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 can be 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 last year.

Impact of this ACS PRF on the graduate student Hyung-Man Cho's progress: This work provides essential insights into the advanced design of lithium-ion battery electrodes with superior high-power and high-energy densities. In addition, it enables the student to develop the integrated understanding between the crystallographic structures and the electrochemistry. In his master course, the study was focused mostly only on the principles and various techniques of the electrochemistry. This work gives him a great opportunity not only to apply the previously obtained knowledge on other research projects, but also to earn a good grasp of the various synthesis methods, the materials characterizations.

During the past twelve months, LiNi0.5Mn1.5O4 spinel materials were successfully synthesized using the sol-gel method. Two types of structures are prepared by different heat treatment procedures. The disordered structure where Ni2+ and Mn4+ are disordered in the spinel framework can be oxidized to the ordered structure by annealing at 700 oC for 48 hours in air. The particle morphology and size distribution of the synthesized powders were examined with the scanning electron microscope (SEM) and the transmission electron microscope (TEM). In order to identify the crystalline phase of the synthesized materials, powder X-ray diffraction (XRD) measurements was done and XRD data analysis was carried out by the Rietveld refinement using the FullProf software package.

As proposed in the original proposal, the nano-structured electrodes have been considering a promising way to achieve utmost power density without much sacrifice of energy density, since it makes it possible to reduce the diffusion lengths and its relatively large surface area will reduce the resistance to interfacial reactions and thus accelerate electrode kinetics. In this work, the nano-structured electrode of spinel materials was prepared via the sol-gel based template synthesis. Commercially available polycarbonate porous membrane of a pore diameter of 200 nm (= 0.2 ?m) as a template was immersed into a precursor sol. Fully soaked templates were placed on a platinum foil current collector, and gelation was carried out at the drying oven. In order to remove the template, the oxygen plasma etching system was utilized. Eventually, as-prepared protruding nano-structured electrode was crystallized with 900 oC heat-treatment. The SEM and TEM examination shows that the particle size of the nano-structured spinel material is in the range of 50-200nm, with well crystalline pure phase. The particle size is about 5-10 times smaller than that obtained in the sol-gel method for power synthesis.

For the electrochemical measurement, a custom-made three-electrode cell configuration was used to enable quantitative electrochemistry analysis. The prepared electrodes and lithium metal were used for the positive and negative electrodes, respectively. For the reference electrode, the copper wire was sandwiched between separators. Before the tests, elemental lithium was cathodically coated on the bare copper wire by taking lithium metal electrode as a counter electrode. The electrolyte was a 1 M solution of lithium hexafluorophosphate (LiPF6) in a 1:1 volume mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). All the cells used for the electrochemical tests was assembled in a glove box filled with purified argon gas (H2O level of <1 ppm). To explore how a dissimilarity of nickel and manganese ordering in the crystal structure affects the elementary reaction steps and their contribution to the total dc polarization, the differentiation of the resistive elements was done by using the electrochemical impedance spectroscopy. The electrochemical impedance measurements were carried out at a cell potential of 4.85 V (vs. Li/Li+) for the frequency range of 100 kHz to 10 mHz. The mechanism-based equivalent circuits which model the elementary reactions and the circuit simulation were adopted for the quantitative analysis of the obtained impedance spectra. Our results clearly demonstrated that the charge-transfer reaction and solid-state diffusion reaction are the first and second largest contributions of the total dc polarization for both the disordered and ordered spinel structures, and further, the contribution of the interfacial charge-transfer resistance of the ordered structure shows considerably higher than that of the disordered structure. This tells that the nickel and manganese ordering in the spinel structure aggravated the charge-transfer reactions.

With the comparison of the cell potential transients at a variety of discharging rates, it is confirmed that the nano-structured electrode could improve the power density of the battery significantly. We conducted detailed analysis on the electrochemical impedance spectroscopy on the nano-structured electrode, the differentiation of the resistive elements and its theoretical analysis was done in the consistent manner as described above for conventional power composite electrodes. Nevertheless, it showed that the contribution from the lithium migration through the multilayer surface films increased. It is well-known that the high voltage operation of the LiNi0.5Mn1.5O4 spinel materials oxidizes the liquid-carbonate electrolyte. This detrimental reaction occurs during the cycling, and thus leads to increase the passivation layers on the surface. The increased surface area of the nano-structured electrode aggravates this undesirable process. We indeed found that the contribution of the interfacial charge-transfer and solid-state diffusion impedances were reduced drastically in such nano-structured electrode. The key factor for enabling high power nano-structured spinel electrode in a practical cell would be to stabilize the electrode/electrolyte interface. This part of the work has been written up in a manuscript that will be submitted in one month.

The next 12 months of the project, the PI and the student will focus on making olivine and layered materials into nano-structured electrodes and investigate the effects of crystallographic orientations on the electrochemistry properties.