Article pubs.acs.org/JPCC
Li/Li7La3Zr2O12/LiFePO4 All-Solid-State Battery with Ultrathin Nanoscale Solid Electrolyte Xufeng Yan,†,‡,∥,⊥,# Zhuobin Li,†,# Zhaoyin Wen,‡,∥,⊥ and Weiqiang Han*,†,‡,§,⊥ †
Ningbo Institute of Industrial Technology, Chinese Academy of Science, Ningbo 315200, China School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China § Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China ∥ Shanghai Institute of Ceramics, Chinese Academy of sciences, Shanghai 200050, China ⊥ College of Materials Science and Optoelectronic Technology, University of Chinese Academy of Sciences, Beijing 100049, China ‡
ABSTRACT: We introduced the nanoconcept in the oxide solid electrolyte Li7La3Zr2O12 (LLZO). All-solid-state Li/LiFePO4 (LFPO) cell using this solid electrolyte with thickness of several micrometers was assembled with appropriate solvent, dispersant, adhesives, and surfactant without cold- or hot-pressing. At room temperature, the Li/LLZO/LFPO cell showed the first discharge capacity of 160.4 mAh g−1, which was 94.4% of the theoretical capacity of LFPO, and the cell provided a discharge capacity of 136.8 mAh g−1 after 100 cycles. At 60 °C, the battery presented more stable electrochemical performance. The capacity loss during cycles 2−100 was only 0.06% (0.7 mAh g−1). The excellent performance could be attributed to the ultrathin solid electrolyte film.
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INTRODUCTION Recent decades, lithium ion batteries (LIBs) have been widely used as power sources for portable electronic devices, such as mobile phones and laptops, and electric vehicles due to their high energy and power density.1,2 However, the common LIBs are based on flammable organic liquid electrolytes, which carry the inherent risks of leakage, fire, and explosion.3 In addition, the development of large-scale LIBs makes these safety issues more serious.4 Therefore, all-solid-state lithium ion batteries (ASSLIBs) have attracted much attention because the replacement of a volatile and flammable organic liquid electrolyte with a nonflammable inorganic solid electrolyte could improve these safety issues.5 For solid electrolyte, there are two main challenges needed to be overcome: One is the low ion conductivity of solid electrolyte, and the other is the large resistance at the electrolyte/electrode interface.6,7 Up to now, many efforts have been made to develop highly conductive materials. For example, a new Li10GeP2S12 phase,8 which was first reported in 2011, had a relatively high ionic conductivity of 1.2 × 10−2 S·cm−1. The conductivity of this sulfide solid electrolyte is comparable to or higher than those of organic liquid electrolytes used in the practical LIBs.9 However, the sulfides are not stable because they can react with ambient moisture to generate H2S gas. On the contrary, the oxide solid © 2017 American Chemical Society
electrolytes are more stable without releasing toxic gases, even in the case of breaking down.10 Among the oxide electrolyte, the garnet type Li7La3Zr2O12 (LLZO) possesses the advantages of excellent chemical stability in contact with lithium metal, which is a key factor to be a promising solid electrolyte.11,12 In recent years, the ion conductivities of LLZO have been highly improved by element doping13−15 or multielement doping;16−18 even some ASSLIBs of LLZO solid electrolyte have been manufactured,19−21 but the low ion conductivities of oxides solid electrolytes limit their applications in ASSLIBs. Fortunately, the deficiency of low conductivity can offset to some extent by minimizing the solid electrolyte thickness.22,23 The lithium diffusion time t and the thickness of electrolyte layer L is proportional to the square, t = L2/D (D, diffusion constant). When L is reduced, t is more largely reduced, and thus the demand for highly lithium ion conductivity of solid electrolyte is dented. By now, the main method of LLZO layer formation was high-pressure powder compacting. The thickness of LLZO pellet prepared by powder compacting was at least several hundred micrometers,11,20,21,24,25 which can hardly reduce the high demand for ionic conductivity. Received: October 11, 2016 Revised: November 29, 2016 Published: January 2, 2017 1431
DOI: 10.1021/acs.jpcc.6b10268 J. Phys. Chem. C 2017, 121, 1431−1435
Article
The Journal of Physical Chemistry C
Figure 1. Schematic illustration of the synthesis procedure. (a) Microscale LLZO particles. (b) Nanoscale LLZO particles. (c) Nanoscale LLZO slurry. (d) Cathode layer of LFPO. (e) LLZO film. (f) All-solid-state battery of Li/LLZO/LFPO.
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Herein, we have successfully developed a simple and low-cost production process to construct an ASSLIB using a garnet type LLZO nanoparticle slurry to form SE layer of several micrometers.
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RESULTS AND DISCUSSION
The scheme (Figure 1) presents the detailed steps from the synthesis of raw materials to the fabrication of ASSLIB. The LLZO nanoparticle slurry was obtained by high-energy ballmilling from microscale LLZO powders and followed by the addition of solvent, dispersant, adhesives, surfactant, and Li salt. Then, the slurry was coated on the LiFePO4 (LFPO) cathodes, which was already mixed with LLZO particles (Figure 1d). Finally, Li foil as anode was assembled as the all-solid-state battery. Figure 2a depicted the XRD spectra of the LLZO nanoparticles before and after ball-milling. As shown in the XRD patterns, the diffraction intensity peaks matched well with the cubic garnet phase Li7La3Nb2O12 (PDF no. 40−0894). The sharp diffraction patterns of the original particles indicate a good crystallization at 1100 °C. Moreover, the peak width of XRD becomes much wider after ball-milling, indicating a reduction of the average size of the LLZO particles. According to Sheller equation, the average particle size can be calculated to ∼61.7 nm after ball-milling. Figure 2b exhibited the scanning electron microscopy (SEM) images of nanoscale LLZO particles. The grain diameter of ball-milled LLZO particles was about 50−80 nm, which is in good agreement with the XRD results. The electrochemical window of the ultrathin LLZO film was characterized by CV at a scanning rate of 0.1 mV s−1 from −0.3 to 5.0 V. As shown in Figure 2c, the electrochemical window of the ultrathin LLZO film was stable, especially in the range of 1.0− 5.0 V. There were several small redox peaks under 1 V, which might result from the side reaction of the remaining dispersant and surfactant or the side reaction at the interfaces between Li and LLZO.25 The temperature dependence of the ionic conductivity was studied in the temperature range from 293 to 373 K in air, as shown in Figure 2d. The ionic conductivity of LLZO thin film was 2.4 × 10−3 mS cm−1 at room temperature and increased to 1.8 × 10−2 mS cm−1 at 373 K. These data were lower than those of the bulk LLZO previously reported.26,27 This was probably due to the existence of more grain boundaries between the nanoparticles. The activation energy (Ea) of lithium ion conduction was 0.25 eV, which was calculated according to the Arrhenius equation
METHODS
LLZO particle was synthesized via the conventional solid-state reaction. The raw materials of Li2O (Alfa Aesar, > 99.5%), La2O3 (Alfa Aesar, > 99.9%, dried at 900 °C for 12 h), and ZrO2 (Alfa Aesar, > 99.9%) were weighted in stoichiometric amounts (15% Li2O excess was used). Al2O3 was added to stabilize the cubic LLZO phase. The mixture of the raw materials was ball-milled for 6 h with zirconia balls. Then, the mixed powder was sintered at 1100 °C for 12 h. Nanoscale LLZO slurry was prepared using high-energy ball-milling technology (Emax, Retsch), with the addition of 1-methyl-2-pyrrolidinone (NMP), LiN(CF3SO2)2 (LiTFSI), SPEEK-PSI-Li, and PiuronicF127 in a glovebox filled with argon. The ultrathin electrolyte film was obtained by wet coating the final slurry on the prepared cathodes and dried at 80 °C in a vacuum oven for 12h. X-ray diffraction (XRD) characterizations were performed on an AXS D8 Advance diffractometer (Cu Kα radiation = 0.154 nm) in the range of 2θ = 10−90°. The microstructure and morphology of the samples were characterized using a fieldemission scanning electron microscope (FESEM, Hitachi, S-4800) and energy-dispersive X-ray spectroscopy (EDS, FEI, Quanta FEG 250). The section of the layer is prepared by the focused ion beam (FIB, Carl Zeiss). The electrochemical measurements were conducted by 2032 coin cell, with lithium foil as the anode, assembled in an argonfilled glovebox. Cathodes were prepared by casting the slurry of NMP, containing 56 wt % LLZO, 24 wt % LFPO, 10 wt % carbon black (Super P), and 10 wt % polyvinylidene fluoride (PVDF) onto Al foil, which would be dried at 80 °C in a vacuum oven overnight. The solid electrolyte slurry was carried out by mixing 45 wt % Li7La3Zr2O12, 45 wt % LiN(CF3SO2)2 (LiTFSI), and 10 wt % for dispersant/adhesives/surfactant. The electrochemical properties were studied with a Land automatic battery tester (Wuhan, China).The cyclic voltammogram (CV) and the impedance were conducted on a Solartron 1470E electrochemical interface (Solartron Analytical, U.K.).
σ= 1432
⎛ −E ⎞ A exp⎜ a ⎟ T ⎝ k bT ⎠
(1) DOI: 10.1021/acs.jpcc.6b10268 J. Phys. Chem. C 2017, 121, 1431−1435
Article
The Journal of Physical Chemistry C
Figure 2. Structure characterizations. (a) XRD patterns of LLZO powder and LLZO slurry. (b) SEM image of the LLZO slurry. (c) Cyclic voltammetry curve of Li/LLZO/Pt cell within the voltage range of −0.3 to 5 V at a scanning rate of 0.1 mV s−1. (d) Arrhenius conductivity plots of LLZO film.
where A is pre-exponential factor, Ea is the activation energy, kb is the Boltzmann constant, and T is the absolute temperature. The calculated Ea value was similar to previous reports.16,28,29 For better elaborating the layer structure of all-solid-state battery, we investigated the section of the layer structure by the FIB. As shown in Figure 3a, the Pt layer on the border of
Figure 4a−e. The CV analysis (as displayed in Figure 4a) of the cell was obtained by scanning at a rate of 0.05 mV s−1 for five cycles between 2.0 and 4.2 V. The cathodic/anodic peaks were corresponding to LFPO.30 These curves overlapped to a great extent, which demonstrated an excellent cycling performance. As shown in Figure 4b, the cell was galvanically charged/ discharged at a current density of 0.1 C. At 0.1 C, the Li/LLZO/ LFPO cell showed a discharge capacity of 136.8 mAh g−1 after 100 cycles, which was 88.6% of the second cycle. During the cycles, the Coulombic efficiency was 89.9% for the first cycle and 99.8% for the 100th. In Figure 4c, for the first cycle, the cell exhibited a discharge capacity of 160.4 mAh g−1, which was the 94.4% of the theoretical capacity of LFPO. The charge and discharge voltage plateaus were 3.47 and 3.39 V, demonstrating a very small polarization. All of the data above indicated the good cycle performance of the Li/LLZO/LFPO cell. Figure 4d showed the rate performance. The Li/LLZO/LFPO cell provided a discharge capacity of 157 mAh g−1 at 0.05 C, 141 mAh g−1 at 0.1 C, 114 mAh g−1 at 0.2 C, and 68 mAh g−1 at 0.5 C. The capacity became 160 mAh g−1, with the current density changed back to 0.05 C, which revealed the excellent rate performance of the Li/LLZO/LFPO cell. To better understand the origin of electrochemical performance of Li/LLZO/LFPO cell, the detailed reaction kinetics was further studied by electrochemical impedance spectroscopy (EIS) measurements. The EIS was measured to analyze the resistance evolution during the cycling performance by changing the frequency from 1 MHz to 0.01 Hz. As shown in Figure 4e, the interfacial resistance of the Li/LLZO/LFPO battery increased mildly, which explained the small offset of the redox peak in the cyclic voltammetry curve (as in Figure 4a).
Figure 3. (a) SEM of the section of LFPO cathode layer and LLZO film. (b) ESB of the section of LFPO cathode layer and LLZO film.
surface and surface was melted by the high energy because the section was polished by FIB. The LLZO ultrathin film was uniformly coated on the cathode layer without obvious cracks or holes. The thickness of the LLZO film was about 3−5 μm, which was much thinner than that of the SE layer formed by powder compacting.9 The energy-selective backscattering (ESB) of the section is shown in Figure 3b; in the SE layer, all of the particles were uniform-mixed: mainly dark color represents Li salt and light color represents nanoscale LLZO. In the cathode layer, the colors white, gray, and black represented LLZO, LFPO, and super P, respectively. The electrochemical performances of Li/LLZO/LFPO cell were tested at room temperature, and the result is presented in 1433
DOI: 10.1021/acs.jpcc.6b10268 J. Phys. Chem. C 2017, 121, 1431−1435
Article
The Journal of Physical Chemistry C
Figure 4. Electrochemical performance of the Li/LLZO/LFPO cell. At room temperature: (a) cyclic voltammetry curve of the initial five cycles of the Li/LLZO/LFPO cell at a scanning rate of 0.05 mV s−1 from 4.2 to 2 V, (b) Cycling performance and Coulombic efficiency, (c) galvanostatic charge/ discharge profiles of the Li/LLZO/LFPO cell at the 1st, 2nd, 10th, 100th cycles, and (d) rate performance. (e) Impedance plots of Li/LLZO/LFPO cell (Z′ and Z″ denote real and imaginary parts, respectively). At 60 °C: (f) cycling performance and Coulombic efficiency.
This work provides a promising way for preparing high performances of solid electrolyte layer of micrometer thickness, which could be used for the industrialization of all-solid-state battery.
Furthermore, the Li/LLZO/LFPO cell was galvanically charged/discharged at 60 °C. As depicted in Figure 4f, at 0.1 C, the cell showed a discharge capacity of 146.2 mAh g−1 after 100 cycles, which was 99.4% of the second cycle. There was only 0.7 mAh g−1 capacity loss during the cycles, which demonstrated that high-temperature performance was more stable than at room temperature. In addition, the Coulombic efficiency of the first cycle was 88.9% and kept 99.3% after the 10th cycle.
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AUTHOR INFORMATION
Corresponding Author
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*E-mail:
[email protected].
CONCLUSIONS We synthesized LLZO nanoparticle slurry by ball milling and assembling with appropriate solvent, dispersant, adhesives, and surfactant without cold- or hot-pressing to form several micrometers thickness of solid electrolyte layer LFPO/LLZO/ Li cell exhibiting good cycling performance. At 0.1 C, the cell showed an excellent cycling stability of 136.8 and 146.2 mAh g−1 after 100 charge/discharge cycles at room temperature and 60 °C.
ORCID
Zhaoyin Wen: 0000-0003-1698-7420 Weiqiang Han: 0000-0001-5525-8277 Author Contributions #
X.Y. and Z.L. contributed equally to this work.
Notes
The authors declare no competing financial interest. 1434
DOI: 10.1021/acs.jpcc.6b10268 J. Phys. Chem. C 2017, 121, 1431−1435
Article
The Journal of Physical Chemistry C
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ACKNOWLEDGMENTS This work was financially supported by the Strategic Priority Research Program of the Chinese Project Academy of Science (No.: XDA09010201), China Postdoctoral Science Foundation (Grant No.: 2015M570529), the National Natural Science Foundation of China (Grant No. 51371186), Ningbo 3315 International Team of Advanced Energy Storage Materials, Zhejiang Province Key Science and Technology Innovation Team (2013TD16). We thank Dr. Hong Li for the helpful discussions and suggestions.
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