Enhanced cyclability of lithium-ion batteries resulting from atomic layer deposition of Al₂O₃ on LiFePO₄ electrodes

Al₂O₃ thin films were deposited on the surfaces of LiFePO₄ cathodes via atomic layer deposition (ALD). Electrochemical characterization was used to evaluate the performance improvement of the cathodes coated with the ALD Al₂O₃ thin films. In rate capability tests, an electrode coated with five ALD cycles of the Al₂O₃ thin film exhibited the best rate performance among the tested samples when the current rate was lower than 2 C. An electrode coated with 15 ALD cycles of the Al₂O₃ thin film delivered the highest average specific capacity—88.9 and 61.3 mAh/g at current rates of 5 C and 10 C, respectively—whereas a bare LiFePO₄ electrode did not have any capacity at the 10 C rate. These results indicate that the Al₂O₃ coating can increase the cathode capacity at high current rates. In a cyclability test, the electrode coated with 15 ALD cycles of the Al₂O₃ thin film retained the largest capacity after 1000 cycles at a rate of 1 C. The specific capacity of the 15ALD sample at the 1000th cycle was approximately 68% of the capacity at the first cycle, whereas this ratio was reduced to 37% for the bare LiFePO₄ electrode. These results indicate that the Al₂O₃ coating maintains the cathode capacity after many cycles. Thicker Al₂O₃ coatings result in a high degree of polarization and increased electrode resistance; therefore, the electrode coated with 25 ALD cycles of the Al₂O₃ thin film exhibited a poorer rate and cycle performance than the 15ALD and 20ALD electrodes. Electrochemical impedance spectroscopy measurements and calculation of the Li⁺ diffusion coefficient revealed that the D_Li of the bare LiFePO₄ electrode was severely reduced after 1000 cycles relative to the electrode coated with 15 ALD cycles of the Al₂O₃ thin film. This led to a faster decay in the capacity of the bare LiFePO₄ electrode after a long-duration test compared with the electrode coated with 15 ALD cycles of the Al₂O₃ thin film. We conclude that the rate capability and cyclability of the LiFePO₄ electrode can be enhanced by depositing Al₂O₃ ALD thin films on its surface. Under the conditions of our ALD process and electrochemical tests, 15 was found to be the optimal number of ALD cycles.