Study on the Electrochemical Reaction Mechanism of ZnFe2O4 by In Situ Transmission Electron Microscopy

A family of mixed transition–metal oxides (MTMOs) has great potential for applications as anodes for lithium ion batteries (LIBs). However, the reaction mechanism of MTMOs anodes during lithiation/delithiation is remain unclear. Here, the lithiation/delithiation processes of ZnFe2O4 nanoparticles are observed dynamically using in situ transmission electron microscopy (TEM). Our results suggest that during the first lithiation process the ZnFe2O4 nanoparticles undergo a conversion process and generate a composite structure of 1–3 nm Fe and Zn nanograins within Li2O matrix. During the delithiation process, volume contraction and the conversion of Zn and Fe take place with the disappearance of Li2O, followed by the complete conversion to Fe2O3 and ZnO not the original phase ZnFe2O4. The following cycles are dominated by the full reversible phase conversion between Zn, Fe and ZnO, Fe2O3. The Fe valence evolution during cycles evidenced by electron energy–loss spectroscopy (EELS) techniques also exhibit the reversible conversion between Fe and Fe2O3 after the first lithiation, agreeing well with the in situ TEM results. Such in situ TEM observations provide valuable phenomenological insights into electrochemical reaction of MTMOs, which may help to optimize the composition of anode materials for further improved electrochemical performance.

electrochemical process remain unclear. Wang's group 24 have reported that the products of the deeply discharged are LiZn and Fe, and the recharged materials are ZnO and Fe 2 O 3 , which is distinguishing from the previous in situ TEM study on Fe 2 O 3 anode 13 . While, Chowdari B.V.R. et al. 25 have suggested that the reaction mechanism of ZnFe 2 O 4 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 scale [26][27][28][29] . Some successes have been achieved on understanding the electrochemical mechanism of SnO 2 30 , Si 31,32 , ZnO 33 , CeO 2 34 , Fe 2 O 3 13 , carbon nanotube (CNT) 35 , graphene 36 , and Co 9 S 8 /CNT 37 in real time through the in situ TEM technique. Up to now, the electrochemical reaction between ZnFe 2 O 4 and Li has not been studied. Here, an all-solid nano-LIB was constructed inside a high-resolution TEM using ZnFe 2 O 4 as working electrode to visualize the microstructure and phase evolution during electrochemical processes. It is found that upon lithiation the ZnFe 2 O 4 nanoparticle was converted into numerous Fe and Zn nanograins within Li 2 O matrix with a severe volume expansion. During delithiation, the anode cannot be converted to its original phase ZnFe 2 O 4 but transformed to Fe 2 O 3 and ZnO. The Fe valence evolution of ZnFe 2 O 4 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 ZnFe 2 O 4 during lithium-ion insertion and extraction.

Results
The microstructure characterization of the obtained ZnFe 2 O 4 /graphene is shown in Fig. 1. Figure 1(a) is the TEM image of ZnFe 2 O 4 /graphene, it indicates there are many ZnFe 2 O 4 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 ZnFe 2 O 4 nanoparticle is given in Fig. 1(b). Obviously, the ZnFe 2 O 4 nanoparticle is primarily composed of nanocrystals with a size ~10 nm. The smaller size of ZnFe 2 O 4 nanocrystals can shorten Li + diffusion pathways, increase the electron/ion conductance, and reduce the volume change induced by lithiation/delithiation, further enable the ZnFe 2 O 4 nanocrystals to show an improved electrochemical performance. The high resolution transmission electron micrograph (HRTEM) was taken along the [114] 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), (131), and (311) planes of the cubic structure of ZnFe 2 O 4 (JCPDS card no. 89-1012). Figure 1(d) presents an ED pattern recorded from the synthesized ZnFe 2 O 4 /graphene. All the diffraction rings can be perfectly indexed as a cubic structure of ZnFe 2 O 4 (JCPDS card no. 89-1012); it further confirms that the resultant products are ZnFe 2 O 4 phase.
To investigate the electrochemical behavior of ZnFe 2 O 4 during lithiation-delithiation cycles, an in situ nanoscale electrochemical device of ZnFe 2 O 4 was constructed, as schematically shown in Fig. 2(a). Briefly, the electrochemical nano-LIB device consists of three essential components: ZnFe 2 O 4 /graphene anode, metal Li counter electrode, and the naturally grown solid electrolyte Li 2 O layer on metal Li. After contact between Li 2 O and ZnFe 2 O 4 /graphene anode was established, a constant potential of − 1.0 V was applied to the ZnFe 2 O 4 /graphene against to the Li counter electrode to drive the first lithiation of ZnFe 2 O 4 . Figure 2(b,c) and Supplementary Movie S1,S2 show the morphological changes of two ZnFe 2 O 4 nanoparticles with diameters of ~196 and 205 nm in the  We next concern about the electrochemical behaviors of ZnFe 2 O 4 /graphene electrode during lithiation/delithiation cycles. A ZnFe 2 O 4 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 ZnFe 2 O 4 /graphene electrode is given in Fig. 4a1. It can be perfectly indexed as the face-centered crystal structure of ZnFe 2 O 4 (JCPDS no. 89-1012). The pristine ZnFe 2 O 4 nanoparticle was inflated and expanded its size to 223 nm after full lithiation (Fig. 4b). The ED pattern of the fully lithiated ZnFe 2 O 4 /graphene electrode is shown in Fig. 4b1; the diffraction rings can be well indexed as Fe, Zn and fcc Li 2 O, suggesting the ZnFe 2 O 4 was transformed to Fe, Zn and fcc Li 2 O 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 Fe 2 O 3 and ZnO, as examined by the ED pattern of the delithiated ZnFe 2 O 4 /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 ZnFe 2 O 4 particle after the second lithiation process is given in Fig. 4c; the marked ZnFe 2 O 4 particle expanded its size to ~240 nm again, just as the first lithiation process. The ED pattern that recorded from the lithiated ZnFe 2 O 4 / graphene electrode after the second lithiation is shown in Fig. 4d1, it indicates the product was Fe, Zn and cubic Li 2 O. The lithiated ZnFe 2 O 4 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 ZnFe 2 O 4 /graphene electrode, and the ED pattern confirms the resultant Fe and Zn nanograins transformed to Fe 2 O 3 and ZnO again in the second delithiation process. As discovered above, all the TEM results indicate the reversible conversion from Fe to Fe 2 O 3 and Zn to ZnO after the first lithiation process.
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 L 2,3 white lines due to the unoccupied 3d orbitals of transition metals 38 . The L 3 /L 2 white− line intensity ratio (I L3 /I L2 ) measured in 3d transition metal is used to determine the occupation number of 3d electrons. Here the I L3 /I L2 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 ZnFe 2 O 4 / 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 ZnFe 2 O 4 /graphene electrode is shown in Fig. 5a, we can see that the L 3 /L 2 intensity ratio of Fe is 5.3, confirming that the valence state of Fe is undoubtedly 3 + 39 . The L 3 /L 2 intensity ratio of Fe in the fully lithiated stage reduced to 2.3, which is smaller than that of Fe 2+ (4.1 ± 0.2), agrees well with the valence state of 0 40 , 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 ZnFe 2 O 4 /graphene anode that shown in Fig. 5c, in which the L 3 /L 2 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 Fe 2 O 3 . It says the Fe element can renew its original state of Fe 3+ after the first delithiation process, further demonstrating good reversibility of Fe metal. Figure 5d displays the L 3 /L 2 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 L 3 /L 2 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 L 3 /L 2 intensity ratio of Fe in ZnFe 2 O 4 was also noticed in the third cycle. The repeated changes in L 3 /L 2 intensity ratio of Fe indicate the complete and reversible electrochemical transition between Fe 0 and Fe 3+ 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 ZnFe 2 O 4 during the electrochemical processes is a reversible phase transition between Fe and Fe 2 O 3 after the first lithiation process.
According to the above in situ TEM analysis, the possible reactions of ZnFe 2 O 4 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 ZnFe 2 O 4 nanoparticles. As Li + insertion continued, the lithiation front in the ZnFe 2 O 4 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 ZnFe 2 O 4 and led to metallic Zn, Fe nanograins and Li 2 O appear in the product followed by the obvious volume expansion. The first lithiation can be expressed as: ZnFe 2 O 4 + 8Li + + 8e − → Zn + 2Fe + 4Li 2 O. During the delithiation process, Li-ion will firstly be extracted from the lithiated ZnFe 2 O 4 leading to volume contraction, and lithiated product metallic Fe and Zn nanograins can be oxidized to metal oxide (Fe 2 O 3 and ZnO) with the presence of Li 2 O through the conversion reaction, ZnFe 2 O 4 is not the initial molecule that can be recovered. So the delithiation process can be described as: Zn + Li 2 O → ZnO + 2Li + + 2e − , 2Fe + 3Li 2 O → Fe 2 O 3 + 6Li + + 6e − . In the following lithiation/delithiation cycles, the reversible conversion reaction of ZnO, Fe 2 O 3 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 + Li 2 O, Fe 2 O 3 + 6Li + + 6e − ↔2Fe + 3Li 2 O. Figure 6 schematically outlines these changes of the ZnFe 2 O 4 nanoparticles during lithiation/delithiation cycles. Therefore, the stable cycling response of ZnFe 2 O 4 (see Supplementary Fig. S1) may be ascribed to not only the synergistic effect of different type metal oxide species (Zn and Fe) on ZnFe 2 O 4 , 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 Fe 2 O 3 convey reversible electrochemical reactivity toward Li and then reveal a reversible phase conversion of Zn-ZnO and Fe-Fe 2 O 3 , accounting for good reversibility and high Coulombic efficiency.

Conclusions
In summary, the electrochemical reaction mechanism of ZnFe 2 O 4 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 ZnFe 2 O 4 , generating ultrafine (1-3 nm) Fe and Zn nanocrystallites within Li 2 O matrix followed by obvious volume expansion. In the first delithiation process, the HRTEM and ED results show that ZnFe 2 O 4 is not the original molecule that can be recovered, but metallic Zn and Fe nanoparticles oxidized to their respective metal oxides ZnO and Fe 2 O 3 with the disappearance of Li 2 O through the complete conversion reaction. The ED patterns and EELS spectra reveal that the electrochemical lithiation/delithiation processes of ZnFe 2 O 4 nanoparticles as anode in LIBs are revealed to be reversible phase transition between Fe, Zn nanograins and Fe 2 O 3 , 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' method 41 . ZnFe 2 O 4 / 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 H 2 O (0.55 g), FeCl 3 (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 ZnFe 2 O 4 anode with the assistance of HRTEM, electron diffraction (ED) and EELS measurements. The ZnFe 2 O 4 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 Li 2 O 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 literature 13 . When ZnFe 2 O 4 /graphene driven by the nanomanipulator of the TEM-STM holder is contact with the Li 2 O layer, a nano-LIBs cell is successfully constructed. After that, a potential of − 1 V was applied to the ZnFe 2 O 4 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.