Enhancing High-Rate and Elevated-Temperature Performances of Nano-Sized and Micron-Sized LiMn2O4 in Lithium-Ion Batteries with Ultrathin Surface Coatings

LiMn2O4 suffers from severe capacity degradation when used as a cathode material in rechargeable lithium-ion batteries, especially when cycled at high rates and elevated temperatures. To enhance its high-rate electrochemical performance at elevated temperature (55 degrees C), we use atomic layer deposition (ALD) to deposit ultrathin and highly conformal Al2O3 coatings (as thin as 0.72 nm) onto micron-sized and nano-sized LiMn2O4 with precise thickness-control at atomic scale. All ALD-modified electrodes exhibit significantly improved capacities and cycling stability compared to bare electrodes. In particular, the effect of ALD coating to improve electrochemical performance of LiMn2O4 is more distinct for nano-sized LiMn2O4 than for micron-sized LiMn2O4, and more distinct for electrochemical cycling at higher charge/discharge rates. For example, nano-LiMn2O4 electrode coated with 6 AlAl2O3 ALD layers delivers higher initial capacity (124.7 mA h/g) and final capacity (106.7 mA h/g) after 100 cycles than bare electrode with an initial capacity of 112.3 mA h/g and a final capacity of only 95.5 mA h/g, when cycled at a very high rate of 5 degrees C at 55 degrees C. In addition, nano-LiMn2O4 electrodes show much better rate performance than micron-LiMn2O4 electrodes at 5 degrees C. The enhanced electrochemical performance of ALD-modified LiMn2O4 is ascribed to high-quality ALD oxide coatings that are highly conformal, dense, complete, and thus protect active material from severe dissolution, and to a formed robust glass layer on the surface of LiMn2O4 that suppress its crystallographic transformation during electrochemical cycling. Surface modifications of LiMn2O4 are also carried out by either ALD coating directly onto the entire LiMn2O4/carbon/PVDF composite electrode or coating only on LiMn2O4 particles in electrode. The former results in more significantly improved electrochemical performance of cathode, possibly because ALD coating onto entire electrode provides better mechanical integrity and preserves contact between LiMn2O4 particles and carbon/poly-vinylidenefluoride network.