Letter Cite This: ACS Appl. Energy Mater. 2018, 1, 3048−3056
www.acsaem.org
Investigation of Rechargeable Poly(ethylene oxide)-Based Solid Lithium−Oxygen Batteries Moran Balaish,†,‡ Michal Leskes,§ and Yair Ein-Eli*,†,‡ †
The Grand Technion Energy Program, Technion- Israel Institute of Technology, Haifa 3200003, Israel Department of Materials Science and Engineering, Technion- Israel Institute of Technology, Haifa 3200003, Israel § Department of Materials and Interfaces, Weizmann Institute, Rehovot 7610001, Israel ‡
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S Supporting Information *
ABSTRACT: Liquid-free solid polymer electrolyte (SPE) Li−O2 batteries are considered advantageous power sources for multiple applications, albeit their cycle performance is far from being acceptable. A most challenging SPE stability in Li−O2 battery operating at 80 °C is described here, presenting possible directions for this battery type future development. Hereby, we investigated poly(ethylene oxide) (PEO) stability in Li−O2 batteries after cycling and determined that the polymer instability is originated from an accumulation of formate-based species, which required high decomposition potential and showed low decomposition efficiency. This poses a key challenging issue of unfavorable round-trip efficiency, dictating a poor cycle performance. KEYWORDS: Li−O2 battery, oxygen, poly(ethylene oxide), polymer electrolyte, CNT, high temperature Li−O2 battery
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INTRODUCTION Nonaqueous Li−O2 battery is predicted to deliver a driving distance of more than 550 km based on its projected practical specific energy of ∼500−900 Wh kg−1.1 Conventional Li−O2 batteries use liquid organic electrolytes, which are considered as the most critical component for achieving a long cycle life. Nevertheless, such electrolytes possess several drawbacks such as high volatility and flammability, unfavorable lifetime, and high cost and are prone to oxidative degradation during ORR, ultimately impeding the realization of a commercial Li−O2 battery. While a great deal of attention and effort has been placed on improving capacity, rate capability, round-trip efficiency, and cycle performance via new concepts and materials, safety has been partially overlooked and the vast majority of Li−O2 battery research has been conducted using volatile and flammable organic liquid electrolytes.2−9 A safer alternative is a liquid-free Li−O2 battery with polymer electrolytes. Polymers are known for their high chemical, thermal, and electrochemical stabilities compared to their liquid counterpart as well as their improved safety, wide operational temperature, flexible configuration, and lightweight. Furthermore, polymer electrolytes can offer excellent processability and flexibility that can help adapt various geometric shapes of batteries. Moreover, polymer electrolytes can ensure battery safety upon cycling, reduce lithium dendrite growth, stabilize lithium metal anode by preventing crossover of H2O and O2 from the air cathode, and help eliminate the use of a separator. While a Li−O2 battery with a liquid-free polymer electrolyte is a very appealing configuration and should be placed at the forefront of electrolyte research for future practical Li−O2 © 2018 American Chemical Society
batteries, its research is still in its infancy. Low ionic conductivity, challenging interfacial chemistry, and poor polymer stability during cycling are major challenges preventing wide practical use of a Li−O2 battery. Significant research efforts are required to overcome these challenges with fundamental understanding of failure mechanisms to develop a long cycle life polymer electrolyte Li−O2 battery. Recent work has highlighted different polymer electrolytes, air-cathode structure, cathode/ solid polymer electrolyte (SPE) design, and experimental conditions.10−13 Yet, all SPE Li−O2 battery systems reported to date suffer from low specific capacity, rate capability, roundtrip efficiency, and an overall unfavorable cycle performance. The technological potential of polymers as solid electrolytes necessitates a comprehensive study to assess their stability during operation of the battery, alongside their capability to support a reversible formation of discharge products. An indepth understanding of the major factors leading to poor cycle performance in SPE-based Li−O2 batteries is essential to develop a liquid-free Li−O2 battery with broad commercial applications. This work strives to advance the understanding of poly(ethylene oxide) (PEO) degradation in the presence of the major discharge product, Li2O2, through operation of liquid-free solid polymer electrolyte Li−O2 battery, operated at a temperature above the melting point of the SPE, i.e., 80 °C.10 By analyzing SPE and air-electrode via liquid- and solid-state Received: May 2, 2018 Accepted: June 19, 2018 Published: June 19, 2018 3048
DOI: 10.1021/acsaem.8b00702 ACS Appl. Energy Mater. 2018, 1, 3048−3056
Letter
ACS Applied Energy Materials
Figure 1. (a) First five discharge/charge cycles (2.2−3.9 V) of SPE Li−O2 battery cycled at 0.05 mA/cm2 operated at 80 °C. (b) Cyclic voltammetry of Li/SPE/CNT cell obtained at a scan rate of 0.1 mV s−1 under argon (dotted line) and oxygen (straight line) at 80 °C. (c) First five discharge/charge cycles of SPE Li−O2 battery limited to 375 mAh g−1 (3 h) at 0.05 mA/cm2 operated at 80 °C. (d) Selected discharge/charge profiles of SPE Li− O2 battery cycled at a current of 0.05 mA/cm2 to limited capacity of 42 mAh g−1 (0.5 h). instruments). Discharge/charge experiments were performed with a battery cycler (Arbin Instruments, BT2000) using 2032 modified coincell configuration with holes, ensuring oxygen penetration to the cathode. Lithium metal (Sigma-Aldrich; 99.9%; ∼0.75 mm thickness) served as the anode and a freestanding binder-free CNT electrode functioned as the air-cathode without the need for a current collector. No separator was used in both T-cell and coin-cell configurations. All cells were held at OCP (open-circuit potential) for 3 h at 80 °C. All experiments of SPE Li−O2 cells were conducted at 80 °C under oxygen (1 atm) environment. Characterization. Morphologies of discharge products subsequent to battery operation were characterized by high resolution scanning electron microscopy (HRSEM; Zeiss Ultra-Plus FEG-SEM) operated at 4 kV. The stability of SPE after discharge and after multiple discharge/charge operation was determined via liquid-state NMR spectroscopy. For 1H and 13C NMR analysis the CNT air-cathode, discharged at 0.05 mA cm−2 to 2.2 V was soaked in deuterated acetonitrile for 24 h (D; 99.8%; Cambridge Isotope Laboratories, Inc.) to dissolve the polymer electrolyte. NMR analysis was performed on a Bruker Avance 500 MHz spectrometer. 1H and 13C NMR spectra were acquired for solutions of approximately 2 wt % solids in CD3CN. The chemical shift scale was set with reference to CD3CN (1H, 1.94 ppm; 13 C, 117.7). Solid-state NMR measurements were performed after discharge and multiple discharge/charge cycles to identify the solid products. Additionally, a reference sample of (PEO)20LiTf dissolved in deuterated acetonitrile was analyzed on a Bruker Avance 300 MHz spectrometer. 1H and 7Li measurements were performed on a Bruker AvanceIII 700 spectrometer using a 1.3 mm fast MAS probe with
NMR spectroscopy, respectively, we were able to determine key factors detrimental for battery operation and early stages’ failure and highlight guidelines and perspectives for stable polymers for Li−O2 batteries. The knowledge gained in this work could prove valuable when considering new polymers for SPE Li−O2 battery at elevated temperatures.
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EXPERIMENTAL SECTION
Solid Polymer Electrolyte. The solid polymer electrolyte was prepared using PEO (Aldrich; Mv 6 × 106) and LiCF3SO3 (99.995%, Sigma-Aldrich) with a salt:polymer mole ratio of 1:20 ((PEO)20LiTf), following a solution casting technique. Both PEO and LiCF3SO3 were dissolved separately in acetonitrile (Aldrich; anhydrous; 99.998%; water content, below 30 ppm) inside an argon-filled glovebox. The solution was mixed for at least 24 h before being cast onto a Teflon cup. The solvent slowly evaporated to leave a freestanding, homogeneous, solid polymer electrolyte film with a thickness of 100−150 μm. Finally, the film was vacuum-dried at 60 °C for 24 h to remove all acetonitrile residues. Electrochemical Measurements. Potentiodynamic experiments were performed in a T-shaped cell with lithium metal (reference electrode), platinum foil (counter electrode), and carbon nanotube (CNT; working electrode) fabric provided by TorTech Nanofibers Ltd. (Koren Industrial Park, Maalot-Tarshicha, Israel). The CNT fabric had a thickness ∼ 30 μm and a weight of 4 × 10−4 g/cm2. Cyclic voltammetry was conducted between 2.2 and 3.6 V at a scan rate of 0.1 mV s−1 using a potentiostat/galvanostat (VSP, Biologic Science 3049
DOI: 10.1021/acsaem.8b00702 ACS Appl. Energy Mater. 2018, 1, 3048−3056
Letter
ACS Applied Energy Materials manual spinning at 20 kHz. 1H MAS spectra were acquired using a rotor synchronized Hahn echo pulse sequence with an RF amplitude of 116 kHz and a relaxation delay that was optimized for each sample and varied between 5 and 12 s. 7Li spectra were acquired with single pulse excitation at an RF amplitude of 143 kHz and a relaxation delay of 12 s. 19 F spectra were acquired on a Bruker AVANCEI 300 spectrometer using a Bruker 2.5 mm double resonance probe. A rotor synchronized Hahn echo sequence was used at 28 kHz MAS, with an RF amplitude of 120 kHz and a relaxation delay of 8 s. 1H−7Li correlation (hector) spectra of SPE following 1 and 10 cycles at 0.05 mA cm−2 was acquired with cross-polarization. Chemical shifts were referenced on adamantane (1H at 1.8 ppm), lithium carbonate (7Li at 0 ppm), and lithium fluoride (19F at −204 ppm). 1H and 7Li signals were normalized by sample weight and number of scans.
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RESULTS AND DISCUSSION
SPE Li−O2 battery is considered a safe alternative to liquidbased Li−O2 battery. Nevertheless, high interfacial resistance at the electrode/SPE interface and overpotential build-up during cycling play a crucial role in the redox processes and may lead to kinetic limitations. A SPE Li−O2 battery with P(EO)20LiTf electrolyte operated at 80 °C, i.e., above the melting point of the polymer, with lower overall cell impedance than a liquid-based Li−O2 cell, offered an advantageous polymer Li−O2 battery configuration with acceptable ionic conductivities on the order of 10−3 S cm−1.10 The cycle performance of SPE Li−O2 battery has proven to be a major setback on the development of polymer-based electrolyte for Li−O2 battery and failed to present acceptable cycle performance unless cycled at relatively high current density (0.2 mA cm−2) to only ∼20% of its full capacity.10 Although PEO degradation after discharge in Li−O2 battery has been studied,10,14,15 there is a dearth of knowledge about polymer stability at elevated temperature during discharge/charge cycles and about major factors responsible for the extremely fast deterioration of SPE Li−O2 battery. Previous publications,14,15 dealing with the chemical and electrochemical stabilities of PEO at room temperature, concluded that imposing potentials above OCP during charging increase the rate of PEO auto-oxidation, leading to side reactions and undesirable products due to spontaneous radical formation. However, it has yet been determined how the chemical structure of PEO is affected by the presence of Li2O2 at elevated temperatures under prolonged battery operation. Panels a and b of Figure 1 present the first five discharge/ charge cycles at a current density of 0.05 mA cm−2 and cyclic voltammetry at a scan rate of 0.1 mV s−1 under argon (dotted line) and oxygen (solid line) environments of SPE Li−O2 battery operated at 80 °C, respectively. When a complete discharge at a current density of 0.05 mA cm−2 was allowed down to 2.2 V, a higher discharge capacity of 2000 mAh g−1 was evident; however, no charging process occurred (Figure 1a). Cyclic voltammetry conducted at a rate scan of 0.1 mV s−1 (Figure 1b) confirmed low reaction reversibility efficiency roughly estimated to be lower than 10% based on the reduction process, observed between 2.4 and 2.7 V (maximum at 2.56 V), and oxidation process, observed between 3.1 and 3.4 V (maximum at 3.23 V). When partially discharged/charged to 375 mAh g−1 (Figure 1c), SPE Li−O2 battery showed rechargeability, yet limited with increasing charge potential from 3.55 to 3.7 V from the first to the fifth cycle. Only when the capacity was limited to even lower values (42 mAh g−1) did the battery demonstrate improved cycle performance of 40 cycles (Figure 1d).10 Moreover, HRSEM images of pristine CNT airelectrode (Figure 2a), and CNT air-electrode after discharge
Figure 2. SEM images of (a) pristine, (b, c) discharged CNT aircathode, and (d) cycled CNT air cathode at 0.05 mA cm−2 in a SPEbased Li−O2 cell.
(Figure 2b,c), and after multiple discharge/charge cycles (Figure 2d), at 0.05 mA cm−2, revealed spherical discharge products with an average diameter of 350 nm, being confirmed as Li2O2 via XRD analysis (Supporting Information Figure S1). However, no apparent thick layer of products, which could have supported the lack of rechargeability (presented in Figure 1), was observed.16,17 Upon cycling, micrometer-sized products were observed at the entire surface of CNT electrode, and it became evident that accumulation of electrochemically irreversible compounds could be responsible for observed limited cycle behavior, especially under low current regime and long operation of battery. Although the nature of Li2O2, a strong base, during operation of Li−O2 cells, can result in unwanted side reactions originating from reactivity between Li2O2 (or LiO2) with the electrolyte and/or with carbon air-electrode, especially at high voltage,17 PEO was determined to be relatively stable (
