(134c) Atomic Layer Deposition (ALD) On the Nanostructured Li-Mn-Rich Composite Li1.2Ni0.13Mn0.54Co0.13O2 Cathode Powder
AIChE Annual Meeting
Monday, November 4, 2013 - 1:00pm to 1:15pm
Atomic Layer Deposition (ALD) on the Nanostructured Li-Mn-rich Composite Li1.2Ni0.13Mn0.54Co0.13O2Cathode Powder
Xiaofeng Zhang,a Ilias Belharouak,a,*
a Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 S. Cass Ave, Argonne, IL 60439, USA, Email: email@example.com(Corresponding author)
Li-Mn-rich (LMR) composite cathode materials Li1+xM1-xO2 (M = Ni, Mn and Co) are of particular interest for plug-in hybrid electric vehicles (PHEVs), in which a high-energy battery is always desired for a higher mileage per charge.[1, 2] In the composite structure, the layered, the spinel and rocksalt structure are compatible at nano-scales, that many composites with different electrochemical properties can be synthesized by simply tuning the Li content in the materials. However, these materials would suffer from voltage decay and capacity fading with cycles, which significantly deteriorate their energy densities and life time for practical applications. In this type of materials, Li content is essential to the arrangement of the transition metal atoms in the composites and also to the structural stability against undesired reactions in a real electrochemical cell. It has been demonstrated that the occurrence of layered-spinel transformation induces the voltage decay. It is because the Li intercalation/deintercalation reactions may preferentially occurs at the tetrahedral site other than octahedral site, which facilitates the spinel formation.[1, 2] Further study by HR-TEM indicated that the composite particle surface could lose the layered feature after a few cycles, and the first-principle calculation suggested a defect-spinel structure forming on the surface due to the migration of transition metal ions into the Li layers. Therefore, the surface chemistry and structure stability appear to be essential to limit the undesired reactions and phase transformation in the composite materials. Some achievements have been made via developing new synthesis processes, new chemistry or surface modifications (e.g., coating), which possibly altered the short-range and long-range ordering of transition metal atoms (Ni and Mn) near the surface, which lead to an improvement in the battery performance.[4-8]
Atomic layer deposition (ALD) is a well-established method to grow ultrathin films on flat surfaces and powders in atomic thickness using sequential, self-limiting surface reactions.[9-14] It has been demonstrated that the surface film can react as a surface barrier between the cathode active material and the electrolyte, which would reduce the loss of active materials in the electrode. [9-14] Consequently, less capacity loss is achieved for the coated cells as compared with the pristine materials. The well-established ALD process normally is using trimethylaluminium (TMA) and H2O as precursors forming an Al2O3 surface film. Experimental results showed that the coated LiCoO2 powders exhibited a capacity retention of 89% after 120 charge–discharge cycles in the 3.3–4.5 V vs. Li/Li+ range. [12, 15-17] The ALD surface coating can also be applied to battery electrodes at a low temperature <150 ºC, which would not degrade the electrodes (e.g., melting of polymer binders) in this temperature range. It is reported that Al2O3-coated LiCoO2 electrodes display superior electrochemical performance in comparison to the uncoated LiCoO2 electrodes. However, Liu et al observed that the ALD surface coated Al2O3 could be lithiated forming passivation LiAlO2 glass on the materials surface using in-situ TEM , which provided new insight into the electrochemical characteristic of the Al2O3surface film against Li.
In this paper, we are aiming for studying the importance of a conformal surface protection film using atomic layer deposition (ALD) technique on the high-capacity Li1.2Ni0.13Mn0.54Co0.13O2 cathode materials in Li-ion batteries, which can be potentially significantly improved in their electrochemical performance. More precisely, we evaluated the effect of coating species, coating thickness and morphology on the rate performance, cycling life and high temperature performance of the high-capacity Li1.2Ni0.13Mn0.54Co0.13O2 cathode materials in half-cell and full-cell configurations. We found that an conformal Al2O3 thin film could be deposited on the powders using 10 ALD cycles of TMA and H2O at 150 °C. TiO2 coating was prepared using 20 ALD cycles of titanium isopropoxide (TTIP) and H2O at 150 °C, which displayed a particulate morphology. Nevertheless, the surface coated TiO2 film was electrochemically active, which could be lithiated forming LixTiO2 as detected by X-ray photoelectron spectroscopy (XPS). The electrochemical reactivity of TiO2 will cause degradation of the surface film, leading to capacity fading of the cell. Nevertheless, it was also found that the Al2O3 surface coating was not electrochemically active after repetitive cycles, which was contrary to the early report by Lui et al that Al2O3 can be lithiated into LiAlO2. To further explorate the phenomena, a systematic study on the ALD coating of Al2O3 and LiAlO2was performed.
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