De-Intercalation Kinetics

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Energy Conversion and Storage; Plasmonics and Optoelectronics

Real-Time Monitoring of Cation Dissolution/De-Intercalation Kinetics from Transition Metal Oxides in Organic Environments Pietro Papa Lopes, Milena Zorko, Krista L. Hawthorne, Justin G. Connell, Brian J. Ingram, Dusan Strmcnik, Vojislav R. Stamenkovic, and Nenad M. Markovic J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b01936 • Publication Date (Web): 30 Jul 2018 Downloaded from http://pubs.acs.org on July 30, 2018

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The Journal of Physical Chemistry Letters

Real-Time Monitoring of Cation Dissolution/DeIntercalation Kinetics from Transition Metal Oxides in Organic Environments Pietro P. Lopesǂ,§, Milena Zorkoǂ,§, Krista L. Hawthorne§,#, Justin G. Connell§,ǂ, Brian. J. Ingram§,#, Dusan Strmcnikǂ,§, Vojislav R. Stamenkovicǂ, Nenad M. Markovicǂ,§,* ǂMaterials Science Division, Argonne National Laboratory, 9700 S Cass Ave., 60439, Lemont, USA § Joint Center for Energy Storage Research, Argonne National Laboratory, 9700 S Cass Ave., 60439, Lemont, USA # Chemical Science and Engineering Division, Argonne National Laboratory, 9700 S Cass Ave., 60439, Lemont, USA *corresponding author: [email protected]

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ABSTRACT

The future of high voltage rechargeable batteries is closely tied to the fundamental understanding of the processes that lead to the potential-dependent degradation of electrode materials and organic electrolytes. To date, however, there have been no methods able to provide quantitative, in situ information about the electrode dissolution kinetics and concomitant electrolyte decomposition during charge/discharge. In this work, we describe the development of such a method, which is of both fundamental and technological significance. Our novel approach enables simultaneous and independent measurements of transition metal cation dissolution rates from different oxide hosts (Co3+/4+ or Cr3+/4+), de-intercalation kinetics of working cations (Mg2+), and the relative rate of electrolyte decomposition.

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The elucidation of physicochemical parameters (descriptors) that control the functional links between the activity and stability of electrochemical interfaces in aqueous environments forms an increasingly active segment of contemporary surface electrochemistry1. Among the various ex situ and in situ experimental techniques suited to establish activity-stability relationships at atomic and molecular levels

1–6

, the stationary probe rotating disk electrode coupled to

inductively coupled plasma mass spectrometry (SPRDE-ICP-MS) method

6–8

has provided the

most detailed information to date. The power of this technique lies in the ability to simultaneously monitor the reaction kinetics on the rotating disk electrode (RDE) and the dissolution dynamics of the transition metal cations (TMC) from an oxide-based electrode material by means of ICP-MS. In doing so it is possible to obtain real-time information about relationships between the activity and stability of anode and cathode materials that are of vital importance to the development of new renewable energy technologies 1,8. Current understanding of activity-stability relationships for TM-based oxides that serve as host frameworks for intercalation/de-intercalation of mobile (working) cations in organic environments remains inadequate; thus far relying solely on separate evaluation of potentialdependent “charging currents” and post mortem determination of the concentration of TMC (e.g. Mn, Ni and Co) dissolved in the electrolyte or deposited at the anode

5,9–12

. Although these ex

situ studies provide some useful information about the potential-dependent stability of TMC during multiple discharge-charge cycles in Li-ion batteries (LIB), such measurements do not allow for reliable quantification of dissolved TMC, nor do they reveal the true dynamics of TMC dissolution. Such measurements of high voltage oxide stability are further complicated by the common use of the two-electrode “coin cell” geometry, which makes it impossible to identify


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electrochemical processes intrinsic to the oxide material due to crossover of decomposition products generated at both electrodes. As a result of this knowledge gap, there is currently no consensus on the mechanisms that may lead to TMC dissolution

5,11–14

. Further complications

arise from the ever-present possibility of electrolyte decomposition, whose individual contribution to the charge/discharge reactions is impossible to measure solely from galvanostatic cycling measurements. What is undisputable in the literature, however, is that the dissolved cations can be deposited on the graphite anode, which, in turn, can lead to voltage fade and early cell failure. Here, by developing the SPRDE-ICP-MS method for use in organic environments, we demonstrate it is possible to easily follow the dynamics of cobalt dissolution from LiCoO2 in 1.2 M LiPF6 in 3:7 ethylene carbonate/ethyl methyl carbonate (EC:EMC) electrolyte. Furthermore, our method can be used to probe the de-intercalation kinetics of divalent cations (e.g. Mg2+) from promising high-voltage oxide materials for Mg-ion batteries (MgCr2O4 spinel 15) by simply using an electrolyte free of Mg2+ (0.2 M LiTFSI). Even though counterintuitive, these experiments offer the unprecedented opportunity to simultaneously measure the rate of Mg2+ deinsertion, the concomitant dissolution of the Crn+ host, and the overall rate of electrolyte decomposition. We conclude that our in situ SPRDE-ICP-MS method opens a new avenue in understanding activitystability relationships in organic environments that are necessary for the development of future energy storage systems. To emphasize the power of the SPRDE-ICP-MS method to monitor the potential-dependent stability of cathode materials in organic environments in situ, Scheme 1 shows the stationary probe around the rotating disk electrode and the modifications required to allow organic samples to be analyzed with the ICP-MS. We note that the high sensitivity of the method is dependent


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mainly on the probe construction around the electrode surface, which is independent of the working electrolyte 6. As commercial ICP-MS instruments are designed to work with aqueous samples, analyzing samples in organic environments such as those encountered in LIB poses a significant challenge. For example, analysis of volatile organic solvents always requires addition of small amounts of O2 in the nebulization gas to help minimize coke formation that foul the vacuum inlet cones. Furthermore, and especially to common organic electrolytes found in LiB, these solutions can dissolve standard tubing materials preventing proper pumping action, thus requiring the use of PTFE-lined peristaltic pumps that feed into an in-line dilution system connected to a temperature controlled PFA-Sapphire sample introduction system (See Experimental Methods and Supporting Information for additional details). The in-line dilution together with temperature control nebulization ensures that the total amount of sample and matrix in the plasma is high enough that is does not compromise the ionization efficiency for the trace level analytes. Together with details pertaining to ICP-MS analysis itself, the electrochemical cell must be contained inside a glovebox with controlled atmosphere to prevent O2 and water contamination, which can have a dramatic effect in the underlying interfacial processes 16.


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Scheme 1. Stationary Probe Rotating Disk Electrode (SPRDE) system coupled to an Inductively Coupled Plasma Mass Spectrometry (ICP-MS), highlighting the transport of species generated at the electrode surface during dissolution of transition metal cation oxide host (e.g., Con+, Crn+, etc.), de-intercalation of working cation (e.g., Li+, Mg2+, etc.), measured by the ICP-MS, and electrolyte decomposition as determined by electrochemical current at the disk.

Such an intricate experimental apparatus is essential for enabling the monitoring of cobalt dissolution from LiCoO2 during electrochemical cycling in 1.2M LiPF6 in 3:7 EC:EMC. Representative current-time curves are summarized in Figure 1 and correspond either to the Con+ “ionic” dissolution current measured by the ICP-MS (in Figure 1a expressed in µA cm-2), or to the total electrochemical current that is simultaneously recorded on the RDE (in Figure 1b expressed in mA cm-2). To the best of our knowledge, this is the first experiment of its kind that provides real-time, quantitative analysis of cation dissolution in organic environments. As we describe further below, it is also possible to determine the excess faradaic charge relative to Li insertion/deinsertion and the simultaneous dissolution of Con+, which can provide important insights into the functional links between cathode stability and electrolyte decomposition.


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We first explore the dynamics of Co dissolution during potential step transients from 3 V vs. Li/Li+ up to a maximum of 5 V (in 0.2 V increments) with 300 sec per potential step. Note that the potential limits were chosen purposely to ensure that both Li deinsertion and insertion take place in order to simultaneously evaluate the electrolyte decomposition currents. Figure 1a reveals that the potential-dependent ionic current response for Co dissolution evolves from transient to steady-state behavior. The first three potential steps (4.0 to 4.4V) exhibit a small, yet clearly discernible, amount of transient Con+ dissolution (nanograms-level) that quickly decays to the same level as the background.

Figure 1. a) In situ dissolution currents for Co ion dissolution (magenta) from LiCoO2 in 1.2M LiPF6 in 3:7 EC:EMC at increasing upper potential values during electrochemical polarization. Corresponding electrochemical current is shown in blue during the same polarization experiment. b) total mass of Co dissolved in each potential step window (left axis) and the corresponding excess positive charge measured on the disk attributed to electrolyte decomposition (right axis).


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Beginning at 4.6 V, however, large, transient Con+ dissolution rates are followed by rapid relaxation to a measurable, steady-state value, signaling that constant Con+ dissolution takes place above 4.6 V. Note that the corresponding current-time profile recorded on the disk electrode shows that measurable disk currents are also only observed above 4.6 V, indicating that there is no measurable electrolyte decomposition below that potential limit. In addition, the results in Figure 1a demonstrate that the Co dissolution current is almost 3 orders of magnitude smaller than the electrochemical current (only ~0.15% of the total current at 5 V), emphasizing the power of the SPRDE-ICPMS method to monitor, almost atom-by-atom, the dynamics of TMC dissolution that are otherwise inaccessible from typical charge/discharge measurements. The potential-dependent rate of dissolution (‫ݎ‬஼௢ ) can be used to calculate the total amount of Co removed during the dissolution process by determining the area under the current-time curve ௧ୀஶ

(Γ஼௢ = ‫׬‬௧ୀ଴ ‫ݎ‬஼௢ ݀‫)ݐ‬. As summarized in Figure 1b, the total mass of Con+ dissolved in the electrolyte increases linearly from 4 to 4.6 V (from 30 to 70 ng cm-2), whereas an almost exponential increase in the total Con+ dissolution is observed from 4.6 to 5 V, with the maximum amount of Con+ removed from the electrode surface at 5 V reaching 800 ng cm-2. Although these values correspond to only a small fraction of the active mass that is lost during polarization, most likely coming from the surface of the material, over many cycles they will have a big impact via increased electrode impedance due to loss of contact to carbon components 13,17, decreased anode performance as Co ions migrate and accumulate at the graphite surface5,9, and ultimately decreased lifetime of the battery. An important consequence of being able to measure Co dissolution rates independently from the electrode current is that it enables correlations between TMC oxide dissolution, the nature of the electrolyte and its decomposition rate. The excess charge observed in Figure 1b can be


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related to electrolyte decomposition by calculating the difference between the charge under the forward potential step (charging step) and the charge passed during the immediate step back to 3V (discharge step), as expected

18,19

. What has not been established before now is that

electrolyte decomposition and Con+ corrosion take place simultaneously, and our independent measurements of electrolyte decomposition and electrode corrosion enable quantitative analysis of their individual contributions to parasitic losses during charge/discharge. Based on this evidence, two distinct corrosion pathways can be proposed. The first is that either electrolyte decomposition triggers Co dissolution or intrinsic LiCoO2 surface instability initiates electrolyte decomposition. The second pathway requires a promoter of cobalt dissolution, e.g. impurity HF or O2 redox from within the oxide host, with HF-induced “attack”20 on cation surface centers (defects) or oxygen recombination21 and release of Con+ and O2 leading to Con+ surface dissolution. In either case the remaining surface defects become the active centers for electrolyte decomposition and, in the case of O2 redox, CO2 release18,19. Our results clearly demonstrate that Co dissolution is potential-dependent, suggesting that a purely chemical corrosion process (e.g., HF attack) is unlikely, as it would take place regardless of electrode polarization. Regardless of the exact corrosion reaction mechanism, our unique capability for monitoring, in situ, Co dissolution and electrolyte decomposition from LiCoO2, as well as the ability to correlate these phenomena in real time, will provide new avenues for exploring the stability of even more relevant high energy oxides, such as those containing Ni, Mn and Co in varying stoichiometry. Furthermore, with the use of well-defined materials and surfaces we can establish at atomic and molecular levels the relationships between electrode morphology, the presence of surface defects, and of electrolyte impurities to electrode stability and electrolyte decomposition processes; an important experimental direction that is ongoing in our laboratory.


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Having established the SPRDE-ICP-MS as a reliable tool for studying the dissolution of TMC in organic environments in situ, we expand our analysis to attempt to directly monitor deinsertion of the working cation from the positive electrode, enabling correlation of deinsertion processes with TMC oxide corrosion and electrolyte decomposition currents. This is particularly important for beyond-Li battery chemistries such as Mg-ion systems, where demonstration of reversible cycling of Mg inside on high-voltage oxide host is extremely challenging due to the lack of electrolytes with appropriate anodic stability. To this end, we select the MgCr2O4 spinel due to its promise as a high voltage cathode material for Mg-ion batteries15,22,23. By continuing to use a Li-based electrolyte but switching the working cation in the oxide host to a chemically distinct species (e.g., Mg2+) it should be possible to directly measure deinsertion of the working ion from the oxide host into the electrolyte. Given the power of our technique to decouple electrolyte decomposition currents from electrode corrosion processes (as discussed above for LiCoO2 system), we explore the kinetic current-time profiles for Cr dissolution and Mg cation deinsertion, as well as the corresponding difference in the total concentrations of dissolved cations, to directly distinguish between TMC surface corrosion, electrolyte decomposition and Mg2+ removal.


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Figure 2. a) Instantaneous concentration levels for Cr ion dissolution and b) Mg ion deinsertion during a potential step experiment shown in (c) from MgCr2O4 spinel in 0.2 M LiTFSI in diglyme electrolyte. d) Comparison between the independently measured disk current and Mg2+ deinsertion current.

In order to detect the ionic current of Mg2+ during deinsertion with high sensitivity, an electrolyte without a background of Mg2+ is required. Therefore, for work with MgCr2O4, 0.2 M LiTFSI in diglyme was used as a Mg-free electrolyte, enabling direct monitoring of Mg2+ ion currents. TFSI- was chosen over PF6- as the working anion, as TFSI- enables significantly more reversible behavior at the Mg anode relative to PF6-

24–26

, and therefore should provide a more

representative electrolyte environment to that encountered in a Mg-ion full cell. To avoid


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unintentional Li insertion into the electrode, the experimental protocol was slightly modified from that used for LiCoO2, instead using consecutive potential steps from 3.7 to 5.1 V vs Li/Li+ (3.0 to 4.4 V vs. Mg/Mg2+, Figure 2c) with no corresponding steps back to lower potential. The ability of the ICP-MS to monitor multiple elements at the same time enabled simultaneous measurements of current-time profiles for Crn+ (Figure 2a) and Mg2+ (Figure 2b) dissolution in the very first “charging cycle”. After polarization above 4.5V vs Li/Li+, we observe a significantly slower rate of Crn+ dissolution than the corresponding Mg2+ dissolution, resulting in an order of magnitude more dissolved Mg2+ relative to Crn+ (~100 µgMg L-1 vs. ~10 µgCr L-1 at 5.1 V; 1:0.06 Mg:Cr molar ratio). Although reversible Mg2+ insertion/deinsertion into the MgCr2O4 spinel has not yet been confirmed experimentally, the fact that Mg2+ dissolution initiates above 3.8 V vs. Mg/Mg2+ (as expected for this material) strongly suggests that the observed Mg2+ dissolution is due to Mg deinsertion. Nonetheless, significant electrolyte decomposition takes place at the potentials required to remove Mg2+ from MgCr2O4, and as shown in Figure 2d, the contribution of Mg2+ deinsertion current to the overall measured faradaic current is ca. 3-4 %. We note that this low overall contribution is still significantly higher than the Crn+ dissolution current (~0.2 %) further supporting the hypothesis that the observed Mg2+ current is the result of electrochemical deinsertion from the spinel lattice. Regardless, the results in Figure 2 unambiguously show that both Crn+ dissolution and Mg2+ deinsertion is accompanied by significant electrolyte decomposition (Figure S1), and the by-products formed can dramatically affect the overall Mg deposition/stripping behavior (Fig. S2). Overall, our results indicate that for any rechargeable Mg battery to function at high voltages, new electrolytes must be developed.


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In summary, the examples presented above, although limited in scope, attest to the broadbased utility of the in situ SPRDE-ICP-MS method as a unique means of extracting activitystability relationships in organic systems that are on a par with those established in aqueous environments. In particular, we have shown that this technique can simultaneously and rapidly measure the onset and dynamic evolution of TMC host corrosion kinetics, deinsertion of the working cation, and electrolyte stability over a wide potential range under experimental conditions that are relevant for real applications. This capability will certainly open a new window of opportunities to rationalize, and ultimately understand, fundamental processes that control the stability and reversibility of both Li-ion and beyond Li-ion battery systems.

EXPERIMENTAL METHODS

Electrochemical measurements. The electrodes were prepared by blade coating the substrate disk with a mixture of active material (LiCoO2 or MgCr2O4), high surface area carbon (Super-P) and PVDF, diluted with N-methyl-2-pyrrolidone (NMP) in the ratio 80:10:10, respectively. A 150 µm thick film of this suspension was coated on substrate disks that were previously polished to a mirror finish. The substrate material was chosen to minimize side reactions, such that we employed Al disks for experiments with LiCoO2, and Mo disks for experiments with MgCr2O4. After coating the substrate surface, the samples were dried in air for 2 hours before overnight drying under vacuum at 80°C. The active material loading was between 5-10 mg cm-2. All surface area normalization is relative to geometric surface area of the electrodes. The electrolyte containing LiPF6 was obtained commercially from Tomiyama, with nominal water and HF content of 20 and 100 ppm, respectively. For the Mg de-intercalation experiments we employed LiTFSI (BASF), dried in a dedicated vacuum furnace at 150ºC overnight, diluted to 0.2M in a diglyme (Sigma Aldrich) solvent. Diglyme was further purified via distillation, as described elsewhere

16,24

, resulting in an H2O content

De-Intercalation Kinetics

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