Accepted Manuscript Title: A facile method of preparing LiMnPO4 /reduced graphene oxide aerogel as cathodic material for aqueous lithium-ion hybrid supercapacitors Authors: Lin Xu, Senlin Wang, Xiao Zhang, Taobin He, Fengxia Lu, Huichang Li, Junhui Ye PII: DOI: Reference:
S0169-4332(17)32906-9 https://doi.org/10.1016/j.apsusc.2017.09.247 APSUSC 37326
To appear in:
Received date: Revised date: Accepted date:
30-6-2017 20-9-2017 28-9-2017
Please cite this article as: Lin Xu, Senlin Wang, Xiao Zhang, Taobin He, Fengxia Lu, Huichang Li, Junhui Ye, A facile method of preparing LiMnPO4/reduced graphene oxide aerogel as cathodic material for aqueous lithium-ion hybrid supercapacitors, Applied Surface Science https://doi.org/10.1016/j.apsusc.2017.09.247 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A facile method of preparing LiMnPO4/reduced graphene oxide aerogel as cathodic material for aqueous lithium-ion hybrid supercapacitors
Lin Xu1, Senlin Wang1,*, Xiao Zhang, Taobin He, Fengxia Lu, Huichang Li, Junhui Ye
College of Materials Science & Engineering, Huaqiao University, Xiamen, Fujian, 361021, People’s Republic of China
*Corresponding author. Fax: +86-595-22693746. E-mail address: [email protected]
Both authors are the first authors.
The reduced graphene oxide supporting LiMnPO4 particles is synthesized.
LiMnPO4/reduced graphene oxide aerogel shows a relatively high mass capacity.
The graphene content has a significant effect on the electrochemical performance.
A lithium-ion hybrid supercapacitor with higher energy density is successfully assembled.
A facile method of preparing LiMnPO4/reduced graphene oxide aerogel (LMP/rGO) as cathodic material was reported here. LiMnPO4 nano-particles were prepared using a facile polyvinyl pyrrolidone-assisted solvothermal route. Then LMP/rGO aerogel 1
was prepared using the accessible restacking method. The influence of the cathodic electrode composition (ratio of rGO to LiMnPO4) on the performance of the LMP/rGO was evaluated by constant-current discharge tests. When compared with 217 C g-1 for the pristine LMP, the best LMP/rGO (the content of rGO is 27.3 wt %) exhibits a higher capacity of 464.5 C g-1 (at 0.5 A g-1), which presenting the capacity enhance of 114 %. Moreover, a lithium-ion hybrid supercapacitor (LIHS) was successfully assembled by using LMP/rGO aerogel as the cathodic electrode and rGO aerogel as the anodic electrode. The LMP/rGO//rGO device achieves excellent specific energy of 16.46 Wh kg-1 at a power density of 0.38 kW kg-1, even under the higher specific power of 4.52 kW kg-1, there still holds the specific energy of 11.79 Wh kg-1. The LMP/rGO//rGO device maintains 91.2% of the initial capacity after 10000 cycles (at 2 A g-1), which displays high rate performance and long cycle life. The 3D LMP/rGO aerogel could be a promising candidate material for the lithium-ion hybrid supercapacitors.
Keywords: Lithium manganese phosphate; Reduced graphene oxideaerogel; Solvothermal reaction; Aqueous lithium-ion hybrid supercapacitor.
1. Introduction Power sources with both superior power capability and superior energy capability have been strongly demanded for their application in electric vehicles (EVs). Lithium-ion batteries (LIBs) and electrochemical capacitors (ECs) are 2
regarded as the promising candidates for such power sources. LIBs are attractive with their large energy capacity (150-200 Wh kg–1), but their power density is rather low (below 1 kW kg–1) mainly due to the large polarization during cycling. By comparison, the conventional electric double layer capacitors (EDLCs) show higher power capability (2-5 kW kg–1) better cycling stability and desirable safety, but lower energy densities (only 5–10 Wh kg–1) [1-4]. Recently, research efforts are directed toward developing a novel electrochemical energy storage system of lithium-ion hybrid supercapacitors (LIHSs), which can bridge the gap between LIBs (e.g. LiCO2, LiFePO4, LiMn2O4, etc.) and conventional EDLC (e.g.activated carbon, graphene, etc.) [5-10]. LIHSs have a relatively high power density and energy density compared to the other energy storage systems. It works with Li+ intercalating/deintercalating into the cathodic electrode and corresponding anions adsorbing/desorbing from the surface of anodic electrode simultaneously . Since the authentication of lithium reversible insertion/extraction for LiFePO4 were reported by Padhi et al. in 1997 . Therefore, LiMPO4 (M = Co, Ni, Fe, Mn, etc.), which have drawn much attention in the research of LIBs . Compared with LiFePO4, low-cost LiMnPO4 (LMP) demonstrates several highlighted characteristics of higher potential plateau, higher theoretical specific energy . Unfortunately, due to the Jahn-Teller effect of Mn3+ sites between MnPO4 and LiMnPO4 with the large volume change, LiMnPO4 sustains drawbacks of inferior ionic diffusivity (<10-6 cm2 s-1) and electro conductibility (<10-10 S cm-1), which limits the rate performance of LiMnPO4 . For purpose of improving the electronic conductivity of LiMnPO4, 3
multiple strategies such as reducing the size of the particles [16-18], carbon coating [19-21] and transition metal doping [22-24] have already been attempted to facilitate the electrochemical properties of LiMnPO4. In recent years, graphene as a promising conductive carbon matrix to immobilize active material for fabricating nanostructure hybrid electrodes, which can preventing its agglomeration and performance decay during the charge/discharge cycling processes [25-27]. Abdulmajid et al.  synthesized (Mn3(PO4)2 hexagonal micro-rods with different graphene foam (GF) by a hydrothermal method, the energy density value of AC//Mn3(PO4)2/100 mg GF asymmetric supercapacitor (ASC) was calculated to be 7.6 Wh kg-1, with corresponding the power density values of 360 W kg-1 (at 0.5 A g-1) in KOH electrolyte. Wanget al.  obtained LiMnPO4 nanoplates cathode material via graphene oxide assisted solvothermal for LIBs, the capacity retains 72.3% after 100 cycles. Compared with stacked graphene, the use of nanostructure graphene as a three dimensional (3D) conducting network enables better dispersion of the active materials and thus improved electronic/ionic diffusion dynamics in the electrode [30-32]. Tianet al. prepared a 3D graphene aerogel and LiFePO4 nanoparticles compound (LFP/GA) by a hydrothermal method, presenting superior electrochemical properties when comparing to the pristine LFP. It is worthwhile mentioning that conductive graphene networks can offer a large number of electronic transmission channels. At the same time, this aerogel structures are advantageous to storage electrolyte for the resupply of Li cations. In
intercalating/de-intercalating processes, the 3D rGO aerogel supporting LiMnPO4 nanoparticles was facilely synthesized as cathodic material in our study. It was prepared by solvothermal treatment and subsequent a restacking process, as schematically illustrated in Fig. 1. Before the recombination process, LiMnPO4 nanoparticles are firstly synthesized via a solvothermal rout and rGO is formed by easily chemical reduction of graphene oxide(GO). Meanwhile, 3D rGO aerogel was also used as anodic electrode in consideration of their high surface area, porous nanostructure, excellent electrical conductivity and high chemical stability [34, 35]. Therefore, a series of LiMnPO4 and reduced graphene oxide hybrid aerogel were fabricated, the ratio of rGO to LiMnPO4 was systemically varied to investigate its effects on electrochemical properties. The charge/discharge process are associated with the transfer of Li+ between two electrodes, the electrode reaction mechanism is similar to that of lithium-ion batteries. The electrolyte mainly plays the role as the ionic conductor. LiMnPO4 have been widely used in the nonaqueous lithium-ion batteries. The use of an aqueous electrolyte can overcome the drawbacks of safety hazards from the use of highly toxic and flammable solvents. We hope to design synthesis of three-dimensional porous LiMnPO4/rGO composite material has the following properties: (1)The rGO three-dimensional skeleton can anchor and uniformly disperse LiMnPO4 nanoparticles, prevent the nanoparticles aggregation together and improve the composites material conductive performance. (2) In turn, the LiMnPO4 nanoparticles embedded in rGO 5
three-dimensional skeleton also can prevent the rGO sheets aggregate into graphite, and increase the specific surface area of the composite material. In short, the three-dimensional porous composite material has good conductivity, high specific surface area and numerous accessible active points. 2. Experimental 2.1. Preparation of LiMnPO4 LiMnPO4 nanoparticles have been synthesized by a facile solvothermal reaction and assisted by polyvinyl pyrrolidone as surfactant. The following chemical agents are analytical grade purity. In this experiment, 15 mmol LiOH·H2O and 5.0 mmol NH4H2PO4 were dissolved into 20 mL and 10 mL water-ethylene glycol (EG) mixture solvent (1:4, v/v), respectively. Afterwards, the above NH4H2PO4 solution mixed with LiOH solution in vigorous stirring followed by generating white suspension. Meanwhile, 5.0 mmol MnAc2·4H2O was dissolved into 20 mL water-ethylene glycol (EG) mixture solvent (1:4, v/v), followed by the addition of polyvinyl pyrrolidone (WPVP = 5% WLMP). Then the mixing MnAc2 solution was dropwise added into the above white suspension with magnetically stirring for 0.5 h, and then transferred into a 100 mL Teflon-lined stainless steel autoclave. This solvothermal reaction was kept at 180 0C for 10 h. Finally, the sample was repeatedly washed with deionized water and absolute ethanol by centrifugation, and then under vacuum drying at 60 0C over night. 2.2. Self-assembled of LMP/rGO aerogel GO suspension was initially prepared using a modified Hummers method, which 6
diluted to 2 mg mL-1 with water-ethanol (1:1, v/v) mixture solvent, subsequently reduced by ascorbic acid to form rGO sheets [36, 37]. For another precursor, the LMP suspension was prepared by dissolving 80 mg LiMnPO4 into 2 mL ethanol with ultra-sonication for 1 h. Finally, the LMP/rGO composites were prepared by dropwise adding given LMP turbid liquid into the rGO suspension under constant acutely stirring for 12 h at room temperature, and then ultra-sonication for 2 h, finally sealed in a 50 mL autoclave, and carried out at 180 0C for 2 h to form the 3D hydrogel. The as-synthesized composite hydrogel was freeze-dried for 24 h.The content of adding rGO was divided into 10 mg, 30 mg and 50 mg (the corresponding composites were named as LG10, LG30 and LG50, separately) to characterize the effect of the added rGO content on the electrochemical properties of the LMP/rGO hybrid aerogel. 2.3. Preparation of the 3D rGO aerogel Adding 200 mg ascorbic acid (the mass ratio ascorbic acid and GO is 2:1) as a reducing agent into the above obtained 50 mL 2 mg mL-1 GO suspension and then stirring for 30min. Subsequently sealed in a 100 mL autoclave, and kept at 180 0C for 2 h to form the 3D hydrogel, freeze-dried for 24 h to obtain 3D rGO aerogel. 2.4. Characterization of material The as-prepared samples were investigated by X-ray diffraction (XRD) (Cu Kα, λ=1.5418 Å), Raman spectrum with an excitation laser of 532 nm. The micro structural and morphology performances were examined by field-emission scanning electron microscopy (FESEM, HITACHI S-4800) and high-resolution transmission electron microscopy (HRTEM, JEM-2100). 7
2.5. Preparation of electrodes and assembling LMP/rGO//rGO devices The used Ni foam substrate (1 cm × 1 cm × 1.6 mm) was pretreated with 5% HCl, ethanol and deionized water for 15 min alternately, in order to remove impurities and oxides layer of the surface then dried in the oven. The working (or cathodic) electrodes containing 80 wt% active materials, 10 wt% polytetrafluoroethylene (PTFE) and 10 wt% acetylene black with ethanol were well mixed. After that, coating the mixture paste onto nickel foam (1cm×1cm), dried at 60
C for 12 h. In a
three-electrode cell, the mass of Ni foam shouldn’t be taken into account for the calculation of electrode materials. The mass loading of active material on nickel foam was about 3.0 mg. The specific capacity of a single electrode can be calculated by the following equation: Cm I t m
Where Cm is the mass specific capacity of single electrode (C g-1), I is the discharge current (A), ∆t is the discharge time (s) and m is the mass of active material (g). As for the charge conservation (the relationship q+ = q-) and both electrodes depend upon its specific capacity (C), the mass of the active material on the cathodic/anodic electrode were calculated by the following formulas: q Cm
m m C C
Here C+, m+ and C-, m- are the specific capacity and the mass of the active materials in cathode and anode electrodes, respectively. Analogously, rGO electrode was obtained by mixing rGO aerogel, PTFE and 8
acetylene black in ethanol with mass ratio (%) of 80:10:10. After that, coating the mixture paste onto nickel foam (1cm×1cm), dried at 60 0C for 12 h. Acording to Eq. (2) and (3), the lithium-ion hybrid supercapacitor LMP/rGO//rGO device was assembled using LMP/rGO as cathode and rGO as anode, which loading active material are 3.2 mg and 3.6 mg, separately. The energy density E (Wh kg-1) and power density P (kW kg-1) of the LIHS can be calculated by the following equations [38, 39]: E I V (t )dt
P E t
Here C is the specific capacity of lithium-ion hybrid supercapacitors (C g-1)，V is the potential range, ∆t is the discharge time (s) . 2.6. Electrochemical measurements The electrochemical performance of the obtained samples was investigated in a three-electrode cell in a 1 M LiOH aqueous solution. The obtained samples were used as the working electrode, a platinum plate (3 cm×3 cm) was used as the counter electrode and Hg/HgO was used as the reference electrode, respectively. Cyclic voltammograms (CV) and galvanostatic charge-discharge (GCD) measurements were carried out on an electrochemical workstation (CHI660E) to evaluate the electrochemical behaviors. And the cycling performance of LIHSs devices was inspected through a LAND battery test system (CT2001A). 3. Results and discussion 3.1. Structure and morphology characterization 9
The crystal phase of the as-synthesized samples were analyzed by X-ray diffraction (XRD), and the corresponding patterns for LMP, rGO and LMP/rGO composites (LG10, LG30, LG50) are displayed in Fig. 2. The typical diffraction peaks of the LMP and LMP/rGO composites are able to be well-indexed to the Pnma space group (JCPDS No. 33-0804) of the olivine-structured LiMnPO4 , which suggested the introduction of rGO sheets has produced less influence on phase construction of the pure LiMnPO4. The sharp characteristic peaks exhibited that obtained samples have well crystallinity. The as-prepared samples are high purity seeing that it is no noticeable diffraction peaks of the other impurities in the spectrum. Moreover, no diffraction peak (002) of rGO (located at around 24.5°) is observed in the diffraction pattern of the LMP/rGO composites, proving the amorphous phase of the carbon source . GO has the sharp diffraction peak (001) which located at around 11.4°. Consequently, it was concluded that the LMnPO4 particles attached onto the rGO sheets prevented restacking of rGO sheets into graphite or graphite nanosheets, which was corroborated by the diffraction pattern. Raman spectroscopy is an efficient assistant measure for describe the graphitization degree of carbon phase which was undetectable by XRD. Consequently, the Raman spectrum of rGO, LG10, LG30 and LG50 are performed for comparison as revealed in Fig. 3. Two strong wide absorption peaks in range of 1200 cm-1–1500cm-1and 1500 cm-1–1700 cm-1 are attributed to carbon phase of the D-band and G-band, separately. D-band stands for the imperfection of graphite structure, while the G band represents the presence of graphite carbon . Accordingly, 10
combination with the result of Raman spectra and XRD pattern can be further proved that the rGO nanosheets composites have been successfully synthesized. The morphology of LMP (a), LG10 (b), LG30 (c), LG50 (d) was characterized by SEM are revealed in Fig.4. In addition, the insets in Fig. 4(a-d) display the corresponding photographs taken by a digital camera. Compared to the powder morphology of LMP, LMP/rGO displays a monolithic cylindrical structure indicating the successful preparation of the aerogel-based materials. Fig. 4(a) indicates that the pristine LMP particles have the olivary morphology, and the particles are slightly agglomerated. The particle size distribution of the LiMnPO4 particles can be further evaluated by Laser Particle Size Analyzer in Fig. S1 (Supporting Information). We can find that the average size of LMP particles is about 97nm., which is match the result of SEM images. For the LMP/rGO composite (Fig. 4b, c and d), LMP particles are well-distributed on the surface rGO sheets. Porous and interconnected graphene networks are clearly observed. This arrangement is beneficial to enlarge the electrode/electrolyte interfaces and facilitate the lithium-ion diffusion during the insertion and extraction processes . As exhibited in Fig. S2 (Supporting Information), demonstrated that these element of Mn, P, O and C are uniformly distributed in LG30 composite., even though the applied carbon support partly interferes with the C element imaging in SEM investigations. These results suggest that the well-dispersed LMP and rGO sheets have been incorporated into the composites. TEM investigations were conducted to further probe the fine structure of LG30 11
composite which revealed in Fig. 5. Fig. 5(a) displays that the LiMnPO4 nanoparticles are homogeneously immobilized tightly on the surface of rGO sheets. As revealed in Fig. S3 (supporting information), the number of rGO layers is in the range of 5-12 layers of each piece on different locations of the particle, which can be roughly estimated by TEM images. These results are in agreement with the SEM image (Fig. 4(c)). As displayed in Fig. 5(b), the noticeable lattice orientation with a width of 0.30 nm which correspond to (200) plane of the orthorhombic-phase LiMnPO4, suggesting that the LMP particles are highly crystalline. High-resolution TEM images of LMP/rGO are shown in Fig. 5(c). Electron diffraction (ED) image in Fig. 5(d) reveals single crystallite diffraction characteristics with light spots verifying olivine crystallite order, which demonstrates the advantage of the synthesis process technique accommodated in this work. These features are considered to be potential materials for the application of electrochemical capacitor. TGA measurements were carried out to evaluate the thermostabilityof the LMP/rGO, and the results are displayed in Fig. S4 (Supporting Information). In the temperature range from 30 0C to 1000 0C, the pristine LiMnPO4 resulting in a little weight loss about 4.99%. For the composite material, the carbon component LMP/rGO is loss of weight about 25.45% in the temperature range of 30-1000 0C. For analyze the porous characteristics, the nitrogen adsorption-desorption isotherms of LG30 were investigated, as revealed in Fig. S5 (Supporting Information). The isotherms of LG30 shows a hysteresis loop that can be attributed to a type IV curve, demonstrating the typical characteristic of a mesoporous structure. The inset of 12
Fig. S5 (Supporting Information) reveals the pore size distribution curves of the LG30, as computed by the BJH formula from the desorption branch. The LG30 exhibits an obvious mesoporous framework with the average pore size of 3.8 nm. The corresponding BJH desorption cumulative pore volume is 0.45 cm3 g-1. As shown in Table S1 (Supporting Information), through the calculation of BET, the specific surface areas of LG30 are counted to be 59.24 m2 g-1. 3.2. Electrochemical measurements of the LMP/rGO aerogel For explore electrochemical performances of the electrode materials, electrochemical measurements were carried out in a three electrode system by cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) measurement. Compares CV curves of LMP, LG10, LG30 and LG50 carried out at 5 mV s-1 in 1M LiOH solution are plotted in Fig. 6(a). Owing to the redox reaction of Mn2+/Mn3+ and Mn3+/Mn4+ in the interface, there are two pair redox peaks in the voltage window of -0.8 to 0.5 V. The Li+ intercalation/deintercalation processes are given below: Li1 x MnPO4 xLi xe LiMnPO4 ch arge
Li cations are deintercalation from LiMnPO4 crystalline phase during charging process, while Li cations are intercalation to form LiMnPO4 crystalline phase during discharging process . Furthermore, the LMP/rGO composites, the area of CV curves is greater than that pure LMP. Among of all curves, the CV curve of LG30 (the content of rGO is 27.3 wt %) exhibited the highest specific capacity than the others. Fig. 6(b) reveals the discharging curves at a current density of 0.5 A g-1 for LMP, LG10, LG30 and LG50. Discharge plateau of the composites is longer than that of 13
pristine LMP owing to the introduction of rGO sheets. The specific capacity of LMP, LG10, LG30 and LG50 can be calculated by Eq.(1), which were found to be 217 C g-1, 319 C g-1, 464.5 C g-1 and 297.5 C g-1, separately. The discharge curve of LG30 shows the best specific capacity, which can match the result of the CV curves in Fig. 6(a). Fig. 6(c) reveals CV curves of LG30 at different scan rates in 1M LiOH solution. The CV curves of asymmetric redox peaks, resulting in the Faraday process of Ohmic resistance and the polarization. However, owing to the internal resistance, the oxidation peaks and reduction peaks shifted toward cathode and anode with the increase of scan rate. The constant current charge/discharge curves of LG30 are shown in Fig. 6(d). The specific capacity of LG30 at different current densities is calculated as 464.5 C g-1 (0.5 A g-1), 328 C g-1(1 A g-1), 274.2 C g-1(2 A g-1), 234C g-1(4 A g-1)and 210 C g-1 (6 A g-1), respectively. As is revealed in Fig. 6(e), on the different current densities of the specific capacities, the composites are greater than that of the pure LMP. That is contributed to the presence of rGO,which can prevent LMP nanoparticles from agglomerating, improve the conductivity of composites and provide more active site for the need of Faradaic reactions. The enhanced electrochemical performance can be further characterized with the electrochemical impedance spectrum (EIS) technique. The impedance charts of rGO, LMP and LG30 are revealed in Fig. 6(f). All curves consist of a semicircle at high-frequency and an inclined line at low-frequency. Of which, The semicircle corresponds to the interfacial charge transfer resistance (Rct), the intercept of horizontal axis represents the solution resistance (Re), the slope of line stands for 14
Warburg impedance (W) . Rct value of LG30 was near to LMP and less than pure rGO. Moreover, the linear slope of LG30 is more vertical in comparison to the LMP, implying that the ion diffusion and reaction kinetics of LG30 are relatively superior due to the restacking of LMP on rGO, resulted in the improvement of conductivity and the formation of porous aerogel framework. 3.3. Electrochemical performance of the LIHS based on the LG30//rGO device In order to further investigate electrochemical properties of the composites, the LIHS of the LG30//rGO device is assembled by using LG30 complex gel as the cathodic electrode and rGO gel as the anodic electrode, compared with the LMP//rGO device. The CV curves of the LG30 electrode and the rGO electrode at a scan rate of 5mV s-1 in a three-electrode system are As revealed in Fig. S6 (Supporting Information), the LG30 electrode exhibits the classic pseudo-capacitance within a potential window of -0.8 to 0.5 V. There are two groups of obvious redox peaks in the curve. However, owing to the behavior of EDLCs, the rGO electrode shows a nearly rectangular shape without redox peaks in the potential window from -1 to 0 V. Consequently, attributing to the combination of EDLCs and pseudo-capacitance, the voltage scope of the LIHS can be partly superimposed as 0-1.5 V. As revealed in Fig. S7 (Supporting Information), it should be noted that the bare Ni foam//Ni foam substrates show a smaller current in comparison to the LMP/rGO//rGO device, indicative of the negligible contribution of the Ni foam substrate towards the capacity under the conditions adopted in the present work. For the two-electrode system, Fig. 7 (a) displays CV charts of the LMP//rGO and 15
LG30//rGO devices at a scan rate of 5mV s-1 under 1.5 V in 1 LiOH electrolyte. LG30//rGO device, the area of the CV curves is larger than the LMP//rGO device as well as in a three electrode system. And the both devices of CV curves tends to a shape of rectangular, indicating that the adding of rGO making the LG30//rGO device has better electrochemical performance than the LMP//rGO device due to the adding of rGO sheets for composites. While the CV curves of the LG30//rGO at various scan rates are exhibited in Fig. 7(c). With the increases of scanning rate, the curve area of LG30//rGO device gradually becomes larger due to the effective combination of EDLCs and pseudo-capacitance, implying that the LIHS has been successfully assembled. Fig. 7(b) reveals the galvanostatic charge/discharge curves at a current density of 0.5 A g-1 for the LMP//rGO and the LG30//rGO within a voltage window of 0 to 1.5 V. Both the LG30//rGO and LMP//rGO show good symmetry, indicating that these materials have good redox reversibility. And the discharge time of the LG30//rGO is much longer than the LMP//rGO, that is the LG30//rGO device has higher specific discharge capacity. The galvanostatic charge/discharge curves of the LG30//rGO at different current densities are shown in Fig. 7(d). Due to good reversibility, LG30//rGO presents the approximative triangular of symmetry. Furthermore, depending on the discharging curves with different current densities, the Cm values of the LG30//rGO and LMP//rGO device were revealed in Fig. 7(e). Therefore, the capacity of the LG30//rGO device is much higher than LMP//rGO device. With the increase of current density from 0.5 A g-1 to 6 A g-1, the capacity of the LG30//rGO 16
device ranging from 79 C g-1 to 56.4 C g-1. While the mass capacity of the LMP//rGO device only from 50.3 C g-1 to 12.6 C g-1. It is found that the LG30//rGO has better capacity property and rate performance. Fig. 7(f) displays the Nyquist plot at a frequency range from 100 kHz to 0.01 Hz for the LG30//rGO and LMP//rGO, demonstrating the conductivity of the studied materials. The impedance caused by the solution resistance (Re) of the electrolyte and the electrode interface can be obtained from the intersection of the semicircle and the transverse axis. The Re reveals LG30//rGO (2.14Ω) < LMP//rGO (2.65Ω) from the impedance chart. There is an inverse relationship between the impedance and the conductivity, and the LG30//rGO has a lower ohm contact resistance, which corresponds to its superior electrochemical performance.The increase in conductivity of the LG30//rGO is mainly due to the availability of electron transport channels from the 3D reduce oxide graphene frameworks, which facilitates the rapid transfer of electrons. Further, the slope of LG30//rGO greater than the LMP//rGO in the Nyquist plot at low frequencies, indicating the LG30//rGO device possesses a smaller ion transfer resistance, attributing to a faster diffusion of the electrolyte. Moreover, the EIS results of two-electrode system are in a good consistent with three-electrode system. To characterize the cycling performances of the LMP//rGO and LG30//rGO, Fig. 8(a) reveals the cycle life of these devices at a current density of 2 A g-1. Due to the large loading quality of the LIHS device at the beginning of the cycle test, which needs a long time for the activation of active substance to reach its highest value. And 17
the line of two devices show the fluctuation decreases until it reaches a steady level. The LMP//rGO and LG30//rGO device are ending up with 77.8% and 91.2% of the initial capacity after 10000 cycles. The LG30//rGO device exhibits prominent cycling property and strong cohesive force on the foamed nickel matrix. Ragone plots of energy density (E) and power density (P) for the LMP//rGO and LG30//rGO devices are displayed in Fig. 8(b). According to Eq. (4) and (5), the equations can be separately calculated the E and P of the lithium-ion hybrid supercapacitors. Therefore, the LG30//rGO device exhibits excellent specific energy of 16.46 Wh kg-1 (at specific power of 0.38 kW kg-1). Even under the higher specific power of 4.52 kW kg-1, there still holds the specific energy of 11.79 Wh kg-1, implying that the devices have feasible and promising applications for electric vehicles. As exhibited in the inset of Fig. 8(b), we have lighted up a LED lamp, in order to investigate the practical application of the LG30//rGO device. Based on the above results, the advantages of LMP/rGO nanocomposite aerogel in electrochemical performance are mainly due to the following factors: (I) LMP nanoparticles attaching tightly and uniformly on rGO flakes, that can prevent the accumulation of 3D rGO sheets, which facilitate the electron transport during the charge-discharge process due to the good electrical conductivity of rGO. (II) The presence of rGO in composites aerogel can effectively prevent the aggregation of LMP nanoparticles, leading to more specific surface area and the high utilization of LMP, which provides more active contacts with the electrolyte and hence accelerate faster Faradaic redox reactions. (III) The composite aerogel unique mesoporous 18
structure obtained by the addition of rGO with the pore diameter 3.83 nm can provide an efficient ion transport pathway, resulting in the penetration of electrolyte into the bulk electrode and the formation of large electrode/electrolyte interfaces to improve the Li+ transport. (IV) The graphene content has a great effect on the morphology and properties of the composites, and further optimizations could lead to even better electrochemical Li-insertion performances of the graphene aerogel-based composite materials. 4. Conclusion In summary, the well-distributed LiMnPO4 nanoparticles anchored on the three-dimensional porous rGO aerogel networks have been successfully synthesized by the LMP/rGO composites have been prepared with the solvothermal and restacking method. Due to the introduction of rGO with the characteristics of large specific surface area, abundant channel and excellent conductivity, which greatly improve the electronic conductivity, enlarge the electrode/electrolyte interface area, increase electrochemical properties of LMP/rGO composites. The graphene aerogel-based LiMnPO4 composites exhibit significantly improved specific capacity, rate capability and cycling stability in comparison to the pristine LMP. In addition, the lithium-ion hybrid supercapacitor (LIHS) was successfully assembled by using LMP/rGO aerogel as the cathodic electrode and rGO aerogel as the anodic electrode. The LG30//rGO device exhibits excellent specific energy of 16.46 Wh kg-1 at the power density of 0.38 kW kg-1. While the LG30//rGO device still keeps to be 11.79 Wh kg-1 under large specific power of 4.52 kW kg-1. The LMP/rGO//rGO device 19
holds 91.2% of the initial capacity after 10000 cycles at a current density of 2 A g-1, which displays high rate performance and long cycle life. Consequently, the LiMnPO4/rGO composite material has been verified to be a promising cathode material for lithium-ion hybrid supercapacitors. Furthermore, it should be pointed out that aerogel-based electrode materials with its predominant characteristics can further extended to various applications in supercapacitors, batteries and sensors, etc.
Acknowledgements The authors would like to acknowledge testing support from Analysis and Testing Center, Xiamen University.
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Fig. 1. A schematic illustration of the preparative process for the LMP/rGO aerogel.
Fig. 2. XRD patterns of rGO, LMP, LG10, LG30 and LG50.
Fig. 3. Raman spectrum of rGO, LMP, LG10, LG30 and LG50.
Fig. 4. FESEM images of (a) LMP, (b) LG10, (c) LG30 and (d) LG50. The insets of (a-d) are the corresponding digital photos of the macroscopic appearance of the samples.
Fig. 5. TEM images (a, b, c) and electron diffraction image (d) of LG30.
Fig. 6. (a) CV curves of LMP, LG10, LG30 and LG50 in1M LiOH solution at a scan rate of 5mV s-1; (b) Discharge curves of LMP, LG10, LG30 and LG50 at a current density of 0.5 A g-1; (c) CV curves of LG30 at different scan rates; (d) Galvanostatic charge–discharge curves of LG30 at different current densities; (e) Specific capacity of the as-prepared samples at different current densities; (f) Nyquist plots of rGO, LMP and LG30 in 1 M LiOH solution.
Fig. 7. (a) CV curves of the LMP//rGO and LG30//rGO at a scan rate of 5 mV s-1; (b) Galvanostatic charge-discharge curves of the LMP//rGO and LG30//rGO at a current density of 0.5 A g-1; (c) CV curves of the LG30//rGO at various scan rates; (d) Galvanostatic charge-discharge curves of LG30//rGO at different current densities; (e) Cm of the LMP//rGO and LG30//rGO at different current densities; (f) Nyquist plots of LMP//rGOandLG30//rGO.
Fig. 8. (a) Cyclic performances of the LMP//rGO and LG30//rGO devices at the current density of 2 A g-1; (b) The plot of the relationship between energy density and power density of the LMP//rGO and LG30//rGO devices.The inset is the digital photos of lighting up a LED lamp.