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Copper ferrites@reduced graphene oxide anode materials for advanced lithium storage applications

Schematic illustration of the synthesis of (a) CuFeO2@rGO and (b) Cu/CuFe2O4@rGO.

Rechargeable lithium-ion batteries (LIBs) with high energy density and power density have been widely used as energy storage devices. There is an increasing demand for LIBs with long-term stability, safety and low cost to meet future requirements for consumer electronics and electric vehicles. Therefore, searching for new anode materials with ultrahigh theoretical capacity and remarkable electrochemical performance is urgently required, due to the low theoretical capacity for the current commercial graphite anodes. Considerable researches have been devoted to the design of transition metal oxides (TMOs) based electrodes including the binary, ternary, and complex metal oxides, for application in high-performance energy storage devices. Among TMOs, the ternary oxides with delafossite structure (ABO2) and spinel structure (AB2O4) have unique layered crystal structures with three-dimensional diffusion pathways, which are benefit for lithium ion insertion and extraction. Recently, CuCo2O4 and ZnFe2O4 have been investigated as anode materials, which exhibit good reversible capacity and cyclability and guide the following study on the ternary oxide anode materials.
Compared with them, copper ferrites including CuFeO2 and CuFe2O4 have been considered as promising anode materials for the advantages of natural abundance, environmental friendliness, high specific and practical availability. The CuFeO2 anode materials for LIBs was first reported by Lu’s group in 201131. CuFeO2 and graphene composites, which had a specific capacity of 670?mAh?g-1, were prepared by a low temperature hydrothermal method32. As the anode for LIBs, pure CuFe2O4 with different morphologies have been investigated. Carbon coated hollow CuFe2O4 spheres with specific capacity of 550?mAh?g-1 was obtained by a polymer-template hydrothermal growth method37. Polypyrrole-coated CuFe2O4 for LIBs with enhanced electrochemical performance was reported by the electrostatic spray deposition technique38. Unfortunately, the application of copper ferrites in LIBs have been impeded by the inherent sluggish kinetic and large volume expansion/contraction during cycling, which eventually leads to rapid capacity fading and poor cycling stability. Some strategies have been achieved to overcome these obstacles, such as downsizing crystal size, designing various porous structures, hierarchical structures.
Recently, the development of nanotechnology provides more approaches to manufacture optimized architecture for enhancing the electrochemical active of copper ferrites. The availability of small crystal size with high specific surface area and facile stress relaxation processes effectively facilitates the Li+ diffusion and makes high rate capability possible42,43,44. Furthermore, graphene with high surface area and great mechanical stiffness have been widely used for energy storage devices as a conductive additive to enhance the electrochemical reactions. In particular, few-layered graphene (FLG) obtained from graphene oxide exhibits a high reversible capacity and excellent Columbic efficiency and very low cycle to cycle capacity fading. The addition of graphene can not only increase electrode-electrolyte contact area and faster electrolyte access to active materials, but also mitigate the volume change and limit structure degradation during cycling. Moreover, the integration of metallic nanocrystals is a new strategy to address the weak charge transfer kinetics for high active surface/interface and robust stability. Therefore, the design of hybrid architectures with improved capability and stability are highly necessary to achieve prominent performances for copper ferrites anodes.
Herein, we report an efficient and scalable hydrothermal method for synthesizing the copper ferrites@rGO composites (CuFeO2@rGO and Cu/CuFe2O4@rGO) with boosted electrochemical performance. As illustrated in Fig. 1, the copper and iron ions could adsorb on the exposed GO surface owing to the oxygen-containing groups after being well dispersed in GO suspension. In the fabrication processes of CuFeO2@rGO composites (Fig. 1a), the primary delafossite CuFeO2 nanocrystals are first formed in the NaOH solution, followed by the oriented attachment growth to construct the sheet-on-sheet CuFeO2@rGO architecture. The synergistic effects of each component improve the capability of CuFeO2@rGO electrode (587?mAh?g-1 at 200?mA?g-1 after 100 cycles). As shown in Fig. 1b, the Cu/CuFe2O4@rGO composite was obtained by means of the reducing and complex characteristics of ethylene glycol (EG) and the stronger complexing ability of ethylenediamine (EN). In this reaction system, part of copper ions form CuFe-precursor combining with iron ions. The rest of copper ions were reduced to metallic copper through reacting with ethylene glycol and ethanediamine in the liquid solution. The similar mechanism for preparing spinel compound oxides by using non-stoichiometric ratio have been reported. Such a phase transformation leads to the well distribution of CuFe2O4 and copper on the surface of rGO, while each of them is interconnected by graphene. It is anticipated that such Cu/CuFe2O4@rGO configuration can gain access to the following advantages: (i) a sufficient electrical contact for rapid electron transfer and a shorter channel for fast lithium ion transport, promoting the electrode reaction kinetic; (ii) large surface area ensures effective contact between the electrolyte and electrode, enhancing the electrochemical actively; (iii) available internal voids can buffer the volume change during lithiation/delithiation processes, increasing structural stability. Unsurprisingly, the as-built Cu/CuFe2O4@rGO electrode exhibited a remarkable rate capability (560?mAh?g-1 at 3200?mA?g-1) and cycling stability (835.2?mAh?g-1 over 100 cycles at 200?mA?g-1), indicating a promising prospect of application in high-end energy storage devices.

The CuFeO2@rGO composites

The crystalline phase and composition of as-synthesized CuFeO2@rGO composites were investigated by XRD measurements. As shown in Fig. 2a, the single delafossite CuFeO2 phase (PDF# 39-0246) of CuFeO2@rGO can be well observed in the scan range of 10–80°, which confirms the good crystallinity of the samples. As for the pure CuFeO2, CuFeO2 is the predominating phase along with a weak impure peak located at around 38°, which may be related to CuO or 2H-CuFeO2. It is hard to determined due to its weak intensity. Pure CuFeO2 displays sharper and stronger diffraction peaks than CuFeO2@rGO, suggesting the smaller crystallite size in CuFeO2@rGO. The d-spacings of the (006) and (110) diffraction peaks indicate that lattice parameters of a?=?0.3031?nm and c?=?1.7141?nm for CuFeO2@rGO sample. In comparison, pure CuFeO2 has lattice parameter values of a?=?0.3027?nm and c?=?1.7161?nm. Raman spectroscopy was adopted to evaluate the graphitic quality, which cannot be detected by XRD data (Fig. 2b). There are three modes at around 105, 342 and 670?cm-1, which correspond to E u , E g and A1g of delafossite CuFeO256. In addition, two well-resolved bands at 1360 and 1590?cm-1 for CuFeO2@rGO are attributed to the D band (k-point phonon of A1g symmetry) and G band (E2g phonon of carbon) of graphene, respectively. Compared with GO (I D /I G ?=?0.86), the increased ratio of the D band to G band (I D /I G ?=?0.97) in CuFeO2@rGO suggests the reduction of graphene, which can be ascribed to smaller but more numerous sp2 domains in carbon57. Moreover, the presence of 2D band at 2694?cm-1 and (D?+?G) band at 2953?cm-1 in Fig. S1 manifests a substantial increase in the disorder degree in graphene sheets. The high ratio I G /I2D indicates the relatively thick graphene layers48, 50. The specific surface area of as-prepared composites was calculated by the Brunauer-Emmett-Teller (BET) method. As shown in Fig. S2, the BET surface area of CuFeO2@rGO is 14.63?m2?g-1, which is very low specific surface area in comparison with graphene51. It indicates that lots of the active surface of graphene have been covered by CuFeO2 crystal with smaller specific surface area16, which may not readily allow nitrogen molecules to get adsorbed onto them unlike pristine graphene. XPS measurements were conducted to detect the composition and chemical state of CuFeO2@rGO. The full XPS spectrum (Fig. S3) reveals the presence of Cu 2p, Fe 2p, O 1?s and C 1?s, with no evidence of impurities. Figure 2d shows the high resolution XPS spectrum of Cu 2p. The dominant doublet peaks positioned at 932.1 and 952.1?eV are ascribed to Cu 2p3/2 and Cu 2p1/2 for Cu+ from CuFeO258. The shoulder peaks located at around 934.5 and 954.9?eV along with two satellite peaks at 943.1 and 961.9?eV correspond to Cu 2p3/2 and Cu 2p1/2 for Cu2+, which should be caused by the easy oxidation of Cu in air atmosphere29. The O 1?s peak located at 530.8?eV (Fig. 2e) further confirms the formation of CuFeO259, whereas the higher binding energy peak positioned at about 532.7?eV is attributed to the surface adsorbed hydroxyl oxygen21, 60. The high resolution XPS spectrum of the Fe 2p doublet (Fig. 2f) with two peaks located at 711.4?eV for Fe 2p3/2 and 725.2?eV for Fe 2p1/2, is characteristic of Fe3+? 60. In addition, the high resolution C 1?s spectrum in Fig. 2c could be deconvoluted into four peaks at 284.4, 285.4, 286.5 and 288.2?eV, corresponding to C-C, C-O, C=O and COOH bonds, respectively. Note that the C-C bond is dominated in the C functional groups, which indicates the possibility of electronic conductivity improvement for CuFeO2@rGO electrode.

(a) XRD patterns and (b) Raman spectra of CuFeO2 and CuFeO2@rGO. High-resolution XPS spectra of (c) C 1?s, (d) Cu 2p, (e) O 1?s, and (f) Fe 2p for CuFeO2@rGO.

The morphology and microstructure of the as-prepared CuFeO2 and CuFeO2@rGO were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Figure 3a reveals the large hexagonal platelet characteristic of 1–2?µm in diameter and 300–600?nm in thickness for bare CuFeO2. Figure 3b–d show that the hexagonal CuFeO2 crystals with the diameter of 200–400?nm and thickness of 40–60?nm are homogeneously anchored on the surface of graphene nanosheets for CuFeO2@rGO. The reduced size of CuFeO2 can be attributed to the functional groups of graphene nanosheets, which can not only restrict the size of CuFeO2 but also act as nucleation centers to facilitate the formation of crystals32, 61. The smaller size of CuFeO2 attached on graphene nanosheets is advantageous to facilitate the lithium ion diffusion and accommodation the large volume changes during cycling, resulting in better electrochemical performance for LIBs. In addition, the selected area electron diffraction (SAED) pattern (inset of Fig. 3c) shows a set of well-defined spots of the prepared CuFeO2 on rGO. A magnified TEM image (Fig. 3e) clearly shows that the primary CuFeO2 nanocrystals are formed on graphene, which further confirms the growth mechanism of CuFeO2. The high-resolution TEM (HRTEM) image reveals clear lattice fringe spacing of 0.25?nm and 0.29?nm (Fig. 3f), which can be readily indexed to the (012) and (006) planes of the delafossite CuFeO2 crystal, respectively.

(a) SEM image of CuFeO2, inset shows the higher magnification. (b) SEM image of CuFeO2@rGO, inset shows the higher magnification. (c,d) TEM images of CuFeO2@rGO, inset of (c) shows the selected area electron diffraction (SAED) pattern. (e) High magnitude TEM image of CuFeO2@rGO. (f) HRTEM image of CuFeO2@rGO.

As a demonstration, the as-prepared CuFeO2 composites were employed as anodes for LIBs. The cyclic voltammetry (CV) analysis was applied to obtain the electrochemical details at a scan rate of 0.5?mV?s-1 within a voltage window of 0.02–3?V (vs. Li/Li+). Figure 4a shows the 1st, 2nd, and 5th CV curves of CuFeO2@rGO electrode. In the first cathodic process, two obvious peaks at about 0.86 and 0.75?V can be ascribed to the decomposition of electrolyte, the formation of solid electrolyte interface (SEI) layer on the electrode surface, as well as the irreversible reduction of CuFeO2 (CuFeO2?+?4Li+?+?4e-???Cu?+?Fe?+?2Li2O). In the reversed anodic process, the broad oxidation peak centered at 1.78?V represents the reversible oxidation of metallic Cu and Fe (Cu?+?2Fe?+?4Li2O???Cu2O?+?Fe2O3?+?8Li+?+?8e-) and Li2O decomposition. In the subsequent cycles, the cathodic peak located at 0.87?V corresponds to the reduction of Cu2O, Fe2O3 and the formation of SEI film31. The corresponding CV curves of CuFeO2 are shown in Fig. S4. The only peak at about 0.73?V in the first cathodic process should be assigned to the reduction of CuFeO2 and the irreversible reaction related to the decomposition of the electrolyte32. The different in the first discharge cycle between CuFeO2 and CuFeO2@rGO may be attributed to the synergistic effects of graphene62. Figure 4b shows the charge/discharge cycling of CuFeO2@rGO electrode in the initial, second, twenty-fifth, fiftieth and hundredth cycles at the current density of 200?mA?g-1. It can be seen clearly that the voltage drops sharply from the open-circuit voltage to about 1.2?V during the first discharge cycle, which is corresponding to the beginning the insertion of Li+ ions63. The discharge profile mainly consists of voltage plateau at about 1.2?V and 0.95?V, agreeing with the first CV curve. At about 0.8?V, the voltage starts dropping with a gentle sloping profile. The first charge cycle has no voltage plateau but a sloping profile that changes at about 1.5?V till about 2.3?V, before changing again. The second charge cycle is analogous to the first charge cycle, which indicates that similar electrochemical reactions are taking place in both cycles. The second discharge cycle has a very different profile in comparison with the first discharge cycle, indicating disparate electrochemical reactions. The voltage plateau originally seen at 1.2?V is no longer seen. The voltage drops slowly from 3 to 1?V, and then slopes downward till 0.02?V. Moreover, the initial discharge capacity of CuFeO2@rGO (985?mAh?g-1) is remarkably higher than the theoretical capacity of CuFeO2 (708?mAh?g-1), which have been found in other metal oxides12, 13, 32, 47, 57. The higher initial discharge capacity may be ascribed to structural destruction upon Li insertion and decomposition of the solvent in the electrolyte, subsequent formation of large area solid electrolyte interphase (SEI) layer and nano Cu and Fe in Li2O matrix. The initial charge capacity of CuFeO2@rGO is 730?mAh?g-1, yielding a coulombic efficiency of 74%. The formation of SEI layer on the surface of active materials has been recognized as the primary cause for irreversible capacity loss, including graphene (Fig. S5)3, 4, 48. Moreover, the high irreversible capacity loss can also be attributed to the volume variations, some undecomposed Li2O phase, along with the irreversible reduction of active materials and electrolyte during the first discharge process11, 64,65,66. Notably, the curves are strongly overlapped for 25, 50 and 100 cycles, suggesting the good stability and reversibility of CuFeO2@rGO electrode.

(a) CV curves of CuFeO2@rGO. (b) Charge/discharge voltage profiles of CuFeO2@rGO at 200?mA?g-1. (c) Charge/discharge voltage profiles of CuFeO2@rGO at different rates. (d) Cycling performance of CuFeO2@rGO and CuFeO2 at 200?mA?g-1 for 100 cycles. (e) Rate capabilities of CuFeO2@rGO and CuFeO2 at different current density. (f) Nyquist plots and equivalent circuit of CuFeO2 and CuFeO2@rGO.

The galvanostatic charge/discharge (GCD) profiles at various rate and the corresponding comparison are shown in Fig. 4c and Fig. S6. One can see that the CuFeO2@rGO electrode delivers discharge capacities of 1078, 561 and 406?mAh?g-1 at 50, 200 and 800?mA?g-1, respectively. In contrast, not only the initial capacity of bare CuFeO2 electrode is lower than the CuFeO2@rGO electrode, but there is also severe capacity fading in a reversible capacity of only 146?mAh?g-1 at 200?mA?g-1. It indicates that the introduction of graphene is beneficial for fast charge transfer and the electrode stable at high rate, leading to the enhancement of electrochemical capability. Fig. 4d shows the cycling performance of CuFeO2@rGO and bare CuFeO2 electrodes at the current density of 200?mA?g-1. The CuFeO2@rGO electrode demonstrates excellent reversibility and cycling stability. A reversible capacity as high as 587?mAh?g-1 is sustained after 100 cycles. It is worth noting that the capacity fading from the first to the tenth cycles can be ascribed to the complicated side-reactions and irreversible structure transformation67, 68. However, in the case of bare CuFeO2, the reversible capacities seriously decline to 222?mAh?g-1, which can be ascribed to the large volume expansion and mechanical stress during lithiation/delithiation processes31. Moveover, the CuFeO2@rGO electrode exhibits a superior cycling capability and stability even at a high rate of 800?mA?g-1 (400?mAh?g-1 after 100 cycles) (Fig. S7). The significantly enhanced cycling performances are closely related to the sheet-on-sheet architecture of CuFeO2@rGO. Specifically, the small size of CuFeO2 along with the high surface area of graphene can provide more electrochemical reaction sites and suppress the aggregation of active materials to keep electrode structure stable69. Impressively, the CuFeO2@rGO electrode behaves robust rate capability (Fig. 4e). Upon cycling at various current densities of 50, 100, 200, 400, 800?mA?g-1, the CuFeO2@rGO electrode exhibits average discharge capacities of 720, 630, 538, 474, 406?mAh?g-1, respectively. Furthermore, the discharge capacity could maintain a discharge capacity of 613?mAh?g-1 when the rate is returned to 100?mA?g-1. In contrast, the CuFeO2 electrode shows inferior rate capability, achieving mere 70?mAh?g-1 at 800?mA?g-1 and poor recovery (178?mAh?g-1 at 100?mA?g-1).
The inspiring rate capability and cycling stability of CuFeO2@rGO electrode originate from the sheet-on-sheet structure. In CuFeO2 electrode, limited by the inherent poor conductivity and sluggish ion transport of disconnected micron-grade crystals, the lithium ion cannot effectively diffuse to the active materials through electrolyte. In contrast, the interconnected CuFeO2@rGO architecture with nanoscale CuFeO2 can shorten the Li+ diffusion pathway for fast electron/ionic transportation. In addition, The conductive graphene boosts the electrical conductivity and the sufficient contact between electrolyte and active materials, promoting charge transfer at the electrode/electrolyte interface. To further understand the electrode kinetics mechanism of CuFeO2@rGO and CuFeO2, electrochemical impedance spectra (EIS) were carried out on the fresh cells from the open circuit voltage. The Nyquist plots are shown in Fig. 4f, all spectrum consist of a depressed semicircle and a straight line. The semicircle is related to charge transfer resistance whereas the line corresponds to solid state diffusion resistance64, 66. The spectra were fitted to an equivalent circuit consisting of resistances (both electrolyte R e and charge transfer R ct ), a constant phase element (CPE), a Warburg impedance (W s ) and an intercalation capacitance (C int ). W s is associated with the solid-state diffusion resistance70, 71. The values of the circuit elements shown in Table S1 (Supporting Information) confirm the easy lithiation kinetics of CuFeO2@rGO electrode. Moreover, the sheet-on-sheet configuration with high surface area can offer more electrochemical reaction sites, which are benefit for the lithiation/delithiation reaction of active materials. The configuration also provides more volume to prevent the aggregation of active materials, ensuing a stable electrode structure. All the aforementioned factors contribute the remarkable electrochemical lithium storage properties of the CuFeO2@rGO composites.

The Cu/CuFe2O4@rGO composites

The conductive additive has shown great success for promoting the capacity and stability of delafossite-type CuFeO2 based anode. Such a strategy should be extended to spinel-type copper ferrite CuFe2O4 with higher theoretical capacity (895?mAh?g-1). However, the active materials could be separated from the add-in graphene for the weak interaction during high-rate cycling, which can be ameliorated by the combination with metal nanocrystals72, 73. In particular, metallic copper with high electrical conductivity has been proven to be an efficiently additive for advanced energy storage. Therefore, the Cu/CuFe2O4@rGO electrode is expected to constructed though a one-step hydrothermal approach, as shown in Fig. 1b. In this reaction process, ethylene glycol medium can act not only as a solvent, but also as a reductant to induce the incorporation of metallic Cu. Moreover, as a cosolvent, ethylenediamine has a stronger chelating ability for the release of isolated iron ions, which influences the crystal growth and generates the CuFe2O4. The integration of metal copper and graphene with CuFe2O4 nanocrystals can prevent the exfoliation of active materials and accelerate the transportation of electrons/ion. Thus, the prospection of higher capacity and high-rate performance for copper ferrites based lithium storage can be achieved.
The crystallographic structure of the as-fabricated Cu/CuFe2O4@rGO hybrid was analyzed by XRD technique. As shown in Fig. 5a, the strong diffraction peaks at around 43° and 51° can be assigned as the (111) and (200) peak of Cu (PDF# 04-0836), respectively. The other diffraction peaks can be indexed as spinel CuFe2O4 (PDF# 25-0283), which confirms the good crystallinity of the products. Moreover, Cu/CuFe2O4 and Cu/CuFe2O4@rGO show the similar lattice parameter values of a?=?b?=?c?=?0.8373?nm. Raman spectroscopy was performed to analyze the coating conditions of rGO layer (Figs 5b and S8). The band at 665?cm-1 corresponds to the A1g vibration of CuFe2O4. Compared with GO (I D /I G ?=?0.86), the increased D/G intensity ratio (I D /I G ?=?0.94) in Cu/CuFe2O4@rGO suggests the reduction of graphene57. The presence of 2D band shows a substantial increase in the disorder degree of graphene with many layers32, 50. The specific surface area of Cu/CuFe2O4 composites show in Fig. S9. The BET value of Cu/CuFe2O4 is 11.96?m2?g-1, similar to the other metal oxides with analogous morphology16, 23. The high specific surface area of Cu/CuFe2O4@rGO (161.39?m2?g-1) is mainly the contribution of graphene, which is connected to large SEI formation, extending up to very high capacity. XPS measurements were conducted to further evaluate the composition of Cu/CuFe2O4@rGO composites. The survey XPS spectrum (Fig. 5c) clearly indicates the presence of Cu, Fe, O and C elements, consistent with the above XRD and Raman results. The high-resolution Cu 2p spectra (Fig. 5d) reveals Cu2+ 2p3/2 and Cu2+ 2p1/2 binding energy peaks at 934.8?eV and 954.9?eV, respectively74. The statellite peaks at 944?eV and 963?eV indicate the existence of metallic copper in Cu/CuFe2O4@rGO hybrid. From Fig. 5e, the peaks at 711.6?eV and 725.4?eV with an energy difference of 13.8?eV, are assigned to Fe3+ 2p3/2 and Fe3+ 2p1/2, respectively. Moreover, the strong C 1?s peak located at 284.5?eV is assigned to the graphitic carbon (C-C) of rGO whereas the weaker peak at 288.5?eV is related to the C in carboxyl (COOH) (Fig. 5f)30. The dramatic loss of oxygen-containing functional groups further indicates the deoxygenation process accompanying the reduction of GO, which is ascribed to the addition of reductive agent in the hydrothermal reaction.

(a) XRD patterns and (b) Raman spectra of Cu/CuFe2O4 and Cu/CuFe2O4@rGO. (c) XPS survey spectrum of Cu/CuFe2O4@rGO. High-resolution XPS spectra of (d) Cu 2p, (e) Fe 2p, (f) C 1?s for Cu/CuFe2O4@rGO.

The morphology and microstructure of Cu/CuFe2O4@rGO composites were examined by SEM and TEM. It can be seen from Fig. 6a and Fig. S10 that the as-prepared Cu/CuFe2O4 has a great irregular CuFe2O4 around large metallic copper crystals. The typical SEM images at different magnifications are shown in Fig. 6b and Fig. S11. It clearly reveals the uniform morphology over the whole surface of Cu/CuFe2O4@rGO, wherein graphene nanosheets have a mass of ultrafine nanoparticles evenly anchored on them. Notably, there is no large aggregations of CuFe2O4 and metallic copper or large vacancies in graphene nanosheets, exhibiting a better distribution and smaller size in comparison with Cu/CuFe2O4. The huge transformation of morphology between the two samples can be ascribed to the synergistic effects of graphene, EN and EG used in the hydrothermal reaction. Graphene can be decomposed by EN and reduced by EG, resulting in abundant active sites, which can control the crystal nucleation and growth of CuFe2O4 and Cu75. In addition, the chelates of EG and the large surface areas of graphene can prevent the agglomeration during the particle growth process76. The TEM images further confirm that graphene nanosheets are decorated by ultrafine nanoparticles with the diameter of 15–25?nm, which interconnected through rGO (Fig. 6c,d). The selected area electron diffraction (SAED, inset of Fig. 6c) shows the polycrystalline diffraction rings of Cu/CuFe2O4@rGO composites. The HRTEM images (Fig. 6e,f) recorded on two different parts further confirm the formation of Cu/CuFe2O4@rGO. The lattice fringes of 0.25?nm and 0.48?nm in Fig. 6e can be ascribed to the (311) and (111) plane of CuFe2O4. The interplanar spacing of 0.21?nm and 0.30?nm in Fig. 6f can be assigned to the (111) plane of Cu and (220) plane of CuFe2O4. The high surface area of Cu/CuFe2O4@rGO combined with uniform distribution of metal copper and ultrafine CuFe2O4 is helpful for fast ion access, stable crystalline structure and efficient electrolyte penetration. Moreover, the hybrid system can promote the electrochemical activities and prevent the separation of CuFe2O4 during cycling, which can result in high-rate lithium storage performance.

(a) SEM image of Cu/CuFe2O4, inset shows the higher magnification. (b) SEM image of Cu/CuFe2O4@rGO, inset shows the higher magnification. (c) TEM image of Cu/CuFe2O4@rGO, inset shows the selected area electron diffraction (SAED) pattern. (d) High magnitude TEM image of Cu/CuFe2O4@rGO. (e,f) HRTEM images of Cu/CuFe2O4@rGO.

The CV profiles of as-prepared CuFe2O4 based electrodes are shown in Fig. 7a and Fig. S12. For Cu/CuFe2O4@rGO electrode, the cathodic sharp peak located at around 0.5?V in the first cycle corresponds to the irreversible reduction of CuFe2O4 (CuFe2O4?+?8Li+?+?8e-???Cu?+?2Fe?+?4Li2O), as well as the growth of SEI layer33. Two broad overlapping anodic peaks positioned at about 1.74 and 1.85?V can be attributed to the reversible oxidation of metallic Cu and Fe (Cu?+?2Fe?+?4Li2O???CuO?+?Fe2O3?+?8Li+?+?8e-), as well as the SEI decomposition34. In subsequent cycles, the cathodic/anodic peaks at around 0.7?V/1.85?V can be observed, corresponding to the improved kinetics as well as the lithiation/delithiation reactions of CuO/Cu and Fe2O3/Fe. Note that the voltammograms are superimposable perfectly after the first cycle, as compared with Cu/CuFe2O4 and CuFe2O4, suggesting better electrochemical reactivity and reversibility for Cu/CuFe2O4@rGO. The initial five voltage profiles of Cu/CuFe2O4@rGO electrode at 50?mA?g-1 and the corresponding comparison are shown in Fig. 7b and Fig. S13. The voltage drops sharply from the open-circuit voltage to about 0.85?V during the first discharge cycle for all electrodes. There is a voltage plateau at about 0.85?V, followed by a sloping profile till just above 0.02?V. The second discharge cycle had a different profile. The voltage dropped initially to about 1?V and has a small voltage plateau at around 0.95?V. At about 0.85?V, the voltage decreased steeply to 0.02?V. The charge cycles are similar and all has a sloping profile that changes at about 1.5?V till 2.3?V. Moreover, the Cu/CuFe2O4@rGO delivers an initial discharge and charge capacity of 1169?mAh?g-1 and 855?mAh?g-1 with a first coulombic efficiency of 73.1%, much higher than those for Cu/CuFe2O4 and CuFe2O4. The enhanced capacity is attributed to the contribution of graphene and metallic copper along with smaller crystal size, which can increase the utilization of active materials. The large irreversible capacity loss is likely ascribed to the consumption of Li+ to form an irreversible SEI layer and the reduction of CuFe2O4. Noticeably, smaller voltage hysteresis for Cu/CuFe2O4@rGO electrode manifests its better electrochemical stability. EIS were carried out on the fresh cells from the open circuit voltage to understand the kinetics of lithiation and delithiation and resistance to charge transfer. The typical Nyquist plots in which semicircles and Warburg line are present are shown in Fig. 7c along with the respective equivalent electrical circuits (the values of the circuit elements are shown in Table S2). The overall low impedance values imply the better reaction kinetics of Cu/CuFe2O4@rGO electrode64. The availability of Cu/CuFe2O4@rGO with Cu and CuFe2O4 nanoparticles can effectively reduce the ion transport dimensions and enlarge the contact surface of electrode-electrolyte, resulting in improved electrochemical performance.

(a) CV curves and (b) charge/discharge voltage profiles of Cu/CuFe2O4@rGO at 50?mA?g-1 for the initial five cycles. (c) Nyquist plots and equivalent circuit of Cu/CuFe2O4 and Cu/CuFe2O4@rGO. (d) Rate capabilities of Cu/CuFe2O4@rGO and Cu/CuFe2O4 at different current density. (e) Rate capabilities of Cu/CuFe2O4@rGO and Cu/CuFe2O4 at varying rate from 50?mA?g-1 to 3200?mA?g-1. (f) Cycling performance of Cu/CuFe2O4@rGO and Cu/CuFe2O4 at 200?mA?g-1 for 100 cycles. (g) Cycling performance of Cu/CuFe2O4@rGO at 800?mA?g-1 for 250 cycles, inset shows the schematic illustration of the half-cell structure during cycling. (h) Selected discharge voltage profiles of Cu/CuFe2O4@rGO at 800?mA?g-

As shown in Fig. 7d, the Cu/CuFe2O4@rGO electrode shows an enhanced rate capability, with the average discharge capacity of 863?mAh?g-1 and 723?mAh?g-1 at rates of 50?mA?g-1 and 800?mA?g-1. Moreover, the capacity can recover to the initial value as long as the rate reverses back to low current density, highlighting the cycling durability. For comparison, the Cu/CuFe2O4 electrode delivers mere 100?mAh?g-1 at 800?mA?g-1 and exhibits poor recovery. To further evaluate the ultrafast electrochemical ability of Cu/CuFe2O4@rGO electrode, high-rate testing was conducted at even higher current densities from 50?mA?g-1 to 3200?mA?g-1 (Fig. 7e). Noticeably, the Cu/CuFe2O4@rGO electrode features a high-rate reversible capability as well as stability and the capacity maintains above 560?mAh?g-1 at 3200?mA?g-1. For comparison, the Cu/CuFe2O4 electrode demonstrates much poor rate capability with negligible discharge capacity of 27?mAh?g-1 at 3200?mA?g-1. In particular, the Cu/CuFe2O4@rGO electrode can quickly recover to its original capacity or even higher (861.1?mAh?g-1 in the 40th cycle) when the rate abruptly switched to 100?mA?g-1, indicating the promising application for advanced energy storage. The significantly boosted rate capacity of Cu/CuFe2O4@rGO electrode is mainly induced by the synergistic effect of graphene sheets and small crystal. The conductive rGO and metallic copper enhance the electronic conductivity and shorten electronic/ionic transport length, resulting in better lithiation/delithiation reaction kinetics. Moreover, the large surface area of Cu/CuFe2O4@rGO provides more surface to the electrolyte and activation sites for electrochemical reactions. In addition, the large surface area can effectively buffer the large volume change during lithium reactions, ensuring a superior high-rate performance. In addition to ultrahigh rate capability, the Cu/CuFe2O4@rGO electrode also possesses boosted cycling performance. As shown in Fig. 7f, the Cu/CuFe2O4@rGO electrode delivers a reversible capacity of 835.2?mAh?g-1 after 100 cycles at 200?mA?g-1, which is much higher than that for Cu/CuFe2O4 (235?mAh?g-1 after 100 cycles). It suggests the extraordinary cycling stability for the highly reversible Li+ insertion/extraction kinetics. Furthermore, the long-term high-rate cycling performance of Cu/CuFe2O4@rGO was evaluated at 800?mA?g-1 for 250 cycles after being activated for 3 cycles at 50?mA?g-1 (Fig. 7g). An ultrahigh discharge capacity of 1102?mAh?g-1 is yielded even after 250 cycles, indicating the robust cyclability even under the long-term and fast discharge/charge cycling. Interestingly, there is a capacity increasing stage after a slow capacity decay, which has been reported for most transition metal oxides62, 77. Such an activation behavior for LIBs originates from the gradually emerging interfacial storage contribution, which can be attributed to the faradaic contribution of pseudo-capacitance and non-faradaic contribution of double-layer capacitance78. In addition, the reversible growth of a polymeric gel-like film around the active materials from electrolyte degradation also can lead to the gradually increased capacity, as shown in the inset of Fig. 7g. The comparison in electrochemical properties between Cu/CuFe2O4@rGo and other related works has been summarized in Table S3. In order to reveal the capacity changes with cycling, some selected discharge profiles of Cu/CuFe2O4@rGO at 800?mA?g-1 are shown in Fig. 7h. According to the CV test at a scan rate of 0.5?mV?s-1 after the frst lithiation/delithiation cycle (Fig. 7a), the discharge processes can be roughly divided into three voltage ranges of 1.2–3.0?V (?C1), 0.5–1.2?V (?C2–?C1), 0.02–0.5?V (?C3–?C2), respectively. Those three stages are corresponding to the formation of gel-like polymer layer at low potentials, the reduction reaction of CuO and Fe2O3, the formation of gel-like polymer layer at high potentials, respectively. Obviously, the capacity increment at high potentials (?C1) is almost negligible. The whole capacity increment (?C3) is mainly dominated by the increasing capacity of the gel-like polymer layer formation at low potentials and the activation CuO and Fe2O3 reduction. The increasing capacity by the activation of CuO and Fe2O3 should be associated with the dispersion of metallic Cu nanoparticles in rGO matrix54. The presence of Cu nanoparticles with high surface activity can enhance the reversible electrochemical reaction of Li2O and the reversibility of Fe back to Fe2O3 during the charge process. In addition, the addition of copper increases extra reversibly convert of Cu2O during discharge cycling, making the capacity increment of Cu/CuFe2O4@rGO electrode with cycling73. Moreover, the capacity increment at low potentials may benefit from the enhanced gel-like polymer growth and the electrolyte decomposition. The formation of CuO and Fe2O3 nanoparticles upon cycling increases the specific surface area of the electrode and enhances the electrolyte decomposition, which could improve the capacity from the gel-like polymer growth12.
Generally, the increased electrochemical performances of Cu/CuFe2O4@rGO electrode can be ascribed to the synergistic effects among metallic Cu, CuFe2O4 particles and graphene nanosheets, originated from the specific electrode configuration. Firstly, the incorporated reduced graphene can offer nucleation sites for smaller crystal growth without any aggregation. The reduced size ensures the sufficient activation of active materials during reactions, resulting in the enhancement of capacity. Secondly, the highly uniform CuFe2O4 contacted with rGO offers interconnected ion diffusion pathways and adequate electrode/electrolyte interfacial area. It can facilitate lithium insertion/extraction to obtain superior rate performance. Thirdly, the highly conductive copper nanoparticles anchored on graphene nanosheets provide more electrochemical reaction sites and accommodate the volume change during cycling, which can lead to better cycling stability. The aforementioned advantages result in the significantly improved electrochemical performance of Cu/CuFe2O4@rGO, showing its potential application for advanced lithium storage.
In summary, the copper ferrites@rGO anode materials for advanced lithium storage have been successfully prepared by a facile hydrothermal approach followed by a calcination process. The well-defined copper ferrites nanocrystals are uniformly capped with curved graphene. The synthetic effects of all components result in the enhancement of lithium storage performance through accelerating the electron/ion transfer and increasing the structural and interfacial stability. The CuFeO2@rGO electrode yields a high rate capability than bare CuFeO2. In particular, the facile fabricated Cu/CuFe2O4@rGO electrode can deliver a high reversible capacity of 1102?mAh?g-1 after 250 cycles at a high current density of 800?mA?g-1 and a remarkable rate capability among 50 to 3200?mA?g-1. The resulting improvement of electron kinetics and appropriate spaces to alleviate the volume change is mainly responsible for the extraordinary performance of Cu/CuFe2O4@rGO. Such outstanding electrochemical performances make the copper ferrites based anode materials promising for the stationary energy storage systems.