Synthesis and characterization of paper-like, nanostructured electrodes for advanced secondary batteries
Final Report Abstract
This project aimed at exploring the mechanical and electrochemical performance of novel paper-like electrodes as a function of their structure and composition. These electrodes were obtained either by selfassembly or layer-by-layer deposition of three different types of oxides, specifically V2O5, SnO2 and LiMnPO4, all of which are good intercalation compounds for Li ions. In order to achieve fast ion diffusion, the oxides were used in the form of nanostructures, in particular nanofibers, -sheets or -particles. Moreover, it was envisioned to ensure a good electrical conductivity of the electrodes through the addition of (reduced) graphene oxide, whose sheet-like structure appears to be well compatible with morphology of the oxide nanostructures. At the same time, it was important to render the electrodes mechanically stable against the stresses introduced upon (de)charging, without requiring the addition of any electrochemically inactive components, such as the often used binders or carbon black. This goal was accomplished through effective nanostructure engineering of the composites. Thus obtained, paper-like electrodes without additives were found to be mechanically stable and highly flexible up to a thickness of 6 µm. The major parameters that determine the papers’ electrochemical performance were identified to be (i) the electrode thickness (ion diffusion is slowed down for an electrode thickness larger than several micrometers), (ii) the interconnection between the oxide nanostructures and their in-plane alignment (needed to facilitate lateral ion diffusion), (iii) the stacking density of the components in both, the lateral and vertical direction (a finite porosity is necessary for efficient ion diffusion), (iv) the content and reduction degree of the GO (to sufficiently enhance the charge transport), (v) a proper balance between in plane an out of plane electrical conductivity, as well as (vi) the size of the GO sheets ( in the range of hundred micrometers are preferable in order to achieve percolation). Importantly, the optimized electrodes reach similar total capacity like other, oxide nanostructure-based electrodes reported in the literature, but at notably faster charging rate. Besides these general achievements, the three different types of electrodes offer the following specific advantages: For the V2O5-based electrodes, the addition of the GO was not necessary to achieve good electrochemical performance, as the V2O5 nanofibers in contrast to the two other oxide nanostructures have sufficient electrical conductivity due to their mixed valence character (V5+/V4+). In addition, it turned out that the natural water content of the electrodes does not limit the performance, but rather is beneficial due to its ability to provide ion diffusion space between the fibers and also to promote the shuttling of Li ions. In the case of SnO2-based electrodes, the developed one-step reduction/crystallization approach yielded selfassembled papers wherein SnO2 nanoparticles are closely connected to rGO sheets, thus enabling effective charge transfer upon Li ion intercalation. It is especially advantageous that while most of SnO2 particle surface is accessible for intercalation, it simultaneously establishes a conductive bridge to the rGO. For the LiMnPO4-based electrodes, carbon coating of the oxide nanoparticles was shown to be an effective strategy to facilitate the charge transport in vertical direction toward the GO sheets, and thus to enhance the electrochemical performance. However, the successful implementation of the carbon-coated particles could only be achieved through the layer-by-layer synthesis approach. As another noteworthy achievement, ice-templating of V2O5 nanofibers was demonstrated to be a valuable approach to a unique type of bio-inspired scaffolds, which combine ultrahigh porosity (up to 99.8%) with the intriguing mechanical characteristics of the structural biomaterial cuttlebone, allowing them to bear more than 1000 times their own weight without failure. At the same time, the multifunctional V2O5 building blocks open up a wide range of possible applications, including sensors, filters, catalysts and rechargeable batteries, which require a large surface area paired with excellent mechanical stability. The paper-like electrodes developed in this project are of interest for further investigations of their suitability as components of flexible Li batteries. There is an increasing demand for such type of batteries for use in wearable electronics, automobile industry or in sustainable energy conservation (solar cells). To this end, it would be highly beneficial to make all required components available in the form of flexible paper-like materials. Our current toolbox already contains several paper-like electrodes that are suitable either as cathode (V2O5 or LiMnPO4) or anode (SnO2). Furthermore, we successfully fabricated paper-like cellulose sheets as separators between the electroactive sheets. The next step is to combine the different paper-like components into layered architectures, and to fine-tune their structure as a route toward highly efficient flexible batteries.
Publications
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Hydrogen-bond reinforced vanadia nanofiber paper of high stiffness, Adv. Mater. 25 (2013), 2468
Z. Burghard, A. Leineweber, P.A. van Aken et al.
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Cuttlebone-like V2O5 Nanofibre Scaffolds – Advances in Structuring Cellular Solids, Nature Scientific Reports 7, Article number: 42951 (2017)
A. Knöller, T. Runcevski, R. E. Dinnebier, J. Bill, Z. Burghard
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Strengthening of Ceramic-based Artificial Nacre via Synergistic Interactions of 1 D Vanadium Pentoxide and 2 D Graphene Oxide Building Blocks, Nature Scientific Reports 7, Article number: 40999 (2017)
A. Knöller, C. P. Lampa, F. von Cube, T. Zeng, D. C. Bell, M. S. Dresselhaus, Z. Burghard, J. Bill
