Anion Effects on Cathode Electrochemical Activity in Rechargeable

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Anion Effects on Cathode Electrochemical Activity in Rechargeable Magnesium Batteries: a Case Study of VO 2

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Ran Attias, Michael Salama, Baruch Hirsch, Reeta Pant, Yosef Gofer, and Doron Aurbach ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b02140 • Publication Date (Web): 06 Dec 2018 Downloaded from http://pubs.acs.org on December 8, 2018

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ACS Energy Letters

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Anion Effects on Cathode Electrochemical Activity in Rechargeable Magnesium Batteries: a Case Study of V2O5 Ran Attias, Michael Salama, Baruch Hirsch, Reeta Pant, Yosef Gofer, and Doron Aurbach* Department of Chemistry and the Institute of Nano-Technology and Advanced Materials (BINA), BarIlan University, Ramat Gan 5290002, Israel

One of the Holy Grails in research and development focused on rechargeable magnesium batteries is development of “conventional” electrolyte solutions that are compatible with both anode and cathode and supports highly reversible magnesium electrochemical activity. In the last couple of years, MgTFSI2, a “simple” salt, attracted considerable attention owing to its high solubility in a range of relevant solvents and apparent compatibility with magnesium anode and cathode materials. Nonetheless, questions were raised regarding the validity of the chemical and electrochemical inertness attributed to the TFSI anion, in particular when electrochemistry of magnesium is in the spotlight. Here, we demonstrate the impact of the TFSI anion on the intercalation kinetics of Mg ions into V2O5. The importance of this work lays on the fact that V2O5 is considered as most attractive high voltage/high capacity cathode for secondary Mg batteries, while MgTFSI2 is considered a very important electrolyte for the same systems.

Magnesium metal is a natural choice as anode material for high energy density rechargeable batteries. Compared with Li metal it possesses high volumetric capacity (3,833 mAh cm-3) and low reduction potential (2.37 V vs. SHE), and exhibits non-dendritic growth. It is also widely abundant, cheap, and environmentally benign.1,2 However, development of rechargeable magnesium batteries (RMBs) faces two major challenges: the slow solid state diffusion of Mg ions in almost all solid hosts, and development of viable electrolyte solutions possessing all critical requirements for battery use. One of the greatest challenges in this respect is to attain compatibility between the anode and cathode electrolyte solutions.3 The small ionic radius of Mg2+ and its bivalency result in a “hard cation”, with high charge density. It is widely accepted that the high charge density results in very slow diffusion kinetics within solid matrices such as intercalation hosts. Of the very few materials that unequivocally exhibit kinetically reasonable

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2 intercalation with Mg ions, orthorhombic V2O5 is very promising as high energy density cathode material. Unfortunately, the majority of studies on Mg2+ intercalation into V2O5 were conducted in electrolyte solutions that are incompatible with Mg anodes (using Mg(ClO4)2 in acetonitrile or organic carbonates based solutions).4–11 TFSI-based electrolyte solutions are getting prodigious attention, as MgTFSI2, among other properties, is the only commercially available “simple” salt that readily dissolves and dissociates in ethereal solvents. No less important, the TFSI anion is considered superior to many other anions in terms of chemical and electrochemical stability. Some publications demonstrated that Mg ions intercalate into V2O5 from MgTFSI2-based electrolyte solutions; in these studies, the electrochemical characterization of V2O5 was conducted over a wide potential window.12–14 In some cases the discharge voltage went as low as 0 V vs. Mg/Mg2+, and even lower. These potential regions are, obviously, impractical for RMBs. More importantly, V2O5 is well-known to undergo multiple phase changes during Li+ intercalation, depending on the voltage limits (intercalation levels). Some of these phase transitions are fully reversible, in both electrochemical and phase terms. Very little is known with respect to magnesium intercalation in V2O5, in contrast with the high levels of depth and precision known for lithium, therefore it is imperative to study this system in a very cautious manner. Thorough and reliable understanding of the Mg-V2O5 system will only be gained by careful and comprehensive studies. For instance, the above-mentioned studies report a serious charge mismatch between the charging and discharging processes. This strongly suggests that some parasitic chemical/electrochemical reactions occur during the charge process. Without thorough understanding of the origin of this charge mismatch, little can be asserted reliably regarding the nature of magnesium intercalation electrochemistry with V2O5. In the current study we focus on aspects associated with interfacial phenomena occurring with V2O5 during magnesium intercalation, namely surface passivation. In this work, we demonstrate, to the best of our knowledge for the first time, that TFSI reacts on the surface of V2O5 when electrochemical intercalation is attempted. We show that the reaction products form a passivation layer that does not allow Mg ions to enter. This phenomenon may also explain the nature of the parasitic reactions seen with MgTFSI2-based solutions mentioned above. One important take-home message from the current study is that due to its electrochemical reactivity, MgTFSI2 is not recommended for screening Mg ions intercalation materials, in particular high voltage oxides. The electrochemical characterizations in this study were carried out in three-electrode flooded cells, composed of monolithic thin film of orthorhombic V2O5 as working electrode and high surface area activated carbon cloth (AC) as reference and counter electrodes. Monolithic V2O5 electrodes were used in order to minimize side reactions related to inert components, such as electron-conducting additives and binders, and composite electrode formulation issues. The feasibility of AC to serve as quasi- reference electrode was examined via comprehensive study, explained in detail in our recent publication15 and in the Supporting Information (SI). Acetonitrile (ACN) was the solvent of choice, due to its high anodic stability and relatively easy Mg+2 desolvation.15 Mg(ClO4)2/ACN electrolyte solution served as a control system, and all electrochemical experiments were conducted in a potential range where the phase transition of V2O5 is fully reversible. This experimental setup allows assessing solely the effect of the TFSI- anion on the kinetics of intercalation of Mg2+ into V2O5 cathodes. Before delving into the focused study, the electrochemical activity of the thin, monolithic V2O5 films deposited on Pt foil electrodes was characterized with a Li-ions based solution. Li ions activity with this


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ACS Energy Letters

3 oxide is not only well-studied and serves as excellent benchmarking system, it is also very forgiving to the experimental conditions, in particular to passivation. Figure 1a presents the cyclic voltammograms (CVs) of V2O5 thin film electrodes in LiTFSI/ACN and LiClO4/ACN solutions. By analyzing these voltammograms two important insights can be deduced. The first relates to the crystal structure and the mass loading of the host (as explained in the SI). The second relates to the effect of the anion on the intercalation process of Li+ into V2O5. The electrochemical signatures indicate that the deposited V2O5 unequivocally assumed the -phase crystal structure, with orthorhombic unit cell structure.16–20 Raman spectra gave further support to this notion (Figure S1). The electrochemical response also clearly reflects the facile intercalation/deintercalation processes (rates), as well as some insights about the thermodynamics (voltages) and phase transitions (sharp current peaks). The redox peaks are located at the same potentials and current densities, and the charge transferred during intercalation is practically identical in both systems. These results indicate, unequivocally, that the electrochemical intercalation process of Li+ ions with V2O5 is indifferent with respect to the anion, in terms of both thermodynamics (reaction potentials) and reaction kinetics (current densities).

Figure 1: (a) Cyclic voltammograms (CVs) of monolithic, thin film V2O5 electrodes in a three-electrode cell comprised of AC as counter and reference electrodes. Electrolytic solutions: LiTFSI/ACN (black line) and LiClO4/ACN (red line). Scan rate: 1 mV s-1 @ RT. (b) CVs of monolithic, thin film V2O5 electrodes in same three-electrode cell configuration with Mg(TFSI)2/ACN (black line) and Mg(ClO4)2/ACN (red line) electrolyte solutions at a scan rate of 0.2 mV s-1 @ RT. Inset: corresponding charge balance of the first (solid line) and third (dashed line) cycles.

In stark difference to the Li-based solutions, the electrochemical response of V2O5 electrodes in Mg-ion based solutions was found to be strongly affected by the anion in the electrolyte solution. Figure 1b shows the CVs of monolithic V2O5 electrodes in Mg(TFSI)2/ACN and Mg(ClO4)2/ACN electrolyte solutions and the charge balance curves corresponding to the first and third cycles. The potential-current dependency provides important insights into the electrochemical reaction kinetics. At the macroscopic level, the currents registered in Mg(ClO4)2/ACN are much higher than in Mg(TFSI)2/ACN solutions. This primary observation indicates that the overall intercalation kinetics of Mg ions into (and out of) V2O5 electrodes is strongly diminished due to the existence of TFSI anions. Such strong effect may originate due to several reasons, such as surface phenomena and solution structure differences. Note that we excluded the possibility that the different electrochemical activity in MgTFSI2 and Mg(ClO4)2 solutions is due to different levels of water contamination, which may be higher in Mg(ClO4)2 solutions. A detailed description of relevant experimental work appears in the SI.



4 However, deeper analysis, at the microscopic level, is needed in order to get better understanding of the mechanism underlying the anion effect on intercalation of Mg ions into V2O5. The voltammograms of the V2O5/Mg(ClO4)2/ACN system exhibit two redox peaks at -0.22 and 0.6 V vs. AC quasi reference electrode (1.88 and 2.7 V vs. Mg/Mg+2). The voltammograms of the V2O5/MgTFSI2/ACN system exhibited weak and poorly resolved, extremely broad, reduction peak. This reflects poor intercalation kinetics into the oxide lattice. An additional interesting observation was made from the corresponding oxidation peaks of this system. The first oxidation peak is located at the same potential as the one for V2O5/Mg(ClO4)2/ACN. However, in the following cycles, the oxidation peak shifts to ever-increasing voltage. In other words, higher overpotential develops after each cycle. Interestingly, the increase in the oxidation peak voltage occurs only from the second cycle. Namely, the first oxidation peak is not shifted to higher potential (relative to the CV in perchlorate), indicating that biasing to lower potential does not cause such shift. At the same rate, the ever-increasing oxidation peak potential is strongly associated with the cycle-by-cycle positive biasing of the electrode. Within the voltage window used in the study, regardless of its origin, this phenomenon is associated with electrochemical oxidation processes rather than reduction. Based on the above results, we hypothesized that TFSI- anions react at high voltages with V2O5 to form a partially impermeable surface film, which hampers intercalation of Mg2+ ions into the solid host. To test the hypothesis, we ran the following experiment. A V2O5 electrode was electrochemically treated in TFSIbased solution; the electrochemical treatment involved single step chronoamperometry measurement to 0.8 V vs. AC to 10 hours in MgTFSI2/ACN electrolyte solution. The electrochemical treatment should cause partial passivation of the electrode surface, according to the hypothesis where it is chemically affected by the TFSI. The electrode was washed thoroughly and transferred to Mg(ClO4)2/ACN solution, and several consecutive CVs were obtained. Figure 2 shows CV curves for the V2O5 electrode in Mg(ClO4)2/ACN after electrochemical treatment in the TFSI-based solution. Interestingly, as can be clearly seen in Fig. 2, the first cycles in the perchlorate solution resemble very much the CVs in TFSI solution. However, continuous cycling shows evolution of CV features that are substantially stronger, and after seven cycles the CV resembles very much that for untreated electrode cycles in perchlorate-based solutions. The treated electrode still exhibits intercalation kinetic limitation, as observed by the washed-out negative half-cycle current wave, and lower charge is associated with the process than with the untreated electrode. The results reveal that the intercalation process from Mg(ClO4)2/ACN solution becomes substantially sluggish after exposing the V2O5 to MgTFSI2/ACN solution, at high voltage of around 2.8 V vs. Mg/Mg2+. In other words, the V2O5 cathode becomes practically blocked due to the electrochemical processing in TFSI-based solution. The results reveal that TFSI anions in the solution have negative effect on the electrochemical activity of V2O5. Moreover, these negative effects are most probably due to formation of impermeable surface film as a result of TFSI oxidation. The improvement in the electrochemical response during cycling in the perchlorate solution can be explained on the grounds of dissolution or breaking of this thin, passivating surface film. The mechanism of this de-passivation process is not yet clear.


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ACS Energy Letters

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Figure 2: The CV curves of monolithic thin film V2O5 in three-electrode cells with Mg(ClO4)2/ACN before (2nd cycle, black line) and after electrochemical treatment in Mg(TFSI)2/ACN solution (1st, 2nd, 4th and 7th cycles, red line). The scan rate was 0.2 mV s-1. Inset: corresponding charge balance for the untreated (2nd cycle, black line) and treated (2nd and 7th cycles, red line) electrodes.

Despite strong evidence for the negative effect of the TFSI anions on the intercalation process of Mg ions into V2O5 host, electrochemistry is not sufficient to elucidate the mechanism at the chemical level. In order to get deeper understanding at the molecular and morphological levels, complementary spectroscopic and microscopic methods were employed. Figure 3 shows the results of X-ray photoelectron spectroscopy (XPS) analysis for V2O5 electrodes after 12 CV cycles in Mg(ClO4)2 and MgTFSI2 based electrolyte solutions. The XPS spectra indicate that there are some differences in the surface chemistry of the V2O5 electrodes processed in the two solutions. The XPS spectra of both systems exhibit peaks around 285 eV associated with carbon C 1S. The smaller, asymmetric peaks at around 289 eV can be identified as carboxylates. In any case, both qualitatively and quantitatively the C 1s spectra are virtually the same in both cases. Both samples exhibit Mg 2p peaks associated with an oxidized form of Mg. Quantitatively, the contribution of this peak is larger for the sample cycled in perchlorate solution, compared with the TFSI solution (see Tables 1 and 2 in the SI). Qualitatively, the Mg 2p features for the two samples appear very similar. Unfortunately, the signal-to-noise ratio is too large to provide intricate qualitative information regarding the Mg chemical environment. The most important peak is found around 685.8 eV, indexed to F 1s. This peak reflects the critical difference between the samples and sheds light on the electrochemical results. This peak was observed, naturally, only in electrodes that were electrochemically processed in MgTFSI2/ACN. From the F 1s binding



6 energy, these peaks are associated with inorganic fluorine anion, in conjunction with electropositive metal cation. The peak position – 685.8 eV – precludes the likelihood of organic-based fluorine, such as a CF3 moiety.21,22 We believe that this peak indicates formation of a very thin insoluble and Mg-ion impermeable surface film of MgF2.23

Figure 3: XPS data obtained from measurements of monolithic thin film V2O5 electrodes after 12 CV cycles in Mg(ClO4)2/ACN (black line) and in MgTFSI2/ACN (red line) electrolyte solutions. Spectra of (a) F,(b) Mg, (c) C, (d) V and O are presented.

Further insights about the surface nature of the electrodes after electrochemical treatment are provided by high-resolution scanning electron microscopy (HR-SEM) measurements. Figure 4 shows HR-SEM images of pristine V2O5 electrodes and of V2O5 electrodes after 6 CV cycles in Mg(TFSI)2/ACN and Mg(ClO4)2/ACN electrolyte solutions. The HR-SEM micrographs clearly indicate that the surface morphology of the pristine sample is slightly different from the electrochemically cycled samples. The cycled electrodes show somewhat smoother and larger crystallite surfaces, and larger cracks. We don’t know yet what causes this difference. While the general morphology of the samples that were cycled in the two electrolytic systems is very similar, there is a very profound difference, with critical consequences. The electrodes cycled in Mg(ClO4)2/ACN are clean from impurities, and only some of the crystallite edges show whiter color, associated with slight charging effects. Similar features are observed also with the pristine electrode, but to a lesser extent. In contrary, the surface of the V2O5 cycled in MgTFSI2/ACN comprises many large islands of areas that show stronger charging effects. It is also possible to discern


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ACS Energy Letters

7 that the surface morphology of these areas exhibits grainy or spongy texture. Due to the decreased electronic conductivity associated with these islands, it is practically impossible to acquire micrographs at much larger magnification (charging). Hence, it is difficult to accurately determine the nature of this surface layer. Obviously, it is very thin. Interestingly, the same kinds of surface features were also found on V2O5 electrodes that were cycled in LiTFSI solutions, however in this case the islands were smaller (Figure S8). It should be noted that formation of LiF deposits on these electrodes occurring in LiTFSI solutions are not supposed to impede pronouncedly Li ions intercalation because most kinds of thin films comprising ionic Li compounds allow migration of Li ions through them since they behave as solid electrolyte interphase for Li ions transport.24

Figure 4: HR-SEM images of monoclinic V2O5 thin film electrodes (thickness ~30 nm) pristine (a, b) and after 6 CV cycles at a scan rate of 0.2 mV/s in (c, d) Mg(ClO4)2/ACN, and (e, f) Mg(TFSI)2/ACN electrolyte solutions.

The above findings strongly support the hypothesis of formation of a thin, passivating layer when V2O5 electrodes are cycled in TFSI-based solutions in the presence of Mg ions. It is well-known that even an exceedingly thin over-layer deposit of stable film may cause serious impedance for Mg intercalation, and even complete passivation. Any single analytical technique on its own cannot provide the robust evidence needed for elucidating the origin of the sluggish electrochemical process of magnesium intercalation in the TFSI-based electrolyte solution. However, the results from electrochemical measurements, coupled with the XPS surface elemental analyses and HR-SEM images lead to an unambiguous conclusion: the TFSI anion reacts electrochemically with V2O5 electrodes to form impermeable/semi-permeable surface films which hamper electrochemical insertion of Mg ions into the oxide host. Chemical analysis supports the hypothesis that this surface film is composed mainly of MgF2. However, the mechanism underlying this surface film formation is not yet clear. The results presented herein, which relate to most important components of rechargeable Mg batteries, are very significant to the field since they will save efforts by omitting research directions that should lead to dead ends.



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ASSOCIATED CONTENT Supporting Information The supporting Information (SI) provides details of the experimental procedures and materials. In addition, the SI section contains Raman spectra of the V2O5 electrodes, current-time curve deduced from chronopamperometry measurements, qualitative and quantitative XPS data, and HR-SEM images of V2O5 in Li ion systems.

AUTHOR INFORMATION Corresponding Author Doron Aurbach; E-mail: [email protected]

Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS A partial support for this work was obtained from the Israel committee of high education and Israel Prime Minister office in the framework of the INREP project. References (1)

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Anion Effects on Cathode Electrochemical Activity in Rechargeable

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