Solid Electrolyte Lithium Phosphous Oxynitride as a Protective Nanocladding Layer for 3D High-Capacity Conversion Electrodes Chuan-Fu Lin,*,†,‡ Malachi Noked,†,‡,§ Alexander C. Kozen,†,‡ Chanyuan Liu,† Oliver Zhao,† Keith Gregorczyk,†,‡ Liangbing Hu,† Sang Bok Lee,§ and Gary W. Rubloff†,‡ Downloaded via MIDWESTERN UNIV on January 27, 2019 at 09:27:32 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
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Department of Materials Science and Engineering, ‡Institute for Systems Research, and §Department of Chemistry, University of Maryland, College Park, Maryland 20742, United States S Supporting Information *
ABSTRACT: Materials that undergo conversion reactions to form different materials upon lithiation typically offer high specific capacity for energy storage applications such as Li ion batteries. However, since the reaction products often involve complex mixtures of electrically insulating and conducting particles and significant changes in volume and phase, the reversibility of conversion reactions is poor, preventing their use in rechargeable (secondary) batteries. In this paper, we fabricate and protect 3D conversion electrodes by first coating multiwalled carbon nanotubes (MWCNT) with a model conversion material, RuO2, and subsequently protecting them with conformal thin-film lithium phosphous oxynitride (LiPON), a well-known solid-state electrolyte. Atomic layer deposition is used to deposit the RuO2 and the LiPON, thus forming core double-shell MWCNT@RuO2@LiPON electrodes as a model system. We find that the LiPON protection layer enhances cyclability of the conversion electrode, which we attribute to two factors. (1) The LiPON layer provides high Li ion conductivity at the interface between the electrolyte and the electrode. (2) By constraining the electrode materials mechanically, the LiPON protection layer ensures electronic connectivity and thus conductivity during lithiation/delithiation cycles. These two mechanisms are striking in their ability to preserve capacity despite the profound changes in structure and composition intrinsic to conversion electrode materials. This LiPON-protected structure exhibits superior cycling stability and reversibility as well as decreased overpotentials compared to the unprotected core−shell structure. Furthermore, even at very low lithiation potential (0.05 V), the LiPON-protected electrode largely reduces the formation of a solid electrolyte interphase. KEYWORDS: solid electrolyte, LiPON, atomic layer deposition, overpotential reduction, SEI reduction, conversion electrodes, artificial SEI Conversion electrodes (e.g., metal oxides, metal fluorides, and metal sulfides) can be lithiated through a reaction that forms a composite of metal nanoparticles embedded in electrically insulating lithium compounds (e.g., Li2O, Li2S, LiF). Through Li-induced reduction of the metal in MXb (where M = metal,
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he demands of electrical energy storage are dramatically increasing in order to power high-performance devices, electric vehicles, and grid-scale energy storage systems.1,2 Other than conventional low-capacity insertion electrodes (e.g., graphite, LiCoO2), enormous efforts have been focused on using high-capacity electrode materials that store lithium through alloying (e.g., Si, Ge, Sn)3,4 or through conversion reactions (e.g., RuO2, Fe2O3)5−9 to build advanced lithium ion batteries that meet the needs of electrical storage energy for nextgeneration batteries. © 2016 American Chemical Society
Received: December 9, 2015 Accepted: January 28, 2016 Published: January 28, 2016 2693
DOI: 10.1021/acsnano.5b07757 ACS Nano 2016, 10, 2693−2701
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RESULTS AND DISCUSSION Fabrication of 3D Core Double-Shell MWCNT@RuO2@ LiPON Electrodes. In this work nanoheterostructures were fabricated with multiwalled carbon nanotube (MWCNT) sponges as 3D scaffolds with large surface areas, high porosity, and excellent electronic conductivity, achieving good mechanical and chemical stability and also serving as the current collector.11,25,26 Three-dimensional nanostructured electrodes have been demonstrated with very high surface areas to significantly increase areal capacity and rate performance9,35 and to reduce electrode overpotentials.36 Additionally, significant enhancement of high-capacity reactive electrode stability has been demonstrated through the use of 3D architectures.37 Finally, we have found the MWCNT scaffold to be particularly valuable because of the ease with which TEM analysis can be done. Atomic layer deposition was used to deposit conformal coatings onto this mesoporous structure.27−31 Figure 1a shows a schematic of the heterostructured electrode fabrication. First, MWCNT sponges grown by chemical vapor deposition (CVD) were cut into a desired and controlled size (1/4 in. diameter, ∼0.5 mm thick) and placed into a homemade cross-flow ALD reactor. Using bis(ethylcyclopentadienyl)ruthenium(II), C14H18Ru (Colonial Metals Inc.), and oxygen (Air Liquide, 99.99%) as the precursors, 300 cycles (GPC ≈ 0.80 Å/cycle) of ALD RuO2 electrode material were deposited at 240 °C on the MWCNT sponge to form core−shell MWCNT@RuO 2 heterostructures. The physical properties of the as-deposited RuO2 were characterized by Raman spectroscopy, XRD, and TEM (Supporting Information Figures S1−S3). After determining the mass of deposited RuO2, samples were loaded into a commercial ALD reactor (Cambridge Nanotech Fiji), where it was coated with 200 ALD cycles of LiPON at 250 °C, to form core double-shell MWCNT@RuO2@LiPON electrodes with a Li+ ion conductivity of 2 × 10−7 S/cm according to previously reported conditions.32 Transmission electron microscopy (TEM) images of MWCNT@RuO2 are shown in Figure 1b,c. These TEM images confirm the conformality and show that the ALD RuO2 layer is ∼24 nm thick, composed of polycrystalline grains with a size of ∼15−20 nm. Figure 1 d,e show the TEM images after subsequent addition of 200 ALD cycles of LiPON (light gray) coating on MWCNT@RuO2 to form the desired MWCNT@ RuO 2 @LiPON electrode. The LiPON coated the RuO 2 electrode conformally, with a thickness of ∼17 nm and an amorphous structure. The variation of thickness of the LiPON layer in the TEM images could be due to different local roughness of RuO2 nuclei and different growth rates of the LiPON at different facets of the RuO2 crystals. Considering these factors, TEM images always suggest some degree of nonuniformity. Most of the underlying RuO2 retains its size and crystallinity during the LiPON deposition process. The as-prepared MWCNT@RuO2@LiPON was directly transferred under UHV conditions to a surface analysis instrument (XPS)thus avoiding air exposureto determine the surface chemistry of the LiPON film. XPS spectra of the LiPON coating on MWCNT@RuO2 are shown in Figure 2. All spectra are calibrated against the Li 1s peak (Figure 2a) binding energy at 55.6 eV, in line with a previously reported value for Li3PO4.32,33 The P 2p3/2 (133.7 eV) peak (Figure 2c) is also consistent with Li3PO4. Doubly coordinated N and triply coordinated N are both observed in our XPS spectra in Figure 2b. In
X = S, O, F, etc.) to its metallic state and subsequent formation of Li-rich insulators, conversion materials can be discharged to 3−5 times the specific capacity found in intercalation materials.10,11 However, the formation of new phases and structures through these complex conversion reactions, MXb + 2bLi+ ↔ M + bLi2X, presents intrinsic limitations to the mechanical stability and reversibility of the system. Mechanically, expansion (lithiation) and shrinkage (delithiation) of the materials induces the formation of cracks and voids in the active material, leading to electrode pulverization and loss of active material. Electrically, the formation of insulating lithium compounds (e.g., Li2S, Li2O, LiF) during the conversion reaction impedes electron pathways between active materials and the current collector, blocking electrical accessibility to the insulating and insulated conducting domains formed in the active material and causing high overpotentials in the system.5,10,11 Unfortunately, despite the high capacity, these intrinsic problems cause poor reversibility and high charge−discharge hysteresis, which impede realization of practical conversion electrodes. Many attempts have been made by applying an atomic layer deposition (ALD) coating (e.g., Al2O3, HfO2) to conformally coat high-capacity electrode materialsboth anode and cathodein order to engineer their stability, to reduce and/or control solid electrolyte interphase (SEI) formation, and to provide mechanical support.12−16 The thickness of ALD layers (Al2O3, HfO2) is critical and needs to be optimized because of the trade-off between obtaining mechanical and chemical stability and reducing ionic impedance. Unfortunately, due to the low ionic conductivity of many ALD films, most commonly oxides, only thin layers (
