Reduction of Electrolyte Components on a Coated Si Anode of Lithium

Jul 7, 2017 - Current efforts to improve energy storage technologies seek to develop materials for rechargeable batteries with higher energy density, ...
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Reduction of Electrolyte Components on a Coated Si Anode of Lithium-Ion Batteries Jose L. Gomez-Ballesteros and Perla B. Balbuena* Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843, United States S Supporting Information *

ABSTRACT: Surface modification of Si anodes in Li-ion batteries by deposition of a thin alucone coating has demonstrated an effective way to help maintain a stable anode/ electrolyte interface and good battery performance. In this work, we investigate the interactions and reactivity of the film with electrolyte components using ab initio molecular dynamics simulations. Adsorption of solvent molecules (ethylene carbonate, EC) and salt (LiPF6) and reduction by two mechanisms depending on the Li content of the film (yielding open EC adsorbed on the film or C2H4 + CO32−) take place near the film/electrolyte and film/anode interfaces. Reaction products incorporate into the structure of the film and create a new kind of solid−electrolyte interphase layer.

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lithiation.21 Further elucidation of the reactivity and interactions between the film and the electrolyte can provide useful information about the film stability and performance as a protective layer of the Si anode.9 In this work, we performed investigations on the reactivity of an alucone thin film deposited on a lithiated silicon surface, based on the model previously developed in our group.21 Using DFT and ab initio molecular dynamics (AIMD) simulations, we seek to elucidate the reduction mechanisms of electrolyte components and additional reactions occurring in the film and at the interfaces with the anode and the electrolyte. Our model consists of a 1.5 nm × 1.5 nm × 3 nm simulation box (Figure 1a), which includes three distinct equally sized regions representing (a) the lithiated silicon anode surface with a LiSi2 composition corresponding to a low lithium content and a voltage > 0.39 V vs Li+/Li metal,22 (b) the structure of the alucone film deposited on the anode surface, and (c) a model electrolyte solution. Two typical components of the electrolyte were considered in our simulations: the liquid solvent, represented by EC (ethylene carbonate; Figure 1b) and the salt represented by LiPF6 (lithium hexafluorophosphate) in the most stable C3v symmetry found in previous DFT studies.23 Further details of the simulation components are given in the Supporting Information (page S-1). The first set of simulations includes only the liquid solvent representing the limit when local salt composition near the electrolyte−film interface is very low; second, the salt is added to represent the case of high concentration of salt near the interface. In addition, variations in the Li loading of the film were also considered. Our previous work demonstrated that saturation with Li occurs approximately at a 1:1 ratio of Li/Al atoms in the film.21 In the current

urrent efforts to improve energy storage technologies seek to develop materials for rechargeable batteries with higher energy density, lower costs, and improved stability and safety.1−4 Si anodes in Li ion batteries can exhibit high capacity (Li22Si5, 4200 mAh g−1) and store approximately 10 times more Li than commercial graphite anodes (372 mAh g−1).5,6 However, large volumetric expansion of Si upon lithiation/ delithiation and irreversible capacity loss due to electrolyte reduction on the surface are challenges that must be addressed to achieve a practical electrode.4,7−9 The solid−electrolyte interphase (SEI) is a layer formed by reduction of the organic solvent and electrolyte components at the anode surface. Although the SEI may passivate the surface, enhancing cycling stability, structural and chemical changes in the anode make it challenging to maintain a stable SEI and avoid capacity loss.4,10,11 Surface modification by atomic and molecular layer deposition (ALD and MLD) has been explored as a solution to this challenge and demonstrated flexibility to accommodate volumetric changes and effectiveness at improving stability while maintaining electric and ionic conductivity.9,12−17 Silicon anodes coated with alucone (aluminum alkoxide) thin films have shown improved cycling stability and high Coulombic efficiency above 99%.18 Characterization of the film using TEM has revealed that the coating exhibits high flexibility, maintaining its structural integrity and electrical conductivity after expansion/contraction of the Si anode while removing the native oxide layer and its negative effects on performance almost completely.19 The alucone coating process of Si is typically done by MLD. Using sequential, self-limited reactions involving trimethylaluminum (TMA) and glycerol, a thin 3D network is created on the Si surface.18,20 Our previous investigations on the film structure using density functional theory (DFT) and Green’s function theory combined with electrochemical testing have revealed details on the process of film formation, structural features, electronic conductivity, and © 2017 American Chemical Society

Received: May 12, 2017 Accepted: July 7, 2017 Published: July 7, 2017 3404

DOI: 10.1021/acs.jpclett.7b01183 J. Phys. Chem. Lett. 2017, 8, 3404−3408

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The Journal of Physical Chemistry Letters

Figure 2. Reduction mechanisms of EC. (a) EC molecule representing the electrolyte solvent. (b) Scission of a C−O single bond to yield oEC−, subsequently adhering to Al of the film. (c) Scission of C−O single bonds on both sides of the EC molecule yields C2H4 and CO32−.

second electron from the Al where o-EC− is adsorbed (a-EC−), as indicated by changes in the bond distances and electronic charges of the atoms involved (Figure S5). The second mechanism involves three steps: (a) breaking of both of the C− O single bonds between the carbonate and ethylene segments of EC (Figure 2c), as a result of the reduction process that conveys two electrons to the carbonate segment (CO32−), (b) adsorption of the carbonate anion to Al in the film, and (c) the resulting ethylene molecule in solution either migrating toward the bulk of the electrolyte (Figure S2b) or remaining near the film−electrolyte interface (Figure S3b). Scission of the C−O bonds in the second mechanism may occur to the EC molecule in solution or in the adsorbed state; therefore, either of the first two steps (bond breaking or adsorption) may occur first; an EC molecule in solution located near the film−electrolyte interface, which may coordinate with a number of Li ions that varies from 1−3 as a result of the overall random molecular motion of EC and the Li ions, can receive two electrons and produce C2H4 and CO32− in solution according to the second mechanism. In this case, breaking of the two C−O single bonds was observed to occur sequentially in our simulations and followed by adsorption of CO32− on Al (Figure S2c). Conversely, an EC molecule adsorbed on the Al of the film may receive two electrons that cause breaking of C−O bonds to produce C2H4 and CO32− adsorbed on Al. In this case, rupture of the two C− O single bonds was observed to occur simultaneously and briefly after the EC molecule was adsorbed on Al (Figure S3c). Overall, both of the reduction pathways observed in our simulations agree with mechanisms previously reported;22,24,27 however, they bring additional information about how reaction products interact with the alucone film. Reduction of EC molecules according to the first reaction mechanism was predominantly observed for the system with an initial Li loading of the film slightly above saturation, whereas the above-discussed second reaction mechanism is more often observed in the system with an initial Li loading of the film at the saturation point. Interestingly, when we monitored the trajectory of Li ions in the film (Figure 3), we observed that Li ions in the film with a Li loading above saturation tend to have a greater degree of mobility and may even escape to the electrolyte region (Figure 3b), in contrast with Li loading at the saturation point, where Li mobility is more restricted (Figure 3a). In the supersaturated case studied, we observed migration of two Li ions to the electrolyte region from the beginning of the simulation time, leaving the film with a number of Li ions slightly below the saturation level. Therefore, the Li content in

Figure 1. (a) Model utilized to describe the anode−film−electrolyte system divided into three regions: LiSi2 surface at the bottom of the simulation box, alucone film in the middle, and liquid (b) ethylene carbonate (EC) representing the electrolyte at the top. Adsorption of EC occurs via attachment of the carbonyl C to (c) Al atoms in the film and (d) Si atoms in the anode surface.

study, we consider the saturation composition (1:1) and a slightly higher ratio (1.1:1). Interactions of the EC molecules with the film are identified and followed throughout the simulation time. The most common interaction occurring in the film and at the interfaces is adsorption of an EC molecule via bonding of the carbonyl O to an AlOx group in the film. The process of adsorption was observed to occur at different locations of the film: near the film−electrolyte interface, inside of the film, and near the film− silicon interface (Figure 1c,d). In the latter case, adsorption may occur on Al of the film (Figure 1c) or Si atoms of the anode (Figure 1d). Both of these interactions may involve a process of charge transfer in which the anode surface cedes approximately two electrons to the EC molecule (Figure S1), which is similar to the two-electron mechanisms reported for adsorption of EC on graphitic anodes, Li metal surfaces, bare lithiated Si anodes, and Al2O3-coated Si.22,24−28 A minimum interatomic distance of 4 Å was observed between Al or Si and the carbonyl O (Figures S2c, S3c, and S4) and similarly between Al or Si and the carbonyl C (Figures S4 and S5), in agreement with the previously reported critical distance for adsorption of EC on lithiated Si surfaces in the absence of a film.22 The charge transfer process takes place on EC molecules that are either in an adsorbed state or in solution coordinated with Li ions, and in both cases, it involves weakening and breaking of O−C single bonds as a step of the EC reduction process, in agreement with charge transfer mechanisms previously reported.22,24,27 Two reduction mechanisms were identified with distinctive reaction products obtained as a result (Figure 2). The first mechanism involves two steps: (a) breaking of one of the two C−O single bonds between the carbonate and ethylene groups of EC to produce an open EC radical anion (o-EC−) and (b) adhesion of the C atom from the open end of o-EC− to Al in the film (Figure 2b). These two steps (bond breaking and adsorption) seem to occur almost simultaneously and involve charge transfer of one electron from the anode surface and a 3405

DOI: 10.1021/acs.jpclett.7b01183 J. Phys. Chem. Lett. 2017, 8, 3404−3408

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Scheme 1. Summary of Reduction Mechanisms of EC Observed in the Alucone-Coated Surface

Figure 3. Trajectories in the z-direction (perpendicular to the anode surface) of Li ions intercalated in the film. The green dashed line represents the film region. (a) Li ion trajectory for loading of Li in the 1:1 Li/Al film, representing Li saturation of the film. (b) Li ion trajectory for Li loading in the 1.1:1 Li/Al film, representing Li loading slightly above saturation. Li ions in the supersaturated film may migrate to the electrolyte region.

Figure 4. Mechanisms of salt decomposition. (a) The original structure of LiPF6 decomposes into various decomposition products: (b) F− ions bonding to o-EC−, (c) F− ions bonding to AlOx groups, and (d) F− ions coordinating with one or two Li+ ions diffusing in the electrolyte phase. P5+ ions may be (e) completely stripped from all F atoms and bond to Si and Al or (f) maintain coordination with 3 F−.

the film seems to have an effect on the reactivity of EC in the alucone film, with reduction of EC producing C2H4 and CO32− near the supersaturated film, versus o-EC− adsorbed on the film as the predominant reaction product for Li content of the film below saturation. One-electron mechanisms of reduction are presumably attributed to slow electron tunneling from the electrode to the electrolyte, in contrast to faster electron transfer that is associated with two-electron mechanisms.29,30 Therefore, the behavior observed in films with different Li loading may indicate different rates of electron transfer depending on Li loading of the film, as discussed in our previous work. 21 Further exploration of the reduction mechanisms of EC in contact with an alucone film containing different loadings of Li ions is currently underway (see Scheme 1). Preliminary results of the incorporation of the salt component (LiPF6, Figure 4a) into our model electrolyte indicate different decomposition mechanisms of the salt depending on its initial location with respect to the film. For a salt unit located in the close vicinity of the film, some F− ions may bond to the open end of adsorbed EC molecules (Figure 4b), while others may bond directly to Al in the film (Figure 4c). In contrast, F− from salt initially located in the electrolyte

region shows preferential coordination with Li ions (Figure 4d). P5+ on the other hand gets easily reduced to P3+ and either coordinates with the Si surface when the salt decomposes in the film (Figure 4e) or remains in solution as PF3 (Figure 4f). In addition, fewer EC molecules were reduced in the presence of the salt. In summary, the reduction mechanisms of EC and LiPF6 in the presence of an alucone film deposited on a lithiated Si anode were characterized using AIMD simulations. We find that there are two main mechanisms by which EC molecules can be reduced and interact with the film near its interface with the electrolyte and with the Si surface. These mechanisms involve electron transfer from the film or anode surface to EC to yield adsorbed open EC (1 e− mechanism) or C2H4 and CO32− (2 e− mechanism). Reaction products from both mechanisms may adsorb on atoms of the film and modify its structure. Preliminary inspection of the effect of Li-ion loading of the film and the presence of salt near the film−electrolyte interface seems to indicate favoring or inhibition of either reduction mechanism. Additional investigations of these effects 3406

DOI: 10.1021/acs.jpclett.7b01183 J. Phys. Chem. Lett. 2017, 8, 3404−3408

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are needed to elucidate the structural changes in the alucone film, and its transformation as a new SEI layer can be formed due to the interactions with electrolyte components and their reduction products, which will provide understanding of the behavior of the protective film and fundamental information to help engineer these materials.

COMPUTATIONAL DETAILS DFT optimizations structure and subsequent AIMD simulations were performed using the Vienna ab initio simulation package (VASP 5.4).31−35 The exchange−correlation functional proposed by Perdew, Burke, and Ernzerhof (GGA-PBE)36 was utilized with plane waves extended to a cutoff energy of 400 eV for valence electrons and the projector augmented wave (PAW) pseudopotentials for ion cores.37,38 Sampling of the Brillouin zone was done with a Γ-point mesh. Electron partial occupancies were described using a Gaussian smearing with 0.05 eV. The energy convergence criteria were set to 10−3 eV for ionic relaxation and 10−4 eV for electronic self-consistent iterations. Classical molecular dynamics simulations, as implemented in the Forcite module of Accelrys Materials Studio V8.0,39 were run to equilibrate the final system using the consistent valence force field (CVFF) prior to running AIMD simulations. AIMD simulations were carried out on the equilibrated systems using the NVT ensemble with the Nose thermostat to maintain the temperature at 400 K and a time step of 1 fs. The simulations were run for approximately 25 ps for the systems without LiPF6 and for 12 ps for those containing both EC and LiPF6. Charge calculations were performed using the Bader analysis scheme.40,41 Further details about the simulation components are provided in the Supporting Information. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b01183. Simulation cell details; charge transfer during adsorption (Figure S1); EC reduction mechanism in solution (Figure S2); EC reduction mechanism at the surface (Figure S3); and tracking interatomic distances during reaction (Figures S4 and S5) (PDF)

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Letter

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Perla B. Balbuena: 0000-0002-2358-3910 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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-EE0007766, of the Advanced Batteries Materials Research (BMR) Program. We acknowledge high-performance computational resources at Texas A&M University (College Station) and the Texas Advanced Computing Center (TACC). 3407

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