Introduction

The ever–growing need for high energy density, power density and stable cyclability has prompted considerable attention to develop promising anode materials for lithium ion batteries (LIBs) to meet the rapid development of portable electronics. Various transition metal oxides (TMOs), such as iron oxides1,2,3, cobalt oxides4,5 manganese oxides6,7 and nickel oxides8,9, have been competitive anode candidates for LIBs due to their stable capability and high reversible capacities (500–1000 mAh g−1)8,10,11. Recently, iron oxides anode has received an upsurge of interest due to their fascinating superiority. However, the poor electrical conductivity, severe volume expansion and higher oxidation potential restrict their applications on LIBs12. Furthermore, the rapid capacity fading of iron oxides anode materials remains a major drawback. In particular, the Fe2O3 anode materials have been found to be irreversible in the first lithiation by in situ transmission electron microscopy (TEM); they undergo a reversible phase conversion between FeO and Fe/Li2O during the lithiation-delithiation cycles13. The Fe2O3 anode materials cannot recover to their original structure after delithiation, which cause a large capacity loss in the first cycle. Recently, Fe–based oxides (AFe2O4, A = Mn14, Co15, Ni16, Cu17 and Zn18, etc) have been studied as promising anodes for LIBs to improve their cyclability. It is anticipated that they can effectively overcome the drawbacks of pure iron oxide anode; then larger reversible capacity, better cyclability and better rate performance can be achieved by the suitable combination of different metal species19.

ZnFe2O4 stands out from other Fe-based spinel series anodes because of its high abundance and high theoretical specific capacity of 1072 mAh g−1 20. Qin et al. first reported the use of ZnFe2O4 as an anode synthesized by a pulsed laser deposition method21 and their initial reversible capacity was 556 mAh g−1 and after100 cycles 78% of the capacity (434 mAh g−1) was still retained. Then the electrochemical performances of diverse ZnFe2O4 nanostructure have been greatly enhanced, such as ZnFe2O4/C hollow spheres22 ZnFe2O4 octahedrons23 ZnFe2O4 nanofibers (ZFO–NF) and nanorods20 and so on. However, a few of the fundamental mechanism concerning the electrochemical process remain unclear. Wang’s group24 have reported that the products of the deeply discharged are LiZn and Fe and the recharged materials are ZnO and Fe2O3, which is distinguishing from the previous in situ TEM study on Fe2O3 anode13. While, Chowdari B.V.R. et al.25 have suggested that the reaction mechanism of ZnFe2O4 is reversible reactions of LiZn to ZnO and Fe to FeO after the first discharge process. Particularly, the dynamic electrochemical reaction of binary transition metal oxide for LIBs is still in a black box.

In situ TEM technique has been recognized as an excellent option to monitor real–time observation of electrode materials with lithium and sodium on the nanometer scale26,27,28,29. Some successes have been achieved on understanding the electrochemical mechanism of SnO230, Si31,32, ZnO33, CeO234, Fe2O313, carbon nanotube (CNT)35, graphene36 and Co9S8/CNT37 in real time through the in situ TEM technique. Up to now, the electrochemical reaction between ZnFe2O4 and Li has not been studied. Here, an all–solid nano–LIB was constructed inside a high–resolution TEM using ZnFe2O4 as working electrode to visualize the microstructure and phase evolution during electrochemical processes. It is found that upon lithiation the ZnFe2O4 nanoparticle was converted into numerous Fe and Zn nanograins within Li2O matrix with a severe volume expansion. During delithiation, the anode cannot be converted to its original phase ZnFe2O4 but transformed to Fe2O3 and ZnO. The Fe valence evolution of ZnFe2O4 nanoparticle is also studied by EELS measurements, which agrees well with the in situ TEM results. Our in situ TEM results for provided the direct experimental evidence of the reaction mechanism of ZnFe2O4 during lithium-ion insertion and extraction.

Results

The microstructure characterization of the obtained ZnFe2O4/graphene is shown in Fig. 1. Figure 1(a) is the TEM image of ZnFe2O4/graphene, it indicates there are many ZnFe2O4 particles with sizes of 120~180 nm anchored on graphene and the transparent nature of the graphene implies that it is fully exfoliated into single or few-layer sheet. A high–magnification TEM image of an individual ZnFe2O4 nanoparticle is given in Fig. 1(b). Obviously, the ZnFe2O4 nanoparticle is primarily composed of nanocrystals with a size ~10 nm. The smaller size of ZnFe2O4 nanocrystals can shorten Li+ diffusion pathways, increase the electron/ion conductance and reduce the volume change induced by lithiation/delithiation, further enable the ZnFe2O4 nanocrystals to show an improved electrochemical performance. The high resolution transmission electron micrograph (HRTEM) was taken along the zone axis with the (220) lattice finger directly seen with a spacing of 0.30 nm as shown in Fig. 1(c); the corresponding fast Fourier transform (FFT) is shown in the inset of Fig. 1(c), in accordance with the (220), and (311) planes of the cubic structure of ZnFe2O4 (JCPDS card no. 89–1012). Figure 1(d) presents an ED pattern recorded from the synthesized ZnFe2O4/graphene. All the diffraction rings can be perfectly indexed as a cubic structure of ZnFe2O4 (JCPDS card no. 89–1012); it further confirms that the resultant products are ZnFe2O4 phase.

Figure 1
Figure 1
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(a) TEM, (b) The high–magnification TEM, (c) HRTEM and (d) ED pattern of the obtained products ZnFe2O4/graphene electrode.

To investigate the electrochemical behavior of ZnFe2O4 during lithiation–delithiation cycles, an in situ nanoscale electrochemical device of ZnFe2O4 was constructed, as schematically shown in Fig. 2(a). Briefly, the electrochemical nano–LIB device consists of three essential components: ZnFe2O4/graphene anode, metal Li counter electrode and the naturally grown solid electrolyte Li2O layer on metal Li. After contact between Li2O and ZnFe2O4/graphene anode was established, a constant potential of −1.0 V was applied to the ZnFe2O4/graphene against to the Li counter electrode to drive the first lithiation of ZnFe2O4. Figure 2(b,c) and Supplementary Movie S1,S2 show the morphological changes of two ZnFe2O4 nanoparticles with diameters of ~196 and 205 nm in the first electrochemical lithiation process. It suggests that the lithiation occurred on all surfaces within 37.0 s, indicating the fast lithium diffusion on the ZnFe2O4 nanoparticle surface. As shown in Fig. 2(b), a ~23% size increase from 196 to 241 nm in the diameter was seen for the ZnFe2O4 nanoparticle anchored on graphene layer and the first lithiation lasted for 21 s, giving a lithiation ratio of ~2.14 nm/s for this ZnFe2O4 nanoparticle. As for another ZnFe2O4 nanoparticle sited on the edge of the graphene (Fig. 2c), its diameter expanded from 205 nm to about 255 nm after full lithiation within 37.0 s and the lithiation ratio of this ZnFe2O4 nanoparticle is about 1.25 nm/s, which is much smaller than that of ZnFe2O4 nanoparticle on graphene layer. This can be attributed to the good Li+ conductivity of graphene. In other words, the good Li+ conductivity of graphene sheets makes the fast diffusion of lithium around ZnFe2O4 nanoparticle.

Figure 2
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Morphological changes of two ZnFe2O4 particles during lithiation with a potential of −1 V.

(a) Schematic illustration of the in situ nano-battery setup. (b–b3) TEM snapshots show the morphological evolution of a ZnFe2O4 particle anchored on graphene with a diameter of ~196 nm. (c–c3) TEM snapshots show the morphological evolution of a second ZnFe2O4 particle sited on the edge of graphene with a diameter of ~205 nm. The red dashed curves demonstrate the reaction front. All the scale bars are 50 nm.

Close view on the microstructure of ZnFe2O4 nanoparticle after full lithiation and delithiation is shown in Fig. 3. Figure 3a,b is the TEM image of an individual ZnFe2O4 nanoparticle anchored on graphene after the first lithiation. It shows that many small nanograins with sizes of 1–3 nm dispersed in Li2O matrix, saying after the first lithiation process Fe and Zn nanocrystals embedded in Li2O matrix uniformly, as evidenced by HRTEM and ED in Fig. 3b,c. From the HRTEM image of the fully lithiated ZnFe2O4 nanoparticle that displayed in Fig. 3b, the lattice spacing recorded from the nanocrystals is about 2.10 Å, agrees with the (110) plane of Fe and (101) plane of Zn. Figure 3c is the ED pattern collected from the lithiated ZnFe2O4/graphene electrode. The diffraction rings corresponding to the (111), (220), (311) lattice planes of Li2O are marked by red lines, while the (110), (101) lattice planes of Fe and Zn have been labeled. Then the potential was reversed to + 3 V to initiate the delithiation process and the TEM images of the electrode after full delithiation are shown in Fig. 3d,e. When the electrode was fully delithiated, the clear lattice fringes (0.25 and 0.27 nm) correspond to (101) plane of phase ZnO and (104) plane of phase Fe2O3 respectively. Also the ED pattern (Fig. 3f) confirms the presence of Fe2O3 and ZnO in the delithiated product, rather than ZnFe2O4. As for ZnFe2O4 electrode, in the most favorable case, the Fe2+ ions in FeO would be further reversibly oxidized to form trivalent Fe3+ ions to obtain Fe2O3 due to the synergetic effects of Zn and Fe in ZnFe2O4 electrode23,24. In addition, no Li2O diffraction ring was detected from the ED pattern, suggesting that the Li2O was almost consumed up during delithiation process.

Figure 3
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(a) TEM and, (b) HRTEM images of an individual lithiated ZnFe2O4 particle, (c) ED pattern of the lithiated ZnFe2O4/graphene electrode, (d) TEM and (e) HRTEM images of delithiated ZnFe2O4 particle, (f) ED pattern of the delithiated ZnFe2O4/graphene electrode.

We next concern about the electrochemical behaviors of ZnFe2O4/graphene electrode during lithiation/delithiation cycles. A ZnFe2O4 particle with a nearly spherical shape and the initial diameter of ~196 nm is selected to check the morphology evolution, as shown in Fig. 4a. The ED pattern of the obtained ZnFe2O4/graphene electrode is given in Fig. 4a1. It can be perfectly indexed as the face–centered crystal structure of ZnFe2O4 (JCPDS no. 89–1012). The pristine ZnFe2O4 nanoparticle was inflated and expanded its size to 223 nm after full lithiation (Fig. 4b). The ED pattern of the fully lithiated ZnFe2O4/graphene electrode is shown in Fig. 4b1; the diffraction rings can be well indexed as Fe, Zn and fcc Li2O, suggesting the ZnFe2O4 was transformed to Fe, Zn and fcc Li2O after the first full lithiation process. Then the potential was reversed to + 3 V to facilitate the delithiation process. Along with the first delithiation process, volume contraction observed throughout the whole nanosphere with the size decreasing from 223 nm to 200 nm. The fully delithiated phase was identified as Fe2O3 and ZnO, as examined by the ED pattern of the delithiated ZnFe2O4/graphene electrode. Then the second lithiation/delithiation cycle was investigated by reversing the applied potential of −1 and + 3 V, as shown in Fig. 4c–e. The TEM image of ZnFe2O4 particle after the second lithiation process is given in Fig. 4c; the marked ZnFe2O4 particle expanded its size to ~240 nm again, just as the first lithiation process. The ED pattern that recorded from the lithiated ZnFe2O4/graphene electrode after the second lithiation is shown in Fig. 4d1, it indicates the product was Fe, Zn and cubic Li2O. The lithiated ZnFe2O4 particle shrunk its size to 211 nm again in the second delithiation process showing the reversible micromorphology change, as displayed in Fig. 4e. Figure 4e1 shows the corresponding ED pattern of the delithiated ZnFe2O4/graphene electrode and the ED pattern confirms the resultant Fe and Zn nanograins transformed to Fe2O3 and ZnO again in the second delithiation process. As discovered above, all the TEM results indicate the reversible conversion from Fe to Fe2O3 and Zn to ZnO after the first lithiation process.

Figure 4
Figure 4
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Morphological and microstructural changes of ZnFe2O4/graphene electrode during the first two lithiation-delithiation cycles.

(a–e) Morphological changes of ZnFe2O4 particle in the first two lithiation-delithiation cycles. (a1–e1) ED patterns recorded from the corresponding ZnFe2O4/graphene electrode in (a–e).

An EELS that assisted in TEM is a useful technique for analyzing the valences of some transition metal elements at the nanoscale. The transition of an electron from a 2p level to 3d orbitals leads to the formation of L2,3 white lines due to the unoccupied 3d orbitals of transition metals38. The L3/L2 white−line intensity ratio (IL3/IL2) measured in 3d transition metal is used to determine the occupation number of 3d electrons. Here the IL3/IL2 of Fe has been obtained to correlate EELS features with the valence states of Fe in the first three lithiated and delithiated states. Figure 5 shows the EELS results of Fe collected from the lithiated and delithiated ZnFe2O4/graphene electrode to confirm the evolution of valence states of Fe elements in the electrochemical lithiation and delithiation cycles. The EELS spectrum of Fe in the original ZnFe2O4/graphene electrode is shown in Fig. 5a, we can see that the L3/L2 intensity ratio of Fe is 5.3, confirming that the valence state of Fe is undoubtedly 3 + 39. The L3/L2 intensity ratio of Fe in the fully lithiated stage reduced to 2.3, which is smaller than that of Fe2+ (4.1 ± 0.2), agrees well with the valence state of 040, as given in Fig. 5b, suggesting the oxide state transition of Fe from 3+ to zero. From the EELS spectrum of Fe recorded from a fully delithiated ZnFe2O4/graphene anode that shown in Fig. 5c, in which the L3/L2 intensity ratio of Fe increased to 5.1, this EELS can be regarded approximately complete oxidation, that is, Fe “3 +” fingerprint. This result confirms that after the first cycle the delithiated product was Fe2O3. It says the Fe element can renew its original state of Fe3+ after the first delithiation process, further demonstrating good reversibility of Fe metal. Figure 5d displays the L3/L2 intensity ratio of Fe is 2.5 after the second full lithiation process, which is similar with that of Fe in the first lithiated stage, corresponding to the valance state of zero. Then the L3/L2 intensity ratio of Fe increased to 5.2 again calculated from the EELS spectrum given in Fig. 5e, it implies that the valance of Fe element is +3 after the second delithiation process. Expectedly, the similar reversal of L3/L2 intensity ratio of Fe in ZnFe2O4 was also noticed in the third cycle. The repeated changes in L3/L2 intensity ratio of Fe indicate the complete and reversible electrochemical transition between Fe0 and Fe3+ during the electrochemical processes, thus leading to high reversibility for iron oxide based anodes. The EELS results are agree well with the ED results shown in Fig. 4a1–e1, which reveal that the electrochemical reaction of iron oxide phase in ZnFe2O4 during the electrochemical processes is a reversible phase transition between Fe and Fe2O3 after the first lithiation process.

Figure 5
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EELS spectra of iron from ZnFe2O4 nanoparticle during the lithiation and delithiation cyclings.

(a) The initial stage, (b) the first lithiated state and (c) the first delithiated state, (d) after the second lithiation and (e) delithiation process and (f) after the third lithiation and (g) delithiation process. The periodic fluctuation of white–line intensity ratio (L3/L2) of Fe in the lithiation/delithiation cycle implies the reversible transformation of Fe0 and Fe3+.

According to the above in situ TEM analysis, the possible reactions of ZnFe2O4 during the lithiation/delithiation process are proposed as follows. During the first lithiation process, Li+ ions can diffuse quickly on graphene sheets, leading to a uniform lithiation take place on the surface of ZnFe2O4 nanoparticles. As Li+ insertion continued, the lithiation front in the ZnFe2O4 gradually propagated and gave rise to a visible interface between the lithiated and unlithiated phases. As a result, lithiation is essentially the destruction of the crystal structure, lithium is intercalated into ZnFe2O4 and led to metallic Zn, Fe nanograins and Li2O appear in the product followed by the obvious volume expansion. The first lithiation can be expressed as: ZnFe2O4 + 8Li+ +  8e− → Zn + 2Fe + 4Li2O. During the delithiation process, Li-ion will firstly be extracted from the lithiated ZnFe2O4 leading to volume contraction and lithiated product metallic Fe and Zn nanograins can be oxidized to metal oxide (Fe2O3 and ZnO) with the presence of Li2O through the conversion reaction, ZnFe2O4 is not the initial molecule that can be recovered. So the delithiation process can be described as: Zn + Li2O → ZnO + 2Li+ + 2e−, 2Fe + 3Li2O → Fe2O3 + 6Li+ + 6e−. In the following lithiation/delithiation cycles, the reversible conversion reaction of ZnO, Fe2O3 and metallic Zn, Fe nanoparticles take place and indicates good reversibility. Thus, after the first cycle, reversible reactions can be expressed as following equations: ZnO + 2Li+ + 2e−Zn + Li2O, Fe2O3 + 6Li+ + 6e−2Fe + 3Li2O. Figure 6 schematically outlines these changes of the ZnFe2O4 nanoparticles during lithiation/delithiation cycles.

Figure 6
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Schematic illustration of the conversion reaction of an individual ZnFe2O4 nanoparticle.

Therefore, the stable cycling response of ZnFe2O4 (see Supplementary Fig. S1) may be ascribed to not only the synergistic effect of different type metal oxide species (Zn and Fe) on ZnFe2O4, but also a facile and easier lithium ion diffusion on graphene during the lithiation/delithiation cycles. Also, in the following lithiation/delithiation cycles, ZnO and Fe2O3 convey reversible electrochemical reactivity toward Li and then reveal a reversible phase conversion of Zn-ZnO and Fe-Fe2O3, accounting for good reversibility and high Coulombic efficiency.

Conclusions

In summary, the electrochemical reaction mechanism of ZnFe2O4 for lithium ion battery anode is investigated by in situ TEM and the results show that in the first lithiation process lithium-ion is intercalated into ZnFe2O4, generating ultrafine (1–3 nm) Fe and Zn nanocrystallites within Li2O matrix followed by obvious volume expansion. In the first delithiation process, the HRTEM and ED results show that ZnFe2O4 is not the original molecule that can be recovered, but metallic Zn and Fe nanoparticles oxidized to their respective metal oxides ZnO and Fe2O3 with the disappearance of Li2O through the complete conversion reaction. The ED patterns and EELS spectra reveal that the electrochemical lithiation/delithiation processes of ZnFe2O4 nanoparticles as anode in LIBs are revealed to be reversible phase transition between Fe, Zn nanograins and Fe2O3, ZnO nanograins. The information obtained from our findings can help to further improve the electrochemical performance of this type material and also is insightful for exploring various types of electrode materials in LIB technology.

Methods

Materials synthesis

Graphite oxide (GO) was synthesized by a modified Hummers’ method41. ZnFe2O4/graphene was prepared by a hydrothermal route. Firstly, GO (100 mg) was dispersed in ethylene glycol (80 ml) with sonication for 30 min to form a homogeneous dispersion. Then, Zn(Ac)2 H2O (0.55 g), FeCl3 (0.81 g) and NaAc (3.6 g) were added into the above solution with stirring for 30 min and the mixture was transferred into a Teflon–lined autoclave with a capacity of 100 mL and maintained at 200 °C for 24 h. The precipitate was isolated by filtration and washed several times after cooling down to room temperature. Finally, the product was obtained by drying the precipitate at 60 °C for 12 h.

In situ TEM electrochemical setup

The nano-LIBs experimental set-up was constructed inside a TEM (JEOL JEM–2100F) to enable the in situ observation on the electrochemical behaviors of ZnFe2O4 anode with the assistance of HRTEM, electron diffraction (ED) and EELS measurements. The ZnFe2O4 nanoparticles anchored on graphene were used as the working electrode and Li metal was coated onto a piezo–driven W probe and regarded as lithium source and counter electrode, an oxide layer of Li2O formed on Li metal when it is exposed to air act as the solid electrolyte. The detailed setup procedure for the nano-LIB can be found in literature13. When ZnFe2O4/graphene driven by the nanomanipulator of the TEM-STM holder is contact with the Li2O layer, a nano-LIBs cell is successfully constructed. After that, a potential of −1 V was applied to the ZnFe2O4 electrode against the Li source to drive Li+ transport to initiate lithiation process and then the bias was reversed to + 3 V to facilitate delithiation. EELS measurements were performed on a TEM with the assistance of Gatan EELS attachments.

Additional Information

How to cite this article: Su, Q. et al. Study on the Electrochemical Reaction Mechanism of ZnFe2O4 by In Situ Transmission Electron Microscopy. Sci. Rep. 6, 28197; doi: 10.1038/srep28197 (2016).