Synthesis and properties of the graphene cathode, GPE-TTF, and graphene/Li anode

Figure 1a schematically presents the structure of the graphene-based quasi-solid-state rechargeable Li-O 2 battery composed of a porous graphene cathode, a GPE containing 0.05 M TTF (GPE-TTF) and a porous graphene/Li metal composite anode. As shown in Fig. 1b, the flexible and free-standing 3D porous graphene cathode has a distinct bicontinuous porous architecture built by seamfree multilayer graphene sheets and interconnected open pore channels (Supplementary Figure 1a). The high electrical conductivity (~1.2 × 104 S/m), large specific surface area (700–1000 m2/g), and ultrahigh porosity ( > 95%), verified by our previous studies16,21,22,30, make this 3D nanoporous graphene a highly promising cathode material for Li-O 2 batteries.

Fig. 1: Configuration and components of quasi-solid-state rechargeable Li-O 2 battery. a Schematic illustration of a quasi-solid-state rechargeable Li-O 2 battery. SEM images of (b) porous graphene cathode and (c) porous graphene/Li anode. Insets show the photographic images. d Optical photograph of GPE-TTF. e, f SEM images of GPE-TTF with different magnifications Full size image

The porous graphene/Li anode is prepared by placing a porous graphene sheet into Li melt. As a result of the lithophilic nature of graphene and the capillary forces generated by porosity, the molten Li penetrates into the entire bicontinuous scaffold in several seconds and uniformly coats the internal surfaces of the tubular graphene sheets (Fig. 1c). The graphene/Li composite inherits the 3D porous structure of the nanoporous graphene matrix. XRD confirms the successful loading of metallic Li in the composite (Supplementary Figure 1b). To assess the Li stripping/plating behaviors of the porous graphene/Li electrode, symmetrical cells are assembled and tested with a fixed capacity of 1 mAh/cm2 at a current density of 0.5 mA/cm2 (Supplementary Figure 2a). Significantly, the cell manifests stable voltage profiles with very slow increase in voltage hysteresis over 150 cycles, which obviously contrasts with the pure metal Li electrode that has almost 10 times the increase in the voltage hysteresis after 50 cycles. Furthermore, the Coulombic efficiency (CE) (capacity ratio of Li stripping/plating) of the porous graphene-based cell can be stabilized quickly from 60.2% in the initial several cycles to a value higher than 95% after 10 cycles and remains stable for 100 cycles. In contrast, the CE of the Cu foil-based cell is rather scattered and declines quickly (Supplementary Figure 2b). These results reveal the excellent reversibility of the porous graphene/Li electrode. The superb property of the 3D porous graphene/Li electrode can be attributed to its large effective surface area, which enlarges the Li/electrolyte contact interface, decreases the effective current density and reduces the charge transfer resistance (Supplementary Figure 2c). Remarkably, the porous graphene/Li composite can also suppress the formation of Li dendrites. As shown in Supplementary Figure 2d, e, the planar Li metal is fully covered by fibrous Li dendrites after cycling, whereas the graphene/Li composite retains the original porous architecture with a smooth and dendrite-free surface.

An ultrathin quasi-solid-state GPE-TTF electrolyte is fabricated by integrating hemi-crystalline poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), nanosized fumed SiO 2 , and 1 M LiClO 4 /DMSO solution with 50 mM TTF additive (see Experimental section). As shown in Fig. 1d–f, the transparent GPE-TTF film is compact and has a thickness of ~10 μm. The GPE-TTF surface is smooth without the obvious presence of large pores. Although TTF was added initially as a redox mediator to assist the charging reaction of the Li-O 2 battery, it was found that it also enhances the amorphization of GPE as revealed by XRD (Supplementary Figure 3 and Figure 4a), which is expected to boost the ionic conductivity of the solid-state electrolyte31. Indeed, electrochemical impedance spectroscopy (EIS) measurements suggest a Li+ ion diffusion resistance of 5.1 Ω for GPE-TTF, lower than the 8.4 Ω for GPE and the 75.4 Ω for a conventional glass fiber separator wetted by liquid electrolyte (glass fiber-LE) (Supplementary Figure 4b). Thermal gravity analysis indicates that GPE-TTF exhibits an elevated DMSO solvent volatilization temperature (Supplementary Figure 4c), demonstrating that GPE-TTF can restrain electrolyte volatilization and has higher thermal stability than the glass fiber-LE. The GPE-TTF also exhibits enhanced electrochemical stability, remaining substantially stable at potentials below 4.75 V (Supplementary Figure 4d and Fig. 5). Moreover, the compact GPE-TTF can serve as a protective layer for the metal Li anode to mitigate air-induced Li passivation (Supplementary Figure 6a, b). Even after 50 cycles, the lithium foil of the GPE-TTF-based Li-O 2 battery remains stable with a metallic luster, while the cycled lithium foil in the Li-O 2 battery using glass fiber-LE loses its metallic luster and becomes partially pulverized (Supplementary Figure 6c). The inhibition of the self-discharge behavior of TTF by GPE-TTF can be demonstrated by permeation tests (Supplementary Figure 7a, c) and electrochemical demonstration (Supplementary Figure 7b, d), revealing the outstanding performance of GPE-TTF in restraining the shuttling of TTF to the anode sides in Li-O 2 batteries. All the advantages mentioned above indicate the highly practical value of GPE-TTF.

Assessment of electrochemical performance in coin cells

By taking the advantages of a porous graphene cathode, a graphene/Li anode and GPE-TTF, a quasi-solid-state Li-O 2 coin cell is constructed with the configuration shown in Fig. 1a. As shown in Fig. 2a–c, galvanostatic discharge-charge tests are conducted at cutoff capacities of 500, 1000, and 2000 mAh/g and a current density of 1000 mA/g, demonstrating the high stability of the quasi-solid-state Li-O 2 battery up to 100 cycles (Supplementary Figure 8a). The discharge potential is ~2.75 V and the charge potential is as low as ~3.60 V, corresponding to a high energy efficiency of 76%. Moreover, with the increase in cutoff capacities from 500 to 2000 mAh/g, the terminal voltages of both discharge and charge remain nearly unchanged (Supplementary Figure 8a). For comparison, the cycling performance of a pure GPE-based Li-O 2 battery is also tested. As indicated in Supplementary Figure 8b, c, the cell with GPE exhibits a consistently higher charge overpotential (~1.65 V) than that of the GPE-TTF-based cell at each discharging/charging cycle, indicating the effectiveness of the introduction of TTF in reducing the charge overpotential. The rate performance shows that the quasi-solid-state Li-O 2 battery manifests stable discharge-charge voltage plateaus and mild polarization for current densities ranging from 100 to 1000 mA/g (Fig. 2d). The exceptional rate capability of the cell can be attributed to the high conductivity and large surface areas of the porous graphene cathode and graphene/Li anode as well as the high ionic conductivity of the ultrathin GPE-TTF. As a control experiment, quasi-solid-state Li-O 2 batteries with a 115 µm thick GPE-TTF tested under identical conditions exhibit obvious decline in discharge-charge voltage plateau (Supplementary Figure 8d), demonstrating the critical role of the GPE-TTF film thickness in rate performance. To investigate the impact of the Li loading amount on the cycling performance of Li-O 2 batteries, we prepared a graphene/Li anode with a higher loading amount of Li (82.5 wt.%, Supplementary Figure 9b, c) by prolonging the contact time between the graphene sheet and the molten Li from 20 s to 40 s. Even though a higher loading amount of Li would enable a higher packing density for the graphene/Li electrode, electrochemical testing shows that increasing the Li proportion from 44.4 wt.% to 82.5 wt.% does not significantly affect the cycling performance of the quasi-solid-state Li-O 2 batteries (Supplementary Figure 9d), suggesting that the graphene/Li anode in this quasi-solid-state Li-O 2 battery is not the determining factor for the cyclic performance.

Fig. 2: Electrochemical performance of the quasi-solid-state Li-O 2 coin batteries. Galvanostatic discharge-charge curves of the quasi-solid-state rechargeable Li-O 2 coin batteries with cutoff capacities of (a) 500 mAh/g, (b) 1000 mAh/g, and (c) 2000 mAh/g at a current density of 1000 mA/g. d Rate capability of the quasi-solid-state rechargeable Li-O 2 coin batteries Full size image

The discharge products on the porous graphene cathode of the quasi-solid-state Li-O 2 batteries were characterized by XRD and SEM. As shown in Fig. 3a, b, characteristic diffraction peaks corresponding to crystalline hexagonal Li 2 O 2 are detected from a discharged cathode with a cutoff capacity of 5000 mAh/g; these peaks disappear after subsequent recharge, implying the formation and complete decomposition of Li 2 O 2 . SEM reveals the formation of particulate Li 2 O 2 with diameters of 300–500 nm uniformly distributed on the surfaces and inner pores of the porous graphene cathode (Fig. 3c). With the assistance of TTF, the Li 2 O 2 particles can be completely decomposed at the low charging overpotential, as evidenced by the clean surface of the recharged porous graphene cathode (Fig. 3d). All these results verify that the reactions in the quasi-solid-state Li-O 2 battery follow the typical Li-O 2 electrochemistry of 2Li + O 2 ⇌ Li 2 O 2 and permit the highly reversible Li 2 O 2 formation and decomposition. For the graphene/Li anodes, the 3D porous structure and the uniform Li coating are well maintained except that the surface becomes slightly rough (Supplementary Figure 10a). No Li dendrites are observed, suggesting the formation of robust and uniform solid electrolyte interphase (SEI) films on the Li surfaces. XPS reveals that the SEI layer is mainly composed of the decomposition products of DMSO and PVDF-HFP, including Li 2 SO 4 , Li 2 S, Li 2 O, LiF, and some organic Li species (Supplementary Figure 10b–f).

Fig. 3: Characterization of discharge products of the quasi-solid-state rechargeable Li-O 2 battery. a Galvanostatic discharge-charge profiles of the quasi-solid-state rechargeable Li-O 2 coin batteries with a cutoff capacity of 5000 mAh/g at a current density of 1000 mA/g. b XRD patterns of pristine, discharged and recharged porous graphene cathodes. SEM images of (c) discharged and (d) recharged porous graphene cathodes. Insets show cross-sectional SEM images Full size image

Graphene-based quasi-solid-state Li-O 2 pouch batteries

To explore the capability of the graphene-based quasi-solid-state Li-O 2 batteries for practical applications, we scaled up the coin battery to a large pouch-type cell with a size of 26 mm × 38 mm × 1.2 mm. As schematically illustrated in Fig. 4a, the quasi-solid-state Li-O 2 pouch cell is assembled by laminating one piece of porous graphene/Li sheet as the anode, two pieces of GPE-TTF as the electrolyte and separators, and two pieces of porous graphene sheets as the cathodes, which are then sealed inside an aluminum-laminated film case with pinholes on both sides. Fig. 4b is a photograph of a prototype pouch-type cell. The cell is mechanically flexible and can be scrolled without degeneration in battery performance (Supplementary Figure 11a). Excellent rate performance has been achieved for the quasi-solid-state Li-O 2 pouch cell. As shown in Fig. 4c, with the increase in current density from 100 to 2000 mA/g, the charge voltage plateau exhibits good reproducibility, and the discharge voltage plateau experiences a gentle and insignificant decrease. Cycling tests of the Li-O 2 pouch cell demonstrate a high durability with stable cycling up to 100 cycles at a cutoff capacity of 1000 mAh/g and a current density of 1000 mA/g (Fig. 4d). Apparently, the excellent performance that was achieved with coin cells can be completely reproduced by large-sized pouch batteries. Importantly, a full discharge capacity up to 33230 mAh/g cathode can be delivered when 70 μm thick porous graphene is used as the pouch cell cathode (Supplementary Figure 11b, Tables 1 and 2). The gravimetric energy density and volumetric energy density of the pouch cell reach 408.48 Wh/kg cell and 174.83 Wh/L cell , respectively, which are calculated based on the total mass and volume of the device (Supplementary Table 2). These values far exceed those of our recently reported liquid electrolyte-based Li-O 2 batteries (260.30 Wh/kg cell , 110.81 Wh/L cell )16. If the pouch-type cell obviates the use of an aluminum-laminated film case (which accounts for a major proportion of the weight and volume in the cell), then the energy densities can reach 727.84 Wh/kg cell without case and 1285.05 Wh/L cell without case (Supplementary Table 2). These metrics that do not consider the packages are of practical significance since the case would not require the same lateral size as the main cell components for the large-scale integration of flexible quasi-solid-state Li-O 2 cells. A preliminary prototype for such a cell is shown in Fig. 4e. The cell laminates were rolled into a rod shape, saving a large volume of package materials. Figure 4f compares the energy densities of the graphene-based quasi-solid-state Li-O 2 pouch cell with those of a commercial Li-ion polymer battery, highlighting the gravimetric energy-density advantage of our quasi-solid-state Li-O 2 pouch batteries. Furthermore, the volumetric energy density calculated without packaging fixation materials far exceeds the value for commercial Li-ion polymer batteries, demonstrating an appealing design for boosting the volumetric energy density of Li-O 2 batteries.