Elsevier

Solid State Ionics

Volume 300, February 2017, Pages 18-25
Solid State Ionics

Synthesis and improved electrochemical performance of LiMn2  xGdxO4 based cathodes

https://doi.org/10.1016/j.ssi.2016.11.026Get rights and content

Highlights

  • Spinel LiMn2O4 powders doped with gadolinium were prepared by sol-gel method.

  • The effect of gadolinium doping content (x = 0.01, 0.04) is reported.

  • A gadolinium doping content of x = 0.04 improves the cycling performance of LiMn2O4.

Abstract

LiMn2  xGdxO4 (x = 0.00, 0.01 and x = 0.04) spinel active material were prepared by sol-gel method. The morphology, crystal structure, thermal stability and electrochemical properties of the synthesized samples were evaluated. Gd-doping maintains the cubic spinel structure of LiMn2O4, leads to a decrease of the average particle size, and has a strong influence on the electrochemical properties of the cathodes. LiMn2  xGdxO4 cathode materials show two pairs of separated redox peaks, independently of the Gd-doping. After 40 charge-discharge cycles at C/2, the discharge capacity was 55.9 mAh·g 1 and 33 mAh·g 1 for x = 0.04 and x = 0, respectively, corresponding to a capacity retention of 80% and 70%, respectively. The Gd-doped with x = 0.04 showed improved cycling performance with respect to pristine LiMn2O4 due the smaller particle size and superior structural stability, showing its suitability for lithium-ion battery applications.

Introduction

The fast growth in portable electronics, plug in hybrid electric vehicle (PHEV) and electric vehicle (EV) markets lead to need for improved energy storage systems in terms of storage capacity and power [1], [2], [3]. In this context, lithium-ion batteries are the major energy storage system for many such applications [4], [5], [6]. High cost and weight of batteries for large scale applications has been a challenge for the scientific community. In this regard, significant advancements have been made in Ni-Cd batteries [7], [8] but these batteries did not get momentum because of cadmium toxicity for large scale applications.

In brief, there are three main components of a typical lithium-ion battery: anode, cathode and electrolyte/separator [7], [8]. The cathode is one of the key component as it determines the cell capacity and cycle life and has been focal point of research. Cathode is constituted by three chemical entities, active material, conductive agent and binder where each component has its specific characteristics and role. The active material acts as lithium reservoir and is typically based on transition metal oxides and phosphates, the binder has the role to binding active material and conductive additive, used to enhance electrical conductivity of electrode [9].

For active cathodes materials, impressive advances have been made using 3d transition metal oxides and phosphates, such as, lithium manganese dioxide (LiMn2O4) [10], [11], lithium cobalt oxide (LiCoO2) [12], [13], lithium nickel oxide (LiNiO2) [14], lithium nickel cobalt manganese oxide (LiNixCoyMnzO2)) [15] and lithium transition metal phosphates (LiMPO4, M = Mn, Co, Ni and Fe) [16], [17]. Out of various materials have been investigated, LiMn2O4 is a material of choice because of high thermal stability, high operating voltage (> 4 V), low cost, and wide availability without any significant environmental concerns [10], [18], [19]. The Lithium Manganese Oxide (LMO) has cubic spinel structure with Fd3m space group where oxygen ions occupy the 32e position, Mn3 + and Mn4 + ions are located at the 16d site and Li at the 8a site [18]. Despite various significant advantages, the capacity fading and durability at high voltage have been a matter of concern [20], [21]. The capacity fading related to multiple chemical and physical processes occurs at wide range of voltage region such as poor stability of organic-based electrolyte, manganese leaching and structural inhomogeneity in spinal framework, formation of solid electrolyte interface (SEI), and Jahn-Teller distortion by Mn+ 3 at octahedral sites [22], [23], [24]. Another important characteristic of LMO is its electrochemical performance, which depend on size, shape, homogeneity and stoichiometry of material, and can be improved using appropriate dopants (metals: Mg, Ti, Fe, Ni, etc.; nonmetals: B, F, S, Br; rare-earth: La, Ce, Pr, Nd, Gd) that stabilize the spinal structure and reduce the capacity fading by enhancing the charge transport [20], [25].

Apart from various dopants reported, rare earth (RE) metals are most promising with a spinal structure LiMn2  xRExO4. It is expected that a controlled amount of RE will bring stability to spinal structure by holding Mn+ 3 and remove JT distortion via loosely bound electron cloud [26], [27], as observed in Gadolinium-doped LMO, LiMn2  xGdxO4, where unit cell parameters significantly changed, results in improved high-rate performance and decreased capacity fading [22]. Important initial reports on RE doped LMO, where LiMn1.99Gd0.01O4 has been synthesized by a molten salt route with a large particle size between 5 and 10 μm [23] did add some value but failed to draw attention because of cumbersome synthetic process at large scale with precise doing. Other reports also followed similar process with other rare-earth elements (Dy, Gd, Tb and Yb) with doping content, supported that Gd doping contributed towards the improvement in cycling performance of LMO [24]. Despite handful reports about RE contributions in Lithium batteries, state of art in controlled doping and a detailed understanding of mechanism of doping behavior is still missing from literature.

Herein, we are presenting the state of art on Gd-doping in LMO, the main objective of this work is to provide a new method for the synthesis of LiMn2  xGdxO4 and to optimize the Gd content in the spinel structure. The current investigation would open doors of new further studies in lithium ion battery research as large scale.

Section snippets

Synthesis of LiMn2  xGdxO4

The preparation of the active materials powders is based on the procedure reported in [24] that is schematically represented in Fig. 1.

In short, the spinel cathode active LiMn2  xGdxO4 materials (x = 0.00, 0.01 and 0.04) were synthesized via sol gel route, using lithium acetate; CH3COOLi; 99.9%, manganese acetate; (CH3COO)2Mn; 99.5% purity, and gadolinium acetate; (CH3COO)3Gd; 99.99%, as precursor materials. All materials were of analytical grade and obtained from Alfa Aesar, USA. The precursor

Cathode active material powder characterization

The morphology of the synthesized LiMn2  xGdxO4 powders are shown in Fig. 2 for x = 0.00 (Fig. 2a), x = 0.01 (Fig. 2c) and x = 0.04 (Fig. 2e), respectively.

Independently of the rare-earth doping content, the resulting active materials show an inhomogeneous truncated octahedral morphology with irregular shape, size and dispersion (Fig. 2).

The particle average size determined from the SEM images is 0.88 ± 0.36 μm for the x = 0.00 particles (Fig. 2a), 0.95 ± 0.25 μm for the x = 0.01 ones (Fig. 2c) and 0.33 ± 0.05 μm

Conclusions

Gadolinium doped samples LiMn2  xGdxO4 (x = 0.00, 0.01 and 0.04) have been successfully synthesized by a sol-gel method. Increasing Gd doping decreases the size of the active material, does not affect the formation of the cubic spinel structure but leads to variations of the overall impedance of the cathodes prepared from these active materials.

The cathodes prepared with the LiMn2  xGdxO4 active materials show charge-discharge curves with significant higher discharge capability at room temperature

Acknowledgment

The authors thank FEDER funds through the COMPETE 2020 Programme and National Funds through FCT - Portuguese Foundation for Science and Technology under Strategic Funding UID/FIS/04650/2013. A. G., C.M.C. and S. F. thank the FCT for grants SFRH/BD/90313/2012, SFRH/BPD/112547/2015 and IF/01516/2013, respectively. The authors thank funding from the FCT under the Indo-Portuguese program of cooperation in science and technology grant INT/Portugal/P-02/2013, 2014–2016. The authors are thankful for

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