Magnetic CoFe2O4 and NiFe2O4 Induced Self-Assembled Graphene Nanoribbon Framework with Excellent Properties for Li-Ion Battery

A magnetically induced self-assembled graphene nanoribbons (GNRs) method is reported to synthesize MFe2O4/GNRs (M = Co,Ni). It is found that MFe2O4 compounds not only locate on the surface of GNRs but anchor on the interlayers of GNRs in the diameter of less than 5 nm as well. The in situ growth of MFe2O4 and magnetic aggregation at the joints of GNRs act as crosslinking agents to solder GNRs to build a nest structure. Additionally, combining GNRs with MFe2O4 helps to improve the magnetism of the MFe2O4. As an anode material for Li+ ion batteries, MFe2O4/GNRs can provide high reversible capacity and cyclic stability (1432 mAh g−1 for CoFe2O4/GNRs and 1058 mAh g−1 for NiFe2O4 at 0.1 A g−1 over 80 cycles).


Introduction
Rechargeable lithium-ion batteries (LIBs) are widely used to meet the ever-increasing demand for fast charging and high-capacity energy storage [1][2][3]. It is particularly important to search for novel anode materials with both high capacity and outstanding safety for high-performance LIBs. Nowadays, graphite is the most commercial anode material for LIBs owing to its abundant natural resources. However, the relatively low theoretical specific capacity (372 mAh g −1 ) and the lithium diffusion coefficient (10 −7~1 0 −10 cm 2 s −1 ) restrict the further application of high-power LIBs [4][5][6]. The choice of magnetic transition metal oxides (TMOs) as anodes for LIBs has received considerable attention due to their high energy density and natural abundance. Apart from binary TMOs, ternary TMOs comprising mixed valence in a single crystalline structure have drawn enormous research attention [7,8]. Regarding cobalt ferrite and nickel ferrite (MFe 2 O 4 , M = Co, Ni), as one of the representative ternary TMOs, better electrical conductivity arises because Co 2+ /Ni 2+ and 1/2 the Fe 3+ cation occupy the octahedral sites. In comparison to conventional spinel oxides, MFe 2 O 4 with a unique inverse spinel structure has higher Li + ion incorporation [9,10]. As a result, MFe 2 O 4 is expected to become an appealing anode material for LIBs. Nevertheless, MFe 2 O 4 suffers from huge volume expansion and material cracking problems during the periodic charge-discharge process, which results in rapid fading capability [11,12]. Two common approaches have been proposed to address the above issues. One is to prepare variously nanostructured (such as hollow nanospheres, nanowires, nanotubes) [13][14][15][16] composite. Nanostructured MFe 2 O 4 can shorten the diffusion paths of Li + ion, leading to higher electrochemical activity.
The other route is to introduce carbon materials, such as carbon nanotubes (CNTs) and graphene, to fabricate MFe 2 O 4 /carbon composites [17][18][19]. Among them, nest structure Figure 1a illustrates the synthesis route of CoFe 2 O 4 /GNRs composites by simple refluxing. CNTs were unzipped to obtain GNRs [30]. After that, both GNRs and cobalt organic salt were dispersed in benzyl ether solution under agitation. Thereafter, aqueous solution was refluxed at 220 • C for 0.5 h, during which Co(acac) 3 decomposed to some CoFeO nanoclusters. Then, borders of GNRs filled with dangling C-C bonds were occupied by the CoFeO nanoclusters. Along with the reaction, CoFeO nanoclusters in situ grew and agglomerated as CoFe 2 O 4 in the surface, interlayers, and edges of GNRs. After drying, the magnetically induced attraction of CoFe 2 O 4 began to focus on the GNRs joints, causing CoFe 2 O 4 /GNRs composites to self-assemble. The synthesis procedure of NiFe 2 O 4 /GNRs is similar to CoFe 2 O 4 /GNRs, except nickel organic is used. The structure of the products was first investigated by SEM. Figure 1b-g represents the SEM images for MFe 2 O 4 /GNRs composites. The hybrids possessed a porous structure in the low-magnified SEM. From the high-magnification SEM (Figure 1c,f), the surface of the GNRs was anchored by metal oxide nanoparticles, and it can be clearly seen that some aggregates of MFe 2 O 4 were in the junctions of GNRs. The aggregates of MFe 2 O 4 acted as a "crosslink agent" that fitted together the GNRs to build a 3D porous structure. As presented in Figure 1b, Figure 1d,g, demonstrating that the C, Ni, Co, Fe, and O are gathered in the junction of GNRs and distributed uniformly along GNRs. The microstructure and composition of the MFe2O4/GNRs were confirmed by the TEM. As shown in Figure 2a,d, there are numerous agglomerated metal oxide nanoparticles at the joints of GNRs. With the cross-linking function of these metal oxide nanoparticles, the GNRs self-assembled into a 3D structure. Additionally, it can be observed from Figure 2b,e that the MFe2O4 in an average size of 3-8 nm was uniformly distributed on both exteriors and interior layers of the GNRs in the hybrids ( Figure S2). It was noted that suspended particles of MFe2O4 were found at the borders of GNRs in Figure 2c,f. It was proven that the unsaturated C-C bond at the edge of the GNRs can facilitate the nucleation and growth of MFe2O4. Figure 2c showed the high-resolution lattice images of CoFe2O4/GNRs nanoparticles in CoFe2O4/GNRs composite. The lattice fringes with a lattice spacing of 0.30 nm and 0.25 nm corresponded to the (220) and (311) planes of CoFe2O4, respectively [31]. Figure 2f showed the high-resolution lattice images of NiFe2O4 nanoparticles. The lattice fringes with a lattice spacing of 0.25 nm were assigned to the (311) planes of NiFe2O4 [32]. The microstructure and composition of the MFe 2 O 4 /GNRs were confirmed by the TEM. As shown in Figure 2a,d, there are numerous agglomerated metal oxide nanoparticles at the joints of GNRs. With the cross-linking function of these metal oxide nanoparticles, the GNRs self-assembled into a 3D structure. Additionally, it can be observed from Figure 2b,e that the MFe 2 O 4 in an average size of 3-8 nm was uniformly distributed on both exteriors and interior layers of the GNRs in the hybrids ( Figure S2). It was noted that suspended particles of MFe 2 O 4 were found at the borders of GNRs in Figure 2c,f. It was proven that the unsaturated C-C bond at the edge of the GNRs can facilitate the nucleation and growth of MFe 2 O 4 . Figure 2c showed the high-resolution lattice images of CoFe 2 O 4 /GNRs nanoparticles in CoFe 2 O 4 /GNRs composite. The lattice fringes with a lattice spacing of 0.30 nm and 0.25 nm corresponded to the (220) and (311) planes of CoFe 2 O 4 , respectively [31]. Figure 2f showed the high-resolution lattice images of NiFe 2 O 4 nanoparticles. The lattice fringes with a lattice spacing of 0.25 nm were assigned to the (311) planes of NiFe 2 O 4 [32].

XRD, Raman, and XPS Analysis
An X-ray diffraction (XRD) analysis was performed to assess the structure, crystallization, and purity of MFe 2 O 4 /GNRs. As shown in Figure 3a In addition, there was also a diffraction peak at 26.5 • (star symbol in Figure 3a), which was assigned to the GNRs [31,32]. Raman measurement was performed in Figure 3b [33,34], which are all consistent with the XRD results. XPS measurements were performed to further study the surface chemical state of the composites (Figure 3d). For MFe 2 O 4 /GNRs, there were several major peaks, assigned to C, O, Fe, Co, and Ni, respectively. In the high-resolution XPS spectra, C 1s of CoFe 2 O 4 /GNRs had a graphitic C-C/C=C peak at 286.7 eV, together with C=O (287.4 eV), as revealed in Figure 3e [32]. The C-C/C=C and C-O peak of NiFe 2 O 4 /GNRs are located at 285.4 eV and 287.6eV, respectively ( Figure 3i). The Co 2p3/2 and Co 2p1/2 peaks of the CoFe 2 O 4 /GNRs were located at 779.4 and 795.8 eV with a 16.4 eV peak-to-peak separation (Figure 3f), indicating the Co(II) oxidation state [35]. The locations of the Fe 2p peaks (Fe 2p1/2 at 724.5 eV and Fe 2p3/2 at 710.4 eV) in Figure 3g verify the presence of Fe 3+ cations in CoFe 2 O 4 . The O 1s spectrum in Figure 3h demonstrates two oxygen attributions. The peaks of 529.7 and 531.2 eV represent a metal-oxygen bond and some surface defect sites, respectively. The deconvolution of the Ni 2p spectrum demonstrates four peaks in Figure 3j. At binding energies of 855.7 eV and 876.3 eV are two main peaks followed by two shake-up satellites. Moreover, the signals of Fe 2p3/2 and Fe 2p1/2 in NiFe 2 O 4 peaks were found at 724.7 eV and 711.9 eV, respectively, indicating that the Fe is in the typical trivalent oxidation state ( Figure 3k). Peaks of O 1s of NiFe 2 O 4 are denoted as crystal lattice oxygen and chemisorbed oxygen in Figure 3l [36,37].

XRD, Raman, and XPS Analysis
An X-ray diffraction (XRD) analysis was performed to assess the structure, crystallization, and purity of MFe2O4/GNRs. As shown in Figure 3a, for CoFe2O4/GNRs composite, the diffraction peaks at 18.3, 30.2, 35.5, 43.2, 57.1, and 62.7 correspond to the lattice planes (220), (311), (400), (511), and (440), respectively, of cubic spinel structure (JCPDS No. 22-1086 and NiFe2O4 with JCPDS card no. 86-2267). In addition, there was also a diffraction peak at 26.5°(star symbol in Figure 3a), which was assigned to the GNRs [31,32]. Raman measurement was performed in Figure 3b,c. D, G, and 2D bands at 1340, 1582, and 2678 cm −1 stand for GNRs [32] on account of the strong disorder carbon's in-plane vibrations and graphitic carbon's in-plane vibrations. The three peaks of GNRs in MFe2O4/GNRs are basically in the same position as those in GNRs alone, but the strength of all three peaks is significantly reduced since GNRs are combined with MFe2O4. In the enlarged part of the figure, both the composites show three major peaks. CoFe2O4 shows at 680, 532, and 320 cm −1 . Further, the peaks of NiFe2O4 are at 672, 470, and 323 cm −1 due to the low Ramanintensity nature of MFe2O4 [33,34], which are all consistent with the XRD results. XPS measurements were performed to further study the surface chemical state of the composites (Figure 3d). For MFe2O4/GNRs, there were several major peaks, assigned to C, O, Fe, Co, and Ni, respectively. In the high-resolution XPS spectra, C1s of CoFe2O4/GNRs had a graphitic C-C/C=C peak at 286.7 eV, together with C=O (287.4 eV), as revealed in Figure  3e

BET and TGA Analysis
The nitrogen adsorption-desorption isotherm and pore-size distribution curves are shown in Figure S3a,b. The Brunauer-Emmett-Teller (BET) specific surface areas of CoFe 2 O 4 /GNRs and NiFe 2 O 4 /GNRs were measured to be 47.7 m 2 g −1 and 27.5 m 2 g −1 , respectively. The sample exhibits a representative type IV curve, indicating a mesoporous structure with distributions ranging from 2 to 10 nm. The foam structure will help the rapid travel of electrolytic ions for accessing the interior MFe 2 O 4 in the lithiation-delithiation process.
The TGA curves for MFe 2 O 4 /GNRs were shown in Figure S4. TGA is performed to analyze the mass ration of MFe 2 O 4 and GNRs. There were several stages of weight loss in Figure S4. The drop in weight below 200 • C can be ascribed to the evaporation of water. The typical weight loss between 200 • C and 550 • C can be ascribed to the oxidation of the GNRs. The one after 550 • C was mainly due to the oxidation of MFe 2 O 4 in air. Based on the weight loss in Figure S4, the weight percentages of CoFe 2 O 4 and NiFe 2 O 4 in MFe 2 O 4 /GNRs were estimated to be~41% and~23%, respectively.

BET and TGA Analysis
The nitrogen adsorption-desorption isotherm and pore-size distribution curves are shown in Figure S3a,b. The Brunauer-Emmett-Teller (BET) specific surface areas of CoFe2O4/GNRs and NiFe2O4/GNRs were measured to be 47.7 m 2 g −1 and 27.5 m 2 g −1 , respectively. The sample exhibits a representative type IV curve, indicating a mesoporous structure with distributions ranging from 2 to 10 nm. The foam structure will help the rapid travel of electrolytic ions for accessing the interior MFe2O4 in the lithiation-delithiation process.

Electrochemistry Performance
Cyclic voltammetry (CV) tests were investigated in half-cell configurations to assess the Li + ion storage properties of the as-prepared MFe 2 O 4 /GNRs. Figure 4a-b showed the CV curves of the CoFe 2 O 4 /GNRs and NiFe 2 O 4 /GNRs in the first three cycles. Two cathodic peaks were observed at 0.57 and 0.53 V in the first cycle, corresponding to multistage electrochemical reduction reactions (lithiation) of MFe 2 O 4 /GNRs with a Li + ion and a partially irreversible solid electrolyte interphase (SEI) layer [32]. The main anodic peaks at 1.5-2.0 V for the two composites were ascribed to the oxidation reactions (delithiation) of MFe 2 O 4 ; in addition, the lithiation and delithiation peaks of GNRs were at 0.13-0.25 V and 0.15-0.24 V, proving that GNRs were helpful for Li + ion storage. The above reactions are as follows [38][39][40][41]: Cyclic voltammetry (CV) tests were investigated in half-cell configurations to assess the Li + ion storage properties of the as-prepared MFe2O4/GNRs. Figure 4a-b showed the CV curves of the CoFe2O4/GNRs and NiFe2O4/GNRs in the first three cycles. Two cathodic peaks were observed at 0.57 and 0.53 V in the first cycle, corresponding to multistage electrochemical reduction reactions (lithiation) of MFe2O4/GNRs with a Li + ion and a partially irreversible solid electrolyte interphase (SEI) layer [32]. The main anodic peaks at 1.5-2.0 V for the two composites were ascribed to the oxidation reactions (delithiation) of MFe2O4; in addition, the lithiation and delithiation peaks of GNRs were at 0.13-0.25 V and 0.15-0.24 V, proving that GNRs were helpful for Li + ion storage. The above reactions are as follows [38][39][40][41]:    In agreement with the CV curves, the potential platform at~0.79 V can be found during the first discharge (lithiation) process. CoFe 2 O 4 /GNRs and NiFe 2 O 4 /GNRs electrodes delivered initial discharge capacities of 1735 mAh g −1 and 1621 mAh g −1 and initial charge capacities of 1573 mAh g −1 and 1502 mAh g −1 , with cycle efficiency of 90% and 92%, respectively. The irreversible capacity losses of the two electrodes in the following cycles were probably associated with the formation of SEI and irreversible lithium loss. It was observed that the second and third cycles of the current curve almost overlapped with each other, proposing that the Li + ion storage of the electrodes was reversible in the following cycles ( Figure S5).
To evaluate the rate capability of MFe 2 O 4 /GNRs, the electrodes were cycled under various current densities from 0.2 to 10 A g −1 , and the results were described in Figure 5a and Table S1. With the increase in current density, the specific capacity of the composites decreases. Because of the slow transportation of Li + ions and insufficient lithiation-delithiation process, a number of "dead" Li + ions are generated, leading to a decrease in the capacity of the electrodes. However, when the current density exceeded 5 A g −1 , the capacity increased instead. This phenomenon was interpreted by the activation process to facilitate Li + ion pathways between the electrolyte and the electrode under small current density cycling [42]. decreases. Because of the slow transportation of Li + ions and insufficient lithiation-delithiation process, a number of "dead" Li + ions are generated, leading to a decrease in the capacity of the electrodes. However, when the current density exceeded 5 A g −1 , the capacity increased instead. This phenomenon was interpreted by the activation process to facilitate Li + ion pathways between the electrolyte and the electrode under small current density cycling [42]. Meanwhile, the capacity of CoFe2O4, NiFe2O4, CoFe2O4/GNRs, and NiFe2O4/GNRs composites can be measured to 937, 867, 1475, and 1235 mA h g −1 , respectively, when the current density returned to 0.2 A g −1 , indicating high reversible rate capability. Even at the higher current density of 5 A g −1 , the discharge capacities of CoFe2O4/GNRs and NiFe2O4/GNRs composite remain high at stable values of 1330 and 960 mAh g −1 , respectively. However, pure CoFe2O4 and NiFe2O4 cannot maintain capacity in such a high rate due to the lack of GNRs skeleton. In addition, we discovered that the CoFe2O4/GNRs electrode shows better rate performance than NiFe2O4/GNRs, CoFe2O4, and NiFe2O4 electrode. The reason might be associated with more stable states and more negative adsorption energy of CoFe2O4/GNRs, which could produce extra capacity through the pseudo-capacitive behavior. Moreover, the larger BET surface area and more metal oxides component of CoFe2O4/GNRs contribute to better performance in the rate test and the following tests.   Figure 5b. The CoFe 2 O 4 /GNRs electrode expresses the best cycle performance after 80 discharge/charge cycles with a reversible capacity of 1432 mAh g −1 . CoFe 2 O 4 (715 mAh g −1 ), NiFe 2 O 4 (569 mAh g −1 ), and NiFe 2 O 4 /GNRs (1085 mAh g −1 ) electrodes with relatively smaller specific capabilities exhibited cycle stability similar to CoFe 2 O 4 /GNRs. When we increased the current density to 1 A g −1 , the results were shown in Figure 5c. After 100 cycles, the discharge capacity of CoFe 2 O 4 /GNRs and NiFe 2 O 4 /GNRs was 1720 mAh g −1 and 1414 mAh g −1 , respectively, while CoFe 2 O 4 and NiFe 2 O 4 were 300 mAh g −1 and 201 mAh g −1 , respectively. Compared to the previously reported MFe 2 O 4 electrodes, the discharge capacity and cycle stability of the electrode materials were superior (Table S2). Furthermore, the capacities that increased gradually in the initial cycles could be the interfacial Li + ion storage or the reversible polymer film produced by Co-, Ni-, and Fe-activated electrolyte degradation [32,42]. In the cycles, network GNRs can be used as the support of MFe 2 O 4 /GNRs, which can inhibit the volume changes in MFe 2 O 4 during the charge-discharge process. In the meantime, the embedded MFe 2 O 4 in the interlayers of GNRs would prevent the re-stacking and provide extra Li + ion storage accommodation. Moreover, the transport channels in porous GNRs can speed up the conduction of electrons/ions. Figure S6 showed the SEM images of MFe 2 O 4 /GNRs electrodes after 100 cycles at 1A g −1 , illustrating that the network remains intact in the process of cycles, which ensured excellent electrochemical performance. However, the CoFe 2 O 4 and NiFe 2 O 4 electrodes detached from the substrate and seriously agglomerated.
To obtain better insight into the mechanism of the electrode reactions in the unique MFe 2 O 4 /GNRs architectures, we performed electrochemical impedance spectroscopy (EIS) for both electrodes, as shown in Figure 5d,e. R ct , R SEI , and R s correspond to the semicircle in the high-frequency region of typical Nyquist plots; CPE relates to the constant phase element [15] on the basis of equivalent electrical circuit mode. The R ct of the CoFe 2 O 4 /GNRs and NiFe 2 O 4 /GNRs electrodes is 9.5 Ω and 16.8 Ω, respectively, which is smaller than CoFe 2 O 4 and NiFe 2 O 4 electrodes, showing that GNRs can effectively reduce the resistance of the anode. In the low-frequency region, the sloping line refers to the Warburg impedance (Zw); the slope of the CoFe 2 O 4 /GNRs electrode is higher than that of the NiFe 2 O 4 /GNRs, CoFe 2 O 4 , and NiFe 2 O 4 electrodes, which implies that the CoFe 2 O 4 /GNRs electrode has lower Li + ion diffusion resistance [31] (Table S3).

DFT Analysis
To further explain the effect of GNRs, we performed DFT calculations focused on the adsorption abilities towards Li + ions and charge distribution after intercalation of Li + ion [43,44]. CoFe 2 O 4 and NiFe 2 O 4 are semiconductors with band gaps of about 1.2 and 1.4 eV, respectively (Figure 6a-d). Since VASP is considered as a pseudopotential, the calculated value is relatively small. After the wrapping of GNRs, the hybrid orbitals appear near the Fermi level, leading to better electrical conductivity, which is consistent with the EIS results. According to Bader charge analysis, the number of transferred electrons for /GNRs were constructed to calculate the corresponding adsorption energy (∆E ads ). As can be seen in Figure 6e, the ∆E ads of lithium adsorption in the CoFe 2 O 4 /GNRs is remarkably higher than that of NiFe 2 O 4 /GNRs, CoFe 2 O 4 , and NiFe 2 O 4 /GNRs, exhibiting that the CoFe 2 O 4 /GNRs can propel the lithium process. Accordingly, the theoretical analysis and experimental results are In a great consistence that the charge/discharge kinetics can be enhanced via GNRs assembling CoFe 2 O 4 and NiFe 2 O 4 to boost the electric conductivity and the advanced intrinsic Li + storage.
The excellence electrochemical performance of MFe 2 O 4 /GNRs electrodes is probably due to the following reasons [17,45]: (1) as an elastic buffer layer, GNRs in the composites can not only avoid the volume expansion of CoFe 2 O 4 and NiFe 2 O 4 during lithiation and delithiation but also efficiently prevent the cracking or crumbling of anode materials [46,47]; (2) GNRs have premium electrical conductivity and are regarded as the electron transport path among CoFe 2 O 4 and NiFe 2 O 4 , reducing the inner resistance of Li + ion batteries; (3) the nano-sized active materials act as nanospacers to refrain the re-stacking of GNRs and therefore maintain their high active surface area, which is beneficial for enhancing the capacity of the composites.

Magnetic Performance Analysis
CoFe 2 O 4 and NiFe 2 O 4 nanoparticles are typical ferromagnetic nanomaterials. To clarify the effect of magnetism of CoFe 2 O 4 /GNRs and NiFe 2 O 4 /GNRs, the magnetic properties were studied by vibrating sample magnetometer system (VSM) at room temperature, as shown in Figure 7. Each CoFe 2 O 4 and NiFe 2 O 4 exhibited superparamagnetic behavior with little remanence and coercivity, implying that there is no remaining magnetization when the applied magnetic field is removed [48]. However, the saturation magnetization (Ms) values of CoFe 2 O 4 /GNRs and NiFe 2 O 4 /GNRs nanocomposites were 11.8 emu/g and 40.4 emu/g, respectively. The saturation magnetization intensity is lower than that of their Molecules 2023, 28, 4069 9 of 13 bulk material. The main reason for the low saturation magnetization could be the size effect of magnetic nanoparticles; the surface spin coupling of nanoparticles is weaker than that of bulk materials. The saturation magnetization of all MFe 2 O 4 /GNRs composites is much higher than that of MFe 2 O 4 and GNRs. It is noted that the saturation magnetization of NiFe 2 O 4 nanoparticles in the NiFe 2 O 4 /GNRs is improved 10 times as much as that on their own. The improved saturation magnetization of NiFe 2 O 4 /GNRs is probably due to the increased electron conductivity by the aid of GNRs. In addition, we also calculated the magnetism enhancement after recombination of GNRs; the reason may be the influence of the built-in electromagnetic field on lithium storage (Table S4). The excellence electrochemical performance of MFe2O4/GNRs electrodes is probably due to the following reasons [17,45]: (1) as an elastic buffer layer, GNRs in the composites can not only avoid the volume expansion of CoFe2O4 and NiFe2O4 during lithiation and delithiation but also efficiently prevent the cracking or crumbling of anode materials [46,47]; (2) GNRs have premium electrical conductivity and are regarded as the electron transport path among CoFe2O4 and NiFe2O4, reducing the inner resistance of Li + ion batteries; (3) the nano-sized active materials act as nanospacers to refrain the re-stacking of GNRs and therefore maintain their high active surface area, which is beneficial for enhancing the capacity of the composites.

Magnetic Performance Analysis
CoFe2O4 and NiFe2O4 nanoparticles are typical ferromagnetic nanomaterials. To clarify the effect of magnetism of CoFe2O4/GNRs and NiFe2O4/GNRs, the magnetic properties were studied by vibrating sample magnetometer system (VSM) at room temperature, as shown in Figure 7. Each CoFe2O4 and NiFe2O4 exhibited superparamagnetic behavior with little remanence and coercivity, implying that there is no remaining magnetization when the applied magnetic field is removed [48]. However, the saturation magnetization (Ms) values of CoFe2O4/GNRs and NiFe2O4/GNRs nanocomposites were 11.8 emu/g and 40.4 emu/g, respectively. The saturation magnetization intensity is lower than that of their bulk material. The main reason for the low saturation magnetization could be the size effect of magnetic nanoparticles; the surface spin coupling of nanoparticles is weaker than that of bulk materials. The saturation magnetization of all MFe2O4/GNRs composites is much higher than that of MFe2O4 and GNRs. It is noted that the saturation magnetization of NiFe2O4 nanoparticles in the NiFe2O4/GNRs is improved 10 times as much as that on their own. The improved saturation magnetization of NiFe2O4/GNRs is probably due to the increased electron conductivity by the aid of GNRs. In addition, we also calculated the  (Table S4). The CoFe2O4/GNRs and NiFe2O4/GNRs have maintained the dispersion and paramagnetism of magnetic nanoparticles, demonstrating their promising applications in magnetic devices.

Synthesis
The synthesis processes of GNRs and MFe2O4/GNRs were shown in supporting in-

Synthesis
The synthesis processes of GNRs and MFe 2 O 4 /GNRs were shown in supporting information [30].

Electrochemical Measurement
Coin cells (CR2430) were used to characterize the electrochemical properties of the sample. The working electrode was prepared by dropping the suspension of composites into a carbon nanofiber paper (CFP, d~16 mm, S~2 cm 2 ) without any binder, then dried in vacuum. Cyclic voltammetry (CV) was carried out on an electrochemical workstation (Solartron Metrology 1260 + 1287, Bognor Regis, UK) with the scanning rate of 0.1 mV s −1 (0.00-3.00 V vs. Li/Li + ). Arbin BT (2000, College Station, TX, USA) was used to measure charge-discharge properties of the samples. All the capacities showed in this work have deducted the contribution from the substrate.

Sample Characterization
X-ray diffraction (XRD, DMAX-2500PC, Tokyo, Japan) was used to record the phase structure of the samples using Cu/Kα radiation (kα = 0.15406 nm). Raman spectra were obtained by a Renishaw Invia Raman Microprobe (Wotton-under-Edge, UK) (100-3000 cm −1 ) using argon ion laser (514 nm). The morphologies of the as-obtained samples were studied using a field emission scanning electronic microscope (FE-SEM, Hitachi S-4800, Tokyo, Japan) and transmission electron microscope (TEM, Zeiss LIBRA 220 FEG, Oberkochen, Germany). X-ray photoelectron spectroscopy (XPS, ESCALAB 250 Thermo Fisher Scientific, Waltham, MA, USA) was carried out to study the surface chemical state of the composites. Brunner-Emmet-Teller (BET) measurements were operated on a Micromeritics ASAP 2020, USA. The content of the sample was analyzed by the thermogravimetric analysis (TGA Perkin ELMER TGA7, Waltham, MA, USA). The measurement of magnetic hysteresis loop was performed using a vibrating sample magnetometer (VSM Lake Shore 7400, Lake Shore Cryotronics, Westerville, OH, USA). All calculations were completed within the framework of the density functional theory (DFT) within the projector's enhanced plane wave method.

Conclusions
A general magnetically induced self-assembled graphene nanoribbons method is proposed to prepare network MFe 2 O 4 /GNRs. The formation of these MFe 2 O 4 /GNRs is due to the surface in situ growth of MFe 2 O 4 on GNRs and aggregation at the joints of GNRs, which is mainly driven by their own magnetic interactions. The unique framework of MFe 2 O 4 /GNRs can effectively prevent the shedding of MFe 2 O 4 , inhibiting the re-stacking of GNRs layers and shortening the Li+ ion transmission path. Consequently, CoFe 2 O 4 /GNRs and NiFe 2 O 4 /GNRs composites exhibited large reversible capacities (1432 and 1058 mAh g −1 at 0.1 A g −1 , 1720 mAh g −1 and 1414 mAh g −1 at 1 A g −1 , respectively), excellent cyclic performance, and good rate capabilities. The reason that the CoFe 2 O 4 /GNRs electrode shows better Li-ion battery performance than NiFe 2 O 4 /GNRs might be associated with the more stable states and more negative adsorption energy of CoFe 2 O 4 /GNRs according to DFT results, which could produce extra capacity through pseudo-capacitive behavior. Additionally, combining GNRs with MFe 2 O 4 helps to improve the magnetism of the MFe 2 O 4 , making MFe 2 O 4 /GNRs suitable for magnetic applications, such as magnetically mediated targeted drug delivery and magnetic resonance imaging.  Figure S1: (a,b) SEM images of as-prepared CoFe 2 O 4 ; (b-d) SEM images of as-prepared NiFe 2 O 4 ; Figure S2 (a-e) TEM images of as-prepared CoFe 2 O 4 /GNRs; Figure S3: The nitrogen adsorption/desorption isotherms and porosity distribution of (a,b) CoFe 2 O 4 /GNRs and (c,d) NiFe 2 O 4 /GNRs; Figure S4: TGA curves of CoFe 2 O 4 /GNRs and NiFe 2 O 4 /GNRs.; Figure S5 Table S1: Rate capability of the samples; Table S2: Comparison of the electrochemical performance of some Li-ion battery anodes; Table S3: The EIS simulation parameters of as-prepared samples; Table S4: Saturation magnetization of as-prepared samples. References [49][50][51] are cited in the supplementary materials.

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Informed Consent Statement: Not applicable.

Data Availability Statement:
The data that support the findings of this study are available from the authors.