# Quantum Chemical Characteristics of Additives That Enable the Use of Propylene Carbonate-Based Electrolytes

Authors: Jaeho Lee; Chaewon Kim; Young-Kyu Han
Journal: International Journal of Energy Research
 (2023)

---

## Abstract

Propylene carbonate- (PC-) based electrolytes are gaining attention as electrolytes in next-generation batteries because of their high stability and excellent temperature characteristics at high voltages. Lithium-ion batteries using PC-based electrolytes with 3-methyl-1,4,2-dioxazol-5-one (MDO) showed excellent capacity retention and lifetime characteristics. Here, quantum chemical methods are used to examine the molecular characteristics of MDO, and they suggest the unique molecular properties of this additive. Our calculations reveal that MDO is reduced prior to ethylene carbonate (EC) and PC solvents and undergoes a remarkably fast reduction decomposition process while producing thermodynamically stable reduction reaction products compared to vinylene carbonate (VC) and fluoroethylene carbonate (FEC) additives. This implies that a thermodynamically stable solid-electrolyte interphase (SEI) can form on the anode surface through a very rapid reaction. Upon reduction, the most preferred thermodynamic reaction between MDO and PC forms Li2CO3, a major SEI component. These reaction characteristics are unique and not observed with VC or FEC. The binding energy with Li+ is lower for MDO than for VC, FEC, or the solvents, making MDO the best choice for desolvation. We demonstrate that the molecular characteristics derived from quantum chemical calculations for MDO can also be applied to various previously reported PC-based electrolyte additives.


---

## Body

## 1. Introduction

The demand for lithium-ion batteries (LIBs) with high capacity and long life is increasing with the growth of the market for various portable electronic devices and electric vehicles [1–3]. This demand is now leading to the active development of different cathode and anode materials for next-generation high-capacity batteries, as well as electrolyte additives that improve the performance and cost-effectiveness of LIBs [4, 5]. Electrolyte additives play different roles, but additives that can form a solid-electrolyte interphase (SEI) on the surface of the anode are being actively studied as they can improve battery lifespan characteristics and stability and prevent electrolyte decomposition [6–10]. The most commonly used electrolytes are ethylene carbonate- (EC-) based types (cyclic carbonate and acyclic carbonate (dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate)) and lithium hexafluorophosphate, but these electrolytes have some limitations because battery stability is degraded due to electrolyte decomposition at a cut-off potential<4.7V vs. Li/Li+ as the temperature increases [11–17]. EC has a high melting point (36.4°C) that imparts low-temperature performance; therefore, a cosolvent, such as dimethyl carbonate (4°C), ethyl methyl carbonate (−14.5°C), or diethyl carbonate (−74.3°C), is used to ensure the electrolyte temperature window. However, these carbonates have low flash points and high volatility, and linear carbonates have a disadvantage in terms of electrolyte stability.Propylene carbonate (PC) has been proposed as a promising alternative electrolyte that can overcome these limitations based on its low cost and excellent temperature characteristics compared to EC (liquid temperature range: −48.8°C–242.0°C) and its tolerance to high voltage [18–20]. However, when used alone, a major limitation of PC is that it exhibits very low battery lifespan characteristics due to graphite exfoliation caused by solvent cointercalation [21–25]. Many approaches have been studied to solve the problems encountered when using pure PC electrolyte, including the introduction of a cosolvent [26–30], molecular engineering to replace PC functional groups [31], and the use of additives [29, 32–40]. Among these strategies, additive use appears to be critical for solving these problems with PC, as additives could simply be added without a need to change the frame of the existing secondary battery system. Vinylene carbonate (VC) [6, 10] and fluoroethylene carbonate (FEC) [25–28], widely used as SEI-forming additives, are known to be ineffective in PC-based electrolytes to improve battery lifespan characteristics. VC cannot be used for high-voltage cathode materials due to its limited oxidation stability [30]. FEC has high oxidation stability, but its use as a sole additive is not practical due to gas generation that arises with increases in the temperature of the battery. He et al. and Kim et al. investigated an FEC-based electrolyte at elevated temperatures [41, 42]. They revealed that HF and acid compounds formed by FEC decomposition cause significant dissolution of transition metal ions into the electrolyte. Rodriguez et al. showed that PC/FEC-based electrolytes cause considerable active material loss when gas generation occurs [43].Li et al. reported that a battery with 3 wt% prop-1-ene-1,3-sultone (PES) added to 1.0 M LiPF6/PC:EMC (1 : 1 by weight) electrolyte showed a capacity loss of only 15% after 50 cycles [44]. Park et al. reported that a battery with 0.5 wt% 1,3-propane sultone (1,3-PS) as a sultone additive to 1.0 M LiPF6/PC:DEC (1 : 1 by volume) showed no capacity loss after 45 cycles [45]. Chen et al. suggested butylene sulfite (BS) as an additive for a pure PC electrolyte battery. A battery with 1.0 M LiClO4/PC:BS (95 : 5 by volume) as the electrolyte showed good reversibility after 20 cycles [34]. Another study reported that a battery with 1,3,2-dioxathiolane 2,2-dioxide (DTD) as an additive to 1.0 M LiPF6 in a pure PC electrolyte showed excellent capacity retention ability until 150 cycles [46]. A battery with 2 wt% vinyl ethylene carbonate (VEC) added to 1.0 M LiPF6 dissolved in pure PC showed 84.4% capacity retention after 100 cycles [25]. Thus far, 3-methyl-1,4,2-dioxazol-5-one (MDO) has shown the best performance as an additive. Röser et al. reported that a battery with 2 wt% MDO added to 1.0 M LiPF6 in a pure PC electrolyte showed 80% capacity retention after more than 450 cycles [38]. The MDO additive prevented graphite exfoliation at the anode by forming an effective SEI at the anode-electrolyte interface. However, the development of film-forming additives for implementation in a pure PC electrolyte has been limited to trial-and-error tests based on the researchers’ experience. In this paper, we asked whether additives might have unique molecular characteristics that would facilitate their use in PC-based electrolytes. To the best of our knowledge, no research has been conducted on the unique molecular properties of additives optimized for use with PC-based electrolytes. We surmised that if these intrinsic properties are understood, this would provide important guidelines for researchers attempting to design new additives for EC-free PC-based electrolytes.In the present study, the molecular properties of MDO were determined using quantum level theory and compared using EC and PC as solvents and VC and FEC as additives. In terms of reduction, MDO is more rapidly reduced than EC or PC and produces reduction reaction products that are more thermodynamically stable than those produced by VC and FEC, but it has a similar oxidation stability to that of PC. Upon reduction, MDO undergoes a rapid reduction decomposition process from a kinetic perspective. After reductive decomposition, MDO forms Li2CO3, which is an important SEI component, and this formation reaction is thermodynamically preferred. These reaction characteristics are not observed with VC or FEC. Of the five molecules examined, MDO has the lowest binding energy with Li+, making MDO most favorable in terms of desolvation. We also show that the results obtained by quantum calculations for MDO can be applied to prop-1-ene-1,3-sultone (PES) [44], 1,3-propane sultone (1,3-PS) [45, 47], butylene sulfite (BS) [34], 1,3,2-dioxathiolane 2,2-dioxide (DTD) [36, 46], and vinyl ethylene carbonate (VEC) [25, 29, 47], which have also been reported as potential PC-based electrolyte additives. All these SEI-forming additives we investigated are known to effectively form SEI at the electrolyte/anode interface through multistep chemical reactions including reductive decomposition to prevent PC cointercalation into the graphite anode and to consequently prevent exfoliation.


## 2. Methods

An extensive computational study based on the Kohn-Sham density functional theory (DFT) was performed to obtain the optimized stationary point and transition states of MDO. The molecular structures were optimized with no symmetry constraints using the B3LYP functional [48, 49] and 6-311G++(d,p) basis set implemented in the Gaussian16 software package [50]. The conductor-like polarizable continuum model, including nonelectrostatic terms, was used to model the solvent effect on the molecular structure and electrical properties [51, 52]. We chose a dielectric constant ε of 20.0, which has been widely used to interpret the experimental results of secondary battery electrolytes [53–59]. Frequency calculations were performed with the same functional and basis sets used for the geometry optimization to confirm the transition states and local minima. The Gibbs free energy was thermally corrected at 298.15 K and 1 atm for all molecular properties. The Gibbs free energy obtained through vibrational frequency calculation was calculated as Ɛ0+Gcorr, the sum of the electronic and thermal free energies, where Ɛ0 is the total electronic energy and Gcorr is the correction of the Gibbs free energy due to internal energy. Gcorr was obtained as Hcorr−TStot, where Hcorr is the correction to the enthalpy due to internal energy, T is the temperature, and Stot is total entropy by St+Sr+Sv+Se (St, entropy due to translation; Sr, entropy due to rotational motion; Sv, entropy due to vibrational motion; and Se, entropy due to electronic motion).The oxidation potential and reduction potentials of molecule M (E°oxidation (Mol) and E°reduction (Mol), respectively), were evaluated from the absolute oxidation and reduction of molecule M relative to an electron at rest in vacuum, with a subsequent subtraction of 1.37 V to convert them to the commonly used Li/Li+ potential scale (Figure 1).
(1)E°oxidationMol=ΔGoxMol+ΔGsolvMol+–ΔGsolvMolF−1.37V,E°reductionMol=−ΔGredMol+ΔGsolvMol−–ΔGsolvMolF−1.37V,where the ΔGox/redMol value is the difference in free energy between a neutral molecule in a gas state and a molecule after oxidation and reduction. ΔGsolvMol+, ΔGsolvMol−, and ΔGsolvMol are the solvation free energies of the cation, anion, and neutral molecules, respectively, calculated by the polarized continuum model, and F is the Faraday constant [60].Figure 1
Free energy cycle for calculating the oxidation/reduction potential. Mol denotes the molecule of interest.


## 3. Results and Discussion

Figure2(a) shows the structures of EC, PC, MDO, VC, and FEC, while Figure 2(b) shows the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for each structure. The most distinctive feature of MDO, a pentagonal ring-shaped cyclic carbonate, is its structural similarity to PC and VC. MDO has one methyl group as in PC and a double bond as in VC. In the HOMO and LUMO distribution lobes, MDO is identical to PC in terms of its electron distribution asymmetry. Still, overall, it shows a much more similar distribution to VC in terms of the location of the HOMO and LUMO.Figure 2
(a) Two-dimensional structures of the solvents and additives. (b) Highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) lobes of EC, PC, MDO, VC, and FEC.
(a)(b)The calculation results in Table1 compare the reduction potential (RP) of Li+(Mol) and the oxidation potential (OP) of (Mol)PF6−; the optimized structures are shown in Figures S1 and S2, respectively. When calculating the redox potential, better accuracy can be obtained by taking the salt into account [60–62]. The RP value is higher for MDO (1.05 V) than for either EC or PC or for additives VC or FEC, suggesting that MDO has superior reduction molecular characteristics. A material that has a low OP can contribute more easily to cathode-electrolyte interphase formation, whereas a high RP value means that an anode-electrolyte interphase is more easily formed. The OP and RP calculations consider the ionization potential and electron affinity, as well as the solvent effect, thereby providing a good qualitative description of the experimental oxidation and reduction potentials. The OP results for the five molecules indicated that FEC with an F element with a high electronegativity has the highest OP (6.52 V), in agreement with the experimental result that FEC has high oxidation stability at the cathode [41]. VC shows the lowest OP value of 6.06 V, which is consistent with its limited application in high-voltage cathodes because of its low oxidation stability [63]. Notably, MDO has a higher OP value than VC and is very similar to PC, indicating that it is unlikely to cause a serious problem at the cathode. In the experimental study by Röser et al., 1.0 M LiPF6 in PC + 2 wt% VC showed that the oxidative decomposition reaction proceeded at ~4.8 V, whereas 1.0 M LiPF6 in PC + 2 wt% MDO was more stable up to ~5.3 V [38]. These results are qualitatively consistent with our calculation results that the OP of MDO is higher than that of VC and can be interpreted as a result of the introduction of N element with high electronegativity.Table 1
Reduction potentials of Li+(Mol) (RP, in V vs. Li/Li+) and oxidation potentials of (Mol)PF6- (OP, in V vs. Li/Li+).
ECPCMDOVCFECRP0.520.471.050.840.85OP6.386.236.236.066.52Figure3 shows the OP calculation results for molecules in which the methyl group and the N element of MDO are substituted with H and C-H, respectively. When the methyl group was substituted with H, the OP value of MDO was lowered by 0.06 V. Interestingly, the OP value of the MDO was lowered by 0.91 V in the case of a molecule in which the N element was substituted with C-H. These results indicate that the oxidation stability of MDO is dominated by the introduction of the N element. As the HOMO of MDO is distributed mainly in the C=N bond (Figure 2(b)), the introduction of the N element is interpreted to play a key role in the greatly enhanced oxidation stability [64–66].Figure 3
Oxidation potentials (OP) of methyl and N atom-substituted MDO (OP, in V vs. Li/Li+).Figure4 shows the reductive decomposition reactions of the five molecules (EC, PC, MDO, VC, and FEC). To understand the structure and energy changes of the five molecules upon reduction, the thermodynamic Gibbs free energies were calculated by optimizing the structure step by step. Only the most energetically favorable reaction path is shown in Figure 4, whereas the structure and energy information for the other reaction paths we considered are shown in Figure S3. The pathways that produce the most stable ring-opened structures after reduction are highlighted with blue lines. The reduction potential results show that the reduction of MDO is the most energetically favorable among the five molecules (step 1→2). Interestingly, the reaction energy of the ring opening (step 2→4) after reduction for MDO is −51.53 kcal/mol, indicating that MDO is the most stable of the five molecules. The ring-opening transition-state energy is also very low (2.61 kcal/mol), implying that this reaction can occur rapidly in terms of kinetics. MDO is also very advantageous in terms of receiving electrons from the anode, as the reductive ring-opening reaction is also energetically favorable and even the reaction rate is extremely high. A large π∗ N–O antibonding orbital population would be reflected in a weak N–O bond [67, 68]. For these reasons, we also performed a natural bond orbital (NBO) analysis.Figure 4
One-electron reductive decomposition reaction energies (ΔG in kcal/mol) and process for (a) ethylene carbonate (EC), (b) propylene carbonate (PC), (c) 3-methyl-1,4,2-dioxazol-5-one (MDO), (d) vinylene carbonate (VC), and (e) fluoroethylene carbonate (FEC). Partial breaks of the bonds of the structures are indicated by dashed lines, with bond lengths shown in Å. The imaginary frequency presented in italic is for transition state (TS).Figure5 shows that the O4–N6 bonding occupancy of MDO has the lowest value and the antibonding occupancy has the highest value. This means that the bond property of N–O appears weaker than the other bonds of FEC (O9–C2) and VC (O7–C3), and it explains the reason for the low energy barrier of the N–O dissociation in the transition-state calculation. In addition, the N–O bond can be easily dissociated by electron reduction, which is interpreted as a repulsion between the lone pair of both the N and O atoms in the N–O bond [69, 70].Figure 5
Natural bond orbital (NBO) analysis: (a) Li(MDO), (b) Li(FEC), and (c) Li(VC).
(a)(b)(c)However, our calculations show that FEC and VC have higher transition states than the solvents. In particular, the very high transition-state energy barrier of VC can be interpreted as electron delocalization in the ring [71]. The Gibbs free energy (step 2→5) of the products of the five molecules after one-electron reduction for EC (−32.04), PC (−32.65), MDO (−51.53), VC (−13.68), and FEC (−28.18 kcal/mol) reveals that MDO eventually forms anionic radicals that are much more stable thermodynamically compared to VC, FEC, or the solvents.We computed and compared the binding energies with Li+ for the five molecules. Generally, SEI-forming additives have a lower binding energy than solvents. A lower binding energy means better desolvation on the anode surface. Since desolvation is the most energy-consuming step in the electrolyte, it requires less energy while Li+ is intercalating into graphite anode, and it is beneficial for the movement of Li+ from the electrolyte to the electrode [72–76]. PC has the strongest binding but creates a problem because the PC molecules are inserted into the graphite after solvation with Li+ and cause exfoliation of the graphite [77]. The insertion of PC into the graphite has also been linked to the strong binding interaction of PC with Li+ [78]. Our binding energy calculation shows that PC (45.3) > EC (43.7) > VC (39.3) ≈ FEC (39.3) > MDO (35.8 kcal/mol), and MDO has a favorable molecular characteristic at the anode side. The rate-determining step in the Li-ion transfer process at a graphitic anode relates to the stripping of Li+ from the solvation sheath, and the desolvation process is directly correlated to the binding energy of the coordinate bond between Li+ and solvents/additives [73, 79]. Our calculation results imply that the overall Li+ migration process from the electrolyte to the graphite anode may proceed faster for the MDO additive than for the other additives due to the lower binding energy that favors desolvation.When MDO, VC, and FEC are used as small additives, the interaction with solvent PC present in large quantities in the LIB system should be considered. Figure6 shows the calculation results for the main reactions for the bonding between the radicals generated after the reduction decomposition reactions of MDO, VC, and FEC, and the PC is used as an electrolyte solvent in the reduction process. For MDO, path A is the most preferred, and the main characteristic of this reaction is that it forms Li2CO3. However, VC and FEC were confirmed to have a more stable reaction path B than the reaction forming Li2CO3 (path A). Generally, an SEI has various components, but Li2CO3, which has high stability and conductivity and is insoluble, appears more helpful for performance improvement than species such as R-O-Li and ROCO2Li in a metastable state [80]. Wu et al. reported the Li2CO3/LiF heterostructure SEI shares the fast Li+ transport kinetics of Li2CO3 and the good electron insulator capability of LiF [81]. Zhang et al. showed that the contact between Li2CO3 and LiF can accelerate space charge accumulation along their interfaces, and this generates a higher ionic carrier concentration and significantly enhances Li+ transport, while decreasing electron leakage [82]. The 1.0 M LiPF6 in the PC + 2 wt% MDO experiment showed increases in the O 1s and C 1s peaks of the XPS survey spectrum of the harvested graphite electrode after first charge/discharge because of the formation of inorganic carbonate, such as Li2CO3. The 1.0 M LiPF6 in the PC + 2 wt% MDO experiment showed increases in the O 1s and C 1s peaks of the XPS survey spectrum of the harvested graphite electrode after first charge/discharge because of the formation of inorganic carbonate, such as Li2CO3. Exfoliation of the graphite anode, which was charged/discharged for only one cycle in the 1.0 M LiPF6 in the PC + 2 wt% FEC, was observed in the broken-up structure, as shown in the top-view SEM micrograph image. The Coulombic efficiency in the first cycle amounted to only 35.8%, 40.7%, and 86.3% for cells containing VC, FEC, and MDO electrolyte additives, respectively [38]. We performed an AIMD calculation of 5 ps to describe solvation sheaths with MDO additives in PC-based electrolytes (Figure S4). MDO dissociated into acetonitrile and carbonate after a ring-opening reaction at 1.5 ps. The carbonate, which is the product of the ring-opening reaction, binds to Li-ion in two coordinates. These results support the calculation results shown in Figure 4(c), which indicate the low energy barrier of MDO, and Figure 6(a), which shows the preference for Li2CO3 formation after combining with PC.Figure 6
Termination reaction energies (ΔG in kcal/mol) of the radical anion (a) MDO, (b) vinylene carbonate (VC), and (c) fluoroethylene carbonate (FEC) in propylene carbonate- (PC-) based electrolytes.
(a)(b)(c)Various molecular-level characteristics of MDO, an additive of PC-based LIBs, have been observed from a quantum chemical perspective. We examined the molecular-level characteristics of various previously reported PC-based additives with improved performance.Figure7 shows the structures of several additives, and their electrochemical characteristics are shown in Table 2. The common characteristics of various PC-based additives, including MDO, are higher RP values than PC and, interestingly, higher RP values than VC currently commercialized in LIBs. Furthermore, due to their higher OP values than VC, all additives are characterized by excellent oxidation stability. In other words, the PC-based additives will cause various chemical reactions at the anode-electrolyte interface based on their higher oxidation stability than VC and their superior reduction ability than PC.Figure 7
Two-dimensional structures of propylene carbonate- (PC-) based electrolyte additives.Table 2
Reduction potentials of Li+(Mol) (RP, in V vs. Li/Li+), oxidation potentials of (Mol)PF6− (OP, in V vs. Li/Li+), binding energy with Li+(in kcal/mol), and experimental cycle numbers for various additives. The optimized structures are summarized in Figures S6 and S7.
RPOPBinding energyExp. cycle numberPES1.086.4640.450 (ref. [44])1,3-PS2.246.3141.450 (ref. [45])BS2.306.2843.020 (ref. [34])DTD2.796.4835.6150 (ref. [46])VEC2.586.1244.4100 (ref. [25])MDO1.056.2335.8450 (ref. [38])VC0.846.0639.3—FEC0.856.5239.3—PC0.476.2345.3—We calculated the decomposition pathways of the PC-based additives to understand the chemical reactions of the additives upon reduction. Figure8 shows the reductive decomposition energy profiles of PES, 1,3-PS, BS, DTD, and VEC additives and shows the results for MDO, VC, and PC for comparison. The optimized structures are listed in Figure S5. Previously, we suggested a fast ring-opening reaction because MDO showed excellent performance in PC-based systems. Similarly, the fast ring-opening reactions occurred for PES, 1,3-PS, BS, DTD, and VEC. Thermodynamically, the five additives form much more stable reduction decomposition products compared to PC and VC, with MDO being the most stable. This characteristic seems to be related to the excellent cycle performance of the battery using MDO as additive, as shown in Table 2. Comparison of the binding energies of the five additives and Li+ in Table 2 shows that all five have lower binding energies than PC. In particular, MDO and DTD are the lowest. These additives are likely to facilitate desolvation at the anode, and this appears to be a common feature of PC-based additives.Figure 8
Reductive decomposition energy profiles of PC-based additives.Finally, when analyzing the chemical interactions between the reduction products of these additives and the PC solvent (see Figure9), path A is clearly preferable for all five additives in the chemical reaction with PC as the electrolyte solvent. Additives containing sulfur have, in common, a preferred reaction that forms Li2SO3, while a reaction pathway that forms Li2CO3 is preferred for VEC. The lithium sulfite (Li2SO3) film improves the electrochemical performance when coated on the surface of an anode electrode [34]. Beheshti et al. reported that the additives containing sulfur form Li2SO3 and lithium alkyl sulfonates (ROSO2Li) as SEI components. SEIs composed of these components have a dense and stable interface that prevents degradation and capacity loss [83]. The Li2SO3 as component of the anode-electrolyte interphase is believed to be beneficial for ionic conductivity [84]. Of course, alkyl sulfide is a component of SEI on the surface of graphite anodes [85], but we draw attention here to the finding that the additives that enable PC-based electrolytes have molecular characteristics that favor the reaction pathway (path A) that commonly forms inorganic materials, such as Li2CO3 and Li2SO3.Figure 9
Termination paths of ring-opened additives (a) prop-1-ene-1,3-sultone (PES), (b) 1,3-propane sultone (1,3-PS), (c) butylene sulfite (BS), (d) 1,3,2-dioxathiolane 2,2-dioxide (DTD), and (e) vinyl ethylene carbonate (VEC) in propylene carbonate- (PC-) based electrolytes.
(a)(b)(c)(d)(e)


## 4. Conclusions

This study performed a quantum chemical examination of the molecular characteristics of an MDO additive that showed excellent performance in PC-based electrolytes. The fundamental molecular characteristics of MDO were (1) high reduction thermodynamic reactivity compared to VC and FEC as well as solvents, (2) high oxidation potential compared to VC, (3) very high reduction kinetic reactivity due to a very low reaction barrier, (4) low binding energy with Li+ compared to solvents, and (5) formation of Li2CO3 as the most favorable reaction between anion radicals and solvent PC.Countless candidate materials are available for the development of organic molecular additive materials. Screening research aimed at understanding the molecular characteristics and exploring the possibility of candidate materials before experiments and molecular design research based on computational chemistry are critical for maximizing the efficiency of the material development process. We identified five molecular characteristics that are also present in other reported PC-based additives. We propose that these molecular characteristics can be used as guidelines for researchers who are designing new materials and screening candidates as additives for high-voltage and high-performance PC-based electrolytes.



---

*Source: 6346995-2023-03-17.xml*