Graphene was synthesized on Cu foil in a thermal furnace at 950°C for 7 min under Argon (Ar): Hydrogen 2 (H₂): methane (CH₄) mixed-gas flow. Subsequently, a pre-prepared poly (methyl methacrylate) (PMMA, Sigma Aldrich) solution was dropped all over the graphene surface, spin-coated at 5000 rpm for 45 s, and then heat treated on a hot plate at 160°C for 3 min. The PMMA solution was prepared by adding 0.5 g PMMA powder to 15 mL chlorobenzene (Sigma Aldrich) and stirring the mixture at 65°C for 8 h. The Cu was removed by dipping the PMMA/graphene/Cu foil stack in 1 mole (M) Iron (III) chloride (F_eCl₃) solution (Sigma Aldrich). The PMMA/graphene stack was then transferred onto glass or polyethylene terephthalate (PET) substrate and heat-treated at 110°C for 3 h. Finally, the PMMA was dissolved by acetone. The following processes were used to fabricate GQDs. Graphene oxide (GO) was prepared from graphite powder using the Hummers method. GO was deoxidized in a horizontal furnace at 350°C–450°C for four hours under Ar to produce reduced graphene oxide powder. About 5.0–6.5 g of graphene oxide powder was oxidized by ultrasonication in concentrated 15 mL sulfuric acid (H₂SO₄) and 40 mL nitric acid (HNO₃) for 20 h. In addition, 350 mL deionized water was used to dilute the mixture. After cooling to room temperature, the resulting powder was redispersed in 35-mL DI water for two hours under ultrasonication. Then, by filtering the resulting suspension through a 200 nanometer (nm) nano-porous membrane, a brown solution was separated. Since the colloidal solution still contained some large graphene nanoparticles (<200 nm) emitting weak blue fluorescence, it was further filtered overnight in a dialysis bag with a cutoff of 3500 dalton (Da) molecular weight, thereby producing strongly fluorescent GQDs. The GQDs were separated into different sizes using several dialysis bags of 1000–50,000 Da and a 20-nm nano-porous membrane.

To fabricate solid-state Li batteries (SSLBs)-based nano-structure devices, after the direct synthesis of the extracted graphene, graphene quantum dots (GQDs) of each shape were directly grown on patterned Li electrodes on a copper (Cu) thin films through a modified chemical vapour deposition (CVD) method. After washing the composite GQD layer in a deionized water cleaning process, heat treatment was performed. Coin cells were assembled in glove box using CR2032 type shells. The modified PEO electrolyte membrane was prepared by drying a mixture of polyethylene oxide (PEO), Lithium bis(trifluoromethanesulphonyl) imide (LiTFSI) powders, and acetonitrile in a vacuum oven at 50°C for 24 h [28–30]. The symmetrical cells with GQD-PEO-LiTFSI hybrid paste were fabricated with two Li disks (thickness of 200 µm) and a donut spacer (thickness of 25 µm). GQD-PEO-LiTFSI hybrid paste was placed in the middle of spacer. The interlayer was built with 10 µL commercial LE (1 m LiPF₆ in ethylene carbonate/diethyl carbonate mixture solvent) or 15 mg paste hybrid on each side of pellet. The interlayer in full cells was built in the same way for symmetric cells [31–33].

Plane- and cross-sectional-view images of the materials and devices were obtained by field emission scanning electron microscopy (FE-SEM, Carl Zeiss, model LEO SUPRA 55). Morphologies of GQDs were analyzed using an High-resolution Transmission Microscopy (HRTEM) (FEI Tecnai F30 S-Twin). To make the HRTEM specimens, the GQDs were dispersed in DI water, drops of which were then put on C- or SiO-coated Cu grids (Tedpella, Inc., Redding, CA, USA) and mica substrate, respectively. The current-voltage (I–V) curve of the device was analyzed using an electrical and electronic workstation (Keithley 2400). Current density-voltage (J-V) characteristics were monitored by forward/reverse scans at 200 ms and 10 mV under standard conditions. The ionic conductivity of the electrolytes was measured on a dielectric spectrometer in the temperature range from −20°C to 100°C in a gold-plated cell with Teflon spacer. Lithium samples were transferred from glovebox to the chamber in a vessel without explosion in air. Raman spectra were observed from a Bruker IFS66 Fourier Transform spectrometer with a FRA106 Raman module. All the heating tests were carried out in Ar-filled glove box. Electrochemical impedance spectroscopy was performed on a Reference 600 (Gamry Instruments) over a frequency range of 0.01 to 1 MHz and the perturbation amplitude was 10 mV.

As shown in Fig. 1, a lithium secondary battery consists of a positive electrode, an electrolyte, and a gold electrode, a polymer separator with thickness of tens to hundreds of micrometers is positioned inside a liquid electrolyte comprising organic solvent and lithium salt. Movements of lithium ions are in opposite directions during charging and discharging. In other words, during discharge, Li⁺ ions move from the negative to the positive electrode, and electrons generated as lithium ionizes also transfer in the same direction. The cause of this Li⁺ ion migration is the system’s pursuit of chemical stability against the potential difference between the two electrodes. Li dendrite growth, which unexpectedly penetrates the separator during charge/discharge operation and causes battery failure and fire, has severely limited the use of next-generation devices as well as various high-performance system applications. Therefore, next-generation batteries require a transition from the existing layered structure to a modified electrode structure that can control the intercalation of lithium between graphite layers.

The electrical properties of the sample according to the I₂ content of the modified poly (ethylene oxide) (PEO) structure are shown in Fig. 2a. For ionic modulation of the PEO structure, the ionic conductivities of electrolytes with different GQD-I₂ contents (1, 1.5 and 2 wt% of PEO-LiTFSI) were measured. It was found that the ionic conductivity of the sample structures depends on the temperature [34]. Compared with pristine PEO-LiTFSI, when 1.5 wt% I₂ was added to the PEO-LiTFSI electrolyte at 50°C, the resulting electrolyte saw its ionic conductivity increase from 9.34 × 10⁻⁶ to 6.28 × 10⁻⁵ S cm⁻¹. The ionic conductivity tends to increase with increasing temperature of the modified PEO-GQD based electrolyte.

In Fig. 2b, Raman spectroscopy was performed to investigate the effect of I₂ on the ionic conductivity. The characteristic band at 190 cm⁻¹ belongs to I₂, but this characteristic peak was not observed in the GQD-modified PEO-LiTFSI-1 wt % I₂ sample. In contrast, in the GQD-modified PEO-LiTFSI-1.5 wt % I₂ sample, a significant band between 130 and 200 cm⁻¹ was observed due to the activation of symmetric stretching mode of the I₃-atomic structural unit. This indicates that iodine exists in I₃ rather than I₂ form in the GQD-modified PEO polymer structure. Through spectroscopic analysis of I₂ dissolved in acetonitrile, the properties of I₃⁻ were confirmed. Unlike the 145 cm⁻¹ peak, related to I₃, the characteristic 190 cm⁻¹ peak of I₂ was dominant. According to this result, I₃⁻ is mainly generated by interaction between I₂ and PEO, in a reaction controlled by GQD.

When I₂ was dissolved in acetonitrile, it received electrons from the modified PEO, an electron donating structure, and generated I⁻ through a reduction reaction. As a result, I⁻ ions generated I₃, an anion, through a chemical reaction with I₂. The interaction between PEO and I₂, as well as the donor-acceptor relationship between iodine species and acceptor, is widely known through the results of previous studies on organic molecular phenomena in water-insoluble solvents. The chemical movement of hydrogen atoms according to different amounts of I₂ in the modified PEO structure (PEO-x wt % I₂, x = 2, 4, 6) is smaller than that in the original PEO structure, and results show that chemical movement increases as the density of I₂ increases. This indicates the complex action at the interface produced by GQDs between I₃-units and PEO molecules.

As a result, the I₂ surface structure modified by GQD reduces the crystallinity of PEO and promotes electron transfer in the PEO hetero-interface structure. GQD-modified I₃⁻ interacts with the carbon-oxygen of the electron-donating substitutes to effectively weaken the bonding force between Li⁺ ions and other oxygen groups, promoting efficient transport of Li⁺. As a result, this process contributes to the enhancement of the ionic conductivity of the GQD-modified electrolyte. Compared with that of the pristine PEO structure, the diffraction intensity of modified PEO-LiTFSI decreased, indicating that there was a change in crystallinity. It was confirmed that I2 doping can significantly reduce the effective contact resistance of the modified GQD-PEO, and can control the crystallinity through the action of GQD. In addition, GQD-based I₂ doping does not affect the electrochemical behavior of the composite electrolyte. We investigate the electrochemical properties of various hetero-interfaces used to stabilize Li metal anodes of Li batteries with symmetric structure. All devices are rated at a fixed capacity of 0.2 mAh cm⁻² and a current density of 0.2 mA cm⁻².

Fig. 3 shows the cell using pristine PEO-LiTFSI electrolyte had an average overpotential of 120 mV and exhibited a limited cycle life of 385 h. Further analysis of the micro-surface morphology of the Li layer by scanning electron microscopy (SEM) revealed that a sharp drop in overpotential occurred during repeated charge/discharge cycles, indicating a short circuit. A porous Li metal surface can be observed in Fig. 3d. The void defects inhibit Li+ ion diffusion and accelerate formation of Li dendrites in the hetero-interface structure [35]. As a result, device performance such as lifetime and durability deteriorated. In contrast, devices using GQD-modified PEO-LiTFSI-x wt% I₂ (x = 1 and 1.5) electrolytes had improved cycling stability. Through the interfacial Li plating and stripping process, dense structural defects were improved at the interface between the GQD-modified PEO-LiTFSI-1.5 wt% I₂ electrolyte and Li metal.

In particular, the GQD-modified PEO-LiTFSI-1.5 wt % I₂ electrolyte had a long lifetime of more than 550 h and a rather low average overpotential of 55 mV, leading to negligible polarization. The Li metal anode showed a generally uniform surface and stable cycling performance without serious defects on the surface or at the interface. The GQD-modified PEO-LiTFSI-1.5 wt% I₂ electrolyte presents a low polarization of 0.2 mA cm⁻². Moreover, the battery performance of devices composed of electrolytes containing 1.5 wt % I₂ ions was superior to those containing I₂ ions at percentages of 0.5, 1.0, and 3 wt %. However, I₃- reacted chemically with the unstable Li structure, composed disproportionately of Li interfaces and GQD-Li₂O. The unstable structure of the I₃-ion caused corrosion of the active Li structure, indicating contact issues between Li layer and GQD and poor battery performance.

Likewise, a moderate reaction of GQD-modified iodine to reduce overpotential is confirmed with different current densities. This indicates a significant enhancement effect of GQD-modified I₂-ion on the interfacial stability between the GQD-electrolyte and the Li layer.

To confirm the mechanism of GQD-modified I₂ doping on the surface and interfacial stability, observed the morphology and composition of the interface at atomic scale by measuring (transmission electron microscopy) TEM. The low-magnification TEM image in Fig. 4a shows the interface observed without I₂ doping. In the high-resolution-TEM (HR-TEM) images of the blue and yellow square region in Fig. 4b, crystalline Li layer and GQD-Li₂O regions are visible. In addition, the high oxygen content and oxygen atoms at the interface between the pristine PEO-LiTFSI electrolyte and the Li anode formed a Lithium oxide (Li₂O) structure [36]. Compared with the case without I₂ doping, it did not change the defect structure at the interface. In the HR-TEM images in Figs. 4c and 4d, Li₂O and GQD nanocrystal regions and an amorphous layer can be seen to be mixed, and an I-doped organic-inorganic layer can be observed.

Crystal structures and grid spacing corresponding to Li, Li₂O, Lithium iodide (LiI), and Lithium iodide oxide (LiIO₃) were can be observed in Figs. 4c and 4d. An Li layer with lattice spacing of 2.48 Å. The lattice spacings of 2.71 and 2.46 Å corresponded to the Li₂O (111) and GQD planes. The LiI layer distributed on the surface of the Li₂O structure and Li₂O domains has been recognized as a solid polymer electrolytes (SPEs) interfacial reactant with Li. The LiI structure forms due to the rapid reaction between I₃- and Li/GQD-Li₂O. In addition, the formation of LiIO₃ took place via different interfacial reactions according to the I₃-doping and Li₂O content. This can be confirmed by the contents of the various nanostructure at the interface after Li formation. The TEM results indicate that GQD-based I₂ doping can achieve uniform formation of Li layer by construction of modified SEI interfacial layer (LiI/GQD-LiIO₃). Controlled I₃ reacts with Li and Li₂O at the interface to generate activated Li+ ions and this leads to a defect-free stable interface, which can enhance Li+ ion conduction at the heterostructure.

The solid-state batteries with GQD-PEO-LiTFSI-1.5 wt% I₂ electrolyte also indicate a good rate capability, as shown in Fig. 5a, High capacities of 152, 148, and 146 mAh g⁻¹ are delivered at, respectively, 50, 100, and 200 μA cm⁻². The capacity recovers to 149 mAh g⁻¹ when the current returns to 100 μA cm⁻². Further evaluation of the electrochemical stability of the GQD-PEO layer was conducted with a cycling test in an GQD-PEO-LiTFSI-1.5 wt% Li cell at 150 μA cm⁻², a stabilized capacity of 149 mAh g⁻¹ is delivered after an activation reaction in the first several cycles.

In Fig. 5b, Raman spectra results show the composition of the modified atomic layer interface and the chemical reaction of the iodine species. The components of the intermediate phase appearing in the PEO-based device structure are Li₂O and an amorphous composite polymer; this intermediate phase becomes a large source of resistance to ion conduction. With GQD-modified I₂ doping, there was no significant change in oxygen content at the heterostructure interface, and the oxygen content of Li₂O, compared to that of Li_6.4La₃Zr_1.4Ta_0.6O₁₂, decreased. As a result, the composition of the GQD-based I-doped hetero-interface, which can control defects between the GQD-modified electrolyte and Li structure.

The properties of elemental iodine on the electrolyte interface before and after the repeated operation indicated that the structure of I₂ deteriorated and the amount of I- component increased, indicating that the LiI₃ moiety was reduced to an LiI layer by Li atoms [37]. The change of the I₃-structure in the modified GQD-PEO-LiTFSI-1.5 wt% I electrolyte was measured by Raman spectra analysis during charge/discharge cycles of transparent LiFePO₄/PEO-LiTFSI-1 wt% I₂ GQD/Li devices. A pathway to stabilize Li was realized by spontaneous formation of a modified LiI layer between the composite polymer/GQD/Li hetero-interface. In the signal range of 0 to 200 cm⁻¹, a peak appeared at 128 cm⁻¹, which corresponds to the symmetric stretching mode of the I₃-structure. The peak at 128 cm⁻¹ decreases during the charge/discharge cycle, indicating that modified I₃⁻ reacts with Li and Li₂O to form a hetero-interfacial passivation layer of LiI and LiIO₃. The reduction of modified I₃⁻ by Li and Li₂O can be described by the Li+ and IO₃-ion reactions.

To design a hetero-interface with improved structural reliability between the Li metal anode and the GQD-modified PEO-LiTFSI-1.5 wt% I₂ electrolyte, a modified transparent LiFePO₄ anode, Li metal anode, and electrolyte were fabricated at 50°C and evaluated via SSLB. The LiFePO₄/Li device to which the pristine PEO-LiTFSI electrolyte was applied showed rapid capacity deterioration, with a capacity retention rate of 43.6% after 100 cycles. In contrast, by applying the GQD-modified electrolyte, the LiFePO₄/Li device showed moderate capacity retention and stable movement at 0.5 C, as can be seen in Fig. 6a. After 100 cycles, the GQD-modified device provides a specific capacity of 148 mAh g⁻¹ and a capacity retention of 94.8%. Moreover, batteries using modified electrolytes with GQDs have capacities higher than those of pristine devices. As a result, the GQD-controlled iodine matrix significantly increased the effective area of Li⁺ ion transport because the hetero-interface and electrolyte inside contribute to battery capacity enhancement [38].

The pristine electrolyte and the GQD-based I-doped electrolyte showed typical charge/discharge characteristics of transparent LiFePO₄ devices, as shown in Fig. 6a. In addition to the voltage of 3.25 and 3.40 V corresponding to LiFePO₄, SSLB containing GQD-PEO-LiTFSI-1.5 wt % I₂ electrolyte modified with GQD also has a 3.14 V discharge characteristic, caused by the state change of modified I₃. This is consistent with the nanostructure properties. The control made possible by use of 1.5 wt % I₂ modified with GQDs revealed the reversible redox properties of LiFePO₄, indicating a reduction of I₃⁻ to I⁻ and oxidation of I- to I₃-due to GQD. This suggests that the GQD-modified iodine structure is involved in the electrochemical change of the device. In addition, the SSLB using the modified GQD-PEO-LiTFSI-1.5 wt % I₂ electrolyte showed superior rate characteristics compared to those of the pristine electrolyte.

Subsequently, using EIS measurement, changes of SSLB interface impedance (R_ct) after repeated cycles were analyzed to determine the hetero-interfacial structural stability between the GQD-modified PEO electrolyte and the Li interface. As shown in Fig. 6b, Rct showed a tendency to increase after continuously changing cycle of the SSLB containing modified PEO-LiTFSI electrolyte. And, the Rct of the device using the modified GQD-PEO-LiTFSI-1.5 wt % I₂ electrolyte was smaller than that of the device using the pristine PEO-LiTFSI electrolyte. After 10, 50, and 100 cycles, the Rcts of the devices were 254, 102, and 86 Ω, respectively. These results indicate that GQD-based modified I₂ doping is responsible for the structuring of the highly conductive modified SEI layer with GQDs, and is related to the formation of the Li/Li₂O structure by iodine redox reaction. The hetero-interface impedance between Li anode and GQD-PEO electrolyte decreased.