Effects of Dimethyl Disulfide Cosolvent on Li-S Battery Chemistry and

Subscriber access provided by ECU Libraries

Article

Effects of Dimethyl Disulfide Cosolvent on Li-S Battery Chemistry and Performance Ethan P Kamphaus, and Perla B. Balbuena Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b04821 • Publication Date (Web): 07 Mar 2019 Downloaded from http://pubs.acs.org on March 8, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chemistry of Materials

Effects of Dimethyl Disulfide Cosolvent on Li-S Battery Chemistry and Performance Ethan P. Kamphaus and Perla B. Balbuena* Department of Chemical Engineering, Texas A&M University, College Station, TX 77843 *e-mail: [email protected]

Abstract Lithium sulfur battery’s (LISB) performance is still held back by undesirable side reactions. One of these side reactions is known as the polysulfide shuttle effect which is a consequence of soluble sulfur reduction products, polysulfides. Polysulfide migration induces other side reactions at the anode surface. In addition, slow kinetics of the main discharge products induces large overpotentials during charge. Researchers have been searching for ways to counteract these effects through many different strategies. One such strategy that could mitigate part of these problems is to use novel electrolytes that contain sulfur that can modify battery chemistry while providing additional energy capacity. Sulfur containing electrolytes like dimethyl disulfide (DMDS) have shown increased performance but the mechanisms are not known. First principles computational chemistry simulations are used to determine different aspects of battery performance to ascertain why this additive helps. We found that DMDS modifies the reduction pathway that leads to a different final product, LiSCH3, than a traditional LISB and this product is more soluble in the electrolyte, which could facilitate the charge reaction. DMDS favorably reacts with S8 to form dimethyl polysulfides (DMPS) in agreement with experimental results. The new final reduction products are electronically insulating but they are more soluble and therefore more reversible than the traditional end product, Li2S, which would increase the battery performance. The results indicate that DMDS will change the sulfur battery chemistry for the better in some ways and similar in others which provide fundamental insight into the experimental evidence. In addition, we provide a schematic for screening novel electrolytes for the lithium sulfur battery.

Introduction Energy storage technology is as relevant and important today as it was at the time of its inception. The technology revolution giving us portable electronics, electric vehicles and more was enabled by the development of high performance rechargeable batteries like the lithium ion battery (LIB). Though this revolution was a big step change for modern society, there is still as strong demand to take battery development further to enable even greater advances. Technologies like long range electric vehicles, longer lasting personal electronics, and even large scale energy storage for green energy applications all require better batteries. 1-4 1 ACS Paragon Plus Environment

Chemistry of Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 27

Currently the leading practical battery technology that sees common use is the LIB 5-6 . Available commercial LiB have a gravimetric energy density around 250 w h kg-1 or 650 W h L-1 7. However to reach the applications that we identified previously, significant advances to the performance of batteries must be made 3. Researchers are exploring several different avenues for increasing battery performance from improving the LIB by using a pure lithium metal anode to switching battery chemistries completely 8-9. There is a limit on the amount of energy storage that a battery can provide which is based on the thermodynamics of the chemistry involved. Since the scientific community is researching the LIB, the battery technology is approaching its maximum theoretical limit which means that sometime in the future, to push batteries farther, different chemistries will have to be explored 10-11. Next generation batteries that utilize other chemistries have been focused on in recent years. Examples include multivalent cation, lithium-air and lithium sulfur batteries (LiSB). Each of these new chemistries requires almost a complete redesign of the battery but they have theoretical limits higher than the LIB 10 . Unfortunately, new chemistries also introduce new and challenging problems as well. Based on a combination of factors including theoretical performance and perceived feasibility of implementation, the LiSB is considered to be potentially the closest technology to the marketplace 12. The LiSB has a theoretical energy density of 2600 W h kg-1 which is a significant improvement over traditional LiB systems 13. The theoretical energy density is usually never obtained so the rule of thumb to estimate overall battery performance is to reduce the theoretical value by a factor of 310. Before the LiSB can transition from the lab or supercomputer into our pockets some of its specific problems need to be solved. The LiSB technology faces three large problems: the use of a lithium metal anode, the polysulfide shuttle effect, and the need of overcoming a significant overpotential during charge 14 . The lithium metal anode is a problem that troubles other battery chemistries like the LiB and the lithium air battery as well. Lithium is a highly reactive metal which creates a solid electrolyte interphase (SEI) layer that is difficult to engineer and manipulate to increase battery performance 11. The polysulfide shuttle effect and the difficulties with the charge reaction are unique to the LiSB chemistry and present very difficult problems to solve 15. These effects arise from the solubility of products generated by discharge reactions. During discharge, the anode is the lithium metal and the cathode is a sulfur/carbon composite. The lithium atoms are oxidized and released into solution while the associated electrons travel to the cathode where they reduce the sulfur that then combines with Li ions coming from the anode. The cathode starts off as elemental sulfur with a S8 crown structure that is reduced into smaller and smaller fragments until the ultimate final product Li2S. These fragments represent reaction intermediates called lithium polysulfides, LixSy. The longer chained lithium polysulfides (y >=6 ) are soluble in the electrolyte so during the discharge of the battery, active cathode material will leave the electrode and diffuse through the electrolyte. Once in the electrolyte, the lithium polysulfides will eventually be transported to the lithium metal anode where the reduction will finish thus generating Li2S. This compound is highly non soluble and has poor oxidation kinetics. In other words, it becomes very difficult to remove the Li2S from the lithium metal anode once a lithium polysulfide reaches there. Moreover, Li2S is also formed on the cathode due to reduction of the PS species that don’t shuttle away thus insulating that electrode as well. This leads to loss of active cathode material and passivation of both electrodes which causes poor 2 ACS Paragon Plus Environment

Page 3 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


cyclability and thus prevents the LiSB from being used in general applications 16-18. In the LiSB scientific community, there are various possible strategies that are being investigated to combat this parasitic effect. Techniques like anchoring the polysulfide to the cathode 19-21, using a solid state electrolyte to prevent polysulfide dissolution 21-22, using novel separators and modifying the electrolyte composition all have been explored experimentally, computationally and theoretically 23-25. One particular method is promising because it is easy to implement and may mitigate at least some of these effects at a fundamental level: modifying the electrolyte 26-28. The electrolyte serves a very important purpose to the operation of any battery technology. Quite simply its role in the battery is to allow ions to dissolve which makes the oxidation of a metal more favorable since a reduced ion has a more favorable energy in solution instead of in the metallic position 29. Electrolytes are chosen for a variety of reasons the main ones being the ionic conductivity and the compatibility of the electrolyte with the electrodes. The ionic conductivity is directly related to the ability of the electrolyte to transport metal ions critical to cell reactions and compatibility with the electrodes is critical to having stable and controlled reactions 29-31. Researchers have explored the use of “non-solvents” like fluorine containing compounds which aim to have a good enough ionic conductivity while greatly restricting the dissolution of polysulfides 30, 32. Others have moved to the idea of using a solid state electrolyte which would solve these problems as well 33-34. One recent idea that shows promise is the use of a sulfur containing electrolyte for use within the LiSB 26, 35-37. Sulfur containing electrolytes are promising because they can attempt to tune the ionic conductivity and the favorability of the dissolution of intermediate lithium polysulfides as well as they add extra sulfur to the battery. The addition of extra sulfur which can potentially be reduced increases the energy capacity of the battery 30. Essentially by adding sulfur, one can make the electrolyte an energetically active battery component. This strategy is to just simply change the battery chemistry to see if that will change performance in an advantageous way. A couple of different sulfur containing molecules have been tested for performance characteristics but there is no clear consensus about which molecules are better or even what all of the effects of the sulfur containing additives are. There are some drawbacks from using a system with a electrochemically active electrolyte. Depending on the ratio of sulfur to electrolyte, the electrolyte could be depleted thus adding to the problems the Li-S system faces. The reduction of the electrolyte components could also cause self-discharge with the Li metal anode. One such sulfur containing additive is dimethyl disulfide (DMDS) and its close family member dimethyl trisulfide (DMTS). Wang’s group has brought this additive to attention and proposed its use in LiSB systems for increased performance 26, 35, 37. DMDS is a well-studied chemical that has found use in other applications unrelated to electrochemical systems. It was researched previously for applications in desulfurization processes that occur in the refining of crude oil because of its nature to react with elemental sulfur and hydrogen sulfide as well as providing some assistance in catalysis 38-39. In a refinery, this is desired so that sulfur can be removed in one of the many steps involved in refining oil. This implies that DMDS would be compatible in a LiSB system and would make chemical changes which could be advantageous. With a combination of different experimental techniques, Wang’s group investigated how the battery chemistry changes and what improvements could be observed. They found that the addition of 3 ACS Paragon Plus Environment


Page 4 of 27

50% by volume DMDS to the electrolyte changes the traditional reduction pathway that lithium polysulfides undergo during discharge. The modified reduction pathway was proposed to give more reversible kinetics, reduce the viscosity of the intermediates and provide additional sulfur loading to the battery since the DMDS is electrochemically active 26, 35. Their findings suggest that the addition of these sulfur containing electrolytes can definitively improve the battery by increasing cycle life and increasing specific capacity. Just like when switching from LIB to LiSB, changing the electrolyte significantly will cause large changes in the battery chemistry and performance. A change to the reduction pathway of the lithium polysulfides can create drastic changes in the operation and design; some of which can provide advantages while others might start new problems. Due to the complexity of electrochemical systems, it can be very difficult for experimental researchers to deconvolute the chemical processes that occur. One way to break down these complicated systems to observe the impact of changing battery chemistry is to explore individual parts of the system and investigate them individually. While this task can be extraordinarily difficult for experimentalists, the power of first principles computational chemistry can help the scientific community explore each part of the battery individually to determine the reasons why this additive shows increased performance 40-41. However, one of the drawbacks of these techniques is the size and scale of what can be simulated. Every interconnected process of the battery chemistry cannot be simulated simultaneously and some assumptions must be made. We have used accurate and sound theoretical methods to explore different aspects of the LiSB system if the traditional electrolyte was modified to include DMDS. Using experimental investigations as a guide, we have explored several different aspects of how the cosolvent could change the battery chemistry and thus performance as well as provide further validation to the experimental results. First principles and ab-initio simulations were used to investigate important behaviors of the LiSB such as the new reduction pathway thermodynamics, solvation and dissolution behavior, and the electronic properties of the new final reduction products. We found that a new reduction pathway is favored by the DMDS which shows a a final reduction product with more favorable dissolution behavior although still is insulating like Li2S. These results agree with Wang’s experimental findings and they add additional insights into the new chemistry. Given the preliminary nature of this work, we did not incorporate the interactions between Li metal and the DMDS and subsequent species though they will be critical to the use of this additive. Future work will examine this and more aspects of the system to make the models increasingly realistic.

Methodology Two different types of computational modeling techniques were used for this work. Vienna abinitio simulation package (VASP) was used for periodic boundary condition (PBC) models where a simulation box is repeated infinitely in the three spatial directions9, 42-43. VASP was used for solid-state and Ab-initio molecular dynamics (AIMD) simulations of the electrolyte. We also utilized Gaussian 09 for non-PBC simulations but with more complex level of theory44. Gaussian was used to calculate the electron affinities and change of free energy of reactions, Δ𝐺𝑟𝑥𝑛𝑠. 4 ACS Paragon Plus Environment

Page 5 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


For all the VASP calculations, the projector augmented wave (PAW) basis set was used with pseudopotentials45-46. The K-point mesh for the basis sets depended on the particular model to ensure accurate results. The Monkhorst-Pack K-point mesh and cutoff energy for each model can be found in Table S147. Density functional theory (DFT) was used to carry out all optimizations and energy calculations for the PBC simulations. We utilized the Perdew–BurkeErnzerhof generalized gradient approximation (PBE-GGA) functional to describe electron structure and behavior48. In all the simulations, the systems were treated as spin polarized so the up and down spin electrons were solved separately. For the solid state calculations, the structures were optimized with DFT+U to ensure accurate structures with the U values taken from literature49. The optimized solid state structures were used to calculate the numerical Hessian matrix of the energy to determine vibrational modes to temperature correct the final energy of the structure. In the AIMD simulations a canonical (NVT) ensemble was used to explore the time evolution of the system. The temperature in all simulations was set at 300 K to represent room temperature. A Car –Parrinello MD algorithm was implemented with a Nose thermostat set to a value of 0.5 to control for temperature variations. The DFT-D3 method with Becke-Jonson damping was also utilized as an empirical dispersion correction technique to capture Van-der-Waals interactions that ab-initio methods miss50. The non-pbc calculations carried out with Gaussian 09 all used the same general methodology, DFT geometry optimizations and frequency calculations done in an implicit solvent field. Based on previous work done in the group and prevalence in literature, hybrid functional B3PW91 was used. For the Δ𝐺𝑟𝑥𝑛, values the 6-311++G(d,p) split valence basis set was used51-52. For the electron affinity calculations, the cc-pvtz basis set was used as described in the SI. The frequencies from the optimized geometry are used to include vibrational corrections to the 0 K DFT ground state energy. All of the non-PBC calculations were done at room temperature. Since all of the relevant chemistry is taking place in the electrolyte, gas phase results would not be as useful as liquid results. We used IEFPCM self-consistent field technique within Gaussian 09 to represent a DME electrolyte53. This method applies a field throughout the simulation that mimics the effect of a bulk solvent without adding significant difficulty to the computation.

Results and Discussion DMDS Dissociation As Wang’s group and other researchers found in systems with sulfur containing electrolytes, the sulfur cosolvents will react with the sulfur to form a new reduction pathway 30. DMDS will react with elemental sulfur to produce a long chained dimethyl polysulfide (DMPS), CH3S10CH3. This reaction has been reported occurring without the presence of an external potential but as already stated DMDS is electrochemically active. Since we are employing first principles method based on sound fundamental theories, we made no assumptions in our models and tested all possible situations. For DMDS to react without an external potential, the S-S bond has to be cleaved generating two SCH3 radicals. The reported value for the bond dissociation energy of DMDS S-S bond is relatively low so this mechanism is plausible 54. These radical fragments would then react with the S8 elemental crown structure giving a linear DMPS molecule.

5 ACS Paragon Plus Environment


Page 6 of 27

With AIMD simulations, we explored the stability, structure and reactivity of DMDS in a simplified electrolyte. In Figure S1, a single DMDS molecule was placed in a box surrounded by DME which corresponds to a 94/6 %v DME/DMDS electrolyte. This was done as a control before using a more realistic model for the electrolyte. In this case no dissociation of the DMDS was observed which means there would be no radical fragments to react with the S8. To gain a more quantitative view of the favorability of the dissociation, non-PBC calculations were used to calculate the thermodynamics of this reaction (1). The Δ𝐺𝑟𝑥𝑛 for this dissociation without any extra electrons is positive or thermodynamically unfavorable. If electrons are added to the system, then the reaction becomes favorable (Table S2). (1) 𝐶𝐻3𝑆2𝐶𝐻3 ↔2 𝑆𝐶𝐻3 Figure S2 used a model closer to the experiments with a 50% v DME/DMDS, and a 50% v DME/DMDS electrolyte with S8 . In the even mixture without S8 , the DMDS still remains intact. This indicates that DMDS will not spontaneously dissociate in the electrolyte mixture. To directly test the experimental reaction pathway, in Figure S3 S8 was placed in the electrolyte. In this case, no dissociation or reaction was observed between species. There could be several reasons for why no reaction was seen. For instance, the reaction conditions (temperature/pressure) could be different or that the nature of the reaction between DMDS and S8 is less understood. For instance, some literature refers to DMDS dissolving S8 but not reacting with it while others use the terms interchangeably 38-39. A close interaction versus a reaction between the species could be difficult to discern especially if the reaction is easily reversible. The previous reactions discussed all lacked an external potential and could be considered to be disconnected from an electrochemical system. As researchers have pointed out, one of the advantages of using sulfur cosolvents in the electrolyte is that they can provide additional energy capacity as they can be reduced just like the elemental sulfur. A reaction product from the reduction of DMDS could be part of a new reduction pathway. In PBC systems, the addition of excess electrons to the system to emulate the effect of an external potential near a polarized surface can be problematic. In a non-PBC simulation, the number of electrons can be specified easily without creating any problems. However, due to interactions of one cell with its “image” (one of the repeat cells of the cell), just adding or removing electrons will introduce inaccuracies into the energies computed from the simulation 55. A relatively simple way around this is to inject an electron via placing a metal atom that is likely to oxidize into the system. Fortunately, this can be done very easily with a lithium atom and it is also representative of a real system, i.e. lithium ions being present in the electrolyte phase. Such a system was used in Figure 1.


A)

1.2 1 0.8 0.6 0.4 0.2 0 -0.2 -0.4 -0.6 -0.8 -1

Li

DME

DMDS 0

AIMD

2

4

6

8

10

12

8

10

12

Time (ps)

B)

Net Bader Charge (e)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



Page 7 of 27

0.2 0 -0.2

SCH3 # 2

-0.4

SCH3 # 1

-0.6

DMDS

-0.8 -1 0

2

4

6

Time (ps) Figure 1: DMDS in DME with 1 Li atom AIMD (94/6 %v DME/DMDS at 300 K).. Snapshot of system is on left (10x10x15 Å). Net Bader charge as a function of time and A) the overall starting components and B) the reduction of DMDS components

From visual observation, at the end of the AIMD simulation the DMDS was reduced. By examining the net Bader charges, extra insight can be gained56-58. The DMDS gained one electron throughout the simulation. The Li atom was oxidized and this electron was localized in the DMDS, thus reducing it into two SCH3 fragments. The corresponding reaction is shown by equation (1b). The charge states of the products are ~-0.5 |e| which is represented by grouping the two SCH3 fragments together. Note that the partial charges are fractional which is nonphysical by quantum chemistry interpretations. This is because no partial charge scheme is perfect and the partial charges are meant to give a descriptive idea of where the electrons in the system lie. (1b) 𝐶𝐻3𝑆2𝐶𝐻3 + 𝑒 ― →(2𝑆𝐶𝐻3) ―1



A)


1.5

Li 1/2

1 0.5

DME

0

-0.5 -1

DMDS

-1.5 -2 0

AIMD

2

4

6

8

10

8

10

Time (ps)

B)

0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 27

-0.2 -0.4 -0.6

SCH3 # 1/2

-0.8 -1

-1.2 -1.4

DMDS

-1.6 -1.8 0

2

4

6

Time (ps) Figure 2: DMDS in DME with 2 Li atom AIMD. Snapshot of system is on left (10x10x15 Å). Net Bader charge as a function of time and A) the overall starting components and B) the reduction of DMDS components

Another lithium atom was added to the system to observe how the dynamics would change if the reduction was a 2e- process. Figure 2 shows the corresponding results for this system. Within the first picosecond of the simulation, both lithium atoms release their electrons which then reduce the DMDS. The DMDS gains two electrons and from the snapshot of the system, the S-S bond was clearly broken. Figure S4 gives a view of the DMDS molecule atom by atom (excluding the hydrogen atoms) to determine where the electrons were transferred to. It can be seen quite clearly that the electrons that reduced the DMDS molecule are primarily centered on the sulfur atoms and both DMDS half fragments are similar in charge states. This reaction corresponds with reaction (1c) identified below. As concluded earlier, the more electrons added to the DMDS the more favorable the reduction becomes. (1c) 𝐶𝐻3𝑆2𝐶𝐻3 +2 𝑒 ― →2 𝑆𝐶𝐻3―1 Based on our models, the spontaneous dissociation of DMDS in solution or DMDS in the presence of elemental sulfur is not favored or observed, but in an electron-rich environment then DMDS is broken by the S-S bonds into two smaller molecules. These SCH3 fragments 8 ACS Paragon Plus Environment

Page 9 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


whether their charge state is radical or ionic, can react with other sulfur containing species that may be present in a LiSB like S8, lithium polysulfides, or even other DMDS molecules. These reactions will be the driving force for creating a new reduction pathway.

New Reduction Pathway As Wang identified previously, the new reduction pathway starts when DMDS reacts with elemental sulfur. In order to run first principle simulations, the geometry of the system is required. This necessitates either using experimental information for the structure of a molecule, or testing a variety of guess geometries to determine which are the lowest energy and thus the most favorable. Instead of guessing different structural configurations, geometries from DFT optimizations were used as initial reactant configurations without making any assumptions about the final structure. Figure 3 shows the input and optimized product. Reaction 2a corresponds to the reaction shown.

Top

Side

Initial

Final

Figure 3: Input and output of reaction between SCH3 and elemental sulfur (2a) 2𝑆𝐶𝐻3 + 𝑆8→𝐶𝐻3𝑆10𝐶𝐻3 Several different initial configurations were tested but the only configuration that caused a reaction is the one shown. The final structure is an example of a DMPS which is very similar in structure to the lithium polysulfides except the extra length and methyl groups that cap the molecules. As described in the previous section, SCH3 can be generated by different types of reactions. This means that based on what method or reaction conditions are present the oxidation state of the SCH3 fragments is variable. Using the optimized DMPS molecule, the ΔGrxn was calculated for the three different possibilities of charge state for the SCH3 molecules: both neutral, one neutral and one radical, and both in radical states. The corresponding ΔGrxns are shown in Table S2. The trend with reaction (2a) is that the neutral SCH3 fragments reacting with the S8 is more favorably than anionic SCH3 fragments. All of these results were calculated with the DMPS being in a neutral charge state. The reactions involving anionic SCH3 fragments might be unfavorable due to the final charge state of the DMPS. Having a reaction involving 9 ACS Paragon Plus Environment


anionic SCH3 fragments and neutral DMPS molecules corresponds to electrochemical reductions as denoted by the change in oxidation state. The geometry of DMPS as shown in Figure 3 is only stable in this configuration with a neutral charge state. As electrons are added to the DMPS molecule and optimized, the molecule will break an S-S bond just like the DMDS did. DMDS is a type of DMPS just shorter chained than the product of the reaction between DMDS and elemental sulfur.

A)


So far we have focused our discussion on the reactions between DMDS and elemental sulfur. But this begs the question; can the SCH3 fragments react with other sulfur species that aren’t S8? We tested this by running an AIMD simulation with two SCH3 fragments and a Li2S8 lithium polysulfide together in the electrolyte. This would represent a case where already formed SCH3 fragments would potentially react with the intermediate reduction products of S8. Figure 4 contains the results of the simulation. 1

Li 2S8

0.8 0.6 0.4 0.2

SCH3 # 2

0

DME SCH3 # 1

-0.2 -0.4 0

2

4

6

8

10

8

10

Time (ps)

AIMD B) Net Bader Charge (e)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 27

1 0.8 0.6 0.4 0.2 0 -0.2 -0.4 -0.6 -0.8 -1 -1.2

Li 2S8

S2 S3CH3 S7CH3 0

2

4

6

Time (ps)

Figure 4: Snapshot of 2 SCH3 neutral fragments with Li2S8 before and after simulation (15x15x15 Å). Net Bader charge as a function of time and a) total components of system and B) the sulfur containing components. Early in the simulation the SCH3 #2 fragment approached the Li2S8 and reacted with the polysulfide. The fragment didn’t add onto the end of the PS but instead reacted with one of the


Page 11 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


non-terminal sulfur atoms. This reaction generated a methylpolysulfide (MPS) and a short chained PS (reaction (3)). (3) 𝐿𝑖2𝑆8 +𝑆𝐶𝐻3→𝑆7𝐶𝐻3―1 + 𝑆2―1 +2𝐿𝑖 + (4)

𝑆2―1 +𝑆𝐶𝐻3→𝑆3𝐶𝐻3―1

The overall Bader charge of the SCH3 fragment doesn’t change noticeably over time but we do observe an increase in partial charge for the Li2S8 and a reduction in charge for the other SCH3 fragment forming S7CH3. This increase in charge is due to the reaction occurring between the Li2S8 and the SCH3. As one of the sulfur bonds breaks and the electrons rearrange, the Li2S8 temporarily loses some electron localization which is balanced by a decrease in the other SCH3 fragment. This is also seen by the S2 partial charge curve; the charge increases before dropping to ~ -1 |e|. The sudden decrease in charge is associated with a reaction with the other SCH3 fragment which forms a 2nd MPS, S3CH3-1 (reaction 4). Throughout the rest of the simulation the two MPS fragments do not react or interchange atoms. From these simulations, the reaction of DMDS to form SCH3 fragments which then react with sulfur species to form DMPS or MPS molecules has been proven to be a possible pathway. Even if the S8 has already been reduced into an intermediate PS species, the DMDS can still modify the reduction process to form MPS/DMPS species. In Wang’s studies, the authors observed a variety of different length DMPS species demonstrating experimentally that the DMPS would reduce and a new reduction pathway was observed. In order to study the reduction of DMPS with DFT, two options are available. One is to guess the different reaction pathways and determine which is the most energetically favorable. Or AIMD can be employed to take as many assumptions out of the model as possible. DMPS can be placed in a simulation cell with lithium atoms and the reduction can be observed stochastically due to the nature of molecular dynamics. Figure S5 represents a control case for the reduction of DMPS. This system was created by taking the optimized structure from the non-PBC calculations and building a cell around it. No electrons are added to the system and no reactions are observed. Because the S-S bond usually is relatively weak, dissociation of the DMPS was a possibility but no dissociation or anything close to it happened in the run. Even with the inclusion of explicit solvent the structure is very similar to that of implicit solvent used in the non-PBC simulations. In Figure 5, we added one Li atom to study a 1 e- reduction mechanism of DMPS.



A)


1 0.8

Li

0.6 0.4 0.2

DME

0 -0.2 -0.4 -0.6

DMPS

-0.8 -1 0

2

4

6

8

10

12

10

12

Time (ps)

AIMD 0.2

B) Net Bader Charge (e)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 27

CH3S7CH3

0

-0.2

CH3S6

-0.4

CH3S4

-0.6 -0.8

S

DMPS

-1

3 0

2

4

6

8

Time (ps)

Figure 5: Snapshot of DMPS in DME with one Li atom is on left before and after simulation (15x15x15 Å). Net Bader charge as a function of time and A) total system components and B) reduction of DMPS components

Based on the control simulation, DMPS should have a neutral overall Bader charge in solution. The simulation takes around 0.5 picosecond before the DMPS moves from a neutral charge state to a charge state near -1|e| while the Li atom loses ~1 |e| worth of charge. To understand the reduction more clearly, we examined the charges of different structures observed in the AIMD. The excess electron quickly reduces the DMPS into two MPS fragments, S6CH3 and S4CH3. These species are stable until ~6 picoseconds when the end SCH3 part of the S4CH3 species attaches itself to the terminal sulfur end of the S6CH3 species forming a S3 fragment and a DMPS molecule, CH3S7CH3. The reactions are summarized below. ―

1

1

―2

(5)

𝐶𝐻3𝑆10𝐶𝐻3 + 𝑒 ― →𝑆6𝐶𝐻3 2 + 𝑆4𝐶𝐻3

(6)

𝑆6𝐶𝐻3 2 + 𝑆4𝐶𝐻3 2→𝐶𝐻3𝑆7𝐶𝐻3 + 𝑆3―1

―

1

―

1

From our earlier DFT calculations, the charge state of the species involved in the reactions is important to the energetics. Since one electron was added to the system, what is the expected charge state of the product MPS fragments? If one were to think in the terms of organic chemistry, they might expect the formation of an anionic S6CH3 fragment along with a radical 12 ACS Paragon Plus Environment

Page 13 of 27

S4CH3 fragment. In the Bader partial charge analysis, these species would have a net partial charge of -1 and 0 respectively. From Figure 5, this was not the observed outcome. Instead both MPS fragments have a partial charge somewhere in between these two cases which implies that the state of the fragments are somewhere between a radical and an anion. Due to how close the two fragments are, this difference doesn’t matter. 1.5


A)

Li #2

1

Li #1

0.5

DME

0

-0.5 -1

DMPS

-1.5 -2 0

2

4

6

8

10

12

8

10

12

Time (ps)

AIMD B)

0 -0.2


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-0.4 -0.6

S5CH3 # 1

-0.8

S5CH3 # 2

-1

-1.2 -1.4

DMPS

-1.6 -1.8 0

2

4

6

Time (ps)

Figure 6: Snapshot of DMPS in DME with two Li atoms is on left before (15x15x15 Å). Net Bader charge as a function of time and components in system are on the right.

Figure 6 shows the results for a DMPS system that is reduced by two electrons. The final products that were observed are two S5CH3 MPS (reaction 7). Compared to the previous system there are some slight differences. One is that the DMPS reduced exactly in half unlike the reduction with 1 e- where the DMPS split into a S=4 and a S=6 fragment. Another important difference is that the MPS fragments do not combine to form a DMPS species, instead they are stable for the rest of the runtime of the simulation. This is most likely due to the charge of the MPS fragments. In the 1 e- simulation, they were both ~ -0.5 |e| compared to ~ -1 |e| in this simulation. The first system is more stable with a DMPS species and 1 e- on a PS molecule versus in the 2nd system, 2 MPS molecules stabilize the energy more favorably. (7)

𝐶𝐻3𝑆10𝐶𝐻3 +2𝑒 ― →2𝑆5𝐶𝐻3―1



Page 14 of 27

Since the operations of batteries create electron rich environments particularly near electrified interfaces, we continued the trend of AIMD simulations. Figure S6 shows the results for a system with 3 e added. The largest difference in this model compared to the previous calculations is that the extent of reactions increases with the addition of another electron. The final reduction products in this case consist of S3, S4CH3 and S3CH3. The trends of the partial charges are very similar to the previous simulations. The Li atoms added to the model rapidly lose their electrons which are transferred to the DMPS species. One important observation about the reaction is that the reduction reactions occur simultaneously. The partial charge curves for the three products all increase in charge at the same time. In this simulation, the DMPS wasn’t split into 2 MPS that then formed 2 MPS and a PS. Instead the DMPS was reduced directly into 2 MPS and a PS. Since these results represent the energetically favorable outcome of a DMPS with 3 added electrons, we can connect the reduction products to the previous models. If we added an additional electron to the 2 e- system, we would expect the formation of 2 MPS and PS in accordance with this simulation. This would have to be accomplished by the reduction of one of the S5CH3 fragments. The results for the 4 electron reduction are given in Figure S7. With this number of electrons, the observed final reduction products were S2, S3, S4CH3 and SCH3. As noted with the previous system, all of the products from the reaction are formed at the same time and not through a series of intermediate reactions. One interesting aspect of this simulation is the presence of a S4CH3 MPS species. The fact that it didn’t get reduced further implies that if more electrons were added to the final results of Figure S6, then instead of the electrons reducing the MPS fragments equally, it would be more favorable to reduce one MPS preferentially.


Page 15 of 27

2


A)

Li # 1-5

1 0

DME

-1 -2 -3

DMPS

-4 -5 0

AIMD B)

2

4

6

Time (ps)

8

10

12

0

S3CH3

S4CH4

-0.5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-1

S2

-1.5

S1

-2

-2.5 -3

-3.5

DMPS

-4

-4.5 0

2

4

6

8

Time (ps)

10

12

14

Figure 7: Snapshot of DMPS in DME with five Li atom is on left (15x15x15 Å). Net Bader charge as a function of time and components in system are on the right.

Figure 7 contains the results for an AIMD simulation with 5 Li atoms added. The final reduction products consist of S1 ,S2, S3CH3 and S4CH3. The charge behavior of the Li atoms behaves the same as previously but the magnitude of the charge that DMPS gains is noticeably “off” by one electron. This can be thought of as a rounding effect. In the simulations, the Li partial charge is not exactly +1. Instead it is more like 0.8 |e| which is missing 0.2 |e| per Li atom. This trend is observable in all of the AIMD simulations. For instance in Figure 13, the DMPS gains a total of less than 3.5 |e| when it should be gaining 4. As described earlier, fractional electrons are nonphysical. The Bader partial charge scheme is just an approximate way to divide the electron density in a familiar and descriptive way. Though the DMPS hasn’t gained a discrete number of electrons, it still gained a significant amount of charge versus the 4 Li atom simulation. The reduction products here favor a fuller reduction of the PS over the smaller fragment MPS species. In the AIMD simulations, several trends were observed. The general reduction mechanism is that DMPS is split into MPS fragments which are then reduced into PS or smaller MPS. Once the MPS is small enough it seems like the PS reduction is more favored than MPS. Another observation is that several polysulfides were generated. This shows that even in a new reduction pathway, it may still be possible to generate traditional polysulfide reduction products. Experimental evidence found that the favored final reaction products were LiS2CH3 and LiSCH3. 15 ACS Paragon Plus Environment


Page 16 of 27

In our simulations we observed the formation of LiSCH3, but not the formation of LiS2CH3. Though AIMD gives an assumption free and stochastic way to explore the DMPS reduction pathway, it unfortunately doesn’t provide the whole picture. For instance, will the polysulfide or the DMPS reduction pathway be favored? What will the final reduction products be? According to Wang et. al. , DMPS is favored over polysulfide reduction pathway but the AIMD simulations would struggle to prove that. One of the goals of this work is to develop a computational procedure that can be used to screen sulfur containing electrolytes. The AIMD simulations we have showed are useful but take significant computational time and power. We have developed another first principles method that can be used to determine if a sulfur containing species will change the reduction pathway of a LiSB.

Electron Affinity Another first principle calculation that can provide great insight into this system is the determination of electron affinity (EA). The electron affinity of a molecule is simply calculated by equation 8 shown below where N is defined as the number of electrons. (8) 𝐸𝐴 = 𝐸𝑁 + 1 ― 𝐸𝑁 In words, the electron affinity is the difference in energy between a molecule with an extra electron added and the neutral case. This concept is closely related to the ionization potential which is the opposite calculation; an electron is removed instead of added. Electron affinity and ionization potentials have a rich history in the field of first principles calculations and computational chemistry. These calculations also can provide great insight especially for electrochemical systems. Researchers have previously quantified a relationship between electrochemical reactivity and electron affinity 59. By calculating the electron affinities, it is possible to discern critical information about the new reduction pathways favorability, reaction steps and relative reactivity. In general, the more favorable the electron affinity; the more reactive the molecule in an electrochemical system that favors electron transfer. Electron affinity provides an advantage over changes in free energy of reactions, ΔGrxn because in these systems with sulfur, the ΔGrxn for reduction is highly favorable and nearly irreversible and thus it is very difficult to draw conclusions. It is worth noting that these Δ𝐺𝑟𝑥𝑛 do not exactly correspond to the reduction and oxidation processes observed in a traditional battery. Those reactions will occur in the presence of an external potential that will change the energetics of the reactions. These Δ𝐺 are determined to see if one of these particular reactions is energetically more viable than another. We calculated the electron affinities for three different types of molecules of all possible lengths with less than ten sulfur atoms: DMPS, MPS and PS. The electron affinities were calculated by first optimizing the neutral species and then adding and electron and optimizing again. This means that our reported electron affinities are adiabatic and not vertical. To make the simulation as accurate as possible we tested four different computational methods: B3PW91/6311++G(d,p) and B3PW91/aug-cc-PVTZ with and without the Yamaguchi spin correction60-61. The results indicated that the use of B3PW91/aug-cc-PVTZ with the spin correction would give the most accurate results (Figure S8). 16 ACS Paragon Plus Environment

Page 17 of 27

1

Electron Affinity (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

MPS

-1

PS

-2

DMPS

-3 -4 -5 0

2

4

6

8

10

# of S atoms Figure 8: Electron affinity comparison for DMPS, MPS and PS species at triple zeta with Yamaguchi spin contamination correction level of theory. The electron affinities reported in Figure 8 give several interesting trends. One is that the DMPS species with CH3SxCH3 (x >2) have lower electron affinities than any of the other species. This means that these DMPS species are more susceptible to reaction than any MPS or PS species. This corroborates the AIMD simulations earlier where the S5CH3 species underwent reduction before the S2CH3 or S3 species. For most of the cases, the electron affinity follows a trend: DMPS

Effects of Dimethyl Disulfide Cosolvent on Li-S Battery Chemistry and

Recommend Documents