Letter pubs.acs.org/NanoLett
Solid-State Lithium−Sulfur Batteries Operated at 37 °C with Composites of Nanostructured Li7La3Zr2O12/Carbon Foam and Polymer Xinyong Tao,*,†,§ Yayuan Liu,§ Wei Liu,§ Guangmin Zhou,§ Jie Zhao,§ Dingchang Lin,§ Chenxi Zu,§ Ouwei Sheng,† Wenkui Zhang,† Hyun-Wook Lee,§ and Yi Cui*,§,‡ †
College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou, 310014, China Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States ‡ Stanford Institute for Materials and Energy Science, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States §
S Supporting Information *
ABSTRACT: An all solid-state lithium-ion battery with high energy density and high safety is a promising solution for a next-generation energy storage system. High interface resistance of the electrodes and poor ion conductivity of solid-state electrolytes are two main challenges for solid-state batteries, which require operation at elevated temperatures of 60−90 °C. Herein, we report the facile synthesis of Al3+/Nb5+ codoped cubic Li7La3Zr2O12 (LLZO) nanoparticles and LLZO nanoparticle-decorated porous carbon foam (LLZO@C) by the onestep Pechini sol−gel method. The LLZO nanoparticle-filled poly(ethylene oxide) electrolyte shows improved conductivity compared with filler-free samples. The sulfur composite cathode based on LLZO@C can deliver an attractive specific capacity of >900 mAh g−1 at the human body temperature 37 °C and a high capacity of 1210 and 1556 mAh g−1 at 50 and 70 °C, respectively. In addition, the solid-state Li−S batteries exhibit high Coulombic efficiency and show remarkably stable cycling performance. KEYWORDS: Lithium sulfur batteries, Li7La3Zr2O12, solid state electrolytes, nanoparticles, carbon foam
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that the most efficient liquid electrolyte for the conventional Li−S batteries is lithium bis(trifluoromethanesulphonyl)imide salt dissolved in a mixture solvent of 1,3-dioxolane (DOL) and 1,2-dimethoxymethane (DME) with LiNO3 as additive to passivate the Li metal anode.18 However, DOL (flashing point of −2 °C; boiling point of 75 °C) and DME (flashing point of −18 °C; boiling point of 42 °C) have serious safety problems due to their intrinsic inflammability and electrochemical instability. Solid-state Li-ion conductors in the absence of flammable organic solvents will be a perfect solution to the safety issue,1,2,4,5,21−24 and all solid-state Li−S batteries have the potential to become the next breakthrough for the energy storage field.2,6,25,26 Solid-state Li-ion electrolytes for advanced Li-ion batteries require high ionic conductivity, good chemical and electrochemical stability, and high mechanical strength.27 Although room-temperature solid-state batteries can be realized based on
nergy density and safety issues are of immense concern in developing state-of-the-art lithium-ion (Li-ion) batteries used for modern electronic vehicles and grid-scale energy storages.1−5 Conventional cathode materials such as LiCoO2, LiNi0.8Co0.15Al0.05O2, LiNi1/3Co1/3Mn1/3O2, and LiFePO4, based on the intercalation mechanism, provide limited energy density due to their low practical capacities that are less than 200 mAh g−1. A promising strategy to improve the energy density is moving from the traditional intercalation chemistry to conversion chemistry.2,6 Lithium−sulfur (Li−S) battery, as one exciting example, can deliver 6-fold specific energy of the existing Li-ion batteries owing to the conversion reaction between Li and S.3,7−10 In addition, sulfur is naturally abundant, low cost, and environmentally benign. Li−S batteries have many challenges regarding materials and chemistry, including the dissolution of intermediate lithium polysulfides in the normal liquid electrolyte, large volumetric expansion (80%) of S upon lithiation, and poor electronic/ionic conductivity of S and lithium sulfide (Li2S). There have been significant developments for designing state-of-the-art Li−S batteries by constructing advanced composite cathode materials in the past two decades.3,7−20 To date, it has been recognized © XXXX American Chemical Society
Received: January 17, 2017 Revised: February 23, 2017 Published: April 7, 2017 A
DOI: 10.1021/acs.nanolett.7b00221 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters the thio-LISICON family such as Li3.25Ge0.25P0.75S4,28 Li3PS4,29 Li7P3S11,30 and Li10GeP2S12,31 the poor chemical/electrochemical and mechanical stability of these sulfide electrolytes hinder their practical application.32 In addition, utilization of only one of the three categories of solid-state Li-ion conductors including ceramics,33−36 glasses, and polymers is not able to meet all the above requirements. Inorganic electrolytes have high conductivity of near 10−4 S cm−1 at room temperature and good chemical stability. However, they are often too rigid and brittle, and show poor contact with electrodes. Polymer electrolytes such as poly(ethylene oxide) (PEO) are promising for solid-state Li−S batteries due to their low cost, reasonable mechanical stability, good compatibility with electrodes, and excellent film-forming ability.37−40 However, the solid-state Li− S batteries based on the PEO electrolyte usually require a working temperature ranging from 60 to 90 °C owing to the poor Li-ion conductivity at low temperature.2,41−43 Herein, we synthesize solid-state composite electrolyte consisting of Li7La3Zr2O12 (LLZO)44−51 and PEO (Figure 1),
Figure 2. Morphology and microstructure of LLZO@C synthesized via the one-step Pechini sol−gel method. (a−c) SEM images of the LLZO@C sample. Panels b and c are the corresponding local magnifications of a and b, respectively. (d−h) TEM images of the LLZO@C sample. Panel e is the high magnification TEM image showing a LLZO nanoparticle on the carbon surface. f and h are HRTEM images corresponding to areas A and B in e, respectively. g is the HRTEM image of the carbon foam. (i) FFT pattern corresponding to the HRTEM image in h. Figure 1. Schematic illustration of an all solid-state Li−S battery based on LLZO nanostructures. The blue background indicates the PEOLiClO4 solid polymer electrolyte. The pink and yellow spheres correspond to the LLZO and S particles, respectively. The construction of S cathode using LLZO@C matrix and PEO binders is aimed to reduce the interfacial resistance between the S and the ion/ electron conductive matrix. The carbon matrix is an LLZO particledecorated porous foam network. S is uniformly dispersed in the porous carbon matrix. The LLZO-PEO-LiClO4 electrolyte is casted onto the composite cathode directly. Meanwhile, the cathode and electrolyte have very close components, which is beneficial to reduce the interface resistance between the solid-state electrolyte and the cathode electrode.
that the LLZO@C sample possesses an interesting cross-linked hierarchical porous structure (Figure 2a−c). The crosssectional SEM image (Figure 2c) indicates that the pore size ranges from several tens of nanometers to several micrometers. The formation of these porous foam-like structures results from the pyrolysis of the citric acid and the nitrates, which releases large quantities of gases.16 The wall of the porous LLZO@C sample consists of abundant carbon nanoflakes decorated by some nanoparticles (Figure 2c). X-ray diffraction (XRD) shows that all peaks can be indexed to cubic LLZO (Figure S1), which match well with the standard pattern of the known garnet phase Li5La3Nb2O12 (JCPDF 45-0109). Transmission electron microscopy (TEM) was used to investigate the microstructure of LLZO@C sample. As shown in Figure 2d, the carbon nanoflake has porous structure. Some nanoparticles with size ranging from 30 to 200 nm can be found on the carbon surface, which is consistent with the calculated results from the Scherrer’s formula (Table S1). Figure 2f,h shows the [111] zone axis HRTEM images, corresponding to areas A and B in Figure 2e, respectively. The spacing between two adjacent lattice fringes is about 0.35 nm, corresponding well to the spacing of the {123} planes of cubic LLZO. The fast Fourier transform (FFT) pattern in Figure 2i indicates that the LLZO nanoparticle is single crystalline. Figure 2g is the HRTEM image of the carbon matrix, revealing the porous and amorphous structure. Some (002) lattice fringes of graphite with distance of 0.34 nm can be found in some local areas (Figure 2g) of the carbon nanoflake. This kind of porous carbon decorated with LLZO nanoparticles will be a perfect matrix for the cathode materials of solid-state Li−S batteries.
which combines the advantages of both polymer and ceramic electrolytes. Moreover, in order to reduce the interfacial resistance between S and the ion/electron conductive matrix due to the poor ionic/electronic conductivities of S and Li2S, we synthesized the LLZO nanoparticle-decorated carbon foam (LLZO@C) by the one-step facile Pechini sol−gel method. The S cathode constructed from the LLZO@C (Figure 1) shows remarkable cyclability and can work at the normal human body temperature of 37 °C. Compared to previous demonstrations of using polymer composites of PEO with ZrO2 for Li−S batteries operated at 70 °C,2 and using PEO polymer with LiTNFSI for Li−S battery at 60 °C,52 the current work represents a step forward toward lower temperature operation due to the improved ionic and electronic conductivity. Figure 2a shows the low magnification scanning electron microscopy (SEM) image of the LLZO@C sample. As indicated by the red arrows, Figure 2b,c is the corresponding local magnification of Figure 2a,b, respectively. It can be found B
DOI: 10.1021/acs.nanolett.7b00221 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters
Figure 3. TEM and elemental mapping of the LLZO@C sample. (a) Low magnification TEM images of the LLZO@C. (b) High magnification TEM image of the corresponding particle indicated by the red arrow in a. (c−h) The element maps of C, O, La, Zr, Al, and Nb.
Usually, LLZO has two kinds of phases including cubic (Ia3d̅ ) and tetragonal (I4(1)/acd). From the thermodynamic point of view, tetragonal phase, possessing a Li-ion conductivity of
