Large-scale energy storage with high energy density is of vital importance for the exploitation of renewable energy resources. Among all present electrochemical storage systems, Li–O₂ battery is a very promising candidate due to the highest theoretical energy density of 3505 Wh/kg. Li–O₂ battery utilizes the Li metal as anode and O₂ as the cathode, following the reaction route of 2Li + O₂ → Li₂O₂.^[1,2] Up to now, mainly four types of Li–O₂ batteries have been widely investigated including aqueous, aprotic, solid-state, and mixed aqueous/aprotic based Li–O₂ batteries systems.^[2] The aqueous and aprotic based Li-air battery exhibits relatively simple structure and has attracted much research attention. However, these two types of Li–O₂ batteries are still faced with many critical challenges. On Li anode side, safety concern due to Li dendrite formation and side reaction with H₂O, O₂, and CO₂ cannot be avoided. While on cathode side, electrolyte decomposition, evaporation, and reaction with O²⁻ and tend to occur.^[3] Owing to the wide electrochemical window and enhanced safety property, solid-state electrolyte based Li–O₂ battery becomes a competitive candidate. Wang et al.^[4] reported Li–O₂/air batteries using Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) solid electrolyte as cathode protective layer, which can prevent organic electrolyte from evaporation and decomposition. Kitaura and Zhou^[5] also reported a Li–O₂ battery using Li_1.575Al_0.5Ge_1.5(PO₄)₃ (LAGP) as solid electrolyte. It can operate from room temperature to 120 °C. However, the application of all-solid-state Li-air battery is still hindered by the poor mechanical flexibility and fragile property. In addition, the large interfacial resistance between solid electrolyte and electrodes further confines the practical application.

Li–O₂ battery using quasi-solid-state electrolyte (QSSE) exhibits the combined advantages comparing to that using liquid and solid-state electrolyte, in terms of high ionic conductivity, high mechanical flexibility and superior safety characteristics. Due to its potential future development, much work has been performed to study QSSE based Li–O₂ battery in recent years. Wu et al.^[6] reported a Li–O₂ battery using super-hydrophobic quasi-solid-state electrolyte (SHQSE) membrane, which can operate under humid atmosphere (RH = 45%) for 150 cycles. Yi et al.^[7] proposed a Li–O₂ battery fabricated with hybrid solid electrolyte (HSE), which offered high Li⁺ transfer number of 0.75 and exhibits long cyclability (> 350 cycles). Yang et al.^[8] reported a flexible Li–O₂ battery with excellent mechanical stability and cycle performance even under various bended and twisted conditions. These pioneering works verified the advantages of QSSE based Li–O₂ battery and demonstrates its enormous potential for future applications. The key issue to realize practical QSSE based Li–O₂ battery is to fabricate QSSE with high ionic conductivity and good interfacial property with cathode materials. One feasible method to fabricate QSSE is to solidify ionic liquid on nano-oxide particles by utilizing the strong confinement of ionic liquid molecules in between the oxide nanoparticles.^[9] Ito et al.^[10] prepared QSSE by compositing EMI-TFSA and nano-silica. They further fabricated a QSSE based Li–LiCoO₂ cell which could charge and discharge up to 10 cycles. The ionic conductivities of Li[G4]TFSI hybridized with different nano-oxide particles have been investigated by Matsuo et al.^[11] Bipolar stacked Li–LiFePO₄ battery, with Li[G4]TFSI-γ-Al₂O₃ as the QSSE, demonstrates a high output potentials of 6.5 V. Unemoto et al.^[12] further assembled a Li–S battery using IL-fumed-silica nanoparticles as the QSSE, which presents improved performance at high sulfur utilization ratios. So far, much work has been carried out on QSSE based Li–O₂ battery. However, further investigations are still needed to improve the electrochemical performances for practical applications.

In this work, we designed and prepared a novel QSSE which composite PYR₁₄TFSI ionic liquid with the nano-fumed silica. The QSSE with optimal composition shows excellent ionic conductivity. A PYR₁₄TFSI-SiO₂-based quasi-solid-state Li–O₂ battery was assembled thereafter, which can operate at both room temperature and high temperature. With the combination of electrochemical, morphological and spectroscopic characterizations, a thorough investigation has been carried out on the PYR₁₄TFSI-SiO₂-based Li–O₂ batteries to understand the degradation mechanisms.

The fabricated PYR₁₄TFSI-SiO₂ quasi-solid-state electrolyte (QSSE) film is soft and transparent as shown in inset of Fig. 2(a). The SEM image (Fig. 2(a)) indicates that the QSSE film is uniform and compact, which would be helpful for partially prohibiting the side reaction between O₂ and Li metal anode. The ionic conductivity of the QSSE was measured by conducting the Electrochemical Impedance Spectroscopy (EIS) experiment with a SS|QSSE|SS cell (SS refers to stainless steel). The conductivity has reached 4.6 × 10⁻⁴ S/cm and 6.0 × 10⁻⁴ S/cm at 25 °C and 75 °C, respectively. The QSSE with such a high ionic conductivity was suitable for the application in Li–O₂ batteries, compared to the aprotic electrolyte (2.5 × 10⁻⁴ S/cm). The Arrhenius plot of ionic conductivity of QSSE at different temperature is shown in Fig. 2(b). According to Arrhenius equation σ = A/T exp(−E_a/κT), where σ, A, T, E_a, and κ refer to ionic conductivity, pre-exponential factor, temperature, activation energy, and Boltzmann constant, respectively, the E_a is calculated to be 0.18 eV. This value is lower than that of the traditional LAGP solid electrolyte (0.31 eV),^[7] indicating a high Li⁺ diffusion ability in the QSSE. The linear sweep voltammetry (LSV) experiment was performed with a Li|QSSE|SS cell and the results are shown in Fig. 2(c). It shows that the QSSE has a wide electrochemical window (> 5.0 V), higher than the most widely-used ether-based electrolyte (usually < 4.5 V). Therefore, the as-prepared QSSE could be a suitable solid-state electrolyte for Li–O₂ batteries, which can operate in the voltage range of 2 V and 4.6 V (vs. Li/Li⁺). Certainly, it can also be used for rechargeable metallic lithium batteries.

Fig. 1.

	Figure Option ViewDownloadNew Window
	Fig. 1. The representative structure of (a) and (b) TFSI−.

Fig. 2.

	Figure Option ViewDownloadNew Window
	Fig. 2. (color online) Characterizations of the PYR14TFSI-SiO2 quasi-solid-state electrolyte (QSSE). (a) The SEM image and the optical image (inset) of the QSSE; (b) Arrhenius plots of Li ionic conductivities of the QSSE; (c) linear sweep voltammetry scan for the QSSE with a scanning rate of 0.05 mV/s.

The electrochemical performances of the QSSE was evaluated in the Li–O₂ batteries. These quasi-solid-state Li–O₂ batteries can operate at both room temperature (25 °C) and high temperature (80 °C). With a current density of 100 mA/g and a lower discharge cut-off voltage of 2.2 V, the QSSE-based Li–O₂ batteries deliver a discharge capacity of 6715 mAh/g and 29942 mAh/g at 25 °C and 80 °C, respectively. The much larger discharge capacity at 80 °C is due to the higher ionic conductivity of QSSE and therefore the smaller reaction polarization in Li–O₂ battery. Figures 3(b) and 3(c) display the typical discharge-charge curve of the QSSE-based Li–O₂ batteries cycled at different current densities with a fixed capacity of 1000 mAh/g. For the electrochemical measurements at 25 °C, the Li–O₂ battery shows a smaller overpotential of 0.60 V and a higher round-trip efficiency of 80% at a lower current density of 50 mA/g, while a larger overpotential over 2.0 V and lower round-trip efficiency of at a larger current density of 500 mA/g. The rate performances are much improved at a higher temperature of 80 °C. The QSSE-based Li–O₂ batteries can work even at a current density of 2000 mAh/g, with an overpotential of only 1.5 V and round-trip efficiency of 64%. At 100 mA/g, the overvoltage between charge and discharge is only 0.45 V, and the corresponding round-trip efficiency is 85%.

Fig. 3.

Figure Option ViewDownloadNew Window

Fig. 3. (color online) Electrochemical performances of the QSSE-based Li–O2 battery. (a) Charge-discharge curves measured at a current density of 100 mA/g with discharge cut-off voltage of 2.2 V at 25 °C and 80 °C; (d) EIS spectra of the QSSE-based Li–O2 battery measured before and after the first discharge and charge process. A current density of 100 mA/g with a fixed capacity of 1000 mAh/g was used for Li–O2 cell cycled at 80 °C; typical discharge-charge profiles of Li–O2 cells cycled with a fixed capacity of 1000 mAh/g at (b) 25 °C and (c) 80 °C, respectively; the terminal voltage recorded with Li–O2 cells cycled at (e) 25 °C and (f) 80 °C, respectively.

In addition, it can be seen that the reaction polarization for the discharge process is less affected when the current density increased from 100 mA/g to 2000 mA/g, but the reaction polarization for the charge process shows the distinct increase, indicating the oxygen evolution reaction (OER) process confines the rate performance of Li–O₂ batteries. The EIS spectra of the QSSE-based Li–O₂ batteries operated at 80 °C (Fig. 3(d)) shows a large increase in impedance after the first discharge and significant decrease after the first charge process. The increase of the impedance can be ascribed to the accumulation of discharge product of Li₂O₂, which is an electronic insulator. After the first recharge process, the majority of the discharged products decompose and the resistance of the cell significantly decreases (20 ohm but still larger than that of the original cell), indicating a good reversibility of the QSSE-based Li–O₂ with controlled discharged capacity. The cycle performances were also evaluated by charging and discharging the cells with a fixed capacity of 1000 mAh/g. The cells can charge and discharge for 100 and 500 cycles at a current density of 100 mA/g and 1000 mA/g at 25 °C and 80 °C, respectively (Figs. 3(e) and 3(f)). The terminal voltage, reflecting the reaction polarization, gradually increases upon prolonged cycling. This is likely due to the accumulation of discharge products and inactive products from side reactions.

In order to investigate the morphology changes of the electrode after the charge-discharge process, SEM measurements were performed. A relatively smooth surface can be observed for the pristine electrode as shown in Figs. 4(a) and 4(d). After the first discharge process, a large cluster of toroid-shaped particles can be observed on the surface of cathode electrode (Figs. 4(b) and 4(e)), implying the high utilization of void volume in the cathode electrode. The formation of large toroid-like particles, consisting of small plates, is started at the active sites of KB^[13,14] and then grows layer-by-layer through a solution-mediated electrochemical process^[15] driven by the partial solubility of discharge intermediate LiO₂ in PYR₁₄TFSI. Figures 4(c) and 4(f) show that after the first recharge process, large toroid-shaped clusters disappear, but numerous small flake-shaped particles still exist on the surface of cathode, indicating that the discharge products can only be partially decomposed.

Fig. 4.

	Figure Option ViewDownloadNew Window
	Fig. 4. The SEM images of the cathode electrode at ((a), (d)) pristine, ((b), (e)) first discharge, and ((c), (f)) first charge states. The Li–O2 battery was tested at a current density of 100 mA/g with a fixed capacity of 1000 mAh/g at 80 °C.

Figure 5 shows the Li 1s and O 1s XPS spectra measured on the cathode electrode before and after charge and discharge processes. The peak located at ∼ 533.2 eV on O 1s XPS spectrum of pristine electrode can be assigned to SiO₂. After the first discharge process, SiO₂ signal disappears, while two new peaks appear. These two new peaks, located at ∼ 531.0 eV and ∼ 532.5 eV, can be assigned to Li₂O₂ and Li₂CO₃,^[16] respectively. This indicates that the formation of a thick layer of discharge products on the cathode electrode. The formation of Li₂O₂ and Li₂CO₃ can be further confirmed by Li 1s spectrum, where the two peaks at ∼ 54.6 eV and ∼ 55.5 eV can be ascribed to Li₂O₂ and Li₂CO₃,^[16] respectively. Therefore, the discharge process can be summarized to follow the reaction of 2Li + O₂ → Li₂O₂, while the formation of Li₂CO₃ is due to the side reaction: 2Li₂O₂ + 2C + O₂ → 2Li₂CO₃.^[17] After the first recharge process, the peaks of Li₂O₂ on the O 1s and Li 1s spectra disappear, indicating the decomposition of Li₂O₂ during the charging process. However, the peak assigned to Li₂CO₃ can still be observed, indicating that the Li₂CO₃ is only partially decomposed. A new peak, located at ∼ 55.0 eV, can be identified and assigned to LiOH. The appearance of LiOH indicates another route of side reaction: 2Li₂O₂ + 2H₂O → 4LiOH + O₂, where H₂O might come from O₂ gas. The accumulation of side reaction products leads to the increase in charge–discharge overpotential, which is the main reason for performance degradation of Li–O₂ batteries.

Fig. 5.

	Figure Option ViewDownloadNew Window
	Fig. 5. (color online) The (a, b, c) Li 1s and (d, e, f) O 1s XPS spectra collected on cathode electrodes at pristine, first discharge and first charge states, respectively. The Li–O2 battery was tested at a current density of 100 mA/g with a fixed capacity of 1000 mAh/g at 80 °C.

In summary, a quasi-solid-state electrolyte (QSSE), made by mixing ionic liquid with nano-fumed SiO₂ particles (80:20 v/v), was successfully fabricated. This electrolyte shows a wide electrochemical window up to 5.5 V. The ionic conductivity of the QSSE is 4.6 × 10⁻⁴ S/cm and 6.0 × 10⁻⁴ S/cm at 25 °C and 75 °C, respectively. The superior ionic conducting properties guarantee the electrochemical performances of the Li–O₂ battery. The overpotential for the first cycle was only 0.6 V and 0.45 V, and the round-trip efficiency was 80% and 85.8%, for Li–O₂ batteries tested at 25 °C and 80 °C, respectively. Although the reaction kinetics and reversibility have been greatly improved, the capacity degradation cannot be avoided owing to the insufficient decomposition of discharge products and side reactions. Therefore, substantial efforts are still needed for further improving the cycle performance of Li–O₂ batteries.