Significant advances in both energy density and rate capability for Li-ion batteries will be necessary for implementation in next generation electric vehicles. We have employed two different methods to improve the rate capability of high capacity electrodes. For example, we previously demonstrated that thin film high volume expansion MoO3 nanoparticle electrodes (~2 μm thick) have a stable reversible capacity of ~630 mAh/g, at C/2 (charge/dicharge in 2 hours). By fabricating thicker conventional electrodes (suitable for vehicular applications) with conductive additive and binder, an improved reversible capacity of ~1000 mAh/g is achieved but, unfortunately, the rate capability decreases. In order to achieve high-rate capability, we applied a thin Al2O3 atomic layer deposition coating to enable the high volume expansion and prevent mechanical degradation. Also, we recently reported that a thin ALD Al2O3 coating with a thickness of ~5 Å can enable natural graphite (NG) electrodes to exhibit remarkably durable cycling at 50˚C. In contrast, bare NG shows a rapid decay. Additionally, Al2O3 ALD films with a thickness of 2 to 4 Å have been shown to allow LiCoO2 to exhibit 89% capacity retention after 120 charge–discharge cycles performed up to 4.5 V vs. Li/Li+. Capacity fade at this high voltage is generally caused by oxidative decomposition of the electrolyte or perhaps cobalt dissolution. We have recently fabricated full cells of NG and LiCoO2 where we coated both electrodes, one or the other electrode as well as neither electrode. In creating these full cells, we observed some surprising results that lead us to obtain a greater understanding of the ALD coatings.
In a different approach we have employed carbon single-wall nanotubes (SWNTs) to synthesize binder-free, high-rate capability electrodes, with 95 wt.% active materials. In one case, Fe3O4 nanorods are employed as the active storage anode material. The highest reversible capacity is obtained using 5 wt.% SWNTs, reaching ~1000 mAh/g at C rate when coupled with a lithium metal electrode. Furthermore, the electrodes exhibit high rate capability and stable capacities of 800 mAh/g at 5C (charge/discharge in 12 minutes), sustained over 100 cycles and 600 mAh/g at 10C (charge/discharge in 6 minutes). Scanning electron microscopy indicates the high-rate capability is achieved because Fe3O4 nanorods are uniformly suspended in the SWNT matrix. Raman spectroscopy was employed to understand how the SWNTs function as a highly flexible conductive additive. Recently, we have also employed this method to demonstrate improved conductivity and highly improved rate capability for a LiNi0.4Mn0.4Co0.2O2 cathode material. These results will be presented in detail.