59th Annual Report on Research 2014

Nano-structured materials have the potential to significantly improve the performance of lithium-ion batteries as energy storage devices. Particle size reduction can result in an increase lithium ion intercalation/de-intercalation rates. This results in materials that have higher power output than their micron-sized analogues. In this work, the spinel-phase LiNi_0.5Mn_1.5O₄ with nanowire morphology was prepared using a sol-gel based template synthesis. The average diameter of each nanowire was determined to be ~140 nm and with a length of approximately 13 μm. TEM images indicate that each nanowire is polycrystalline in structure with nano-size crystallites (about 47 nm in size with a range of 30 - 80 nm) of different crystallographic orientations.

In order to characterize the structure of LiNi_0.5Mn_1.5O_4, the as-prepared nanowires were mechanically separated from the Pt foil current collector, and the powder XRD pattern was obtained. The disordered structured LiNi_0.5Mn_1.5O₄ spinel material has a cubic crystal structure (space group Fd-3m) with lithium occupying the tetrahedral sites (Wyckoff position: 8a) and disordered nickel and manganese atoms occupying the octahedral sties (Wyckoff position: 16d). Electrochemical characterization consisted largely of galvanostatic cycling experiments. Charge/discharge curves of the nano-sized LiNi_0.5Mn_1.5O₄ showed similar voltage plateau regions with that of typical micro-sized composite electrodes. However, the irreversible capacity fade occurs not exclusively in the first cycle, but also in consecutive cycles. It was determined that is the result of the electrolyte decomposition on the particle surface, thereby forming a passivation layer on the surface of the electrode material at high voltage regions, (> 4.3 V (vs. Li/Li⁺)). In addition, during cycling, reduction of manganese (III) ions in LiNi_0.5Mn_1.5O₄ results in the formation of manganese (II) ions, which can dissolve in the liquid electrolyte. This phenomenon will increase the internal resistance of the cell and result in poor battery performance.

In order to prevent the deleterious effects of electrolyte decomposition and cation dissolution, the interface between the electrolyte and the active materials was modified using atomic layer deposition (ALD). Electrodes particles were deposited layer by layer with either TiO₂ or Al₂O₃ thin films (< 5 nm of ALD thickness) were found to have a profound reduction in irreversible capacity fade. In order to correlate thin film deposition and changes in the electrochemical reaction mechanism, electrochemical impedance spectroscopy (EIS) measurements were performed in the charged state at 4.77 V (vs. Li/Li⁺). It is particularly noteworthy that the resistance of the lithium ion migration of nanowire LiNi_0.5Mn_1.5O₄ coated with TiO₂ and Al₂O₃ surface films reduced by approximately 44 % and 67 %, respectively when compared to that of the non-coated nanowire electrodes. Elemental analysis of the electrolyte was performed using inductively coupled plasma optical emission spectrometry (ICP-OES). It was determined that manganese ion dissolution into the electrolyte is alleviated when electrodes are coated with either TiO₂ and Al₂O₃.

From our extensive studies, we found that the increased surface area of one-dimensional nanowire electrode enable to deteriorate the side reactions at the interface, notwithstanding important benefits of the nano-sized structure. As the protective layer at the surface for high voltage operation, the TiO₂ and Al₂O₃ thin film were successively deposited by using ALD method. Our work shed remarkable insights on the importance of interface protection for nano-structured electrode materials, particularly the ones that operate at voltage higher than 4.3 V (vs. Li/Li⁺). All this work will be written up in a manuscript and submitted in one month.