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Graphene-Oxide-Assisted Synthesis of Ga2O3 Nanosheets/Reduced Graphene Oxide Nanocomposites Anodes for Advanced Alkali-Ion Batteries Mingzhi Yang, Changlong Sun, Tailin Wang, Fuzhou Chen, Minglei Sun, Lei Zhang, Yongliang Shao, Yongzhong Wu, and Xiaopeng Hao ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00826 • Publication Date (Web): 08 Aug 2018 Downloaded from http://pubs.acs.org on August 10, 2018

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ACS Applied Energy Materials

Graphene-Oxide-Assisted Synthesis of Ga2O3 Nanosheets/Reduced Graphene Oxide Nanocomposites Anodes for Advanced Alkali-Ion Batteries Mingzhi Yang,†,# Changlong Sun,†,# Tailin Wang,† Fuzhou Chen,† Minglei Sun,‡,§ Lei Zhang,† Yongliang Shao,† Yongzhong Wu† and Xiaopeng Hao*,† †

State Key Lab of Crystal Materials, Shandong University, Jinan, 250100, Shandong,

P. R. China ‡

School of Mechanical Engineering, Southeast University, Nanjing, 211189, Jiangsu,

P. R. China §

Institute of High Performance Computing, A*STAR, Singapore 138632, Singapore

*E-mail: [email protected] [#] These authors contributed equally to this work.

1

ACS Paragon Plus Environment

ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT Herein, we propose a practical way for direct preparation of gallium oxide nanosheets/reduced graphene oxide (Ga2O3 NSs/rGO) nanocomposites via oxygenic groups contained hydrophilic graphene oxide (GO) template with subsequent annealing treatment. Benefiting from the two-dimension (2D) nanoarchitecture of Ga2O3 NSs and rGO, Ga2O3 NSs/rGO nanocomposites exhibit enhanced kinetics and improved cycling stability in Li-ion and Na-ion batteries (LIBs/SIBs). The pseudocapacitance performance is authenticated through kinetic analysis. Ex-situ XRD and XPS measurements prove that the reversible Li-ion storage in Ga2O3 NSs/rGO electrodes is conversion reaction and alloying mechanism. In addition, the full cells fabricated by coupling Ga2O3 NSs/rGO anode and LiFePO4/C or Na3V2(PO4)3/C cathodes exhibit outstanding electrochemical performances. In general, such a new approach, which is not specific to Ga2O3 NSs/rGO nanocomposites, offers great opportunities for 2D oxide nanomaterials or nanocomposites as high-performance electrodes.

Keywords: graphene oxide; Ga2O3 nanosheets; nanocomposites; Li-ion batteries; Na-ion batteries 2


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1. INTRODUCTION To cater the requirements for rapid electrochemical energy storage, the explorations of battery materials with tremendous capacity and superb stability are vital,1,2 such as metal oxides,3-6 metal sulfides,7,8 and metal nitrides.9,10 In particular, metal oxides (MO) attract a lot of attention because of their tremendous capacity, abundant reserves and environmental friendliness. It is quite different with the intercalation reaction for graphite (372 mAh g−1, theoretical specific capacity),11 the energy storage mechanism of MO is a conversion process that can provide a specific capacity of about 1000 mAh g-1, which is much higher than graphite.12 However, MO electrodes still beset by inferior cycling performance and poor reversible capacity which result from the large volume changes and lack of electrically conductive pathways.13 To overcome these issues, one efficient strategy is to construct MO electrode materials with nanoscaled structure to maintain good structural integration, which can provide abundant available channels for fast lithium/sodium diffusion.14,15 Gallium oxide (Ga2O3) is an important functional material with unique physical and chemical

properties

and

diverse

applications,

such

as

lasers,

visible

photoluminescence, catalyses and sensors.16,17 Moreover, Ga2O3 shows favorable capability for lithium/sodium storage, suggesting its applicability for electrochemical energy storage.18-21 Compared to other metal based electrodes, the Ga-based materials show a liquid self-rehabilitation behaviour and could mitigate adverse effects from environmental changes.22,23 However, Ga2O3 electrodes are still being troubled by ordinary rate performance and inferior cycling stability caused by intrinsically inferior 3



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electronic mobility, slow reaction kinetics and severe volume changes in cyclic procedures. Thus, improving the conductivity and stability of the Ga2O3 electrodes is still a big challenge. Two-dimensional (2D) nanostructure is an effective architecture for energy storage due to its high surface area leading to strong affinity with other nanomaterials

and

sufficient

surface/interface

interaction

with

the

electrode/electrolyte.15,24-26 Besides, the sheet-like structure could shorten the conduction paths of ions and electrons to improve the reaction kinetics and mitigate the volumetric expansion of active materials in cycling procedures.25,27,28 In particular, when hybridized with reduced graphene oxide (rGO), the conductivity and structural stability of hybrid materials can be further enhanced because of the superior conductivity and mechanical properties of the rGO.3,29,30 Therefore, it is highly desirable to design and develop Ga2O3 nanosheets/rGO (Ga2O3 NSs/rGO) nanocomposites with extraordinary rate and cycling performances for electrochemical energy storage. Herein, we designed Ga2O3 nanosheet structure and successfully fabricated Ga2O3 NSs/rGO nanocomposites via a scalable but straightforward method. Due to the ultrathin and fine 2D nanostructure of Ga2O3 NSs and the presence of rGO, the pseudocapacitance is greatly enhanced, and responsible for the fast kinetics in conversion reaction and alloying mechanism of Ga2O3 NSs/rGO electrodes. The Ga2O3 NSs/rGO electrodes show outstanding cycling stability and rate performance both in half and full cells of LIBs and SIBs. This work not only introduces GO as template for the synthesis of metal oxide 2D nanomaterials but also demonstrates that 4


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the great potential applications of metal oxide nanosheets/rGO nanocomposites as advanced anodes in energy storage field.

2. RESULTS AND DISCUSSION 2.1. Morphology and Structural Characterizations

Figure 1. (a) Synthetic diagram of the Ga2O3 NSs/rGO nanocomposites. (b, c) SEM, EDS and elemental mappings of C, Ga, O. (d) XRD pattern. (e, f) TEM, HRTEM and SAED (inset) images. Scale bars: inset of (f) 10 1/nm. (g) Nitrogen adsorption-desorption isotherm and pore distribution. Ga2O3 NSs were synthesized onto the GO layers through precipitating method. Figure 1a shows the synthetic diagram of the precipitating reaction. Due to its high hydrophilicity and surface oxygen groups, GO sheets could disponible as templates 5



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for the preparation of 2D nanostructure.31,32 The GO sheets with functional groups can adsorb the formed GaOOH by precipitation process in the presence of NH3·H2O (Figure S1). Due to the limitation from GO sheets, the formation of Ga2O3 was confined when calcined in an Ar atmosphere (Figure S2). Ga2O3 NSs have a similar 2D morphology as GO, suggesting a successful replica of the GO sheets (Figure 1b). The corresponding energy-dispersive spectroscopy (EDS) microanalysis and elemental mappings reveal the homogeneous distribution of carbon, gallium and oxygen (Figure 1c). X-ray diffraction (XRD) pattern (Figure 1d) and selected area electron diffraction (SAED) image (Figure 1f inset) can distinctly illustrate that the material corresponds to hexagonal Ga2O3. Transmission electron microscopy (TEM) image reveals the combination of rGO sheets and Ga2O3 NSs in Figure 1e. And, a lattice fringe of 0.25 nm corresponding to (110) facet of Ga2O3 is explicitly shown in high-resolution TEM (HRTEM) image (Figure 1f). The Brunauer-Emmett-Teller (BET) and pore size distribution results demonstrate that Ga2O3 NSs/rGO exhibit a large specific surface area (127 m2 g−1) and mesoporous structure (Figure 1g). The intricate pore structure of the nanocomposites clearly offers abundant nanochannels to contact the electrolyte through surface-interface interactions. The D band around 1355 cm-1 (attributed to unordered carbon) as well as G band around 1580 cm-1 (first-order scattering originating of the tensile vibration mode E2g in the sp2 carbon domain)33 of Raman pattern (Figure S3) also demonstrate the presence of rGO and the thermogravimetric analysis (TGA) indicates the weight content of carbon in Ga2O3 NSs/rGO nanocomposites is ~19 wt. % (Figure S4). 6


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2.2. Electrochemical performances of Ga2O3 NSs/rGO electrodes in LIBs

Figure 2. Electrochemical performances of Ga2O3 NSs/rGO electrodes in LIBs. In half-cells: (a, b, c, d, e, f) CV curves, charge/discharge profiles, Nyquist plots, cycling, rate and long-term cycling performances. In full-cells: (g, h, i) Charge/discharge profiles, cycling and rate performances (Insert are optical images of the LED lights powered by the full cells.). The current density and gravimetric capacity are counted by Ga2O3 NSs/rGO. CV curves of first three cycles at 0.1 mV s−1 are displayed in Figure 2a. The first weak cathodic peak around 0.7 V correspond to the reduction reaction: Ga2O3 + 6Li+ + 6e− → 2Ga + 3Li2O, as well as the emergence of solid electrolyte interphase (SEI).20 In addition, the stronger cathodic peak around 0.3 V is the formation of LixGa, which is consistent with the alloying process: Ga + xLi+ + xe− → LixGa.20,34 During

7



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the charge process, the anodic peaks around 0.25 and 0.85 V can be ascribed to dealloying reaction20: LixGa → Ga + xLi+ + xe−. The cathodic/anodic peaks in the subsequent cycle curves tend to be stabilized, demonstrating a favorable reversibility of Li+ storage. The pure Ga2O3 electrodes (Figure S5a) show the same behavior as that in Ga2O3 NSs/rGO in CV curves, it is believed that they share the same charge/discharge reactions. Galvanostatic charge/discharge curves of hybrid electrode at 0.1 A g-1 are shown in figure 2b. Due to the inevitable emergence of SEI, leading to the capacity decrease in the 1st cycle,35 the discharge and charge capacities are 1112.73 and 843.55 mAh g−1, respectively. In the subsequent two cycles, the discharge capacities are retained at 828.97 and 749.72 mAh g−1, respectively. However, the pure Ga2O3 electrode exhibits a relatively low original capacity (Figure S5b). In addition, alternating-current impedance tests are applied to verify the Li+ and electrical conductivity in the materials (Figure 2c). The reduced Rct and steeper incline suggest faster electrons and ions migration in Ga2O3 NSs/rGO electrodes, which benefit by the 2D structures of Ga2O3 NSs and rGO. Figure 2d shows the cycle ability about Ga2O3 NSs/rGO under 0.1 A g−1. The incipient Coulombic efficiency is 75.8 %, while increase to above 97.1 % and maintain in ~100 % in subsequent 200 cycles. After the decrease in previous cycles, the discharge capacity increased in subsequent cycles and remains at ~834 mAh g-1 at 200th cycle. According to previous reports, the increasement of capacity is a frequent case in oxide electrodes.36-40 Firstly, the activation process is ordinary during the 8


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electrochemical reaction.41 Secondly, the disintegration of adsorbed electrolyte on the external of materials play a positive role for the extra capacity contribution through “pseudo-capacitance-type behavior”.42,43 Furthermore, the interfacial lithium storage which is related to the presence of rGO and the unique 2D structure of the Ga2O3 NSs also can contribute to the enhanced electrochemical performances.38,44 For comparison, the pure Ga2O3 electrode only retains 181.4 mAh g−1 at 200th cycle (Figure S5b). The excellent electrochemical properties of Ga2O3 NSs/rGO are also reflected on its rate and high current cycling performances. As shown in Figure 2e, Ga2O3 NSs/rGO electrodes provide reversible capacities of about 864, 807, 697, 613, 564 and 395 mAh g−1 as the current densities increase from 0.1 to 5.0 A g-1. The discharge capacity can still be retained at 283 mAh g−1 (10 A g−1) and the initial capacity could be recovered at 0.1 A g−1, indicating highly reversible Li+ storage. This result also suggesting that the stable Ga2O3 NSs/rGO electrodes promote transmissions of both electrons and Li-ions. The long-term cycling stability was tested at 2.0 A g−1 (Figure 2f). After 2000 cycles, the Ga2O3 NSs/rGO electrode still deliver a reversible capacity of ~540 mAh g−1, which also disclosing excellently reversible Li-ion kinetics.44 The reduction of the semi-circle of the electrochemical impedance spectroscopy (EIS) spectrum (Figure S6) indicates that the transfer performances of electrons and lithium ions are effectively improved through activation process during cycles.45 In addition, the 2D structure of Ga2O3 NSs/rGO electrode has been well-preserved after 2000 cycles, which is favorable to the long-cycle and high-rate performances (Figure S7). 9



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These result in the good charge and discharge performances of Ga2O3 NSs/rGO nanocomposites in LIBs. In consideration of its performances in the practical application, the Ga2O3 NSs/rGO electrodes were assembled with the cathode material of LiFePO4/C as full cells. Before that, the commercial LiFePO4/C powders are first checked (Figure S8). Figure 2g illustrates the charge/discharge curves of the Ga2O3 NSs/rGO // LiFePO4/C full cell at 0.1 A g−1 (1−4 V). It shows a voltage plateau at ~2.5 V , as well as high initial charge/discharge capacities of 821.16 and 703.69 mAh g−1anode, respectively. Because of the initial irreversible capacity, the initial Coulombic efficiency is 85.7 % and hold at about 100 % in subsequent 100 cycles with the capacity of ~570 mAh g−1anode (Figure 2h), which corresponds to an average capacity loss per cycle of 2.5 %. The rate performance of the full cell at 0.1, 0.2, 0.5 and 1.0 A g−1 are shown in Figure 2i, and its capacities are 623, 507, 391 and 315 mAh g−1anode. Exhilaratingly, when the current density returns to 0.1 A g−1, the discharge capacity of the full cell increases back to 613 mAh g−1anode. To facilitate the potential applications of full cells, a single button cell is used to power different color LED lights (2.2−2.4 V, 0.5 W, Figure 2i inset). All the results illustrate that the Ga2O3 NSs/rGO is a suitable electrode material for LIBs.

10


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Figure 3. Pseudocapacitance analysis and Li-ion storage mechanism of Ga2O3 NS/rGO electrode in LIBs. (a) CV profiles. (b) Relationship between anodic peak current and scan rates. (c) Bar chart of capacitive capacities vs. scan rates. (d) Ex-situ XRD of the electrode at the first cycle. (e) Ex-situ XRD patterns at 0.01 V and 3.0 V. (f) Ex-situ Ga 3d XPS patterns of the lithated/delithiated hybrid electrode. (g) Schematic diagram of lithiation process. The superior electrochemical properties of Ga2O3 NSs/rGO electrodes attract our attentions to explore the kinetic principle based on the CV analyses.46-50 Figure 3a shows the CV curves of Ga2O3 NSs/rGO electrode from 0.1 to 20 mV s−1. The CV curves show similar shapes but slight peak shift with the increase of scanning rate. According to the log(v)-log(i) plots (Figure 3b and Figure S9), the b-value is 0.86, 11



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demonstrating the positive capacitive kinetics of Ga2O3 NSs/rGO electrode.51 As in Figure 3c, the capacitive contribution increases and the diffusion contribution decreases. In general, because of the unique pseudocapacitance kinetics of the Ga2O3 NSs/rGO electrode, the cells exhibit excellent rate and cycling performances. Compared to Ga2O3 NSs/rGO electrode, pure Ga2O3 electrode shows a lower contribution of pseudocapacitance (37.1% at 20 mV s-1, Figure S10), resulting in poor electrochemical performance. To understand the Li-ion storage mechanism, ex-situ XRD and XPS characterizations are performed. Figure 3d shows the XRD patterns of Ga2O3 NSs/rGO electrode acquired from discharged/charged cells up to the retaining voltages in CV profile (Figure S11). The Ga2O3 NSs/rGO electrode shows a decrease in Ga2O3 peak intensity up to 0.5 V, whereas a new peak including around 34.8° is observed, corresponding to Ga (JCPDS No. 71-0505).52 The Ga2O3 peaks disappear below 0.16 V, demonstrating the lithiation of the Ga2O3 during the discharge process.34 In a continued Li+ insertion process up to 0.01 V, the XRD peaks attributed to Li2O (JCPDS No. 12-0254) are observed (Figure 3e and Figure S12). The results reveal Ga2O3 decompose into Ga and Li2O, similar with the previous report.53 Besides, the presence of the Ga and Li2Ga peaks suggest that the decomposed Ga reacted with lithium ions to form Li2Ga alloy at ~0.3 V.54 The diffraction peaks of Ga2O3 are recovered after charge to 3.0 V, suggesting that the conversion of Ga2O3 to be partially reversible.20 The recovery of the original crystal structure supported the stable electrochemical cycling of the Ga2O3 NS/rGO hybrid electrode. For more discussion 12


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of the valance state of electrochemical process, ex-situ Ga 3d XPS spectra are conducted. As shown in Figure 3f, pristine Ga2O3 NS/rGO electrode shows a single Ga 3d (Ga3+) peak at 20.3 eV. After first discharging to 0.01 V, the observed peaks at 18.2 and 19.1 eV are assigned to the metallic Ga (Ga0, 18.2 eV) and Li−Ga alloys (~19.1 eV). Broadening and shifting in the binding energy towards a lower energy are also observed. The slight negative shift is responsible for the formation of metal Ga by the decomposition of Ga2O3. This scenario unambiguously reveal the different chemical position of Ga in the fully lithiated electrode. At the charged state of 3.0 V, peaks belong to Ga2O3 are partially regained. It was found that the electrodes did not completely go to its initial stage after 1st cycle which was already proved by the XRD and CV. According to the above analysis and previous report,20 the whole electrochemical process is similar to those of GaS anode.34 Upon discharge, the Ga2O3 gradually turns into metallic Ga and Li−Ga alloys with the formation of Li2O. And the Li−Ga alloys recover to related oxides when it is recharged to 3.0 V. The electrochemical reactions of Ga2O3 NS/rGO are as follows: Ga2O3 + 6Li+ + 6 e− → 2Ga + 3Li2O

(1)

Ga + 2Li+ + 2e− ↔ Li2Ga

(2)

The schematic illustration of conversion and alloying reaction mechanism is demonstrated in Figure 3g. According to the conversion and alloys mechanism, the theoretical specific capacity is around 1145 mAh g−1. 2.3. Electrochemical performances of Ga2O3 NSs/rGO electrodes in SIBs 13



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Figure 4. Electrochemical performances of Ga2O3 NSs/rGO electrodes in SIBs. In half-cells: (a, b, c, d, e, f) CV curves, charge/discharge profiles, Nyquist plots, cycling, rate and long-term cycling performances. In full-cells: (g, h, i) Charge/discharge profiles, cycling and rate performances (Inset is optical images of the LED lights powered by the full cells.). The current density and gravimetric capacity are counted by Ga2O3 NSs/rGO. Stimulated by the excellent performances of Ga2O3 NSs/rGO electrodes in LIBs, the cells have been assembled to study the electrochemical performances of the hybrid electrodes for SIBs. According to CV and charge/discharge profiles (Figure 4a and 4b), it can be found that Ga2O3 NSs/rGO electrodes exhibit similar electrochemical behavior in SIBs as in LIBs, corresponding to the conversion and alloying process of Ga2O3.19 The AC impedance characterization in SIBs also confirms the better ion and 14


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electron conductivity in Ga2O3 NSs/rGO electrodes (Figure 4c). As shown in Figure 4d, after initial several cycles of descent, the discharge capacity is maintained in a stable state and retains 554.93 mAh g−1 after 100 cycles (the pure Ga2O3 just remains 212.07 mAh g−1, Figure S13), as well as the coulombic efficiency more than 98%. Due to the favorable electrolyte accessibility of the unique 2D structure of Ga2O3, the combination of the good electronic conductivity of the Ga2O3 NSs−rGO and the excellent ion diffusion，Ga2O3 NSs/rGO electrodes present excellent rate and long cycle performances in SIBs which reveal highly reversible Na+ intercalation kinetics. As shown in Figure 4e, the reversible capacities are 582, 513, 435, 358, 310, and 263 mAh g−1 at 0.1, 0.2, 0.5, 1.0, 2.0, and 5.0 A g−1, respectively. Even at 2 A g−1, the capacity stabilizes around 302 mAh g−1 after 1000 cycles (Figure 4f). Similarly, Ga2O3 NSs/rGO electrodes show decent electrochemical performance in the full cells of SIBs when assembled with Na3V2(PO4)3/C (Figure 4g-4i, Figure S14). In Figure 4h, after 100 cycles, the full cell still retains a reversible capacity of 451.5 mAh g−1anode at 0.1 A g-1, as well as excellent rate performance (Figure 4i). Both of these results demonstrate that the Ga2O3 NSs/rGO nanocomposites also exhibit outstanding electrochemical performances in SIBs.

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Figure 5. Pseudocapacitance analysis of Na+ storage. (a) CV curves. (b) Relationship between anodic peak current and scan rates. (c) Capacitive capacity (red) and diffusion capacity (gray) at 20 mV s−1. (d) Bar chart of Pseudocapacitance behavior . The kinetic principle results demonstrate that the Ga2O3 NSs/rGO electrodes show excellent pseudocapacitance properties in SIBs (Figure 5). According to the results of CV curves (Figure 5a), the b-value is 0.79 can be calculated (Figure 5b), suggesting a more favored capacitive kinetics of Ga2O3 NSs/rGO. In Figure 5d, within the 0.1 to 20 mV s−1, the capacitive capacity increased, the diffusion contribution restrained, and the capacitive contribution is 67.6% at 20 mV s−1. Overall, the pseudocapacitance kinetics of the Ga2O3 NSs/rGO electrode makes it excellent electrochemical performances in both LIBs and SIBs.

3. CONCLUSIONS 16


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In a summary, a straightforward production of Ga2O3 NSs/rGO nanocomposites is demonstrated in this paper. Such self-established nanocomposites can reduce the transport pathways of alkali metal ion (Li+/Na+), and suppress the volume expansion during cycles. Owing to the effect of 2D structure of Ga2O3 and rGO, the Ga2O3 NSs/rGO electrodes exhibit excellent electrochemical performances both in LIBs and SIBs. It exhibit the reversible capacity of ~540 mAh g−1 and ~302 mAh g−1 at 2.0 A g−1 after 1000 and 2000 cycles in the half cells of LIBs and SIBs, respectively. Even in the full cells, Ga2O3 NSs/rGO // LiFePO4/C and Ga2O3 NSs/rGO // Na3V2(PO4)3/C show outstanding cycling and rate performances. The charge storage mechanisms and pseudocapacitance analysis demonstrate that unique 2D structure of Ga2O3 NSs and the presence of graphene can accelerate the transfer of alkali metal ion. All of the results indicate the promising potential of Ga2O3 NSs/rGO nanocomposites as a candidate material for high-performance anode in energy storage filed.

ASSOCIATED CONTENT Supporting Information

Experimental

details

of

Ga2O3

NSs/rGO

nanocomposites,

equipment

and

characterization techniques; SEM images and XRD pattern of GaOOH NSs/GO, pure Ga2O3 nanoparticles, LiFePO4/C, and Na3V2(PO4)3/C powders; TGA of Ga2O3 NSs/rGO and pure Ga2O3; CV, EIS and galvanostatic discharge/charge tests of Ga2O3 NSs/rGO and pure Ga2O3 in LIBs and SIBs. This material is available free of charge via the Internet at http://pubs.acs.org. 17



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AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

Author Contributions #

M.Y and. C.S. contributed equally to this work.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by NSFC (Contract 51572153, 51602177) and the Major Basic Program of the Natural Science Foundation of Shandong Province (Contract ZR2017ZB0317).

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For Table of Contents Only

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Figure 1. (a) Synthetic diagram of the Ga2O3 NSs/rGO nanocomposites. (b, c) SEM, EDS and elemental mappings of C, Ga, O. (d) XRD pattern. (e, f) TEM, HRTEM and SAED (inset) images. Scale bars: inset of (f) 10 1/nm. (g) Nitrogen adsorption-desorption isotherm and pore distribution. 121x86mm (300 x 300 DPI)


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Figure 2. Electrochemical performances of Ga2O3 NSs/rGO electrodes in LIBs. In half-cells: (a, b, c, d, e, f) CV curves, charge/discharge profiles, Nyquist plots, cycling, rate and long-term cycling performances. In full-cells: (g, h, i) Charge/discharge profiles, cycling and rate performances (Insert are optical images of the LED lights powered by the full cells.). The current density and gravimetric capacity are counted by Ga2O3 NSs/rGO. 112x74mm (300 x 300 DPI)



Figure 3. Pseudocapacitance analysis and Li-ion storage mechanism of Ga2O3 NS/rGO electrode in LIBs. (a) CV profiles. (b) Relationship between anodic peak current and scan rates. (c) Bar chart of capacitive capacities vs. scan rates. (d) Ex-situ XRD of the electrode at the first cycle. (e) Ex-situ XRD patterns at 0.01 V and 3.0 V. (f) Ex-situ Ga 3d XPS patterns of the lithated/delithiated hybrid electrode. (g) Schematic diagram of lithiation process. 139x114mm (300 x 300 DPI)


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Figure 4. Electrochemical performances of Ga2O3 NSs/rGO electrodes in SIBs. In half-cells: (a, b, c, d, e, f) CV curves, charge/discharge profiles, Nyquist plots, cycling, rate and long-term cycling performances. In full-cells: (g, h, i) Charge/discharge profiles, cycling and rate performances (Inset is optical images of the LED lights powered by the full cells.). The current density and gravimetric capacity are counted by Ga2O3 NSs/rGO. 119x83mm (300 x 300 DPI)



Figure 5. Pseudocapacitance analysis of Na+ storage. (a) CV curves. (b) Relationship between anodic peak current and scan rates. (c) Capacitive capacity (red) and diffusion capacity (gray) at 20 mV s−1. (d) Bar Chart of Pseudocapacitance behavior. 127x95mm (300 x 300 DPI)


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