Electrochemical and structural study of the 3.3 V reduction step in defective LixMn2O4 and LiMn2O(4−y)Fy compounds

doi:10.1016/S0378-7753(99)00233-5

Journal of Power Sources

Volumes 81–82, September 1999, Pages 627-631

https://doi.org/10.1016/S0378-7753(99)00233-5 Get rights and content

Abstract

Non-stoichiometric or fluorine-substituted Li_xMn₂O₄ are known to present a reversible redox step at 4.5 V as well as a reduction step at 3.3 V appearing to the expense of the usual 4.0–4.15 V. We present results of very slow stepwise potentiodynamic studies which allowed to localise the oxidation step associated to the 3.3 V reduction level close to 3.95 V. We propose the cubic spinel lattice parameter value as the key parameter for knowing a priori if these redox states will be present or not, with 8.215 Å as threshold value. In situ XRD studies performed across the 3.3↔3.95 V redox step indicate that these additional states are probably associated to structural features.

Introduction

The LiMn₂O₄ spinel phase has been given intensive studies over the last years as positive electrode material for Li-ion batteries. Due to its 4.0 and 4.1 reversible redox states, it seems to be the best candidate to replace LiCoO₂ in commercial lithium-ion batteries, for cost and toxicity reasons. But when prepared with conditions which lead to non-stoichiometric compounds, this system was shown to present an additional reversible redox step at 4.5 V [1], as well as a reduction step at 3.3 V [2]. Although these states have attracted attention because they seem to be related to poor performance for batteries made with such compounds, their physico-chemical origin is still unclear. From studies of samples prepared in various conditions, Tarascon et al. [1]suggested that the 4.5 V step is related to Li–Mn cation mixing with Mn³⁺ on the spinel 8a tetrahedral sites, whereas Gao and Dahn [2]gave evidence that the presence of the 3.3 V reduction step is related to that of the 4.5 V redox one, and that their amplitudes are related to the amount of oxygen deficiency. Then, Amatucci et al. [3]determined that these two steps have equal capacities, which tends to prove that they have a common cause.

We have done an extensive study of these anomalous states on a series of defective Li_yMn₂O₄ (0.925≤y≤1.0) and high temperature heat-treated samples, as well as on a partly fluorinated sample LiMn₂O_3.74F_0.26 [4], which all appear to show similar 4.5 and 3.3 V peaks [5]. Results are presented in this paper on a quenched Li_0.925Mn₂O₄ and the fluorinated compounds, determining the oxidation level corresponding to the 3.3 V reduction step which was previously said irreversible [2]. In situ XRD has been performed on these compounds in order to look for a possible structural signature of this step.

Section snippets

Experimental

Li_yMn₂O₄ (0.925≤y≤1.075) were prepared as reported elsewhere [1]. The fluorine-substituted sample LiMn₂O_4−zF_z was synthesized according to Amatucci et al. [4]. From the determination of its Mn oxidation state, its formula was LiMn₂O_3.74F_0.26. For obtaining samples with a wide range of lithium deficit, some of the Li_yMn₂O₄ compounds were heat-treated at 1100°C for 24 h, cooled down to 820°C, allowed to rest 15 h at this temperature and then quenched to room temperature.

Electrochemical studies

Fig. 1 shows the potential/composition curve obtained on the first galvanostatic cycling of a Li_0.925Mn₂O₄ sample performed at a nominal C/20 rate in the 4.8 V↔2.8 V potential window. On first charge, the additional high potential oxidation step is well observed at 4.6 V after the usual 4.0 and 4.15 V plateaus. On first discharge, both additional states are observed at 4.5 V and 3.2 V. The plateaus at 2.8–3.0 V correspond to cycling partly on the cubic↔tetragonal transition, Li₁Mn₂O₄↔Li₂Mn₂O₄.

In situ XRD studies of the 3.3 V step

We looked for a possible structural signature of this 3.3–3.95 redox state during a extensive in situ study of these spinel systems 8, 9. Several XRD patterns were recorded at given intervals over a 4-day oxidation at 3.96 V constant potential. The as-prepared fluorinated compound shows a splitting of all reflections except the (hhh) ones into three which seems to exist also for Li_yMn₂O₄ (y<1) compounds, though in a much lesser extent. This splitting can be assigned to a small fraction of a

Discussion

The most intriguing question at the beginning of this study was the fact that both cation defective spinels (as samples of nominal formula Li_yMn₂O₄ with y<1) and anion defective ones (i.e., LiMn₂O_3.74F_0.26) show the additional 4.5 V and 3.3 V redox steps. Cation-defective compounds present these states when the oxidation states are lower than 3.53 2, 3, which is also the case for the fluorinated sample. Recent investigations [10]are in agreement with this trend, but they determined a maximum Mn

Conclusion

An extensive study was performed on the additional 4.5 V and 3.3 V redox states that can be observed in some non-stoichiometric Li_yMn₂O₄ spinels as well as in fluorine substituted ones. We have first determined the oxidation potential of the 3.3 V reduction step which is close to 3.95 V. Two linear relationships have been found between the amplitude of these redox states and the spinel lattice parameter value, for low temperature and high temperature treated samples respectively. The cubic

Acknowledgements

We acknowledge C. Masquelier and G. Rousse for very interesting and fruitful discussions. One of us (M.R.P.) is grateful to the Generalitat de Catalunya for a RED contract.

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Electrochemical and structural study of the 3.3 V reduction step in defective LixMn2O4 and LiMn2O(4−y)Fy compounds

Abstract

Introduction

Section snippets

Experimental

Electrochemical studies

In situ XRD studies of the 3.3 V step

Discussion

Conclusion

Acknowledgements

J. Power Sources

J. Power Sources

J. Solid State Chem.

J. Solid State Chem.

J. Power Sources

J. Electrochem. Soc.

J. Electrochem. Soc.

IBA Fall Meeting 1996

Prog. Batt. Batt. Mat.

Electrochemical and structural study of the 3.3 V reduction step in defective Li_xMn₂O₄ and LiMn₂O_(4−y)F_y compounds