Introduction

With the rapid development of society, the growing energy markets urgently require lithium-ion batteries with higher energy density [1,2,3]. However, the commercial graphite anode materials are far from the market demands because of the low theoretical specific capacity (372 mAh g−1) [4,5,6]. Therefore, seeking for high-performance, low-cost, and easy-to-prepare anode materials for lithium-ion batteries have become a popular research area for researchers [2, 7,8,9,10]. As novel anode materials, bimetallic oxides with high theoretical capacity were widely studied in the past decades [11,12,13,14,15,16,17,18,19]. Bimetallic oxides often undergo several chemical reactions during charge and discharge processes [20,21,22]. Due to the nature of metal, these reactions occur at different potentials, which can suppress the volume changes of the electrode material during reactions in some degree [22, 23]. As a common bimetallic oxide, ZnMn2O4 (ZMO) not only possesses the above advantages but also has a lower discharge voltage platform, and thus results in a higher output voltage, which drive researchers to study it extensively [13, 14, 24,25,26,27,28,29,30,31,32]. Yan’s group [31] obtained a high-performance mesoporous ZMO microtubule via a biomorphic strategy. The ZMO mesoporous microtubules exhibited larger surface area and faster reaction kinetics, which led to high capacity, rate capacity, and cycling stability in both lithium-ion and sodium-ion batteries. Zhang et al. [33] designed a 2D-on-3D structured ZnMn2O4@CMK-3 hybrid through a common hydrothermal method. Benefiting from its unique nanostructure, the ZnMn2O4@CMK-3 hybrid exhibited a superior lithium storage performance.

However, the volume changes of the bimetallic oxide during the charge and discharge processes are still inevitable [34]. After long-term cycles, electrode materials will pulverize and fall off from the current collector, resulting in fatal capacity degradation [35]. Moreover, the poor conductivity of bimetallic oxide hinders the ion diffusion and transfer during charge/discharge process. To overcome the above inherent drawbacks, the modifications of bimetallic oxides electrode materials are mainly focused on nanosization and carbon coating. Shen’s group [36] synthesized a unique hierarchical 3D ZnCo2O4 nanowire with high capacity, excellent cycling performance, and good rate capability. Lu et al. [37] prepared a thin carbon layer coated ZnSn2O4 mesoporous nanocube for LIBs and also achieved an excellent cycling performance. Furthermore, Jiang et al. [38] reported that the Ag/mesoporous carbon nanotube hybrids showed a better cycling performance than that of mesoporous carbon nanotube. Both experimental results and theoretical calculations indicated that the presence of Ag nanoparticles on mesoporous carbon nanotubes could inhibit the continuous generation of solid electrolyte interphase (SEI) film, which led to high initial coulombic efficiency (CE) and enhanced cycling performance. Inspired by this and enlightened by the intrinsic merits of ZMO, it is thus highly desirable to judiciously integrate Ag nanoparticles with ZMO in a close integration manner which would take full advantage of the synergistic interaction between them giving rise to promising electrochemical performances. However, to the best of our knowledge, the addition of Ag nanoparticles in ZMO to improve the electrochemical performances and cycle stability of ZMO as anode material in lithium ion battery has not been reported yet.

Herein, a simple and rapid coprecipitation method has been designed to synthesize ZnMn2O4–Ag (ZMO–Ag) hybrids with various weight ratios of Ag as anode materials for lithium ion battery. Ag/ZMO hybrids can be rapidly prepared with a relatively low cost and more likely to be applied in industrial production. The decoration of Ag nanoparticles not only can improve the conductivity of ZMO but also can effectively inhibit the continuous generation of SEI film, which definitely will benefit for the improvement of the initial CE. The electrochemical measurements verify that the optimum Ag/ZMO hybrid (ZMO–Ag–2) demonstrates higher reversible capacity and better cycling stability compared with that of pure ZMO. After 100 cycles at 100 mA g−1, the ZMO–Ag–2 still exhibits an ultra-high reversible capacity of 1300 mAh g−1, which proves that ZMO–Ag–2 probably can be considered a promising anode material for lithium-ion batteries in the future. Furthermore, this modification method provides a new idea for the follow-up design and modification of electrode materials.

Experimental

All of the reagents used in this article are analytical grade and used without further purification.

Synthesis of ZMO

Firstly, according to the mole ratio of the element, a certain amount of Zn(NO3)2 × 6H2O and Mn(NO3)2 were dissolved in 100 mL water and labeled as solution A. Another 100 mL saturated oxalate ethanol solution was prepared and labeled as solution B. Then, solution A was injected into solution B at a rate of 5 mL min−1 under continuous stirring. Subsequently, the precipitation was centrifuged, washed with DI water 3 times, and dried in the oven at 100 °C. ZMO was obtained after being calcined in tube furnace at 550 °C for 2 h.

Synthesis of ZMO–Ag

Briefly, an appropriate amount of AgNO3 was dissolved in solution A. Then, the rest of the process was similar to the preparation of ZMO. According to the theoretical weight ratio of Ag:ZMO = 0.5%, 1%, 3%, the obtained Ag/ZMO hybrids were labeled as ZMO–Ag–1, ZMO–Ag–2, ZMO–Ag–3, respectively.

Material characterizations

The crystal structure and bonding information of the as-prepared materials were characterized by XRD (Rigaku Ultima IV; Cu Kα radiation, 1.5406 Å) and Xplora plus, respectively. FESEM (Hitachi S–4800) was used to characterize the morphology features and EDS analysis was carried out in the SEM to determine the approximate element ratio. In addition, HRTEM (Tecnai, F30) was applied to characterize the crystal structure. XPS measurements were carried out on the XPS apparatus (PHI QUANTUM 2000) and all binding energies were referred to the C 1 s peak of 284.8 eV as an internal standard.

Preparation of electrodes and electrochemical characterization

Typically, 70 wt% as-prepared Ag/ZMO hybrids, 20 wt% acetylene black, and 10 wt% sodium carboxymethylcellulose were mixed into homogeneous slurry. Then, the black slurry was coated with copper foil collectors and dried at 120 °C under vacuum overnight to obtain anode electrodes. All of the electrochemical properties were tested with typical CR2032 coin half-cells. Metal lithium foil was used as the counter electrode, cellgard 2400 was used as the separator, and 1 M LiPF6 dissolved in ethylmethyl carbonate, ethylene carbonate, and diethyl carbonate (1:1:1 v/v/v) served as the electrolyte. The galvanostatic charge–discharge test was performed on LAND2001 with a voltage range of 0.01–3.0 V. The cyclic voltammetry (CV) was measured with electrochemical workstation (CHI630E) at a scan rate of 0.1 mV s−1 between 0.01 and 3.0 V. AC voltage amplitude of 10 mV was employed to measure electrochemical impedance spectroscopy by the Autolab electrochemical workstation (PGSTAT302N) within the frequency ranging from 10 mHz to 100 KHz.

Results and discussion

Figure 1 briefly describes the preparation process of Ag/ZMO hybrids. Precipitate containing uniform distributed MnC2O4, ZnC2O4, and Ag2C2O4 can be obtained by simply injecting aqueous solution A containing Ag+, Zn2+, and Mn2+ into a saturated oxalic acid ethanol solution. After being calcinated at 550 °C, mesoporous structured Ag/ZMO hybrids can be obtained due to the generation of massive CO2 and H2O gases.

Fig. 1
figure 1

The schematic illustration of the preparation process of Ag/ZMO composites

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Figure 2 shows the field-emission scanning electron microscopy (FESEM) images and elemental distribution of the ZMO, ZMO–Ag–1, ZMO–Ag–2, and ZMO–Ag–3, respectively. Figure 2a–h show the morphology of the ZMO, ZMO–Ag–1, ZMO–Ag–2, and ZMO–Ag–3 at different magnifications, which indicate that with the increase of Ag content, no morphology change has been observed. As can be seen from Fig. 2i–l, Zn, Mn, O, and Ag are uniformly distributed in the Ag/ZMO hybrids. In addition, energy-dispersive X-ray spectroscopy (EDS) analysis for all samples is conducted and the results are shown in Fig. S1. The EDS results demonstrate that the approximate weight ratio of Ag in the ZMO–Ag–1, ZMO–Ag–2, and ZMO–Ag–3 are 8.10%, 11.59%, and 21.04%, respectively, which is probably due to the difference of the solubility product constant of MnC2O4, ZnC2O4, and Ag2C2O4. During the reaction process, the concentration of oxalic acid will gradually decrease, resulting in incomplete precipitation of Zn2+ and Mn2+ in the limited reaction time. Therefore, the designed weight ratios of Ag:ZMO are 1%, 3%, and 5%, but the actual weight ratio are 8.10%, 11.59%, and 21.04%, respectively.

Fig. 2
figure 2

SEM images of the as-prepared ZMO (a, e), ZMO–Ag–1 (b, f), ZMO–Ag–2 (c, g), and ZMO–Ag–3 (d, h). Element mapping of the as-prepared ZMO (i), ZMO–Ag–1 (j), ZMO–Ag–2 (k), and ZMO–Ag–3 (l)

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The microstructure and morphology of ZMO–Ag–2 were further explored by high-resolution transmission electron microscopy (HRTEM). Combine the dark-field scanning transmission electron microscopy (DF-STEM) image (Fig. 3a) and elemental mapping images (Fig. 3d), Ag nanoparticles can be clearly observed and they are decorated on the ZMO matrix. Lattice fringes with d spacings of 0.232 and 0.303 nm are observed (Fig. 3c), which are in accordance with the (111) and (112) crystal planes of cubic Ag and spinel ZnMn2O4, respectively.

Fig. 3
figure 3

Dark-field STEM image (a), HRTEM images (b, c), and elemental mapping (d) of ZMO–Ag–2

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X-ray diffraction (XRD) patterns of the as-prepared materials and purchased Ag powder are shown in Fig. 4a. The characteristic peaks at 38.1°, 44.3°, and 64.4° can be accurately indexed to the (111), (200), and (220) planes of Ag (black line, JCPDS card No. 24–0783), respectively. While the peaks at 18.2°, 29.3°, 31.2°, 33.0°, 36.4°, 38.9°, 44.8°, 50.7°, 51.9°, 54.4°, 56.7°, 59.0°, 60.8°, and 65.2° are ascribed to the (101), (112), (200), (103), (211), (004), (220), (204), (105), (312), (303), (321), (224), and (400) planes of ZMO (red line, JCPDS card No. 24–1133), respectively. The diffraction peaks of the as-prepared materials can be well matched with ZMO and Ag, and no obvious shift of the diffraction peaks are observed with the increase of the weight ratio of Ag. Such results indicate that the obtained materials are the hybrid of bimetallic oxide ZMO and pure Ag. To further characterize the bonding information, normal Raman measurement is conducted on these samples with a 638-nm laser. As the Raman spectra shows in Fig. 4b, 324 cm−1, 383 cm−1, and 681 cm−1 are the main characteristic peaks of all samples. The peak at 681 cm−1 is assigned to the A1g symmetry, 324 cm−1 and 383 cm−1 are characteristic peaks of the octahedral BO6 site [14, 32, 39,40,41]. Comparing the Raman spectra of these three samples, the characteristic peaks of ZMO are gradually broadened as the increase of Ag ratio, which indicates that the crystallization of ZMO can be weakened with the addition of Ag. In addition, no characteristic peaks attributed to Ag2O2 and Ag2O [42, 43] are detected, which further proves that silver is in the form of pure Ag metal rather metal oxides in the hybrids.

Fig. 4
figure 4

XRD patterns (a) of purchased Ag powder, as-prepared ZMO, ZMO–Ag–1 ZMO–Ag–2, and ZMO–Ag–3. Raman spectra (b) of as-prepared ZMO, ZMO–Ag–1 ZMO–Ag–2, and ZMO–Ag–3. XPS spectra of (c) full scan, (d) Ag 3d of the as-prepared ZMO–Ag–2

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X-ray photoelectron spectroscopy (XPS) was conducted to further analyze the composition and elemental chemical states of Zn, Mn, and Ag in the as-prepared ZMO–Ag–2 hybrid. Figure 4c shows the survey XPS spectrum of the ZMO–Ag–2 hybrid which is mainly composed of Zn, Mn, Ag, and O elements. The peaks at 49.4 eV, 641.9 eV, 653.9 eV, 88.7 eV, 139.8 eV, 1021.4 eV, and 1044.3 eV correspond to Mn 3p, Mn 2p3/2 Mn 2p1/2, Zn 3p, Zn 3 s, Zn 2p3/2, and Zn 2p1/2, respectively [14, 35, 44, 45]. As shown in Fig. S2a and S2b, the spin energy separation between Mn 2p3/2 and Mn 2p1/2 is 12 eV, while energy separation between the Zn 2p3/2 and Zn 2p1/2 is 22.9 eV, which indicates that the oxidation states of Mn and Zn are trivalent and bivalent, respectively [46]. The two peaks at binding energy of 368.2 eV and 374.4 eV corresponded to Ag 3d5/2 and Ag 3d3/2, respectively. The spin energy separation between Ag 3d5/2 and Ag 3d3/2 indicates that the valence of Ag is zero [47].

In order to investigate the performance of the Ag/ZMO hybrids as anode materials in lithium-ion batteries, the electrochemical performances of the ZMO, ZMO–Ag–1, ZMO–Ag–2, and ZMO–Ag–3 were measured. Figure 5a and c are the first three representative cyclic voltammogram (CV) curves of ZMO and ZMO–Ag–2 at a scan rate of 0.1 mV s−1 in the voltage range of 0.01–3.0 V (vs. Li/Li+). During the first cathodic scan of ZMO, the first reduction peak appears at around 1.25 V, corresponding to the process of Mn3+ being reduced to Mn2+ [11]. Then, a wide reduction peak appears at 0.8 V, which is due to the formation of SEI film during the first discharge process [44]. The strongest reduction peak is around 0.3 V, which corresponds to the process of Mn2+ and Zn2+ being reduced to Mn0 and Zn0 and the forming process of ZnLix alloy, respectively. During the anodic scan process, two obvious broaden oxidation peaks fall in 1.25 V and 1.55 V, which correspond to the fact that Mn0 and Zn0 are oxidated to Mn2+ and Zn2+, respectively. In the two subsequent cycles, the main reduction peak shifts to 0.5 V, and the peak position of the oxidation peak does not change significantly [48, 49]. Compared with the CV curves of ZMO, a pair of redox peaks can be found at 0.1 V and 0.37 V in the CV of ZMO–Ag–2 (Fig. 5c), which are corresponding to the alloying and de-alloying process of Ag and Li. In order to verify this conclusion, purchased pure Ag powder was used to fabricate electrode and it was assembled in CR2032–coin batteries to evaluate its electrochemical performances. As shown in Fig. S3a, a pair of redox peaks at 0.1 and 0.37 V are observed which is consistent with that of ZMO–Ag–2. Also, the first three charge/discharge curves of Ag are measured at current density of 100 mA g−1 (Fig. S3b). The discharge platform is around 0.1 V, and two charging platforms are located at 0.15 V and 0.37 V, respectively, which may be attributed to the progressive de-alloying process of AgLix alloy [50,51,52]. The CV and charge/discharge characteristics of ZMO–Ag–1 and ZMO–Ag–3 (Fig. S4 and S5) are similar to that of ZMO–Ag–2.

Fig. 5
figure 5

CV curves of ZMO (a) and ZMO–Ag–2 (c) at a scan rate of 0.1 mV s−1. Charge/discharge profiles of ZMO (b) and ZMO–Ag–2 (d) at the current density of 100 mA g−1

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Comparing the first three charge/discharge curves of the as-prepared materials and purchased Ag powder (Fig. 5b, d, S4b, S5b and S3b), the initial CE of ZMO, ZMO–Ag–1, ZMO–Ag–2, ZMO–Ag–3, and Ag are 67.1%, 73.2%, 74.5%, 79.2%, and 67.7%, respectively. It is clearly observed that the first cycle CE of Ag/ZMO hybrids increases along with the increase of Ag content. This indicates that the existence of Ag may have synergistic effect with ZMO and inhibits the continuous generation of SEI film in the first discharge process.

The electrochemical impedance spectroscopy (EIS) measurements were conducted to evaluate the electron conductivities of ZMO and Ag/ZMO hybrids. In Fig. 6a, the semicircle at high frequency decreases along with the increase of Ag content, which indicates that the charge transfer resistance of the battery is reduced with the addition of Ag particles. After 30 cycles at current density of 500 mA g−1, the EIS spectrum of these electrodes was shown in Fig. S6. The cycled cell exhibits a much lower charge transfer resistance at high frequency compared with the uncycled cell, which is probably attributed to the activation of electrode materials.

Fig. 6
figure 6

Electrochemical impedance spectroscopy of the as-prepared materials (a). Rate performance (b), and cycle performance (c) of purchased Ag powders and as-prepared materials

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The rate capacity displayed in Fig. 6b indicates that the ZMO–Ag–2 exhibits the best electrochemical performance. At 50, 200, 500, 1000, and 2000 mA g−1, the ZMO–Ag–2 exhibits average capacity of 980, 810, 760, 740, and 700 mAh g−1, respectively, whereas the ZMO exhibits average capacity of 970, 780, 680, 600, and 520 mAh g−1. Therefore, it can be visualized that the addition of Ag particles can effectively improve the rate capacity of ZMO. Compared with ZMO, ZMO–Ag–1, and ZMO–Ag–2, both Ag and ZMO–Ag–3 have better cycle performance at high current density. With the increase of current density, only slight decreases of the capacities of Ag and ZMO–Ag–3 can be observed. Figure 6c displays the cycling performance of Ag, ZMO, ZMO–Ag–1, ZMO–Ag–2, and ZMO–Ag–3 at the current density of 100 mA g−1. As shown in Fig. 6c, in the first 40 cycles, the capacity of ZMO is a little higher than that of ZMO–Ag–2. However, with the further increase of cycle number, the capacity of ZMO–Ag–2 gradually increases, while the capacity of ZMO decreases rapidly. The rapid decrease of the capacity of ZMO is probably due to the continuous growth of SEI film and gradual consumption of electrolyte during the charge and discharge processes. On the other hand, the decoration of Ag may lead to the formation of a thinner SEI film and may have a certain synergistic effect with ZMO during the lithiation/de-lithiation processes. After 100 cycles, an ultra-high capacity of about 1300 mAh g−1 can be obtained for the ZMO–Ag–2 hybrid. This cycle performance is better than most of the similar materials, as shown in Table S1. The cycling performance of different electrode materials are measured at 1000 mA g−1 and the results are shown in Fig. S7. Before applying 1000 mA g−1, a small current density of 50 mA g−1 was applied to activate the electrode materials in the first three cycles. Compared with the other electrode materials, ZMO–Ag–2 exhibits the best cycle performance. After 150 cycles, ZMO–Ag–2 still has a capacity of 560 mAh g−1, while the capacity of ZMO, ZMO–Ag–1, and ZMO–Ag–3 decreases to 440 mAh g−1, 356 mAh g−1, and 378 mAh g−1, respectively. Although the cycle performances of ZMO–Ag–3 and Ag are ultra-stable, the capacities of ZMO–Ag–3 and Ag are much lower than that of ZMO–Ag–2, which is mainly attributed to the decrease of theoretical specific capacity of the composite with the increase of Ag content. The above lithium ion storage performances of the four as-prepared materials demonstrate that the addition of Ag can effectively improve the electrochemical performances of ZMO, and ZMO–Ag–2 shows the best properties than other Ag/ZMO composites.

To further prove that Ag nanoparticles can enhance the initial CE and cycling stability of ZMO, post-mortem analysis was conducted. The morphology analysis results of the cycled materials were shown in Fig. S8. It is obvious that the particle size of ZMO is greatly increased after 30 cycles, while no obvious increase of particle size in cycled Ag/ZMO hybrids are observed. Furthermore, there are a lot of creaked particles on the electrode surface of ZMO. On the contrary, cycled Ag/ZMO hybrids basically maintain the same shape as before the cycle. This may be one of the main reasons that Ag/ZMO hybrids show higher cycle stability than that of ZMO. Further in situ spectral and structural investigation is under progress to study the improvement mechanism by Ag addition.

Conclusions

In summary, Ag/ZMO hybrids with excellent cycle performance were designed and synthesized by a facile and scalable coprecipitation method, which was considered a promising method for industrial application. The electrochemical performance of different electrode materials showed that the addition of Ag can significantly improve the cycle stability of ZMO. After 100 cycles, the ZMO–Ag–2 could still maintain ultra-high capacity of 1300 mAh g−1 at 100 mA g−1. The addition of Ag can effectively improve the conductivity of ZMO. Moreover, the existence of Ag may lead to the growth of a thinner SEI film on the surface of the electrode material which could greatly improve initial CE during charge/discharge process. Post-mortem analysis indicates the decoration of Ag nanoparticles may have a certain synergistic effect with ZMO during the lithiation/de-lithiation process and then result in a higher cycle stability. It is anticipated that our work may open up a new avenue to rapidly and massively produce various anode materials decorated with Ag nanoparticles for lithium-ion batteries.