LiCoO 2 surface layer is proposed and prepared through sol-gel method. The physical and electrochemical performances of pristine LiMn 2 O 4 and LiCoO 2 -coated LiMn 2 O 4 cathode materials were investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, electrochemical measurements respectively. Comparing with the pristine LiMn 2 O 4 , the LiCoO 2 - coated LiMn 2 O 4 phase significantly improved cycling stability, especially at 55°C. Additionally, the thermal safety of LiMn2O 4 is greatly enhanced after being coated by LiCoO 2 . ICP-AES measurement, structural analysis, and impedance experiments indicate that the improved electrochemical property of LiCoO 2 -coated LiMn 2 O 4 should be attributed to the alleviated dissolution loss of manganese, strengthened structural stability.
Lithium ion battery (LIB) has become the most widely used power supply for electronics. Safer electrode material has been pursued by researchers for years [
LiMn2O4 powder was purchased from Hebei Strong-Power Li-ion Battery Technology Co. Ltd. (D98, China). LiCH3COO∙2H2O (1.03 g), Co (CH3COO)2∙4H2O (2.53 g) with a stoichiometric ratio (1:1) were dissolved in distilled water. An aqueous solution of ethylene glycol and citric acid (1:4) as a chelating agent was added to the mixtures. pH value at 7.0 - 7.5 was achieved by Ammonium hydroxide. Then slowly add the LiMn2O4 powders (50 g) to the sol and vigorously stirred at 85˚C for 5 h. As the evaporation of water proceeding, the sol was turned into a viscous transparent gel. After drying and sieving, the powder was sintering in air at 350˚C for 3 h and 650˚C for 3 h to obtain LiCoO2-coated LiMn2O4. For a comparison, pristine LiMn2O4 was also heat-treated in the same condition.
X-ray diffraction patterns were recorded on a DX-2700 diffract meter (Siemens D-5000, Mac Science MXP 18) equipped with Cu Kα radiation of λ = 0.154145 nm. The diffraction patterns were recorded between scattering angles of 15˚ and 80˚ at a step of 4˚/min. The morphology was studied by a scanning electron microscopy (S4700, Hitachi) and transmission electron microscope (JEOL-1200EX). After cycling, the batteries were disassembled in glove box and the electrodes and membrane were washed by EC/DMC for several times. The cathode was used to examine the changes in structure by XRD and the obtained solution was diluted to suitable concentration to detect the content of Mn element. Inductively coupled plasma atomic emission spectrometry analysis was conducted on IRIS Intrepid П XSP inductively coupled plasma emission spectrometer (THERMO).
To obtain working electrode, 85 wt% active materials, 6 wt% polyvinylidene fluoride and 9 wt% acetylene black were homogeneously mixed in NMP. Then the resulting slurry was spread on an Al foil and thoroughly dried. The electrodes were punched in the form of 14 mm diameter disks, and the typical active material mass loading was about 6 mg/cm2. The electrolyte was 1 M LiPF6 dissolved in a mixture of ethylene carbonate and dimethylene carbonate with the volume ratio of 1:1. The anode of the battery is Li electrode. The assembly process was conducted in an argon-filled glove-box with the content of H2O and O2 less than 1 ppm.
Before electrochemical tests, the batteries were aged for 24 h to ensure good soakage. The cells were charged and discharged on a battery tester (CT-3008W, NEWARE) between 3.3 and 4.35 V at the rate of 2C at elevated temperatures (55˚C ± 2˚C) in dry oven (A201113, Shanghai).
Scanning electron microscopy has been shown in
present a uniform particle distribution, ranging from 3 to 6 μm. The pristine spinel crystals are smooth with well-defined facets, as observed in
The further investment of the surface of LiMn2O4 by transmission electron microscope is displayed in
To further identify the homogeneity of coating layer, the element distribution is determined by EDS mapping, which is displayed in
The XPS is shown in
observation in TEM and EDS element mapping.
The structure of pristine LiMn2O4 and LiCoO2-coated LiMn2O4 cathodes after cycling 100 times (55˚C) was examined. The results are given in
pattern of the cycled LiCoO2-coated LiMn2O4, the peak intensity declines a polarization, we still should ascribe them to the LiCoO2 on the surface of LiMn2O4.
In
To further verify the effects of surface coating on manganese ions dissolution, the quality of the manganese element was directly determined by using ICP-AES. Li metal anode was washed by dilute hydrochloric acid after 100th cycle at 55˚C ± 2˚C. It can be seen in
In summary, the surface of LiMn2O4 sample was modified by LiCoO2 using a sol-gel method. TEM and XPS results confirm the existence of LiCoO2 layer. A uniform and dense layer about 3 - 5 nm was coating on the
Samples | The quality of Mn ions on Li anode (μg/cm2) |
---|---|
Pristine LiMn2O4 | 22.54 |
LiCoO2-coated LiMn2O4 | 10.17 |
surface of pristine LiMn2O4. The LiCoO2-coated LiMn2O4 sample exhibits much better cycling stability at elevated temperature (55˚C) compared with the pristine sample. These results demonstrated that this is an effective way to improve the high-temperature cyclic performance of spinel LiMn2O4.
This work was supported by National Science Foundation of China (No. 50672026). This work was also supported by Shanghai Nano Technology Promotion (No. 12ZR1448800).