Initial Steps in PEO Decomposition on a Li Metal Electrode

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Initial Steps in PEO Decomposition on a Li Metal Electrode Amina Mirsakiyeva, Mahsa Ebadi, Carlos Moyses Araujo, Daniel Brandell, Peter Broqvist, and Jolla Kullgren J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b07712 • Publication Date (Web): 20 Aug 2019 Downloaded from pubs.acs.org on August 24, 2019

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The Journal of Physical Chemistry

Initial Steps in PEO Decomposition on a Li Metal Electrode Amina Mirsakiyeva1, Mahsa Ebadi1, C. Moyses Araujo2, Daniel Brandell1, Peter Broqvist1,*, and Jolla Kullgren1,* 1

Structural Chemistry Program, Department of Chemistry – the Ångström Laboratory, Uppsala

University, Box 538, SE-751 21 Uppsala, Sweden. 2

Materials Theory Division, Department of Physics and Astronomy, Uppsala University, Box 516,

SE-751 20 Uppsala, Sweden. Corresponding Author *corresponding authors: [email protected], [email protected]

Abstract

Poly(ethylene oxide) (PEO) is the most widely used compound as a solid-state (solvent-free) polymer electrolyte for Li-batteries, mainly due to its low glass transition temperature (Tg) and ability to dissolve Li-salts. It is also frequently suggested that its cathodic stability renders it possible to operate with Li-metal anodes in the design of high energy density storage devices. However, little is still known about the true interfacial chemistry between Li-metal and PEO, and how these two materials interact with each other. We are here exploring this relationship by the

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means of Density Functional Theory (DFT) based modelling. Using bulk structures and isolated PEO chains, we have found that there is a strong thermodynamic driving force to oxidize Li-metal into lithium oxide (Li2O) when PEO is decomposed into C2H4 and H2, irrespectively of the PEO oligomer length. Explicit modelling of PEO on a Li(100) surface reveals that all steps in the decomposition are exothermic and that the PEO/Li-metal system should have a layer of Li2O between the polymer electrolyte and the metal surface. These insights and the computational strategy adopted here could be highly useful to better tailor polymer electrolytes with favorable interfacial properties.

KEYWORDS Density functional theory, solid-electrolyte interface, polymer decomposition, ab initio thermodynamics


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Introduction Energy storage devices in terms of rechargeable batteries employing Li-metal electrodes are on the verge of large-scale commercial breakthrough1–4. Li-metal, which is ideal for high energy density storage in terms of negative electrode capacity and operating voltage, is however problematic due to its high reactivity and its ability to form dendrites during battery cycling. These shortcomings render a poor coulombic efficiency and large safety concerns, respectively. Of the many strategies to mitigate these problems, the use of solid electrolytes is perhaps the most straight-forward route toward Li-metal stabilization in battery devices5. Solid polymer electrolytes (SPEs), comprising a salt dissolved in a solid polymer matrix, constitute one of the two major categories of solid electrolyte materials; the other being ceramic electrolytes. While the bulk ionic conductivity of SPEs is comparatively low, the flexibility of polymers render them superior in terms of surface adhesion and mechanical compatibility with the electrodes, not least considering the volume changes experienced during battery operation6. The low bulk conductivity could in principle be mitigated by addition of liquid solvents into the electrolyte matrix, but this comes with large compromises with respect to electrochemical stability and Li-metal surface passivation7. Since the ionic transport in truly solvent-free SPEs generally is associated with the polymer segmental motion, low-Tg polymers with Li+-coordinating motifs are sought. This is primarily why poly(ethylene oxide) (PEO) is dominating this research field since its dawn in the early 1970s8. While the anodic stability of PEO is considered problematic, this material is generally considered to have a good cathodic stability, thereby being able to operate at the low electrochemical potential associated with metallic Li9. This has been shown repeatedly through


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electrochemical measurements, such as cyclic (CV) and linear sweep voltammetry (LSV), impedance spectroscopy, cycling of symmetric Li-Li cells, and also from the implementation of PEO-based electrolytes in commercial solid-state batteries10,11. The exact mechanisms and reactions occurring at the Li-metal/PEO interface are, however, not well understood. Thereby, little is known about potential improvements of this system and how its chemistry can be tailored further. Moreover, due to its large degree of crystallinity at ambient temperatures, PEO-based SPEs are generally operated at elevated temperatures (60-80 °C), which can cause instabilities in both the Li-metal electrode, the PEO-electrolyte, and any interfacial layer between them. There are likely very reactive sites on the surface of the highly reducing Li-metal surface, where chemical and electrochemical reactions with PEO could occur. It is also less known to what degree there exist any passivating solid electrolyte interphase (SEI) layer on the Li-metal electrode when utilizing SPEs. It could well be speculated that PEO decomposes to a thin passivating, yet ionically conductive, film on the metal surface – such as on conventional graphite anodes – and that this film is less prone to dissolution in the bulk SPE phase. It is known from experimental studies employing x-ray photoelectron spectroscopy (XPS) that Limetal has a native surface layer of both adsorbed and reacted species, even before any exposure to electrolyte12. This includes nitrogen, molecular carbon oxide and carbonate species, and a substantial part of oxides, which are primarily considered to be Li2O. On a very reducing metal such as Li, Li2O is a likely reaction product, but could then itself change the chemical potential at the surface which, in turn, cause the formation of other products such as superoxide ions13,14, or even peroxide ions. Any surface layer formed is thereafter presumably modified significantly, first by exposure to electrolyte, and then during repeated cycling. The bulk SPE should then encounter a chemical interplay with both the native surface layer, Li-metal and other formed decomposition


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products during battery operation. From the limited number of experimental XPS studies of cycled SPEs on Li-metal, a large amount of LiOH have been detected from PEO-based SPEs15, but could well be an artefact of substantial amounts of H2O residuals in PEO, since little LiOH was found for other polymer hosts16. Nevertheless, these studies clearly indicate the existence polymer decomposition products. In this context, computational techniques can be instrumental in gaining insight into the issues mentioned above. Density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations have, for example, been employed to study the reactions of Li-metal with liquid17–21 and ceramic22–26 electrolytes, and also for modelling the SEI layer formed on the surface of Li metal. The surface reactions between Li-metal and SPEs on the other hand have been largely neglected. To the best of our knowledge, there are only two recent studies addressing this important topic. In the first one, classical molecular dynamics (MD) simulations have been carried out with the aim to understand ionic transport in the vicinity of a Li metal/PEO interface27. In the second study, density functional theory based calculations and AIMD simulations have been employed to get atomic level insights into the early stages of SEI formation. In this context, several representative material classes, such as polyethers, polyalcohols, polyesters, polycarbonates, polyamines and polynitriles have been studied. For polycarbonates, it was found that, analogous to low-molecular-weight organic carbonates, decomposition pathways through Ccarbonyl–Oethereal and Cethereal–Oethereal bond cleavage are energetically favorable28. In this work, we intend to continue filling the literature void by investigating the thermodynamics of decomposition reactions involving PEO on a Li-metal surface forming Li oxides (Li2O, Li2O2 or LiO2) by an analysis of the PEO-Li system using thermodynamic data for the isolated bulk and molecular components.


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This analysis is followed by explicit simulations of PEO adsorption and the following PEO decomposition on the Li metal surface.

Computational Methods Electronic structure calculations The electronic structure calculations presented in this work were performed using the Density Functional Theory (DFT) in the implementation with a plane wave basis set and pseudopotentials. The exchange correlation energy was described using the derivation proposed by Perdew, Burke and Ernzerhof, i.e. the PBE density functional29,30. To describe the core electrons and the corevalence electron interactions, the pseudopotential of the Projector Augmented Wave (PAW) type was used31,32. In the calculations, the Li 1s2 2s1, C 2s22p2, H 1s1 and O 2s22p4 electrons were treated explicitly. Thus, for Li, the semi-core states, which are important for the oxidized atoms, were also included. In the simulations, a plane wave cut-off of 600 eV was used. In geometry optimizations, structures were deemed to be converged when the maximum forces on all atoms were below 0.01 eV/Å. In all simulations, a Gaussian smearing of 0.025 eV for our electronic states was used. The k-point sampling was adopted to the unit cell used in the simulations. For bulk calculations, 888, 888, 884 and 666 k-point meshes for bulk lithium metal, lithium oxide, lithium peroxide and lithium superoxide were used, respectively. Spin-polarized calculations were utilized when necessary (e.g. for O2(g)). For the geometry optimizations performed in the screening of local minimum structures for PEO adsorption on the Li(100) surface, the Brillouin zone was sampled at


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the Γ-point only. All calculations were carried out using the Vienna Ab initio Simulation Package (VASP)33–36. Model systems The bulk Li metal (Li(s)) was modelled enforcing a bcc structure; for bulk lithium oxide (Li2O(s)) an anti-fluorite structure; for bulk lithium peroxide (Li2O2(s)) a hexagonal structure (P63/mmc); and finally, for lithium superoxide (LiO2(s)), an orthorhombic structure (F4/mmm). The lattice parameters were optimized for the various bulk compounds, obtaining the following results: Li(s), a=b=c= 4.34 Å; Li2O(s), a=b=c=4.62 Å; Li2O2(s), a=b= 2.85 Å and c=8.04 Å; LiO2(s), a= 4.78 Å, b= 5.05 Å, and c= 7.78 Å. In addition to the bulk structures, also oxide, peroxide and superoxide layer formation on the Li(100) surface were studied. The oxide layer formation was modelled using a 5 atomic layer thick p(22) supercell. In each case a number of oxygen atoms or O2 entities, corresponding to a monolayer of the respective oxide layer was placed on the Li(100) surface which was subsequently optimized. The bottom two layers of the Li(100) surface were kept fixed at their optimized bulk values. PEO was built from ethylene oxide (C2H4O) and consist of H2C-O-CH2 monomer units. Gas phase PEO properties were calculated for single H3C-(H2C-O-CH2)n-CH3 chains with varying n in a large supercell model, ensuring isolated non-interacting polymer chains. In all calculations, we use terminal -CH3 groups at the oligomer end points to terminate the chains, thereby rendering an ethoxy end-group capping. For the adsorption studies on the Li(100) surface, PEO with a chain length of n=3 was chosen. It has previously been shown that this chain length effectively captures the rather “localized” character of the electronic structure of the PEO28, which makes it suitable


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for use in DFT studies. In the adsorption simulations, we used a p(6×6) Li(100) supercell. Given the large flexibility for this comparatively small oligomer, we performed a total of 37 geometry optimizations with different starting geometries to map the potential energy surface for oligomeric PEO on Li(100) to ensure that the calculated adsorption energies corresponds to a low-lying minimum energy structure. Results and discussion Thermodynamics based on bulk and surface data We start our analysis by establishing the oxidizing power of PEO(g) as a function of chain length, n. This is done by calculating the relative chemical potential µO (in eV per O) for PEO(g) as a function of n using the following formula:

𝜇𝑂 (𝑛) =

𝐸𝑃𝐸𝑂 − 𝐸𝐻2 − (𝑛 + 1)𝐸𝐶2 𝐻4 𝑛

where 𝐸𝑃𝐸𝑂 , 𝐸𝐻2 and 𝐸𝐶2 𝐻4 are the DFT total energies (at T=0K) for a PEO oligomer of size n, H2(g) and C2H4(g), respectively. On this scale and under the same approximation, µO for O(g) and O2(g) are -1.90 eV and -4.94 eV, respectively. The decomposition reaction is quite different from the reversal of the polymerization reaction since the oxygen in the oligomer are used to oxidize the Li metal. The decomposition likely leads to the formation of gaseous ethene (C2H4(g)) and H2(g), where the latter (and the +1 C2H4(g) in the formula above) comes from decomposing (and reaction of) the two CH3 end groups of the polymer chains. That fact that only T = 0 K data are considered is clearly a limitation of the current study and is a consequence of the difficulty in determining the entropic contributions for the PEO oligomer. Nevertheless, we are convinced that the T = 0 K data will be indicative of the general trends, and that they still constitute a relevant


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estimate of the oxidative power of the polymer. It should also be noted that as we consider one PEO oligomer in the gas phase, the calculated μO(n) should be considered an upper limit of the oxidative power at T = 0 K. Forming the solid phase of PEO(s) should lower EPEO, and thereby lower µO, i.e. weakening the oxidative power. The resulting μO(n) is plotted in Fig. 1a and shows that the oxidative power of the oligomer rapidly converges to the asymptotic value of -7.1 eV when increasing the chain length (i.e. 𝜇(∞) = −7.1 eV). At n=3, which is smallest chain length considered here, the oxidative power becomes -7.7 eV (i.e. 𝜇(3) = −7.1 eV). Thus for 𝑛 larger than 3 we have −7.7 < 𝜇𝑂 < −7.1 eV. This range of 𝜇𝑂 -values corresponds to a relatively low oxidative power. To investigate whether the oxidizing power of PEO is still enough to oxidize the lithium metal into Li2O, Li2O2 or Li2O2, we have calculated the formation energy of these compounds as a function of µO: 𝐸𝑓 (𝜇𝑂 ) = 𝐸𝑜𝑥𝑖𝑑𝑒 − 𝐸𝑚𝑒𝑡𝑎𝑙 − 𝑛𝑂 𝜇𝑂 . 𝐸𝑜𝑥𝑖𝑑𝑒 is here the total energy of either of the bulk oxides, 𝐸𝑚𝑒𝑡𝑎𝑙 is the total energy of Li(s), and nO is the number of oxygen atoms. In the formula, 𝐸𝑜𝑥𝑖𝑑𝑒 and 𝐸𝑚𝑒𝑡𝑎𝑙 must contain the same number of Li atoms. The result for 𝐸𝑓 (𝜇𝑂 ) is shown in Fig. 1b for all oxides. In the figure, we also indicate the oxidative power of PEO together with that of O2(g) for reference. In range of oxidative power permitted by PEO (−7.7 < 𝜇𝑂 < −7.1 eV), only Li2O and Li2O2 are thermodynamically stable (Ef < 0). Thus, based on the T=0K formation energies, we expect that PEO chains in contact with Li(s) will lead to the formation of bulk Li2O(s). However, we cannot rule out that the Li2O2(s) phase might form at the surface where there will be more space to accommodate the larger peroxide ions.


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Figure 1. (a) The oxidative power μO of PEO(g) as function of chain length n. (b) Formation energy for lithium oxide, peroxide and superoxide versus oxygen chemical potential. (c) Monolayer interface energy upon oxidation of the Li(100) surface as function of oxygen chemical potential (same legend as in (b)). The filled zones in (b) and (c) corresponds to the chemical potential for individual PEO chains of varying lengths (resulting from (a). The dashed horizontal line in (c) is the surface energy for the clean Li(100) surface.

To investigate the “surface effect” in more detail, we have computed the energy of formation for a monolayer of Li2O and Li2O2, on top of the Li(100) metal surface, as shown in Fig. 2. In this analysis, we used a five-layered (2×1)Li(100) slab supercell and calculated the interface energies according to:

𝐸𝑖𝑛𝑡 (𝜇𝑂 ) =

𝐸𝑜𝑥𝑖𝑑𝑒 − 𝐸𝑚𝑒𝑡𝑎𝑙 − 𝑛𝑂 𝜇𝑂 𝐴

where 𝐸𝑜𝑥𝑖𝑑𝑒 and 𝐸𝑚𝑒𝑡𝑎𝑙 are the total energies of the slab with and without 𝑛𝑂 oxygen atoms, respectively, and where A is the corresponding surface area.


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Figure 2. The various surfaces considered in the comparison of surface oxidation. Li is represented by purple balls and oxygen with red. (a) shows the clean Li(100) surface, (b), (d) and (e) show the Li(100) with one monolayer (ML) of oxide, peroxide and superoxide, respectively. (c) shows the two ML oxide which in terms of stoichiometry is equivalent to the one ML peroxide surface. Fig. 1c compares the calculated interface energies of the monolayer formed on top of the pristine (neat) metal surface as a function of 𝑛𝑂 . As for the bulk compounds, the Li2O monolayer formation comes out as the most stable. In this particular structure, the oxide ions are coordinated to four Li ions, i.e. half the number compared to the coordination in the bulk Li2O structure. For comparison, we also consider an interface with two monolayers of oxide. Since the stoichiometry of this interface (two layers of Li2O) is identical to the one monolayer peroxide interface, a direct comparison can be made. We find that the two monolayer oxide interface is more stable by 0.67 eV/Å2. This result is not unexpected, given that both Li(s) and Li2O(s) are cubic structures with a relatively small lattice mismatch, and implies that even on a Li-metal template; the oxidative power of PEO is large enough to also penetrate in to the subsurface of the Li metal.


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In conclusion, the analysis based on bulk data and monolayer oxidation suggest that lithium oxide (Li2O) should be the primary product when PEO interacts with a Li metal surface. This conclusion finds support in the XPS measurements in Refs15,37, which proposes Li2O as a component in native layers formed on a Li metal electrode. However, to unravel the mechanisms that may underlie such transformations and to discuss possible kinetic limitations for oxide growth, an explicit interaction of PEO with a Li metal surface needs to be considered. Explicit interaction between PEO and the Li(100) surface To simulate the adsorption and decomposition processes of explicit PEO in more detail, larger surface models are necessary. Furthermore, we also need to limit the size of the PEO oligomer. In this part of the work, we use a chain length of n= 3, with the H3C-(H2C-O-CH2)3-CH3 formula, thereby including three oxygen atoms and the possible formation of four C2H4(g) and one H2(g) molecule upon decomposition. The intention is to gain insight into possible reaction mechanisms and reaction intermediates that could form during the decomposition to being able to discuss possible kinetic limitations. Adsorption of intact PEO oligomers First, the initial adsorption of the PEO oligomer on the Li metal surface is considered. For this purpose, 37 different initial structural configurations were constructed, and each of these were allowed to relax to their nearest local minimum energy structure. The relative energies with respect to the most stable adsorption configuration is shown in Fig. 3a. It can be noted that all molecules are adsorbed as intact molecules and differ by at most 0.9 eV, and that the majority of the computed adsorption energies are found to be within the range of approximately 0.3 eV with respect to the most stable adsorption configuration, which is shown in Fig. 3b. This suggests that the lowest energy configuration is most likely a good representative.


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The adsorption energy calculated with respect to the Li metal and the PEO oligomer in gas phase is 0.94 eV. In the most stable configuration found, PEO is adsorbed parallel to a {100} direction and binds to the surface through its three oxygen atoms, which are coordinated to three different Li atoms in the surface. The Li-O bond lengths are in the range 1.954-1.994 Å. Similar Li-O motifs are found in many other of the adsorption configurations. We find no stable configurations where the oxygen moieties coordinates more than one Li. This is likely a steric effect which can be appreciated by inspecting the stable configuration shown in Fig. 3b.

Figure 3. (a) Relative stability of 37 configurations of an adsorbed PEO oligomer at the Li(100) surface. The energies are plotted with respect to the energy of the most stable configuration, which is shown in (b). Decomposition of PEO and oxidation of the Li(100) surface Starting from the most stable configuration for the adsorbed PEO on the Li surface, as shown in Fig. 3b, two tentative routes to its decomposition and simultaneous oxidation of the surface are


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considered. Neither of these leads to the formation of the ethylene oxide monomer unit or -CH3 end groups, but instead to C2H4(g) and adsorbed -C2H5 molecules in conjunction with the oxidized surface. During the desorption process, we assumed that the two unsaturated C2H5 formula units form C2H4(g) and H2(g). Thus, guided by the results of the previous section, it can be expected that the oxygen atoms strives to increase their coordination number with respect to Li if given the opportunity. Fig. 4 illustrate the two tentative decomposition routes for PEO on the Li(100) surface. Both involves the forced breaking of C-O bonds in the oligomer (Fig. 4a), which allow oxide ions to slide down between Li atoms, and thereby increase their coordination (Fig. 4b-d). These simulations are always started by breaking the C-O bond following directly after the first ether group along the oligomer chain (1 in Fig. 4a), which liberates a C2H5O unit (c.f. Fig. 4b-c). Thereafter, we consider one route (route I) were the consecutive C-O bonds along the oligomer chain are broken, liberating one C2H4 unit every second C-O bond is broken, and one route (route II) where the C-O bonds are broken “outward in” (c.f. Fig. 4a).


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Figure 4. (a) Two tentative routes for the decomposition of PEO on the Li(100) surface based on scissoring of C-O bonds along the oligomer. The numbers in circles at the bottom of the panel indicates the different bond that are being scissored in two decomposition routes considered in this work. In route I, the bonds are scissored in the order: 1, 2, 3, 4, and in route II they are scissored in the order: 1,4,2,3. Bottom panels: Starting from the lowest energy configuration (b) of the intact PEO oligomer, the decomposition involve consecutive scissoring of C-O bonds along the polymer. The first step is shown in (c). The two routes both leaves three oxygen ions embedded in the surface (d). In the latter case (route II), the first C2H4 unit is liberated after breaking three C-O bonds. All PEO decomposition fragments are kept adsorbed until the very last step of the tentative reaction routes. Before desorption, the last two C-O bonds needs to be broken, which liberate two C2H5


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units. As mentioned above, when calculating the energy of the final state, it is assumed that the two -C2H5 groups are decomposed into two C2H4(g) and one H2(g) molecules. The full decomposition routes leaves 3 oxygen atoms incorporated in the top layer of the Li slab (Fig. 4d) and the full process, i.e. the following reaction: H3C-(H2C-O-CH2)3-CH3(g) + Li(100)(s)  Li-slab-O3(s)+ 4C2H4(g) + H2(g)

is strongly exothermic with an average energy gain of 2.42 eV/O atom (ΔE = -2.42 eV). All steps in the decomposition, except the desorption of the fragments, in the two routes are exothermic with reaction energies larger than 0.38 eV and 0.64 eV in route I and II, respectively. Thus, the results from the explicit interaction of the polymer corroborate the results obtained from the thermodynamic analysis of bulk and gas constituents presented above. Adsorption and decomposition of PEO on the oxidized Li(100) surface In the previous section, we found, in agreement, with the bulk and surface thermodynamics, that the Li(100) surface will be readily oxidized when brought in contact with PEO. There is a large energy gain associated with the PEO decomposition on the surface. From these simulations, we find that the initial and the full oxidation of Li will be thermodynamically favorable. However, to understand the kinetics underlying these reactions, we need to obtain a picture of what will happen in between. We have therefore performed simulations of PEO adsorption and decomposition, following the same route as in Fig. 4 but for a partly (or fully) oxidized Li(100) surface. Simulations of a PEO oligomer adsorbed on top of a p(66) supercell of a 2 ML Li2O surface, formed on top of the Li(100) surface, have been undertaken. We started from the most stable configuration for PEO on the pristine (neat) Li(100) surface and subsequently re-optimize the


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structure. The optimized adsorption configuration is displayed in Fig. 5a. We have also calculated the energy for the first step in the decomposition of this structure in an analogous way as for the pristine Li(100) surface. This decomposition process is associated with a large exothermic energy (ΔE = -3.48 eV), similar to the first oxygen abstraction on the clean Li(100) surface. This time also accompanied by a severe distortion of the surface (see Fig. 5b). For the fully oxidized Li2O(111) surface, the initial decomposition step become endothermic (E = +2.71 eV), following the process illustrated in Fig. 5c-d. This is expected, since the oxidation would require the formation of a Li ion with a higher oxidation state than +1 or the formation of superoxide- or peroxide ions, and consistent with the thermodynamic data presented above.


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Figure 5. (a) PEO adsorbed on a Li(100) surface with two ML of oxide, and (b) the first step towards its decomposition following the routes described in Figure 4. (c) PEO adsorbed on a Li2O(111) surface, and (d) the first step towards its decomposition following the routes described in Figure 4. The structure in (d) is highly unstable with respect to the intact PEO on the surface. This fact is illustrated by the faded colors in (d). Conclusion We have presented an analysis based on thermodynamic data for bulk and surfaces to predict the outcome of the interaction of PEO and the pristine and partially (or fully) oxidized Li(100) surface. By calculating the oxidizing power of the gaseous PEO oligomers when it undergoes


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decomposition into C2H4(g) and H2(g), we show that there is a strong thermodynamic driving force to form Li2O. The analysis also shows that the more oxidized phases, lithium peroxide and superoxide, do not form under these conditions. These results are corroborated by explicit firstprinciples simulations of PEO adsorption and decomposition on the pristine, partially and fully oxidized Li(100) surfaces. These explicit simulations suggest that the decomposition of PEO is strongly exothermic as long as there are metallic Li, i.e. for the pristine and partly oxidized Li(100) surfaces. Simulations using a fully oxidized Li2O(111) surface, show that the PEO decomposition is an endothermic process. Based on our findings, we anticipate that PEO/Li-metal systems should have an interphase layer of Li2O between the polymer electrolyte and the metal surface with a thickness limited by kinetics. Further studies are needed to address the details in the kinetics, both when it comes to the decomposition reaction and the growth of the oxide layer, i.e. the buildup of the SEI. ACKNOWLEDGMENT We acknowledgement financial support from STandUP for Energy, eSSENCE, the Swedish Energy Agency (grant no. 39036-1), and ÅForsk (JK).The computations were performed on resources provided by the Swedish National Infrastructure for Computing (SNIC) at NSC and UPPMAX. REFERENCES (1) (2) (3) (4)

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TOC GRAPHICS

TOC Schematic illustration showing the different scenarios that could follow the adsorption and interaction of PEO on a Li metal surface. Depending on the oxidative power of the polymer, which will depend on length and temperature, lithium oxides with varying oxygen content is expected to form.


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Figure 1. (a) The oxidative power μO of PEO(g) as function of chain length n. (b) Formation energy for lithium oxide, peroxide and superoxide versus oxygen chemical potential. (c) Monolayer interface energy upon oxidation of the Li(100) surface as function of oxygen chemical potential (same legend as in (b)). The filled zones in (b) and (c) corresponds to the chemical potential for individual PEO chains of varying lengths (resulting from (a). The dashed horizontal line in (c) is the surface energy for the clean Li(100) surface.



Figure 2. The various surfaces considered in the comparison of surface oxidation. Li is represented by purple balls and oxygen with red. (a) shows the clean Li(100) surface, (b), (d) and (e) show the Li(100) with one monolayer (ML) of oxide, peroxide and superoxide, respectively. (c) shows the two ML oxide which in terms of stoichiometry is equivalent to the one ML peroxide surface.


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Figure 3. (a) Relative stability of 37 configurations of an adsorbed PEO oligomer at the Li(100) surface. The energies are plotted with respect to the energy of the most stable configuration, which is shown in (b).



Figure 4. (a) Two tentative routes for the decomposition of PEO on the Li(100) surface based on scissoring of C-O bonds along the oligomer. The numbers in black circles at the top and bottom of panel indicates the different routes characterized by different order in the scissoring of the bonds. Bottom panels: Starting from the lowest energy configuration (b) of the intact PEO oligomer, the decomposition involve consecutive scissoring of C-O bonds along the polymer. The first step is shown in (c). The two routes both leaves three oxygen ions embedded in the surface (d).


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Figure 4. (a) Two tentative routes for the decomposition of PEO on the Li(100) surface based on scissoring of C-O bonds along the oligomer. The numbers in circles at the bottom of the panel indicates the different bond that are being scissored in two decomposition routes considered in this work. In route I, the bonds are scissored in the order: 1, 2, 3, 4, and in route II they are scissored in the order: 1,4,2,3. Bottom panels: Starting from the lowest energy configuration (b) of the intact PEO oligomer, the decomposition involve consecutive scissoring of C-O bonds along the polymer. The first step is shown in (c). The two routes both leaves three oxygen ions embedded in the surface (d).



TOC. Schematic illustration showing the different scenarios that could follow the adsorption and interaction of PEO on a Li metal surface. Depending on the oxidative power of the polymer, which will depend on length and temperature, lithium oxides with varying oxygen content is expected to form.


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Initial Steps in PEO Decomposition on a Li Metal Electrode

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