Operando study of Fe3O4 anodes by electrochemical atomic force microscopy

Applied Surface Science 426 (2017) 217–223

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Operando study of Fe3 O4 anodes by electrochemical atomic force microscopy Shuwei Wang a,1 , Weicong Zhang b,c,1 , Yuanning Chen c,d , Zhengwu Dai c,d , Chongchong Zhao a , Deyu Wang a,∗ , Cai Shen a,∗ a

Ningbo Institute of Materials Technology & Engineering, Chinese Academy of Sciences, 1219 Zhongguan Road, Ningbo, China Foundation Department, Zhejiang Wanli University, 8 South Qianhu Road, Ningbo, China c Ningbo Baohong Information Technology Co., Ltd., 7 Entrepreneurial Avenue, Ningbo, China d Ningbo Micro Energy Technology Co., Ltd., 7 Entrepreneurial Avenue, Ningbo, China b

a r t i c l e

i n f o

Article history: Received 17 May 2017 Received in revised form 20 July 2017 Accepted 21 July 2017 Available online 24 July 2017 Keywords: Lithium-ion batteries Anode Iron oxide Solid electrolyte interphase Atomic force microscopy

a b s t r a c t Present study provided visual evidence of solid electrolyte interphase (SEI) layer formation on Fe3 O4 anode during charge and discharge using in situ electrochemical atomic force microscopy. AFM images show that SEI layer formed on Fe3 O4 electrode from fluoroethylene carbonate (FEC)-based electrolyte was more stable and compact than that formed from ethylene carbonate (EC)-based electrolyte. In addition, presence of surface cracks on the electrodes indicated poor formation of an intact SEI layer. This observation was more apparent in the EC-based electrolyte. Lack of an intact SEI layer resulted in decomposition of electrolytes which were reflected by presence of large air bubbles and dendrites on the electrode during CV. Although FEC-based electrolyte improved the performance of Fe3 O4 anodes in lithium ion batteries, its protective effects were far from perfect. To accelerate the application of Fe3 O4 or other metal oxide anodes in lithium ion batteries, better electrolytes and sophisticated carbon coating techniques are needed. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Numerous research efforts have been directed at researching for new electrode materials to develop safe rechargeable lithium ion batteries (LIBs) with high energy density and long cycle life. Transition metal oxide (conversion type anode) is one of the most studied anode materials during the past decades [1–4]. Among them, Fe3 O4 is regarded as one of the most promising anodes because of its high theoretical capacity (928 mAh g−1 ), low cost, and environmental benignity [5–12]. Compared to silicon, Fe3 O4 also has moderate volume change during lithiation/delithiation. By designing nanostructural Fe3 O4 and constructing Fe3 O4 -based composites, it is possible to shorten the travel distance of the electrons and lithium ions, and to buffer the volume change of Fe3 O4 during charge/discharge [13–15]. Despite that, Fe3 O4 anode has not been successfully commercialized to-date due to poor cycling per-

∗ Corresponding authors. E-mail addresses: [email protected] (D. Wang), [email protected] (C. Shen). 1 Equal contribution. http://dx.doi.org/10.1016/j.apsusc.2017.07.206 0169-4332/© 2017 Elsevier B.V. All rights reserved.

formance. Moreover, Fe3 O4 anode has surface instability due to the lack of stable solid electrolyte interphase. Solid electrolyte interphase (SEI), a sacrificial layer reduced from electrolyte solution, enables commercial lithium-ion batteries (LIBs) to operate at a proper potential window range (usually 0–4.2 V, vs. Standard Hydrogen Electrode) in the presence of organic electrolyte [16]. Ideally, SEI layer should be compact, insoluble, and irreversibly adhere to the active surface of electrode in order to prevent further decomposition of the electrolyte. Controlling SEI formation on anode has been considered an important approach for safe and stable battery operation. Understanding the evolution of SEI layer as well as the structural changes on electrode surface is important for interpreting the performance of conversion type anode. However, up-to-date, the role of SEI layer in performance deterioration of Fe3 O4 has not been given much attention. In situ scanning probe microscopy (SPM) is a powerful method to investigate morphological changes on the surface of electrode materials during electrochemical process in liquid electrolyte solutions [17–19]. It has the outstanding advantages of being real time, non-invasive and atomic resolution [20–24]. Atomic force microscopy (AFM) technique has been used to study the SEI layer on model system with flat surface such as HOPG [20,21,25–30], silicon anode [31–35], MnO [36,37] and embedded

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single particle (LiNi0.5 Mn1.5 O4 ) electrode [30]. To the best of our knowledge, the structure of SEI layer on Fe3 O4 anode has not been characterized using direct visualization method. Present study aimed to investigate the surface structure evolution of SEI layer on Fe3 O4 formed from EC and FEC-based electrolytes by in-situ AFM. Understanding of the evolution of SEI layer and Fe3 O4 particles structure will further improve our understanding on how SEI layer of metal oxides affects battery performance.

2.4. Characterization Ex-situ XPS analysis was performed on Kratos Axis Ultra Xray photoelectron spectrometer using 1253.6 eV Mg K␣ X-rays. Samples were removed from the cells and rinsed with dimethyl carbonate (DMC) to remove residual salt and solvent in an argonfilled glovebox. Samples were then transported to XPS facility in sealed bags to avoid contact with air. The analyzed area of electrode was 300 × 700 ␮m2 . The binding energies were referenced to the hydrocarbon C1s photoelectron peak at 284.8 eV.

2. Experimental section 2.1. Preparation of Fe3 O4 electrode Fe3 O4 electrodes were prepared using our previously method [38]. Fe3 O4 films were deposited directly onto copper electrodes (14 mm in diameter) by applying a constant cathodic current (5 mA) for 600 s. The mixed electrolyte solution of 0.05 M Fe2 (SO4 )3 , 1 M NaOH and 0.1 M triethanolamine (TEA) was used for the deposition and kept in an oil bath at 90 ◦ C in air. Copper disk, carbon paper and platinum wire was used as working electrode, counter electrode, and reference electrode, respectively. After deposition, Cu disks were cleaned with water and ethanol, then dried in air. All electrodeposition experiments were performed using an electrochemical workstation (CHI660D, Shanghai Chenhua instrument, Co. Ltd.). 2.2. Battery performance test Electrochemical measurements were carried out using a 2032type coin cell system. Weights of the Fe3 O4 films before and after deposition were determined using analytical balance to the accuracy of 0.01 mg. The average loading of Fe3 O4 was ∼0.8 mg cm−2 . 2032-type coin cells were assembled in argon-filled glovebox with oxygen and moisture concentrations below 0.1 ppm. Fe3 O4 film was used as working electrode and lithium wire was used as counter electrode. Celgard 2400 polypropylene was used as separator. The electrolytes used were comprising of a solution of 1 M lithium hexafluorophosphate (LiPF6 ) dissolved ether in a mixture of EC/EMC/DMC (1:1:1, volume ratio) or FEC/EMC/DMC (1:1:1, volume ratio). Charge/discharge measurements were carried out galvanostatically at 0.5C over a voltage range of 0.01–2.5 V (vs. Li/Li+ ) using a commercial battery test system (LAND model, CT2001A) at room temperature. 2.3. Electrochemical performance and image scanning using EC-AFM Electrochemical impedance spectroscopy (EIS) spectra were collected with potentiostat/galvanostat 1470E equipped with a frequency response analyzer 1455A from Solartron. In situ AFM (Bruker Icon) experiments were conducted in argon-filled glovebox (MBRAUN, H2 O ≤ 0.1ppm, O2 ≤ 0.1ppm) at room temperature. The Li-Fe3 O4 cell was composed of Fe3 O4 substrate as working electrode (WE) and Li wire as counter and reference electrodes (CE and RE). Electrolytes used were 1 M LiPF6 dissolved either in a mixture of ethylene carbonate or fluoroethylene carbonate/dimethyl carbonate (EC/DMC or FEC/DMC, volume ratio of 1:1) (Shanshan Corporation). In order to study the SEI layer formation, the Li-Fe3 O4 cell was studied by cyclic voltammetry (CV) at a scanning rate of 0.5 mV s−1 between 3.0 and 0 V. AFM topography was collected simultaneously in ScanAsyst mode using nitride coated silicon probes (tip model: SCANNASYST-FLUID with k = 0.7 N m−1 , Bruker Corporation).

3. Result and discussion Fig. 1 shows the surface structural evolution of Fe3 O4 electrode during the first lithiation-delithiation cycle in EC-based electrolyte between 3.0 and 0.0 V (vs Li+ /Li) by CV method (OCV is about 2.5 V). XRD has been conducted in our previous work to confirm the crystal structure of Fe3 O4 [38]. Fig. 1a shows a typical surface structure of Fe3 O4 electrode for potential between 3.0 to 2.82 V. No obvious changes in the surface structure of Fe3 O4 electrode can be observed for potential between 3.0 to 1.0 V. When the potential was cycled down to about 0.8 V, very thin SEI layer can be observed (Fig. 1b, indicated by the white arrows). Similar finding can be observed in the first CV curve of Fe3 O4 electrode cycled in EC based electrolyte at room temperature between 0 and 3.0 V (vs Li+ /Li) at a scan rate of 0.5 mV s−1 (Fig. S1). A sudden increase in cathodic peak at around 0.8 V can be ascribed to decomposition of electrolytes and the formation of SEI layer [30]. Decreased reduction potential leads a further growth of the SEI layer (Fig. 1c) and Li insertion. The thick SEI can be found at some area of the particle (indicated by the red arrow in Fig. 1c). Line profiles of Fig. 1 a and c shown the height of SEI layers is ∼0–50 nm (indicated in Fig. S2). Fig. 1a’ and c’ showed after the growth of SEI layer, the Fe3 O4 particle size changed from 1.62 ␮m to 1.77 ␮m. During the reverse scan of potentials, SEI layer formed in EC-based electrolyte was soft and could be modified by AFM tip during imaging. Surface of the SEI layer became smoothened due to repeated scanning by AFM tip (Fig. 1d–h). One should note that during the whole CV process, the volume change (except the cracks) is far less than theoretical predication. This is similar to what have been observed in Si anodes [32,39]. The exact mechanism of this difference is not clear yet. During discharge process, no obvious Fe3 O4 particles broken or surface cracks can be observed (Fig. 1a–c). However, obviously particles broken and surface cracks can be observed in the charge process (Fig. 1d–h). Some particles have been broken seriously (indicated by white square area in Fig. 1h). Fe3 O4 particles showed the volume change as indicated by the arrows, at which the particle size changed from 1.77 ␮m to 1.87 ␮m. Also, line profiles of Fig. 1a’ and h’ shown the height of swelling is ∼125 nm (indicated in Fig. S2). Surface cracks (indicated by white arrow in Fig. 1f–h) can be observed during lithium extraction when the potential was cycled back to 3.0 V which continue to increase in sizes until CV completed. Thus, SEI layer formation mainly caused the thickness change while volume change can lead to surface cracks. Cracking of the SEI layer resulted in exposure of fresh Fe3 O4 particles to electrolyte; hence, continuous decomposition of electrolyte. Decomposition of electrolyte was also accompanied by formation of large air bubbles (Fig. S3a) and lithium dendrites on the Fe3 O4 anode (white precipitates in Fig. S3b). Bubble formation is a direct evidence of electrolyte decomposition which is due to poor formation of a proper SEI layer on Fe3 O4 electrode. Existence of dendrites on the first cycle of charge/discharge is also another indication of poor formation of SEI layer on Fe3 O4 anode. Our previous study showed that SEI layer formed on HOPG surface in FEC based electrolyte is denser and harder than that the

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Fig. 1. AFM images of the structural evolution of SEI layer on Fe3 O4 in 1 M LiPF6 EC/DMC electrolyte cycled at a scanning rate of 0.5 mV/s under potential range of (a) 3–2.82 V; (b) 0.80–0.47 V; (c) 0.25–0–0.15 V; (d) 1.04–1.48 V; (e) 1.48–1.83 V; (f) 1.83–2.15 V; (g) 2.17–2.48 V; (h) 2.9–3.0 V. The scale bars are 1 ␮m.

Fig. 2. AFM images of the structural evolution of SEI layer on Fe3 O4 in 1 M LiPF6 FEC/DMC electrolyte cycled at a scanning rate of 0.5 mV/s under potential range of (a) 3–2.79 V; (b) 0.60–0.42; (c) 0.07–0–0.15 V; (d) 2.78–3.0 V. The scale bars are 1 ␮m.

one formed from EC-based electrolyte [40]. Thus, we examined if FEC based electrolyte can help to improve the performance of Fe3 O4 anode by cycling Fe3 O4 electrodes in 1 M LiPF6 FEC/DMC electrolyte. Fig. S4 shows the first CV curve of Fe3 O4 electrode cycled in FEC-based electrolyte at room temperature between 0 and 3.0 V (vs Li+ /Li) at a scan rate of 0.5 mV s−1 . Fig. 2a shows a typical surface structure of Fe3 O4 electrode for potential between 3.0 and 2.79 V.

When the potential was cycled down to about 0.77 V, SEI layers can be observed to be deposited on the Fe3 O4 particles (indicated by white arrow in Fig. 2b’). Compact SEI was found to cover the whole surface when the potential was reduced to zero (Fig. 2c’). The morphology of surface remained unchanged during the anodic sweeping (Fig. 2d). The SEI layer formed on Fe3 O4 electrode consisted of closely packed nanometer-sized particles (Fig. 2c’ and d’).

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Fig. 3. AFM images of Fe3 O4 anode after the 1st CV (a), and the surface morphology after scratched (b) in 1 M LiPF6 EC/DMC electrolyte. AFM images of Fe3 O4 anode after the 1st CV (c), and the surface morphology after scratched (d) in 1 M LiPF6 FEC/DMC electrolyte.

Fig. 4. (a) Cycling performance comparison EC-based and FEC-based electrolyte. (b) EIS analysis of Fe3 O4 anode after first cycle voltammetry.

Fe3 O4 particles showed SEI layer formation as indicated by the arrows, at which the particle size changed from 445 nm to 575 nm. Line profiles of Fig. 2a’ and d’ shown the height of SEI layers is 15–35 nm (indicated in Fig. S5). During the whole CV, no obvious Fe3 O4 particles broken (indicated by the two white square area in Fig. 2a–d) or surface cracks can be observed, which indicating a good SEI layer formation on the Fe3 O4 particle surface. To further explore the property of SEI layer, SEI layers were formed under the same conditions as Figs. 1 and 2 by ex-situ experiments. Precipitates (SEI layer) formed on Fe3 O4 electrode obtained from LiPF6 /EC/DMC electrolyte (Fig. 3a) can be scraped off by repeated AFM scanning in the contact mode with a force of 175 nN (Fig. 3b). However, SEI layer formed on Fe3 O4 particles obtained

from LiPF6 /FEC/DMC electrolyte (Fig. 3c) can resist the scrapping and keep complete intact even when the force was increased to 245 nN (Fig. 3d). Such result indicated that the SEI layer formed under FEC based electrolyte is harder than that under EC based electrolyte. We further investigated the electrochemical performance of Fe3 O4 electrode in EC and FEC-based electrolytes using CR2032 coin cells assembled in argon filled glove box. The as prepared Fe3 O4 electrode was used as working electrode. Fig. 4a shows battery cycling performance of the Fe3 O4 electrodes tested in EC and FEC-based electrolytes. The capacities of the first cycle were ∼1200 mAh/g in both EC and FEC-based electrolytes, and initial coulombic efficiencies were of 48.9% and 51.9 respectively. The large irreversible capacity lost can be ascribed to the formation of SEI layer.

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Fig. 5. Model diagram of Li ion diffusion during the redox reaction in 1 M LiPF6 EC/DMC electrolyte (a) and in 1 M LiPF6 FEC/DMC electrolyte (b).

Fig. 6. XPS spectra of the SEI on Fe3 O4 electrode cycled at a scanning rate of 0.5 mV/s between 3.0 and 0 V in 1 M LiPF6 EC/DMC electrolyte and 1 M LiPF6 FEC/DMC electrolyte.

Battery cycling performance (after 25 cycles) of the Fe3 O4 electrode demonstrated that FEC sample was more stable than that of EC sample (Fig. 4a). Because this electrodeposited Fe3 O4 thin film have no conductive agent and binder, so the capacity decreased faster than Fe3 O4 electrodes prepared from slurry paste method. The electrochemical impedance spectra for the two samples was investigated after the first CV. The semicircles at high and medium frequencies consist of the SEI film (RSEI ) and the charge-transfer resistance (Rct ). Obviously, the SEI (RSEI ) resistance of Fe3 O4 electrode cycled in FEC based electrolyte were much larger than that of Fe3 O4 electrode cycled in EC based electrolyte. This is due to the fact that a harder and denser SEI layer formed on Fe3 O4 electrode surface in FEC based electrolyte. On the basis of the aforementioned findings, a model diagram shown in Fig. 5 compares the differences in SEI layer formed in EC and FEC-based electrolytes. In LiPF6 /EC/DMC electrolyte (Fig. 5a), the Fe3 O4 particles were covered with inhomogeneous and defective deposited SEI layer. In the process of conversion reaction, Li ion could migrate through the defective SEI film easily. Fast insertion and disinsertion of Li ion can lead to sudden stress increasing, as a result, the integrity and mechanical property of the particle will be damaged. In LiPF6 /FEC/DMC electrolyte (Fig. 5b), dense and compact (uniform) SEI was found to cover the whole Fe3 O4 particles. During the conversion reaction, Li ion can tunnel through the SEI layer steadily. Thus, we can infer that the structure of SEI can suppress Fe3 O4 particles broken and cracking.

Table 1 Atomic concentrations of C, O, Li and F obtained from XPS. atomic concentration (%)

Fe3 O4

EC/DMC FEC/DMC

C

O

Li

F

47.69 47.28

49.93 48.09

1.51 1.80

0.87 2.83

Fig. 6 shows the XPS analysis of the electrodes after first CV. The C 1s spectra exhibits a main component 284.8 eV indicates C H and C C bonds, which is attributed to contaminated hydrocarbon and carbon atoms of organic species bound to carbon or hydrogen only. The shoulders at 285.2 eV, 286.8 eV, 288.2 eV, 289 eV are the presence of C O H, C O C, COO− , CO3 2− bonds, which indicated alkyl carbonates ROCO2 Li could be the main organic component of SEI. The O 1s spectra, the presence of C O C and CO3 2− bonds could be associated with ROCO2 Li species. The F 1s and Li 1s spectra revealed the LiF is the main inorganic component of SEI. Both samples have high atomic concentration of organic component composed of carbon and oxygen. Inorganic species such as LiF were also found in both samples with higher atomic concentration in FEC sample than that in EC sample. The atomic concentrations of C, O, Li, and F were summarized in Table 1.

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4. Conclusions In situ AFM technique is able to provide direct visualization of the SEI layer formation on Fe3 O4 electrodes. The SEI layer formed on Fe3 O4 electrode from EC based electrolyte was looser than that formed from FEC based electrolyte. The collapse of original structure were observed on Fe3 O4 electrode surface during charging and discharging probably due to inhomogeneous and defective deposition cover on Fe3 O4 particles, which allow for iron ion migrate out of SEI layer. Cracking of SEI layer resulted in continues electrolyte decomposition which was supported by presence of large air bubbles and dendrites on Fe3 O4 electrode during the CV. In FEC based electrolyte, passive films closely packed formed on Fe3 O4 particles is effective in preventing the migration of iron ion. Due to the dense and compact SEI layer on Fe3 O4 electrode surface, FEC based electrolyte improved the performance of Fe3 O4 anodes in lithium ion batteries. But its protective effects were far from perfect. It has been known that SEI layer on carbon is compact and stable, thus, it might be helpful to form a carbon coating layer on the surface of Fe3 O4 , and to improve its cycling performance. To accelerate the application of Fe3 O4 anode or other metal oxide anodes in lithium ion batteries, better electrolytes and sophisticated carbon coating techniques are needed.

Acknowledgements We thank the National key research and development program (Grant No. 2016YFB0100106), Ningbo international cooperation project (2016D10015), Ningbo Enrich the people project (2017C10010) and the Youth Innovation Promotion Association (CAS) for financial supports.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2017.07. 206.

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