|Discharge capacity as a function of cycle number of the cells with: (a) 0, (b) 10, (c) 50, (d) 100, and (e) 500 Al2O3 ALD cycles performed on the LiCoO2 electrode. Credit: ACS, Cheng et al. Click to enlarge.|
A team led by researchers from the National Taiwan University of Science and Technology has shown that a thin Al2O3 coating layered on a LiCoO2 by atomic layer deposition (ALD) can effectively eliminate capacity fading during repeated charging and discharging. However, a TiO2 ALD coating led to significant improvement only at high cycle numbers.
Their work, accepted for publication in ACS’ Journal of Physical Chemistry C, suggests a criterion for judging the suitability of potential ALD coatings on Li-ion cathode materials.
LiCoO2 has been used as the cathode material of choice in many commercial lithium-ion batteries because of its high capacity and good rate capability, the team notes. However, if more than half of the lithium-ions are extracted, Co4+ will dissolve into the electrolyte, resulting in structural changes to the LiCoO2 particles and deterioration of the battery’s performance.
Using metal oxides as a coating on the electrode helps prevent Co dissolution and electrolyte decomposition, thereby reducing the decrease in the lithium-ion battery’s energy density and the working voltage. Some of the metal oxides reported as surface coating candidates are: Al2O3, TiO2, ZnO, SiO2, and ZrO2.
This study uses the ALD technique to directly deposit Al2O3 and TiO2 films on LiCoO2 electrodes and then investigates the electronic band structures of the coatings and active materials, during charging and discharging, to illustrate differences in the electrode’s cycle performance. Finally, we suggest a criterion appropriate for judging the suitability of potential ALD coating on the cathode materials.—Cheng et al.
The team prepared electrode materials for testing and assembled coin-type cells. A porous Celgard 2320 film and lithium metal served as the separator and anode, respectively. The electrolyte consisted of 1.0 M LiPF6 dissolved in an ethylene carbonate/propylene carbonate/diethyl carbonate (3:2:5 in volume) mixed solvent.
They found that the capacities of the Al2O3 ALD-LiCoO2 electrodes were higher, for large charge-discharge cycles, compared to a bare electrode, provided that the ALD cycle number was less than 100. The Al2O3 ALD-LiCoO2 electrode formed using 10 ALD cycles had the best capacity retention. However, the cycle performance of Al2O3-coated electrode would be poorer with 500 ALD cycles. With TiO2, 50 ALD cycles on LiCoO2 electrode provided the greatest improvement, whereas the improvement is important only when the charge/discharge cycle number is larger than 33 when compared with the bare electrode.
Based on their analysis of the differential capacity vs. potential curves, they suggested that the poorer cycling performance of TiO2 could be related to the participation of the TiO2 thin film in the redox reaction. They suggested that the redox current is impeded at the Al2O3-LiCoO2 junction, while electrons and holes were energetic enough to flow into the TiO2, due to the smaller band gap energy. The barrier between the valence band maxima of TiO2 and LiCoO2 expands as the charge-discharge cycle number increases, eventually making TiO2 redox-inactive.
From this study, we conclude that the metal oxide with the higher band gap energy would be the better LiCoO2 electrode ALD coating layer to diminish capacity fading. Additionally, this study suggests a criterion for suitable ALD coatings; If the charge/discharge potential range is from 3.0 to 4.5 V vs. Li/Li+ (i.e. a 1.5 V voltage window), the thin film deposited on LiCoO2 (Eg=2.4 eV) should have a band gap energy larger than 1.5+2.4=3.9 eV to accommodate the band structure shift of LiCoO2 during charging/discharging to avoid the ALD coating undergoing a redox reaction. Therefore, large band gap materials, such as: MgO (Eg=7.8 eV), Li2O (Eg=7.99 eV), and SiO2 (Eg=9 eV) may be potential candidate ALD coatings on the cathode materials.—Cheng et al.
Ho-Ming Cheng, Fu Ming Wang, Jinn P. Chu, Raman Santhanam, John Rick, and Shen-Chuan Lo (2012) Enhanced Cycleabity in Lithium Ion Batteries: Resulting from Atomic Layer Deposition of Al2O3 or TiO2 on LiCoO2 Electrodes. The Journal of Physical Chemistry C doi: 10.1021/jp210551r