Reports: DNI1051311-DNI10: Functional Nanostructured Oxides for Electrochemical Energy Storage
Ying Shirley Meng, PhD, University of California (San Diego)
This research funding was awarded to the PI when she was in her second year at the Department of NanoEngineering, UCSD. The impact of this ACS PRF on the PI's academic career is demonstrated in two aspects: Firstly, the scope of the work is distinctly different from the PI's postdoctoral and doctoral work, which enables the PI to establish new research direction in nano-structured electrodes for high power batteries. Secondly, the winning of the prestigious ACS PRF fund has enabled the PI gain credential and attract more research funding in the past two years. The PI's research group has grown to 12 graduate students and 3 postdocs as of Fall 2013. Impact of this ACS PRF on the graduate student Hyung-Man Cho's progress: This work gives a good opportunity for Cho to learn various advanced nano-technologies, such as template-assisted synthesis and atomic layer deposition (ALD). It enables the student to obtain essential insights of these nano-structured electrodes in order to achieve utmost power density without much sacrifice of energy density. At the same time, the student also has the opportunity to collaborate with an exchange student Hao Liu from Hong Kong to explore the synthesis methods developed for Si and CuO anode type of nano-structured electrodes. LiNi0.5Mn1.5O4 spinel materials with disordered (space group, F d -3 m) and ordered structure (space group, P 43 3 2) were prepared and their elementary polarizations were quantitatively analyzed in order to clarify how the differences in crystallographic structure affect the rate performance of the cathode materials. A comparative analysis from a combination of electrochemical impedance spectroscopy and theoretical analysis of the equivalent circuit disclosed that the nickel and manganese ordering in the spinel-framework aggravates the charge-transfer reactions. In order to minimize the diffusion lengths and maximize the active surface area, the one-dimensional nano-structured electrode which consists of the disordered spinel was prepared and revealed that both charge-transfer and solid-state diffusion resistances reduced, but the resistance of lithium migration through the interfacial film regions increased significantly. As-prepared one-dimensional nano-structured electrode gives rise to improved lithium-ion transport, but unfavorable side reactions are more pronounced due to the larger surface-to-volume ratio. Since the disordered spinel contains Mn3+ in the structure, the Jahn-Teller distortion of Mn3+ causes serious mechanical stress and its disproportion reaction (2Mn3+ = Mn4+ + Mn2+) results in Mn2+ dissolution from the active materials. The cation dissolution is therefore possible during cycling, which results in permanent loss of positive electrode materials and decrease of capacity. The scanning electron microscopy (SEM) and transmission electron microscope (TEM) examination shows that the morphology and particle size of the as-prepared nano-structured electrode. The protruding nano-structured electrode was crystallized with 800 oC calcination. The average diameter of the nano-wires and length were about 160 nm and 12 um, respectively. Each nano-wire is constructed with the nano-sized particles in the range of 10-50 nm, with well crystalline phase. The electrochemical performance of nano-structured electrode was evaluated in the custom-made cell configurations. The cell was cycled galvanostatically between 3.5 and 4.85 V (vs. Li/Li+) at a current rate of 0.2 C. The electrochemical charge and discharge curves shows typical plateaus at around 4.7 V (vs. Li/Li+) for the Ni2+/Ni4+ redox couples and at around 4.0 V (vs. Li/Li+) for Mn3+/Mn4+ redox. However, significant irreversible loss continues through the consecutive cycles, meaning that the coulombic efficiency is poor. The ICP-OES analysis of the electrolyte which was re-collected from the disassembled cell after cycles revealed that the cation dissolution occurs severely. To maintain the structural integrity of nano-structured LiNi0.5Mn1.5O4 spinel, we hypothesize that a protective coating layer can be added on the surface of the nano crystalline particles to protect the materials from side reactions with the electrolyte. Despite the numerous reports dedicated to wet-chemical methods, it is apparent that the lack of control over surface coverage, thickness, and uniformity prohibits wet-chemical methods for nano-structured materials. Atomic layer deposition (ALD) is a method to coat conformal thin films with atomic thickness using sequential, self-limiting surface reactions. Al2O3 and TiO2 films were grown directly on the as-prepared nano-structured electrodes using ALD (Beneq TFS200) at 250 oC. Due to the self-limited reactions in ALD, the thickness of the as-deposited film is simply controlled by the number of cycles. Since ALD has the advantage of depositing a layer-by-layer homogeneous film coating on a three-dimensional structure, the protective coating layer of TiO2 and Al2O3 was prepared on the as-prepared nano-structured electrodes using ALD. To estimate the morphology and thickness of the coating layer, TEM was performed. In the TEM images, the lattice fringes for the bulk region are identical to pure phase of LiNi0.5Mn1.5O4 spinel materials. In contrast, conformal coating film which has clearly different from the bulk region is clearly observed for TiO2 and Al2O3 ALD coated nano-structured electrodes. It also shows that the coating layers cover the all the surface of the nano-structure uniformly. In addition, with the various ALD cycles, it has different thickness of the coating layers. That is, the cycle number of ALD employed is reflected in the thickness of the coating. In case of TiO2 and Al2O3 ALD, the estimated growth rate is 0.388 Å / cycle and 0.9 Å/ cycle, respectively. We found that 2 nm TiO2 coating layer shows the significantly improved coulombic efficiency (56.7 %, 89.5 %, 91.6 %, 90.4 %, and 88.4 % for initial five cycles). The surface modification with the application of the protective layer at the surface maintains the structural integrity of the nano-structured electrode and show better cycleability with the enhanced coulombic efficiency. All this work will be written up in a manuscript and submitted in the next few months.