Enhanced ionic conductivity with Li7O2Br3 phase in

Enhanced ionic conductivity with Li7O2Br3 phase in Li3OBr anti-perovskite solid electrolyte Jinlong Zhu, Shuai Li, Yi Zhang, John W. Howard, Xujie Lü, Yutao Li, Yonggang Wang, Ravhi S. Kumar, Liping Wang, and Yusheng Zhao Citation: Applied Physics Letters 109, 101904 (2016); doi: 10.1063/1.4962437 View online: http://dx.doi.org/10.1063/1.4962437 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/109/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Three-dimensional ionic conduction in the strained electrolytes of solid oxide fuel cells J. Appl. Phys. 119, 174904 (2016); 10.1063/1.4948694 Correlation of anisotropy and directional conduction in β-Li3PS4 fast Li+ conductor Appl. Phys. Lett. 107, 013904 (2015); 10.1063/1.4926725 Ionic conductivity and dielectric permittivity of PEO-LiClO4 solid polymer electrolyte plasticized with propylene carbonate AIP Advances 5, 027125 (2015); 10.1063/1.4913320 Retraction: “Lithium-ion conducting La2/3–x Li3x TiO3 solid electrolyte thin films with stepped and terraced surfaces” [Appl. Phys. Lett. 100, 173107 (2012)] Appl. Phys. Lett. 102, 089902 (2013); 10.1063/1.4794148 Lithium-ion conducting La2/3− x Li3 x TiO3 solid electrolyte thin films with stepped and terraced surfaces Appl. Phys. Lett. 100, 173107 (2012); 10.1063/1.4709402

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APPLIED PHYSICS LETTERS 109, 101904 (2016)

Enhanced ionic conductivity with Li7O2Br3 phase in Li3OBr anti-perovskite solid electrolyte €,2 Yutao Li,3 Jinlong Zhu,1,a),b) Shuai Li,1,a) Yi Zhang,1 John W. Howard,1 Xujie Lu 1 1 1 Yonggang Wang, Ravhi S. Kumar, Liping Wang, and Yusheng Zhao1,4,b) 1

High Pressure Science and Engineering Center, University of Nevada, Las Vegas, Nevada 89154, USA Center for Integrated Nanotechnologies and Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA 3 Materials Research Program and The Texas Materials Institute, University of Texas at Austin, Texas 78712, USA 4 Department of Physics, South University of Science and Technology of China, Guangdong 518055, China 2

(Received 18 July 2016; accepted 26 August 2016; published online 7 September 2016) Cubic anti-perovskites with general formula Li3OX (X ¼ Cl, Br, I) were recently reported as superionic conductors with the potential for use as solid electrolytes in all-solid-state lithium ion batteries. These electrolytes are nonflammable, low-cost, and suitable for thermoplastic processing. However, the primary obstacle of its practical implementation is the relatively low ionic conductivity at room temperature. In this work, we synthesized a composite material consisting of two antiperovskite phases, namely, cubic Li3OBr and layered Li7O2Br3, by solid state reaction routes. The results indicate that with the phase fraction of Li7O2Br3 increasing to 44 wt. %, the ionic conductivity increased by more than one order of magnitude compared with pure phase Li3OBr. Formation energy calculations revealed the meta-stable nature of Li7O2Br3, which supports the great difficulty in producing phase-pure Li7O2Br3 at ambient pressure. Methods of obtaining phasepure Li7O2Br3 will continue to be explored, including both high pressure and metathesis techniques. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4962437]

Lithium ion batteries (LIBs) have become profoundly important energy storage devices, particularly for their widespread applications to electric vehicles and portable electronic devices.1–4 Most commercial LIBs make use of a liquid electrolyte, having multiple disadvantages such as toxicity, corrosiveness, and flammability.5,6 To this end, inorganic solid state electrolytes have drawn extensive attention and are expected to eliminate the safety and environmental drawbacks of liquid electrolytes.7–10 An additional advantage of using solid electrolytes in LIBs is in the aversion of lithium dendritic growth11 while using lithium metal as the anode, which can maximize the energy/power density. Nevertheless, solid electrolytes must surpass or compete with liquid electrolytes in terms of ionic transport, low activation energy, negligible electronic conductivity, large electrochemical window, and electrode compatibility.8 Currently, there are few known solid electrolytes possessing the desired properties, none of which meet all of the criteria.12–16 For instance, the newly developed lithium superionic conductor Li10GeP2S12, having a three-dimensional framework, has a Liþ conductivity of 1.2 102 S/cm at room temperature (RT), which is the highest ionic conductivity reported in a solid electrolyte, but suffers from long-term electrode instability.15 The garnet-type Li7La3ZrO12 has a high bulk ionic conductivity but enormous grain boundary resistance.13,14 Nanoporous b-Li3PS4 was reported to have a conductivity of 1.6 104 S/cm at RT.16 NASICON (sodium (Na) Super Ionic CONductor)13 and LISICON (LIthium Super Ionic a)

J. Zhu and S. Li contributed equally to this work. Authors to whom correspondence should be addressed. Electronic addresses: [email protected]; [email protected] and [email protected]

b)

0003-6951/2016/109(10)/101904/5/$30.00

CONductor)17,18 compounds are widely studied solid state fast lithium-ion conductors. There is still no known solid electrolyte that can now fulfill all of the requirements necessary to replace liquid electrolytes in LIBs. A primary goal in exploring a new class of solid electrolytes is not only to enhance the flexibility of full solid-state battery design, but to meet the requirements of portable energy storage and transportation in a more environmentally friendly fashion. The recently reported antiperovskite electrolyte family,19,20 adapted from perovskites (K,Na)MgF3, could be a promising system due to their crystal structure and Liþ superionic conductivity. The advantages of Li3OA (A ¼ halogens) include the large electrochemical window, low electronic conductivity,21,22 and stability against a lithium metal anode.23 The relatively low ionic conductivity at lower temperatures must be further enhanced by structural/chemical manipulations, such as cationic doping and Liþ depleting. In this work, we demonstrated that a layered antiperovskite structure compound (Li7O2Br3), which is thermal dynamically unstable compared to Li3OBr, was kinetically quenched as a secondary phase during the Li3OBr synthesis process. We found that with the percentage of Li7O2Br3 increasing in the composite mixture, the ionic conductivity increased and the activation energy decreased. The maximum layered structure weight percentage reached was 44% (44:56 Li7O2Br3:Li3OBr), with the ionic conductivity reaching 0.24 104 S/cm, more than one order of magnitude higher than the pure phase Li3OBr compound. Pure phase cubic antiperovskite Li3OBr and composite phase Li7O2Br3:Li3OBr were synthesized using a solid state reaction inside a glovebox with starting materials of Li2O (Alfa Aesar, 99.5%) and LiBr (Alfa Aesar, anhydrous, 99%).

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The reactions were performed under the protection of argon gas with oxygen and moisture levels below 10 ppm; the appropriate molar ratios were weighed, mixed, and ground inside the glovebox. The mixed raw materials were then loaded into alumina crucibles. The mixtures were heated at 480 C for 16 h, ball milled for 1 h, and repeated for a total of three cycles. For the ball milling process, the powders were loaded into sealed ball milling jars inside the glovebox and then transferred to the ball milling instrument. The basic reaction pathway is shown in Equations (1) and (2) Li2 O þ LiBr ! Li3 OBr;

(1)

2Li2 O þ 3LiBr ! Li7 O2 Br3 :

(2)

The X-ray powder diffraction (XRD) patterns were recorded on an X-ray diffractometer (Bruker D8 Advance) with Cu K-a radiation. The as synthesized powders were sealed in a zero background air tight sample holder during the x-ray diffraction process. The crystal parameters and phase ratios were analyzed by Rietveld refinement using the GSASþEXPGUI software package.24 Electrochemical impedance spectroscopy (EIS) measurements were also performed inside the glovebox. The samples were melted in between two pieces of Au foil electrodes forming symmetric cells. The thickness of the sample is 300 lm, the dimension is 0.6 cm, and the area is 0.28 cm2. The sample is assembled in a sealed sample holder (with slight pressure applied) and placed in a compact furnace for heating inside a glovebox filled with argon gas. The cells were allowed to stabilize for 1 h after reaching the target temperature in the furnace at each set point prior to each impedance measurement. The impedance spectra data were collected on a Solartron 1260A instrument at OCV (Open Circuit Voltage), with applied AC perturbation voltage of 10 mV in the frequency range from 1 MHz to 1 Hz, as function of temperature (from 25 C to 180 C). The crystal structures of Li3OBr, LiBr, and Li7O2Br determined from refinement were employed as the initial structure model to study the total energy as a function of cell volume. The the stability calculations for Li7O2Br3 were performed by using CASTEP software25 at the GGA level of theory and energy cutoff of 380 eV, by considering the reaction 2ðLi3 OBrÞ þ LiBr ¼ Li7 O2 Br3 : As reported and shown in Fig. 1, Li3OBr crystallizes into the cubic antiperovskite structure with space group Pm-3m ˚ , with corner-sharing OLi6 and cell parameter a ¼ 4.0347(6) A octahedrons and Br anions located on the A-site.26 The layered Li7O2Br3 is a tetragonal anti-perovskite with space group ˚ and c ¼ 21.4811(5) A ˚ , and the I4/mmm, a ¼ 4.0148(5) A refined cell parameters are listed in Table I. According to the theoretical calculation, both Li3OBr27 and Li7O2Br3 are meta-stable phases compared with the starting Li2O and LiBr materials. Furthermore, Li7O2Br3 is even less stable than the Li3OBr phase. The calculated total energy of Li3OBr þ LiBr mixture is 22.49957 meV lower than Li7O2Br3 per formula unit. This indicates that the layered Li7O2Br3 could be meta-stable relative to Li3OBr since

FIG. 1. Powder X-ray diffraction pattern of composite phase Li3OBr:Li7O2Br3; the empty black circles and solid red lines are from experiment and refinement. The blue ticks correspond to the indexing of cubic Li3OBr with space group Pm-3m; the red tricks correspond to the peak positions of layered Li7O2Br3 with space group I4/mmm. Peaks marked by solid circle and by diamond are from LiBr and Li2O, respectively. The insets give the schematic representation of the cubic and layered antiperovskite structures, and the green, brown, and red balls (in the center of the octahedrons) represent the Li, Br, and O atoms, respectively.

LiBr is a very stable compound. Therefore, it is a challenge to synthesize Li7O2Br3 and avoiding the competing phase mixtures Li3OBr-LiBr and Li2O-LiBr. However, by kinetic control, both phases can be quenched to RT. Fig. 1 gives a representative diffraction pattern with 44 wt. % Li7O2Br3. For the current effort, the maximum weight percentage of Li7O2Br3 in the two-phase composite is 44 wt. %, as shown in Fig. 2. For each of the reactions, there are un-reacted or decomposed Li2O and LiBr impurities, as indicated in Figs. 1 and 2. Because the phase purity has an influence on the ionic conductivity, the comparison of the three composites with 39 wt. %, 41 wt. %, and 44 wt. % Li7O2Br3 indicated the sudden increase of the ionic conductivity is an intrinsic property of the compound, other than from the impurities. Fig. 3 gives all the impendence measurements of composite phase samples from RT to 180 C. In Fig. 3(f), the typical impedance spectra of Li3OBr at RT consists of two semi-circles, the high frequency arc from the bulk resistance, and low frequency arc from the grain boundary.28–32 To obtain the resistance, equivalent circuits (inset of Fig. 3(f)) were used to fit the impedance spectra. In the equivalent circuit, one set of resistance (R) paralleled with a constant phase angle element (CPE) represents the impedance from the bulk, and the other set represents the impedance from the grain boundary. After running the fitting, the bulk and grain boundary resistances were obtained. It should be noticed that the grain boundary resistance vanished with temperature higher than 100 C for all the samples. The conducting activation energy is calculated by fitting the Arrhenius relation of conductivity as function of temperature: rT ¼ ro exp[Ea/(kT)], where Ea is the activation energy for conduction, T is the absolute temperature, and ro is the pre-exponential factor. Therefore, we get: ln(rT) ¼ Ea/k 1/T þ A. By linear fitting the relationship of ln(rT) 1/T, to obtain the slope d, Ea ¼ d k. At a low temperature, particular at RT, Fig. 3 suggests that the grain boundary resistance dominates the total impedance, especially at lower layered Li7O2Br3 content. For instance, the bulk and grain boundary resistance of Li3OBr


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TABLE I. Refined crystal parameters for Li3OBr and Li7O2Br3 at room temperature; the occupancy for all the atoms and the thermal parameters for all Li atoms were fixed during the refinement. Coordinates f equivalent positions Formula Li3OBr (56 wt. %)

Li7O2Br3 (44 wt. %)

˚) Lattice parameters (A a ¼ 4.0347(6)

a ¼ 4.0148(5) c ¼ 21.4811(5)

Li O Br Li1 Li2 Li3 Br1 Br2 O

x

y

z

Occupancy

Thermal parameters (Uiso)

0.5 0.5 0 0 0 0 0 0 0

0 0.5 0 0.5 0 0 0 0 0

0 0.5 0 0.093(2) 0.1865(1) 0 0.3121(3) 0.5 0.09806(3)

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

0.02 0.02(2) 0.018(2) 0.02 0.02 0.02 0.02(1) 0.02(1) 0.03(1)

FIG. 2. X-ray diffraction patterns of pure cubic phase Li3OBr and with phase mixture of Li7O2Br weight percentage up to 44 wt. %. Peaks marked by solid circle and by diamond are from LiBr and Li2O, respectively.

was 8.8 kX and 83.0 kX, respectively, with the ratio of 1:10. The two mixed phase samples (14 wt. %, 39 wt. % Li7O2Br3) also showed similar ratio. However, for the 41 wt. % and 44 wt. % Li7O2Br3 sample, as the result of huge decrease of grain boundary resistance, the ratio between the grain and grain boundary resistance are both 1:1, indicating the Li7O2Br3 phase improved the grain boundary conductivities. To explore the intrinsic ionic conductivity of the material, the bulk conductivity of these samples were derived and analyzed. The Arrhenius plot of the derived bulk conductivities of these samples are shown in Fig. 4(a). Fig. 4(b) gives the bulk conductivities as the function of Li7O2Br3 phase fraction at RT. The ionic conductivity at RT changed from 106 S/cm for Li3OBr (consistence with reported in Ref. 27) to 0.24 104 S/cm for the composite sample with 44 wt. % Li7O2Br3, more than an order of magnitude increase. In the temperature range studied, the bulk conductivities of all the samples obey the Arrhenius equation, and the derived active

FIG. 3. Impendence measurements of pure Li3OBr (a) and two phase composite samples ((b)–(e)); (f) The impedance spectrum of Li3OBr at 25 C, and equivalent circuit fitting result.


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FIG. 4. (a) Arrhenius plots of the bulk conductivities of composite phases of Li3OBr and Li7O2Br3 between 25 C and 180 C with Au foil as electrodes; the activation energies Ea are derived by linear fitting of ln(rT) versus 1/T. (b) and (c) give the ionic conductivities at room temperature and activation energies as a function of Li7O2Br3 weight percentage in the composite phase samples.

energies Ea are plotted in Fig. 4(c). It is interesting that with the Li7O2Br3 weight percentage increasing in the composite phase samples, the ionic conductivity at RT increased slowly and the activation energy decreased slowly until the turning point at the weight percentage of 40 wt. %. After that, the ionic conductivity increased rapidly, and correspondingly, the active energy decreased suddenly with Li7O2Br3 content. In the view point of crystal structure, as shown in the inset of Fig. 1, the layered anti-perovskite Li7O2Br3 can be viewed as a depleted structure from cubic anti-perovskite Li3OBr, that is, depleting one Li2O layer in every three OLi6 octahedral layers. Therefore, the layered structure can generate more defects (Li vacancies and O vacancies) compared with the cubic Li3OBr. As a result, more channels are created in the layered Li7O2Br3, facilitating Liþ migrating. The increased ionic conductivity and decreased activation energy is contributed by the increase of layered structure Li7O2Br3 in the mixed phase. It is well established the ionic conductivity transport pathway is along the edge of the octahedron via vacancies, either O2 in perovskite33 or Liþ in antiperovskite.21 As indicated by the calculation results, the Li3OX with stoichiometric crystal lattice is not a good Liþ conductor.22 The high lithium content in the lattice confines the mobility of Liþ and leads to low Liþ conductivities in this kind of materials. However, in the layered anti-perovskite Li7O2Br3 structure, the octahedral polyhedrons permit the existence of more Liþ defects and vacancies, thereby enhancing the ionic conductivity. It is also worth noting the turning point of the bulk conductivity at RT (Fig. 4(b)) and the conducting activation energy (Fig. 4(c)) at around 40 wt. % layered phase. Based on the experiment results, we propose that at lower layered content, the cubic Li3OBr is the main phase and its 3D framework dominates the ionic conducting. When the layered content reached to 40 wt. %, the fast Liþ transporting pathway connected, resulting in the rapid increase of ionic conductivity and decrease of conducting activation energy. As mentioned above, the contribution from the grain boundary to the total resistance also decreased suddenly to a low level when the layered content reached to 40 wt. %. It indicates that the Liþ ionic migrating is dominated by Li7O2Br3 phase at this point. With a further increase of layered Li7O2Br3 phase, the conductivity increased and the activation

energy decreased. Summarily, the scenario indicated that the pure phase layered structure Li7O2Br3 has higher intrinsic ionic conductivity and lower activation energy for Liþ transporting. Further efforts of making pure phase Li7O2Br3 is being explored by traditional solid state reaction methods, pulse laser deposition, and high pressure techniques. In this work, pure phase Li3OBr, as well as its phase mixing composites with layered Li7O2Br3, was synthesized via solid state reactions. Li7O2Br3 is a meta-stable phase as indicated by experiment and theoretical calculation; however, its depleted layered structure permits large amount of Li vacancies and forms fast Liþ transporting pathway. Its promising high ionic conductivity makes it worthy of further experimental investigation on layered anti-perovskite Li7O2Br3. Our future goals include obtaining a pure phase layered structure sample (manipulate synthesis parameters), determining the Li ionic transport mechanism/pathway (neutron diffraction) and microstructural influences (TEM). The authors are grateful for the financial support from the ARPA-E project (0670-3052). 1

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