Tabuchi et al. (2016) optimized cation ratio and calcination atmosphere of iron Fe and nickel Ni substituted lithium manganese oxide (Li2MnO3) based on electrochemical properties obtained under application of stepwise charging. The study is published in Journal of Power Sources,
The Fe- and Ni-substituted Li2MnO3 (Li1+x(FeyNiyMn1-2y)1-xO2, 0<x<1/3, y= 0.1, 0.15, 0.2) was prepared using coprecipitation-calcination (0.25mol/batch). The dried precursor after calcination was then calcined at 8500C for 3h in air or nitrogen atmosphere after pulverization. To obtain homogeneous precursor, coprecipitation under cooling at -10deg. C and air bubbling processes must be needed. Cell tests started from galvanostatic charging, up to 4.8V under a fixed current density per mass of active material of 40mAg-1 and afterwards, cells were kept at 4.8V till low current density of 10mAg-1 (constant current-constant voltage mode) before being discharged at 40mAg-1 down to 2.0V.
Cell test was conducted at 300C for stepwise charging as it increased gradually from 80mAhg-1 at 40mAhg-1 and discharge characteristics under high current densities to 2.0V evaluated from 40 to 2400mAg-1 after charging up to 4.8V at 40mAg-1. Discharge behavior at 0 and -200C was collected to 2.0V at a fixed current density of 40mAg-1 after charging to 4.8V at 300C.
Results showing the effects of charging mode and calcination in electrochemical properties examined at y value=0.15 in the chemical formula showed a brown color when calcined in nitrogen atmosphere and dark brown color when calcined in air. X-ray diffraction patterns show the formulation of Li2MnO3-type phase independence of calcination temperature but lattice parameter and transition metal occupancy of both samples were mutually similar. There was also a successful control average oxidation achieved as chemical analysis data revealed that a nominal cation ratio was maintained.
When examining the effect of mode of initial charging, sample of y=0.15 calcined in air and selected as a positive electrode material showed poor electrochemical performance when galvanostatic mode was selected. Initial efficiency and cyclability up to the 20th cycle were only 53% and 79% respectively. Selecting stepwise charging resulted to high initial discharge capacity and energy density with a better cyclability of 97% which showed that stepwise charging mode must be selected for iron and nickel substituted Li2MnO3.
Discharge capacity values for the 5th to 24th cycle when selecting stepwise charging mode showed a higher value for sample calcined in nitrogen atmosphere compared to those calcined in air atmosphere when y=0.15. This shows that reduction in average oxidation states of transitional metal ions favors improvement of electrochemical performance of sample calcined in nitrogen atmosphere.
Fifth discharge capacity after the stepwise charging was greater than initial capacity after galvanostatic charging for sample where y value was equal to 0.1 or 0.15 but was almost equal to initial capacity after galvanostatic charging for which y equals 0.2. This result show that degree of improvement of discharge capacity depends on initial charge capacity at Li2O extraction part above 4.5V.
Sample for which y value equals 0.15 was seen to have a better high-rate performance when compared to others but better low-temperature performance was obtained for sample where y value equals 0.1 which is also attractive as a positive electrode material if change of the discharge curve shape with increasing number of cycles were suppressed.
According to the authors, Li1+x(FeyNiyMn1-2y)1-xO2 samples (0<x<1/3) for y equals 0.1 and 0.15 has optimal cation ratios for use as positive electrode materials which differ from previous reports with sample y values of 0.2 or 0.25.
Tabuchi, M., Kageyama, H., Shibuya, H., Doumae, K., Yuge, R., Tamura, N. Stepwise charging and calcination atmosphere effects for iron and nickel substituted lithium manganese oxide positive electrode material, Journal of Power Sources 313 (2016) 120-127.
Mitsuharu Tabuchi1 , Hiroyuki Kageyama1, Hideka Shibuya2, Kyosuke Doumae2, Ryota Yuge3, Noriyuki Tamura3Show Affiliations
- National Institute of Advanced Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka, 563-8577 Japan
- Tanaka Chemical Corp., 45-5-10 Shirakata, Fukui, Fukui, 910-3131, Japan
- NEC Corp., 34 Miyukigaoka, Tsukuba, Ibaraki, 305-8501, Japan
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