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Understanding Interlayer Deprotonation of Hydrogen Titanium Oxide for High-Power Electrochemical Energy Storage Simon Fleischmann, Kristina Pfeifer, Mathias Widmaier, Hwirim Shim, Öznil Budak, and Volker Presser ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00363 • Publication Date (Web): 18 Apr 2019 Downloaded from http://pubs.acs.org on April 18, 2019
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ACS Applied Energy Materials
Understanding Interlayer Deprotonation of Hydrogen Titanium Oxide for High-Power Electrochemical Energy Storage Simon Fleischmann,1,2,‡ Kristina Pfeifer,3,‡ Mathias Widmaier,2,3 Hwirim Shim,1,2 Öznil Budak,1,2 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
Robert Bosch GmbH, Robert-Bosch-Campus 1, 71272 Renningen, Germany
‡
equal contributions
* Corresponding author’s eMail:
[email protected] Abstract Negative electrode materials that possess fast lithium insertion kinetics are in high demand for high power lithium-ion batteries and hybrid supercapacitor applications. In this work, hydrogen titanium oxides are synthesized by a proton exchange reaction with sodium titanium oxide resulting in the H2Ti3O7 phase. We show that a gradual water release in four steps yields intermediate phases of hydrogen titanate with different degrees of interlayer protonation. Further, a synthesis route using zinc nitrate is explored yielding H2Ti3O7 with a high rutile content. This material dehydrates already at a lower temperature, resulting in a lamellar rutile titania phase. The hydrogen titanate materials with partially protonated interlayers are tested as negative electrodes in a lithium-ion battery and hybrid supercapacitor setup, showing an improved performance compared to the fully protonated phases. The performance in half-cells reaches around 168 mAh/g, with high retention of 42 mAh/g at 10 A/g. This translates to an energy of 88 Wh/kg for a full-cell with a maximum power of 12.76 kW/kg and high cycling stability over 1000 cycles, as demonstrated for half-cell and full-cell configurations. Keywords: lithium-ion battery; hybrid capacitor; anode; hydrogen titanate; interlayer protons
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1. Introduction Electrochemical energy storage devices are required to facilitate the transition from fossil to renewable energy sources.1 Careful cell design and the adaptation of nanostructured materials are important and promising pathways towards next-generation devices.2-5 Lithium-ion batteries (LIBs) are currently applied in numerous commercial products like portable electronics or electric vehicles.6 Most LIBs rely on graphite anodes that present an insertion potential below ca. 0.2 V vs. Li/Li+,6 which is favorable to achieve high cell voltages and, in turn, high specific energy. The low anode potential, however, is outside the electrochemical stability window of carbonate-based electrolytes. Consequently, electrolyte reduction takes place during the first charging cycle and the solid electrolyte interphase (SEI) is formed.7 This process has several negative consequences, such as the irreversible consumption of lithium ions, resulting in a loss of active material,8 reduced lithium ion transfer kinetics,7 and an increased risk of dendrite formation during fast charging.9 Alternative anode materials operating at a higher insertion potential (>1 V vs. Li/Li+) to circumvent SEI formation are in high demand as they offer higher power, better stability, and increased safety.10 Particularly promising candidates are polymorphs of titania or titania/carbon hybrids,11-12 such as anatase TiO2,13-14 bronze TiO2,15 or rutile TiO2.16-17 The lithiation reaction occurs according to Equation 1:
(Eq. 1)
𝑇𝑖𝑂2 +𝑥𝐿𝑖 + +𝑥𝑒 ― ↔𝐿𝑖𝑥𝑇𝑖𝑂2
with a theoretical specific capacity of 335 mAh/g for full lithiation, with x=1.16 Compared to spinel lithium titanate (Li4Ti5O12, LTO, theoretical capacity of 175 mAh/g for fully lithiated Li7Ti5O12),18 these titanates offer a higher theoretical capacity and require no inactively bonded lithium in their crystal structure. The maximum capacity of titanate materials is dependent on the crystal structure, size, and shape of the polymorph.19 Bulk rutile exhibits a limited lithium storage capacity of
