Effects of Al substitution for Ni and Mn on the electrochemical properties of LiNi0.5Mn1.5O4

doi:10.1016/j.electacta.2011.03.093

Electrochimica Acta

Volume 56, Issue 18, 15 July 2011, Pages 6554-6561

https://doi.org/10.1016/j.electacta.2011.03.093 Get rights and content

Abstract

The effects of Al substitution for Ni or (and) Mn in LiNi_0.5Mn_1.5O₄ spinel on the structures and electrochemical properties are investigated. Powders of LiNi_0.5Mn_1.5O₄, Li_0.95Ni_0.45Mn_1.5Al_0.05O₄, LiNi_0.475Mn_1.475Al_0.05O₄ and Li_1.05Ni_0.5Mn_1.45Al_0.05O₄ are synthesized by a thermopolymerization method. Their structures and electrochemical properties are studied by X-ray powder diffraction, scanning electron microscopy, infrared spectroscopy, cyclic voltammetry and galvanostatic charge–discharge testing. The introduction of Al in these LiNi_0.5Mn_1.5O₄ samples has resulted in structure variation, and greatly improved their cyclic performance and rate capability. The effects of Al substitutions for Ni and Mn in the LiNi_0.5Mn_1.5O₄ are different. Compared with LiNi_0.5Mn_1.5O₄, Li_0.95Ni_0.45Mn_1.5Al_0.05O₄ demonstrates higher specific capacity at room temperature but faster capacity fading at elevated temperatures. Li_1.05Ni_0.5Mn_1.45Al_0.05O₄ displays a lower discharge capacity but better capacity retention at 55 °C. Moreover, the cyclic performance and rate capability of the Ni-substituted Li_0.95Ni_0.45Mn_1.5Al_0.05O₄, Ni/Mn co-substituted LiNi_0.475Mn_1.475Al_0.05O₄ and Mn-substituted Li_1.05Ni_0.5Mn_1.45Al_0.05O₄ at room temperature are similar, and have improved substantially compared with the Al-free LiNi_0.5Mn_1.5O₄ sample.

Highlights

► Aluminum doped LiNi_0.5Mn_1.5O₄ powders are synthesized by a thermopolymerization process. ► The comparison of Al doping between Mn sites and Ni sites is made for the first time. ► Properly Al-doped LiNi_0.5Mn_1.5O₄ shows excellent cycling stability at 55 °C with the fading rate as low as 0.015% per cycle. ► At room temperature, the capacity fading rate may be even slower (less than 0.01%) and the rate capability is very good (114 mAh g⁻¹ at 10 C).

Introduction

With the ongoing global warming and depletion of crude oil, the environment and energy crisis exert great pressure on the existing energy infrastructures. Consequently, we have been largely increasing the utilization of renewable energies such as solar and wind, and, very vigorously developing electric vehicles (EV) and hybrid electric vehicles (HEV) to reduce CO₂ emissions. Also, large-scale energy storage systems are essential to store energy and provide stable energy output. At the forefront of these efforts is to seek an appropriate electricity storage device. Owing to their high energy density and excellent cycling performance, lithium-ion batteries become the most attractive candidates. They have proved to be the best power sources for portable devices [1], [2], [3]. Nevertheless, we need to improve further their performance substantially to meet the more demanding requirements of these large energy storage systems, especially on energy density and power density. If considering only the contribution from the cathode material, we prefer to use a material with a high capacity (in Ah kg⁻¹) and a high working potential (in V). The product of both can be defined as the energy density of that material (in Wh kg⁻¹). Fig. 1 compares the energy density of several cathode materials for lithium-ion batteries. In this figure, the integration area of the discharge curve represents the energy density of corresponding material. Thanks to its 4.7 V high operating voltage, which is higher than 3.4 V for LiFePO₄, 3.9 V for LiCoO₂ and 4.1 V for LiMn₂O₄, the spinel-structured LiNi_0.5Mn_1.5O₄ delivers an energy density of 650 Wh kg⁻¹, about 20–30% higher than the others. In addition, the three-dimensional lithium diffusion pathways in the spinel lattices are beneficial to provide a high power density. Therefore, LiNi_0.5Mn_1.5O₄ has attracted great attentions during the past decade [4], [5], [6], [7], [8], [9], [10], [11].

LiNi_0.5Mn_1.5O₄ displays a flat voltage profile at 4.7 V corresponding to the redox reactions of Ni²⁺/Ni³⁺ and Ni³⁺/Ni⁴⁺ redox couples, with the theoretical capacity of 146.7 mAh g⁻¹ [12], [13]. It is actually regarded as a special case of doped lithium magnetite spinels LiM_xMn_2−xO₄ (M = Cr, Fe, Co, Ni, Mg, Cu, etc.) [14], [15], [16], [17], [18], [19]. This composition with the Ni:Mn molar ratio of 1:3 has attracted more attention than the others because it shows a dominant potential plateau around 4.7 V, while other compounds (LiM_xMn_2−xO₄, M: Cr, Fe, Co, Cu) exhibit two plateaus at 4.0 and 5.0 V [19]. Usually, the capacity at 5.0 V or above is not utilizable because no suitable electrolyte system is available at present. Thus, LiNi_0.5Mn_1.5O₄ has become one of the most important cathode materials for lithium-ion batteries. Unfortunately, even this compound (LiNi_0.5Mn_1.5O₄) still has a non-negligible capacity fading during cycling, especially at elevated temperatures, due to the structural and chemical instabilities resulted from the presence of high spin Mn³⁺ ions [20].

A number of groups have worked on improving the electrochemical properties of this interesting cathode material, especially on its cycling stability, by means of surface coating [20], [21], [22], [23] and doping [4], [11], [24], [25], [26], [27]. These methods, especially the doping, are proved to be quite effective. Aklalouch et al. have synthesized LiCr_2γNi_0.5−γMn_1.5−γO₄ (0 < γ < 0.2) by a sucrose-aided combustion method [24]. The samples with γ ≤ 0.1 have capacity retention of 96% after 40 cycles at 55 °C. Ito et al. have increased the lithium diffusion coefficient by Co doping [25]. The LiNi_0.5−xCo_2xMn_1.5−xO₄ (0 ≤ 2x ≤ 0.2) exhibits improved cyclic performance at a high rate and at elevated temperatures. Park et al. have investigated LiNi_0.5−xMn_1.5Cr_xO4 (x = 0.00, 0.01, 0.03 and 0.05) and found that the initial capacity and capacity retention are improved as the Cr content increases [4]. Oh et al. have investigated a series of doped system LiNi_0.5−xMn_1.5−yM_x+yO₄ (M = Cr, Al and Zr) and found that the electrical conductivity increase with Cr- and Al-doping [26]. We can see that the foreign ions can substitute either Mn ions or Ni ions, or both of them. But to the best of our knowledge, the difference between the substitutions of Ni or (and) Mn has not been studied yet. To perform this study, we design three different spinels of Al substitutions, viz. Li_0.95Ni_0.45Mn_1.5Al_0.05O₄, LiNi_0.475Mn_1.475Al_0.05O₄ and Li_1.05Ni_0.5Mn_1.45Al_0.05O₄. The content of Al is controlled to be 0.05 and the content of Li is altered to keep charge neutrality. LiNi_0.5Mn_1.5O₄ is also studied as a contrast. In the end, we have found LiNi_0.475Mn_1.475Al_0.05O₄ gives rise to the best performance.

Section snippets

Experimental

The LiNi_0.5Mn_1.5O₄, Li_0.95Ni_0.45Mn_1.5Al_0.05O₄, LiNi_0.475Mn_1.475Al_0.05O₄ and Li_1.05Ni_0.5Mn_1.45Al_0.05O₄ powders were synthesized by a thermopolymerization method. Stoichiometric amounts of lithium nitrate (LiNO₃, 5% excess), aluminum nitrate (Al(NO₃)₃·9H₂O), nickel nitrate (Ni(NO₃)₂·6H₂O) and manganese acetate (Mn(CH₃COO)₂·4H₂O) were dissolved in deionized water to obtain a 0.2 M solution. Then acrylic acid (AA) was added to form an AA-H₂O (1:2, v/v) solution. The solution was then kept in an oven

Results and discussion

The X-ray diffraction patterns of the spinel powders are shown in Fig. 2. The results reveal that the products are well-defined cubic spinels and no trace of impurity phase (such as Li_xNi_1−xO) is detected. The structures are assigned to either $Fd \bar{3} m$ (F-type) or P4₃32 (P-type) space group, which depends on the ordering of transition metal cations. It is well known that spinels showing cations ordering on the octahedral sites have P4₃32 type symmetry [28]. In this structure the Ni²⁺ and Mn⁴⁺

Conclusions

LiNi_0.5Mn_1.5O₄ and Al-doped Li_0.95Ni_0.45Mn_1.5Al_0.05O₄, LiNi_0.475Mn_1.475Al_0.05O₄ and Li_1.05Ni_0.5Mn_1.45Al_0.05O₄ spinels have been synthesized by a thermopolymerization method. The Al-doping changes the space group of LiNi_0.5Mn_1.5O₄ from ordered P4_–332 to disordered $Fd \bar{3} m$ under the same heat treatment conditions. It can significantly improve the cycling stability of LiNi_0.5Mn_1.5O₄. The capacity retentions of Al-doped spinels are over 99% after 100 cycles at room temperature. Even at 55 °C, the

Acknowledgments

This study was supported by National Science Foundation of China (grant nos. 20971117 and 10979049) and Education Department of Anhui Province (grant no. KJ2009A142). We are also grateful to the Solar Energy Operation Plan of Academia Sinica.

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Effects of Al substitution for Ni and Mn on the electrochemical properties of LiNi0.5Mn1.5O4

Abstract

Highlights

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgments

J. Power Sources

Electrochim. Acta

Electrochim. Acta

Electrochim. Acta

J. Power Sources

J. Power Sources

Electrochem. Commun.

Electrochim. Acta

J. Power Sources

Electrochem. Commun.

J. Power Sources

J. Power Sources

J. Alloys Compd.

Electrochim. Acta

J. Power Sources

J. Power Sources

J. Electrochem. Soc.

Appl. Phys. Lett.

Effects of Al substitution for Ni and Mn on the electrochemical properties of LiNi_0.5Mn_1.5O₄