First-Principles Study on the Initial Oxidative Decompositions of

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First-Principles Study on the Initial Oxidative Decompositions of Ethylene Carbonate on Layered Cathode Surfaces of Lithium Ion Batteries Xueping Qin, Perla B. Balbuena, and Minhua Shao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b02096 • Publication Date (Web): 24 May 2019 Downloaded from http://pubs.acs.org on May 25, 2019

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The Journal of Physical Chemistry

First-Principles Study on the Initial Oxidative Decompositions of Ethylene Carbonate on Layered Cathode Surfaces of Lithium Ion Batteries Xueping Qin1,2, Perla B. Balbuena2*, Minhua Shao1* 1Department

of Chemical and Biological Engineering, The Hong Kong University of Science and Technology, Hong Kong

2Department

of Chemical Engineering, Texas A&M University, College Station, TX, 77843, United States

Abstract Understanding the solvent decomposition mechanisms at the electrode-electrolyte interface is of great importance to mitigate capacity fading and improve cycling performance of batteries. Firstprinciples calculations were conducted to study the oxidative decomposition reactions of ethylene carbonate (EC) on the (110) surfaces of LiCoO2 (LCO) and LiNi1/3Co1/3Mn1/3O2 (NCM) in lithium ion batteries (LIBs). All the possible oxidative decomposition reaction steps of EC on cathode surfaces, including the H-abstraction reaction and ring-opening reactions caused by Cc-Oe and/or Ce-Oe bond cleavage, were analyzed from both thermodynamic and kinetic aspects. Our calculation results indicated that EC decompositions were initiated by the ring-opening reaction as the first step reaction on both LCO and NCM surfaces, which was caused by Cc-Oe bond cleavage

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with the activation energy of 0.96 eV and 0.57 eV, respectively. In the second step, the Habstraction reaction was prone to occur on LCO surfaces with the reaction barrier of 0.90 eV and reaction energy of -1.51 eV. However, the proton was much easier to be transferred from EC to NCM cathode surfaces with one small reaction barrier of 0.26 eV and the reaction energy (-1.64 eV) was similar to that on LCO. We concluded that the main oxidative decomposition reactions of EC on layered cathode surfaces were initiated by the ring-opening reaction, followed by proton transfer reaction contributing to the hydroxyl group (-OH) formation. Additionally, the Lireleasing behavior was observed on both cathode surfaces, which might contribute to the capacity fading of LCO and NCM. 1. Introduction Lithium ion batteries (LIBs) are currently used as the energy storage devices to power the electric vehicles (EV) and hybrid electric vehicles (HEV), normally including the transition metal oxides as cathodes and organic-solvent based liquid electrolytes.1-3 With the demand for lithium ion batteries increasing, high capacity and good cycle performance of batteries are required desperately. Increasing the operational voltage of LIBs could allow them to store quite a significant amount of energy. However, during the processes of lithiation and delithiation between the anode and cathode materials, the degradation reactions of organic-solvents based electrolytes would be accelerated by improving the operational voltage of batteries at both the anode-electrolyte interface and the cathode-electrolyte interface because of the instability of electrolyte components,4-6 leading to electrode degradation, capacity fade and poor cycling stability of batteries and also some safety issues.7 Thus, understanding the interfacial reaction mechanisms at the electrode-electrolyte interface (EEI) is of great importance to provide more effective strategies to help design interfacial layers in LIBs, alleviating capacity fading and improving the cycle performance of batteries.


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The commonly used electrolyte (LiPF6 dissolved in a mixture of ethylene carbonate and other linear carbonate solvents) is reduced below 0.8V at the anode-electrolyte interface and oxidized above 5.0 V vs. Li+/Li at the cathode-electrolyte interface, thus forming the solid electrolyte interphases (SEI).8-11 At the anode-electrolyte interface, including the graphite, silicon as well as the Li metal, the reductive decomposition pathways of EC-based electrolyte have been investigated intensively via theoretical simulations.12-17 However, EC-based electrolyte decomposition reactions at the cathode-electrolyte interface have been rarely studied,18-20 and the oxidative reaction pathways are still unclear. For the electrolyte decomposition reactions at the cathode-electrolyte interface, several experimental techniques including in situ differential electrochemical mass spectrometry (DEMS) and subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIR) have been conducted on layered cathode materials (including LiCoO2, LiNiO2, LiNi1/3Co1/3Mn1/3O2 and some Li-rich cathode materials) to detect the gas products of side reactions.21-25 To better understand the interfacial decomposition reactions, theoretical simulation has become one powerful technic to explore the reaction mechanism from the atomic level besides its wide applications in studying the properties of both electrode materials and SEI layers.26-29 Previous DFT-based simulations were also conducted that provided valuable insights in the kinetic aspects about the EC decomposition reactions on cathode surfaces in LIBs. Gauthier et al.30 proposed that initial ring-opening of cyclic carbonate could be triggered by nucleophilic attack of surface oxygen ions in the layered LiMO2 cathode. However, a direct proton abstraction reaction was predicted on delithiated Ni0.5Mn1.5O4 (NMO) spinel (100) and (111) surfaces by Borodin et al.18 EC decomposition reactions on spinel material LiMn2O4 (100) and (111) surfaces were simulated by Leung’s group, including both the ring-opening reaction and proton transfer reaction.7,19 They


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concluded that the reaction steps were surface dependent and also related to the state of charge (Li content). Tebbe et al.20 modelled the EC ring opening reaction on LiCoO2 (101̅4) surface, which could be activated by cathode surface, PF6- anions as well as other EC solvents. Another recently published paper by Xu et al.31 simulated the EC decomposition reaction on Li(Ni, Mn, Co)O2 (101̅4) surfaces, and the simulation results showed that the first decomposition step of EC was the ring-opening reaction initiated by Cc-Oe bond cleavage. Such a ring-opening reaction of EC with a tiny reaction barrier (only 17 meV on bare cathode surface) might occur once in contact with liquid electrolytes, and cannot be the rate-limiting step in the whole decomposition reaction scheme of the electrolyte. The authors discussed that a possible rate-limiting step may exist in the following decomposition reactions where the ring-opened EC might be attacked by PF6- anions or other EC solvent molecules. More recently, Giordano et al.32 employed DFT simulations to compare four different reactions of EC energetically on both layered (LixMO2) and rocksalt (MO) oxide surfaces, and the simulation results indicate that the transition metal ion in oxides could be used as the chemical reactivity descriptor of oxide-electrolyte interfacial reactions. The surfaces of late transition metal oxides were prone to be reduced by the EC decomposition reaction, and their Fermi level was closer to O 2p band center. Most of these above theoretical simulations at cathode-electrolyte interface focused on the first decomposition step of EC on cathode surfaces, and more comprehensive oxidative decomposition pathways, including both the first and following reaction steps, are still much less known. While Li’s group33 explored all the possible oxidative decomposition pathways of EC in LIBs, these simulations were conducted in the absence of cathode surfaces, which should have significant influence on the electrolyte decompositions. In this work, we explored both the first and second decomposition steps of EC on the LiCoO2 (LCO) and LiNi1/3Co1/3Mn1/3O2 (NCM) (110) surfaces using first-principles DFT-based


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simulations. Previous publications focused on EC decompositions on spinel cathode materials like LiMnO2 and LiNi0.5Mn1.5O4. Our simulations were focused on two commonly commercialized layered cathode materials LCO and NCM, which would be more meaningful. Since LCO and NCM have a similar structure, we believe that a systematic theoretical study on the initial decompositions of EC on both surfaces could contribute to the understanding of general decomposition pathways. These findings could also help to design more complex electrolyte systems to investigate the passivation layer formation and gas by-products. For all possible reactions in initial and following decomposition steps, both the reaction energies and activation energies were calculated and analyzed. We found that the EC oxidative decompositions on layered cathode surfaces (both LCO and NCM) were initiated by the Cc-Oe cleavage reaction, boosting the following proton transfer reactions. By comparing the reaction barriers on LCO and NCM, EC decompositions are much easier on NCM cathode surfaces especially the second H-abstraction reaction. Additionally, both the Li-releasing and -OH formation phenomena were observed on LCO and NCM, and the released Li ions could contribute to the capacity fading of cathode materials by being involved in the passivation layer formation. 2. Computational details DFT Calculations. Theoretical calculations were performed using density functional theory (DFT) by the Vienna Ab Initio Simulation Package (VASP) code.34 The projector augmented wave method (PAW)35,36 pseudopotentials were used for all the atoms in the models, and the wave functions for valence electrons were expanded in a plane wave basis set with 500 eV as the kinetic energy cutoff. The valance electron configurations for atoms in this study were 1s1 for H, 1s22s1 for Li, 2s22p2 for C, 2s22p4 for O, 3d84s1 for Co, 3p64s13d6 for Mn, and 3p64s13d9 for Ni. The generalized gradient approximation (GGA-PW91)37 was applied to the transition-metal cathode


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materials to evaluate exchange-correlation energy with the Hubbard U correction,38,39 which could guarantee more accurate electrochemical as well as d-orbital electronic properties. The U values for Co, Ni, and Mn (4.91 eV, 6.7 eV and 4.64 eV, respectively) were selected from previous reports.40-42 Spin-polarization was considered in all calculations. The convergence criteria in the structure relaxations for the electronic self-consistent iterations and the ionic relaxation loops were set to 10−5 eV and 0.02 eV Å−1, respectively. Van der Waals interactions were considered by using the vdW-DF2 function proposed by Klimes.43,44 The LCO cathode bulk was fully optimized (Figure S1a), and NCM bulk was modelled by the (3 x 3) supercell, corresponding to 27 replications of the reduced primitive LiNi1/3Co1/3Mn1/3O2 cell (Figure S1b). After optimization, the lattice parameters with the ratio of c/a = 4.909 (a = 2.94 Å and c = 14.42 Å) and c/a = 4.983 (a = 2.92 Å and c = 14.55 Å) for LCO and NCM respectively, were in good agreement with the experimentally examined values.45-49 In the DFT simulations of electrolyte decomposition reactions on cathode surfaces, both the minimum energy path (MEP) and transition state (TS) could be searched by conducting the climbing image nudged elastic band (CINEB) method.50-52 Reaction barriers were defined as the energy difference between the saddle point (representing the TS) and the reactant. Bader charge analysis was performed to analyze the charge for atoms in the optimized structures along the simulated reaction pathways.53,54 All the optimized structures and charge density difference diagrams were carried out with the VESTA software.55 Slab models. For layered cathode materials, (110) and (104) surfaces were studied as the most stable surfaces with the lowest surface formation energies.56-58 For both LCO and NCM, two common commercialized cathode materials for LIBs with quite similar structures, (110) surface was selected to explore the EC oxidative decomposition mechanisms with the objective of trying


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to deepen the understanding of electrolyte decomposition reaction mechanism on layered materials with one certain facet. Due to the computation limitations, the slab thickness convergence study must be conducted in the models, thus slabs with different atomic layers were considered, including three layers with 108 atoms, four layers with 144 atoms and five layers with 180 atoms. The corresponding surface energies of the three surface models were calculated and at least four layers were selected, which was enough for the surface simulations by providing the convergence of the surface energy within 0.004 meV/Å2. Here, five-layer LCO (Figure 1) and four-layer NCM were used for slab simulations (Figure S2). All the atoms in the top three layers were fully relaxed and the rest were fixed to the bulk atom positions representing the bulk cathode materials. The extra 16 Å vacuum slab was added in the z direction of all slab models to avoid the spurious interactions between the adsorbate molecules and the backside of periodic slabs. Γ-point Brillouin zone sampling was used in the slab calculations, and 2 x 2 x1 sampling was also included in the convergence tests. The corresponding results of slab energy calculations indicated that Γ-point Brillouin zone sampling in large supercells of slab models guaranteed convergence of total energy within 0.03 meV/atom.

Figure 1. Slab model of LCO (110) surfaces. Li atoms are represented in green, Co in dark blue, and O in red, respectively.


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3. Results and discussion 3.1. EC adsorptions on layered cathode (110) surfaces Before studying the decomposition mechanisms of solvent molecules, the adsorption structure was examined by screening the possible sites for EC on LCO and NCM surfaces. Three kinds of adsorption orientations (including parallel, edge-on and up adsorptions) and different active sites in each adsorption orientation were considered, resulting in 5 and 21 possible adsorption structures on LCO (110) and NCM (110), respectively. The optimized structures were shown in Figure S3 and Figure S4, and corresponding adsorption energies were calculated and summarized in Table S1 and S2 (see ESI†). Figure 2a showed the most stable adsorption structure of EC on LCO (110). In this parallel adsorption model, the binding energy was -1.26 eV with the formation of Oc-Lisurf (1.98 Å) bond between EC and LCO surfaces. For EC adsorption on NCM surfaces, the dashedblack triangle in Figure 2b connecting two Ni and one Mn on the cathode surface represented the most active adsorption site (denoted as 2NiMn site), and the corresponding stable adsorption structure was shown in Figure 2c. The binding energy of EC on the NCM (110) surfaces was -1.83 eV and one bond (Oc-Mnsurf) was formed between the cathode surface and the EC solvent molecule with the bond length of 2.19 Å. Comparing the adsorption structures of EC on these two layered cathode surfaces, NCM showed stronger affinity to EC molecules with the larger binding energy of -1.83 eV due to the existence of Mn and Ni as the surface transition metal species, which might contribute to the following oxidative decompositions of solvents.


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Figure 2. (a) EC adsorption on LCO (110); (b) the most active adsorption site of EC on NCM surface (dashed-black triangle): 2NiMn site. EC will adsorb on the NCM cathode surface within this triangle region in parallel orientation; (c) the most stable adsorption structure of EC on NCM (110) surfaces. The carbonyl oxygen (Oc) in EC and Li on the LCO cathode surface (Lisurf) are shown in Figure 2a; similarly, Mn on NCM surface (Mnsurf) is shown in Figure 2c. Li atoms are represented in green, Ni in dark grey, Co in dark blue, Mn in purple, O in red, C in dark brown and H in white, respectively. 3.2. Initial decomposition reaction pathways of EC on layered cathode (110) surfaces Since NCM and LCO have very similar structures, for clarity here we present the simulation models of EC decompositions on NCM surfaces and more structures on LCO could be found in supporting information (see ESI†). For the first decomposition step of EC on cathode surfaces, four possible reactions were proposed based on the analyses of atomic structures, charge density distribution as well as previous theoretical simulations, as shown in Figure 3. The middle structure in Figure 3 was the most stable adsorption configuration of EC on the cathode surface with the largest adsorption energy corresponding to Figure 2c, where Cc, Oc, Ce, and Oe represent the carbonyl carbon, carbonyl oxygen, ethylene carbon and ethylene oxygen in the adsorbed EC molecule, respectively. All the proposed reaction pathways included one proton abstraction reaction (Figure 3a)7,18,19 and three ring-opening reactions (Figure 3b, 3c, 3d)7,30,59. Figure 3a showed that one proton was transferred from adsorbed EC to the oxygen on the cathode surface,


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forming a hydroxyl group. The ring-opening reaction of EC was initiated by Ce-Oe bond cleavage (Figure 3b) and the different Cc-Oe cleavages led to the rest two ring-opening reactions (Figure 3c and 3d). Both Figure 3c and 3d showed that Cc bonded to the oxygen on cathode surface (Osurf) and Oe bonded to the nickel atom (Nisurf) after the cleavage of Cc-Oe bond. The difference between the products of Cc-Oe cleavage (1) (Figure 3c) and Cc-Oe cleavage (2) (Figure 3d) is that Nisurf and Osurf is bonded with a bond length of 2.2 Å in the former while there is a larger distance of 3.86 Å between Nisurf and Osurf in the Cc-Oe cleavage (2) product.

Figure 3. All four possible reaction pathways for the first step of EC oxidative decompositions on the NCM (110) surface. The middle structure is the stable optimized adsorption configuration of EC on cathode (110) surface, and the other four structures represent different possible reaction mechanisms: (a) proton abstraction reaction; (b) ring-opening reaction due to the Ce-Oe bond cleavage; (c) ring-opening reaction due to the Cc-Oe bond cleavage (1); (d) ring-opening reaction due to the Cc-Oe bond cleavage (2). The names of specific atoms in EC are carbonyl carbon (Cc), carbonyl oxygen (Oc), ethylene carbon (Ce) and ethylene oxygen (Oe), as denoted in the middle adsorption structure. The oxygen atom (Osurf) and Ni atom (Nisurf) on the NCM cathode surface are shown in 3c. Atomic color representations are the same as those in Figure 2.


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Reaction energies (ΔE) and reaction barriers (Barriers) were calculated separately to compare the four reaction pathways as the first step of EC decompositions from both thermodynamic and kinetic aspects. The reaction energy was calculated by the energy difference between the product and the reactant, and the reaction barrier was defined as the energy difference between the transition state (TS) and the reactant. During TS searching in the initial decomposition of EC on NCM surfaces, CINEB method was used to explore the minimum energy path (MEP) and calculate the activation energy. Figure 4 showed the energy curves during the TS searching along the different reaction pathways (as indicated in Figure 3) and the corresponding structures including reactant, TS and product along the MEP are shown in Figure S5. Apparently, the Cc-Oe bond cleavage (2) in Figure 4d had the smallest reaction barrier among four reactions. To be more specific, both the reaction energies and reaction barriers for the four proposed pathways were presented in Figure 5 by inserting the corresponding reaction products (data were summarized in Table 1). Reactions with negative ΔE are exothermic, which are the energetically favorable, while the positive ΔE means endothermic reactions. As we can see clearly from Figure 5, the Habstraction reaction of all four pathways was the only endothermic step with a reaction energy of 0.42 eV and the corresponding energy barrier was 1.13 eV, indicating that the proton transfer between the adsorbed EC and the cathode surface was unfavorable both thermodynamically and kinetically. Conversely, the three ring-opening reaction pathways caused by C-O bond cleavage were all exothermic and Cc-Oe cleavage (2) was the most favorable one with the lowest activation energy of only 0.57 eV, which is predicted as the first decomposition step of EC on cathode NCM (110) surfaces.


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Figure 4. Energy curves along the reaction pathways during the transition state searching: (a) Habstraction; (b) Ce-Oe bond cleavage; (c) Cc-Oe bond cleavage (1); (d) Cc-Oe bond cleavage (2).

Figure 5. Reaction energies (ΔE) and reaction barriers (Barriers) for four possible reaction pathways as the first decomposition step for EC on NCM (110) surfaces. The corresponding products for different pathways are inserted in the figure. The reaction energies are denoted as


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black filled circles. The reaction barriers are denoted as red empty circles. Atomic color representations are the same as those in Figure 2. Table 1. The reaction energies (ΔE) and activation energies (Barriers) for all four possible initial reaction pathways.

1st step

Habstraction

Ce-Oe cleavage

Cc-Oe cleavage (1)

Cc-Oe cleavage (2)

ΔE (eV)

0.42

-1.02

-0.65

-0.70

Barriers (eV)

1.13

2.72

0.80

0.57

The product of the first ring-opening reaction caused by Cc-Oe cleavage (2) step is shown in Figure 6a and 6b from two different views. It was interesting that one Li ion (denoted as Li1 in Figure 6a and 6b) was pulled out of the cathode surface and bonded to three O atoms, two of which belonged to the ring-opened EC species shown in Figure 6b. These three Li1-O bonds had lengths of 1.83 Å, 1.88 Å and 2.00 Å, respectively, which were formed mainly due to the strong attractive interactions between O atoms and Li1 atom. One similar Li-releasing behavior was also reported by Ogata’s group.59 These authors carried out the ab initio molecular dynamics (AIMD) simulations to investigate the EC decomposition reactions on LiCoO2 (110) surfaces. They found that first decomposition step of EC was also the Cc-Oe cleavage reaction. However, Xu et al.31 did not find Li release in the decomposition reactions of EC on NCM (101̅4) surfaces, although the first step of EC decomposition was also the Cc-Oe broken ring-opening reaction, in agreement with our simulation results to some extent. Similarly, such a Li-releasing phenomenon was not mentioned in the simulation of EC degradation on LiCoO2 (101̅4) surfaces conducted by Musgrave’s group.20 Additionally, taking EC decompositions on LCO (110) surfaces into


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consideration as we will discuss in the following section, the almost same behavior of releasing Li was observed in the first step reaction (Figure S7c), and there is only slight difference in the bond length of three O-Li bonds (1.82 Å, 1.91 Å and 1.96 Å). We concluded that the Li-releasing behavior during the EC decompositions on layered cathode surfaces (LCO and NCM) was facetdependent and not material-dependent, and this released Li was involved in the initial decomposition reaction of EC, which might contribute to the Li consumptions even in the condition of the electrolyte in contact with the cathode surface at open circuit voltage. Consequently, the released Li ions could be involved in the formation of passivation layers, degrading the cathode materials and reducing the battery capacity.

Figure 6. Structure of the reaction product after the Cc-Oe cleavage (2) as the first reaction step of EC decomposition on the NCM (110) surface from two view directions in 6a and 6b; charge density difference distribution of the adsorption structure of EC on the NCM (110) surface in 6c. Li1 in Figure 6a and 6b is denoted as the released Li ion. Black dashed lines in Figure 6b show these three O-Li bonds. The blue dashed lines in Figure 6c represent the possible atomic interactions between adsorbed EC and cathode surfaces. In Figure 6c, the cyan and yellow represent the electron-depletion and electron-rich regions, respectively. Atomic color representations are the same as those in Figure 2.


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Additionally, static DFT calculations were conducted to analyze the charge density distribution of the intact adsorption structure, as shown in Figure 6c. It was clear that the charge density was redistributed to a large extent at the interface between EC and the NCM (110) surface. Once EC was adsorbed on the cathode surface, the Oc-Mnsurf bond formed (Figure 2c) due to the strong electrostatic interactions between Oc and Mnsurf, where the electron density was largely increased as indicated by the yellow region in Figure 6c. Meanwhile, the charge density was reduced on the Cc-Oc bond, and this is the electron-depletion region represented by the cyan colored area. Consequently, Cc had a quite large tendency to be easily attacked by the Osurf atom, leading to the Cc-Oe bond breaking and almost simultaneous Oe-Nisurf bond formation (Figure 6a and 6b). Since there was an obvious charge redistribution in the adsorbed EC on NCM (110) surfaces, Bader charge analysis was conducted to analyze the charge transfer between the EC molecule and NCM cathode surfaces before and after the ring-opening reaction. The charge analysis result was shown in Table S3. By comparing the total electron numbers of EC molecule in the reactant (34.06 e-) and product (34.06 e-), we found that no charge transfer occurred from the cathode to electrolyte during the initial ring-opening reaction. Combining the above analysis results, including the reaction energies (ΔE = -0.7 eV) and activation energies (energy barrier = 0.57 eV) shown in Table 1, we concluded that initial ring-opening reaction caused by Cc-Oe cleavage (2) was the first decomposition step of EC on NCM (110) surfaces. Such one fast reaction happens once the electrolyte solution becomes in contact with the cathode surface and it is independent on the operation voltage of batteries. 3.3. Following decomposition reaction pathways of EC on the NCM (110) surface Ring-opened EC species adsorbed on the cathode surface after the Cc-Oe scission in the first decomposition step, and the following possible decomposition reactions were proposed and


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simulated, including H-abstraction and Ce-Oe cleavage. More details about structure models (including reactants, TS and products) and reaction energy curves in the 2nd step reaction pathways could be found in Figure S6. Both reaction energies and activation energies of the 2nd steps were summarized in Table 2. As shown in Table 2, an exothermic H-abstraction reaction with a small reaction barrier of only 0.26 eV indicated that the ring-opening reaction of EC in the first step largely facilitated the subsequent proton transfer from EC to the NCM surface, leading to the hydroxyl group formation. However, the formation of epoxy ethane caused by the 2nd Ce-Oe cleavage following the initial Cc-Oe scission was unlikely to occur on the cathode surface due to the quite large activation energy (2.69 eV) and endothermic reaction energy (1.43 eV). Table 2. The reaction energies (ΔE) and activation energies (Barriers) for the 2nd step predicted reaction pathways. Ce-Oe cleavage

2nd step

H-abstraction

ΔE (eV)

-1.64

1.43

Barriers (eV)

0.26

2.69

(epoxy ethane)

The reaction energy profile of the H-abstraction reaction as the second step was shown in Figure 7. After the ring opening reaction in the initial decomposition step, one proton was quite close to the oxygen on the cathode surface at a distance of 2.45 Å, as shown in the reactant structure in Figure 7. The transition state (TS) was searched corresponding to the image 2 along the reaction path, where the transferred proton had very similar distance to adsorbed EC and cathode surfaces (1.3 Å and 1.4 Å, respectively). Based on the calculation results in Table 2, this proton transfer reaction was both energetically and kinetically favorable once the C-O bond cleavage in the initial decomposition reaction, and the hydroxyl group formed in the product shown in Figure 7. Bader


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charge analysis results (Table S4) indicated that 1.45 electrons were transferred from adsorbed EC to NCM surfaces along with the proton transfer, which exactly followed the so-called protoncoupled electron transfer mechanism (PCET). This PCET mechanism is a well-known class of reactions and has been reported recently by several research groups.60-63 More recently, the Han group60 investigated the EC decomposition on the spinel LiMn2O4 cathode surface by AIMD simulations. But according to their simulation results, only the proton transfer without ringopening step was found and regarded as the initial decomposition step for EC on Li2MnO4 (111) surfaces.

Figure 7. Reaction energy profile of H-abstraction reaction as the second step of EC oxidative decomposition reactions on cathode NCM (110) surfaces was shown below. Three structures (top) were presented along the reaction path, including the reactant (image 0), TS (image2) and product (image 6). The black dashed line in the top structures shows the distances between the transferred proton and oxygen on the cathode surface as well as the ethylene carbon in the ring-opened EC.


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3.4. Comparing the decomposition reaction pathways of EC on pristine NCM and LCO (110) surfaces As we mentioned before, another layered cathode material, LiCoO2 (110) was also included for the investigation of the EC decomposition mechanism. All possible decomposition steps were simulated (see ESI†). Similarly, the oxidative decomposition of EC on LCO (110) was also initiated by Cc-Oe cleavage with the activation energy of 0.96 eV, which was much smaller compared to the H-abstraction (1.88 eV) and the Ce-Oe cleavage (2.09 eV). For the second step after the EC ring-opening reaction on LCO (110), only the H-abstraction reaction was simulated due to the impossibility of epoxy ethane formation on cathode surfaces33. Unlike the quick proton transfer (Ebarrier = 0.26 eV) from EC to NCM cathode surfaces, the H-abstraction reaction for ringopened EC on LCO (110) resulted in a high energy barrier of 0.90 eV, indicating the proton could still be transferred to LCO forming the hydroxyl group (-OH) but more difficult compared that on NCM surfaces. Comparing the structures of reactant and product during H-abstraction on both cathode surfaces, we found that the product was relatively more stable on NCM with a higher reaction energy of -1.64 eV than that on LCO (-1.51 eV) and the distance between H and Osurf in reactant was shorter on NCM (2.4 Å) than LCO (2.6 Å), indicating larger tendency of following H transfer on the former cathode surfaces. Therefore, the difference of activation energies for subsequent H-abstraction step on NCM and LCO could be mainly ascribed to the different ringopened structures after the initial decomposition as well as the product stability after proton transfer. All the reaction barriers for the possible decomposition steps of EC on NCM and LCO (110) were summarized in Table 3, and the same oxidative decomposition pathway for EC on both layered cathode surfaces was found. On NCM and LCO surfaces, EC decompositions were both initiated by Cc-Oe bond cleavage and the following H-abstraction reaction occurred. This ring-


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opening reaction by the C-O bond breaking largely facilitated the subsequent H-abstraction reaction by decreasing the reaction barrier from 1.13 eV to 0.26 eV on NCM surfaces (from 1.88 eV to 0.90 eV on LCO surfaces). Table 3. Comparison of the energies barriers (eV) for predicted reaction pathways (including 1st and 2nd steps) on NCM and LCO surfaces. 1st step

2nd step

Habstraction

Ce-Oe cleavage

Cc-Oe cleavage (1)

Cc-Oe cleavage (2)

Habstraction

NCM

1.13

2.72

0.80

0.57

0.26

LCO

1.88

2.09

/*

0.96

0.90

*One thing to be noticed here is that Cc-Oe cleavage (1) was not found on LCO as the 1st step reaction, and the transition state could be located. During the initial ring-opening reaction and following H-abstraction reaction of EC on layered cathode surfaces, two critical phenomena were observed, including releasing Li along the C-O cleavage (Figure 6) and producing the hydroxyl group (-OH) after proton transfer (Figure 7). Interestingly, Li-releasing behavior could be recovered during the proton transfer process with the formation of -OH on both NCM and LCO surfaces (Figure 7 and Figure S8). On NCM surfaces, the H-abstraction reaction could happen very quickly with a small reaction barrier of 0.26 eV and the released Li ion could reinsert into the layered cathode material, protecting the Li from dissolving into the electrolyte and being consumed. However, the proton transfer from ring-opened EC to LCO surfaces was much more difficult than that on NCM surfaces with the reaction barrier of 0.90 eV, which makes the Li reinsertion slow and challenging. Therefore, the Li-releasing was more severe on LCO compared to the NCM cathodes, and these Li ions could be involved in the formation of passivation layers due to the existence of other solvent molecules or EC fragments, degrading the cathode materials and reducing the battery capacity finally. Meanwhile, for EC on


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NCM surfaces, the decomposition reactions were easier and -OH species was formed quickly. The hydroxyl group might weaken the transition metal-oxygen (TM-O) bonds and induce the transition metal dissolution from cathode materials to electrolyte solutions64,65, which has been experimentally observed on NCM and Li-rich NCM cathodes 66-68. 3.5. Effect of de-lithiated cathode surfaces on the initial decompositions of EC During the cycling of lithium ion batteries, the high state of charge of batteries could greatly influence the electrolyte decomposition reactions. Hence, the initial decomposition mechanisms of EC on de-lithiated LCO and NCM were also investigated. The de-lithiated LCO and NCM models were shown in Figure S9, and the most stable EC adsorption structures were screened. On the de-lithiated cathode surfaces, both ring-opening reactions and H-abstraction reactions were considered, and it was found that H-abstraction reactions were favorable on both LCO and NCM with the reaction energies of -3.10 eV and -3.45 eV (Table S5), respectively. These values are consistent with literatures.18,69 The kinetic barriers were calculated to be 0.98 eV and 0.48 eV for de-lithiated LCO and NCM, respectively. By comparing the reaction barriers of H-abstraction on pristine LCO (1.88 eV) and NCM (1.13 eV), one may conclude that the high state of charge (eg. de-lithiated state) could contribute to more severe electrolyte decompositions by lowering the activation energies. For de-lithiated cathode slabs, it was obvious that LCO could maintain the ordered layer structure, while the Co atom in NCM has the displacement leading to the structure rearrangement. One more interesting phenomenon was that the surface oxygen in NCM came out of the top layer with the presence of dangling bonds, which might trigger the proton transfer and lead to the Mn dissolution from cathode materials.64,65,68


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In summary, EC follows the same decomposition mechanism consisting of the initial ringopening and the subsequent H-abstraction reactions on both pristine LCO and NCM surfaces. NCM could induce the faster and easier decomposition reactions of solvent molecules with small reaction barriers and the produced -OH could lead to transition metal dissolutions. On LCO surfaces, the second H-abstraction reaction has almost the same barrier (0.90 eV) as the initial ring-opening reaction and the released Li tends to be consumed or migrates into the electrolyte, resulting in capacity fading. While on de-lithiated LCO and NCM surfaces, the H-abstraction reaction was favorable from both thermodynamic and kinetic aspects. It is expected that other electrolyte components including the lithium salt (LiPF6) and other solvents (dimethyl carbonate and dimethoxyethane), and even the dissolved transition metal ions could contribute to further decomposition reactions, leading to the cathode-electrolyte interphase (CEI) formation and gas products. Our simulation results may not be adequate to explain the formation mechanism of CEI and gas by-products. Here we mainly focused on the initial decomposition reactions of EC on cathode surfaces aiming to understand the reaction mechanisms on both layered LCO and NCM. The initial decomposition pathways of solvent molecules are very important to study the following formation of CEI and gas evolutions. We believe that the theoretical explanations for finally formed CEI or gas by-products need to introduce the lithium salt and more solvent molecules or even additives. Indeed, several simulations were tested to check if the CO2 could be produced when one EC molecule was adsorbed on both perfect and de-lithiated cathode surfaces, but it turned out the reaction was very difficult to converge to locate the transition state. That means the simulation system needs to be updated by introducing more complex electrolyte system based on the initial decomposition mechanisms in this work, which will be addressed in our future publications. 4. Conclusions


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Using first-principles DFT calculations, we investigated the thermodynamics and kinetics of the oxidative decomposition reaction mechanisms of EC on layered cathode materials NCM (110) and LCO (110) surfaces. Our simulation results indicate that the decomposition reactions of EC on both cathode surfaces are initiated by Cc-Oe cleavage (ring-opening reaction) and followed by transferring one proton to cathode (H-abstraction reaction), forming an hydroxyl group by bonding to oxygen ions on (110) surfaces. However, EC decomposition reactions are relatively easier on NCM (110) than those on LCO (110) surfaces by comparing the activation energies of both the first ring-opening reaction and subsequent H-abstraction reaction (0.57 eV and 0.26 eV for NCM; 0.96 eV and 0.90 eV for LCO). Both the Li-releasing behavior and hydroxyl group formation were observed on both layered cathode surfaces, and released Li could reinsert into cathode materials after H-abstraction reaction. These released Li atoms on LCO surfaces have large tendency to be consumed by other solvent molecules or fragments before the 2nd step proton transfer, while Habstraction reaction on NCM surfaces happens very fast with the small barrier of 0.26 eV, leading to the -OH formation. The Li releasing and -OH formation might provide some explanations to the capacity fading of LCO and transition metal dissolution phenomenon on NCM, respectively. Our findings are expected to offer additional insights into the understanding of electrolyte decomposition mechanism at the cathode-electrolyte interface in Li-ion batteries. Associated Content Supporting Information Available Bulk structures of layered cathode materials; slab model of NCM; EC adsorption models and adsorption energies on LCO and NCM surfaces; models including reactants, transition states and products along reaction pathways of EC on LCO and NCM; energy curves and Bader charge analysis results.


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Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] ORCID Xueping Qin: 0000-0001-5895-4972 Perla B. Balbuena: 0000-0002-2358-3910 Minhua Shao: 0000-0003-4496-0057 Notes The authors declare no competing financial interest. Acknowledgements The authors acknowledge the support from Research Grant Council (26206115) and Innovation and Technology Fund (ITS/161/16FP) of the Hong Kong Special Administrative Region. X. Qin thanks the financial support of Overseas Research Awards from The Hong Kong University of Science and Technology. National Supercomputer Center in Guang Zhou (Tianhe-2) and High Performance Computer Center in HKUST are acknowledged. References (1) Goodenough, J. B.; Park, K.-S. The Li-Ion Rechargeable Battery: A Perspective. J. Am. Chem. Soc. 2013, 135, 1167-1176. (2) Tarascon, J.-M.; Armand, M. In Materials for Sustainable Energy, p 171-179. (3) Scrosati, B.; Garche, J. Lithium Batteries: Status, Prospects and Future. J. Power Sources 2010, 195, 2419-2430. (4) Aurbach, D.; Gofer, Y.; Langzam, J. The Correlation Between Surface Chemistry, Surface Morphology, and Cycling Efficiency of Lithium Electrodes in a Few Polar Aprotic Systems. J. Electrochem. Soc. 1989, 136, 3198-3205. (5) Etacheri, V.; Marom, R.; Elazari, R.; Salitra, G.; Aurbach, D. Challenges in the Development of Advanced Li-Ion Batteries: A Review. Energy Environ. Sci. 2011, 4, 3243-3262. (6) Gabano, J.-P. Lithium batteries. London and New York, Academic Press, 1983, 467 p. (7) Leung, K. First-Principles Modeling of the Initial Stages of Organic Solvent Decomposition on LixMn2O4(100) Surfaces. J. Phys. Chem. C 2012, 116, 9852-9861.


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First-Principles Study on the Initial Oxidative Decompositions of

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