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Atomic layer deposited molybdenum oxide/carbon nanotube hybrid electrodes: Influence of crystal structure on lithium-ion capacitor performance Simon Fleischmann, Marco Zeiger, Antje Quade, Angela Kruth, and Volker Presser ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03233 • Publication Date (Web): 11 May 2018 Downloaded from http://pubs.acs.org on May 14, 2018
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ACS Applied Materials & Interfaces
Atomic layer deposited molybdenum oxide / carbon nanotube hybrid electrodes: Influence of crystal structure on lithium-ion capacitor performance Simon Fleischmann,1,2 Marco Zeiger,1,2 Antje Quade,3 Angela Kruth,3 and Volker Presser1,2,*
1
INM - Leibniz Institute for New Materials, 66123 Saarbrücken, Germany
2
Department of Materials Science and Engineering, Saarland University, 66123 Saarbrücken, Germany
3
Leibniz Institute for Plasma Science and Technology, 17489 Greifswald, Germany
* Corresponding author’s eMail:
[email protected] Abstract Merging of supercapacitors and batteries promises the creation of electrochemical energy storage devices that combine high specific energy, power, and cycling stability. For that purpose, lithium-ion capacitors (LICs) that store energy by lithiation reactions at the negative electrode and double-layer formation at the positive electrode are currently investigated. In this study, we explore the suitability of molybdenum oxide as a negative electrode material in LICs for the first time. Molybdenum oxide/carbon nanotube hybrid materials were synthesized via atomic layer deposition and different crystal structures and morphologies were obtained by post-deposition annealing. These model materials are first structurally characterized and electrochemically evaluated in half-cells. Benchmarking in LIC full-cells revealed the influences of crystal structure, half-cell capacity, and rate handling on the actual device level performance metrics. The energy efficiency, specific energy, and power are mainly influenced by the overpotential and kinetics of the lithiation reaction during charging. Optimized LIC cells show a maximum specific energy of about 70 Wh·kg-1 and a high specific power of 4 kW·kg-1 at 34 Wh·kg-1. The longevity of the LIC cells is drastically increased without significantly reducing the energy by preventing a deep cell discharge, hindering the negative electrode from crossing its anodic potential limit.
Key words Lithium ion capacitor; hybrid materials; electrochemical energy storage; asymmetric supercapacitor; molybdenum oxide
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1. Introduction In the face of global warming and climate change, the mitigation of CO2 emissions is considered a key challenge during the next decades.1 Therefore, a transition from fossil to renewable energy sources in the electricity and mobility sectors has to be realized, requiring the availability of efficient, fastresponding electrochemical energy storage devices.2-3 Electrical double-layer capacitors (EDLCs) employ high surface area electrodes that store energy by formation of the electrical double-layer via adsorption of ions at the charged interface to the electrolyte.4 Most commonly, carbons with high internal porosity are used as electrodes, such as activated carbons5-6 or carbide-derived carbons7-8, offering a high surface area (2000-3000 m2·g-1) for ion electrosorption. Carbon nanomaterials like carbon nanotubes (CNTs)9 and carbon onions10-11 with a large outer surface area find use in high power applications, as they enable even faster double-layer formation by offering shorter diffusion paths to the electrolyte ions.12 Though offering high specific power (>10 kW·kg-1) and long lifetimes (>100,000 cycles), the main drawback of EDLCs is their low specific energy (100 Wh·kg-1).13 Commonly found Faradaic materials are metal oxides such as MoO2,14 V2O5,15 MnO2,16 Nb2O5,17 or LTO,18 which are combined with conductive carbons to form a composite electrode due to their oftentimes poor electrical conductivity. Yet, intercalation reactions are kinetically limited by solid state diffusion and may cause significant volumetric changes to the electrode materials during operation19, thereby limiting the specific power (
