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J. Phys. Chem. C 2010, 114, 9528–9533
Conductive Surface Modification with Aluminum of High Capacity Layered Li[Li0.2Mn0.54Ni0.13Co0.13]O2 Cathodes Jun Liu, B. Reeja-Jayan, and Arumugam Manthiram* Electrochemical Energy Laboratory and Materials Science and Engineering Program, The UniVersity of Texas at Austin, Austin, Texas 78712 ReceiVed: March 6, 2010; ReVised Manuscript ReceiVed: April 19, 2010
The electrode films fabricated with the high capacity layered oxide Li[Li0.2Mn0.54Ni0.13Co0.13]O2 have been surface modified with metallic aluminum by a thermal evaporation process. The morphology and electrochemical performances of the bare and the Al-coated cathodes have been investigated by scanning electron microscopy (SEM) and charge-discharge measurements. Compared to the bare Li[Li0.2Mn0.54Ni0.13Co0.13]O2 cathode, the Al-coated cathodes (coating time e 30 s) exhibit higher discharge capacity with lower irreversible capacity loss, better cyclability, and higher rate capability. Specifically, the 20 s Al-coated cathode exhibits the highest capacity (278 mAh/g at C/20 rate) and the best rate capability (157 mAh/g at 5C rate), while the 30 s Al-coated cathode displays the best cyclability (268 mAh/g with a capacity retention of 98% in 50 cycles). An analysis of the charge-discharge capacity values, surface conductivity evaluation, and electrochemical impedance spectroscopy (EIS) measurements reveal that the improved electrochemical performances are due to the suppression of both the oxygen vacancy elimination at the end of the first charge and side reactions with the electrolyte and the decrease in charge transfer polarization by the Al-modification layer. Introduction Lithium ion battery technology has played a key role in the portable electronics revolution, and it is vigorously pursued for vehicle applications. However, the presently available cathodes have limited capacity ( 10 s Al-coated cathode > 30 s Al-coated cathode > 20 s Al-coated cathode, which follows the exact increasing order of rate capability. The results imply that the differences in the rate capability are predominantly due to the differences in the charge transfer polarization. It should be noted that the decreasing order of Rct is not exactly the same with the increasing order of surface conductivity. The reason is that the charge transfer kinetics are affected by both the electron migration rate and the lithium ion diffusion rate in the surface layer. Because Al is a good electronic conductor but a poor lithium-ion conductor, increasing the Al coating time will increase the surface electron migration rate (surface conductivity) while decreasing the surface lithium-ion diffusion rate. The 20 s Al-coated cathode appears to have high surface electron migration rate without compromising too much the surface lithium-ion diffusion rate, resulting in the smallest Rct and the best rate capability. Conclusions The high capacity layered Li[Li0.2Mn0.54Ni0.13Co0.13]O2 cathode has been coated with various amounts of aluminum by a thermal evaporation method. Electrochemical data reveal that the Al coating increases the discharge capacity, decreases the irreversible capacity loss in the first cycle, improves the cyclability, and enhances the rate capability. The increase in capacity is due to the suppression of oxygen vacancy elimination at the end of first charge, the improvement in cyclability is due to the suppression of both oxygen vacancy elimination and the side reaction with the electrolyte in the subsequent cycles, and the enhancement in rate capability is due to the enhanced surface conductivity by the Al coating layer. Acknowledgment. Financial support by NASA Glenn Research Center is gratefully acknowledged. References and Notes (1) Lu, Z.; Beaulieu, L. Y.; Donaberger, R. A.; Thomas, C. L.; Dahn, J. R. J. Electrochem. Soc. 2002, 149, A778. (2) Kang, S. H.; Sun, Y. K.; Amine, K. Electrochem. Solid-State Lett. 2003, 6, A183.
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