A Hierarchical SnO2@Ni6MnO8 Composite for High-Capacity Lithium-Ion Batteries

Semiconductor-based composites are potential anodes for Li-ion batteries, owing to their high theoretical capacity and low cost. However, low stability induced by large volumetric change in cycling restricts the applications of such composites. Here, a hierarchical SnO2@Ni6MnO8 composite comprising Ni6MnO8 nanoflakes growing on the surface of a three-dimensional (3D) SnO2 is developed by a hydrothermal synthesis method, achieving good electrochemical performance as a Li-ion battery anode. The composite provides spaces to buffer volume expansion, its hierarchical profile benefits the fast transport of Li+ ions and electrons, and the Ni6MnO8 coating on SnO2 improves conductivity. Compared to SnO2, the Ni6MnO8 coating significantly enhances the discharge capacity and stability. The SnO2@Ni6MnO8 anode displays 1030 mAh g−1 at 0.1 A g−1 and exhibits 800 mAh g−1 under 0.5 A g−1, along with high Coulombic efficiency of 95%. Furthermore, stable rate performance can be achieved, indicating promising applications.


Introduction
Given the numerous applications of lithium-ion (Li-ion) batteries in our daily lives, the development of high-performance Li-ion batteries has attracted increasing attention, as they are important sources of clean and renewable energy.The demands of next-generation Liion batteries include sufficient energy density and improved safety and cycle life.However, at present, graphite anodes with a low theoretical capacity are dominant, restricting the development of emerging Li-ion batteries [1][2][3][4].In addition, lithium dendrite formed under high current density is associated with safety problems.Therefore, commercial graphite anodes cannot satisfy current demands.In recent years, researchers have developed several promising anode materials with increased capacity and safety with potential to replace graphite to improve performance [5,6].Many nanostructured anodes have been reported, such as metal oxides (e.g., MnO 2 , GeO 2 , Co 3 O 4 , Fe 2 O 3 , SnO 2 , etc.) [7][8][9][10] and alloyingdealloying mechanism-based anodes (e.g., Si, Si/SiC, Si/C/SiC, Si/Ge, etc.) [11][12][13], many of which exhibit satisfactory capacity and rate performance compared to graphite-based anodes, given their high theoretical capacities.The application of such anodes is still limited by defects, such as large volume change and poor conductivity [14][15][16][17][18][19][20][21].
SnO 2 has been widely considered one of the most promising anodes among metal dioxides in recent years, owing to its abundance, environmental friendliness, and Li-ion storage performance.Nevertheless, its theoretical capacity (782 mAh g −1 ) and the volume change of SnO 2 in lithiation/delithiation lead to a rapid decrease in capacity.The low conductivity of SnO 2 also leads to poor rate performance and low capacity retention.Furthermore, large volume change leads to electrode pulverization [22][23][24].Liu et al. reported a carbon-coating-layered SnO 2 hollow sphere, which evenly coated SnO 2 with a carbon nanolayer [25].The hollow structure and carbon buffer layer improved the capacity significantly.Zhou et al. synthesized a hollow SnO 2 @C combining SnO 2 with carbon.The hollow sphere ensured structural stability during long-term charge and discharge, exhibiting stability of 1628 mAh g −1 under 0.1 A g −1 [26].Liu and colleagues reported a graphene-mesoporous SnO 2 combining SnO 2 with conductive graphene, which exhibited improved conductivity [27].
In addition, bimetal oxides such as CuCo 2 O 4 , ZnFe 2 O 4 , CoMn 2 O 4 , ZnCo 2 O 4 , and ZnMn 2 O 4 exhibit satisfactory electronic conductivity and reversible capacity, which may be caused by the heterovalent cations and corresponding general redox reactions [28,29].Zhang et al. prepared a yolk-shell CoMn 2 O 4 microsphere using a carbon template with a solvothermal method [30].The yolk-shell structures of CoMn 2 O 4 microspheres buffered the volumetric change in Li + insertion/extraction, thereby reducing electrochemical pulverization.CoMn 2 O 4 -based anode exhibited 1643 mAh g −1 at 0.1 A g −1 .Ren et al. presented ZnCo 2 O 4 @reduced graphene oxide nanocomposites; ZnCo 2 O 4 nanoparticles were anchored uniformly and compactly on reduced graphene oxide.High specific surface area and mesoporous structure provided a large contact area with an electrolyte and shortened the transferring pathway of the Li + ions [31].Chen et al. prepared a hollow panpipe-like ZnMn 2 O 4 nanocomposite, which was conducive to the penetration of the electrolyte during the charge/discharge process [32].However, the intercalation/deintercalation of Li + in those composites would cause considerable volume change during cycling, resulting in serious agglomeration of the materials and the exfoliation of the electrode materials from the collector, rapidly reducing the performance.Engineering optimal structures to efficiently accommodate the volume change has become an important strategy to improve Li storage properties [33,34].
Here, we developed a hierarchical SnO 2 @Ni 6 MnO 8 composite comprising Ni 6 MnO 8 nanoflakes growing on the surface of a three-dimensional (3D) SnO 2 , as illustrated in Figure 1.The SnO 2 @Ni 6 MnO 8 composite was prepared through a two-step hydrothermal method.After calcination, hollow SnO 2 was formed, providing space for the penetration of electrolytes, benefitting Li + ion diffusion and accommodating the volumetric change during cycling.The special structure and composition of composites have rarely been reported.Composite-based anodes achieve positive electrochemical performance.The capacity remains 1030 mAh g −1 after cycling 50 times under 0.1 A g −1 and 800 mAh g −1 under 0.5 A g −1 , in addition to high Coulombic efficiency exceeding 95% and recoverable rate performance.SnO2 has been widely considered one of the most promising anodes among metal dioxides in recent years, owing to its abundance, environmental friendliness, and Li-ion storage performance.Nevertheless, its theoretical capacity (782 mAh g −1 ) and the volume change of SnO2 in lithiation/delithiation lead to a rapid decrease in capacity.The low conductivity of SnO2 also leads to poor rate performance and low capacity retention.Furthermore, large volume change leads to electrode pulverization [22][23][24].Liu et al. reported a carbon-coating-layered SnO2 hollow sphere, which evenly coated SnO2 with a carbon nanolayer [25].The hollow structure and carbon buffer layer improved the capacity significantly.Zhou et al. synthesized a hollow SnO2@C combining SnO2 with carbon.The hollow sphere ensured structural stability during long-term charge and discharge, exhibiting stability of 1628 mAh g −1 under 0.1 A g −1 [26].Liu and colleagues reported a graphene-mesoporous SnO2 combining SnO2 with conductive graphene, which exhibited improved conductivity [27].
In addition, bimetal oxides such as CuCo2O4, ZnFe2O4, CoMn2O4, ZnCo2O4, and ZnMn2O4 exhibit satisfactory electronic conductivity and reversible capacity, which may be caused by the heterovalent cations and corresponding general redox reactions [28,29].Zhang et al. prepared a yolk-shell CoMn2O4 microsphere using a carbon template with a solvothermal method [30].The yolk-shell structures of CoMn2O4 microspheres buffered the volumetric change in Li + insertion/extraction, thereby reducing electrochemical pulverization.CoMn2O4-based anode exhibited 1643 mAh g −1 at 0.1 A g −1 .Ren et al. presented ZnCo2O4@reduced graphene oxide nanocomposites; ZnCo2O4 nanoparticles were anchored uniformly and compactly on reduced graphene oxide.High specific surface area and mesoporous structure provided a large contact area with an electrolyte and shortened the transferring pathway of the Li + ions [31].Chen et al. prepared a hollow panpipe-like ZnMn2O4 nanocomposite, which was conducive to the penetration of the electrolyte during the charge/discharge process [32].However, the intercalation/deintercalation of Li + in those composites would cause considerable volume change during cycling, resulting in serious agglomeration of the materials and the exfoliation of the electrode materials from the collector, rapidly reducing the performance.Engineering optimal structures to efficiently accommodate the volume change has become an important strategy to improve Li storage properties [33,34].
Here, we developed a hierarchical SnO2@Ni6MnO8 composite comprising Ni6MnO8 nanoflakes growing on the surface of a three-dimensional (3D) SnO2, as illustrated in Figure 1.The SnO2@Ni6MnO8 composite was prepared through a two-step hydrothermal method.After calcination, hollow SnO2 was formed, providing space for the penetration of electrolytes, benefitting Li + ion diffusion and accommodating the volumetric change during cycling.The special structure and composition of composites have rarely been reported.Composite-based anodes achieve positive electrochemical performance.The capacity remains 1030 mAh g −1 after cycling 50 times under 0.1 A g −1 and 800 mAh g −1 under 0.5 A g −1 , in addition to high Coulombic efficiency exceeding 95% and recoverable rate performance.reactor and reacted at 180 • C for 6 h.Then, the obtained precipitate was washed with water and ethanol alternately by centrifugation, then dried in an oven.

Preparation of SnO 2 @Ni 6 MnO 8 Composite
An amount of 0.1 g of the SnO 2 /C precursor was ultrasonically dispersed in 40 mL of deionized water.Then, 300 mg of Ni(NO 3 ) 2 •6H 2 O, 530 mg of Mn(NO 3 ) 2 •4H 2 O, 140 mg of hexamethylenetetramine, and 29 mg of trisodium citrate dihydrate were subsequently added into the above solution under ultrasonic treatment.The solution was placed in the reactor at 140 • C for 6 h.Then, it was taken out and cooled.The sample was washed and dried at 60 • C and annealed at 500 • C for 1 h in air with a heating rate of 5 • C per min.

Electrochemical Measurement
A coin cell system was used to measure electrochemical performance.First, 3D SnO 2 @Ni 6 MnO 8 , carbon black, and carboxymethyl cellulose were mixed (mass ratio = 8:1:1) and coated on Cu foil.After drying at 60 • C in a vacuum overnight, the sample was cut into small discs.2032-typed cells were assembled by using a glove box (Mikrouna, Super 1220/750/900).Li foil was used as the counter electrode, and the Celgard polypropylene film was used as a separator.Electrolytes contained 1 M LiPF 6 in ethyl carbonate and diethyl carbonate (volume ratio = 1:1).Galvanostatic charge-discharge was tested on a measuring system (Netware, CT-4008).
Figure 2f,g are SEM images of SnO 2 @Ni 6 MnO 8 calcined at 500 • C. The morphology remains unchanged compared to that before heat treatment.Dense nanoflakes are coated on the surface without breaking.Figure 2h shows a TEM image of the composite.Because the tubular structure is covered by thick and dense nanoflakes, it is difficult to observe the SnO 2 inside.The TEM image shows thin nanoflakes, which are beneficial in terms of providing large active sites for reaction with Li + ions in Li-ion batteries.The XRD pattern of SnO 2 @Ni 6 MnO 8 is displayed in Figure 2i.Compared with the diffraction profile shown in Figure 2c, diffraction peaks at 37.3 • , 43.4 • , and 63.1 • are observed, corresponding to the (222), (004), and (044) planes of Ni 6 MnO 8 (JCPDS card #49-1295), respectively, which verify a secondary growth.The BET measurements of SnO 2 @C and SnO 2 @Ni 6 MnO 8 are shown in Figure 3. Owing to the coating of dense Ni 6 MnO 8 nanoflakes, the BET surface area of SnO 2 @Ni 6 MnO 8 is as high as 69.6 cm 2 g −1 compared to SnO 2 @C (10.3 m 2 g −1 ).The inserts show that pore distributions of SnO 2 @C and SnO 2 @Ni 6 MnO 8 dominate at about 3 nm and 15 nm.The increased surface area and large pore volume promotes electrolyte penetration.
coral-like structure with a diameter of 400-500 nm.The XRD pattern of SnO2/C precursor is presented in Figure 2c.Four diffraction peaks were detected at 26.6°, 33.9°, 51.8°, and 65.9°, corresponding to the (110), ( 101), (211), and (301) planes of tetragonal SnO2 (JCPDS card #41-1445), respectively [35,36]. Figure 2d,e show the SnO2@Ni6MnO8 after in situ growth of Ni6MnO8 on SnO2/C precursor.The surface of SnO2/C is completely coated by dense Ni6MnO8 nanoflakes.The 3D profile is well-maintained, indicating a robust structure.Figure 2f,g are SEM images of SnO2@Ni6MnO8 calcined at 500 °C.The morphology remains unchanged compared to that before heat treatment.Dense nanoflakes are coated on the surface without breaking.Figure 2h shows a TEM image of the composite.Because the tubular structure is covered by thick and dense nanoflakes, it is difficult to observe the SnO2 inside.The TEM image shows thin nanoflakes, which are beneficial in terms of providing large active sites for reaction with Li + ions in Li-ion batteries.The XRD pattern of SnO2@Ni6MnO8 is displayed in Figure 2i.Compared with the diffraction profile shown in Figure 2c, diffraction peaks at 37.3°, 43.4°, and 63.1° are observed, corresponding to the (222), (004), and (044) planes of Ni6MnO8 (JCPDS card #49-1295), respectively, which verify a secondary growth.The BET measurements of SnO2@C and SnO2@Ni6MnO8 are shown in Figure 3. Owing to the coating of dense Ni6MnO8 nanoflakes, the BET surface area of SnO2@Ni6MnO8 is as high as 69.6 cm 2 g −1 compared to SnO2@C (10.3 m 2 g −1 ).The inserts show that pore distributions of SnO2@C and SnO2@Ni6MnO8 dominate at about 3 nm and 15 nm.The increased surface area and large pore volume promotes electrolyte penetration.Figure 2f,g are SEM images of SnO2@Ni6MnO8 calcined at 500 °C.The mor remains unchanged compared to that before heat treatment.Dense nanoflakes ar on the surface without breaking.Figure 2h shows a TEM image of the composite.the tubular structure is covered by thick and dense nanoflakes, it is difficult to obs SnO2 inside.The TEM image shows thin nanoflakes, which are beneficial in t providing large active sites for reaction with Li + ions in Li-ion batteries.The XRD of SnO2@Ni6MnO8 is displayed in Figure 2i.Compared with the diffraction profil in Figure 2c, diffraction peaks at 37.3°, 43.4°, and 63.1° are observed, correspondin (222), (004), and (044) planes of Ni6MnO8 (JCPDS card #49-1295), respectively, wh ify a secondary growth.The BET measurements of SnO2@C and SnO2@Ni6M shown in Figure 3. Owing to the coating of dense Ni6MnO8 nanoflakes, the BET area of SnO2@Ni6MnO8 is as high as 69.6 cm 2 g −1 compared to SnO2@C (10.3 m 2 inserts show that pore distributions of SnO2@C and SnO2@Ni6MnO8 dominate at nm and 15 nm.The increased surface area and large pore volume promotes ele penetration.An SEM image of SnO 2 @Ni 6 MnO 8 and the elemental distribution are displayed in Figure 4. Sn, Ni, Mn, and O elements are evenly distributed throughout the composite.The EDS spectrum shown in Figure 4f confirms the presence of Sn, O, Mn, and Ni.The signal of Si is ascribed to the substrate used for measurement [37].The profiles of Mn and Sn do not match well with the profile, which could be caused by the covering of signals by Ni and O, as has previously been reported in some surface-coated composites [38]. signal of Si is ascribed to the substrate used for measurement [37].The profiles of Mn and Sn do not match well with the profile, which could be caused by the covering of signals by Ni and O, as has previously been reported in some surface-coated composites [38].The XPS spectra of SnO2@Ni6MnO8 were measured.The coexistence of Mn, Ni, C, O, and Sn is verified by the XPS survey spectrum (Figure 5a).The XPS spectrum of C 1s (Figure 5b) is characterized by three peaks located at 284.4, 286.2, and 288.4 eV and indexed to C-C, O-C, and C═O groups, respectively [39].The Ni 2p spectrum shows four peaks at 853.4, 861.5, 870.4,and 879.9 eV, corresponding to Ni 2+ (Figure 5c). Figure 5d displays the Mn 2p peaks centered at 637.7 and 653.7eV, which are ascribed to Mn 2p3/2 and Mn 2p1/2, respectively.The Sn 3d spectrum (Figure 5e) verifies the existence of Sn 3d3/2 and Sn 3d5/2, confirming Sn 4+ of SnO2 [27].Some shifts of Sn elements occurred, which may be ascribed to the impact of the Ni6MnO8 coating on the surface of SnO2 [40].In the O 1s spectrum (Figure 5f), 529.4 eV is associated with metal-oxygen bonds, whereas the other peaks are attributed to the lattice oxygen of Ni6MnO8 (530.9 eV) and surface-adsorbed oxygen (531.8 eV) [33, 41,42].Furthermore, the atomic percentages of Sn, Ni, Mn, and O are 1.46%, 0.99%, 1.72%, and 71.49%, respectively.signal of Si is ascribed to the substrate used for measurement [37].The profiles of Mn and Sn do not match well with the profile, which could be caused by the covering of signals by Ni and O, as has previously been reported in some surface-coated composites [38].The XPS spectra of SnO2@Ni6MnO8 were measured.The coexistence of Mn, Ni, C, O, and Sn is verified by the XPS survey spectrum (Figure 5a).The XPS spectrum of C 1s (Figure 5b) is characterized by three peaks located at 284.4, 286.2, and 288.4 eV and indexed to C-C, O-C, and C ═ O groups, respectively [39].The Ni 2p spectrum shows four peaks at 853.4, 861.5, 870.4,and 879.9 eV, corresponding to Ni 2+ (Figure 5c). Figure 5d dis-plays the Mn 2p peaks centered at 637.7 and 653.7eV, which are ascribed to Mn 2p3/2 and Mn 2p1/2, respectively.The Sn 3d spectrum (Figure 5e) verifies the existence of Sn 3d3/2 and Sn 3d5/2, confirming Sn 4+ of SnO2 [27].Some shifts of Sn elements occurred, which may be ascribed to the impact of the Ni6MnO8 coating on the surface of SnO2 [40].In the O 1s spectrum (Figure 5f), 529.4 eV is associated with metal-oxygen bonds, whereas the other peaks are attributed to the lattice oxygen of Ni6MnO8 (530.9 eV) and surface-adsorbed oxygen (531.8 eV) [33,41,42].Furthermore, the atomic percentages of Sn, Ni, Mn, and O are 1.46%, 0.99%, 1.72%, and 71.49%, respectively.O groups, respectively [39].The Ni 2p spectrum shows four peaks at 853.4, 861.5, 870.4,and 879.9 eV, corresponding to Ni 2+ (Figure 5c). Figure 5d displays the Mn 2p peaks centered at 637.7 and 653.7eV, which are ascribed to Mn 2p 3/2 and Mn 2p 1/2 , respectively.The Sn 3d spectrum (Figure 5e) verifies the existence of Sn 3d 3/2 and Sn 3d 5/2 , confirming Sn 4+ of SnO 2 [27].Some shifts of Sn elements occurred, which may be ascribed to the impact of the Ni 6 MnO 8 coating on the surface of SnO 2 [40].In the O 1s spectrum (Figure 5f), 529.4 eV is associated with metal-oxygen bonds, whereas the other peaks are attributed to the lattice oxygen of Ni 6 MnO 8 (530.9 eV) and surface-adsorbed oxygen (531.8 eV) [33,41,42].Furthermore, the atomic percentages of Sn, Ni, Mn, and O are 1.46%, 0.99%, 1.72%, and 71.49%, respectively.signal of Si is ascribed to the substrate used for measurement [37].The profiles of Mn and Sn do not match well with the profile, which could be caused by the covering of signals by Ni and O, as has previously been reported in some surface-coated composites [38].The XPS spectra of SnO2@Ni6MnO8 were measured.The coexistence of Mn, Ni, C, O, and Sn is verified by the XPS survey spectrum (Figure 5a).The XPS spectrum of C 1s (Figure 5b) is characterized by three peaks located at 284.4, 286.2, and 288.4 eV and indexed to C-C, O-C, and C═O groups, respectively [39].The Ni 2p spectrum shows four peaks at 853.4, 861.5, 870.4,and 879.9 eV, corresponding to Ni 2+ (Figure 5c). Figure 5d displays the Mn 2p peaks centered at 637.7 and 653.7eV, which are ascribed to Mn 2p3/2 and Mn 2p1/2, respectively.The Sn 3d spectrum (Figure 5e) verifies the existence of Sn 3d3/2 and Sn 3d5/2, confirming Sn 4+ of SnO2 [27].Some shifts of Sn elements occurred, which may be ascribed to the impact of the Ni6MnO8 coating on the surface of SnO2 [40].In the O 1s spectrum (Figure 5f), 529.4 eV is associated with metal-oxygen bonds, whereas the other peaks are attributed to the lattice oxygen of Ni6MnO8 (530.9 eV) and surface-adsorbed oxygen (531.8 eV) [33,41,42].Furthermore, the atomic percentages of Sn, Ni, Mn, and O are 1.46%, 0.99%, 1.72%, and 71.49%, respectively.The electrochemical properties of 3D SnO 2 @Ni 6 MnO 8 nanocomposite and SnO 2 were measured from 0.01 to 3.0 V.The cycling performance at a current density of 0.1 A g −1 is presented in Figure 6a.The SnO 2 @Ni 6 MnO 8 anode shows a high specific capacity of 914 mAh g −1 .After 50 cycles, the SnO 2 @Ni 6 MnO 8 anode maintains a reversible capacity of 1030 mAh g −1 , and its Coulombic efficiency exceeds 96%.The first-cycle capacity of SnO 2 is 876 mAh g −1 .However, the capacity decreases rapidly to 509 mAh g −1 after the 50th cycle, indicating that the Ni 6 MnO 8 coating considerably enhances the capacity and stability.The slight increase in capacity is ascribed to the activation of the nanoscale composite.Figure 6b shows corresponding charge-discharge profiles of the SnO 2 @Ni 6 MnO 8 anode.There are two plateaus on the discharge curves, which correspond to Li + ion insertion in SnO 2 @Ni 6 MnO 8 .In the charging process, there are three plateaus at around 0.5-1.0V, 1.2-2.0V, and 2.0-3.0V, which are assigned to Li + ion extraction.Figure 6c shows the cycling performance at 0.5 A g −1 .After cycling 50 times, the SnO 2 anode shows a low capacity of 437 mAh g −1 .In contrast, the capacity of SnO 2 @Ni 6 MnO 8 is maintains as high as 800 mAh g −1 , with a Coulombic efficiency of about 95%, indicating a sufficient reversibility.Figure 6d shows the cycling curves with a similar shape to those presented in Figure 6b, representing the same electrochemical behaviors at a relatively high current density.Compared to some other reported anodes, the SnO 2 @Ni 6 MnO 8 anode exhibits a competitive performance, as shown in Table 1.The electrochemical properties of 3D SnO2@Ni6MnO8 nanocomposite and SnO2 were measured from 0.01 to 3.0 V.The cycling performance at a current density of 0.1 A g −1 is presented in Figure 6a.The SnO2@Ni6MnO8 anode shows a high specific capacity of 914 mAh g −1 .After 50 cycles, the SnO2@Ni6MnO8 anode maintains a reversible capacity of 1030 mAh g −1 , and its Coulombic efficiency exceeds 96%.The first-cycle capacity of SnO2 is 876 mAh g −1 .However, the capacity decreases rapidly to 509 mAh g −1 after the 50th cycle, indicating that the Ni6MnO8 coating considerably enhances the capacity and stability.The slight increase in capacity is ascribed to the activation of the nanoscale composite.Figure 6b shows corresponding charge-discharge profiles of the SnO2@Ni6MnO8 anode.There are two plateaus on the discharge curves, which correspond to Li + ion insertion in SnO2@Ni6MnO8.In the charging process, there are three plateaus at around 0.5-1.0V, 1.2-2.0V, and 2.0-3.0V, which are assigned to Li + ion extraction.Figure 6c shows the cycling performance at 0.5 A g −1 .After cycling 50 times, the SnO2 anode shows a low capacity of 437 mAh g −1 .In contrast, the capacity of SnO2@Ni6MnO8 is maintains as high as 800 mAh g −1 , with a Coulombic efficiency of about 95%, indicating a sufficient reversibility.Figure 6d shows the cycling curves with a similar shape to those presented in Figure 6b, representing the same electrochemical behaviors at a relatively high current density.Compared to some other reported anodes, the SnO2@Ni6MnO8 anode exhibits a competitive performance, as shown in Table 1. Figure 7a,b show the rate performance of the 3D SnO2@Ni6MnO8 nanocomposite. Figure 7a shows that the capacities are 819, 775, 655, and 339 mAh g −1 at 0.1, 0.2, 0.5, and 1.0 A g −1 , respectively.Once the rate returns to 0.1 A g −1 , the capacity recovers to 760 mAh g −1 .
The galvanostatic charge-discharge curve for each current density with a similar profile Figure 7a,b show the rate performance of the 3D SnO 2 @Ni 6 MnO 8 nanocomposite.Figure 7a shows that the capacities are 819, 775, 655, and 339 mAh g −1 at 0.1, 0.2, 0.5, and 1.0 A g −1 , respectively.Once the rate returns to 0.1 A g −1 , the capacity recovers to 760 mAh g −1 .The galvanostatic charge-discharge curve for each current density with a similar profile is shown in Figure 7b, indicating satisfactory reversibility.The excellent rate performance and cyclic reversibility of the SnO 2 @Ni 6 MnO 8 composite are attributed to the hierarchical morphology with nanoflakes growing in situ on a 3D structure, which provides efficient space to alleviate the volumetric expansion and enable rapid transport of electrons and ions.
is shown in Figure 7b, indicating satisfactory reversibility.The excellent rate performance and cyclic reversibility of the SnO2@Ni6MnO8 composite are attributed to the hierarchical morphology with nanoflakes growing in situ on a 3D structure, which provides efficient space to alleviate the volumetric expansion and enable rapid transport of electrons and ions.CV curves are shown in Figure 8.There are three peaks in the cathodic process at 0.34, 0.96, and 1.39 V, indicating Li + ion insertion in the SnO2/Ni6MnO8 anode.In contrast, the peaks at 0.61, 1.38, and 2.31 V are assigned to the extraction of Li + , in accordance with the charge-discharge curves shown in Figures 6 and 7.The Nyquist plots presented in Figure 9 show the charge transfer resistances for SnO2/Ni6MnO8 and SnO2/C anodes.The fitting values before and after cycling are about 130 and 200 Ω, respectively, indicating an improved conductivity after coating with Ni6MnO8.An interesting line of future research would be to obtain the diffusion coefficient through EIS spectra [43].CV curves are shown in Figure 8.There are three peaks in the cathodic process at 0.34, 0.96, and 1.39 V, indicating Li + ion insertion in the SnO 2 /Ni 6 MnO 8 anode.In contrast, the peaks at 0.61, 1.38, and 2.31 V are assigned to the extraction of Li + , in accordance with the charge-discharge curves shown in Figures 6 and 7.The Nyquist plots presented in Figure 9   CV curves are shown in Figure 8.There are three peaks in the cathodic process a 0.34, 0.96, and 1.39 V, indicating Li + ion insertion in the SnO2/Ni6MnO8 anode.In contras the peaks at 0.61, 1.38, and 2.31 V are assigned to the extraction of Li + , in accordance wit the charge-discharge curves shown in Figures 6 and 7.The Nyquist plots presented i Figure 9 show the charge transfer resistances for SnO2/Ni6MnO8 and SnO2/C anodes.Th fitting values before and after cycling are about 130 and 200 Ω, respectively, indicating a improved conductivity after coating with Ni6MnO8.An interesting line of future researc would be to obtain the diffusion coefficient through EIS spectra [43].

Conclusions
In summary, a hierarchical SnO 2 @Ni 6 MnO 8 composite was developed for a secondary battery anode with satisfactory Li-storage performance by the hydrothermal method.The presented approach is controllable and simple; however, we expect that a higher-yield preparation can be achieved in the future.The 3D structure provides a large space to buffer volumetric change during charge and discharge.The hierarchical surface shortens the transferring distance of Li + ions and electrons, and the Ni 6 MnO 8 coating on SnO 2 improves conductivity.Compared to SnO 2 , the coating of Ni 6 MnO 8 considerably enhances the discharge capacity and stability of the anode.The capacities of SnO 2 @Ni 6 MnO 8 anodes remain 1030 and 800 mAh g −1 after 50 cycles at 0.1 and 0.5 A g −1 , respectively, and the Coulombic efficiency continues to exceed 95%.Moreover, a high and reversible rate performance is also achieved.We believe that the 3D structure presented here and the high electrochemical performance opportunities to engineer emerging composites for secondary battery systems.However, potential challenges should also be considered in future investigations, such as extending the applicable materials, reducing the cost of reagents and steps during synthesis, avoiding the toxicity, etc.

2 . Experimental 2 . 1 .
Preparation of 3D SnO 2 /C Precursor First, 20 mmol sucrose was dissolved in 40 mL of deionized water.Then, 10 mmol SnCl 4 •5H 2 O (Aladdin Co. Ltd., Beijing, China) was added to the solution with continuous stirring.After complete dissolution, it was transferred into a PTFE-lined stainless-steel Materials 2022, 15, x FOR PEER REVIEW 4 of 11

Figure 2 .
Figure 2. (a,b) SEM images and (c) XRD pattern of SnO2/C precursor.SEM images of SnO2@Ni6MnO8 precursor (d,e) before and (f,g) after annealing.(h) TEM image of the Ni6MnO8 nanoflakes on the surface of SnO2.(i) XRD pattern of the SnO2@Ni6MnO8 composite.

Figure 4 .
Figure 4. (a) SEM and corresponding mapping images of (b) Ni, (c) Mn, (d) Sn, and (e) O. (f) EDS spectrum.The XPS spectra of SnO 2 @Ni 6 MnO 8 were measured.The coexistence of Mn, Ni, C, O, and Sn is verified by the XPS survey spectrum (Figure 5a).The XPS spectrum of C 1s (Figure 5b) is characterized by three peaks located at 284.4, 286.2, and 288.4 eV and indexed to C-C, O-C, and C
show the charge transfer resistances for SnO 2 /Ni 6 MnO 8 and SnO 2 /C anodes.The fitting values before and after cycling are about 130 and 200 Ω, respectively, indicating an improved conductivity after coating with Ni 6 MnO 8 .An interesting line of future research would be to obtain the diffusion coefficient through EIS spectra [43].

Table 1 .
Comparison of electrochemical performance of SnO 2 anodes.