Layered xLiMO2·(1−x)Li2MO3 electrodes for lithium batteries: a study of 0.95LiMn0.5Ni0.5O2·0.05Li2TiO3

https://doi.org/10.1016/S1388-2481(02)00251-5Get rights and content

Abstract

The electrochemical properties of 0.95LiMn0.5Ni0.5O2·0.05Li2TiO3 have been investigated as part of a study of xLiMO2·(1−x)Li2MO3 electrode systems for lithium batteries in which M=Co, Ni, Mn and M=Ti, Zr, Mn. The data indicate that the electrochemically inactive Li2TiO3 component contributes to the stabilization of LiMn0.5Ni0.5O2 electrodes, which improves the coulombic efficiency of Li/xLiMn0.5Ni0.5O2·(1−x)Li2TiO3 cells for x<1. The 0.95LiMn0.5Ni0.5O2·0.05Li2TiO3 electrodes provide a rechargeable capacity of approximately 175 mAh/g at 50 °C when cycled between 4.6 and 2.5 V; there is no indication of spinel formation during electrochemical cycling.

Introduction

Significant efforts have been made over several years to find an alternative positive electrode for LixC/Li1−xCoO2 lithium-ion batteries to reduce their cost and improve their safety, particularly for large-scale applications, such as electric vehicles. These efforts have included, for example, the substitution of Mn or Ni for Co in the layered LiCoO2 structure [1], [2], [3], the exploitation of manganese oxides, particularly those with the spinel structure [4], [5], [6] and, more recently, the use of LiFePO4 with an olivine-type structure [7], [8]. All of these systems have certain disadvantages: (1) nickel-based electrodes are limited by the instability of Ni4+ ions at high potentials, which compromises the safety of lithium-ion cells, (2) Li1+xMn2−xO4 spinel electrodes suffer from solubility problems, notably at elevated temperature, e.g., 50 °C, (3) layered LiMnO2 transforms to a spinel-like structure during electrochemical cycling, and (4) LiFePO4 has a relatively low theoretical capacity (170 mAh/g) and intrinsically low ionic and electronic conductivity. Despite these limitations, considerable progress has been made in stabilizing layered and spinel electrode structures by selective cation substitution [9], [10] and by applying a protective coating on the electrode particles [11]; the electronic conductivity of LiFePO4 has been significantly improved by coating the surface of the electrode particles with carbon [12], [13].

We have adopted an approach to stabilize layered electrode structures by using solid-solution electrodes of general formula xLiMO2·(1−x)Li2MO3 that contain an electrochemically active LiMO2 component (M=Co, Ni, Mn) and an electrochemically inactive Li2MO3 component (M=Mn, Ti, Zr); the latter component can be represented in layered LiMO2 notation as Li(Li0.33M0.67)O2. Our primary goal is to stabilize an electrode structure in which manganese and/or nickel are the electrochemically active ions. Our research of xLiMO2·(1−x)Li2MO3 structures originated from the discovery that lithia (Li2O) could be leached from Li2MnO3(Li2O·MnO2) by acid treatment to yield a Li2−2xMnO3−x(xMnO2·(1−x)Li2MnO3) structure without dislodging the manganese ions from their layers [14], [15], [16]. Relithiation of this compound in an electrochemical cell yields the discharged composition xLiMnO2·(1−x)Li2MnO3. The relationship between the structural components in these types of electrodes and the electrochemical reaction pathway for lithium extraction/insertion are shown in a schematic illustration of a LiMO2–MO2–Li2MO3 phase diagram in Fig. 1. Previous reports have indicated that the Li2MnO3 component helps to suppress the transformation of the layered LiMnO2 component to a spinel configuration; moreover, it has been demonstrated that the partial substitution of Mn by Zr in the Li2MnO3 component increases the stability of a delithiated xMnO2·(1−x)Li2Mn1−yZryO3 electrode in air [15]. However, it is impractical to synthesize discharged xLiMnO2·(1−x)Li2MnO3 electrodes for lithium ion cells by the two-step procedure of (1) acid treatment, and (2) chemical relithiation, because it is difficult to control the reactions and the ultimate composition of the final product. Furthermore, the reaction of Li2MnO3 with acid is accompanied by some H+ exchange for Li+ [17], [18], thereby introducing a water component to the electrode which is difficult to remove without structural decomposition. It stands to reason, therefore, that it is preferable to manufacture xLiMO2·(1−x)Li2MO3 electrodes in one step under dry conditions.

Li2MnO3, Li2ZrO3 and Li2TiO3 are electrochemically inactive components and therefore do not contribute to the electrochemical capacity of xLiMO2·(1−x)Li2MO3 electrodes. In this respect, it has been recently demonstrated that the layered compound Li(Li0.2Mn0.4Cr0.4)O2, which falls within the class of xLiMO2·(1−x)Li2MO3 structures at x=0.5 (i.e., when the M:M ratio is 1:1), provides excellent capacity and cycling stability, particularly at 50 °C [19]. In Li(Li0.2Mn0.4Cr0.4)O2, the tetravalent manganese ions are electrochemically inactive; all the capacity is derived by lithium extraction during which the Cr3+ ions are oxidized to Cr6+ [20]. Similarly, it has been demonstrated that an electrode can be fabricated from a LiCoO2–Li2MnO3 solid solution in which the cobalt ions are electrochemically active and the manganese ions are inactive [21]. In another development, it has been shown that Ni2+ ions can be substituted for Mn and Li in the Li2MnO3[Li(Li0.33Mn0.67)O2] structure to induce electrochemical activity [22], [23]. The end-members of this system, which can be represented by either Li(Li0.33–0.67xMn0.67–0.33xNix)O2 (0<x<0.5) or yNiO·(1−y)Li2MnO3 (0<y<0.5), are Li2MnO3 and Li(Mn0.5Ni0.5)O2. It has been reported that the nickel and manganese ions in the fully discharged electrodes are divalent and tetravalent, respectively, and that the electrochemical capacity is obtained from a Ni2+/Ni4+ couple [23].

Because nickel-based electrodes are prone to oxygen loss at the top of charge, our approach has been to use Li2MO3 structures, notably Li2TiO3 and Li2ZrO3 that have strong Ti–O and Zr–O bonds relative to Mn–O, Ni–O and Co–O, to stabilize LiMO2 electrodes. In this paper, we report on our initial investigation of the synthesis and electrochemical properties of 0.95LiMn0.5Ni0.5O2·0.05Li2TiO3 electrodes.

Section snippets

Experimental

Three compounds within the xLiNi0.5Mn0.5O2·(1−x)Li2TiO3 system corresponding to the compositions LiNi0.5Mn0.5O2 (x=1.00), 0.95LiMn0.5Ni0.5O2·0.05Li2TiO3 (x=0.95) and 0.90LiNi0.5Mn0.5O2·0.10Li2TiO3 (x=0.90) were prepared by reacting stoichiometric amounts of Ni0.5Mn0.5(OH)2, Ti[OCH(CH3)2]4 and LiOH first at 480 °C for 12 h and then at 900 °C for 10 h in air; thereafter, the samples were rapidly quenched (also in air). The Mn0.5Ni0.5(OH)2 precursor was prepared by precipitation from a basic NaOH

Results and discussion

The powder X-ray diffraction patterns of LiNi0.5Mn0.5O2 (alternatively NiO·Li2MnO3), 0.95LiMn0.5Ni0.5O2·0.05Li2TiO3, and 0.90LiMn0.5Ni0.5O2·0.10Li2TiO3 are shown in Figs. 2(a)–(c). The X-ray diffraction patterns of these compounds are consistent with layered structures with Li2MNO3-type character (M=Mn, Ti, Zr), as evident from the small peak at approximately 21° 2θ in Figs. 2(a)–(c). Although the precise crystal symmetry of these compounds is still unknown, all the major peaks could be

Conclusions

The data presented in this paper provide evidence that an electrochemically inactive Li2TiO3 component contributes to the stabilization of the active LiNi0.5Mn0.5O2 component of 0.95LiMn0.5Ni0.5O2·0.05Li2TiO3 electrodes; the superior coulombic efficiency of Li/0.95LiMn0.5Ni0.5O2·0.05Li2TiO3 cells has been attributed to a decrease in the oxygen activity at the surface of delithiated electrode particles at high states of charge. These data, although preliminary, hold promise for tailoring the

Acknowledgements

J.T. Vaughey is thanked for determining the lattice parameters of the compounds. Support from the Office of Advanced Automotive Technologies of the United States Department of Energy, under Contract No. W31-109-Eng-38 is gratefully acknowledged.

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