J. Cent. South Univ. Technol. (2011) 18: 1844-1848
DOI: 10.1007/s11771-011-0912-2
Al2O3 coating for improving thermal stability performance of
manganese spinel battery
LIU Yun-jian(刘云建)1, 2, GUO Hua-jun(郭华军)2, LI Xin-hai(李新海)2
1. School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, China;
2. School of Metallurgical Science and Engineering, Central South University, Changsha 410083, China
? Central South University Press and Springer-Verlag Berlin Heidelberg 2011
Abstract: The synthesis of Al2O3-coated and uncoated LiMn2O4 by solid-state method and fabrication of LiMn2O4/graphite battery were described. The structure and morphology of the powders were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The electrochemical and overcharge performances of Al2O3-coated and uncoated LiMn2O4 batteries were investigated and compared. The uncoated LiMn2O4 battery shows capacity loss of 16.5% after 200 cycles, and the coated LiMn2O4 battery only shows 12.5% after 200 cycles. The uncoated LiMn2O4 battery explodes and creates carbon, MnO, and Li2CO3 after 3C/10 V overcharged test, while the coated LiMn2O4 battery passes the test. The steadier structure, polarization of electrode and modified layer are responsible for the safety performance.
Key words: LiMn2O4 battery; Al2O3 coating; overcharge test; cyclic performance
1 Introduction
The demand for high energy rechargeable batteries for electric vehicle application has accelerated the development of lithium ion battery materials. The important factors governing their applications are low price, high safety, high specific energy density and environment friendliness. Spinel type LiMn2O4 has attracted particular interest, because it is environmentally benign, inexpensive and abundant in its raw materials. However, our recent research result shows that the manganese spinel battery is not safe especially at abuse using, such as 3C/10 V overcharge test [1].
Extensive applications of lithium ion batteries generate increasing safety concerns and many studies are focused on the safety of lithium ion batteries [2-4], especially on overcharge [5-6]. There are several reports on a built-in and chemical prevention of the overcharge, namely, electrolyte additives [7-11]. But all the additives will affect the electrochemical performances especially the cyclic performance.
Surface modification was a groovy method to be used to improve the cyclic performance especially at elevated temperature [12-13], but the method using surface modification to improve the thermal stability, especially the overcharge test, is few. In this work, Al2O3 was coated on the surface of LiMn2O4 via heterogeneous nucleation process. The effect of Al2O3 content coated onto LiMn2O4 powders was investigated. The electrochemical and 3C/10 V overcharge performances of the uncoated and surface modified LiMn2O4 battery were characterized. The possible mechanism for the improvement in the electrochemical and safety property was presented.
2 Experimental
Spinel LiMn2O4 was synthesized by a solid-state method. Stoichiometric amounts of lithium carbonate (Li2CO3) and electrolytic manganese dioxide (MnO2, Aldrich) were firstly mixed together and ground in an agate mortar for 30 min, then the mixture was preheated at 500 °C for 5 h before being sintered at 750 °C for 10 h in air.
A heterogeneous nucleation process was used to prepare the 0.5% (mass fraction) Al2O3-coated LiMn2O4 materials. LiMn2O4 and aluminum nitrate (AlNO3·9H2O) were dissolved into distilled water and ethanol under stirring to form a clear solution. The pH of the solution was controlled at 5.0. Then the mixture was sintered at 400 °C for 10 h.
The X-ray diffraction (XRD, Rint-2000, Rigaku) measurement using Cu Kα radiation was employed to identify the crystalline phase of the synthesized LiMn2O4and exploded powders. The particle size and morphological properties of the prepared LiMn2O4 exploded powders were measured by scanning electron microscope (SEM, JEOL, JSM-5600LV) with an accelerating voltage of 20 kV.
Cyclic performance tests in charge-discharge were performed at 0.5C at room temperature with BK-7064 battery testing instrument. The voltage range was 3.0-4.2 V. Electrochemical impedance spectroscopy (EIS) of the cell was carried out using CHI660A (Chenghua, Shanghai). The amplitude of the AC signal was 5 mV over the frequency range between 100 kHz and 0.01 Hz.
A 325680-type size cell (10 A·h) was introduced for this study. The LiMn2O4 powders, carbon black, PVDF and NMP were mixed together in a high-speed mixer at a certain ratio, and then the viscous slurry was coated onto an aluminum foil current-collector and dried at 120 °C under vacuum for 24 h. The cathode was then pressed and divided into small patches. The cathode and anode were assembled through convoluting, then enclosed in stainless steel can, and dried under vacuum for 36 h. Celglard 2400 microporous membrane and 1.0 mol/L LiPF6 in mixture of EC, EMC, DMC (1:1:1, in volume ratio) were chosen as separator and electrolyte, respectively. Expressly, the semi-?nished batteries were charged/discharged in the voltage range of 3.0-4.2 V and 0.02C as cut-off current of charge. The eligible batteries were selected after two cycles.
The overcharge performance of 325680-type battery (10 A·h) was tested by the DC power supply. Current limiting or temperature trip safety devices (e.g. PTC) were not used in the experimental cells.
3 Results and discussion
3.1 XRD and SEM comparison analysis
X-ray diffraction patterns of pristine LiMn2O4 and Al2O3-coated LiMn2O4 are presented in Fig.1. All the diffraction peaks are confirmed to be well-defined spinel phase with the space group of Fd3m. No impurity phase is detected for Al2O3-coated LiMn2O4, which suggests that Al2O3 has modified only the surface of the active material without changing the crystal structure of the bulk active material. Diffraction peaks of LiMn2O4 shift to higher angle after coating (as shown in Fig.1(b)), indicating the shrinkage of crystal lattice. The shrink of lattice parameter for the Al2O3-coated spinel is due to the decrease in the amount of Mn3+ and the substitution of Al3+ which has a smaller ionic radius relative to Mn3+. The Al3+ may be doped onto the surface of LiMn2O4 and the Li-Al-Mn-O layer is formed during the sintered procedure. The stability of LiMn2O4 spinel structure is improved, which can be concluded from above results.
Fig.1 XRD patterns of pristine LiMn2O4 and Al2O3-coated LiMn2O4: (a) Full profiles; (b) Partial profiles (35°-40°)
Figure 2 shows the surface morphology changes of LiMn2O4 after coating with Al2O3. As shown in Fig.2(b), Al2O3 particles are observed on the surface of the Al2O3-coated LiMn2O4 after surface modification treatment. However, the small particles adhered to surface on the pristine LiMn2O4 disappear after coating.
It can be expected from the SEM results that the surface modification of LiMn2O4 with Al2O3 could decrease the direct contact area between spinel cathode material and electrolyte, accordingly prevent the electrolyte from decomposition and obstruct the spinel LiMn2O4 from dissolving into the electrolyte.
3.2 Electrochemical characterization
Figure 3 shows the charge/discharge curves of the pristine and Al2O3 coated LiMn2O4 performed at 25 °C. The initial discharge capacity of pristine is 105.4 mA·h/g. By contrast, the Al2O3-coated LiMn2O4 exhibits lower initial discharge capacity of 98.6 mA·h/g. The insulating Al2O3 on the LiMn2O4 surface decreases the total conductivity of the materials and hinders the extraction and insertion of Li ions through the interface during the first cycle, which results in the lower initial discharge capacity of the cathode.
Fig.2 SEM images of pristine LiMn2O4 (a) and 0.5% Al2O3- coated LiMn2O4 (b)
Fig.3 Discharge curves of LiMn2O4
The cyclic performance of the pristine and the Al2O3-coated LiMn2O4 is shown in Fig.4. It shows clearly that the capacity of the bare LiMn2O4 decreases faster than the Al2O3-coated LiMn2O4. After 200 cycles, the capacity losses of pristine and the Al2O3-coated LiMn2O4 are 12.5% and 16.5%, respectively. The result indicates that the cyclic performance of the Al2O3-coated LiMn2O4 is better than that of the uncoated-LiMn2O4. XIA et al [14] reported that the decomposition of the electrolyte occurred on the surface of spinel LiMn2O4 which led to the fast capacity decline. In the present work, it is believed that the Al2O3 coating on the surface of LiMn2O4 will reduce the dissolution of Mn.
Fig.4 Cyclic performance of LiMn2O4
3.3 Safety performance
Figure 5 shows the overcharge test curves of the LiMn2O4 batteries. In Fig.5(a), the temperature of the battery surface of pristine LiMn2O4 rises with the voltage rising. The battery explodes when the voltage arrived at 10 V, and the temperature of the battery surface rises to 290 °C in 12 s. In Fig.5(b), the temperature of the battery surface of the Al2O3-coated LiMn2O4 rises slowly with the voltage rising. When the voltage arrives 10 V, the temperature of the battery surface is 127 °C. Then, the temperature descends with time. The temperature descends to 89 °C after 25 min. The battery passes the 3C/10 V overcharge test. The results indicate that the battery using Al2O3-coated LiMn2O4 shows better thermal stability during the overcharge test.
Fig.5 Overcharge curves of LiMn2O4 battery: (a) Pristine LiMn2O4; (b) Al2O3-coated LiMn2O4
Figure 6 shows the SEM images of electrode after 3C/10 V test. In Fig.6(a), amounts of small sintered particles are observed. The result indicates that LiMn2O4 reacts with electrolyte, and creates new materials. And in Fig.6(b), the surface of LiMn2O4 is soaked by electrolyte seriously. However, the Al2O3-coated LiMn2O4 keeps the particle well after overcharge test.
Fig.6 SEM images of electrode after overcharge: (a) Pristine LiMn2O4; (b) Al2O3-coated LiMn2O4
Figure 7 shows X-ray diffraction patterns for the electrode of LiMn2O4 battery after overcharged test. The diffraction peaks of the electrode of Al2O3-coated LiMn2O4 battery can be indexed as spinel phase with a space group of Fd3m. But in the diffraction pattern of the electrode of pristine LiMn2O4 battery, the peaks of carbon, MnO, and Li2CO3 are indexed. This indicates that the LiMn2O4 battery explodes and creates C, MnO and Li2CO3. The appearance of C, MnO and Li2CO3 was discussed in Ref.[1].
Fig.7 XRD patterns of overcharged cathode electrode of Al2O3- coated LiMn2O4 (a) and pristine LiMn2O4 (b)
3.4 Possible mechanisms
To compare the surface state of LiMn2O4 before and after being coated in more detail, AC impedance measurement was carried out using three-electrode configuration. In Fig.8, the impedance spectra of LiMn2O4 before and after being coated are combinations of two depressed semicircles in high frequency region and a straight line in low frequency region. An intercept at the Zreal axis in high frequency region corresponds to the ohmic resistance (Rs). The depressed semicircle in the high frequency range is related to the Li-ion migration resistance (Rf) through the solid electrolyte film (SEI) film formed on the cathode surface. The second semicircle in the middle frequency range indicates the charge transfer resistance (Rct). The inclined line in the lower frequency range represents the Warburg impedance (W), which is associated with Li-ion diffusion in the LiMn2O4 particles. It is obvious that the ohmic and migration resistances produced by the coated LiMn2O4 electrode exceed the pristine ones. The resistance and polarization of LiMn2O4/electrolyte are increased.
The thermal stability of LiMn2O4/graphite battery has been improved by surface modification. The reason can be concluded as follows: First, the LiMn2O4 is separated from the electrolyte by Al2O3 coating layer, and the reaction between the cathode and electrolyte is restrained during the overcharge test, which results in less caloric; Second, the crystal lattice of the coated LiMn2O4 shrinks, which results in shorter Mn—O bond and steadier spinel structure; Third, the resistance and polarization of coated LiMn2O4/electrolyte are increased, which indicates that the potential energy of Li-ion emersion during the overcharge test is increased.
Fig.8 AC impedance of LiMn2O4 electrode
The Al2O3-coated LiMn2O4 not only improves the thermal stability of battery, but also improves the cyclic performance of battery. It is a novel method to improve the thermal stability of LiMn2O4/graphite battery with large capacity.
4 Conclusions
1) The LiMn2O4 modified with Al2O3 and 325680- type LiMn2O4/graphite battery (10 A·h) is fabricated. The crystal lattice of LiMn2O4 shrinks after coating. After 200 cycles, the capacity losses of the pristine and coated LiMn2O4 battery are 12.5% and 16.5%, respectively.
2) The pristine LiMn2O4 battery explodes after 3C/10 V overcharge test, and the cell surface temperature arrives at 290 °C, while the coated LiMn2O4 battery passes the overcharge test. Carbon, MnO, and Li2CO3 are detected in the exploded powders, while Al2O3-coated LiMn2O4 still keeps spinel phase.
3) The performances of LiMn2O4 battery especially the safety performance are improved by Al2O3 coating. The steadier structure, polarization of electrode and modified layer are responsible for the safety performance.
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(Edited by HE Yun-bin)
Foundation item: Project(10JDG041) supported by the Advanced Person Fund of Jiangsu University, China; Project(2007CB613607) supported by the National Basic Research Program of China
Received date: 2010-12-06; Accepted date: 2011-02-20
Corresponding author: LIU Yun-jian, PhD; Tel: +86-13505282025; E-mail: lyjian122331@yahoo.com.cn
