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Exploring the redox decomposition of ethylene carbonate–propylene carbonate in Li-ion batteries

A small difference caused by a single methyl group between EC and PC could cause a large difference in lithium alkyl carbonates.


Introduction
Rechargeable batteries are among the most promising candidates to obtain high practical energy density for portable electronics, electric vehicles, and energy-storage devices, in the current, sustainable energy-based economy. 2 Electrolytes, as their main component, play an essential role in the critical properties of lithium-ion batteries (LIBs) and lithium-metal batteries (LMBs), such as safety, cycle-life, and power density. 3,4 Designing application-oriented electrolytes have become an efficient way to enhance the performance and the safety of LIBs and LMBs. 5,6 The oxidation stability and the reductive inertness are two essential criteria to design electrolytes for improving the efficiency of a battery system. Electrolytes, owning high oxidation potentials, such as sulfones, ionic liquids, nitriles, and fluorinated compounds, were mainly designed to fit the highvoltage electrode materials. [7][8][9][10][11] The as-desired high energy density and excellent safety can be achieved by using high voltage and non-flammable electrolytes. On the other hand, due to its low thermodynamic potential (0∼0.20 V vs. Li), Li/Li + is reactive toward most of the electrolytes, which induces the sacrificial decomposition of the electrolytes to form solidelectrolyte interphase (SEI) or cathode-electrolyte interphase (CEI). The inertness of electrolytes could only be reached through robust and stable SEI or CEI film, which prevents the decomposition of the electrolyte and guarantees electronically blocking and Li + -conductive. [12][13][14] Several efforts were focused on designing electrolytes with additives to form an SEI with enhanced properties, enabling long-term cycling. 12,15 Although many new electrolytes were experimentally developed for LIBs and LMBs, the design of an "ideal" electrolyte with afferent SEI systems is still lack of essential information. Even the most commonly used carbonate-based solvents, such as ethylene-or propylene carbonate (EC and PC), and their derivatives, their disparity in forming SEI on the graphitic anode of LIBs from electron-level is still not elucidated. 4,16,17 The efforts of developing new electrolytes need more investigations focusing on the mechanisms, to reduce the consumption of resources originating from semi-empirical trial-and-error. Although Borodin et al. [18][19][20] investigated the electrochemical properties of many electrolytes/solvents, direct and comprehensive EC and PC studies are rare.
In this work, we focused on the insights of the intrinsic disparity of EC and PC in LIBs, using an approach based on electron-level calculations. Fluoroethylene carbonate (FEC) with better oxidative stability and SEI formation capability was considered to highlight the difference between EC and PC (Fig.  1). The three carbonates own similar physical properties (dipole moments and dielectric constants in Table S1). The electrochemical properties of EC and PC with a minimal structural difference caused by a single methyl group were revealed. Moreover, we focused not only on single EC and PC molecule but also on their reductive products (lithium alkyl carbonate complexes) as the main SEI components. Current work is expected to guide the design of enhanced electrolyte and SEI systems in LIBs.  Table 1 Oxidation potentials (E ad ) and the difference between vertical (E vert ) and adiabatic (E ad ) oxidation potentials of solvates from density functional theory (DFT) calculations using M05-2X/6-31+G(d,p) method and SMD (water and acetone) implicit solvent model. The Li/Li + oxidation potential is 1.4 V. 19 The definition of E vert and E ad can be found in Supporting Information.

Computational method
Density functional theory (DFT) calculations were performed using the Gaussian 09 program package. 21 M05-2X density functional 22 was used to evaluate the oxidation potential due to its advantages in transferability and describing the localized hole in solvent oxidation. 23 The B3LYP/6-31G* 24-28 level of theory was chosen to investigate the pathway of the reduction reactions. The SMD implicit solvation model 29 using water and acetone parameters were also employed to calculate the oxidation potentials. The calculation details can be found in supporting information.

Results and discussion
Oxidation stability of electrolytes is the key feature of Li-ion batteries (LIBs). The cell voltage of common LIBs is ≈3.8 V. 30 The computed oxidation potentials of EC, PC, and FEC were above 6.5 V vs. Li/Li + (Table 1), which is higher than the working potential (3.8 V) of LIBs, also showing excellent oxidation stability. Specifically, the oxidation potentials of EC and FEC were slightly higher than that of PC. The experimental values (EC: 5.5-6.7 V; PC: 6.0-6.8 V), [31][32][33][34] being sensitive to the experimental conditions, may hide the small difference between the oxidation potential of EC and PC.
Besides the thermodynamic stability of electrolytes, the kinetic stability was also evaluated, based on Marcus electron transfer theory, using the difference between the adiabatic and vertical oxidation potentials (E vert -E ad ) (see details in Supporting Information). 19,20 Large (E vert -E ad ) value means a high energy barrier and a low oxidation reaction rate, which is beneficial for the oxidation stability of the electrolyte. [18][19][20] It was also found that PC was more stable than EC and FEC in the oxidation reactions from kinetics (E vert -E ad in Table 1), although PC owns a lower oxidation potential. Fluorine could enhance the oxidation potential (EC: 6.78 or 6.84 V; FEC: 7.21 or 7.45 V), but it had a weak effect on the dynamics of the oxidation process. The possible trade-off relationship between the oxidation potential and the (E vert -E ad ) was observed, also described by Borodin et al., 18,20 which points out that an attractive highvoltage electrolyte owns both high oxidation potential and increased (E vert -E ad ) value, which needs to break out the limitation of traditional materials system.
Other criteria for choosing proper electrolyte in LIBs was the reduction stability. The reduction potential of the solvent molecules mainly depends on the coordinated position of the Li + cations. It was found that the reduction potentials of EC, PC, and FEC are <1 V, except the formation of LiF in FEC (2.25 V). 20 Considering the relatively high working potential (3.8 V) of standard Li-ion batteries, it is easy to trigger the single electron reduction of EC, PC, and FEC. However, electrolytes' reduction is only the first step of the electrolyte decomposition to form the lithium alkyl carbonates as solid−electrolyte SEI in LIBs.
The decomposition of EC, PC and FEC corresponded to a regular ring-opening reaction, which usually requires high driving force (> 60 Kcal/mol, Fig. S2 and S3). We also found that the coordinated Li + cations could assist the opening of the fivemember ring. The energy barriers of ring-opening in Li + EC (TS-E1, Fig. 2), Li + PC (TS-P1, Fig. 3) and Li + FEC (TS-F1 and TS-F2, Fig.   Fig. 1 Chemical structures of EC, PC and FEC. Fig. 2 The calculated profile of free energy (ΔG) of EC decomposition assisted by Li ion. The hydration energy of the solvated electron in water was -1.63 eV. 1 The shade areas denote the radical carbon and oxygen. 4) were found to be 5.9, 7.7 and (9.5 and 6.5) Kcal/mol respectively. A bit higher energy (1.8 Kcal/mol) was required for the Li + PC complex, in comparison with the Li + EC complex. This difference can be attributed to the only methyl group in PC molecule. Also, it was more challenging to break the C-O bond, attached to the terminated F (9.5 Kcal/mol) than that which was far from the F atom (6.5 Kcal/mol) in FEC (Fig. 4). This also indicates that the terminated group in PC and FEC could modify the rigidity of the conformation and then the energy barrier of the ring-opening reactions.
The resulting intermediates (Int-E2, Int-P2, Int-P3, Int-F2, and Int-F3) owned unsaturated C atoms (pink area in Fig. 2-4). There were two possible ways to saturate these active sites: (i) the adjacent C atoms provided the empty orbitals to form the C=C double bond. This pathway required the C-O bond breakage, shown in TS-E2, TS-P2, TS-P3, TS-F3, TS-F4. The energy barrier for cutting the C-O bond is ~17 Kcal/mol in these intermediates. The products of LiCO 3anion and small molecules of ethene, propene, and fluorinated propylenebased agents were obtained; (ii) the active LiCO 3anion product of the step mentioned above could directly attract the unsaturated C atoms of Int-E2, Int-P2, Int-P3, Int-F2, and Int-F3 to form lithium alkyl carbonates (Fig. 5).
The small structural difference caused by a single methyl group between EC and PC could cause different lithium alkyl carbonates ( Fig. 5 and 6). The EC based lithium alkyl carbonate showed centrosymmetric conformation (Dihedral angle 1-2-3-4 = 180° of Product-E1 in Fig. 5a). Once one methyl substituent in PC appears, the main chain was distorted to fit the asymmetric conformation (Dihedral angle 1-2-3-4 = 64.4° of Product-P1 in Fig. 5b). As a comparison, the strong electron-withdrawing group substituent in FEC showed a similar effect as the EC on the distorted conformation for lithium alkyl carbonates (Dihedral angle 1-2-3-4 = 177.8° of Product-F1). This indicates that the geometry of the side termination can be the reason for the symmetry loss of lithium alkyl carbonate molecule (Fig. 5b).
Besides the difference of the molecule conformation, the EC-(Product-E1) and PC-based (Product-P1) lithium alkyl carbonates showed distinct electronic transition behaviour. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) can be responsible for the electronic transitions (Fig. 6). The HOMO of Product-E1 was delocalized on the whole organic part of the complex, particularly on the O atoms coordinated with Li. On the other hand, its LUMO was localized on the terminated Li atom sites. However, the molecule orbitals of Product-P1 presented the character of localization and asymmetry. The HOMO and LUMO of Product-P1 were localized on the two sides of the lithium alkyl carbonate, respectively, caused by one methyl substituent, present in the structure of PC. The molecule orbitals of Product-F1 also presented the same characters as the Product-P1 (Fig.  S4).
Molecule orbitals affected the chemical reactivity of the lithium alkyl carbonate complex. HOMO had the priority to provide electrons, while LUMO accepted the electrons. 35 The localized HOMO and LUMO pairs of Product-P1 (Fig. 6) and   Product-F1 (Fig. S4) provided two side sites for donating and withdrawing electrons. Thus, it was preferential to form more distorted conformation for a single Product-P1 (ring formation in Fig. S5). However, Product-E1 owned delocalized HOMO, and then the electron-donation sites were mainly distributed on the four O atoms of the two sides of the complex. The Li atoms, as the main LUMO distribution played the role of withdrawing electrons. Thus, it was preferential to form the dimer Product-E1 via the Li-O interactions between the two complexes (Fig. 7). Further, this linking mode increased the possibility of extending the dimer complex by coordinating more Product-E1 complexes and then forming a two-dimensional layered structure ( Fig. 7 and S6).
Experimental results 1, 17 already showed that EC-based electrolyte could form protective interphase on the anode of LIBs, while the reversible Li + intercalation/deintercalation was enabled. The Product-E1 complex owned the potential to form an ordered and continuous layered structure, which was a useful component of the protective SEI. The space and the terminated O atoms between adjacent Product-E1 complexes benefit from capturing and transporting Li + interaction. Also, the possible Product-E1 layer's flexibility could tolerate the volume change due to the intercalation/deintercalation of Li + , while one group substituent in PC would result in the symmetry loss of the lithium alkyl carbonate complexes. The small difference of one methyl group substituent in PC was further expanded when assembled many complexes and then contributed to the two interphase extremities between EC and PC-based electrolytes in LIBs.
The smaller difference (trans−cis) between trans-butylene carbonate (t-BC) and cis-butylene carbonate (c-BC) than that of EC-PC exhibited the similar relationship to the EC−PC mystery. 36,37 We found only a small difference (0.5 kcal/mol) between the energy barrier of the decomposition of single t-BC and c-BC molecule (Fig. S8). However, the dimerization of t-BC shows relatively lower energy barrier than that of c-BC (Fig. 8), which indicates that the former is preferable. The reported experimental results show that the dimerized products are primary components of a robust SEI film. 37,38 Current study indicates that the higher number of dimerized t-BC can be obtained, which may contribute to the reversible reaction of graphite anodes (as EC). Whereas fewer dimerized c-BC causes the exfoliation of graphite (as PC).
SEI was mainly composed of the inner (closer to the solid electrode) and an outer layer (closer to the liquid electrolyte), depending on the reduction state. 39 Lithium alkyl carbonate complexes were the nucleus of the outer less-reduced layer of SEI. As the component of the inner layer, fully reduced inorganic products were possible missing in EC and PC. While, the LiCO 3anion tended to attract Li + FEC to form the fully reduced inorganic product (LiF) as the nucleus of the inner layer of SEI (Fig. S7). It was also helpful to understand the phenomenon of fluorination, which can significantly improve the formation capability of SEI, while FEC can form a more robust and stable SEI film than that of EC. 1

Conclusion
In summary, we systematically examined the oxidation stability, the reduction reaction mechanisms, and the reduced complexes of EC and PC in LIBs, using the DFT calculation method. The oxidation of EC was easier to be driven than that of PC considering the kinetic stability, based on the fact that the difference of their thermodynamic stability is relatively small. During the reduction reactions, both of EC and PC generated active, less-reduced forms of organic intermediates and the LiCO 3anions, which could form symmetrical and distorted lithium alkyl carbonates. Analysis of molecule orbitals indicated that the EC-based complex owned delocalized electron distribution and preferred to form multi-complex assemble, which was the basis of a robust SEI film. While, the PC-based complex with localized electron distribution tended to form a ring by itself, taking away the opportunity of interacting with another complex. Fluorination could enhance the oxidation potential and improve the SEI formation capability by producing LiF. This electron-level understanding of the intrinsic disparity of EC−PC could help in the design of better electrolytes and interphases for application oriented battery setups.