English

• 0.   Introduction

  As a promising energy storage technology for consumer electronics, electric vehicles, and stationary energy storage systems, rechargeable lithium - ion batteries (LIBs) have been widely studied owing to their high energy density, long cycle life, and low cost^[1]. Nevertheless, the commercial graphite anodes still have a low theoretical specific capacity (< 360 mAh·g^-1) and rate performance, which is a challenge to meet the everincreasing power demand and higher energy density.

  Graphene, a single layer of sp² - hybridized in a hexagonal honeycomb lattice, has received persistent attention due to its unique electrical, mechanical, and thermal properties. For the LIB application, the theoretical capacity of graphene can reach up to 744 mAh·g^-1 due to the formation of a Li₃C structure^[2]. Very recently, it was revealed that the assembly of two - dimensional (2D) graphene sheets into three-dimensional (3D) architectures can provide resultant graphene-based composites with stronger mechanical strengths^[3], lighter weight, and more rapid electron transport kinetics^[4].

  So far, a number of researches have been focused on developing high-capacity and long cycle life anodes, such as carbonaceous materials, conducting polymers, and transition metal oxides (TMOs) ^[5-6]. As a usual TMO, δ-MnO₂ has been used widely as the LIB anode for its high capacity, low cost, environmental friendliness, and natural abundance^[7-8]. However, the charge transfer in a common δ - MnO₂ anode is hindered by both inherently poor electrical conductivity and ionic conductivity. The common δ - MnO₂ anode also suffers severe volume expansion (>300%) during the Li-ion insertion process, and undesired aggregation of particles and formation of thick solid electrolyte interphase (SEI) layer during the discharge - charge cycling process^[9]. Therefore, it is necessary to combine the advantages of δ - MnO₂ nanoparticles (NPs) and reduced graphene oxide aerogel (RGOA) in order to boost the conductivity, as well as to relieve the aggregation and reduce the volume expansion of TMO anodes. To the best of our knowledge, there have been no any reports on the electrochemical performance of 3D network RGOA/δ-MnO₂ anodes.

  Currently, there are two types of electrochemical energy storage devices: LIB and supercapacitor. The LIB owns a large energy density but a slow charge rate, while the supercapacitor displays the merits of fast charging and long-term cycling stability but low energy density. Apparently, they also work in different mechanisms: the LIB is dominated by a diffusion - controlled process because the Li ion diffusion rate is much lower than the charge transfer rate, while the supercapacitor follows a pseudocapacitive - controlled mechanism due to the typical ionic absorption effect. Currently, it is necessary to increase the pseudocapacitive contribution of LIBs in order to satisfy the requirements of a fast charging vehicle.

  In this research, a simple self-assembly route was developed to synthesize the RGOA/δ-MnO₂ hybrids for the LIB applications, where the ultrathin δ-MnO₂ nanosheets (NSs) were successfully integrated into the RGOA. The effect of δ-MnO₂ content on the device performance was investigated systematically. After constructing a LIB, the specific capacity of the RGOA anode can be significantly improved from 720.8 to 1 701.9 mAh·g^-1 by incorporating a suitable quantity of δ - MnO₂ NPs. The excellent performance of RGOA/δ-MnO₂-160 mg can be attributed to both the high theoretical capacity of MnO₂, rapid Li - ion transport channel and charge transport network of RGOA. Additionally, the pseudocapacitive contribution cann′t be neglected, and it would be helpful for achieving a good rate performance.

  1.   Experimental

  Hydrogen peroxide (H₂O₂, 30%, w/w, AR), hydrochloric acid (HCl, 36%~38%, AR), potassium permanganate (KMnO₄, ≥99.5%), natural graphite (99%), anhydrous sodium nitrate (NaNO₃, ≥99.5%), concentrated sulfuric acid (H₂SO₄, 96%~98%, AR). Glycine (Gly) was purchased from Aladdin Chemical (Shanghai, China).

  1.1   Preparation of 3D RGOA anodes

  Briefly, the reduced graphene oxide (RGO) was prepared from natural graphite flakes through the modified Hummers method^[10]. First, 70 mL H₂SO₄ was poured into a 1 000 mL beaker and then placed in an ice bath. Afterward, NaNO₃ (0.5 g) and natural graphite (1.0 g) were slowly added into the above solution and stirring for 20 min. Finally, KMnO₄ (5 g) was slowly added into the beaker and kept in an ice bath environment for 12 h, which was followed by a dilution process with a large amount of deionized (DI) water. After cooling to room temperature (RT), HCl (10 mL) and H₂O₂ (10 mL) were sequentially added to the above solution until appearing a bright yellow color. Afterward, the obtained products were naturally precipitated and washed several times by DI water.

  The RGOA was prepared via a facile hydrothermal route as reported in 3D graphene/δ - MnO₂ aerogels^[10], which indicated a large potential for removing the heavy metal ions. Briefly, 200 mg Gly was added to a 72 mL aqueous dispersion of graphene (1.4 mg·mL^-1) and stirred magnetically for 2 h. The resulting suspension was then transferred to a Teflon - lined autoclave and reacted at 160 ℃ for 16 h to obtain the RGOA. After cooling to RT, the obtained column samples were washed by DI water and then dried at 80 ℃ in a vacuum for 2 h.

  1.2   Preparation of RGOA/δ⁃MnO₂ composites

  The RGOA/δ - MnO₂ composites were prepared by a redox reaction. Firstly, different weights of KMnO₄ powders (80, 120, 160, 200, and 240 mg) were dissolved in 80 mL DI water and heated to 80 ℃. The pH value of the solution was adjusted to 2.0 by adding several millilitres of 1 mol·L^-1 HCl solution. Then, 50 mg RGOA was added to the above solution and kept for 2 h at 80 ℃ under water bath heating. The resulting samples were then washed with DI water and dried at 80 ℃ overnight. Five samples were obtained for the LIB anodes, and they were marked as RGOA/δ - MnO₂ - 80 mg, RGOA/δ - MnO₂ -120 mg, RGOA/δ - MnO₂ -160 mg, RGOA/δ -MnO₂-200 mg, and RGOA/δ-MnO₂-240 mg by terms of the actual KMnO₄ adding amounts.

  1.3   Material characterization

  The surface morphology was characterized by a field emission scanning electron microscope (FESEM, SU8020, Hitachi) at an accelerating voltage of 5 kV. The structure was measured by X- ray diffraction (XRD) (D8 Advance) diffractometer with a Cu Kα source (λ = 0.154 056 nm) and a scanning range from 5° to 70°. X-ray tube voltage and current were set at 40 kV and 30 mA, respectively. Transmission electron microscopy (TEM) images and energy dispersive X - ray spectrum (EDS) were obtained with a Tecnai G2F20 instrument at an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) was used to measure the surface chemical states and compositions by using the ESCALAB 250Xi instrument (Thermo Fisher Scientific). The Brunauer - Emmett - Teller (BET) specific surface area was measured using a JW - BK112 analyzer at 77.4 K.

  1.4   Li⁃ion battery measurements

  The CR2025 coin cells were assembled in an Ar-filled glovebox with both concentrations of moisture and oxygen below 10^-6. A paste was prepared by mixing the active material (RGOA/δ - MnO₂ with various weight ratios), acetylene black, and binder (carboxy-methyl cellulose) at a weight ratio of 6∶3∶1. The anodes were fabricated by coating the above mixture paste on a Cu foil and dried at 55 ℃ for 48 h. The cathode was a circular Li foil (1.4 cm in diameter, 99.99%). 1 mol·L^-1 LiPF₆ in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) was used as the electrolyte, while a celgard 2400 microporous polypropylene membrane was used as a separator.

  The LIB batteries were galvanostatically charged and discharged (GCD) on a Land CT 2001A cell test instrument (Wuhan LAND Electronic Co., Ltd.). The cyclic voltammetry (CV) tests were conducted at a voltage interval of 0.005~3.0 V (vs Li/Li⁺) at a potential scan rate of 0.1 mV·s^-1, by using an electrochemical workstation (CHI660C, Shanghai Chenhua Instrument Co., Ltd.). All the above electrochemical measurements were carried out at RT.

  2.   Results and discussion

  The synthetic procedure for the RGOA/δ - MnO₂ composite is shown in Scheme 1. Typically, the GO was synthesized by using a modified Hummer′s method, and then self - assembled into a 3D interconnected hydrogel through hydrothermal treatment. Second, the as - prepared RGOA was undergone a vacuum freeze - drying procedure to obtain a 3D hierarchical architecture. Moreover, the obtained RGOA was laid into an acidic KMnO₄ solution to ensure in situ deposition of δ - MnO₂. The above reactions can be simplified as following^[11]:

  $\begin{array}{l} {\rm{4KMn}}{{\rm{O}}_{\rm{4}}}{\rm{ + 3C + }}{{\rm{H}}_{\rm{2}}}{\rm{O}} \to \\ \;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;4\delta - {\rm{Mn}}{{\rm{O}}_{\rm{2}}}{\rm{ + 2KHC}}{{\rm{O}}_3}{\rm{ + }}{{\rm{K}}_{\rm{2}}}{\rm{C}}{{\rm{O}}_{\rm{3}}} \end{array} $

  (1)

  Scheme 1

  

  Scheme 1.  Schematic illustration on preparation process of RGOA/δ-MnO₂ composite

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  During this process, the RGOA (with a main composition of carbon) acts as a reducing agent, which converts the permanganate brine solution (MnO₄^-) to insoluble MnO₂ and then deposits MnO₂ on the RGO surface. It is worth noting that the deposition of ultrathin MnO₂ NPs can also inhibit further permeation of MnO₄^- into the RGO layer.

  Fig. 1 shows the FESEM images of the as-prepared RGOA and RGOA/δ -MnO₂- 160 mg samples. As can be seen in Fig. 1a, the RGOA shows a 3D porous frame-work structure that not only facilitates the catalytic reactions but also becomes an interfacial reactive template for the growth of δ-MnO₂^[12]. In addition, it is also found that the RGOA/δ - MnO₂ composite still maintained a honeycomb 3D porous network structure (Fig. 1b). A few ultrathin δ-MnO₂ NPs were observed on the RGOA surface from the SEM images with a higher magnification (Fig. 1c). In other words, these δ - MnO₂ NPs were wrapped by the wrinkle graphene sheets. Fig. 1e presents the EDS mappings of the RGOA/δ - MnO₂ sample. The elements of C, O, and Mn were distributed uniformly in the RGOA/δ - MnO₂ (Fig. 1f, 1g, and 1h), implying that the δ-MnO₂ sheets were successfully loaded into the graphene surface^[13].

  Figure 1

  

  Figure 1.  FESEM images of (a) RGOA (inset: photograph of RGOA) and (b, c) RGOA/δ-MnO₂-160 mg and (d) TEM image of RGOA/δ-MnO₂-160 mg; EDS mappings of RGOA/δ-MnO₂-160 mg: (e) survey image, (f) C, (g) O, and (h) Mn

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  The N₂ absorption- desorption measurements were applied to compare the specific surface areas of the samples, as shown in Fig. 2. The pristine RGOA sample exhibited a high BET specific area of ~140 m²·g^-1, while a value of 149 m²·g^-1 was observed in the RGOA/δ- MnO₂ - 160 mg hybrid sample. It suggests that there was no obvious change in the surface area of RGOA even if loading enough δ-MnO₂ NPs.

  Figure 2

  

  Figure 2.  Typical N₂ adsorption-desorption isotherms of as-grown RGOA and RGOA/δ-MnO₂-160 mg samples

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  Fig. 3 indicates a typical low - magnification TEM image of RGOA/δ-MnO₂-160 mg hybrid and its corresponding EDS spectrum. It is seen clearly that all the MnO₂ NPs were wrapped by the wrinkle graphene sheets with a considerable dispersibility, while the signal of Mn element is identified in the EDS spectrum, confirming the existence of MnO₂ in the hybrid again. It is worth noting that the signal of Cu is from the copper grids, while the K peak might be associated with the use of KMnO₄ during the synthesis process.

  Figure 3

  

  Figure 3.  (a) TEM image and (b) corresponding EDS spectrum of RGOA/δ-MnO₂-160 mg hybrid

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  Fig. 4 compares the FESEM images of RGOA/δ - MnO₂ hybrids with various MnO₂ contents. As can be seen, when the MnO₂ content was less than 160 mg, the hybrid displays a characteristic of wrinkle 3D graphene framework. The MnO₂ NPs cann′t be distinguished due to their small sizes and good insertion into RGOA. That is to say, most of MnO₂ NPs were wrapped by the wrinkle graphene nanosheets in this case. Nevertheless, when the MnO₂ content exceeded 160 mg, a serve aggregation of NPs occurred, leading to the direct exposure of MnO₂ particles in the air or electrolyte. The 3D network feature of RGOA disappeared due to an influence from the excessive MnO₂. In other words, the MnO₂ particles cann′t form a good contact with the conductive graphene in this case.

  Figure 4

  

  Figure 4.  FESEM images of RGOA/δ-MnO₂ with different adding amounts: (a) 80 mg, (b) 120 mg, (c) 160 mg, (d) 200 mg and (e) 240 mg

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  The phase structure of RGOA and RGOA/δ-MnO₂- 160 mg samples were investigated by both the XRD patterns and Raman spectra. Fig. 5a displays that all the diffraction peaks of RGOA/δ-MnO₂ -160 mg can be assigned to the phases of graphene and tetragonal δ - MnO₂ (PDF No. 42 - 1316) ^[12]. Fig. 5b shows that the RGOA owns two sharp peaks at 1 348 and 1 587 cm^-1, which correspond to the D and G bands of graphene, respectively. Moreover, two weak peaks around 2 650 and 2 900 cm^-1 are assigned to G′ and D+D′ of graphene, implying that there might be a few defects in the RGOA. For the RGOA/δ - MnO₂ - 160 mg sample, a sharp peak appears at 633 cm^-1 in addition to the D and G peaks from the graphene, and this additional peak can be attributed to the tensile vibration of Mn—O lattice, indicating the existence of crystallized δ - MnO₂ NPs again^[14].

  Figure 5

  

  Figure 5.  (a) XRD patterns and (b) Raman spectra of as-prepared RGOA and RGOA/δ-MnO₂-160 mg

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  To further investigate the chemical composition and surface electronic state of the RGOA/δ-MnO₂ - 160 mg hybrid sample, the XPS were performed, and the results were shown in Fig. 6. By a careful inspection of wide region spectroscopy and elemental analysis, the characteristic signals attributed to C1s, O1s, and Mn2p can be observed clearly (Fig. 6a), indicating the coexistence of C, O and Mn components. It is worth noting that the carbon signal (C1s, 284.9 eV) originates from RGOA (Fig. 6b), the peak at 529.8 eV (Fig. 6c) is associated with the O1s signal in anhydrous (Mn—O—Mn) and hydrated (Mn—O—H) manganese oxides, respectively. In the Mn2p spectrum displayed in Fig. 6d, two peaks located at 641.9 and 653.5 eV correspond to the binding energies of Mn2p_3/2 and Mn2p_1/2, in good accordance with a previous report^[15].

  Figure 6

  

  Figure 6.  XPS spectra of RGOA/δ-MnO₂-160 mg: (a) full spectrum; (b) C1s; (c) O1s; (d) Mn2p

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  A series of electrochemical tests were also conducted to study the Li - ion storage properties of a LIB anode in the half lithium-ion cell. Fig. 7a shows the CV plots of a RGOA/δ-MnO₂ -160 mg anode at a scan rate of 0.1 mV·s^-1 for the first three cycles. During the nega-tive scan process of the first cycle, a sharp cathodic peak appeared at 0.1 V, corresponding to the intercalation/deintercalation processes of Li ions. Meanwhile, there was also a broad peak at ~0.7 V, implying the formation of a SEI layer on the surface of anode. In total, the above electrochemical processes can be described as the following reactions^[16-18]:

  $\text{Mn}{{\text{O}}_{\text{2}}}\text{+4Li }\rightleftharpoons \text{ 2L}{{\text{i}}_{\text{2}}}\text{O+Mn} $

  (2)

  Figure 7

  

  Figure 7.  (a) CV and (b) GCD plots of RGOA/δ-MnO₂-160 mg anode; (c) Cycling performance profiles of RGOA/δ-MnO₂ anodes at a current density of 0.1 A·g^-1; (d) Rate performance of RGOA/δ-MnO₂ anodes

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  From the second cycle, the cathodic peak at 0.1 V turned to 0.3 V, suggesting an irreversible structural or textural changes due to the formation of Li₂O and metallic manganese^[19]. The disappearance of the cathodic around 0.7 V implies that the formation of SEI layer has been accomplished during the second cycle. Similarly, during the positive scan process of the first cycle, there were two anodic peaks at 1.3 and 2.1 V, corresponding to the oxidation of Mn⁰ to Mn²⁺ and Mn⁴⁺, respectively. The CV curves can be overlapped well after the second cycle, implying that this RGOA/δ - MnO₂ anode owns an excellent structural stability and electrochemical reversibility.

  Fig. 7b also shows the discharge - charge voltage profiles of a RGOA/δ - MnO₂ - 160 mg anode, which delivered an initial discharge and charge (or reversible) capacity of 2 192.2 and 1 493.8 mAh·g^-1, respectively. It determines an initial Columbic efficiency of 68.1% at 0.1 A·g^-1. Such an initial capacity loss might be caused by the formation of SEI layer. This process refers to both the electrolyte decomposition and the reaction of the oxygen containing functional groups on 3D RGOA/δ-MnO₂ with lithium ions. Despite the initial capacity loss, the well - overlapped charge/discharge profiles after the second cycle demonstrates an excellent reversibility of the 3D RGOA/δ-MnO₂ anode. Additionally, a large voltage plateau located at ~0.50 V was observed in the first three cycles, being consistent with the reduction (or cathodic) peaks of voltages described in the CV profiles.

  In order to find a suitable loading amount of δ - MnO₂, the GCD profiles of the RGOA/δ - MnO₂ electrodes with various compositions are displayed in Fig. 7c. These electrodes were measured at a current density of 0.1 A·g^-1 and undergone 70 charge/discharge cycles. The result shows that the specific capacity of RGOA/δ - MnO₂ increased firstly and then decreased with rising the δ - MnO₂ loading amount, reaching a maximum value of 1 701.9 mAh·g^-1 after 70 cycles at a δ-MnO₂ amount of 160 mg. Compared to the single RGOA electrode, the enhanced specific capacity can be attributed to a high theoretical capacity of MnO₂ (1 230 mAh·g^-1). In contrast, the specific capacity of RGOA was just 538 mAh·g^-1. It′s worth noticing that the specific capacity of RGOA/δ-MnO₂-160 mg is much higher than the theoretical capacity of MnO₂, which is a result of the enhanced conductivity and the suppressed volume expansion after using a 3D porous framework RGOA matrix^[12]. However, a further increase of δ - MnO₂ content to 200 mg would also result in a decrease of the specific capacity, and the RGOA/δ-MnO₂- 240 mg anode even displays a lower specific capacity than the single RGOA one^[5]. The capacity of RGOA/δ-MnO₂-160 mg increased and RGOA/δ-MnO₂-240 mg decreased, whereas other materials are mostly stable with the cycle performances. This phenomenon might be associated with the environment or state of δ- MnO₂ NPs. With increasing δ- MnO₂ content, more and more δ - MnO₂ NPs cann′t be wrapped completely by the graphene in RGOA, leading to a direct exposing of these particles to the electrolyte. As a result, the volume expansion and particle aggregation would be taken place, which will prevent forming a good electric contact of MnO₂ with RGOA or the collector (Cu foil), leading to an unavoidable reduction of the conductivity. On the other hand, the exposed MnO₂ particles will also result in a thickening of the SEI layer, which lengthens the transport distances of Li ions. In total, both the above two reasons are responsible for a poor battery performance after incorporating excessive δ-MnO₂ NPs.

  Fig. 7d displays the rate performance of the RGOA/δ-MnO₂ anodes with varying the current density from 0.1 to 5 A·g^-1 then to 0.1 A·g^-1. For the RGOA/δ -MnO₂ -160 mg, the reversible charge capacities were 1 426.5, 1 128.4, 897.1, 723.6, 534.6 and 386.2 mAh· g^-1 at 0.1, 0.2, 0.5, 1, 2 and 5 A·g^-1, respectively. Obviously, the lower the current density is, the more active sites the Li ions seize, which leads to higher specific capacity under lower current density. Herein, the current density reflects the charge/discharge rate. It means that the increase of charge rate would result in a reduction of specific capacity. When it recovered to 0.1 A·g^-1, the RGOA/δ-MnO₂-160 mg still maintained a high reversible capacity of 1 200.7 mAh·g^-1 (~84% compared to initial capacity). In addition, the RGOA/δ-MnO₂-160 mg anode also indicates much higher specific capacity than other ones at a current density of 5 A·g^-1, implying that this material is more appropriate for working at a rapid charging/discharge condition. It also suggests that a suitable ratio of δ-MnO₂ to RGOA is crucial to achieve both the good rate performance and cyclic stability. The ideal case is that the δ -MnO₂ NPs should be distributed on the surface of RGOA dispersively.

  It is speculated that the excellent rate performance of RGOA/δ- MnO₂-160 mg might be related with a pseudocapacitive contribution. Accordingly, the CV curves at incremental scan rates (0.1 to 2 mV·s^-1) are shown in Fig. 8a. The similar shapes of the CV curves at different scan rates, indicating that the RGOA/δ-MnO₂ electrode has good reversibility. The capacitive effects of the total charge storage can be revealed according to the following equation^[20]:

  ${{i}_{\text{p}}}=a{{v}^{b}} $

  (3)

  Figure 8

  

  Figure 8.  Electrochemical kinetics analysis of RGOA/δ-MnO₂-160 mg: (a) CV curves at different scan rates from 0.1 to 2.0 mV·s^-1; (b) relationships between ln i_p and ln v; (c) plots of i/v^1/2 vs v^1/2 and (d) normalized contribution ratios of pseudocapacitive (blue) and diffusion-controlled (gray) capacities at various potentials

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  For a better analysis, the equation can be reorganized as follows:

  $\ln {i_{\text{p}}} = b\ln v + \ln a $

  (4)

  Where i_p represents the peak current density, v means the sweep rate, both a and b are adjustable parameters. Apparently, i obeys a power law relationship with v, while b can be obtained from the curve slope by plotting ln i_p against ln v. When b approaches to 0.5, the system is dominated by a diffusion -controlled process; while b approaches to 1.0, the charge storage is mainly controlled by pseudocapacitance^[21]. As is shown in Fig. 8b, the slope b values for peak 1 (cathodic peak) and peak 2 (anodic peak) were calculated to be 0.65 and 0.77, respectively. It means that the capacitance contribution cann′t be neglected for a RGOA/δ-MnO₂ - 160 mg electrode.

  Furthermore, the pseudocapacitive contribution can be quantified by another equation^[22]:

  $i = {k_1}v + {k_2}{v^{1/2}} $

  (5)

  For better fitting, this equation can be rewritten by:

  $i/{v^{1/2}} = {k_1}{v^{1/2}} + {k_2} $

  (6)

  Where i represents the measured current density, k₁ and k₂ are the adjustable parameters. Herein, k₁ represents the capacitance - controlled contribution while k₂ stands for diffusion - controlled contribution. Typically, the linear profiles of i/v^1/2 vs v^1/2 are plotted in Fig. 8c. The values of k₁ and k₂ can be determined at various potentials (0.1 to 2.0 V) for determining the diffusion coefficient of Li⁺ ions in the electrodes. Accordingly, the normalized results are displayed in Fig. 8d. It is observed clearly that the electrochemical reactions occurred at the potentials except 0.4 and 0.5 V are dominated by a capacitance -controlled process. Dominant pseudocapacitive contribution means more Li ions involved in surface reactions during electrochemical process. At 0.4 or 0.5 V, the battery displays a poor capacitive contribution (18% and 46%, respectively), which might be related to the lithiation/delithiation reactions occurred in this potential range. As is mentioned previously in Fig. 7b, a large voltage plateau appeared a ~0.50 V in the discharge profile of a battery, corresponding to the insertion process of Li ions.

  Furthermore, we also compare the RGOA/δ-MnO₂- 160 mg with other reports (Table 1). It is found clearly that a charge/discharge rate of 0.1 A·g^-1 is the most commonly used for characterizing the actual specific capacity of a LIB anode. Besides, nearly all the LIBs containing transitional metal oxide anodes display high discharge and reversible capacities during the first cycle. Then, a sharp decrease of specific capacity usually exhibited between first and second discharge process, which can be ascribed to large amount of SEI formation caused by defects in graphene and irreversible conversion upon lithiation. Therefore, it is not reliable to compare the first specific discharge/charge capacity of a LIB device. We always make a comparison of the actual LIB performance after 70~100 cycles. As shown in Table 1, it is worth mentioning that the RGOA anodes referred in this work display both excellent rate performance and longterm stability.

  Table 1

  

  Table 1.  Comparison on the actual Li⁺ storage performance of 3D graphene anodes

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  Anode	Specific capacity/(mAh·g-1)	Cycle number	Current density/(A·g-1)	Ref.
  RGOA/δ-MnO2-160 mg	1 701.9	70	0.1	This work
  3D porous carbon nanosheets/SiO2	635.2	120	0.1	[23]
  3D RGOA/ultrafine SnO2	778	100	0.1	[24]
  3D RGOA/porous hollow SnO2	620	200	0.05	[25]
  3D N-doped RGOA/CuO	725.3	100	0.1	[26]
  3D RGOA/ultrafine Fe3O4	2 136	100	0.5	[27]

  Furthermore, the normalized contribution from the pseudocapacitance-controlled process is also displayed in Fig. 9a. In other words, the calculation result will rep-resent an average value from the contribution of differ-ent potentials. After an integral calculation, the LIB device made with the RGOA/δ-MnO₂-160 mg electrode delivered a pseudocapacitive contribution of ~76.56% to whole current at a moderate scan rate of 1 mV·s^-1. The main charge-discharge mechanism is thus a pseudocapacitive behavior, which is an ultrafast charge storage process and contributes to the high rate performance. Additionally, the long-term cyclic stability and high charge/discharge performance of RGOA/δ - MnO₂- 160 mg electrode were also examined and the results were indicated in Fig. 9b. As can be seen, the measurements were conducted at a high current density of 5 A· g^-1 for 600 cycles. After undergoing 600 discharge/charge cycles, a large reversible charge capacity of 241.1 mAh·g^-1 remained in the Li - ion battery, implying the RGOA/δ - MnO₂ electrode displays an excellent recycling stability.

  Figure 9

  

  Figure 9.  (a) CV curve with the pseudocapacitive contribution shown by the shaded area at a scan rate of 1 mV·s^-1; (b) Long-term cycling performance and Columbic efficiencies of RGOA/δ-MnO₂-160 mg anode at 5 A·g^-1 (Note that the profiles in (b) is initialized at the 75th cycle for avoiding the large fluctuation during these cycles)

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  3.   Conclusions

  In summary, the RGOA/δ- MnO₂ composites with an interconnected 3D structure were fabricated hydrohermally by in situ loading ultrathin δ - MnO₂ NPs in RGOA, while KMnO₄ was used as the raw for synthesizing δ-MnO₂ NPs. The influence of KMnO₄ adding amounts (namely, the MnO₂ contents) was investigated on both the morphology and electrochemical performance of the RGOA/δ-MnO₂ composite products. After optimizing the composite compositions, the RGOA/δ - MnO₂ - 160 mg anode displayed the most excellent reversible specific capacity of 1 701.9 mAh·g^-1 even after 70 cycles at a charge/discharge rate of 0.1 A·g^-1 and delivered reversible capacity as high as 210.5 mAh·g^-1 even after 600 cycles at 5 A·g^-1. The excellent performance of RGOA/δ-MnO₂ -160 mg is attributed to the synergistic effect of pseudo-capacitance contribution (~76.56% at 1 mV·s^-1), small size of δ-MnO₂ and porous network structure of RGOA after a deep and comprehensive analysis.

  
  
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  Scheme 1  Schematic illustration on preparation process of RGOA/δ-MnO₂ composite

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  Figure 1  FESEM images of (a) RGOA (inset: photograph of RGOA) and (b, c) RGOA/δ-MnO₂-160 mg and (d) TEM image of RGOA/δ-MnO₂-160 mg; EDS mappings of RGOA/δ-MnO₂-160 mg: (e) survey image, (f) C, (g) O, and (h) Mn

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  Figure 2  Typical N₂ adsorption-desorption isotherms of as-grown RGOA and RGOA/δ-MnO₂-160 mg samples

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  Figure 3  (a) TEM image and (b) corresponding EDS spectrum of RGOA/δ-MnO₂-160 mg hybrid

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  Figure 4  FESEM images of RGOA/δ-MnO₂ with different adding amounts: (a) 80 mg, (b) 120 mg, (c) 160 mg, (d) 200 mg and (e) 240 mg

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  Figure 5  (a) XRD patterns and (b) Raman spectra of as-prepared RGOA and RGOA/δ-MnO₂-160 mg

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  Figure 6  XPS spectra of RGOA/δ-MnO₂-160 mg: (a) full spectrum; (b) C1s; (c) O1s; (d) Mn2p

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  Figure 7  (a) CV and (b) GCD plots of RGOA/δ-MnO₂-160 mg anode; (c) Cycling performance profiles of RGOA/δ-MnO₂ anodes at a current density of 0.1 A·g^-1; (d) Rate performance of RGOA/δ-MnO₂ anodes

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  Figure 8  Electrochemical kinetics analysis of RGOA/δ-MnO₂-160 mg: (a) CV curves at different scan rates from 0.1 to 2.0 mV·s^-1; (b) relationships between ln i_p and ln v; (c) plots of i/v^1/2 vs v^1/2 and (d) normalized contribution ratios of pseudocapacitive (blue) and diffusion-controlled (gray) capacities at various potentials

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  Figure 9  (a) CV curve with the pseudocapacitive contribution shown by the shaded area at a scan rate of 1 mV·s^-1; (b) Long-term cycling performance and Columbic efficiencies of RGOA/δ-MnO₂-160 mg anode at 5 A·g^-1 (Note that the profiles in (b) is initialized at the 75th cycle for avoiding the large fluctuation during these cycles)

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  Table 1.  Comparison on the actual Li⁺ storage performance of 3D graphene anodes

  Anode	Specific capacity/(mAh·g-1)	Cycle number	Current density/(A·g-1)	Ref.
  RGOA/δ-MnO2-160 mg	1 701.9	70	0.1	This work
  3D porous carbon nanosheets/SiO2	635.2	120	0.1	[23]
  3D RGOA/ultrafine SnO2	778	100	0.1	[24]
  3D RGOA/porous hollow SnO2	620	200	0.05	[25]
  3D N-doped RGOA/CuO	725.3	100	0.1	[26]
  3D RGOA/ultrafine Fe3O4	2 136	100	0.5	[27]

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