatment of MgzMnO4 gives ionexchange sites specific to lithium ions, but the chemical analysis and pH titratibn studies have suggested MgO dissolution in addition to the Mg2+/H+ion exchangeduring the acid treatment. A similar reaction, Liz0 dissolution, may proceed in the present case. Such a dissolution reaction well explainsthe smaller number of insertion sites in M-0.5-400 than in M-0.5-850. X-ray analysis of the precursor (Li/Mn = 0.2,850 "C) gave the diffraction pattern of a mixture of LiMnzO4 and Mn304; both have a spinel structure. Transmission electromicrographic observation showed a formation of a mixed crystal instead of a simple mixture of LiMnzO4 and Mn304 particles. The acid treatment causes the dissolution of Mn2+ from MnsO4 in addition to the Li+ extraction. The Mn2+dissolution results in the formation of the pores in the particlesF3 thus causing the production of an hydroxyl group on the I
-0-Mn-O-
I
site at the pore surface. Such a hydroxyl group may correspond to the nonspecific ion-exchange site in the present case. A comparativelylarge amount of Li+-specific ion-exchange sites in M-0.2-850 can be explained by the fact that the tetrahedral Li+-specific site near the pore surface can be easily protonated by the adsorbed water.
04.4120
(20) Blasse, G. Philips Res. Rep., Suppl. 1964,3,1. (21) Jansen, V. M.; Hoppe, R. 2.Anorg. Allg. Chem. 1973,397,279.
Conclusion The Li+-insertion sites in the spinel-type manganese oxide can be classifiedinto three types (redox,Li+-specific ion-exchange, and nonspecific ion-exchange sites). The proportion of each site varies depending on the preparation conditions. The oxidation state of manganese in the heattreated precursor plays an important role in the formation of the insertion sites. Registry No. Li&Os, 554-13-2; MnOOH, 1202599-9; Li, 743993-2; manganese oxide, 11129-60-5. (22) Miyai, Y.; Ooi, K.; Katoh, S . J. Colloid Interface Sci. 1989,130, 535. (23) Hayashi, T.; Iwasaki, T.; Onodera, Y. Nippon Kagaku Kabhi 1988,1906.
