Direct visualization of solid electrolyte interphase on Li₄Ti₅O₁₂ by in situ AFM

Abstract

Whether Li₄Ti₅O₁₂ has a solid electrolyte interphase (SEI) layer on the electrode surface has been the subject of controversy for a long time due to the delicate nature of this SEI layer and the lack of reliable characterization tools. In this paper, we report direct visualization of SEI layer formation on an LTO electrode surface by in situ atomic force microscopy under potential control. Our results showed that no SEI layer formed from EC/DMC based electrolyte in the potential range of 2.5–1.0 V. However, by extending the reduction potential down to zero, it is possible to grow a SEI layer on the LTO surface. The strategy of forming an SEI layer by discharging an LTO anode down to 0 V in the first cycle and then operating the battery in the normal range of 2.5–1.0 V might be a facile method to improve LTO battery performance. Various additives such as Vinylene Carbonate (VC), ethylene sulfate (ES) and fluoroethylene carbonate (FEC) were used as additives to evaluate their effect on SEI layer formation.

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Introduction

Li-ion battery (LIB) systems with high energy density and long cycle life are important power sources for modern portable electronic devices as well as energy storage.^1,2 However, anode materials such as graphite or lithium metal have the risk of lithium plating at high current densities which could eventually short-circuit the battery.^3,4 Silicon, another important anode, suffers from large volume expansion (∼300%) during lithium insertion/extraction.^5,6

Spinel lithium titanate (Li₄Ti₅O₁₂, LTO), with a theoretical capacity of 175 mA h g⁻¹, is one of the attractive negative electrode materials for high-performance LIBs because of its zero-strain characteristic during charge/discharge.^7,8 In addition, LTO has a high redox potential at around 1.5 V vs. Li⁺/Li, preventing any lithium plating. However, batteries with LTO anode are still not widely used by the industries mainly due to the unexpected severe gassing^9–11 during charge/discharge cycles despite the fact that LTO has a conventional operating voltage (>1 V vs. Li⁺/Li), which is higher than that of the electrolyte decomposition.

It is well-known that solid electrolyte interphase (SEI) layer formed on graphite anode at around 0.8 V vs. Li⁺/Li to prevent further electrolyte decomposition. Ideally, SEI layer should be compact, insoluble and irreversibly adhere to the active surface. Upon formation, SEI layer should be stable enough to stop further decomposition of the electrolyte solution.^12,13 Although, SEI formation on graphite has been well-studied, knowledge regarding formation of SEI on LTO anode is still controversial. For example, D. W. Dees and D. Gonbeau et al. found no SEI formation on the surface of LTO from the impedance of cells with LiNi_0.8Co_0.15Al_0.05O₂-based positive and LTO-based negative electrodes.^14,15 J. K. Kim et al. conducted experimental and simulated EIS study and found SEI film can be formed on the surface of LTO anode even above 1 V.¹¹ A. Benayad et al. revealed formation and dissolution of the SEI layer during charge and discharge under the voltage range of 1.0–2.5 V, respectively.¹⁶ Some researcher hypothesized that although no SEI layer formation above 1.0 V, such protection layer can be formed under 1.0 V, where most electrolytes can be reduced. For example, C. B. Yue et al. revealed that electrolytes decomposed irreversibly below 1.0 V and SEI film was formed on LTO surface at 0.7 V in the first discharge process.¹⁷ So far, if LTO really constitutes a SEI layer is not thoroughly experimentally verified.^18,19 The publications that cover this area are not in complete agreement yet.

Therefore, it is necessary and important to study reduction reactivity of electrolyte on LTO anode above and below 1 V to understand SEI formation and prevent gas generation of LTO anode batteries.

In situ analysis by atomic force microscopy (AFM) with electrochemical control on the electrode allows direct study of surface reaction and topography evolution, while maintaining conditions that closely simulate real-life devices with minimal destructive impact.^20–30 It allows direct visualization of changes in SEI layers as a function of applied electrochemical potentials and/or electrolyte compositions/additives.^30,31 Therefore, electrochemical AFM has became an important tool to clarity the whole reactions during the SEI formation.^32,33

Here, we report the morphological differences of SEI layer formed from ethylene carbonate (EC)-based electrolytes on LTO electrode. Vinylene carbonate (VC), ethylene sulfate (ES) and fluoroethylene carbonate (FEC) were used as additives to evaluate their effect on SEI layer formation. We provide direct evidence of SEI layer formation during cyclic voltammetry (CV) process which brings new insight for understanding of SEI formation on LTO electrode.

Experimental

Electrochemical performance and image scanning using EC-AFM. CR2032-type coin cells were assembled to evaluate the electrochemical performance. The coin cells consisted of LTO powders as the cathode material, lithium foil as the anode, and polypropylene as the separator. The cathode consisted of 80 wt% LTO, 10 wt% Super-P carbon black and 10 wt% poly(vinylidene fluoride) (PVDF) binder to form an active material of about 2 mg cm⁻², excluding the copper current collector. As a control experiment, Super-P electrode was also prepared with 90 wt% Super-P carbon black and 10 wt% poly(vinylidene fluoride) (PVDF) binder (Fig. S1†). In situ AFM (Bruker Icon) experiments were conducted in an argon-filled glovebox (MBRAUN, H₂O ≤ 0.1 ppm, O₂ ≤ 0.1 ppm) at room temperature. The LTO/Li cell consisted of LTO as working electrode (WE) and Li wire as counter and reference electrodes (CE and RE). Electrolyte solution used was 1 M LiPF₆ dissolved in a mixture of ethylene carbonate/dimethyl carbonate (EC/DMC, volume ratio of 1 : 1) (Zhangjiagang Guotai-Huarong New Chemical Material Corporation) with 20% vinylene carbonate (VC) or ethylene sulfate (ES) or fluoroethylene carbonate (FEC) by volume (Fu Jian Chuang Xin Science and Develops Corporation). The EC cell was covered with a thin rubber skirt which attached to scanner. During AFM imaging, the circulation of the grove box was stop to prevent solvent evaporation. The EC cell working under this condition can last for >24 h without changing the concentration of the electrolyte. In order to form SEI layer, the LTO/Li 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). Contact mode was applied to scratch the surface using the same probe.

Results and discussion

Fig. 1 shows the CV of LTO sample in the first few cycles in the voltage range of 2.5–1.0 V. All CV curves exhibited similar redox peaks around at 1.46 V and 1.65 V, which were attributed to the redox of Ti⁴⁺/Ti³⁺. The result is in consistent with that reported in literature.^19,34 No other obvious reduction peaks were observed in the voltage range of 2.5–1.0 V.

Fig. 1 CVs of LTO/Li cell with no additive cycled between 2.5 and 1.0 V with a scan rate of 0.5 mV s⁻¹.

Surface morphology evolution of the LTO was monitored by peak force tapping mode under potential control. Peak force tapping mode performed a very fast force curve at every pixel in the image. The peak force of each curves was then used as the imaging feedback signal to provide direct force control (for example, minimal repulsive force). This allows it to operate at even lower forces than tapping mode to protect the delicate samples. Our previous paper demonstrated that AFM operated under peak force tapping mode enable perfect imaging of SEI layer.³⁵ AFM images shown in Fig. 2 revealed no obvious structural change of LTO particles. The particle with size of 790 nm × 1050 nm remained unchanged after the CV and no SEI layer formation was identified. The result is consistent with the CV, in which no reduction peak of electrolyte was detected. A careful examination of the LTO particle revealed nano-cavities on the LTO particle surface, which might originated from the evaporation of gases from the reaction sites. Gas formation was observed during the CV process using the optical microscopy integrated to the AFM. One should note that such gassing behaviour should not be related with the missing reduction peak in the CV. It is well-known that gassing occurs even when the LTO-based battery is not cycle; and store at room temperature.³⁶

Fig. 2 In situ AFM images of LTO electrode cycled between 2.5 and 1.0 V with a scan rate of 0.5 mV s⁻¹ during first CV. (a) Before CV (b) after CV.

SEI layer formation on graphite is known to occur at potential around 0.8 V.^27,35 With the aim of forming SEI layer on the surface of LTO in the first cycle (reduce the electrolyte below 1.0 V) and then operating the battery in normal working potential range (for example 2.5–1.0 V), we extended the CV voltage down to 0.0 V. Fig. 3 shows the CV of LTO electrode cycled between 2.5 and 0.0 V. A broad and irreversible peak appeared at around 0.7 V during reduction process, which has been ascribed to the electrolyte decomposition (SEI film formation) on the LTO electrode.³⁴

Fig. 3 CV of LTO/Li cell cycled between 2.5 and 0.0 V with a scan rate of 0.5 mV s⁻¹.

Unlike SEI layer formed on graphite electrodes, where SEI layer usually in the range of 20 to few hundred nm has been detected,³⁵ only slight morphological change of LTO particles was observed by AFM. Small amount of LTO particles showed signs of SEI layer formation as indicated by the arrows, at which the particle size changed from ∼295 nm to 330 nm after CV (Fig. 4). Considering the bubble formation observed during the CV process, it is assumed that the electrolyte mostly decomposed rather than forming precipitates on the LTO electrode. No further structural change was observed during the following CVs.

Fig. 4 AFM images of LTO electrode before (a) and after (b) cycled between 2.5 and 0.0 V.

Electrolyte additives are an effective and economic method to enhance Li-ion battery performance.^11,37 These electrolyte additives usually display a higher reduction potential than the other electrolyte components and thus can be preferentially reduced. The reduction products usually precipitate at the graphite surface leading to formation of SEI layer to prevent further decomposition of electrolyte.^38,39 Certain electrolyte additives have been reported to assist formation of SEI layer on electrode surface. For example, addition of 2% VC into 1 M LiPF₆/EC + DMC + EMC has been reported to help formation of a protective SEI film on LTO electrode and improve its performance.¹¹ To verify the effect of additives on formation of SEI layer, several common additives have been added into the 1 M LiPF₆/EC + DMC electrolyte. Fig. 5 shows the CV of LTO/Li cell cycled in the 1 M LiPF₆/EC + DMC electrolyte with 20% of VC between 2.5 and 0.0 V. A reduction peak was observed at around 0.6 V, which is similar to that found in 1 M LiPF₆/EC + DMC electrolyte without additives. Moreover, a weaker peak around 0.9 V was observed, which was ascribed to the reduction of VC. Highly reactive VC tends to polymerize during cell cycling; which favours the formation of passivating film on the graphite anode.^40–42 However, VC polymerization was found to occur on the graphite anode side in the LiFePO₄/graphite cell; and no evidence of polymerization was found on the cathode side. So far, the polymerization mechanism induced by the VC additive is still not well understood.

Fig. 5 CV of LTO/Li cell cycled in the 1 M LiPF₆/EC + DMC electrolyte with 20% of VC between 2.5 and 0.0 V with a scan rate of 0.5 mV s⁻¹.

Although significant reduction peak was detected in the CV, the no obvious polymerization was observed on LTO electrode. AFM images before and after cycling did not show much differences in terms of surface morphology (Fig. 6a and c). Careful zoom in of the images (Fig. 6b and d) showed formation of scattered and non-uniform precipitates (diameter of ∼200 nm; height of 20 nm) on the LTO surface after the CV. Moreover, we observed gassing behaviour during the CV process indicating different reduction mechanism of VC additive on LTO as compared to the graphite anode. This is probably due to the catalytic effect of LTO. Detailed reason is still unclear at the moment.

Fig. 6 AFM images of LTO electrode before (a and b) and after (c and d) cycled in the 1 M LiPF₆/EC + DMC electrolyte with 20% of VC between 2.5 and 0.0 V.

ES has also been regarded as another effective additives for SEI film formation on graphite. ES was reported to reduce prior to EC at the interface between graphite and electrolyte which helped the formation of SEI layer.^43,44 The effects of ES on formation of SEI layer on LTO electrode is not much investigated to date. We performed experiment to verify the effects of ES on formation of SEI film on LTO surface.

Fig. 7 shows the CV of LTO/Li cell cycled in the 1 M LiPF₆/EC + DMC electrolyte with 20% of ES between 2.5 and 0.0 V. A reduction peak was observed at around 0.6 V, which is similar to that found in aforementioned experiments. No other obvious reduction peak was observed. It is possible that reduction peak of ES is overlapping with the redox peak of Ti⁴⁺/Ti³⁺. The second and the third CV cycled from 2.5 to 1.0 V showed no obvious change, indicating stability of the surface structure.

Fig. 7 CVs of LTO/Li cell cycled in the 1 M LiPF₆/EC + DMC electrolyte with 20% of ES between 2.5 and 0.0 V with a scan rate of 0.5 mV s⁻¹.

Fig. 8a and b shows AFM images of LTO electrode before and after the first CV. Precipitates were clearly formed on LTO surface.

Fig. 8 AFM images of LTO electrode before (a) and after (b) the first CV in the 1 M LiPF₆/EC + DMC electrolyte with 20% of ES between 2.5 and 0.0 V. Surface morphology of LTO surface after the 3^rd CV (c), and the surface of (c) after scratched (d). Scale bar: 500 nm.

Height profiles analysis shown in Fig. S2† revealed SEI layer has a height variation of about 80 nm, which was ∼40 nm rougher than pristine LTO surface. One should note that because LTO is a zero strain material, thus increased in height can only be ascribed to SEI layer formation. Fig. 8c shows AFM image of LTO surface after 3rd CV. It was found that SEI layer formed can be partially removed after repeating scanning, indicating instability of these layers. Indeed, SEI layer can be removed by AFM tip in contact mode. Fig. 8d shows LTO surface scratched by AFM tip under a force of 70 nN in contact mode. The exposed surface demonstrated similar structure as that before cycling (Fig. 8a), indicating the scratched top layer was indeed SEI layer.

SEI formed from FEC-based electrolyte on anode is known to improve cyclability and shelflife of LIBs.^45,46 This is mainly due to preferential reduction of FEC over other electrolyte species which led to formation of an electrochemically stable SEI layer on the anode.^47–51 Fig. 9 shows the CVs of LTO/Li cell cycled in the 1 M LiPF₆/EC + DMC electrolyte with 20% of FEC between 2.5 and 0.0 V. Similar to VC additive, two clear reduction peaks were observed at around 0.6 V and 0.95 V. Higher reduction potential (0.95 V) is mainly due to introduction of F group into FEC which reduced both HOMO and LUMO of FEC. It is also interesting to note that oxidation peak located at round 1.65 V is much stronger than those of VC and ES additives (Fig. S3†). Moreover, the sample with FEC additive had the smallest potential differences among the samples (VC, ES, and FEC). This indicates that LTO anode with FEC additive had the lowest electrode polarization.

Fig. 9 CVs of LTO/Li cell cycled in the 1 M LiPF₆/EC + DMC electrolyte with 20% of FEC between 2.5 and 0.0 V with a scan rate of 0.5 mV s⁻¹.

Fig. 10 shows AFM images of LTO electrode cycled at various potential stage. As indicated by arrows, SEI layer formed as protrusions with height of about 20–30 nm can be identified on the LTO surface. The coverage of the SEI layer is much lesser than that formed from ES additive but more than that formed from VC additive. FEC can easily decomposed into VC and HF, the later can react with Li ion forming a good SEI layer of LiF. Thus, it is no surprise that FEC additive formed a thicker SEI layer than VC additive. In comparison to SEI layer formed on graphite electrode (Fig. S4†) (thickness in the range of about 300–500 nm), SEI layer formed on LTO surface was thinner. The result demonstrated different polymerization mechanism exists between graphite and LTO electrodes. Further research using complementary tools such as XPS, IR, Raman and gas chromatography-mass spectrometry would answer these unknown questions.

Fig. 10 AFM images of LTO electrode before (a) and after (b) cycled in the 1 M LiPF₆/EC + DMC electrolyte with 20% of FEC between 2.5 and 0.0 V. Scale bar: 500 nm.

Conclusions

We provide direct evidence of SEI layer formation on LTO surface by in situ AFM. Although our results showed no SEI layer formation from the EC/DMC electrolyte at the potential range of 2.5–1.0 V, we showed further growth of SEI layer on the LTO surface was possible by extending the reduction potential down to zero. Such strategy could be applied in future to firstly form a protective SEI layer at a more negative potential (for example, down to 0.0 V) and then charge/discharge LTO batteries at the normal working potential range (for example 2.5–1.0 V). VC, FEC, and ES were used as additives to evaluate their effect on SEI layer formation. It was found that ES additive assisted formation of a dense SEI layer on LTO surface. Meanwhile, electrolyte containing FEC additive has the smallest electrode polarization. A combination of both ES and FEC additives might be a solution to form proper protective SEI layer on LTO while maintaining good battery performance.

Acknowledgements

We thank the National Natural Science Foundation of China (Grant No. 21303236), National key research and development program (Grant No. 2016YFB0100106), and project of “Investigation on the synthetic technologies of sub-micro-size Li₄Ti₅O₁₂ utilized for energy-storage batteries” from State Grid Corporation of China. Cai thanks the financial support from the Ningbo 3315 plan, the Youth Innovation Promotion Association, CAS, and SRF for ROCS, SEM.

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Footnote

† Electronic supplementary information (ESI) available: AFM image of electrode composes only Super-P and PVDF, graphite, and height profiles of LTO surface. See DOI: 10.1039/c6ra16208h

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