Exploring the LiOH Formation Reaction Mechanism in Lithium–Air

Jan 16, 2018 - Identifying the electronic factors that enhance or hinder the formation of primary discharge/charge products or secondary parasitic spe...
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Cite This: Chem. Mater. 2018, 30, 708−717

Exploring the LiOH Formation Reaction Mechanism in Lithium−Air Batteries Ana E. Torres* and Perla B. Balbuena* Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843, United States S Supporting Information *

ABSTRACT: Identifying the electronic factors that enhance or hinder the formation of primary discharge/charge products or secondary parasitic species is crucial for defining the fundamental chemical reactions that may take place within the complex electrolytic media of a lithium−air battery. For example, several reaction mechanisms have been proposed to explain the formation of the LiOH discharge product in the presence of LiI and water; however, none of these have been demonstrated or fully understood. A similar situation exists for the decomposition reaction, which leads toward molecular oxygen evolution. Herein we present a mechanistic theoretical study of the reactions taking place in the electrolytic media of a Li−O2 battery with dimethoxyethane (DME) in the presence of both water and the LiI additive. The results reveal that water is the most energetically favorable source of protons yielding LiOH. The effect of iodide in lithium peroxide bond scission can be ascribed to a halogen-bond interaction. The reaction pathway involving hydrogen peroxide was found to be a viable route accounting for the lithium hydroxide decomposition.

1. INTRODUCTION Lithium-based batteries are promising alternatives to circumvent existing energy storage issues toward a sustainable energy mix shift.1 Some of the features of lithium-ion batteries that make them a game changer in the energy field are their higher voltage, higher energy density, and longer cycle life compared with the Ni−Cd and Pb−acid rechargeable batteries.2 The high specific density requirements for modern applications as power sources in electric vehicles demand values approaching 13 000 W h/kg, which corresponds to the energy density of gasoline. The theoretical specific energy density for a nonaqueous lithium−air (Li−O2) battery is 11 680 W h/kg which is close to the gasoline value whereas it exceeds around 10 times that ascribed for some lithium-ion batteries.3 A Li−O2 battery main structure consists of a lithium-metal negative electrode (anode) and a porous positive electrode (cathode) (e.g., reduced graphene oxide). Both electrodes are separated by a lithium-ion conducting electrolyte solution containing a lithium salt such as lithium bis(trifluoromethyl) sulfonylimide (LiTFSI) and an organic solvent like dimethoxyethane (DME). During discharge, the lithium metal from the anode is oxidized to lithium ions; thus the released electrons travel through an external circuit to the cathode. Simultaneously the lithium ions flow through the electrolytic media to the cathode guided by an electrochemical potential gradient. Meanwhile at the cathode side, the reduction and lithiation of oxygen takes place. This process is reversed during the charging process.2 Some of the challenges in Li−O2 battery development are the electrolyte and electrode stability, the overvoltage for the © 2018 American Chemical Society

oxygen evolution reactions, and a limited cycle life. Also, a mandatory requirement for their applicability as rechargeable batteries is the reversibility of the oxygen reduction reactions during discharge and the oxygen evolution reactions which take place in the charging process.4 In the discharge process the oxygen reduction of molecular oxygen occurs mainly, but not exclusively, through the following reactions: Li+ + O2 + e− ↔ LiO2 +

−

(1)

LiO2 + Li + e ↔ Li 2O2

(2)

2LiO2 ↔ Li 2O2 + O2

(3)

The dominant discharge product within an aprotic electrolyte is Li2O2. Indeed, theoretical calculations at B3LYP/6-311+ +G(d,p) level of theory and the Poisson−Boltzmann Finite element method (PBF) have shown that the favored reaction pathway for the Li2O2 formation in DME is the direct reduction, represented in reaction 2.5 The lithium peroxide deposits on the cathode forming an insulating and insoluble (in nonaqueous solvents) layer which prevents achieving the theoretical electrochemical capacity.6−10 Moreover, it is known that the electrode potentials influence the generation of oxygen-containing intermediate species and determine the Li2O2 formation mechanism. It has been proposed that at high potentials (low overpotentials) Li2O2 forms by means of a solution-mediated disproportionation Received: September 21, 2017 Revised: December 6, 2017 Published: January 16, 2018 708

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anhydrous Li−O2 battery, that produces Li2O2 as the discharge product with LiI, the minimum potential to oxidize Li2O2 is 2.96 V.17 The thermodynamic analysis showed that LiOH could not be the active species during charging, but Li2O2 oxidation seems more plausible at the I3−/I− potential, thus suggesting that other reactions different than direct oxidation of LiOH in the presence of iodide might take place.17 Moreover, the observed low reduction potential during charge (ascribed to LiOH direct oxidation) was rationalized in terms of the contribution of the activity of water in the LiI/ LiTFSI/DME electrolyte at standard conditions.18 Despite these interesting results, the chemical reaction decomposition mechanism of LiOH remains unknown. Up to now, the main suggested decomposition pathway accounts for oxygen (O2) recovery while a second one that might also take place involves hypoiodite formation and disproportionation reactions leading to iodate (IO3−) and some other products. Some reports have shown evidence of parasitic reactions associated with the LiI additive in a Li−O2 battery with ethereal solvents.9 For non-carbonate-based electrolytes, such like glyme solutions (i.e., dimethoxyethane, DME), the degradation might take place through acid−base reactions between the discharge oxygen-containing products (Li2O2) and the ether solvent molecules. The iodide from the additive could facilitate subsequent hydroperoxide (LiOOH) decompositions. Further reactions between LiOOH and iodine-containing species (LiOI) may lead to the regeneration of LiI along with the formation of O2 and water.10 These two last products correspond with those obtained during the Li−O2 battery charging process. Kwak and co-workers19 proposed a mechanism for the LiOH formation initiated by the hydrogen abstraction from the nucleophilic lithium peroxide to the β-methylene carbon of the ether-based solvent molecule. This reaction along with an E2 concerted elimination may lead toward the generation of LiOOH, LiOCH3, and other subproducts of the electrolyte decomposition as shown in Scheme 1. The suggested

mechanism, whereas at low potentials (high overpotentials) the formation of Li2O2 occurs from electrochemical reduction via a surface-mediated mechanism.11 The influence of the discharge overpotential over the morphology of the discharge product (Li2O2) has been reported, and more recently Franco and coworkers found that the nucleation process could play an important role in the lithium peroxide morphology and discharge capacity of Li−O2 aprotic batteries.12 The reverse lithium peroxide oxidation reaction is not fully understood so far. However, the following elementary reactions have been proposed within a charge model in which the coexistence of thin-film small particles (Li2O2(f)) and large particles (Li2O2(p)) is considered:13 Li 2O2(p) → Li+ + e− + LiO2(s)

(4)

Li 2O2(f) → Li+ + e− + Li+ − O2−

(5)

Li+ − O2− → Li+ + e− + O2

(6)

LiO2(s) → Li+ − O2−

(7)

Taking into account the fast lithium-oxide ion-pair (Li+− O2−) decomposition, the oxidation of large particles could be expressed as a two-electron electrochemical reaction:14 Li 2O2(f) → 2Li+ + O2 + 2e−

(8)

which leads toward the molecular oxygen evolution and subsequent cycling process. With the aim of reducing the overpotential needed to oxidize Li2O2, redox mediators such as LiI which can undergo electrochemical oxidation are used. Thus, in the process, LiI transfers electrons to lithium peroxide (Li2O2). The redox couples, which play a crucial role in the electrolytic solution, are mainly I3−/I− and I2/I3−. However, some negative points are attributed to the iodide additive. For example, it could induce parasitic reactions during discharge, and some authors claim that it does not participate in the oxygen reduction process.7 Even more, recently there was a report of an aprotic lithium− air Li−O2 battery with excellent features such as high energy efficiency, low overpotential, and long cycle performance by addition of water into the iodide-containing aprotic battery for which LiOH is the discharge product.8 It was found that the lithium ions react with oxygen at the cathode producing lithium hydroxide instead of the insoluble lithium peroxide, that grows in the electrode and is hard to remove during the battery charge. Conversely, LiOH decomposes more easily throughout this process in the presence of water and iodide. It has been suggested that water may be the proton source which leads toward the formation of the LiOH discharge product.8 The viability of the LiOH oxidation mechanism was questioned based on its equilibrium potential (3.4 V vs Li+/ Li under standard conditions) which exceeds the experimentally observed one (∼3), thus suggesting that LiOH formation/ decomposition seems difficult to occur.15 It has been claimed that I−/I3− redox couple is involved in the charge potential. In this line, it was proposed that posterior discharge cycles are associated with I3− reduction back to I−.15 In response to this controversy, it was asserted that LiOH decomposition is promoted by iodide through a chemical pathway and that other iodine-based polyanions such as hypoiodite (IO−) might be involved in the complex equilibria.16 In a further contribution, it was highlighted that, for an

Scheme 1. Decomposition Reaction Mechanism of Dimethoxyethane in Li−O2 Batteries Containing LiI As Proposed in Ref 19

mechanism is based on experimental detection of LiOCH3.19 However, this assumption is not supported by any detailed mechanistic study. Besides, irreversible parasitic routes need to be avoided to allow long-term rechargeability in a Li−O2 battery. It must be highlighted that Qiao and co-workers20 did not detect oxygen evolution during charging in nonaqueous conditions, because LiOH does not oxidize. However, when water is added LiOH transforms to Li2O2 which is oxidized by the I−/I3− redox couple promoting the reversible evolution of oxygen.20 DME has an intermediate donor number (DN ∼ 24) that at high potentials can exhibit contributions from solution and surface pathways during discharge. The oxygen-based species 709

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Chemistry of Materials (O2− or LiO2) formed after the reduction steps at the cathode surface can be distributed between adsorbed species on the electrode surface and dissolved species in the electrolyte leading to solvated ions, ion pairs, or clusters.21 It has been suggested that, in the presence of small amounts of water and LiI, the solution pathway is promoted given the Lewis acidity of H2O.22−24 In summary, there are manifold competing reactions within this complex system; thus mechanistic studies are required to get insights into the occurrence of the chemical reactions that lead to parasitic products and species that could undergo separately through electrochemical reactions and may help to explain the observed charge/discharge voltage. This work explores and analyzes basic chemical reactions that might take place in the electrolytic media of a lithium−air battery where dimethoxyethane (DME) is used as aprotic solvent in the presence of lithium iodide and water. Only the fundamental chemical reactions were included to explore the chemical pathways and possible intermediate species that could be relevant in the formation of LiOH. The present calculations offer an explanation for the LiOH formation in gas phase or the LiOH unit (seed for the LiOH crystal) within a field as that given by a continuum solvent model, not near the electrode surface, as shown in Figure 1. Indeed, we did not address the

To include the solvent effect in the energetics, the gas-phase energy minima were reoptimized including the density-based SMD solvent model.29 The static dielectric constant of dimethoxyethane and the solvent radius were taken from ref 30. In a recent work, the solvation free energy of lithium with dimethoxyethane was calculated using a Poisson−Boltzmann model with a fitted ionic radius which reproduced experimental solvation free energies for Li+ and O2− in organic solvents.31 The solvation free energies of the lithium ion were calculated at the SMD (DME)- M06L/6-31+G(d,p) level of theory using the monomer cycle shown in Figure S1 provided in the Supporting Information. The thermodynamic properties and the solvation free energies are presented in Tables S1 and S2. It is worth to mention that the solvation free energies (ΔG0(solv)) calculated using a mixed cluster-continuum model for two and three molecules of DME bicoordinated to the lithium ion are in agreement with those calculated by Kwabi and co-workers.31 The stationary points detected along the potential energy surface were confirmed as energy minima or transition structures through vibrational frequency calculations. Intrinsic reaction coordinate (IRC) calculations were carried out to verify the connection between the transition structures and the minima.32,33 Representative electrostatic pathways were calculated through relaxed coordinate scan for some electrostatic adducts detected along the potential energy curve following the electrostatic bond vector as reaction coordinate, until each fragment is far apart from each other (distance >6 Å). All calculations were carried out using Gaussian 09 Rev D.01 software.34

3. RESULTS AND DISCUSSION 3.1. Hydrogen Abstraction from Water Molecule. Based on several experiments it has been proposed that the most probable starting point for the formation of LiOH in solutions containing LiI is lithium peroxide.3,19,20,35 In fact, when low concentrations of LiI are used the major detected discharge product is Li2O2.20 The peroxide anion is a strong nucleophile and is highly reactive. The O−O bond dissociation energy in hydrogen peroxide is 47 kcal/mol compared to 118 kcal/mol in the oxygen molecule.36,37 Thus, when it interacts with labile hydrogen-containing species it tends to abstract it. In an initial stage, we explored the reaction mechanism of the Li2O2 molecule interacting with water molecule, and then, the LiI additive was included. The calculated reaction pathways in gas phase and with implicit solvent (dimethoxyethane) are presented in Figure 2. It should be noted that LiI is shown in the first steps of the reaction pathway because it was included to calculate the relative energy and to meet the reaction stoichiometry. However, after reaching the electrostatic minima between LiOH and LiOOH, as further discussed, it did play an active role in the reaction. We found some electrostatic minima between Li2O2 and the water molecule when they initially interact. In gas phase, Li2O2 formed a bonding electrostatic interaction with the water molecule stabilized by a hydrogenbond-type interaction. This interaction vanished after including the continuum solvent model reflected in a (7)H−(5)O bond elongation. Also, the O−H bonding of the water molecule interacting with an oxygen atom of the lithium peroxide in absence of the solvent increased (1.0 Å) when compared to the bond distance of 0.96 Å present in the free water molecule. It was found that the electrostatic minimum destabilizes when including the implicit solvent; even its geometry distorts from a bent structure to an extended one. This could be ascribed to the poorly polar nature of the DME solvent in contrast to the highly electrostatic nature of most of the interactions considered. Indeed, the energy of the corresponding energy

Figure 1. Schematic description of the considered species in the electrolytic solution (sol) for the quantum calculations.

electrochemical reactions that might take place nor the explicit solvation shell over the reactant species, although these factors are worthy of further investigation. Quantum chemical calculations performed herein aim to detect the most favorable pathway for (a) the LiOH formation and (b) the O2 regeneration. Finally, the chemical role of iodide in the lithium peroxide bond scission is analyzed.

2. COMPUTATIONAL METHODS We performed quantum mechanical calculations based on the density functional theory (DFT) using the M06L local meta-GGA functional.25 We performed geometry optimizations of the energy minima and stationary points detected along the investigated reaction pathways. The Pople 6-31+G(d,p) basis set26 was used for carbon, hydrogen, and oxygen atoms while the LANL2DZ basis set and pseudopotential27 were used for iodine. We calculated a lithium− acetone binding energy of 44.3 kcal/mol at M06L/6-311+G(d,p) level of theory which is in good agreement with the experimental value (44 kcal/mol28). Likewise, this basis set facilitated the convergence of the solution-phase geometry optimizations of the intermediate species and transition structures on the potential energy surfaces. 710

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Figure 2. Minimum energy pathways for the reaction of Li2O2 and water, with LiI addition to obtain the final product LiOH. Relative energies (in kcal/mol) of the gas-phase reaction pathway and when the SMD continuum solvent model is included are shown in blue and black, respectively. The dotted lines link each electrostatic minimum with the separated fragments; the electrostatic pathways are presented in the Supporting Information (Figures S2 and S3). The discontinuous orange lines show the barrier height. The energies of the reactants, lithium peroxide, water, and lithium iodide, were taken as reference for the relative energies.

Figure 3. Lithium peroxide−water electrostatic minima: (a) gas phase, (b) optimized structure in DME continuum solvent.

minimum shown in Figure 3 destabilized 5.9 kcal/mol when the energy values are compared between the solvent free model and the continuum solvent calculations. Next, a hydrogen abstraction of the water molecule by the Li2O2 fragment takes place readily. This process, as shown in Figure 4, occurs through the abstraction of the water-bonded hydrogen atom into the Li atom out-of-plane of the lithium peroxide molecule leading to the lithium hydroperoxide (LiOOH)−lithium hydroxide (LiOH) electrostatic product through an almost barrier-less pathway. In this first step the LiOH molecule is first formed and seems to be reversible given the similar stability of the intermediate species and the small barrier. Even with the viability of this chemical process, it is hindered somewhat in the presence of the ether-based solvent increasing the proton abstraction barrier height from 2.7 to 4.5 kcal/mol. Due to the electrostatic nature of the species therein formed, the products remain interacting through Coulombic forces.

Figure 4. Hydrogen abstraction induced by lithium peroxide over the water molecule, transition structures from DFT calculations (TS1 in Figure 2). (a) Gas-phase optimized transition structure. (b) Analogue structure under implicit solvation.

They should be separated to continue subsequent chemical reactions. Thus, the electrostatic pathways, which lead to detachment of the isolated moieties LiOH and LiOOH, were 711

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static interactions. As previously described for the LiOOH− LiOH electrostatic intermediate, the relaxed scan simulating the detachment of the LiOI−LiOH fragments, which can be found in the Supporting Information, showed an uphill trajectory with minor barriers of 2.9 and 7.2 kcal/mol in gas phase and with solvent, respectively (Figure S3). 3.3. Hydrogen Abstraction from the Dimethoxyethane Solvent Molecule. In order to evaluate the competing set of reactions exerted by lithium peroxide with the dimethoxyethane solvent molecule, we calculated the abstraction of hydrogen from the DME molecule. Lithium peroxide formed a bicoordinated electrostatic adduct with DME as shown in Figure 6. This initial electrostatic structure exhibited a preference to interact with the DME molecule; indeed, it is more stabilized than its analogue with a water molecule in ∼5 kcal/mol when the continuum solvent is included. Unlike the Li2O2 electrostatic adduct with a water molecule which preserves its planar geometry, the peroxide moiety suffered an angular distortion when it interacts with DME alleviating the stress tension of the Li2O2 fourmembered-planar-ring original structure. The geometries presented in Figure 7a,a′,b,b′ illustrate that the Li−O−Li angle (in lithium peroxide) contractions are 12.6° and 33.1° in gas phase and with implicit solvent, respectively. Next, a proton is abstracted from the methylene (−CH2−) within DME by Li2O2 oxygen, and the resulting OH moiety simultaneously detaches from one of the lithium ions after surpassing a barrier of 31.3 kcal/mol (solvent (DME) path, Figure 6). Based on the SMD-M06L results, the Li−O(H) distance within the lithiated moiety in the lithium hydroperoxide (LiOOH)−DME(−H) product increased 40% from its initial value of 1.8 Å in Li2O2−DME (see Figure 7b′,c′). Conversely, the interaction between the lithiated and the etherbased fragments reinforced after the described reaction since the Li−O distance slightly decreased. For comparison, Table 1 shows calculated energies of the Li2O2·DME energy minima, as well as those corresponding to hydrogen abstraction barriers (from DME by Li2O2), using different theoretical models at DFT level of theory. Interestingly, for the models ranging from gas phase, gas phase including continuum solvent model, to cluster models and solid state under periodic boundary conditions, the reaction barriers exhibit similar values. Subsequently, the hydrogenated product was detected to undergo a concerted 1,2-hydrogen migration within the carbon subunit and a CO bond scission in which methoxide acts as leaving group while being attracted to the lithium ion. This process is exothermic, and it takes a high barrier of 31.7 kcal/ mol to get the methyl vinyl ether and the lithium methoxide− lithium hydroperoxide products. It must be mentioned that the product moieties remain interacting together through Coulombic forces. Hence, the detachment of the three isolated fragments was modeled in two steps: first, the separation of methyl vinyl ether (CH2CHOCH3), and then, the split-up of LiOCH3 from the LiOOH fragment. The corresponding trajectories can be found in the Supporting Information (Figures S5 and S6). As can be seen in Figure S5 the separation of CH2CHOCH3 from the LiOOH−LiOCH3 fragment exhibited a negligible barrier of 0.5 kcal/mol in vacuum which becomes slightly more pronounced (1.3 kcal/ mol) when the implicit solvent model was used. Overall this first process requires only ∼10 kcal/mol to be achieved.

calculated. As can be seen in Figure 2, the energetics of the separated fragments stabilized when the solvent is included. This electrostatic pathway can be found in the Supporting Information (Figure S2) and is represented in the reaction pathway as a dotted line instead of a continuous line that denotes a chemical reaction pathway. 3.2. Effect of Iodine in the Peroxide Bond Scission. It is known that an iodide ion can catalyze the decomposition of H 2 O 2 to O 2 and H 2 O through the formation of IO − intermediate species.38 Based on experimental observations it has been claimed that the LiOH formation proceeds through the decomposition of hydroperoxide intermediate promoted by iodide.20 However, nothing has been discussed about the possible origin of its catalytic activity. In recent works, the halogen bonding has been proposed as a possible source of catalytic activity in organic reactions.39 Next, we analyzed the effect of iodide in the decomposition of lithium hydroperoxide. The nucleophilic hydroperoxide moiety readily forms an ionic compound with LiI where the electropositive lithium ions interact with oxygen and iodine. The Coulombic interactions between fragments diminished in the presence of the solvent field. As shown in Figure 5, the Li−

Figure 5. Electrostatic minima between LiOOH and LiI and TS2; transition structures of the O−O bond scission within the LiOOH fragment (a) in solvent free conditions and (b) with continuum solvent.

O and the I−Li distances increase slightly for the SMD-M06L optimized geometries. The LiI···LiOOH intermediate has the iodine atom just in the middle of both lithium ions, located in the corner of a squared distorted structure similar to Li2O2. Once the iodine atom approaches oxygen, the O−O peroxide bonding breaks. In addition, it was detected that the O−I distance of 2.7 Å in the transition structure (Figure 5b, TS2) is shorter than the sum of the van der Waals radii (3.54 Å) reflecting a strong nucleophile−(iodide−)substrate interaction which in turn comprises the stability of the peroxide bond. This bimolecular nucleophilic substitution is driven by electrostatic lithium bonding interactions. As product of this reaction LiOI and LiOH are obtained which remain connected by electro712

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Figure 6. Minimum energy pathways for the reaction of Li2O2 and DME in kcal/mol. Relative energies of the gas phase and the SMD-M06L reaction pathways are shown in blue and black, respectively. The dotted lines link the electrostatic minima with the separated fragments; the electrostatic pathway is presented in the Supporting Information (Figures S5 and S6). The discontinuous orange lines show the barrier height. The energies of the reactants, lithium peroxide and dimethoxyethane, were taken as reference to calculate the relative energies.

Figure 7. Electrostatic minima between Li2O2 and DME (b, b′) and TS1 (in Figure 6); transition structures of hydrogen abstraction from DME by Li2O2 (c, c′). The Li2O2 isolated molecules were presented for reference (a, a′). Parts a, b, and c correspond to the gas-phase optimized structures while the primed labels (a′, b′, c′) are with continuum solvent.

By comparing both pictures presented in Figures 2 and 6 it could be said that lithium peroxide has a slight preference to interact with DME molecule; however the energetics of the

hydrogen abstraction step favored the reaction with the water molecule since its barrier is markedly smaller when compared to the DME pathway (4.5 vs 31.7 kcal/mol). The LiOOH 713

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product obtained for the Li2O2 + DME reaction could react in a similar way as discussed previously in Section 3.1 with LiI and lead to the formation of LiOI and LiOH. Up to this point the final discharge product LiOH could be formed through the reaction with DME but more easily and facilitated by kinetic factors with water through the active role of iodide enabling the peroxide bond scission. Nevertheless, the parasitic LiOI subproduct is formed, and if iodide plays a catalytic role it must regenerate back to the LiI compound. So, we explored a chemical pathway through quantum mechanical calculations whereby molecular oxygen is recovered, whereas lithium iodide is regenerated as presented below. 3.4. Reaction Paths That Connect the LiOH Formation with O2 and LiI. The regeneration routes of O2 and LiI were explored. The starting point for the mechanistic study was the parasitic LiOI product. It was proposed that once this subproduct is formed it could react with the remaining lithium hydroperoxide via a peroxide bond scission guided by iodide. The reaction pathway is depicted in Figure 8. LiOI and LiOOH formed a highly stable electrostatic compound. Then, the latter fragment suffered an O−O bond scission, which could be promoted presumably by a halogenbond-type interaction as can be seen in Figure 9. The iodine atom does not interact directly with the peroxide bond; it tends to attack the oxygen on which the lone pairs reside forming a halogen-type bonding interaction thus weakening the O−O bond and forming the LiOOI and LiOH species. These interacting fragments undergo several rearrangements, interestingly, forming a six-membered-ring stable intermediate (LiOOI−LiOH′ in Figure 8) which can distort to a LiOOI

Table 1. Relative Energies of Intermediates and Transition Structures Corresponding to the Hydrogen Abstraction from Dimethoxyethane (DME) by Li2O2 Calculated through Different Theoretical Models relative energy (kcal/mol) calculation details A B C D E F G H

a

Li2O2·DME

[Li2OO···H···DME(−H)]‡

−20.4 −19.0

20.7 31.3 27.9 26.8 22.6 33.4 22.4 22.1

−27.2 −35.1

a

(A) M06L/6-31+G(d,p) gas-phase calculations presented herein; (B) SMD-M06L/6-31+G(d,p) continuum solvent calculations presented herein; (C) B3LYP/6-311+G(2df,p) calculated energy barrier from (Li2O2)4 cluster in the singlet electronic state from ref 40; (D) MP2/6311+G(2df,p) calculated energy barrier from (Li2O2)4 cluster in the singlet electronic state from ref 40; (E) Superoxide terminated Li2O2 at optB88-vdW level of theory under periodic boundary conditions taken from ref 41; (F) Peroxide terminated Li2O2 at optB88-vdW level of theory under periodic boundary conditions taken from ref 41; (G) Superoxide terminated Li2O2 at optB88-vdW level of theory under periodic boundary conditions using a continuum solvent model under an electric field applied away from the surface taken from ref 41; (H) Superoxide terminated Li2O2 at optB88-vdW level of theory under periodic boundary conditions using an explicit solvent model under an electric field applied away from the surface taken from ref 41;

Figure 8. Minimum energy reaction profiles for the interaction of LiOI and LiOOH molecules in kcal/mol. Relative energies of the vacuum and the SMD-M06L reaction pathways are shown in blue and black, respectively. The dotted lines link the electrostatic minima with the separated fragments; the electrostatic pathway is presented in the Supporting Information (Figures S4 and S7). The discontinuous orange lines show the barrier height. The energies of the initial reactants, lithium peroxide, water, and lithium iodide (and LiOOH), were taken as reference for the relative energies. To preserve the stoichiometry of the reaction, the energy values of two LiOH fragments were added to all the species along the studied pathway while calculating the relative energies. 714

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Figure 9. LiOOH peroxide bond scission triggered by LiOI intermediate, transition structures from DFT calculations (TS1, Figure 8). (a) Vacuum optimized transition structure. (b) Analogue structure when an implicit solvation model was included.

planar squared structure after surpassing a barrier of 17.9 kcal/ mol (TS2, Figure 8) under the solvent field. Even more, a less favorable path that connects the parasitic iodine derived products with the LiOH compound was found. It is worth to mention that the chemical reactions and geometry rearrangements preceding LiOOI formation take place with energies below the energy of the reactants. Thus, LiOOI is detected as a secondary product that might form and be present in the electrolytic media when iodide is included in the electrolytic medium given that its subsequent decomposition requires a great energy. Indeed, SMD-M06L results show that the posterior oxygen molecule formation requires surpassing a barrier exceeding 50 kcal/mol (TS3, Figure 8) to get an electrostatic adduct between O2 and LiI through a moderate endergonic process. This result reflects the iodine atom preference to interact strongly with the oxygen nucleophilic sites. Moreover, the chemical route explains the regeneration of the lithium iodide catalytic additive and a chemical route that leads to the molecular oxygen regeneration, alternative to electrochemical pathways that might take place during the recharging process. The results presented herein revealed that lithium hydroxide can be formed through three possible chemical routes: (I) hydrogen abstraction from the water molecule by Li2O2, (II) hydrogen abstraction from the DME molecule by Li2O2, and (III) LiOOH decomposition triggered by the LiOI secondary product. However, the first one seems more plausible based on the calculated energy profiles. On the other hand, in the aprotic lithium−air battery containing water and LiI, the LiOH product must decompose during recharging cycles, and oxygen must be recovered. 3.5. LiOH Chemical Decomposition Pathway. Experimental determinations reported that, during discharge, when the water content increases, the presence of hydrogen peroxide is detected. Furthermore, this is accompanied by an increase in the amount of Li2O2 detected while LiOH decreases. Also, the former product has been claimed to be the only one that decomposes with iodide.20 H2O2 has been detected as a resultant product of the reaction between Li2O2 and water in Li−O 2 batteries with similar electrolytes (LiCF 3 SO 3 / TEGDME).42 Based on these findings, the reaction mechanism of LiOH and H2O2 was studied theoretically, and the results are presented next. For this interaction, it was found that just the reagent fragments are stabilized by the solvent field. In turn, the barrier involved in the hydrogen abstraction from the H2O2 exerted by the hydroxyl moiety increased in 15 kcal/mol when compared to the gas-phase calculated value (see TS1, Figure 10). Then, the reaction thrived to the lithium hydroperoxide and water

Figure 10. Lithium hydroxide decomposition pathway. Minimum energy paths for the reaction of LiOH and hydroperoxide. Relative energies (in kcal/mol) of gas phase and SMD-M06L reaction pathways are shown in blue and black, respectively. The dotted lines link the electrostatic adduct with the separated fragments. The discontinuous orange lines show the barrier height. The energies of the reactants, lithium peroxide and two water molecules, were taken as reference to calculate the relative energies and to preserve the stoichiometry of the reaction.

electrostatic adduct which then is separated in a similar procedure as aforementioned for other electrostatic minima to yield the LiOOH isolated product. Finally, the hydroperoxide can interact in a further step with LiOH through a shallow TS1 transition state (presented in Figure 2); this intermediate could evolve smoothly to the Li2O2 + H2O original reagents through an exergonic process. This arises by following the reverse pathway connecting TS1 in Figure 2, which is energetically accessible. This last mechanism presented herein (Figure 10) shows the relevance of the hydrogen peroxide formed in the electrolytic media since it proves the chemical reversibility of the formation−decomposition pathway of lithium hydroxide. Once the Li2O2 compound is regenerated, it can suffer posterior oxidation that leads to the recovery of molecular oxygen and metallic ions. In fact, LiOI and LiOOI were detected as the most energetically stable products along the studied reaction pathways linked to the LiOH formation. It is well-known that in aqueous, basic solutions hypoiodite (OI−) readily undergoes a disproportionation reaction:

3IO− ⇌ IO3− + 2I−

(9)

The presence of LiIO3 at voltages below 3.4 V has been previously reported.35 Thus, the studied reaction pathway suggests an alternative route that might explain the iodate formation. Concerning the O2 regeneration process, the proposed mechanism favors the conversion of LiOH to LiOOH via H2O2; otherwise results displayed in Figure 8 suggest that LiOH might react with LiOOI intermediate species leading to LiOOH and hypoiodite. An oxygen evolution reaction has been recently proposed involving LiOOH·H2O in the presence of I3− which may contribute to lowering the charging overpotential.43 It is worth to mention that if hydrogen peroxide is present in 715

DOI: 10.1021/acs.chemmater.7b04018 Chem. Mater. 2018, 30, 708−717

Article

Chemistry of Materials

Technologies of the U.S. Department of Energy under Contract DE-EE0006832 under the Advanced Battery Materials Research (BMR) Program. The authors acknowledge the computational resources provided by Texas A&M High Performance Research Computing and Laboratory for Molecular Simulation (LMS). A.E.T. gratefully acknowledges CONACYT for the postdoctoral scholarship (364788/ 245467).

the electrolytic medium, it might interact with hypoiodite ion (IO−) through the following reaction: H 2O2 +OI− ⇌ I− +H 2O + O2

(10)

In aqueous phase, the above-mentioned reaction takes place and could account for the oxygen recovery within the Li−O2 battery context. A general comment must be made in the case of the adducts found along the four explored reaction pathways. Given the electrostatic nature of the different adducts formed, they are susceptible to disaggregation by the application of an electric field which was not included here, but it might affect the energy needed to separate each fragment.

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4. CONCLUSIONS In the present work, the chemical basic interactions that help to rationalize the experimental detected products in the electrolytic media of the Li−O2 battery were studied theoretically using DME as solvent in the presence of both water and the additive LiI. Water was found as the most energetically accessible source of protons to obtain the precursors of LiOH given that the energy barriers to accomplish the Li2O2 decomposition exhibited meaningful lower values when compared to the DME path. They do not exceed 20 kcal/mol until LiOOI is formed. The subsequent reaction to regenerate oxygen seems prohibitive based on the results obtained with the theoretical model presented herein. The catalytic effect of iodide in the peroxide bond scission is shown to be possible due to a halogen-bond interaction between the iodine atom and an oxygen nucleophilic site which leads to the O−O bond breakage. Finally, the oxygen chemical recovering pathway involving hydrogen peroxide is an alternative and more plausible route than that having iodine-containing secondary products.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b04018. Thermodynamic properties, relaxed scan pathways from an electrostatic adduct to the separated fragments, and geometries of relevant intermediates and transition structures (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Perla B. Balbuena: 0000-0002-2358-3910 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle 716

DOI: 10.1021/acs.chemmater.7b04018 Chem. Mater. 2018, 30, 708−717

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