Solubility of the Solid Electrolyte Interphase (SEI) in Sodium Ion

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Letter

On the Solubility of the Solid Electrolyte Interphase (SEI) in Sodium-ion Batteries Ronnie Mogensen, Daniel Brandell, and Reza Younesi ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00491 • Publication Date (Web): 09 Nov 2016 Downloaded from http://pubs.acs.org on November 11, 2016

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ACS Energy Letters

On the Solubility of the Solid Electrolyte Interphase (SEI) in Sodium-Ion Batteries

Ronnie Mogensen, Daniel Brandell, Reza Younesi*

Department of Chemistry – Ångström Laboratory, Uppsala University, Box 538, SE-75121 Uppsala, Sweden

e-mail: [email protected]

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Abstract

It is often stated that formation of a functional solid electrolyte interphase (SEI) in sodium-ion batteries is hampered by the higher solubility of SEI components such as sodium salts in comparison to the lithium analogues. In order to investigate these phenomena, SEI formation and functionality, as well as cell self-discharge, are studied for the sodium-ion system with comparative experiments on the equivalent lithium-ion system. By conducting a set of experiments on carbonaceous anodes, the impact of SEI dissolution is tested. The results show that the SEI layer in sodium-ion cells is inferior to that in lithium-ion counterparts with regards to self-discharge; sodium cells show a loss in capacity at a dramatic rate as compared to the lithium counterparts when they are stored at sodiated and lithiated states, respectively, for long time with no external applied current or potential. Also, synchrotron-based hard x-ray photoelectron spectroscopy measurements indicate that the major factor leading to increased self-discharge is dissolution of significant parts of the sodium based SEI. Furthermore, the influence of fluoroethylene carbonate (FEC) electrolyte additive on self-discharge is tested as a part of the work.


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Table of Content (TOC) graphic

Sodium ion batteries (SIBs) and lithium ion batteries (LIBs) are both subject to the same limitations in the anode-electrolyte interfacial reaction; the solid electrolyte interphase (SEI) on an anode with the electrochemical potential below ∼1 V vs. Na+/Na is vital to make a SIB kinetically stable.1,2 An ideal SEI must fulfil several criteria such as; i) being electronically insulating to prevent continuous decomposition of the electrolyte; ii) being ionically conducting to permit transport of Na+ or Li+ ions while blocking other species such as solvent molecules; iii) being insoluble and inert in the chemical environment existing in the cell to prevent continuous reactions.3,4 The focus of this work is on the functionality of the SEI and its relation to selfdischarge in SIBs in comparison to that in LIBs. The SEI should ideally only be formed once during early cycling and then act as a passive component over time of battery life as its creation consumes electrolyte and depletes the supply of alkali ions in the cell. Although there are some studies on the subject5–9, the solubility of the SEI components for LIBs is not studied intensively and there is even less literature on the issue regarding SIBs. The popularity of lithium has led to a stagnation in the knowledge of surface chemistry in other alkali metal systems such as sodium


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when used in electrochemical cells utilizing organic solvents. The properties of lithium deviate significantly from sodium because of the significant difference in charge density; some significant and unique properties of lithium include higher stability in organometallic compounds and higher degree of covalence in the bonds to halogens10. The fundamental properties of the cation are extremely important as even small differences such as the solubility and electrochemical stability of carbonates could make the difference between a stable and a unstable interfacial chemistry, as well as rendering different morphologies of the SEI11. It is hard to discern any solid trend to reliably explain and foresee the solubility of alkali salts in organic solvents, but it is not too controversial to state that sodium does not form short strong and covalent bonds with other small atoms. This partly explain the discrepancy between cell stability compared to the otherwise identical lithium systems11,7. There are good empirical reasons to suspect that the dissolution of SEI in SIBs is more of a challenge than that in LIBs; firstly, sodium salts can have very different solubilities than their lithium analogues in the same solvent12,13; secondly, there are some evidence from propylene carbonate systems – such as the work by Moshkovich et al.7 – where sodium perchlorate in propylene carbonate is revealed to give a substantial amount of soluble decomposition products while LiClO4 and KClO4 do not. However, the question still arises: does the SEI dissolution prevent SIBs utilizing “common” electrolytes such as the popular NaPF6 salt dissolved in alkyl carbonates solvents? In order to shed some light on this matter, we have investigated SEI formation and functionality in a sodium-ion system with comparative experiments utilising equivalent sodium and lithium systems. The electrochemical experimental tests of this work is inspired by the set of experiments performed by Dahn et al.14 to investigate the impact of additives on self-discharge in graphite anodes. In short, the experiment aims to measure the self-


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discharge by combining galvanostatic cycling and long pause periods, and by conducting the experiments on anodes consisting of high surface area carbon black (Super P) or low surface area hard carbon (often used as anode material for SIBs), the impact of the any surface area dependent dysfunction of the SEI can be tested in a relatively simple system. Carbon black materials such as Super P are not practical anodes but possess some properties that motivate this choice of active material.15 The high surface area of Super P will enhance any effect related to SEI formation, and Super P is moreover electrochemically active towards both sodium and lithium, thus enabling comparative experiments using the same electrodes. Figure 1 shows a schematic picture of Na and Li cells used in this study and the cycling regime applied on the cells. In practice, the protocol starts with 10 galvanostatic cycles aimed at stabilizing the capacity and forming a robust SEI, followed by 5 pauses in fully sodiated/lithiated state separated by 5 galvanostatic cycles each (see figure 1b). By cycling cells in a program that contains pauses after sodiation/lithiation, it is possible to monitor the static stability of the SEI without any influences from volume expansion and polarization. The potential of the carbon electrode is monitored in situ by sampling of the cell voltage. When the voltage increases during a pause time, it means that it is being drained of its alkali ions14 and this in turn means that the SEI is not passivating the electrode sufficiently. It should be noted that it is probable that if the reactions that consume alkali ions are not stopped, then neither are the processes that break down the electrolyte without consuming the alkali cations. In figure 1, the full cycling regimes as well as the charge and discharge capacities for equivalent lithium and sodium cells using Super P working electrode and electrolytes of 1 M LiPF6 or NaPF6, respectively, dissolved in EC:DEC (1:1) are shown. The amount of ions being consumed is determined

with

two

different

methods:

the

first

is

to

compare

the

charge


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(delithiation/desodiation) capacity for the cycle prior to the pause step with the charge capacity for the charge following the pause step. In order to rule out any miscalculation arising from degradation of the electrode during the pause, the capacity is crosschecked between the last cycles before the pause with the second cycle after the pause. A second way to determine lost capacity is to measure the potential increase and correlating this value to the “Voltage vs. Capacity” plot for the electrode’s charge cycle prior to the pause (see figure 1C-1H).

Figure 1. A) Schematic of the cell construction used in the experiments. B) The complete cycling protocol used to measure self-discharge. A typical performance of lithium (C & D) and sodium (E & F) ion cells cycled galvanostatically with pause time during cycling. Overview of voltage


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evolution vs. time (left) and the charge and discharge capacities (right) for lithium and sodium cells using Super P working electrode and electrolytes of 1 M LiPF6 or NaPF6 dissolved in EC:DEC (1:1). G) Voltage increase during the first 100 h pause for the sodium and lithium cells. H) Capacity loss calculation for the cells in (G) using the voltage increase during pause in combination with the 10th cycle charge curve.

A comparison between the two techniques gives a deviation of less than 7% for the capacity loss, i.e. less than 7 mAh/g deviations for the most inconsistent data points. While the change in charge capacity is a reliable and straightforward technique, the determination of capacity loss via the voltage profiles enables the speed of self-discharge to be assessed at any point during the pause and thereby show rate changes in the dissolution (see Figure S1-S3). The comparison between sodium and lithium cells (Figure 1G) shows that during a 100 h pause, the former obtained a voltage increase of 0.679 V while the voltage increase for the latter under the same condition amounted to merely 0.147 V. The charge capacity lost as a consequence of the 100 h pause amounts to 71 and 38 mAh/g for the sodium and lithium cells respectively, which is almost 80 % of the sodium cell charge capacity and only 20 % of the lithium cell charge capacity. Following the experimental setup described above, we also studied hard carbon electrodes as well as 10 wt% FEC electrolyte additive in the standard electrolyte (EC:DEC 1:1 1M NaPF6). We also included lithium cells that were cycled to 0.3 V vs. Li+/Li which is comparable to the potential of almost 0 V vs. Na+/Na in order to truly have the same conditions for the SEI formation in the lithium and sodium cells. All systems investigated in this study show self-discharge behaviour to some extent (see figure 2) but the magnitude is dependent on parameters such as type of carbon used, cut off/hold potential, and whether FEC is used or not.


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The most significant parameter in this study, however, appears to be the cation and the results shows that sodium SEI development might need some unique solutions; in agreement with the findings of Moshkovich et al.7 for the PC-NaClO4 system. The best performing cell with regards to self-discharge was hard carbon with a low surface area. This is expected and also the motivation to use super P as the main substrate, as it will enhance any impact of surface reactions and give good signal to noise ratios. The lowest selfdischarge in the Super P system was displayed by the lithium-Super P cells that were cycled between 0.3 V and 2.5 V vs. Li+/Li, while the second lowest self-discharge in the Super P system was lithium cells cycled at the full range of 0.1 V - 2.5 V vs. Li+/Li. For sodium, the cells using 10 % FEC additive showed reduced self-discharge but this improvement came at the cost of increased polarization, in agreement with a recent study by Dugas et al.16


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Figure 2. Left) Voltage increase during the first 100 h pause for all cell types with charge capacities for cycles adjacent to the first 100 h pause used to calculate the self-discharge, right) Charge and discharge capacities.


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The results show that the potential difference between sodium and lithium is not the parameter that gives rise to the different self-discharge behaviour in Li and Na systems. In fact, the potential limited Li-super P cells were similar to the Na-super P cells in terms of capacity; 97 vs. 94 mAh*g-1 respectively but with much lower amount of self-discharge (16 vs. 71 mAh*g-1). These results indicate that the higher electrochemical potential of sodium might not be detrimental to the formation of a functional SEI layer but is instead an advantage judging from the improvement seen for lithium when utilizing a higher cut-off potential, similar to that of sodium. Furthermore, it is also shown that the self-discharge behaviour can be altered and improved by additives even though in this case the FEC arguably changes the problem from high self-discharge to high polarization. The increased polarization can be seen as a larger raise in potential after pausing, as the cell relaxes in comparison to the standard sodium electrolyte (Figure S1). Since the capacity of super P in sodium cells is different than the capacity in lithium cells, the comparisons are made either in absolute values as in figure 3A or in normalized capacity/coulombic efficiency as in figure 3B and as can be seen the trend of higher selfdischarge in SIB-Super P systems is quite clear in both types of plot.


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Figure 3. Charge capacity loss in relative and absolute terms: A) Absolute loss in mAh/g, B) Loss in percent of charge capacity. The self-discharge in the Na/Li-Super P system is readily shown and quantified; see figure 2 and 3. Therefore, the results clearly show that the self-discharge is a more pronounced problem in the tested SIBs compared to the that in analogues LIBs. However, the solely electrochemical results are not enough to show that the SEI is soluble, since there is no guarantee that this is the only mechanism for self-discharge. Other often attributed possible sources of self-discharge in these half-cells involve redox-shuttles, SEI dissolution or cracking/delamination and solvent permeability of the SEI layer. Although the redox-shuttle seems unlikely in this simple system, there may be ‘cross-talk’ between the highly reactive sodium anode and the high-surface area carbon electrode. Lee et al.17 have proposed several mechanisms for solvent decomposition at the metallic sodium surface, leading to reactive species that might diffuse to the other electrode and react. However, since the hard carbons display extremely low self-discharge, a redox-shuttle mechanism seems less likely. The reasoning behind this conclusion is that the self-discharge rate with this type of mechanism should be much less affected by surface area of the carbon than


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other possible mechanisms; if assuming that the redox-shuttle activity is not limited by bulk diffusion or surface kinetics. Cracking or delamination is also highly unlikely as the experiment is purposely designed to test the static stability of the SEI by minimizing material expansion and contraction. The electrochemical data presented thus far do not disprove the possibility of an SEI with porous morphology causing self-discharge by failing to block the solvent molecules from making contact with the electrode surface and reacting. If the SEI is non-soluble and solvent permeable, the SEI is expected to grow in thickness during pauses as more decomposition products are produced without any mechanism of removal. To determine if the SEI problems stem from the solubility of SEI component or permeability of solvent, synchrotron-based hard x-ray photoelectron spectroscopy (HAXPES) measurements were performed on super P electrodes that were paused for different amount of time, 72 h and 168 h, using two different photon energies of 2005 eV and 6015 eV; see Figure 4. The higher photon energy results in deeper probing depth of the SEI. It can be seen that the main peak in the pristine spectra, representing the Super P, has almost disappeared in the cycled electrodes, while new peaks originating from SEI component appear (the details of deconvoluted spectra are presented in the SI). For the 72 h samples the C-C peak is only visible when using the high energy radiation of 6015 eV, while for the 168 h pause samples the 284.4 eV peak appears in the more surface sensitive C1s 2005 eV spectra. This shows that the thickness of SEI is not increasing during pauses, but is instead decreasing. These results strengthen that the SEI solubility is the primary cause of self-discharge, rather than that a more solvent-permeable SEI is formed. It should be noted, however, that the severity of self-discharge is enough to encompass both these processes but that the results show that removal of SEI (dissolution process) is faster than new SEI formation (solvent permeability).


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The changes in the thickness and nature of the SEI in NIBs during storage (pause) time should also be taken into account when preparing samples for ex-situ characterization of SEI; in other words, samples that have inadvertently been kept in contact with electrolyte for long time (e.g. sample preparation or transportation) may not present a conclusive picture of the SEI.

The C-C peak from the electrode becomes more visible after long pause-time

Figure 4. C1s HAXPES spectra of super P electrodes that were sodiated and paused in sodium half-cells containing 1 M NaPF6 in EC:DEC (1:1) electrolyte. Measurements were performed using two excitation photon energies of 2005 eV (left) and 6015 eV (right).

In summary, the results show that the differences between Na and Li decomposition products are affecting the properties of the respective SEI layers. The differences in cell performance with regard to columbic efficiency and self-discharge do not seem to arise from the 0.3 V difference in potential. It is therefore suggested that the primary reason for inferior performance is dissolution of the SEI components in the sodium ion batteries. Having targeted the main SEIstability problems in NIBs, the next step would be to implement measures to counteract this shortfall in properties and thus enable practical NIBs. Additives such as FEC or difluoroethylene carbonate (DFEC)18 are shown to mitigate the problem of self-discharge, and if the optimal


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concentration of FEC additive is found the problem with increased polarisation might be negligible. As mentioned earlier, lithium forms more stable organometallic compounds and one use for these compounds is their use as an anionic polymerization initiator10; this means that there should be more species ready to polymerize at the interface in LIBs compared to SIBs. Combined with the more capable polymerization ability of lithium resulting from its higher charge density, this might be the reason for the increased solubility of sodium SEI components as the organic species that are not crosslinked or polymerized will dissolve and leave the rest of the SEI exposed to solvent and thereby to dissolution. This line of argumentation can also explain why only a few of the film forming additives from LIBs are effective in SIBs19 as, for example, the vinylene group seems to require presence of the more capable carbanion of organolithium compounds to function well.20 Finally, it should be mentioned that we have cycled Na-super P cells in conventional nonpause cycling programs, and at a glance the results indicate that super P is performing rather well with excellent capacity retention (see Fig. S4). It is not until the static stability is tested through slow cycling or pauses that flaws such as electrolyte failure and capacity fading appear. Therefore, it is detrimental that electrochemical tests such as galvanostatic cycling or cyclic voltammetry are accompanied with pause (ageing) tests, such as performed in this study, in order to evaluate the performance of anode materials in sodium-ion batteries.

Acknowledgments: The authors are grateful to the Swedish Energy Agency (Batterifonden, project number: 40468-1) for financial support. We thank the Helmholtz-Zentrum Berlin (HZB) for the allocation


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of synchrotron radiation beamtime. Fredrik Lindgren and Julia Maibach are also acknowledged for scientific discussions. Supporting Information Available: Details regarding of cell fabrication and cycling; sample preparation routines for HAXPES measurements; primary data from pause experiments; long term cycling data, and capacity loss rate plots.

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Impact of So-Called SEI-Forming Electrolyte Additives. J. Electrochem. Soc. 2013, 160, A709–A714. (15)

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