Influence of cobalt ions on the electrochemical properties of lamellar ...

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structures with the selected content of 0.15 Co per mole of oxide as the optimum composition. In ..... cobalt in the lamellar structure is illustrated by a notable.
470

Ionics 6 (2000)

Influence of Cobalt Ions on the Electrochemical Properties of Lamellar Manganese Oxides a

a

S. F r a n g e r ,

S. B a c h ,

9

J.P. P e r e l r a - R a m o s

a

and N. Baffier b

~Laboratoire d'EIectrochimie, Catalyse et Synth~se Organique, C.N.R.S. UMR 7582 2, rue Henri-Dunant 94320 Thiais, France bLaboratoire de Chimie Appliqu6e de l'Etat Solide, C.N.R.S. UMR 7574 11, rue Pierre et Marie Curie 75231 Paris, France Abstract. Evaluation of Co-doping on the electrochemical properties of the sol-gel birnessite and

the new lithiated manganese oxide Li0.45MnO2+5 is reported. For both compounds the synthesis of Co-doped materials via a solution technique is described. We demonstrate the interest of Co-doped structures with the selected content of 0.15 Co per mole of oxide as the optimum composition. In the case of Li0.45Mnl_yCOyO2+8, prepared at 300 ~ a mixture of a lamellar phase and a cubic one is identified while the Co-doped birnessite appears as a single phase. A probable substitution of Mn by Co ions explains the better specific capacity of 185 mAh/g found and the excellent stability observed over 40 cycles in the voltage range 4.2-2.0 V.

1. I n t r o d u c t i o n Due to their various crystalline forms, manganese oxides offer a wide field of research as Li intercalation compounds usable for Li-ion batteries and secondary lithium batteries [1-8]. Extensive studies have been performed on the preparation, structure and electrochemical properties of these materials. Besides the conventional high temperature solid state synthesis procedure, low temperature processes such as hydrothermal, ion-exchange and sol-gel techniques have been employed to form materials with new structures and properties. Spinel LiMn20~ [1-3], layered LiMnO2 [9,10] and layered MnO 2 [11-14] are among the most promising compounds. The main strategy for improving the cycling stability of the spinel compound is to add dopants such as Co, A1, Cr, and Li to alleviate the Jahn-Teller effect which is believed to be one of the reasons for cycling instability [3,15,16]. Anion doping of F has also been used to improved the high temperature performance of the spinel compound [17]. Conversely, this strategy has been little applied to improve the cycling properties of layered phases. Our research group synthesized a large variety of 2D phases using various synthesis ways based on the reduction of a AMnO4 aqueous solution by different kinds of

reducing agents [18,19]. In acidic medium we have previously shown that incorporation of Bi 3+ ions during the building of amorphous lamellar phases was possible and attractive in terms of electrochemical behaviour [20]. Indeed, the interest of doping these manganese oxide structures was illustrated by a gain of 15% on the faradaic yield obtained during cycling with an excellent stabilization of the specific capacity [20]. In contrast, in alkaline medium, birnessites corresponding to the chemical compositions A0.2sMnl.9602.nH20, (A = Li, Na),

A0.HMnl.9602.nH20(A = Pb, Mg),

A~.o9Mnl.9602.nH20 (A = A1), obtained from an ion-exchange procedure of the sodium birnessite Na0.zsMnl.9602 did not allow any significant improvement [12, 21]. More recently, the partial substitution of Mn ~+ by Co 3+ in LiMnO2 to get the composition LiMn0.9Co0.jO 2 has been shown to limit the structural instability of LiMnO2 and to improve its cycling performance with a stable capacity of about 200 mAh/g after 20 cycles on the voltage range 4.8-2.6 V [9,10]. This promising result prompted us to evaluate the effect of the Co-substitution on two lamellar phases we already investigated as rechargeable cathodic materials (i) the lithiated oxide Li0.4sMnO2+a (0.1 < 5 < 0.125) prepared through an ion-exchange procedure from the sol-gel

Ionics 6 (2000) lamellar compound Nao.45MnO2 [22] (ii) the sol-gel birnessite MnO1.84 0.6.H20 [11]. In this work we report on the synthesis of their Co-doped forms and the impact provided on their electrochemical behaviour. The results

471 contains the degradation products of the fumaric acid oxidation (alcaline oxalate and hydroxide). The xerogel is then heat-treated at 600 ~

in air for 10 hours. The

are discussed in comparison with those obtained for the

resulting product is the ternary oxide Na0.7MnO 2. An acidic treatment (H2SO 4 1 Mol.L -t) performed on trivalent

parent oxides and other lamellar phases.

manganese oxide phases provokes the release into the

2. Experimental Details The mean oxidation state of manganese (ZMn) was determined by a chemical titration using ferrous ammonium sulphate and potassium permanganate with an accuracy of + 2% [23] : the powder sample is dissolved in an aqueous solution containing 50% (vol) concentrated H2SO 4 and an excess of ferrous ammonium sulphate, During the dissolution of the powder, the Mn(IV) ions from the sample are reacted with Fe(II) and finally the excess of Fe 2§ ions are titrated with an aqueous solution of MnO4-. Chemical composition of the compound was made by elemental analysis using ICP - MS (Inductively Coupled P l a s m a - Mass Spectrometry). XRD experiment was performed with a Siemens D 5000 diffractometer using the CoK~ radiation (L = 1.7889 ,~,). The morphology and the grain size of the sample were performed with a Philips XL 30 scanning electron microscope. Electrochemical studies were carried out in Swagelok| - type two electrodes cells for the galvanostatic measurements and cycling tests. The working electrode consisted of a stainless steel grid with a geometric area of 1 cm 2 on which the cathodic material was pressed. The cathode was made of a mixture of active material (80% wt), graphite (7.5% wt), acetylene black (7.5% wt) and teflon as binder agent (5% wt). The film is obtained, in 10 minutes, by mixing the oxide powder, carbon and teflon. The electrolyte used was 1 mol.L -1 LiC104, dried under vacuum at 170 ~ for 15 h, dissolved in twice distilled propylene carbonate obtained from Fluka| Galvanostatic measurements were made with a MacPileII apparatus. The working composition was changed by coulometric titration. 3.

Results and Discussion

3.1. Synthesis. The sol-gel birnessite

solution of Na + ions followed by disproportionation of the Mn 3+ into soluble Mn 2+ and insoluble Mn 4+ ions. Thus, in this case, the

solid

network progressively trans-

forms into birnessite compound 3.68).

MnO1.84.0.6H20 (Z =

The Co-doped birnessite Mno.ssCoa1501.84"0.6 1-120 Cobalt is introduced in the desired ratio Co/Mn = 0.176 from Co(CH3CO2)2.4H20 in the aqueous solution containing both the reducing agent and the sodium permanganate salt. The preparation described above for the SGBir is also available for the Co-doped birnessite (Z = 3.7)

Co-doped lithiated oxide Lia45Mn1_yCoyO2+a, Three main synthesis steps are required. Each step corresponds to the synthesis of the following materials described

below:

i:

sol-gel

~-Na0.TMnO2,

ii:

Na~I.45MnO2.0.6H20 and iii: Lio.45MnvyCOyO2+~,. i) The synthesis of the sol-gel c~-Na0.TMnO2 has been already described above as a part of the preparation way of the SGBir. ii) In this case the oxidation treatment applied to c~Na0.TMnO2 in order to get the SGBir is replaced here by a moderate treatment. It consists of a water washing [24]. After one hour the reaction is over and leads to the final product Na0.45MnO2.0.6H20. After filtration the remaining solid was washed with water and air dried at 60 ~

The water content of the product is

about 0.6H20. The final material exhibits a hexagonal symmetry with the following lattice parameters, a = 2.84 ]~, c = 7.27 ~. iii) A quantitative ion exchange reaction was carried out by refluxing Na0.45MnOe.0.6H20 (5 g) for 2 days in aqueous solution containing a large excess of lithium hydroxide (1 mol.L -j) with or without a metallic cation (Me = Co 2+ from Co(CH3CO2)2.4H20 ). After cooling and filtration, the remaining solid was washed with water and dried at 60 ~

The same water content as in

The sol-gel birnessite (SGBir) is prepared via the reduction

Na0.45MnO2.0.6H20 is observed. The anhydrous final

of a sodium permanganate salt in aqueous solution by an

compound is obtained after a thermal treatment at 300 ~ during 10 hours and corresponds to the following

organic acid such as the fumaric acid [18]. Using an initial ratio M n O 4 / C 4 H 4 0 4 = 3, a dark brown gel is obtained in approximatively 5 minutes. The natural deshydratation at room temperature of this gel gives the xerogel which

formula: Lio.45Mnl.yfoyO2+~, (0.075 < ~' < 0.1) with 0 < y < 0.2. Z = 3.77 for the undoped compound while 3.7 < Z < 3.74 for the Co-doped materials.

472

Ionics 6 (20001

Fig. 1. Scanning electron micrographs of Lio.45MnO2§ ~ (a, b) and Lio.45Mnll.ssCoo.jsO2+~. (c, d).

birnessite with the following parameters : a = 2.84 A, c --

3.2. Structure and Electrochemical Properties.

Lio.45-

9.84 A while the other diffraction peaks can be ascfbed t~

Mnl_yCoy02+6. (0.075 < r < 0.1 ; 0 < y < 0.2). Particle

a spinel phase with a = 8.16 ]~. All these diffraction peak~ are found again in the XRD pattern of the Co-dope~

3.2.1. The Co-doped lithiated manganese oxides

morphology of Li0.45MnO2+5 and Li0.45Mno.85Co0.1502+~, has been examined by scanning electron microscopy (Fig. 1). Figure 1 clearly shows the homogeneity of the particle size distribution probably due to the solution technique

sample with broader lines indicating a more disorderex structure. No additional phase such as a Co-rich phas~ appears. Hence the structure of the parent oxide is un

used which combines the sol-gel process and an ion-

changed and all these results are consistent with a pro

exchange procedure. Particles consist of platelets of about 3 ~tm wide. The specific surface area of the doped and the

b a n e substitution layers.

undoped samples is 1.7 m2/g,

of Mn

by

Co

ions

in

MnO

As previously reported for the undoped material (Fig

The XRD patterns of Li0.45MnO2+~ and Li0.45Mn0.85Co0.1502+~, obtained after a thermal treatment at 300 ~

3), the heat-treatment at 600 ~ instead of 300 ~ unam biguously leads to a well cristallized cubic phase (a = 8.2]

for 10 h are reported in Figs. 2a and 2b, respectively. Figure 2a clearly indicates a mixture of two phases. The diffraction lines, 002 : d = 4.92 ,~, 101 : d = 2.38 A can

,~) which confirms manganese ions are substituted b 3 cobalt ions in the core structure [22]. The very smal additional peak at 24.24 ~ (20) reflects the presence o

be indexed on the basis of a hexagonal cell of the

Li2MnO3.

Ionics 6 (2000)

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Fig. 4. Influence of the Co-content in Lio.45Mnl.yCoyO2+~, with 0.11 < y < 0.19 on the specific capacity and the faradaic yield (cycling limits 4.2-2.0 V, C/20).

Figure 4. In spite of close values of the oxidation state of Mn (3.7 < ZM,< 3,74), the cells tested with various

a 2 Hexagonal . a=2.84 A, c=9.84 A

amounts of Co in the cathodic material delivered different specific capacities. The disparity observed in the results is difficult to explain as a function of the Co-content. Except for y = 0.15, the initial capacity is rather low, between 90 mAh/g for y = 0.17 and 0.13, and 120 mAh/g for y 0.11 and 0.19. For y = 0.17, the capacity first increases and then stabilizes around 120 mAh/g between cycles 10 and 20. The best result is obtained for the composition

10

20

30

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70

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Fig. 2. Comparison of XRD diffraction patterns (~CoK~) of (a) Lio.45Mn02+~ (300 ~ h) and (b) Lio.45Mno.8~Coo.ls02+~, (300 ~ / 10 h).

The influence of the Co-content in Li0.45Mnl_yCOyO2+~, with 0.11 < y < 0.19 on the cycling properties recorded at a C/20 rate in the voltage window 4.2-2.0 V is shown in

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Li0.45Mn0.~sCoo.1502+~, since the corresponding cell delivers a remarkable specific capacity of 185 mAh/g over 20 cycles at a C/20 rate. A typical discharge-charge profile of Li0.4sMn0.ssCo0AsO2+a, at C/20 rate is reported in Fig. 5. Li insertion takes place in one step around 2.8 V and involves the reduction of around 0.6 Mn 4+ into Mn >, During the charge, a larger faradaic yield is recovered due to the extraction of 0.1 additional Li ion near 4 V. The successive dischargecharge cycles are superimposed showing the high re-

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Fig. 5. Chronopotentiometric curves for the two first reduction oxidation at low current density (C/20 rate) of Lio4sMnt_yCOyO2§~.heat-treated at 300 ~ h in a 1 mol.L -~ LiC104 Propylene Carbonate.

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Ionics 6 (2000 0,7

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Number of cycles Fig. 6. Evolution of the specific capacity and the faradaic yield as a function of the number of cycles of Lio45MnO2+~ h [cycling limits 4.2-2.0 V, (n) heat-treated at 300 ~ C/20] and Lic~.45MnvyCoyO2+~.heat-treated at 300 ~ h [cycling limits 4.2-2 V, (g) C/20 and (t) C/5].

versibility of the Li insertion-extraction reaction in Li045Mn0.85Co0.1502+~,. The lack of polarization in the cycling profiles demonstrates the stability of the Co-doped structure. Figure 6 confirms the attractive properties of this material insofar as it can be seen it can sustain a relatively high discharge-charge rate (C/5) without any significant capacity loss : 175 mAh/g can be still retained after 20 cycles. At C/20 rate, the benefit of the presence of cobalt in the lamellar structure is illustrated by a notable enhancement of the capacity with 185 mAh/g (+ 20%)in comparison with the behaviour exhibited by the undoped material.

3.2.2. The sol-gel Co-doped birnessite. SEM micrographs of Fig. 7 indicate a less homogeneous distribution in the particle size of the Co-doped birnessite. This may stem from the use of the more severe oxidation treatment applied with H2SO4. The mean grain size observed is 4 I.tm. The X-rays diffraction pattern of the MnOj.84.0.6H20 (SGBir) is shown in Fig. 8. The most intensive lines correspond to the (0 0 l) planes. The structure of the birnessite can be described as a lamellar phase of hexagonal symmetry with the following cell parameters: a = 2.84 A, c = 7.27 A. From the observation of Fig. 8, it is apparent that the sol-gel compound exhibits an important preferred orientation since only the 001 and 002 peaks appear with noticeable intensities. This indicates a stacking of the layers parallel to the (a, b) plane over large domains. Comparison of XRD patterns recorded for the undoped and the Co-doped birnessites shows there is no change in the positions and intensities of diffraction lines, except only a more important background which seems to

Fig. 7. Scanning electron micrographs of Mno.ssCoo.~O1.84-0.6H20.

indicate a slight disorder in the Co-doped structure. 11 means that cobalt ions are well incorporated in SGBil without any secondary phase.

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lonics 6 (2000)

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clusions can be drawn from these results: i) Cobalt ions are probably responsible for a better structural stability which ensures a longer cycle life of birnessite ii) the presence of structural water does not prevent the use of hydrated oxides as rechargeable cathodic material for secondary lithium batteries. In another paper to be published we also point out the faster kinetics of lithium transport in

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Fig. 9. Chronopotentiometric curves for the two first reduction-oxidation at low current density (C/20 rate) of Mn08sCo0~sO~s4.0.6HzO in a 1 mol.L-' LiCIO4 / Propylene Carbonate. The first two discharge-charge cycles of the Co-doped birnessite are reported in Fig. 9. From an initial potential of 3.6 V, a S-shape curve is obtained for the reduction process involving the reduction of 0.6 Mn 4+ ions instead of 0.85 for the undoped SGBir [11]. This is explained by the

lower content

of

Mn 4+ ions

in

Mn0.~sCoo.~s-

Ohs4.0.6H20 (~ 70 %) compared to that in the SGBir (85 %). Once again as in the case of the pristine material, a large hysteresis appears between the charge and the discharge curves even there is an excellent efficiency of the charge process. A maximum voltage of 4.2 V is needed to

4. Conclusion We have proposed a convenient technique to synthesize Co-doped layered manganese oxides with the optimum composition of 0.15 Co per mole of oxide. We have established that Co incorporation takes place inside the structure of the parent oxides, probably through a substitution of Mn by Co ions as reported for high temperature doped forms of spinel oxides. This strenghtening of the host lattice is responsible for the excellent electrochemical properties of compounds. However, a possible limitation of the dissolution process cannot be discarded as a promoting effect induced by the presence of cobalt. Electrochemical results show that with a stable capacity of 170 - 185 mAh/g over 20 cycles lbr the Co-doped forms, Mn0.ssCo0.15Ohs4.0.6H20 and Li0.45Mno.85Co0.1502+5, compare very well with the best materials known to operate in

extract all the lithium ions from this layered host lattice. Further cycling slightly reduces the hysteresis but the

the voltage window 4.2-2.0 V.

main effect provided by the dopant is the remarkable sta-

5. Acknowledgement The financial support by the CEA/CEREM is gratefully acknowledged.

bility of the high specific capacity achieved over 30 cycles: 170 mAh/g (Fig. 10). Conversely, a sharp decrease of the capacity takes place in the first cycles for the undoped birnessite. This capacity decay is slower thereafter to reach 155 mAh/g at the 30th cycle. Two main con0,8 0,75 0,7 9

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Mc. Kinnon, and S. Colson, J. Electrochem. Soc. 138, 2859 (1991).

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6. References [1] M.M. Thackeray, W.I.F. David, P.G. Bruce, and J.B. Goodenough, Mater. Res. Bull. 18, 4611 (1983). [2] T. Ohzuku, M. Kitagawa, and T. Hirai, J. Electrochem. Soc. 137, 769 (1990). [3] J.-M. Tarascon, E. Wang, F.K. Shokoohi, W.R,

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Number of cycles Fig. 10. Evolution of the specific capacity and the faradaic yield as a function of the number of cycles of (a) MnOI.84.0.6H20 [cycling limits 4.2-2.0 V, (o) C/20] and (b) Mno.ssCool.~Ol.s4.0.6H20 [cycling limits 4.2-2.0 V, (n) C/20].

[4]

P.M. De Wolff, Acta Cryst. 12, 341 (1959).

[5]

S. Sarciaux, A. Le Gal La Salle, A. Verbaere, Y. Piffard, and D. Guyomard, J. Power Sources 8 1 -

[6]

82, 656 (1999). T. Nohma, T. Saito, N. Furukawa, and H. Ikeda, J. Power Sources 26, 389 (1989).

476 [7]

[8] [9] [10]

[11] [12] [13] [14] [15]

[16]

Ionics 6 (2000) T. Nohma, Y. Yamamoto, K. Nishio, I. Nakane, and N. Furukawa, J. Power Sources 32, 373 (1990). L. Li, G. Pistoia, Solid State Ionics 47, 231 (1991). P.G. Bruce, A.R. Armstrong, and R.L. Gitzendanner, J. Mater. Chem. 9, 193 (1999). Y. Shao-Horn, S.A. Hackney, A.R. Armstrong, P.G. Bruce, R. Gitzendanner, C.S. Johnson, and M.M. Thackeray, J. Electrochem. Soc. 146, 2404 (1999). S. Bach, J.P. Pereira-Ramos, and N. Baffler, Electrochimica Acta 36, 1595 (1991). S. Bach, J.P. Pereira-Ramos, and N. Baffler, J. Electrochem. Soc. 143, 3429 (1996). P. Strobel, C. Mouget, Mater. Res. Bull. 28, 93 (1993). F. Leroux, D. Guyomard, and Y. Piffard, Solid State Ionics 80, 299 (1995). C. Sigala, D. Guyomard, A. Verbaere, Y. Piffard, and M. Tournoux, Solid State Ionics 81, 167 (1995). L. Guoha, H. Ikuta, T. Uchida, and M. Wakihara, J. Electrochem. Soc. 143, 178 (1996).

[17] M.R. Palacin, G.G. Amatucci, M. Anne, Y. Chabre, L. Seguin, P. Strobel, J.-M. Tarascon, and G. Vaughan, J. Power Sources 81-82, 627 (1996). [18] S. Bach, M. Henry, N. Baffler, and J. Livage, J. Solid State Chem. 88, 325 (1990). [19] S. Bach, J.P. Pereira-Ramos, and N. Baffler, J. Power Sources 81-82, 273 (1999). [20] S. Bach, J.P. Pereira-Ramos, C. Cachet, M. Bode, and L.T. Yu, Electrochimica Acta 40, 785 (1995). [21] P. Le Goff, N. Baffler, S. Bach, and J.P. PereiraRamos, Mater. Res. Bull. 31, 63 (1996). [22] S. Franger, S. Bach, J.P. Pereira-Ramos, and N. Baffler, J. Electrochem. Soc. 147, 3226 (2000). [23] M.J. Katz, R.C. Clarke, and W.F. Nye, Anal. Chem. 28, 1956 (1956). [24] P. Le Goff, S. Bach, J.P. Pereira-Ramos, N. Baffler, and R. Messina, Solid State Ionics 61, 309 (1993).

Paper presented at the 7th Euroconference on lonics, Calcatoggio, Corsica, France, Oct. 1-7, 2000. Manuscript rec. Oct. 3, 2000; acc. Dec. 1, 2000.

ERRATA "PMMA Based Protonic Polymer Gel Electrolytes" S. Chandra, S.S. Sekhon and Narinder Arora Ionics 6, 112-118 (2000) 1. Fig. 2 and Fig. 4 are to be interchanged (not figure captions) 2. Fig. 3 and Fig. 5 are to be interchanged (not figure captions)