Electrochemical performance of

SrF₂-coated LiMn₂O₄ cathode material for Li-ion batteries

LI Jian-gang(李建刚)¹, HE Xiang-ming(何向明)², ZHAO Ru-song(赵如松)¹

1. School of Chemical Engineering, Beijing Institute of Petrochemical Technology, Beijing 102617, China;

2. Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing100084, China

Received 15 July 2007; accepted 10 September 2007

Abstract: SrF₂-coated LiMn₂O₄ powders with excellent electrochemical performance were synthesized. The electrochemical performance of SrF₂-coated LiMn₂O₄ electrodes was studied as function of the level of SrF₂ coating. With increasing the amount of the coated-SrF₂ to 2.0% (molar fraction), the discharge capacity of LiMn₂O₄ decreases slightly, but the cycleability of LiMn₂O₄ at elevated temperature is improved obviously. In view of discharge capacity and cycleability, the 2.0% (molar fraction) coated sample shows optimum cathodic behaviors. When being cycled at 55 ℃, as-prepared LiMn₂O₄ remains only 79% of its initial capacity after 20 cycles, whereas the 2.0% (molar fraction) coated sample shows initial discharge capacity of 108 mA?h/g, and 97% initial capacity retention.

Key words: lithium ion batteries; LiMn₂O₄; surface coating; SrF₂

1 Introduction

Spinel LiMn₂O₄, with economical and environmental advantages, is considered to be one of the most promising cathode materials for rechargeable lithium-ion batteries. Its drawbacks include the significant capacity fading during cycling at elevated temperature due to several probabilities, e.g., manganese dissolution[1-2], Jahn-Teller distortion[3] and electrolyte decomposition[4]. Partial replacement of Mn in LiMn₂O₄ by some low valent metal ions such as Li⁺, Ni²⁺, Co³⁺, Al³⁺ and Cr³⁺[5-9] can effectively enhance its cycleability. On the other hand, another appropriate method to reduce capacity fades of LiMn₂O₄ is surface coating of the spinel to avoid Mn dissolution. Metal oxides, such as lithium boron oxide glass (LBO)[10], SiO₂[11], ZnO[12], ZrO₂[13], Al₂O₃[14] and TiO₂[15], have been reported to be very interesting materials for this purpose. The coated metal oxides are insoluble in water, but cannot remain stable unfortunately under the corrosion of HF. Recently, SUN et al[16] reported the surface modification of LiCoO₂ by AlF₃. The AlF₃ coating layer could reduce the formation of LiF films that increase cathode/electrolyte interfacial impedance and also suppress Co dissolution by covering LiCoO₂ surface from HF attack, therefore improving the capacity retention of LiCoO₂ at 4.5V cut-off cycling.

It is very necessary to study the effects of fluoride-coating on the cycleability of LiMn₂O₄. To our best knowledge, there are still no literatures on this issue. In this study, SrF₂ that is insoluble in HF was investigated to be used as coating material of LiMn₂O₄. Herein, the effects of coated-SrF₂ amount on the structural and electrochemical properties were discussed.

2 Experimental

LiMn₂O₄ sample was first prepared by annealing the well-ground mixture of stoichiometric EMD and Li₂CO₃ at 800 ℃ in air for 20 h. For preparing the SrF₂-coated LiMn₂O₄ cathode material, the prepared LiMn₂O₄ powders were suspended in the Sr(NO₃)₂ solution, and then NH₄F solution was dropped slowly into the suspended solution. After being stirred for 2 h, the coated LiMn₂O₄ powders were filtrated and heated under N₂ for 5 h. The heat treatment temperature was 300, 400, 500, 600 and 700 ℃, respectively. The amount of coated SrF₂ corresponds to 0.5%, 1.0%, 2.0% and 4.0% (molar fraction) of the LiMn₂O₄ powders. Powder X-ray diffraction (XRD7000) was used to characterize the structure of the powders. Particle morphology of the powders was observed using a scanning electron microscope (HITCHI-s3500). The electrochemical characterization was performed using CR2032 coin-type test cells. The cell consisted of a cathode with the composition of 88% coated or uncoated LiMn₂O₄, 6% carbon black, and 6% PVDF (mass fraction), a lithium metal anode separated by a Celguard 2400 microporous film. The electrolyte was 1.15 mol/L LiPF6/EC+DMC+ DEC (3?3?1 in volume ratio). The charge-discharge tests were galvanostatically performed over 3.3-4.3 V at different current densities. AC-impedance measurements were performed using a CHI660B impedance analyzer over the frequency range from 100 kHz to 1 mHz with the amplitude of 10 mV.

3 Results and discussion

3.1 Structural variation after SrF₂-coating

XRD patterns of the 4.0% SrF₂-coated LiMn₂O₄ sample heat-treated at 300, 400, 500, 600 and 700 ℃ respectively for 5 h under N₂ are shown in Fig.1. It is obvious that all samples show main peaks attributed to well-defined spinel phases and small amount of SrF₂ peaks as impurity. The SrF₂ peaks become sharp with increasing the heat-treated temperature from 300 to 700 ℃. This indicates that the coated SrF₂ exists as a separate phase, and SrF₂ crystallization extent enhances with increasing heat-treated temperature. The crystal lattice constants of the coated samples, as listed in Table 1, clearly reveal that the values remain almost unchanged at 300-500 ℃ and decrease obviously over 600 ℃. This indicates a possible surface reaction between SrF₂ and LiMn₂O₄ during firing over 600 ℃, which may result in the formation of a thin LiMn_2-xSr_xO_4-yF_y solid-solution layer. In order to avoid the reaction between SrF₂ and LiMn₂O₄, the heat-treated temperature of the following studied SrF₂-coated LiMn₂O₄ samples are selected to be 400 ℃.

Fig.1 XRD patterns of 4.0% SrF₂-coated LiMn₂O₄ samples

Table 1 Lattice parameters of 4.0% SrF₂-coated LiMn₂O₄ sample heat-treated at different temperatures

XRD patterns of 0, 0.5%, 1.0%, 2.0% and 4.0% SrF₂-coated LiMn₂O₄ samples are shown in Fig.2. The SrF₂ peaks can be detected when the coated SrF₂ amounts are up to 2.0%, and become stronger with increasing the coated SrF₂ amounts to 4.0%. However, the lattice parameters of LiMn₂O₄ show no detectable changes before and after coating, supporting the postulate that SrF₂ exists as a separate phase on the LiMn₂O₄ particle surface.

Fig.2 XRD patterns of SrF₂-coated LiMn₂O₄ samples

3.2 Morphology of SrF₂-coated LiMn₂O₄

Fig.3 shows SEM micrograph and EAX(energy dispersed X-ray analysis) image of 2.0% SrF₂-coated LiMn₂O₄. The powders are agglomerated, consisting of many small crystal particles. As can be seen in EPMA image, the distribution of Sr on the powder is fairly uniform.

3.3 Electrochemical characteristics of SrF₂-coated LiMn₂O₄

Fig.4 shows charge-discharge curves for the as- prepared LiMn₂O₄ and various amounts of SrF₂-coated

Fig.3 Scanning electron micrograph of 2.0% SrF₂-coated LiMn₂O₄ (a) and Sr-distribution map obtained by energy dispersed X-ray analysis (b)

Fig.4 Charge-discharge curves of SrF₂-coated LiMn₂O₄ samples

LiMn₂O₄ at room temperature. The as-prepared LiMn₂O₄ shows discharge capacity of  114.6 mA?h/g, which is very close to the data reported in Refs.[11-15]. Compared with the base one, the SrF₂-coated LiMn₂O₄ samples show lower capacity. The discharge capacity decreases slightly with increasing amount of SrF₂ coating to 2%, but reduces remarkably to 104.4 mA?h/g with increasing amount of the SrF₂ coating to 4%. With increasing amount of the SrF₂ coating, the polarization (voltage difference between the charge and the discharge curves) enhances. Because the SrF₂ coating is inactive, it can be speculated that the coating may increase the contact resistance between inter-particles and the charge-transfer resistance, leading to the decrease of the capacity. The supposition can be confirmed by the EIS spectra of the as-prepared LiMn₂O₄ and various amounts of SrF₂-coated LiMn₂O₄, as shown in Fig.5.

Fig.5 EIS spectra of SrF₂-coated LiMn₂O₄ samples

The cycleability of the spinels was tested at 55 ℃, using a voltage window of 3.3-4.3 V and a current density of 110 mA/g. The resulting data are plotted in Fig.6. When being charge-discharged for 20 cycles at 55 ℃, the as-prepared LiMn₂O₄ sample shows very fast capacity fading, and only 79% of its initial capacity remains. With increase of the amount of the coated SrF₂ from 0% to 2.0%, the discharge capacity of LiMn₂O₄ decreases slightly, but the cycleability of LiMn₂O₄ is improved obviously. As mentioned in other papers, surface coating of LiMn₂O₄ by some stable substances, e.g. SiO₂[11], ZrO₂[13] and Al₂O₃[14], can prevent the direct contact between LiMn₂O₄ particles and electrolyte, and hence lessen the possibility of spinel dissolution and electrolyte decomposition that are the major cause of capacity loss. The AlF₃ coating layer of LiCoO₂ also have been reported to reduce the formation of LiF films that increase the cathode/electrolyte interfacial impedance and also suppress Co dissolution by covering LiCoO₂ surface from HF attack[16]. SrF₂ is very stable even in HF, so it can be speculated that the surface coating of LiMn₂O₄ by SrF₂ can improve the cycleability of LiMn₂O₄ cathode due to the same reason. 2.0% coated sample exhibits the best electrochemical performance. This shows initial discharge capacity of 108 mA?h/g, and 97% initial capacity retention after 20 cycles. Although 4.0% coated sample also shows very good cycleability, its lower capacity is unsatisfied due to higher impedance resulted from too much coated SrF₂. The 2.0% coated sample shows optimum cathodic behaviors in view of discharge capacity and cycleability.

Fig.6 Cycleability of SrF₂-coated LiMn₂O₄ samples at 55 ℃

4 Conclusions

1) SrF₂ is coated uniformly on the surface of LiMn₂O₄ particles by using chemical deposition, followed by heat-treatment at 400 ℃.

2) Significant improvement of the electrochemical performance of LiMn₂O₄ is achieved by coating SrF₂. With increasing the amount of the coated SrF₂, the discharge capacity of LiMn₂O₄ decreases slightly, but the cycleability of LiMn₂O₄ at elevated temperature is improved obviously.

3) In view of discharge capacity and cycleability, the 2.0% coated sample shows optimum cathodic behaviors. It shows initial discharge capacity of 108 mA?h/g, and 97% initial capacity retention after 20 cycles at 55 ℃.

4) The SrF₂ coating is favorable to improving the cycleability of LiMn₂O₄ at elevated temperatures.

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(Edited by YANG Bing)

Foundation item: Project(KM200710017001) supported by the Scientific Research Common Program of Beijing Municipal Commission of Education, China

Corresponding author: LI Jian-gang; Tel: +86-13681208073; E-mail: lijiangang@bipt.edu.cn