Boosting the Cycle Life of Li–O2 Batteries at Elevated Temperature by

Boosting the Cycle Life of Li−O2 Batteries at Elevated Temperature by Employing a Hybrid Polymer−Ceramic Solid Electrolyte Jin Yi,† Yang Liu,† Yu Qiao,† Ping He,‡ and Haoshen Zhou*,†,‡ †

Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1, Umezono, Tsukuba 305-8568, Japan ‡ National Laboratory of Solid State Microstructures & Center of Energy Storage Materials and Technology, Nanjing University, Nanjing 210093, China S Supporting Information *

ABSTRACT: With the increasing demands of electric vehicle and grid storage, the solid-state Li−O2 battery is generally considered as an alternative cost-effective energy-storage device because of its high energy density and safety. However, there are several challenges that need to be overcome to meet the stringent requirements imposed by diverse applications, especially at high temperature. In this work, an ion-conducting hybrid solid electrolyte (HSE) integrating polymer electrolyte with ceramic electrolyte (1:1 w/w) has been successfully designed and prepared, which displays high Li+ transference number (0.75) and ionic conductivity (0.32 mS cm−1) at room temperature. The solid-state Li−O2 battery enabled by the as-prepared HSE delivers a superior long life (350 cycles, >145 days) at 50 °C to that of the conventional ether-based nonaqueous Li−O2 battery. The use of a HSE could lead to a new avenue for the development of highperformance solid-state Li−O2 batteries. ith the flourish of electric vehicles and grid storage, exploitation of high energy density and cost-effective energy storage devices is urgently required. Owing to the intrinsically limited energy density, the available energy storage devices, including Li-ion batteries (LIBs) and supercapacitors, fail to satisfy the requirements. Substantial efforts have been made to develop high-energy storage devices in the past decades. In recent years, it has widely been shared that the rechargeable lithium−oxygen (Li−O2) battery is an appealing energy storage device for its high theoretical specific energy density (3505 Wh kg−1) and thus has attracted increasing attention.1−5 Although a considerable amount of efforts are being spent on the research of Li−O2 batteries throughout the world, including the mechanism, material, and system, practical application of the Li−O2 battery still meets thorny problems.6 In addition, as a hot topic, the safety issue of energy storage devices has captured extensive attention, especially for future applications of electric vehicles. On account of the leakage and volatilization of liquid electrolyte under an open operation system for a nonaqueous Li−O2 battery, particularly at high temperature (HT), the desiccation of liquid electrolyte during cycling would give rise to deterioration of cycling performance, as evidenced by comparing results of the cycling performance of the typical Li−O2 batteries using the conventional ether-

W

© 2017 American Chemical Society

based liquid electrolyte obtained at room temperature (RT) and HT shown in Figure 1. It can be seen that the conventional nonaqueous Li−O2 battery assembled with a single-walled carbon nanotube (SWCN)-based cathode and 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in tetraethylene glycol dimethyl ether (TEGDME) ether-based electrolyte can stably operate for more than 15 cycles, while at HT (50 °C), the evaporation rate of liquid electrolyte would be accelerated, triggering the desiccation of liquid electrolyte and battery “death” (only 6 cycles). Moreover, it should also be noted that the intrinsically flammable organic liquid electrolyte may result into a fire hazard or even explosion when the liquid electrolytebased Li−O2 batteries are operated at HT. It is thereby easy to visualize that the safety issue has become the bottleneck for the development of aprotic Li−O2 batteries and their future practical application. Fortunately, solid electrolytes including polymer and ceramic electrolytes have been universally acknowledged as competitive alternatives to liquid electrolytes for Li−O2 batteries with Received: April 4, 2017 Accepted: May 7, 2017 Published: May 8, 2017 1378

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http://pubs.acs.org/journal/aelccp

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ACS Energy Letters

Figure 1. Comparison on the typical discharge−charge profiles of conventional nonaqueous Li−O2 batteries based on the same amount of ether-based liquid electrolyte (1 M LiTFSI/TEGDME) and SWCNs cathodes: (a) 25 and (b) 50 °C.

Figure 2. (a) Digital images of HSE under different states. (b) Arrhenius plots of Li ionic conductivities of the HSE at elevated temperature. (c) Comparison of the activation energy of the LAGP pellet, PPE, and HSE. (d) Current variation with time during polarization of a Li/HSE/ Li symmetrical cell with an applied potential of 50 mV and EIS (inset) before and after polarization.

enhanced safety.7−10 To overcome the disadvantages of the polymer and ceramic electrolytes, such as low ionic conductivity and poor mechanical strength, a hybrid solid electrolyte (HSE) integrating the benefits from different components and addressing the disadvantages of each would be a proming strategy for the development of a solid-state Li− O2 battery.11,12 Among a series of ceramic electrolytes, Li1.6Al0.5Ge1.5(PO4)3 (LAGP) possesses high lithium ion conductivity (over 10−4 S cm−1), favorable chemical stability against lithium, and a wide electrochemical window together with high stability in air and is considered as a promising candidate for solid-state Li-based batteries. 13−15 More importantly, it has been reported that the LAGP can offer adsorption of oxygen molecules on to its suface followed by the

reduction of oxygen and the formation of Li2O2.16,17 In an attempt to shed more light on their application under unfavorable conditions, herein, an ion-conducting HSE integrating poly(methyl methacrylate-styrene) (PMS) and LAGP (1:1 w/w) with high ionic conductivity and Li+ transference number (tLi+) has been designed and prepared. Meanwhile, it is well-known that temperature has remarkable effects on the electrochemical behavior of the Li−O2 battery, including improvement in ionic conductivity of the electrolyte and modification of the electrical conductivity of the intrinsic semiconductor Li2O2 and the interface chemistries between Li2O2 and the electrolyte.18−22 Consequently, it is overwhelmingly paramount to investigate the electrochemical behavior of the solid-state Li−O2 battery at HT. Hence, on 1379

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conventional polymer electrolytes, such as poly(ethylene oxide) (PEO) electrolyte with lithium salts (LiX, X is the anion), display a typical tLi+ of ∼0.2−0.3.26−28 According to the Bruce−Vincent−Evans equation (eq 2),29 a high tLi+ (0.75, Figure 2d) for the HSE has been obtained with the Liconducting NASICON solid electrolyte (LAGP) and anion trapping, which promote the movement of free Li ions.30,31 Compared with the tLi+ of PPE (0.51) and 1 M LiTFSI/ TEGDME (0.40), as displayed in Table S1, the high tLi+ of the HSE would decrease the concentration gradients at electrode surfaces and enable achieving a high power capability.

the basis of the above-mentioned challenges for solid-state Li− O2 batteries, the purpose of this study, in particular, is to unravel the electrochemical behavior of the HSE-based Li−O2 battery at HT. Furthermore, the feasibility of the HSE in applications of next-generation flexible/wearable electronic devices is also discussed. The ceramic electrolyte LAGP was prepared through a conventional solid-state reaction. Afterward, its structure and morphology were determined using X-ray diffraction (XRD) pattern and SEM evaluation, as displayed in Figure S1 (Supporting Information). It can be found that the XRD pattern corresponding to the NASICON-type phase can be indexed to the hexagonal structure (LiGe2(PO4)3, JCPDS 410034) without impurity and the agglomerations of microsize LAGP after calcination are observed.13,14 After intensively mixing the PMS with LAGP in tetrahydrofuran (THF), the HSE, in the form of a flexible thin film with a thickness of ∼30 μm, was prepared through the phase inversion process and reinforced by a polyethylene supporter, as shown in Figure 2a, which shows outstanding bendability and mechanical stability endowing promising application in flexible electronic devices. The ionic conductivity of HSE was obtained by the electrochemical impedance spectroscopy (EIS) method. Compared with the ionic conductivities of PMS-based polymer electrolyte (PPE) (0.03 mS cm−1), 1 M LiTFSI/TEGDME (0.25 mS cm−1), and a LAGP plate (0.10 mS cm−1), as summaried in Table 1, the high ionic conductivity of HSE (0.32

t Li+ =

ionic conductivity (mS cm−1, RT)

Li+ transference number (tLi+)

0.32 0.03 0.2532

0.75 0.51 0.40

HSE PPE 1 M LiTFSI/ TEGDME LAGP pellet LAGP powder

0.10

∼133

mS cm−1) is beneficial from the participation of a high ionic conductive LAGP ceramic electrolyte and enhanced Li+ migration. Furthermore, as presented in Figure 2b, the Arrhenius plots of Li ionic conductivities of HSE at various temperatures manifest the typical Arrhenius behavior and can be well described by the Arrhenius equation (eq 1), where σ(T), A and Ea are the ionic conductivity, pre-exponential factor, and activation energy, respectively, while κ and T are the Boltzmann constant and temperature, respectively.23 σ=

⎛ E ⎞ A exp⎜ − a ⎟ ⎝ κT ⎠ T

(2)

ΔV is the applied polarization voltage (ΔV = 50 mV), Io and Ro are the initial current and interfacial resistance before polarization, respectively, and Iss and Rss are the steady-state current and interfacial resistance after polarization for 10000 s, respectively. The electrochemical stability is another key property to affect the HSE application for Li−O2 batteries. Figure S2 presents the result of linear sweep voltammogram (LSV) and cyclic voltammetry (CV) profiles of the HSE using Li metal and stainless steel as the counter/reference electrode and working electrode, respectively. The HSE exhibits a stable window up to 5.2 V vs Li/Li+, and scarcely any reactions take place within the operation voltage window (from 2 to 4.8 V vs Li/Li+), implying pleasurable electrochemical stability of the HSE during cycling. The HSE with remarkable properties and flexibility could ensure its application under various bending and twisting conditions, encouraging us to explore the possibility of HSE as a flexible solid electrolyte for the Li−O2 battery. As the main challenge for the development of solid-state Li− O2 batteries, how to circumvent sluggish lithium ion transport and poor interface kinetics between the HSE and cathode is non-negligible.34,35 The construction of a polymeric gel-like interphase between the HSE and cathode would be effective in enhancing the lithium ion transport.36 In order to improve the lithium ion transport, according to the reported works in our group, a unique cross-linked network gel consisting of a Li saltmodified SWCN and ionic liquid (IL), hereafter named LSI, was constructed as the cathode for Li−O2 batteries.37−39 Due to the properties of high ionic conductivity, negligible vapor pressure, and low flammability, ILs have captured rising attention for their application in Li−O2 batteries.40−42 The electrochemical behaviors of Li−O2 batteries based on etherbased liquid electrolyte and LSI at 50 °C are given in Figure S3; the restriction of Li+ migration triggered by the desiccation of ether-based solvent can be circumvented with the introduction of an IL to the cathode at HT, resulting in improved cycling performance (more than 80 cycles). To facilitate offering efficient channels for electrons, ions, and oxygen, the LSI cathode is expected to address the obstacle of Li-ion migration at the interface between the HSE and cathode, as supported by the following experiments and results. From the electrochemical performance of a solid-state Li−O2 battery constructed with a HSE and LSI at RT, as dispalyed in Figure S4, it can be found that more than 160 cycles can be obtained. Besides, to obtain further insight concerning the discharge product of the solid-state Li−O2 battery at RT, the XRD technique was carried out to determine the component of the discharge product; as illustrated in Figure S5, three typical

Table 1. Comparison on the Properties of Various Electrolytes Involved in This Study sample

Iss(ΔV − IoR o) Io(ΔV − IssR ss)

(1)

On the basis of the Arrhenius equation, the total Ea is calculated to be 0.2 eV for the HSE, lower than 0.31 and 0.36 eV for LAGP14 and PPE, respectively, indicating fast charge transfer and Li diffusion at the interface in the HSE. The decreased Ea (illustrated in Figure 2c) may be caused by the formation of an ionically conducting liquid medium on the ceramic and polymer surfaces via NASICON Li-ion hopping, which results from interfacial interaction and the consequential splitting of the Li+ and TFSI ion pairing.24,25 Additionally, tLi+ is a critical factor, and a high transference number is prerequisite for achievement of high power output capability. Generally, 1380

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Figure 3. Electrochemical performance of solid-state Li−O2 batteries based on the HSE and LSI cathode under dry O2 at 50 °C: (a) power output capability at different current densities, (b) typical discharge−charge profiles at a current density of 200 mA g−1 with a fixed capacity of 1000 mAh g−1, and (c) the corresponding terminal voltage with cycle life.

S7, it can be found that the main discharge products are still composed of Li2O2 with toroid morphology together with some impurities, which are derived from the undesired side reaction at HT, and the above discharge products can be decomposed during the charge process at HT.43−45 In order to quantitatively evaluate the byproducts derived from attack of the highly reactive O2− and O22− produced during discharge processes on carbon materials and other components of the solid-state Li− O2 battery at HT, chemical titration processes were employed according to our previous works.46,47 From the obtained results, it is found that numerous obtained Li2O2 (47.9%) would attack other components of the solid-state Li−O2 battery (carbonbased cathode, polymer-based electrolyte, etc.), leading to the formation of a multitude of Li2CO3-like byproducts (Li2CO3, LiOH, etc.) and thereby resulting in a high charge voltage (>4 V, Figure 3c). The energy efficiency of the solid-state Li−O2 battery (based on output electric energy/input electric energy) decreases from 65% in the initial cycle to 57% after 350 cycles. A layer derived from the aggregation of LiCO3 on the cathode surface tends to form based on the following reactions (eqs 3 and 4), which would be accelerated at HT.21,22,48

diffraction peaks that occurred at 32.9, 35, and 58.7° could be assigned to the (100), (101), and (110) signals of Li2O2, respectively.32 Meanwhile, the above diffraction peaks disappear after charge, inferring the formation/decomposition of Li2O2 with pine straw morphology responses for the reversible discharge/charge capacities. In order to investigate the power capability of the solid-state Li−O2 battery at HT, solid-state Li−O2 batteries assembled with HSE (or PPE) and LSI were operated at 50 °C, and the corresponding results obtained under different current densities are provided. As displayed in Figure 3a, it can be seen that the solid-state Li−O2 battery can stably operate at 50 °C with a voltage gap of 1.83 V even at a high current density of 2000 mA g−1, showing favorable power capability. However, due to the unfavorable tLi+ of PEE, the PPE-based solid-state Li−O2 battery delivers poor rate capability, as evidenced from Figure S6. The high power capability may be attributed to the high tLi+ of the HSE with decreased concentration gradients at electrode/electrolyte interfaces and the LSI cathode with high efficiency transport channels for electrons, ions, and oxygen. What is more, Figure 3b displays the typical discharge−charge profiles for the solidstate Li−O2 battery cycled at a current density of 200 mA g−1 with a capacity limit of 1000 mAh g−1. Unexpectedly, a record long-life cycling performance (350 cycles, >145 days) is achieved with negligible decay for the solid-state Li−O2 battery. The observed performance improvement is associated with the employment of this as-prepared HSE. Compared with the component of the discharge product at RT, from the XRD result collected after discharge at HT and presented in Figure

Li 2O2 + C + 1/2O2 → Li 2CO3

(3)

2Li 2O2 + C → Li 2O + Li 2CO3

(4)

Through comparison on the Raman spectra collected at pristine and cycled cathodes, as given in Figure S8, the results reveal that the existing Li2CO3-like byproducts may mainly account for the increased overpotential and the final “death” of the 1381

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Figure 4. Flexible solid-state Li−O2 battery based on a HSE while bent: (a) schematic of the proposed flexible solid-state Li−O2 battery, (b) digital images of the fabricated flexible Li−O2 battery powering a commercial LED lamp, (c) power output capability at different current densities at 50 °C, (d) typical discharge−charge profiles, and (e) cycle life at a current density of 200 mA g−1 with a capacity limit of 1000 mAh g−1 at 50 °C.

purposes, to demonstrate its potential application in flexible electronic devices, a flexible solid-state Li−O2 battery was proposed and fabricated under being bent (as illustrated in Figure 4a), in which the as-prepared HSEs were sandwiched between the Li anode and LSI cathode. The flexible solid-state Li−O2 battery can be successfully used to power a commercial LED lamp while bent (Figure 4b). Furthermore, it can be seen that the flexible solid-state Li−O2 battery also displays high power output capability even while bent at 50 °C, revealing that the power output capability of the HSE-based flexible Li−O2 battery is scarcely affected by external bending (Figure 4c). In addition, the as-fabricated flexible solid-state Li−O2 battery can discharge−charge for 90 cycles without any decay in capacity at a current density of 200 mA g−1 at 50 °C (Figure 4d,e). Combining these results, it becomes evident that the asprepared HSE shows feasible application in flexible solid-state Li−O2 batteries. Given the compositional tunability of the HSE and interface controllable cathode, these results herald the advent of a new generation of flexible solid-state Li−O2 batteies whose compositions and interfacial structures can be tuned rationally. From a broader perspective, this strategy could be

HSE-based solid-state Li−O2 battery. To some degree, this limitation is the result of cathode instability-related issues that need to be solved for the development of a solid-state Li−O2 battery at HT.21,22,49,50 The stability of the cathode with highly reactive O2− and O22− has been a tough challenge that needed to be addressed. In this regard, the carbon-free cathodes give a lot space to further improve the cycle life of future solid-state Li−O2 batteries at HT.51,52 From the above results and discussion, it can be concluded that a long-life solid-state Li−O2 battery has been successfully developed at HT based on the asprepared HSE with high t Li + and ionic conductivity. Furthermore, how to avoid the side reactions related with carbon materials in Li−O2 batteries at HT is a difficult problem yet to be adequately resolved in this work. The obtained results further emphasize how crucial possible side reactions are a major limiting factor in the development of long-life solid-state Li−O2 batteries. Besides addressing the challenges of the HSE for achieving enhanced electrochemical performances, it is indispensible to make feasible the widespread application of solid-state Li−O2 batteries for flexible energy storage devices. For illustrative 1382

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(3) Yi, J.; Wu, S.; Bai, S.; Liu, Y.; Li, N.; Zhou, H. Interfacial Construction of Li2O2 for a Performance-Improved Polymer Li-O2 Battery. J. Mater. Chem. A 2016, 4, 2403−2407. (4) Jung, J.-W.; Im, H.-G.; Lee, D.; Yu, S.; Jang, J.-H.; Yoon, K. R.; Kim, Y. H.; Goodenough, J. B.; Jin, J.; Kim, I.-D.; et al. Conducting Nanopaper: A Carbon-Free Cathode Platform for Li−O2 Batteries. ACS Energy Lett. 2017, 2, 673−680. (5) Shui, J.; Lin, Y.; Connell, J. W.; Xu, J.; Fan, X.; Dai, L. NitrogenDoped Holey Graphene for High-Performance Rechargeable Li−O2 Batteries. ACS Energy Lett. 2016, 1, 260−265. (6) Li, F.; Zhang, T.; Zhou, H. Challenges of Non-Aqueous Li-O2 Batteries: Electrolytes, Catalysts, and Anodes. Energy Environ. Sci. 2013, 6, 1125−1141. (7) Li, F. J.; Kitaura, H.; Zhou, H. S. The Pursuit of Rechargeable Solid-State Li-Air Batteries. Energy Environ. Sci. 2013, 6, 2302−2311. (8) Yi, J.; Guo, S.; He, P.; Zhou, H. Status and Prospects of Polymer Electrolytes for Solid-State Li-O2 (Air) Batteries. Energy Environ. Sci. 2017, 10, 860−884. (9) Wu, S.; Yi, J.; Zhu, K.; Bai, S.; Liu, Y.; Qiao, Y.; Ishida, M.; Zhou, H. A Super-Hydrophobic Quasi-Solid Electrolyte for Li-O2 Battery with Improved Safety and Cycle Life in Humid Atmosphere. Adv. Energy Mater. 2017, 7, 1601759. (10) Sun, J.; Zhao, N.; Li, Y.; Guo, X.; Feng, X.; Liu, X.; Liu, Z.; Cui, G.; Zheng, H.; Gu, L.; et al. A Rechargeable Li-Air Fuel Cell Battery Based on Garnet Solid Electrolytes. Sci. Rep. 2017, 7, 41217. (11) Zhou, W.; Wang, S.; Li, Y.; Xin, S.; Manthiram, A.; Goodenough, J. B. Plating a Dendrite-Free Lithium Anode with a Polymer/Ceramic/Polymer Sandwich Electrolyte. J. Am. Chem. Soc. 2016, 138, 9385−9388. (12) Chinnam, P. R.; Wunder, S. L. Engineered Interfaces in Hybrid Ceramic−Polymer Electrolytes for Use in All-Solid-State Li Batteries. ACS Energy Lett. 2017, 2, 134−138. (13) Fu, J. Fast Li+ Ion Conducting Glass-Ceramics in the System Li2O−Al2O3−GeO2−P2O5. Solid State Ionics 1997, 104, 191−194. (14) Xu, X.; Wen, Z.; Wu, X.; Yang, X.; Gu, Z. Lithium IonConducting Glass−Ceramics of Li1.5Al0.5Ge1.5(PO4)3−xLi2O (x = 0.0− 0.20) with Good Electrical and Electrochemical Properties. J. Am. Ceram. Soc. 2007, 90, 2802−2806. (15) Kitaura, H.; Zhou, H. S. Electrochemical Performance and Reaction Mechanism of All-Solid-State Lithium-Air Batteries Composed of Lithium, Li1+xAlyGe2‑y(PO4)3 Solid Electrolyte and Carbon Nanotube Air Electrode. Energy Environ. Sci. 2012, 5, 9077−9084. (16) Balaish, M.; Kraytsberg, A.; Ein-Eli, Y. A Critical Review on Lithium-Air Battery Electrolytes. Phys. Chem. Chem. Phys. 2014, 16, 2801−2822. (17) Kumar, B.; Kumar, J. Cathodes for Solid-State Lithium−Oxygen Cells: Roles of Nasicon Glass-Ceramics. J. Electrochem. Soc. 2010, 157, A611−A616. (18) Tan, P.; Shyy, W.; Zhao, T. S.; Wei, Z. H.; An, L. Discharge Product Morphology versus Operating Temperature in Non-Aqueous Lithium-Air Batteries. J. Power Sources 2015, 278, 133−140. (19) Song, M.; Zhu, D.; Zhang, L.; Wang, X. F.; Mi, R.; Liu, H.; Mei, J.; Lau, L. W. M.; Chen, Y. G. Temperature Characteristics of Nonaqueous Li-O2 Batteries. J. Solid State Electrochem. 2014, 18, 739− 745. (20) Park, J.-B.; Hassoun, J.; Jung, H.-G.; Kim, H.-S.; Yoon, C. S.; Oh, I.-H.; Scrosati, B.; Sun, Y.-K. Influence of Temperature on Lithium-Oxygen Battery Behavior. Nano Lett. 2013, 13, 2971−2975. (21) Ottakam Thotiyl, M. M.; Freunberger, S. A.; Peng, Z.; Bruce, P. G. The Carbon Electrode in Nonaqueous Li−O2 Cells. J. Am. Chem. Soc. 2013, 135, 494−500. (22) McCloskey, B. D.; Speidel, A.; Scheffler, R.; Miller, D. C.; Viswanathan, V.; Hummelshøj, J. S.; Nørskov, J. K.; Luntz, A. C. Twin Problems of Interfacial Carbonate Formation in Nonaqueous Li−O2 Batteries. J. Phys. Chem. Lett. 2012, 3, 997−1001. (23) Kim, J.-K.; Lim, Y. J.; Kim, H.; Cho, G.-B.; Kim, Y. A Hybrid Solid Electrolyte for Flexible Solid-State Sodium Batteries. Energy Environ. Sci. 2015, 8, 3589−3596.

used as a general approach to design and develop solid-state Li−O2 batteries at elevated temperature. In summary, with the aim to achieve high performance of solid-state Li−O2 batteries at HT, in this study, a HSE integrating polymer electrolyte with a ceramic electrolyte (1:1 w/w) was designed and prepared, which offered high tLi+ and ionic conductivity with flexibility. In contrast to the conventional nonaqueous Li−O2 batteries based on ether-based liquid electrolyte, the solid-state Li−O2 battery assembled with the asprepared HSE and SWCN-based cathode delivers a superior long life (350 cycles, >145 days) at 50 °C. Moreover, the asfabricated flexible solid-state Li−O2 battery can operate for 90 cycles with negligible capacity fading even under being bent at 50 °C, demonstrating promising application in flexible electronic devices under unfavorable conditions. It should be noted that the carbon-based cathode used in this work is unstable at HT in the presence of highly reactive O2− and O22−; therefore, if the proposed flexible solid-state Li−O2 battery is based on a carbon-free cathode, the electrochemical performance of this battery would be dramatically improved. These results, in addition to shedding light on the benefits of a HSE for performance-improved solid-state Li−O2 batteries under high temperature, highlight the appealing application of flexible solid-state Li−O2 batteries in next-generation electronic devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.7b00292. Experimental section, Table S1 showing the parameters to determine the transference number, and Figures S1− S8 showing XRD patterns, linear sweep voltammograms, discharge−charge patterns, cycle lives, rate capabilities, SEM images, and Raman spectra (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] or [email protected]. ORCID

Jin Yi: 0000-0001-6203-1281 Yu Qiao: 0000-0002-2191-3875 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for partial financial support from the Advanced Low Carbon Technology Research and Development Program (ALCA) of Japan project, the National Basic Research Program of China (2014CB932300), and the National Natural Science Foundation (NSF) of China (21633003, 21373111).

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REFERENCES

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ACS Energy Letters

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DOI: 10.1021/acsenergylett.7b00292 ACS Energy Lett. 2017, 2, 1378−1384