Enhancement of the electrochemical properties of o-LiMnO2 cathodes at elevated temperature by lithium and fluorine additions

doi:10.1016/j.jpowsour.2005.03.193

Journal of Power Sources

Volume 154, Issue 1, 9 March 2006, Pages 268-272

https://doi.org/10.1016/j.jpowsour.2005.03.193 Get rights and content

Abstract

Orthorhombic LiMnO₂ cathodes suffer severe capacity fading due to accelerated manganese dissolution at elevated temperatures. Fluorine-modified LiMnO₂ cathodes prepared by a solid-state reaction show less cation mixing and a well-defined crystallinity with a larger grain/powder morphology. The cathodes exhibit improved capacity retention and rate capability at the elevated temperature (55 °C) compared with undoped counterparts.

Introduction

Lithium-manganese-oxide is a candidate material to replace the commercial LiCoO₂ cathode in lithium-ion secondary batteries, because it has the advantages of lower toxicity and cost [1], [2], [3], [4], [5]. Unfortunately, however, one of the choices, LiMn₂O₄, delivers a lower discharge capacity (110–120 mAh g⁻¹) than LiCoO₂ (140–150 mAh g⁻¹) [1], [2], [3], [4], [5]. Moreover, it faces severe capacity fading problems due to accelerated dissolution of manganese on cycling at elevated temperature (>50 °C), and structural instability due to Jahn–Teller (J–T) distortion [5], [6], [7], [8], [9]. Orthorhombic LiMnO₂ (o-LiMnO₂) has a high theoretical capacity (∼285 mAh g⁻¹) and a better cycleability than LiMn₂O₄ over a wide voltage range [10], [11], but its cycling performance is poor at the elevated temperature [11].

It has been reported that o-LiMnO₂ transforms to a phase with a spinel-like structure during cycling [12]. The dissolution of manganese is again the cause of poor cycling performance at the elevated temperature and leads to defective spinels [13]. The dissolution is induced by acids generated from reaction of LiPF₆ salts with the water impurities present in the cell [14], [15]. Robertson et al. [16] suggested that the capacity decay is due to disproportionation of the spinel electrode into acid-soluble species.

Croguennec et al. [17] reported that small crystallites of 1–10 μm are more easily transformed into a spinel-like structure. They showed that o-LiMnO₂ has stacking faults (local regions of monoclinic ordering) that are caused by structural disorder between the Li and Mn sites, and that these stacking faults induce a broadening of the (1 1 0) diffraction peak [17], [18]. Orthorhombic LiMnO₂ cathodes with more stacking faults underwent an easier phase transformation from an orthorhombic to a spinel structure with more severe capacity fading on cycling, compared with samples with less stacking faults [19].

To stabilize the structural instability for elevated-temperature performance, cation substitution such as o-LiAl_1−xMn_xO₂ has been studied. While capacity retention was found to be better than that of o-LiMnO₂ on cycling tests at 55 °C, Al substitution (o-LiAl_1−xMn_xO₂) did not prevent J–T distortion [20]. An alternative approach to improving the structural stability is by coating the o-LiMnO₂ surface with metal oxides, which might minimize the structural instability from HF attack [8], [9].

Recently, improvements on cycleability through the co-doping of cations (Li, Mg, Al, etc.) and anions (F, S, etc.) into LiMn₂O₄- or LiNiO₂-based cathodes have been reported [21], [22], [23], [24], [25], [26], [27]. A sintering agent during synthesis can cause large and smooth powders [28], [29]. Dahn's group [30] obtained dense LiNi_xCo_1−2xMn_xO₂ materials at 900 °C by adding LiF as a sintering agent. This study reports enhancement of the electrochemical properties of o-LiMnO₂ cathodes by Li and F additions through a solid-state reaction.

Section snippets

Experimental

Li_1+xMn_1−xO_2−yF_y (0 ≤ x ≤ 0.07, 0 ≤ y ≤ 0.3) samples were prepared by a solid-state reaction with LiOH·H₂O, Mn₂O₃ and LiF. Heat treatment was performed at 800 °C for 20 h in an argon atmosphere. An excess amount of lithium was used to compensate for the amount lost during heat treatment. The lithium composition was analyzed by inductively coupled plasma atomic emission spectrometry (ICP–AES).

X-ray diffraction (XRD) with Cu Kα radiation (M18XHF-SRA, MAC Science) was used to determine the phases and the

Results and discussion

The XRD patterns for various lithium-manganese-oxide samples with anion-dopant fluorine before cycling are presented in Fig. 1. All the samples have a predominantly orthorhombic structure (with a space group of Pmnm) based on the o-LiMnO₂ structure with small amounts of m-Li₂MnO₃ phase [1]. The formation of the Li₂MnO₃ phase is due to reaction with oxygen in the atmosphere. Anion substitution does not cause any particular change in the lattice constants (a, b and c) of o-Li_1+xMn_1−xO_2−yF_y. The

Conclusions

Fluorinated lithium-manganese-oxide has been synthesized through a solid-state reaction with Li and F additions. The o-Li_1.07Mn_0.93O_1.92F_0.08 cathode has a well-defined crystallinity with less stacking faults and a larger grain/powder morphology, than undoped o-LiMnO₂. The fluorine-modified LiMnO₂ cathode exhibits improved capacity retention and rate capability at an elevated temperature (55 °C).

Acknowledgements

The authors thank Jisuk Kim and Mijung Noh at Kumoh National Institute of Technology for their experimental support and Cheil Industries for the supply of the electrolytes. This work was supported by KOSEF through the Research Center for Energy Conversion and Storage at Seoul National University, by the National R&D Program of the Ministry of Science and Technology, and by the Basic Research Program (R01-2004-000-10173-0) of KOSEF.

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Short communicationEnhancement of the electrochemical properties of o-LiMnO2 cathodes at elevated temperature by lithium and fluorine additions

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgements

Mater. Res. Bull.

Mater. Res. Bull.

J. Power Sources

Mater. Res. Bull.

J. Power Sources

J. Fluorine Chem.

J. Power Sources

J. Power Sources

J. Power Sources

Electrochim. Acta

J. Eur. Ceram. Soc.

Mater. Sci. Eng. B

Physica C

J. Electrochem. Soc.

Solid State Ionics

Rev. Chim. Miner.

Short communication
Enhancement of the electrochemical properties of o-LiMnO₂ cathodes at elevated temperature by lithium and fluorine additions