Letter Cite This: J. Phys. Chem. Lett. 2017, 8, 6048−6052
pubs.acs.org/JPCL
Understanding How Nitriles Stabilize Electrolyte/Electrode Interface at High Voltage Huozhen Zhi,† Lidan Xing,*,† Xiongwen Zheng,† Kang Xu,*,‡ and Weishan Li*,† †
Engineering Research Center of MTEES (Ministry of Education), Research Center of BMET (Guangdong Province), Engineering Lab. of OFMHEB (Guangdong Province), Key Lab. of ETESPG (GHEI), and Innovative Platform for ITBMD (Guangzhou Municipality), School of Chemistry and Environment, South China Normal University, Guangzhou 510006, China ‡ Electrochemistry Branch, Sensor and Electron Devices Directorate, Power and Energy Division, U.S. Army Research Laboratory, Adelphi, Maryland 20783, United States S Supporting Information *
ABSTRACT: Nitriles have received extensive attention for their unique ability in stabilizing electrolytes against oxidation at high voltages. It was generally believed that their anodic stability originates from a monolayer of chemisorbed nitrile molecules on transition-metal oxide surface, which physically expels carbonate molecules and prevents their oxidative decomposition. We overturn this belief based on calculation and experimental results and demonstrate that, like many high voltage film-forming electrolyte additives, nitriles also experience an oxidative decomposition at high voltages, and the high oxidation stability of nitrile-containing electrolytes is merely the consequence of a new interphasial chemistry. This important mechanistic correction would be of high significance in guiding the design of new electrolytes and interphases for the future battery chemistries.
I
associated with nitrile oxidation during the linear sweep voltammetry (LSV), nitrile has been regarded as thermodynamically stable against oxidation.4−12 However, such mechanistic claims raise various suspicions. It is well-established that solid electrolyte interphases (SEI) formed on anode surfaces typically consist of two layers, with an outer layer of more organic and an inner layer of more inorganic natures.13 While electrolyte components could infiltrate through the porous outer layer, the dense inorganic layer physically prevents any direct contact between electrolytes and electrode surface.14 Therefore, it is very unlikely that an adsorbed monolayer without covalent-linkages could be electrochemically tough as the dense SEI inner layer, which must resist the thermodynamic driving force and suppress carbonate oxidation up to ∼6 V versus Li/Li+. Meanwhile, the absence of observable redox events in voltammetry does not necessarily indicate thermodynamic stability, as the sensitivity of this dynamic technique to the decomposition processes significantly depends on the reaction kinetics relative to the limited time frame of experiments. One good example would be those wellestablished additives such as vinyl carbonate (VC), which forms dense interphases on both anode and cathode surfaces but does not exhibit any characteristic reduction or oxidation peaks in voltammetry.15,16 Thus the mechanism of how nitrile stabilizes cathode/electrolyte interface remains unclear, and
ncreasing operating voltage is one of the key approaches to promote the energy densities of electrochemical energy storage devices such as batteries and capacitors. However, this approach is confined by the electrochemical stability of electrolytes, which reduce at low and oxidize at high potentials. While most carbonate-based electrolytes can effectively stabilize at potentials of Li through the formation of an interphase, their stability against oxidation often encounters difficulty at or above 4.5 V versus Li. Hence, new electrolytes of high anodic stability are critical for the development of next-generation battery chemistries of high-energy densities.1 Among the limited choices of polar aprotic molecules suitable as electrolyte solvents, sulfones and nitriles have been credited for their extraordinary anodic stabilities, which have found various applications in lithium ion batteries (LIBs) and supercapacitors.2,3 However, in the absence of molecular-level understanding of how they stabilize the electrolyte/cathode surface at high potentials, current efforts of electrolyte development are not entirely based on science but mainly rely on semiempirical trial-and-error. The high anodic stability of sulfones has been ascribed to their inertness toward lithium salt and cosolvent at high voltage when used as bulk solvent.3 On the contrary, the anodic stability of nitrile-containing electrolytes was believed to arise from the preferential chemisorption of nitrile molecules on transition metal (TM) oxide surface, which generates a layer of (−CN−TM) complexes that physically expels carbonate solvents from intimate contact with cathode.4−12 Moreover, because no apparent oxidation peak can be identified to be © 2017 American Chemical Society
Received: October 15, 2017 Accepted: November 30, 2017 Published: November 30, 2017 6048
DOI: 10.1021/acs.jpclett.7b02734 J. Phys. Chem. Lett. 2017, 8, 6048−6052
Letter
The Journal of Physical Chemistry Letters
conclude that SN would oxidize instead of remaining thermodynamically stable against oxidation. The structure of SN in SN−PF6− complex after oxidation is significantly different from that of isolated SN or SN−Li+. H transfer is observed in SN−PF6− after oxidation, but fortunately without HF generation that often occurs in the similar complexes formed by carbonate molecules (Figures S3 and S4). The H+ that breaks away from −CH2 group of SN is captured by its −CN group instead of PF6−, which should ascribe to the high negatively charge-distribution of N element (see Figure S1). Meanwhile, the calculated oxidation potential of SN−Li+−PF6− is very similar to that of SN−PF6−, indicating that the effect of Li+ should have been overwhelmed by PF6−. In other words, the oxidation stability of SN is dictated by its association with anion. Analysis of the aftermath of SN−carbonate or solvent−Li+− PF6−−SN oxidation shows that although the oxidation stability of isolate SN is obviously higher than that of carbonate solvents, the electron lost during the oxidation process is mainly from SN instead of carbonate (Figures S5 and S6). It can be found from the optimized structures that H transfers from SN to carbonate after oxidation. Most importantly, there has been no HF generation in the entire oxidation processes, in sharp contrast with the conventional carbonate electrolytes. The absence of HF would have a profound effect on the cycling stability of such electrolytes with high-voltage electrodes. Oxidation of SN leads to SN−e radical cation, SN−e−H radical, and carbonate−H radical, all of which could terminate to create −CN-containing interphase that suppresses the sustained electrolyte oxidation. Probable radical termination reactions can be found in Figure S7. LSV was performed using Pt and Co3O4 as working electrodes. The former is a standard nonporous substrate that is often used in electrochemical measurement due to its inertness against reduction or oxidation, and the latter was used to simulate a porous composite electrode often used in actual electrochemical devices such as batteries and capacitors, where the high surface area would magnify any parasitic process. At only 5% concentration, SN effectively postpones the oxidation limit of baseline electrolyte up to 5.7 and 6.0 V versus Li/Li+ on Pt and Co3O4 electrodes (Figure 2A,C), respectively. No oxidation event can be detected in the voltammetry for SN decomposition. A few caveats should be mentioned here when viewing the data presented in Figures 1 and 2. First of all, the observed oxidation potential of baseline electrolyte on both Pt and Co3O4 electrode (Figure 2) is obviously lower than that of calculated potentials shown in Figure 1. The difference is caused by the absence of electrode material in the calculation models. Borodin et al. demonstrated that the oxidation potential of solvent would change in the presence of electrode material and surface area.25,26 Therefore, the oxidation activity order of solvents shown in Figure 1 is far more important than the absolute values therein. Second, the lower oxidation onset potentials as observed in LSV on Co3O4 surface as compared with Pt surface just reflect the difference in the respective areas of these two electrodes, with the values measured on Co3O4 being more close to reality. Third, unlike the reduction process on anode surface, which mainly involves the reaction of electrolyte components, the mechanism of oxidation process on cathode is quite complicated and may involve the active species of cathode materials themselves, often as reflected in transitionmetal reduction and dissolution13 or oxidation of oxygen layer.27 However, as long as electrolyte oxidation provides the
one needs to unequivocally answer these questions of critical importance: Does nitrile remain oxidative stable at high potentials? Or does it oxidize preferentially at low potential to form a protective interphase? Furthermore, are the (−C N−TM) species detected on electrode surface the monolayers of chemisorbed nitriles on transition metal? Or are they the result of electrochemical reaction? In this work, using succinonitrile (SN) as a representative for nitriles that has been widely used in LIBs as electrolyte additive,6,8,12,17,18 cosolvent19 or solid electrolyte,20 we attempt to answer those questions. Via DFT calculations and electrochemical as well as spectral experiments, we rigorously investigated how SN interacts with other electrolyte components and the effect of such interactions on anodic stabilities of SN at cathode surfaces. It has been well-established that the stability of a solvent molecule should never be evaluated in isolation; instead, the presence of any cosolvent and salt anion would cast significant influence on the overall electrolyte stability.3,21−23 Thus the optimized structures before and after the single-electron oxidation of various solvents, with or without cosolvent and anion interactions, are presented in Figures S1−S6, with the corresponding calculated oxidation potential (Eox) shown in Figure 1. These oxidation potentials of the isolated solvents
Figure 1. Calculated oxidation potential of solvents, solvent−Li+, solvent−PF6−, solvent−Li+−PF6−, solvent−SN, and solvent−Li+− PF6−−SN complexes.
decrease in the order of SN > ethylene carbonate (EC) > ethylmethyl carbonate (EMC) > dimethyl carbonate (DMC), which is in good agreement with the reported literature that SN shows the highest oxidation stability.12 The presence of Li+ slightly increases the oxidation stability of all solvents without changing the order, which should be ascribed to the electronwithdrawing ability of Li+ from these solvent molecules, resulting in lower electron availability and hence higher resistance against oxidation. Conversely, the salt anion hexafluorophosphate (PF6−) lowers the oxidation stability of all solvents,3,21 but this effect seems to be especially severe on SN because its complex with PF6− has an oxidation potential even lower than the corresponding complexes between PF6− and carbonates. This conflicts with all reports that claimed higher anodic stability of SN than carbonates. Considering that, in an actual electrochemical device, an anion−solvent complex (SN−PF6−) would be the species that most probably appears at the cathode surface, especially when the potential of cathode is polarized toward positive extremes,24 we can confidently 6049
DOI: 10.1021/acs.jpclett.7b02734 J. Phys. Chem. Lett. 2017, 8, 6048−6052
Letter
The Journal of Physical Chemistry Letters
Figure 2. LSV curves with a scan rate 0.1 mV/s of fresh Pt (A) and Co3O4 (C) electrodes cycling in baseline and SN-containing electrolytes. LSV curves of soaked and prescanned Pt (B) and Co3O4 (D) electrodes in baseline electrolyte.
Figure 3. Snapshots of Li/soaked Co3O4 and Li/prescanned Co3O4 V-type cells before (A,D) and after (B,E) LSV scanning, together with the N 1s XPS spectra of Soaked Co3O4 (C) and prescanned Co3O4 (F) after LSV scanning in baseline electrolyte.
adsorbed SN would make a difference in terms of anodic stability. However, both electrodes (Figure 2B,D) behave similarly to those electrodes without surface-soaking (fresh electrodes, Figure 2A,C). Furthermore, surface analysis using Xray photoelectron spectra (XPS) also failed to detect any Ncontaining species on the surface of these presoaked electrodes (Figure 3C). Pictures of the Li/presoaked Co3O4 cell before and after LSV (Figure 3A,B) show apparent electrolyte discoloration, where the light orange after working electrode is polarized to 6 V and may arise from the oxidation of electrolyte and Co dissolution from Co3O4. Experiments provide evidence of both: The content of Co dissolved in electrolyte is determined to be 0.032 mg/L by inductively coupled plasma (ICP) analysis, while electrolyte oxidation can
product, transition metal or oxygen layer could be viewed only as electrochemical mediators, and their eventual reduction leads to the loss of electron from electrolyte solvents. In this sense, the order of oxidation stability as shown in Figure 1 still stands as a useful guideline. Additional experiments were designed to reveal whether SN oxidatively decomposes at high potentials (Figure S8). It was first assumed that SN indeed adsorbs on the electrode surface once the electrode is directly exposed to electrolyte and forms strong association with the transition-metal oxide arrays on the surface. Thus using baseline electrolytes containing no SN, LSV was carried out for both Pt and Co3O4 electrodes that were presoaked in SN-containing electrolytes with or without solvent rinsing. If the adsorption hypothesis is correct, then the 6050
DOI: 10.1021/acs.jpclett.7b02734 J. Phys. Chem. Lett. 2017, 8, 6048−6052
Letter
The Journal of Physical Chemistry Letters
stability of isolate SN against oxidation, its interaction with salt anion significantly reduces its resistance against oxidation, thus making it possible to form N-containing interphase before carbonate molecules decompose. Differing from carbonates, the HF-free oxidation process of SN might be the key for the cycling stability of nitrile-containing electrolytes with highvoltage cathode materials.
be visually observed from the TEM images (Figure 4B). These results indicate beyond any doubt that SN could not stabilize
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b02734. Computational and experimental details, Figures S1−S8 (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*L.X.: Tel.:+86 20 39310256. E-mail:
[email protected]. *K.X.:
[email protected]. *W.L.:
[email protected].
Figure 4. TEM images of fresh Co3O4 (A), soaked-Co3O4 electrode (B), and prescanned Co3O4 (C,D) electrodes after LSV scanning in baseline electrolytes.
ORCID
Lidan Xing: 0000-0002-3642-7204 Weishan Li: 0000-0002-1495-4441
the cathode/electrolyte interface by merely adsorbing on electrode surface, nor will it remain intact at high potentials. Next, we assume that SN stabilizes the cathode/electrolyte interface via oxidative decomposition and the subsequent interphase formation, as suggested by the above DFT calculations. According to Figure 2C, the onset oxidation potential of baseline carbonate electrolyte is ∼4.5 V; therefore, to prevent sustained carbonate oxidation, the formation of an SN-originated interphase must start before 4.5 V. Preformation of such interphase is attempted by polarizing both Pt and Co3O4 electrodes in 5% SN-containing electrolyte at 4.5 V, followed by washing with DMC solvent. These electrodes with preformed interphase were then exposed to baseline electrolytes containing no SN, and anodic polarization was conducted again, whose LSV curves are shown in Figure 2B,D. The oxidation of baseline electrolyte is significantly suppressed in the presence of the preformed interphases originated from SN, confirming that SN could only provide stabilization when it is electrochemically oxidized. XPS confirms this hypothesis with clear N-containing species on the Co3O4 surface (Figure 3F). Differing from Figure 3B, neither discoloration (see Figure 3E) nor co-dissolution occurs in the baseline electrolyte after the Co3O4 electrode with preformed interphase was polarized to 6.0 V, which convincingly indicates that the oxidative decomposition products of SN not only suppress the electrolyte oxidation but also inhibit the Co on Co3O4 surface from dissolution at high potentials. TEM reveals a thin and uniform film that evenly covers the surface of charged Co3O4 electrode (Figure 4C,D), which further confirms the filmforming reaction of SN-containing electrolyte. In conclusion, the mechanism of how SN stabilizes electrolytes at the high-voltage cathode surfaces has been rigorously investigated by means of theoretical and experimental methods. The previously established hypothesis that the chemisorption of SN on transition-metal oxide surface prevents electrolyte decomposition is shown to be incorrect. Instead, the oxidative decomposition of nitrile leads to the formation of a cathode interphase, which is responsible for the high-voltage stability of nitrile-containing electrolytes. DFT calculations demonstrated that despite the intrinsically high
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant No. 21573080), the Pearl River S&T Nova Program of Guan gzho u (G ran t No . 201506010007), the Key Project of Science and Technology in Guangdong Province (Grant No. 2016B010114001), Guangdong Program for Support of Top-notch Young Professionals (2015TQ01N870), and Distinguished Young Scholar (2017B030306013).
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DOI: 10.1021/acs.jpclett.7b02734 J. Phys. Chem. Lett. 2017, 8, 6048−6052
