Formation and Growth Mechanisms of Solid-Electrolyte Interphase

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Formation and Growth Mechanisms of SolidElectrolyte Interphase Layers in Rechargeable Batteries Fernando A. Soto, Yuguang Ma, Julibeth M Martinez de la Hoz, Jorge M Seminario, and Perla B. Balbuena Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b03358 • Publication Date (Web): 17 Nov 2015 Downloaded from http://pubs.acs.org on November 22, 2015

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Chemistry of Materials

Formation and Growth Mechanisms of Solid-Electrolyte Interphase Layers in Rechargeable Batteries Fernando A. Soto1, Yuguang Ma1, Julibeth M. Martinez de la Hoz1,2, Jorge M. Seminario1,2,3 and Perla B. Balbuena1,2* 1 Department of Chemical Engineering, 2Department of Materials Science and Engineering, 3 Department of Electrical Engineering Texas A&M University, College Station, TX 77843 *e-mail: [email protected] Abstract Battery technology is advancing rapidly with new materials and new chemistries; however materials stability determining battery lifetime and safety issues constitutes the main bottleneck. Electrolyte degradation processes triggered by electron transfer reactions taking place at electrode surfaces of rechargeable batteries result in multicomponent solid-electrolyte interphase (SEI) layers, recognized as the most crucial yet less well-understood phenomena impacting battery technology. Electrons flow via tunneling from the bare surface of negative electrodes during initial battery charge causing electrolyte reduction reactions that lead to SEI nucleation, but the mechanisms for further growth beyond tunneling-allowed distances are not known. Our first-principles computational studies demonstrate that radical species are responsible for the electron transfer that allows SEI layer growth once its thickness has evolved beyond the electron tunneling regime. In addition, the composition, structure, and properties of the SEI layer depend on the electrolyte, especially on the extent to which they are able to polymerize after reduction. Here we present a detailed study of polymerization mechanisms and propose mechanistic differences for electrolytes yielding a fast and a slow SEI growth. This new understanding leads to firm guidelines for rational electrolyte design.

Introduction Numerous experimental and theoretical studies are oriented to elucidate details of the formation of a solid-electrolyte interphase layer due to the reduction of the electrolyte at the surface of anodes of lithium-ion and lithium-sulfur batteries1-5. State-of-the-art non-aqueous liquid electrolytes are composed of a lithium salt that is dissolved in a solvent or mixture of organic carbonate solvents. Many insights have been obtained from the analysis of the first stages of 1 ACS Paragon Plus Environment


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electrolyte decomposition on various electrode surfaces6. It is clear that electron transfer from the negative electrode surface during charge of the battery produces the decomposition of the electrochemically unstable solvents and salt near the electrode surface7, and as a result a multicomponent SEI film is formed that contains inorganic compounds such as LiF, Li2O, Li2CO3, and organic species such as lithium ethylene dicarbonate (Li2EDC) and many others including oligomeric and polymeric compounds4. An effective SEI layer is responsible for the chemical and mechanical stability of the electrode; it should be able to let Li ions go through and passivate the electrode surface thus drastically reducing the number of undesired electron transfer side reactions1. However, because a certain fraction of Li ions are trapped inside the SEI layer by combining with fragments of the electrolyte decomposition instead of intercalating in the electrode structure or being deposited on its surface, there is always an irreversible capacity loss (ICL) associated with these phenomena. The crucial role of the SEI layer on the battery lifetime depends on the extent of the ICL, which is directly related to the structure and composition of the film, together with the layer chemical and mechanical protective abilities. Depending on the reactivity of the solvent, additive, and salt components a great variety of radical anions and neutral species are formed after initial electron transfer from the surface to the adsorbed molecule or to intact or open molecules in the liquid phase8-12. Our earlier analyses of solvent reduction showed that after the initial reduction of an ethylene carbonate (EC) molecule, two different radical anions EC•- may be formed following ring opening13. These radical anions paired to a Li ion can combine with itself or with several other moieties to form various different products13. Li2EDC is formed via dimerization occurring by nucleophilic attack of the radical center by oxygen13. Similarly, a radical anion is formed from VC ring opening14 which can dimerize forming lithium vinylene dicarbonate (Li2VDC) and lithium divinylene dicarbonate (Li2DVDC) among other products14. Such dimers may further aggregate forming oligomers of 3, 4, or more units via O..Li..O interactions15. The free energies of formation of dimers of Li2EDC, Li2VDC, and that derived from propylene carbonate (Li2PDC) are -2.72 eV, -3.40 eV, and -3.66 eV respectively, thus very exothermic reactions. Previous studies reported similar exothermic reaction energies for other higher order oligomers15. Therefore, for the organic fragments of solvent decomposition, the oligomerization reaction competes with adsorption on the surface and with further polymerization reactions. Other successful additive, fluorinated ethylene carbonate (FEC), may produce F radicals along with organic radical anions and neutral species but also 2 ACS Paragon Plus Environment

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VC-like radicals9. Again, the organic reactive product may compete to forming a polymer or oligomer film with double-bonded characteristics or become adsorbed on the surface. LiPF6 salt decomposition has been recently investigated on silicon surfaces12. It was concluded that the initial products are F• radicals that recombine rapidly nucleating LiF on the surface. Radical O species are also detected from salt decomposition; these radicals can be precursors of Li2O or silicate formation. Besides the organic products (oligomers and polymers), LiF, and Li2O, a frequently found product is Li2CO316,17. Radical species can also recombine or react with other components forming polymers. Mass spectroscopy evidence has indicated the formation of long chain oligomers in the SEI18-20. Experimental studies by Ota et al.21 also showed that VCderived films contain poly(VC), an oligomer of VC, a ring opening polymer of VC, and polyacetylene.

In summary, a large collection of experimental and theoretical studies have

contributed to elucidating the structure and composition of the SEI layer, including recent attempts of combinatorial screening22. Regarding electron transfer, a comprehensive study of Leung and collaborators23 explained how the presence of insulating layers in a natural or artificial SEI slows down the side reactions because electron transfer or tunneling fall in the nonadiabatic regime. However, one crucial remaining question is how does the SEI layer grow after its thickness goes beyond electron tunneling lengths? And why is the growth rate so critically dependent on the nature of the solvent/additive used in the electrolyte formulation? Understanding these fundamental issues would form a solid basis for a rational electrolyte design. Here we investigate the electrochemical stability of key SEI components and the formation of polymeric species via density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations. Our study provides a detailed analysis of the role of radical species and their involvement in the SEI growth from which a clear picture of electron transfer mechanisms in post-tunneling regimes emerges.

From our analysis and the available

experimental and theoretical information, we also present a detailed characterization of fast and slow SEI layer growth. Results and Discussion Electron transfer through the SEI layer. It is generally accepted that the SEI layer structure may be approximated by an “inner” (close to the electrode) inorganic layer that is composed by LiF, Li2O, and Li2CO3 as the main components and an “outer” (in contact with the electrolyte) more

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porous organic layer. Such an organic layer may be composed of dimer species as Li2EDC, Li2VDC and Li2DVDC, or higher order oligomers that result from successive monomer additions. It could be speculated that as these constituents nucleate on the anode surface or on the nascent SEI, channels may be formed that could allow the transfer of charged species and, therefore, the growth. We tested this hypothesis and determined the formation of chemical bonds at the interfaces between inorganic phases such as LiF, Li2O, and Li2CO3 and lithiated anodes, as well as between the nascent SEI “phases” developing compact multicomponent blocks that allow Li ion but no anion diffusion. DFT analysis and evaluation of the minimum contact distances between interfacial atoms indicate that the surfaces are strongly interactive, and there are not any voids or channels. Although gas molecules are formed, their proportion is not enough to affect film porosity significantly. Examples of the interfacial distances in the models studied are provided as Supplementary Information (Figure S1). One important issue, however, was found regarding the stability of the organic phases tested on a system where a thin Li2EDC film has grown directly on a Li104Si32 model anode surface as shown in Figure S2. AIMD simulations reveal that electron transfer from the anode surface to an ethylene dicarbonate (EDC) anion from a Li2EDC molecule causes its decomposition into two carbonate anions (CO3) and one ethylene molecule (C2H4). Time evolution of the charges shown in Figure S3 clearly shows charge transfer from the anode to the Li2EDC film. Bader charge analysis of the decomposed anion was performed at different times (Figure 1), leading to formulation of reactions (I) and (II) as the path followed by the EDC anion of Li2EDC during its decomposition: -1.9

EDC -0.5

EC

-0.5

EC + 1e-

-1.4

+ CO3 -1.5

CO3

+ C2H4

(I) (II)

Details of the electron transfer from the surface is observed in the successive frames (Figure 1) yielding elongation of the C(ethylene)-O(carbonate) bond from 1.445 to 1.596 Å at 68 fs and then to 1.804 Å at 74 fs and consequent increase of the negative charge resulting in bond splitting and formation of a CO3 anion and an open EC also negatively charged (reaction I). The open EC anion receives an additional electron (reaction II) ending in separation of the other CO3 anion and a neutral ethylene molecule at 1363 fs (Figure 1). We emphasize that the fractional electronic charges reported in equations (I) and (II) and Figure 1 are artifacts of the Bader


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algorithm. Moreover, electronic charges are not physical observables; however, the values provide a good estimate of the probability of a certain electronic distribution in the system.

Figure 1. Decomposition of a Li2EDC molecule shown at different times along with charges calculated for the different fragments of the decomposition of the EDC anion. Color code: Red: O; Grey: C; White: H.

To investigate whether these oligomer decomposition reactions may also occur far from the anode surface we built a multicomponent SEI model anode/X1/X2 formed by two or more blocks (X1, X2) stacked over the anode surface Li13Si4. The models emulate the initial stages of SEI formation after some X1, X2 compounds (such as LiF or Li2O) have nucleated and crystalized over the surface. The choice of LiF and Li2O was based on these being the most common products found experimentally24,25 and theoretically12,26 in Si anodes. AIMD simulations were performed to evaluate the time evolution of the system after addition of a radical species: vinyl C2H3• and hydroxyl OH• radicals were tested in various locations of the simulated system. These radicals only have in common the presence of an unpaired electron and 5 ACS Paragon Plus Environment


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their subsequent extreme reactivity. In both cases, a Li atom was also added to the system, representing the Li ions that become reduced at the anode surface during charge. On top of the anode/X system, we deposited a thin organic layer of a given oligomer (Li2EDC or Li2VDC). AIMD simulations were run for times up to 75 ps. The initial location of the radical was varied to test different environments. Oligomer decomposition events were detected when the radical species is located at the inorganic/organic interface. Figure 2 shows one of these events: a Li2EDC molecule is decomposed by interaction with a vinyl radical, resulting in scission of one C(carbonate)-O(ethylene) bond and formation of CO2 and a new radical OC2H4CO3. The Bader charge analysis (Figure 2e) follows four Li2EDC molecules and the C2H3 radical showing large oscillations in the charges of both C2H3 and the decomposing Li2EDC molecule. The inset reflects the electron propagation effect not only to the decomposing molecule but also in the surrounding Li2EDC environment during the event. The electron transfer to the decomposing Li2EDC molecule is clearly detected analyzing the partial density of states of the structures before and after the decomposition (Figure S6) that shows a significant increase of the electron population in the vicinity of the Fermi level.

Figure 2. (a) Initial configuration of a model anode/SEI given by Li13Si4/LiF/Li2EDC surface. A C2H3 radical and a Li atom were added near the LiF surface in the Li2EDC region. (b) After 2485 fs, one of Li2EDC molecules close to the LiF surface starts to be reduced, and the C(carbonate)-O(ethylene) bond elongated. (c) At 2500 fs the C(carbonate)-O(ethylene) bond is broken (shown by a red arrow) forming CO2 and an OC2H4CO3 radical anion. (d) Snapshot of the system at 50 ps. e) Bader charges evolution for the decomposing molecule (dark orange) and the surrounding Li2EDC molecules as well as that of C2H3 (light blue). The inset amplifies a 10 fs interval right before and right after the decomposition event. Color code: Li: purple; F: light blue; Si: yellow; O: red: C: grey; H: white.


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A different mechanism takes place in the decomposition of Li2VDC as shown in Figures 3 and 4, where the C(vinyl)-O(carbonate) bond breaks leading to the separation of one (Figure 3) or two CO3 groups (Figure 4). These decompositions have similarities with that in Figure 1. The difference is that the electron transfer that triggers the reaction comes from the anode surface in Figure 1 whereas here the additions of the vinyl and the Li atom radical species cause a rapid electron propagation in their vicinity as shown by the charge evolution in Figures 3 and 4 that induces the bond breaking.

Figure 3. (a) Initial configuration of a model anode/SEI: Li13Si4/Li2O/Li2VDC surface. A C2H3 radical has been added near the Li2O surface in the Li2VDC region. (b) After 10104 fs, one of Li2VDC molecules close to the Li2O surface starts to be reduced, and the C(vinyl)-O(carbonate) bond elongated. (c) At 10115 fs the C(vinyl)-O(carbonate) bond is broken forming CO3 and a C2H2CO3 radical anion. (d) Bader charges evolution for the decomposing molecule (red line) and the surrounding Li2VDC molecules as well as that of C2H3. Color code as in Figure 2.

The induced electron-rich region is clearly observed in an electrostatic potential map shown in Figure 4 (blue regions). From the AIMD trajectories, it is observed that one of the CO3 groups becomes adsorbed at the surface while the other has an excess amount of Li atoms around it, hindering its mobility. On the other hand the neutral C2H2 molecule becomes very mobile within the bulk of the organic block. The zoomed-in snapshots reveal that the radical Li atom plays a role in the dissociation of this molecule. At 4031 fs, one Li positions itself in a “bridge” configuration. That is, between the acetylene group and the carbonate group. Due to the



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similarities with the reaction in Figure 1, this decomposition may also occur at the anode surface if the oligomer would be located close to that surface.

Figure 4. Electrostatic potential map follows a reaction occurring at the solid/electrolyte interphase region of the Li13Si4/LiF/Li2VDC surface. The sequence illustrates the events preceding a 2-step decomposition of Li2VDC yielding 2 CO3 groups and C2H2. One CO3 anion separates first from the whole molecule (at approximately 3931 fs) and short after (at 4031 fs) the other CO3 group breaks apart from the acetylene molecule. Color code: Li: purple; Si: yellow; F: green; C: grey; O: red, H: white.

To better understand the nature of these reactions, DFT calculations of bond decomposition analysis were carried out in four dimers: Li2EDC, Li2VDC, lithium butylene dicarbonate (Li2BDC) and lithium divinylenedicarbonate (Li2DVDC). The energy needed to break a specific bond was determined by the energy difference between the molecule and that of the remaining fragments after bond breaking. The intrinsic chemical stability is tested by analyzing the bond strengths of the intact molecule, whereas the chemical response to a stimulus is evaluated by the changes in bond strengths occurring when the molecule interacts with an OH radical. Unless otherwise stated, the calculations of bond dissociation energies (BDE) were done as homolytic cleavages. In a homolytic cleavage where a bond is broken and no other bonds are formed, these energies are also the barriers for the bond dissociation process. Thus, reducing the BDE the 8 ACS Paragon Plus Environment

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barrier is reduced, i.e., making the process kinetically favorable

27

. Figure 5 shows that the

intrinsic bond energies of a Li2VDC molecule range between 67 and 115.9 kcal/mol, indicating that in the absence of a stimulus the molecule is chemically very stable. However, the addition of the OH radical interacting with the end Li ion stabilizes the molecule by 21.2 kcal/mol. Note that the weakest bond is the C(carbonate)-O(vinyl). Its breaking requires 67 kcal/mol in the intact molecule (in gas phase) and such energy is reduced to 18.2 kcal/mol after the OH radical attaches to one of the Li ions, remaining the weakest bond and the one that has the largest percent of bond strength decrease (~27%).

The other cases included as Supplementary

Information (Figure S4) reveal that the same C(carbonate)-O(vinyl) is the easiest to break in the Li2DVDC molecule, and the equivalent C(carbonate)-O(ethylene) is the weakest in Li2EDC and Li2BDC. These results are in agreement with the findings from AIMD shown in Figure 2. However, a different bond: O(carbonate)-C(vinyl) and the O(carbonate)-C(ethylene) bond were found to break in the AIMD simulations shown in Figures 1, 3, and 4, leading to the separation of carbonate groups and therefore to the formation of an experimentally commonly found compound, Li2CO3.



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Figure 5. B3PW91/6-31g(d) calculated bond energies (in kcal/mol) required to break specific bonds. a) For Li2VDC before (top) and after the addition of an OH radical to its most favorable site (bottom). Bonds broken are shown by green arrows. The weakest bond is the C(carbonyl)-O (vinyl) and the attachment of OH facilitates the breaking. b) One of the Li2VDC carbonate terminal groups reorients to interact with two Li ions. That configuration facilitates the breaking of the O(carbonate)-C(vinyl) bond. Color code as in Figure 2.

The reason for the different bond-breaking mechanisms may originate from the influence of the surrounding environment which may induce alternative molecular conformations, and high temperature (450K) in which the reaction takes place in the AIMD simulation.

A new

calculation was conducted with the configuration found for the Li2VDC molecule immediately before the reaction shown in Figure 4 to verify this hypothesis. Two Li ions become close to one of terminal carbonate groups resulting in the weakening of the O(carbonate)-C(vinyl) bond to 10.41 kcal/mol thus leading to the separation of the carbonate group (Figure S5). This also explains the formation of carbonate molecules and its aggregation that may occur between blocks of different SEI components. The role of Li2CO3 as “glue” had been proposed in earlier studies28. To understand whether there is a solvent effect on the bond dissociation energy of the oligomer we included solvation effects (represented with EC properties) with the polarizable continuum model (PCM) and repeated the calculation at the B3PW91/6-31g(d) theory level for the oligomer shown in Figure 5b (Li2VDC interacting with two Li ions). The calculated BDE is 15.39 kcal/mol, which is slightly higher than the BDE with no solvation effects (10.41 kcal/mol). Since this bond breaking leaves as products two neutral molecules, solvation effects are expected to be small. Despite this increase in bond stability, the BDE remains low; therefore, we can conclude that our approach is a useful tool to determine the stability of the oligomers. We note that the gas phase reaction in Figure S5 was not set to reproduce that in the electrolyte medium observed in the AIMD trajectories, but only to evaluate the change in BDE due to the interaction with the radical species. The geometry of the reactant for the BDE calculation in Figure S5 was taken from AIMD right before the reaction where Li is forming a complex with the Li2VDC molecule. The reaction that takes place in the AIMD simulation is Li2VDC + Li Li2CO3 + CHCHOCO2Li is exothermic with a calculated energy of reaction of -92 kcal/mol. Before we leave this section it is important to point out that the specific voltage of the simulation cell has not been evaluated, although it is mainly defined by the degree of lithiation in the model anode. However, as remarked by Leung

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interactions of the anode surface with the electrolyte solution and this variation could be a cause for inducing the observed reactions. Polymerization reactions. Another crucial point relates to the effect that the nature of the electrolyte has on SEI growth. In the previous section we proved the low stability of several organic species derived both from a typical solvent (EC) and a typical additive (VC). We note that additives like VC are known to slow down side reactions and form much stable compact films21,30,31.

Therefore, the empirically found clearly different SEI performance between

electrolytes without additives (for example EC) and those containing beneficial additives (such as VC or FEC) should be attributed to the fast polymerization rate of the additives21,30,31. Once the initial decomposition products polymerize, a much lower amount will be available for the formation of unstable oligomer species that may lead to uncontrolled growth because of rapid electron propagation.

Thus, solvents/additives able to generate stable polymer-based and

inorganic (LiF, Li2CO3) films21,30,31 should be responsible for much more stable SEI layers where growth will fall within the much slower non-adiabatic electron tunneling regime23. To investigate possible polymerization mechanisms we conducted a series of new computational studies. Figures 6 to 9 depict the chain initiation mechanisms that we examined via B3PW91/6311+G(3df) analysis of structures and reaction energies. Structural details for each of the reactions are provided in Figures S7 to S13. The initial one-electron reduction reaction results in two possible open radical anions after ring openings a and b of Figure 6, which differ in the radical localization and excess charge in each structure. In gas phase, pathway b is thermodynamically more favorable than pathway a for VC and is the only one favorable pathway for EC (Figure S7). Noticeably, The reaction energy difference of a and b is only ~8 kcal/mol for VC. Figure 6 illustrates all the possible chains initially formed by reaction of the anion radical formed by reaction b with an intact EC(VC) molecule. All the reaction energies are more favorable for EC, with pathway 1b being the most exothermic (∆Ereac= -27.3kcal/mol). The result agrees with Tavassol et al.’s study, in which a C-C bond formation and a C-O bond breaking were reported.32

The pathway is also the most thermodynamically favorable one for VC.

However, the final product for VC results in 2 CO2 and a OC4H4O radical (Figure S9). Similarly one terminal CO2 group tends to separate for the VC products of reactions 2a, 2b, and 3 (Figure



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S10) due to the effect of the nearby C=C double bond. Such bond breaking resembles those discussed in relation of Li2EDC and Li2VDC decompositions.

a

+eO

14.1 -3.8

O

O O

O

O

.

a

- O b

-26.0 -11.8

O

-

a +

O

-

O O

-10.8 -11.9

3

2O

2b

O

O

-

c

O O

1b 1c

-11.1

O

O

O

-

O

O O.

O. O

-

O

CH 2.

-3.9 -73.6

3

O

O O

1

O

C.

O

O O

O

b

. -27.3 -95.6

2a

O

O O

-

5.4 -10.3 O

b

O

O-

O

O

O

O CH 2.

O

O

Figure 6. Initial polymerization mechanisms for EC (VC). B3PW91/6-311+G(3df) calculated reaction energies are reported in kcal/mol with red (blue) labels for EC (VC). There are two possible ring opening events (a and b); b is more favorable. Reaction energies for pathways 1b, 1c, 2a, 2b, and 3a are shown for reaction of radical anion EC(VC) generated via ring opening b with an intact EC(VC) molecule. Pathway 1c was not found for VC. Structures are shown for EC reactions. Additional information for VC is included as Supplementary Information.


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-34.0 -14.9

O

O

O

-CO2

O

O

-

a CH2. +

.

2O

O

O

1c

O

-

O

O CH 2.

-19.0 -84.2

3

-12.0

O

O

C.

O O

O

-

O

-

O.

O

O

CH 2.

O

O

O

O O

.

O

-

O

O.

-11.8

2a

O O

O

O -

O

c

2b -

1b

b 1

0.9 -41.0

-51.7 -105.9

3

+ O

-

O O

O

O

-

O

. Chain Termination

Figure 7. Radical anion EC(VC) generated via ring opening b (Figure 6) loses a CO2 group and reacts with an intact EC(VC) molecule. B3PW91/6-311+G(3df) calculated reaction energies are reported in kcal/mol with red (blue) labels for EC (VC). Breaking mode 1c is not found for VC. Chain termination is another possible event (see text).

Alternatively, the same open EC(VC) anion radical may lose a CO2 (structural details in Figure S11) and the resulting radical undergoes several pathways upon reaction with an intact EC(VC) as shown in Figure 7. Structural details of each pathway are provided in Figure S12. Again, much higher exothermicity is detected for the VC reactions, although some of the VC products (from 1b and 2a) lose the terminal CO2 or are completely unstable as in pathway 2b (Figure S12). Two open anion radicals may also combine with each other, leading to chain termination as shown in Figure 7. The process is also highly exothermic, with reaction energies of 51.1kcal/mol for EC and -84.7 kcal/mol for VC. Thus, termination competes with chain formation for the case of EC, but pathway 1b is more favorable than termination for VC.



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. O

-44.0 -81.6

O

O

a O-

+

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3

O

1b

2O

O

O

b

O 1

c

1c

-1.8

O

O

-7.8 3.93

O

O

-

O O

O

O C.

O O

O.

O

O

O

O

98.3 101.5

O

.

O

-

O

O

CH 2. O

2b

-

O

O

-7.0 -23.9

3

O.

O O

O

O

-

2a

-10.8 -3.2

O

-

O-

-CO +e-

O

-

O

O

CH 2.

-

(Termination)

Figure 8. Radical anion EC(VC) generated via ring opening a (Figure 6) reacts with an intact EC(VC) molecule. B3PW91/6-311+G(3df) calculated reaction energies are reported in kcal/mol with red (blue) labels for EC (VC). Route 1c was not found for VC.

The alternative EC(VC) open radical anion produced by pathway a (Figure 6) undergoes similar chain formation reactions with an intact EC(VC) molecule yielding a variety of chains as illustrated in Figures 8 and 9. It is clear that pathway 1b is again the most favorable for both species, where the VC product loses a terminal CO2 group (Figure S13), and a CO group separates from the VC product of pathway 2a, the 2nd most favorable. The radical termination reaction shown in Figure 8 possesses high positive reaction energies (Figure S14) and therefore is unlikely to proceed.


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Figure 9. Radical anion EC(VC) generated via ring opening a (Figure 6) loses a CO group and reacts with an intact EC(VC) molecule. B3PW91/6-311+G(3df) calculated reaction energies are reported in kcal/mol with red (blue) labels for EC (VC). Pathways 2a and 1c were not found for VC. The chain termination reaction is not thermodynamically favorable (75.1 kcal/mol) for the EC product and the product is not stable for the VC derived anion radicals.

The same radical anion obtained from EC(VC) via pathway a can lose a CO group (Figure S15) becoming a shorter radical anion shown in Figure 9, which may react with an intact EC(VC); however such reaction is thermodynamically favorable only for VC.

The most favorable

pathway is 1b where the VC product loses a terminal CO2 group (Figure S16). The EC product of route 2a is unstable and decomposes into small fragments (Figure S16), and similarly the EC and VC products of route 2b are unstable.



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Figure 10. Chain growth mechanisms of radical anion intermediates EC(VC) generated via ring opening b followed by pathway 1b (Figure 6) reacting with an intact EC(VC) molecule. The calculated reaction energies are reported in kcal/mol with red (blue) labels for EC (VC). Route 1c was not found for VC.

Figures 6 to 9 illustrate formation of possible initial radical anion chains which may propagate the chain reactions by attacking another intact EC/VC molecule. As the intermediates of 1b are the most energetically stable for EC and VC, we studied further chain growth reactions from this pathway, as illustrated in Figures 10 and 11. The EC/VC initiators derived from a and b ring openings are examined. Not surprisingly, the reactions at site 1 and breaking the C-O bond 16 ACS Paragon Plus Environment

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(pathway 1b) still have the most exothermic energies. They are the only thermodynamically favorable reaction for each EC/VC derived intermediate (Figure 10). However, the energetic calculations show that the further chain growth reactions are much less exothermic compared to the first step EC/VC addition. For example, for the radical intermediates generated via ring opening b, the 1st EC addition via pathway 1b has a reaction energy of -27.3 kcal/mol. The value decreases to -8.7 kcal/mol in the 2nd EC addition, and drops from -95.6 to -11.3 kcal/mol in the VC addition reaction. Moreover, VC and EC additions in this growth step generate comparable reaction energies. This is very different from the first EC/VC addition, in which the VC reactions are much more exothermic.



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Figure 11. Chain growth mechanisms of radical anion intermediates EC(VC) generated via ring opening a followed by pathway 1b (Figure 6) reacting with an intact EC(VC) molecule. The calculated reaction energies are reported in kcal/mol with red (blue) labels for EC (VC). Route 1c was not found for VC.

In summary, in the initial polymerization steps, VC shows much greater chain formation reaction energies than EC does. This strongly suggests that the presence of VC may accelerate the polymerization process. Although the attack of intact molecules could take place on many sites, 1b is the most energetically favorable site for both radical anions (obtained through ring openings a and b in Figure 6). The O-O bonding is not stable in the polymerization process. Also there is a trend for the initial chains derived from VC to lose a CO2 or a CO terminal group which is attributed to the role of the conjugated structures on the stability of the chain. In all these reactions (Figures 6 to 11) we have always considered a reaction of VC with an open VC or that of an EC reacting with an open EC. To complete our analysis, we investigated the crossproducts (open VC reacting with intact EC and open EC reacting with intact VC). The results are summarized in Table 1. Table 1. Calculated reaction energies for reactions of open VC with intact EC or open EC reacting with intact VC Reaction

∆Ereaction (kcal/mol)


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O

O

O

C. O

O

-

++

O

O

b1

O

O

1

O.

-40.4

-

O

O

O

O

O

C. O

O

-

+

O

O

O

-

O

O.

O

O O

O

O

+

-

-81.6

O.

O

-

O O

O

-58.3

O

O

CH.

O

O

O

O

-

+

O

O

O

-

-95.6 O.

CH.

O

O

. - +

O

O

O O

O

O

O.

-74.1

-

O

O

O

O

O

O

O

. O-

O

+

O

O

O

-

O

O

O.

O

O O

- +

O

.

O. O

O

-44.0

-

-49.8

O O

O

O

O

O

O O

O

.

O

-

+

O

O

O

-

O O

O.

-27.3

O

From Table 1 we conclude that the reactions where VC radicals generated via ring openings a or b attack intact EC molecules are much less exothermic than those where open VC attacks intact VC molecules. Therefore, VC radicals will tend to react with VC molecules first. For EC radical initiated reactions, the radicals (generated via either ring openings a or b) also prefer to bind with



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a VC molecule for the first step, in agreement with previous findings 33. This shows that even if EC radicals are formed, VC is also heavily involved into the chain formation. In summary, our analysis suggests that polymerization reactions initiated by open VC or open EC radical anions reacting with intact VC molecules are thermodynamically more favorable than those reacting with intact EC molecules, which suggests the greater polymerization tendency of VC-derived species. The fact that intact EC molecules would be involved in these reactions indicates that less EC would be further reduced thus producing much less ethylene gas, in agreement with Lucht et al’s34 comparative analysis between electrolytes containing additives (such as VC) and EC-solutions. They concluded that the formation of poly(VC) in VC-based electrolytes inhibits EC reduction which is detected by the reduced formation of ethylene and Li2EDC and suggested two possible polymerization pathways for poly(VC) formation. We also tested this hypothesis (Figure S17) and found that the only thermodynamically favorable pathway starts from open VC radical anion that reacts with an intact VC (Figure S18). The other alternative, which starts form the radical stabilized after losing CO2 forms a different intermediate and the further steps are not favorable (Figure S18). Thus, in spite of the low stability of the R-O-(CO)-O- groups, formation of poly(VC) may be an alternative to formation of linear chains.

In addition, cross-polymerization and secondary reactions may occur. For

example, Shkrob et al.35 suggested that secondary radicals such as those occurring from reactions of ring opening radical products with neutral species

would be the main players in

polymerization reactions derived from EC. According to their study based on photolytically and radiolytically induced radical reactions, secondary radicals would result in branched polymers connected by carbonate groups.36 Moreover, such reactions would result from radical instead of anionic polymerization that would be conducive only to linear chains. The high population of radicals and radical anions resulting from the numerous decomposition reactions not only of the solvents and salts but also of the organic products such as short-chain oligomers shown here supports the existence of alternative pathways conducive to branched polymers. We note that this analysis of possible polymerization pathways is based exclusively on thermodynamics. However, many of these reactions might be defined by kinetic considerations and these require another computational effort that we plan to address in future work. On the other hand, besides poly(VC), our analysis suggests that short-chain oligomers based on linear chains would also be thermodynamically favorable. But due to their chemical nature, these oligomers should be also 20 ACS Paragon Plus Environment

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chemically unstable and would be easily decomposed by a radical attack as we showed for Li2EDC, Li2VDC, Li2BDC and Li2DVDC in the first part of this study. However, we have shown that the radical anion VC•-, initial product from VC decomposition (previous to the formation of Li2VDC) is easier to polymerize than EC•-, the anion radical product from EC. Therefore a polymer film is rapidly formed after VC decomposition and only small amounts of Li2VDC would remain intact. On the other hand, the polymerization rate of EC products is comparatively slower than that of the VC products, and EC•- can be dimerized forming Li2EDC which is more prone to aggregate with other units forming a Li2EDC amorphous phase. Thus Li2EDC agglomerates and constitutes its own block in EC-derived SEI layers whereas the organic products from VC are mainly polymers and oligomers. These ideas are schematically displayed in Figure 12 that illustrates the main features of growth in electrolytes containing additives that lead to controlled growth vs. those that result in unstable external layers that are easily attacked by radical species promoting sustained growth.

Figure 12. Principles that rule SEI growth based on the analyses discussed in this study. Successful additives such as VC or FEC lead to compact SEI layers (usually containing stable polymer species) and to relatively stable outer organic SEI layer, thus to controlled growth. In contrast, electrolytes that do not



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contain these additives lead to an outer layer containing unstable species prone to radical attack and continued growth.

Conclusions Electron transfer from the anode via tunneling would not be possible after the SEI layer grows beyond ~10 Å37. However, a large number of radicals are produced in the initial stages of SEI nucleation. Some of them get adsorbed on the anode surface; others remain in the liquid phase and are able to propagate the reaction before the film is formed. Once some of the blocks start to nucleate on the anode, these radicals which have extremely high reactivity can readily transfer charge to their surrounding environment. However, if the film is too dense, only small radicals such as Li atoms would be allowed. Thus, electrons will travel through blocks transported by Li atoms as reaction-inducing agents and further radical formation at the local level. Radical propagation would proceed especially through the organic layers, and once they reach a molecule that can be attacked and decomposed (a solvent, a salt, or an unstable product such as Li2EDC or Li2VDC) new radicals will be generated, and the reaction continues. Note that the radical-based mechanism of electron transfer does not need high porosity or an electronically conductive medium to propagate the reaction since electrons will travel via small radicals38. The potential of Li atoms as charge transfer agents had been suggested by Harris et al

39

. Due to their much

smaller size, Li atoms can migrate through outer SEI regions thus becoming the reducing agent of the electrolyte and of the unstable SEI components. However since this electron transfer mechanism would occur only at the interface between the outer SEI layer and the electrolyte, it will be dominant when the outer organic layer is electrochemically unstable as shown for several oligomers in this study.

The great advantage of VC and FEC as additives is their fast

polymerization. Polymers are much less prone to be reduced by a radical attack. Here we examine various initiation and propagation polymerization reactions.

We conclude that

polymerization reactions initiated by open VC or open EC radical anions reacting with intact VC molecules are thermodynamically more favorable than those reacting with intact EC molecules, which suggests the greater polymerization tendency of VC-derived species. The fact that intact EC molecules would be polymerized indicates also that these molecules would not be further reduced thus producing much less ethylene gas in agreement with experimental evidence. We also found a possible pathway for poly(VC) formation to be thermodynamically favorable. Thus, we postulate that VC-containing solutions produce more polymeric species and less short 22 ACS Paragon Plus Environment

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oligomer products, in contrast with pure EC solutions that would produce a thick organic layer of unstable oligomer products. Therefore, if the outer organic layer contains less of the electrochemically unstable species (as in a VC or FEC-based electrolyte), the SEI rate growth will significantly decrease (as observed experimentally) compared with those based on VC (or FEC)-free electrolytes. Summarizing, any species with a high reduction potential that upon reduction generates products that are electrochemically stable will be a favorable additive for the SEI. Thus, we emphasize that the stability of the electrolyte decomposition products is the determinant factor of the SEI growth. The concept that we introduce in this paper, that the stability of the products is more crucial than that the stability of the electrolyte components is totally new and can be applied to any electrode surface. The important element that the surface provides is its reactivity, which will define a set of appropriate electrolyte components. Such set however has to be carefully investigated to determine the stability of their decomposition products. This concept should be one of the key elements for a rational design of electrolytes for long-lived rechargeable batteries. Computational methods AIMD simulations were carried out using the NVT ensemble at 450 K and a time step of 1 femtosecond using the Vienna ab initio simulation package VASP40-44, with the Perdew-BurkeErnzerhof functional (GGA-PBE)45 and the projector augmented wave (PAW) pseudopotentials provided in the VASP databases describing electron-ion interactions46,47. Tritium masses are substituted for protons to permit this time step. The Nose thermostat was used to control the temperature oscillations during the simulation with a Nose-mass parameter of 0.5, which gives a frequency of oscillation corresponding to 176-time steps. Simulations carried out at 310 K yielded similar results as reported in this work. A Γ-point Brillouin zone sampling was applied in this case with a plane-wave energy cutoff of 400 eV. Convergence criteria for optimizations are set to 10-3 and 10-4 eV, for ionic relaxation loop and electronic self-consistent iteration, respectively. A Gaussian smearing with a width of 0.05 eV was employed and a Γ-point Monkhorst–Pack48 mesh sampling was used on the surface Brillouin zone. Bader charge analyses were used to perform charge calculations21,49. Within this method, the total electronic charge of an atom is approximated by the charge enclosed within the Bader volume defined by zero flux surfaces. The introduction of a radical species such as C2H3 into the simulation cell was done by



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adding the C2H3 molecule near the interface between a solid layer (such as Li2O) and the electrolyte. This radical species has an unpaired electron located on one of the C atoms. To induce the formation of the radical anion, we added a Li atom. The interaction between these two species determines an electron transfer from the Li (becoming a cation) to the C2H3 species that becomes a radical anion. The Bader charges were monitored throughout the simulation, and the charge evolutions shown in Figures 2 and 3 followed the interval before and after the Li2EDC decomposition and includes not only the reacting molecule but also its neighbors.

The

simulations were repeated from different initial configurations and the events are reproducible, although the reactions may occur at different times. Further details on the preparation of the simulation cell are included as Supporting Information. DFT calculations of bond strengths and molecular stability and polymerization reaction energies were done using the hybrid B3PW91 method in the framework of the generalized gradient approximation (GGA), as implemented in the Gaussian 09 suite of programs.

The hybrid

B3PW91 functional is composed of the Becke’s three parameter gradient-corrected exchange functional50 and the Perdew-Wang 91 correlation functional51. The hybrid functional, B3PW91, has shown its reliability for the electronic structure calculations in lithium-ion batteries

11

. For

the polymerization reaction energies, both the geometry optimization and the single-point calculations were carried out using a 6-311+G(3df) basis set. Vibrational frequencies were computed for verifying local minima and transition states, and yielding zero-point energy (ZPE) and thermal corrections, at the same level of theory. Supporting Information Available Snapshots and evolution of the interfacial structure of the oligomer/inorganic/lithiated Si anode systems (Figures S1-S3). Bond decomposition reactions of oligomer species (Figures S4 and S5). Partial density of states of reacting Li2EDC molecule shown in Figure 2 illustrating changes in the electron population before and after reaction (Figure S6). Energetics and structures of polymerization reactions (Figures S7 to S18). This information is available free of charge via the Internet at http://pubs.acs.org. Acknowledgements


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This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231, Subcontract No. 7060634 under the Advanced Batteries Materials Research (BMR) Program. Computational resources from Texas A&M Supercomputing Center, Brazos Supercomputing Cluster at Texas A&M University and from Texas Advanced Computing Center at UT Austin are gratefully acknowledged.

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Table of Contents Graphic


Formation and Growth Mechanisms of Solid-Electrolyte Interphase

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