Research Article Cite This: ACS Appl. Mater. Interfaces 2017, 9, 36377-36384
www.acsami.org
Degradation Mechanism of Dimethyl Carbonate (DMC) Dissociation on the LiCoO2 Cathode Surface: A First-Principles Study Liyuan Huai, Zhenlian Chen,* and Jun Li* Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, People’s Republic of China S Supporting Information *
ABSTRACT: The degradation mechanism of dimethyl carbonate electrolyte dissociation on the (010) surfaces of LiCoO2 and delithiated Li1/3CoO2 were investigated by periodic density functional theory. The high-throughput Madelung matrix calculation was employed to screen possible Li1/3CoO2 supercells for models of the charged state at 4.5 V. The result shows that the Li1/3CoO2(010) surface presents much stronger attraction toward dimethyl carbonate molecule with the adsorption energy of −1.98 eV than the LiCoO2(010) surface does. The C−H bond scission is the most possible dissociation mechanism of dimethyl carbonate on both surfaces, whereas the C−O bond scission of carboxyl is unlikely to occur. The energy barrier for the C−H bond scission is slightly lower on Li1/3CoO2(010) surface. The kinetic analysis further shows that the reaction rate of the C−H bond scission is much higher than that of the C−O bond scission of methoxyl by a factor of about 103 on both surfaces in the temperature range of 283−333 K, indicating that the C−H bond scission is the exclusive dimethyl carbonate dissociation mechanism on the cycled LiCoO2(010) surface. This study provides the basis to understand and develop novel cathodes or electrolytes for improving the cathode−electrolyte interface. KEYWORDS: periodic density functional theory, Madelung matrix, dimethyl carbonate electrolyte dissociation, LiCoO2 surface, kinetic analysis chemistry composition evolved as a function of the potential.14 The kinetic study of side reactions by Chen et al.15 showed that the reaction rate had a strong dependence on the upper cutoff potential and a significant change of the reaction mode was found at high voltage of 4.5 V. Further, the oxygen released from the cathode material during the cycling was confirmed to enhance the reactivity of the electrolyte and even to form CO2,16,17 which varied in amounts as a function of the cycling conditions. Previous experimental works have concentrated on the studies of the formation, composition, and properties of side reactions on the CEI. It is also important to understand the surface property and its reactivity at the atomic level. However, the first-principles modeling of the CEI is often a challenge of the theoretical study of the surface and interface reaction, which requires atomistic models of the organic molecule−cathode surface. Only recently, calculations of ethylene carbonate (EC) dissociation emerged in the study of CEI on the oxide cathodes, including layered LiCoO2.5,18,19 However, no single molecular solvent can satisfy the many conflicting requirements for the electrolyte of LIB. Cyclic EC and linear carbonates, dimethyl
1. INTRODUCTION Rechargeable lithium-ion battery (LIB) has been widely recognized as one of the competing energy storage technologies due to its high energy and power densities at a reasonable cost.1,2 The degradation of cell performance has been observed and attributed largely to the surface deterioration of the electrodes since the invention of LIB. The cathode−electrolyte interface (CEI) plays a crucial role in the lithium-ion transfer and the health of battery at the charged states. The side reaction of electrolyte oxidation on CEI can result in serious problems in the battery life and safety, including capacity fading, reduced cycle life, gas generation, and loss of electrolyte.3−6 The presence of the CEI layer on Li1−xCoO2 (LCO) cathode material was first suggested by Goodenough et al.,7 and later numerous studies have explored its composition and origin.8 The formation of the carboxylate group O−CO on cycled Li1−xCoO2 was found by Kanamura et al. using in situ FTIR.9 Aurbach et al. reported the presence of semicarbonates ROCO2Li species on cycled Li1−xMO2 and Li1−xMn2O4 electrode surfaces, which were products of reactions between cathodes and carbonate molecules.10,11 The electrolyte degradation process was reported to depend on elevated temperatures and potentials by Thomas et al.12 and Lu et al.13 The in situ study of the CEI layer showed that the surface© 2017 American Chemical Society
Received: June 28, 2017 Accepted: September 29, 2017 Published: September 29, 2017 36377
DOI: 10.1021/acsami.7b09352 ACS Appl. Mater. Interfaces 2017, 9, 36377−36384
Research Article
ACS Applied Materials & Interfaces
the accuracy and the computational cost, we used the computationally less expensive cutoff energy of 400 eV in the present study. The conventional hexagonal cell with the space group of R3̅m was adopted for the initial structure of the layered oxide LiCoO2. The Brillouin zones were sampled with the Monkhorst−Pack k-points29 of 11 × 11 × 2 for bulk calculation. The optimized lattice parameters for LiCoO2 are 2.836 and 14.102 Å, respectively, which agree well with the previous theoretical (2.863 and 14.073 Å)30 and experimental (2.816 and 14.050 Å)31 results. To construct a proper surface model to investigate the dissociation mechanism of DMC, we considered three possible low index facets of (001), (110), and (010). The (001) surface exposing CoO6 with the O layer in the outermost surface is the most stable surface, but DMC only could be physically adsorbed. The calculations showed that the (010) surface is more stable than the (110) surface and the adsorption energy of DMC on the (010) surface is −1.17 eV, about 0.59 eV larger than that on the (110) surface. Therefore, the (010) surface was selected as the active model for the study of the side reaction of DMC dissociation, which would be the likely one worth attention in reducing its activation for the side reaction of DMC dissociation from this study. In the present study, we investigated the structural stability with three stacking models of O1, O2, and O3 for the delithiated phase of Li1/3CoO2. The results showed that the O3−Li1/3CoO2 structure is the most favorable structure, agreeing with the GGA result of Van der Ven et al.32 and consistent with early experiments.33 Therefore, the stable O3-type structure was used to investigate the DMC dissociation mechanism. Detailed information of the Li-vacancy configurations are compared for the three stacking models in the Supporting Information (SI) file. From the extended √3 × √3 × 1 supercell, the (010) surface of LiCoO2 or Li1/3CoO2 was constructed by a (1 × 3) supercell, which consisted of nine LCO layers with a vacuum thickness of 15 Å. The bottom six LCO core layers (simulating the bulk structure of LCO) were fixed, and the rest were allowed to relax during optimizations. Geometry optimizations were performed until the forces acting on all unconstrained ions were less than 0.05 eV/Å. Brillouin zones were sampled with a grid of 2 × 3 × 1 Monkhorst−Pack special k-points. The transition state (TS) was determined using the climbing nudged elastic band method.34,35 The harmonic vibrational frequency calculations were performed to characterize the nature of all reactants, products, and transition states. In this procedure, all of the surface layers were fixed and the adsorbates were allowed to relax. The reactant and product possess all real frequencies, and the transition state possesses a single imaginary frequency. All possible reaction pathways for each reaction were considered, but only the reaction path with the lowest energy barrier was reported. Zero-point energy (ZPE) corrections were considered for all calculations. The adsorption energy (Ead) for the adsorbate (DMC) was calculated on the basis of
carbonate (DMC) and ethyl methyl carbonate (EMC), are commonly used components in the mixing electrolyte solvents of the LIB due to their distinct molecular properties of dielectric constant, viscosity, and conductivity. It is well known that cyclic and linear organic molecules are often very different in their chemical properties; cyclic EC and linear DMC, EMC will contribute differently to the performance of LIB. To some degree, DMC is the most vulnerable component in the electrolytes due to its lowest boiling point and flash temperature among all of the often-used nonaqueous solvents. Previously calculated highest occupied molecular orbital energies for the single EC, EMC, or DMC electrolyte are −0.31005,20 −0.29905,20 and −0.2980 au (this work), respectively. The results indicated that DMC electrolyte is the easiest one to be oxidated on the cathode surface. Yet, there is no calculation on linear DMC available so far. Furthermore, the development of 5 V Li-ion chemistry demands new understanding of the CEI on high-voltage cathode surfaces, which need to resolve the difficulties not only in the construction of big supercell structures but also in the identification of a representative surface model from vast configurations of lithium-vacancy distribution associating with the high-voltage, charged phase. In the present study, the first-principles calculations and kinetic analysis were performed to study the adsorption and side reaction mechanisms of CH3OCOOCH3 (DMC) electrolyte dissociation on the (010) surfaces of LiCoO2 and delithiated Li1/3CoO2 (to simulate the charged state at 4.5 V). Because of the fundamental difference between cyclic and linear molecules, one should expect a different dissociation mechanism from that of DMC with respect to that of EC.5 Furthermore, this work will extend the pristine surface to a high-voltage delithiated surface, whose oxidation toward electrolyte becomes much more critical to 5 V Li-ion chemistry. To resolve the challenge in modeling charged structures, the high-throughput Madelung matrix calculation was employed to systematically evaluate possible Li1/3CoO2 supercells without loss of generality.21 This work aims to elucidate the degradation mechanism of DMC on the cathode surface and offer insights on the cathode−electrolyte interface in high-voltage domain, which may promote rational design of the interface to improve the performance, cycling stability, and calendar life of the battery.
2. COMPUTATIONAL DETAILS The Vienna ab initio simulation package was employed to perform all of the density functional theory (DFT) calculations.22,23 The electron−ion interactions were described by the projector-augmented-wave pseudopotentials.24,25 The exchange-correlation energies were calculated by generalized gradient approximation (GGA), with the function of Perdew− Burke−Ernzerhof (PBE).26 The PBE + U method was used to correct the self-interaction error for the transition metal Co, where the effective parameter U − J was 3.3 eV.27 Spinpolarized calculations were performed, and the plane wave cutoff energy of 400 eV was used for the expansion of wave functions. The effect of cutoff energy on the theoretical results was checked in the test calculations. We found that if the cutoff energy was increased to 550 eV, as in the previous studies,17,28 the adsorption energy of DMC and the reaction energy barrier changed slightly (
