Journal of Nanomaterials

Volume 2012 (2012), Article ID 743236, 7 pages

doi:10.1155/2012/743236

Research Article

Surface Modification of LiMn₂O₄ for Lithium Batteries by Nanostructured LiFePO₄ Phosphate

B. Sadeghi, R. Sarraf-Mamoory, and H. R. Shahverdi

Department of Materials Engineering, Tarbiat Modares University, P.O. Box 4838-141, Tehran, Iran

Received 17 May 2012; Revised 31 August 2012; Accepted 31 August 2012

Academic Editor: Christian Brosseau

Copyright © 2012 B. Sadeghi et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

LiMn₂O₄ spinel cathode materials have been successfully synthesized by solid-state reaction. Surface of these particles was modified by nanostructured LiFePO₄ via sol gel dip coating method. Synthesized products were characterized by thermally analyzed thermogravimetric and differential thermal analysis (TG/DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDX). The results of electrochemical tests showed that the charge/discharge capacities improved and charge retention of battery enhanced. This improved electrochemical performance is caused by LiFePO₄ phosphate layer on surfaces of LiMn₂O₄ cathode particles.

1. Introduction

Since the birth of the lithium ion battery in the early 1990s, its development has been very fast and it has been widely applied as power source for a lot of light and high value electronics due to its significant advantages over traditional rechargeable battery systems [1, 2]. Due to its low cost and low toxicity, the spinel LiMn₂O₄, the cathode for Li-ion batteries, has been extensively investigated. The spinel LiMn₂O₄ has a cubic structure with the space group of Fd3m symmetry in which lithium and manganese ions occupy tetrahedral (8a) sites and octahedral (16d) sites, respectively, within a cubic close packed oxygen array with oxygen ions in 32e sites. Many reports revealed that the spinel LiMn₂O₄ offers a potentially attractive alternative to the presently commercialized LiCoO₂. However, a key problem prohibiting LiMn₂O₄ from commercialization is its severe capacity and cycling performance fading during cycling [3, 4]. Several factors cause capacity fade of Spinel LiMn₂O₄, as it had been reported by some investigators [5–11].

(1) Dissolution of Mn³⁺. At the end of discharge, the concentration of Mn³⁺ arrives at its highest level. Meanwhile, after cycling or storage, the surface of LiMn₂O₄ is rich in Mn³⁺, contrary to the bulk structure. The Mn³⁺ at the surface may disproportionate according to the following reaction:

then, Mn²⁺ ions from this reaction dissolve in the electrolyte solutions.(2) Jahn Teller effect. At the end of discharge, the Jahn Teller effect happening at first on the surface of some particles may expand into an overall composition of Li[1+δ]Mn₂O₄. Thermodynamically speaking, this system is not really at equilibrium. The phase transition from a cubic into a tetragonal symmetry is a first order process. Even though this kind of distortion is small, it is big enough to destroy the structure to form a tetragonal structure, which is low in symmetry and high in disorder.(3) In organic solvents, the highly delithiated particles are not stable at the end of discharge; namely, the high oxidation ability of Mn⁴⁺ will lead to a decomposition of the solvents.

Synthesis approaches of the spinel LiMn₂O₄ are various, including solid reaction [12–14], sol-gel reaction [15, 16], and Pechini process [17]. However, specific capacity of the LiMn₂O₄ seems to be independent of the synthesis approaches, which typically is around 120 mAh/g between 3.5 and 4.3 V. Compared to sol-gel and Pechini approaches, in which additional starting materials and synthetic procedures are generally needed for preparation and separation of the targeted Li-Mn precursors, solid reaction is simpler and easier in being handled. Therefore, in this work we selected solid reaction to synthesize spinel LiMn₂O₄.

Recent research demonstrated that the importance of surface structural features of electrode materials for their electrochemical performance so, an effective strategy, coating the spinel LiMn₂O₄ with organic and inorganic compounds, has been investigated. Jiang et al. [18] coated LiMn₂O₄ spinel with 2 wt. %Li-M-PO₄ (M = Co, Ni, Mn) and improved the discharge test by showing the cycling and rate capacities of the spinel LiMn₂O₄ cathode materials. LiFePO₄ due to its potentially low cost, environmental benignness, and the belief that it could have a major impact in electrochemical energy storage, is the subject of many researches. Also, it can be a good candidate for improving electrochemical properties of conventional cathodes like LiMn₂O₄.

In this study, we coated the LiMn₂O₄ cathode with nanostructured LiFePO₄ layer. The modified LiMn₂O₄ can be protected from Mn dissolution, as the LiFePO₄ nanostructure is formed on the surface of the spinel LiMn₂O₄ cathode. The cycling and rate capacity of LiMn₂O₄ cathode materials were significantly enhanced by stabilizing the electrode surface with LiFePO₄ nanostructure.

2. Experiment

2.1. Synthesis of LiMn₂O₄

Spinel LiMn₂O₄ powder was prepared by a solid-state reaction. All starting materials for the synthesis of LiMn₂O₄ were purchased from Merck Company. To prepare this spinel, first manganese oxide (II) and lithium hydroxide with molar ratio of 7 : 3 were mechanically mixed. To achieve a more homogenous mixture and lowering calcification time, the mixture was prepared using a planetary mill with higher power, ball to power ratio of 10 : 1, and rotation speed of 200 rpm. Milling was stopped after 15 min and the obtained powder was heated in an electrical tube furnace for 5 hours. The hearting rate of the furnace was 10°C min⁻¹. It must be mentioned that the other parameters of the mill was supposed as constant.

2.2. Synthesis of LiFePO₄ Particles and Nanostructure Layers

Sol-gel synthesis is a low temperature, wet chemical approach, which is often used for the preparation of metal oxides or especially thin film. Standard sol-gel synthesis involves the formation of a sol, that is, a stable colloidal suspension of solid particles in a solvent and the gelation of the sol to form a gel consisting of interconnected rigid skeleton with pores made of colloidal particles. The properties of the gel are determined by the particle size and cross linking ratio. The gel can then be dried to form xerogel, which shows reduced volume. To obtain the final products, all liquids need to be removed from the surface of pores by a heat treatment carried out at elevated temperatures [19].

As X-ray diffraction peaks of LiFePO₄ thin film layers cannot be detected due to low-weight percentage, first we synthesized phosphate powders by sol gel process to determine optimum synthesis condition. Stoichiometric amounts of lithium phosphate (Li₃PO₄, Aldrich 33,889-3) and phosphoric acid (H₃PO₄, Merck, 100563) were dissolved in 200 mL water by stirring for 60 min. Separately, citrate iron (III)(Merck, S3657400 219) was dissolved in 300 mL of water by stirring for 60 min.The two solutions were mixed together and obtained a transparent gel which was dried at 70°C for 30 h at atmosphere of 99.999% argon. After grinding with a mortar and pestle, the obtained materials were calcined for 1 h. Calcination temperature was measured using thermogravimetry and differential thermal analysis (TG/DTA) result of the dried gel. The heating rate was 10°C min⁻¹. Same sol was used for sol gel dip coating of samples. Three layers of phosphate coat were applied with drawl speed of 10 mm/min, remaining time of 2 min and up speed of 2 mm/min. After each coating step, the sample was dried 70°C for 30 h. Final coated samples were calcined at 670°C for 1 h. Reaction conditions used for preparing different sample of LiMn₂O₄ electrodes and LiFePO₄ sol were listed in Table 1.

Table 1: Reaction conditions for the preparation of LiMn₂O₄ and LiFePO₄ sol.

Reaction conditions for the preparation of LiMn₂O₄ electrode
Reaction conditions for the preparation of LiFePO₄ sol

Material used
Mn : Li ratio
Sample
Reaction temp (°C)
Reaction time (hrs)
Li precursor
Fe precursor
P precursor
Solvent
Molar ratio

MnO₂ + LiOH
7 : 03
A
900
5
LiOH·H₂O
C₆H₅FeO₇·2H₂O
H₃PO₄
Deionized water
1 : 03 : 02

B
800

C
500

D
200

2.3. Structural and Morphological Characterization of Synthesized LiMn₂O₄ Powder and LiFePO₄ Sol

The thermal decomposition behavior of the gel was examined with a thermogravimetric analyzer (TGA, Perkin Elmer, TAC 7/DX) under N₂ flow. Structural analysis of the obtained products was carried out using an XRD instrument (Philips Expert) with radiation source of Cu-Kα. The surface morphology and energy dispersive spectrometry (EDAX) of the coated particles were taken with a SEM (Philips XL30) microscope. The cyclic voltammogram properties of LiFePO₄-coated LiMn₂O₄ sample were teted.

2.4. Electrochemical Measurements

Charge and discharge diagrams and cyclic performance were conducted using the AUTOLAB-302 machine. In order to produce LiMn₂O₄ cathode with coated LiFePO₄, first synthesized LiMn₂O₄ as well as carbon black and PTFE binder was mixed together with ratio of 85 : 15 : 5 and was placed in a nickel mesh as current collector. Then, using the dip coating method this mesh was placed in the LiFePO₄ sol to contaminate the surface of materials in the mesh. At the end, the mesh was heated at optimum calcination temperature for 1 h in Ar atmosphere. For the negative electrode, the graphite as well as carbon black and PTFE binder was mixed together with ratio of 85 : 15 : 5 and was placed in a nickel mesh as current collector. The used electrolyte was a 1 M solution of LiClO₄ in EC: DMC (1 : 1 ratio by volume). Charge/discharge and cyclic voltammetry experiments were carried out in a two-electrode glass cell that was built for this purpose. It must be noted that the battery assemble as well as all electrochemical experiments was performed within the glove box at the presence of Ar atmosphere.

3. Results and Discussion

Figure 1 presents DTA and TG diagrams produced for LiFePO₄ gel from sole-gel process. The figure exhibits three distinct weight loss zones. At the first step, a weight drop of 6.4% was observed which is due to evaporation of the physically absorbed water and degradation of organic components with the sole. The second loss in within the temperature range of 200 to 500°C, where weight drop is about 33.4% which can be due to release of water during the crystallization as well as pyrolysis of citrate and other organic components. The last weight loss of the given gel will gradually initiate from the temperature of 522°C and continue up to 800°C. The weight drop in this zone is 11% which is due to pyrolysis of remained organic components. All events during the heating of LiFePO₄ gel in zones 1 and 2 took place at the temperatures of 132 and 372°C, respectively. These reactions are endothermic, while events occurred in zone 3 are at the temperature of 653°C and are exothermic. So we selected the temperature 670°C as calcination temperature.

Figure 1: TG and DTA diagrams of gel LiFePO₄ at the range of room temperature to 850°C in N₂ atmosphere with heating rate of 10 mL/min.

Figure 2 shows the progression of the reaction between MnO₂ and LiOH mixed in a 7 : 3 ratio from 200°C to 900°C. As can be seen, the peaks of LiMn₂O₄ without any impurities have seen in 800°C. Also, MnO₂ and Mn₂O₃ impurities can be exist at 200°C and 500°C, respectively. The parameters like insufficient temperature and time, decomposition of unreacted MnO₂ can be reasons such impurities, respectively. On the other hand, the XRD patterns at 800°C show clearly the characteristic peaks of the spinel LiMn₂O₄ structure, for example, the (111), (311), (222), (400), (331), and (511) peaks. When the heating temperature exceeds 800°C, some characteristic peaks of Mn₃O₄ appear due to excess manganese oxide present in original sample. Given these observations, it could be deduced that with increasing heating temperature, the crystallites of the spinel LiMn₂O₄ grow and become ordered and at 800°C a well-ordered spinel structure LiMn₂O₄ is produced.

Figure 2: Powder XRD patterns of LiMn₂O₄ products after reaction at various temperatures.

Figure 3 displays XRD pattern for uncoated LiMn₂O₄, phosphate components of LiFePO₄, and LiMn₂O₄ coated by phosphate components of LiFePO₄. The lattice constants of bare LiMn₂O₄ and modified LiMn₂O₄ were calculated from the XRD spectrums which are 0.821 and 0.823 nm, respectively. The modified samples have a larger lattice constant than bare spinel, According to Hu et al. [20], LiFePO₄ may form not only a thin layer on the surface of spinel but a solid solution by interacting with spinel. In addition, the decrease in the lattice parameter of the modified sample may be due to substitute iron ion with manganese. The increase of lattice constant can make the crystal structure more stable, leading to the stable cycling performance of LiFePO₄-modified LiMn₂O₄ during charge/discharge process. Figure 4 shows the surface and cross-section SEM images of LiFePO₄ thin film prepared by dip coating sol gel deposited on LiMn₂O₄ substrate at 670°C under argon atmosphere with the thickness of about 300 nm. As shown in Figure 4, average grain size of LiMn₂O₄ is about 100 nm. Figures 4(b) and 4(c) show SEM and TEM images of LiFePO₄ coating. As can be seen in Figures 4(b) and 4(c), the thickness of film is about 300 nm. It is interesting to note if the oxide coating is too thin, it may fail to protect the cathode material over long electrochemical cycling; if the oxide coating is too thick, the capacities of the cathode material may be reduced because the oxide coating has higher electronic resistance and higher diffusion resistance for lithium ions [21].

Figure 3: XRD pattern of synthesized products uncoated bare LiMn₂O₄, LiMn₂O₄ coated with LiFePO₄.

(a)

(b)

(c)

Figure 4: SEM image (a) surface image, (b) cross-section image of LiFePO₄ thin film deposited on LiMn₂O₄ substrate, (c) TEM image of the LiFePO₄ nanoparticle.

So, based on Figure 4(c) we can conclude that the particle size of LiFePO₄ is less than 100 nm, and LiFePO₄ film is nanostructure.

Figure 5 shows EDX analysis of the LiFePO₄ nanostructure film. Each peak on the spectrum represents a transition with a characteristic energy. Every element has its own “fingerprint” of peaks so we can deduce that it is a represent. The P element can be clearly observed on the surface of modified materials.

Figure 5: EDX analysis of LiFePO₄ nanostructure film.

The main difference of this study is applying nanostructured LiFePO₄ coatings directly on cathode surface instead of surface of cathode particles. Figure 6 shows the initial discharge profile with c-rate performances of the bare LiMn₂O₄ and modified by LiFePO₄ nanostructure in the voltage range of 3.4–4.3 V. Two plateau features of LiMn₂O₄ can be seen in all curves which is the special characterization of lithium manganese dioxide. Figure 7 shows performance of the bare LiMn₂O₄ which has been modified by LiFePO₄ nanostructure. This cell was tested for 100 cycles at a constant current density of 120 mAh/g (1 C rate: 1 C rate means that the cell has been charged for 1 h upto 4.3 V) and the voltage between 3.0 and 4.3 V. It is clear that surface modification significantly improves the cyclability of LiMn₂O₄. Both samples show initial discharge capacity of more than 115 and 125 mAh g⁻¹, respectively. It can be accepted that presence of LiFePO₄ phosphate layer leads to a better electrical conductivity of the LiMn₂O₄ particles as well as lowering or hindering the additional reactions of cathode materials with the electrolyte. All in all, this not only improves the cyclic capacity of the battery at the higher rates, but also results in higher discharge rates of cathode materials of LiMn₂O₄ coated with phosphate layer of LiFePO₄.

Figure 6: Initial discharge capacities for LiFePO₄-coated LiMn₂O₄, and pristine LiMn₂O₄.

Figure 7: Cycle properties of LiMn₂O₄ modified by LiFePO₄ nanostructure.

Figure 8 shows the cyclic voltammogram (CV) of the LiMn₂O₄ and LiFePO₄-coated LiMn₂O₄ electrodes. The voltammetric traces were recorded at a scan rate of 0.05 mVs⁻¹, and lithium served as both the counter and reference electrodes. The bare LiMn₂O₄ and LiFePO-coated LiMn₂O₄ materials exhibited typical voltammograms with different kinetic properties. The CV curves of these samples give two pairs of well-separated reversible oxidation/reduction peaks at around 3.98 and 4.13 V, which are attributed to the splitting of the original Li position in tetrahedral sites (8a). An extra peak at 3.4 V has been observed only for modified one. The presence of the shoulder like appearance at around 3.4 V during the cathodic and anodic sweep corresponded to one of the electrochemical characterizations of LiFePO₄ and confirmed the presence of the LiFePO₄ layer. [22]. Additionally, the appearance of the dual oxidation and reduction peak corroborated to the coexistence of the LiMn₂O₄ phase during cathodic and anodic sweep. From these results, the thin LiFePO₄ layer was strongly believed to hamper the Mn dissolution during lithiation, improving the electrochemical performance of LiFePO₄-coated LiMn₂O₄.

Figure 8: Cyclic voltammograms for bare LiMn₂O₄ and LiFePO₄-coated LiMn₂O₄.

4. Conclusions

Effects on surface modification of the spinel LiMn₂O₄ by a LiFePO₄ nanostructure as a cathode for Li-ion batteries have been studied as an approach to investigate the electrochemical performance. The results of electrochemical tests showed better capacity retention due to coating layer on the surface of bare LiMn₂O₄ material.

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