Influence of Mg2+ doping on the structure and electrochemical performances of layered LiNi0.6Co0.2-xMn0.2MgxO2 cathode materials

doi:10.1016/j.jallcom.2016.02.119

Journal of Alloys and Compounds

Volume 671, 25 June 2016, Pages 479-485

https://doi.org/10.1016/j.jallcom.2016.02.119 Get rights and content

Highlights

•
Mg-doped sample exhibits better electrochemical performance.
•
The change of crystal structure by Mg doping are studied.
•
The Mg doping improves the lithium ion diffusion coefficient.

Abstract

Introducing the Mg ion into host lattice is applied to improving the electrochemical performance of LiNi_0.6Co_0.2Mn_0.2O₂. The effect of Mg substitution for Co on the structure, morphology, electrochemical properties and Li⁺ diffusion coefficients are investigated in details. Rietveld refinement results reveal that Mg is incorporated into the bulk lattice, which results in reduced cation mixing and expand c-lattice parameter. All Mg-doped sample exhibit better cycle and rate performances, although the Mg substitution for Co led to decreasing a part of capacity. The Li diffusion coefficients obtained by galvanostatic intermittent titration technique (GITT) are increased with increases of Mg content.

Graphical abstract

Introduction

Lithium-ion batteries (LIBs) have been widely utilized as a power source for portable electronic devices. In particular, further increasing the energy density and reducing the cost are main directions of developments in LIBs to be used for mobile electronics and transportation [1]. Cathode materials, as one of the most critical components of lithium ion batteries, play a critical role in determining the performance of LIBs [2]. Among of cathode materials, those in the series LiNi_1−x−yCo_xMn_yO₂ have been intensively investigated as high-energy density cathodes [3], [4], [5], [6]. Considering the higher specific capacity and lower cost, LiNi_0.6Co_0.2Mn_0.2O₂ is considered to be one of the most promising cathode materials for next generation of commercial materials [7], [8]. Unfortunately, this material still has significant drawbacks. Due to the similar radius of Li⁺ (0.76 Å) and Ni²⁺ (0.69 Å), Ni²⁺ ions would occupy the sites in Li layer to form non-stoichiometric structures, known as cation mixing. It causes various problems including capacity loss, structural deterioration and block the pathway of lithium diffusion [9]. The drawback should be overcome before LiNi_0.6Co_0.2Mn_0.2O₂ realizes the commercial application [10].

These drawbacks could be greatly mitigated by substituting other ions such as Mg²⁺ [11], [12], Al³⁺ [13], Cr³⁺ [14], [15], Na⁺ [16]. Among them, Mg is one of the most attractive doping elements. Due to the similar ionic radius of Mg²⁺ ion (0.72 Å) with Li⁺ (0.76 Å) ion, it can occupy both Li sites and transition metal sites. On the one hand, the Mg²⁺ ions substitution for transition metal ions can reduce cation mixing. Furthermore, Mg²⁺ ion plays in a role in screening the O^2--O^2- repulsions during the charge, thus preventing the interslab collapse [17]. For instance, Pouillerie et al. reported that the substitution of Mg²⁺ ions could enhance the electrochemical performance in the LiNi_1−xMg_xO₂ system, due to the less structural collapses during cycling [18]. Sun et al. proposed that the structural stability and cycling behavior are improved by a small amount of Mg substitution for Ni in the Li [Li_0.15Ni_0.275Mn_0.575]O₂ system [19]. Woo et al. proposed that Mg incorporation into LiNi_0.8Co_0.1Mn_0.1O₂ can enhance electrochemical and thermal properties [20]. Huang et al. reported that Mg could suppress the phase transition and improve electrochemical properties at higher upper cutoff potentials [21]. What's more, Co is the most expensive and toxic functional element in cathode materials, it should be replaced by low cost and environmental-friendly elements. Therefore, it is expected that a partial substitution of Co with Mg would have a positive effect to improve battery performances.

In this paper, aiming to understand the impact of partial substitution of Co with Mg in LiNi_0.6Co_0.2Mn_0.2O₂, the effects of substitution Mg for Co on the morphology structure, surface chemical states and electrochemical performance of the materials were investigated systematically.

Section snippets

Synthesis of materials

LiNi_0.6Co_0.2-xMn_0.2Mg_xO₂ (x = 0, 0.01, 0.03, 0.05) cathode materials have been prepared by co-precipitation and high-temperature solid state method. First, the precursors of (Ni_0.6Co_0.2-xMn_0.2Mg_x) (OH)₂ were synthesized by typical hydroxide coprecipitation method. An aqueous solution of 1 M NiCl₂•6H₂O, CoCl₂•6H₂O, MnCl₂•4H₂O and MgCl₂•6H₂O (molar ratio of Ni: Co: Mn = 0.6: 0.2-x: 0.2: x) was added into a continuously stirred flask using a peristaltic pump under Ar atmosphere. At the same time,

Results and discussion

The chemical compositions of prepared samples are determined by ICP-AES, and the results are shown in Table 1. It is noted that the chemical composition for each element is close to the target values. Fig. S1 shows SEM images of the as-prepared samples. It shows no significant differences in particle size and morphology, indicating that the Mg-doping has no effect on the particle agglomerations.

The structures were characterized using XRD. Fig. 1 shows the XRD patterns of the prepared samples.

Conclusion

LiNi_0.6Co_0.2-xMn_0.2Mg_xO₂ (x = 0, 0.01, 0.03 and 0.05) materials have been synthesized via hydroxide co-precipitation method. The Rietveld refinements of XRD data indicate that the introduce of Mg into host lattice enlarges the inter-slab distance and reduces the Li/Ni disorder, which is in favour of the diffusion of the Li⁺ ion. Electrochemical tests results suggest that the Mg-doped samples deliver a higher capacity retention and excellent rate capability. GITT results confirm that Mg

Acknowledgments

This work was supported by the National Basic Research Program of China (973 Program, 2014CB643406).

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Citation Excerpt :
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Influence of Mg2+ doping on the structure and electrochemical performances of layered LiNi0.6Co0.2-xMn0.2MgxO2 cathode materials

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Synthesis of materials

Results and discussion

Conclusion

Acknowledgments

J. Power Sour.

J. Alloy. Compd.

Electrochim. Acta

J. Alloy. Compd.

Electrochim. Acta

Electrochim. Acta

J. Alloy. Compd.

J. Alloy. Compd.

J. Alloy. Compd.

J. Alloy. Compd.

Electrochim. Acta

Ceram. Int.

J. Alloy. Compd.

Electrochim. Acta

J. Solid State Electrochem.

The Li-ion rechargeable battery: a perspective

J. Am. Chem. Soc.

Influence of Mn content on the morphology and electrochemical performance of LiNi1-x-yCoxMnyO2 cathode materials

J. Mater. Chem.

Influence of Mg²⁺ doping on the structure and electrochemical performances of layered LiNi_0.6Co_0.2-xMn_0.2Mg_xO₂ cathode materials

Influence of Mn content on the morphology and electrochemical performance of LiNi_1-x-yCo_xMn_yO₂ cathode materials