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: dimethyl carbonate electrolyte dissociation; kinetic analysis; LiCoO2 surface; Madelung matrix; periodic density functional theory;

•NCM conserves high structural reversibility without phase changes up to 4.8 V.•NCM shows clear and stable Co3+/Co4+ couples between 4.5–5.0 V for cycles.•Li+ diffuses like in a thin layer in deep delithiated states.•The poor inter-cluster conductivity may result in a 10% capacity lose in electrodes.Understanding the structure properties in deep delithiated states and electrochemical kinetics in the high potential window of LiNi1/3Co1/3Mn1/3O2 (NCM) cathode materials is essential to advance their performance in rechargeable lithium-ion batteries for 5 V chemistry. Here we report a layered single-phase NCM showing great structural reversibility and without H3 phase formation when charged up to 4.8 V at least for 5 cycles. However, the poor cluster scale conductivity results in inactive material in thick electrode during cycling, approximately 10% (w/w), may corresponding to the reduced capacity respective to thin electrode. The Co3+/Co4+ redox peaks are found clear and stable for cycles, lying between 4.5 ∼ 5.0 V. That is tightly correlated to the fast Li-ion kinetic, i.e., the electrochemical behaviour of the NCM material is not limited by Li+ diffusion but behaving like in a thin layer. That is very different from previously reports on charge transfer mechanisms of cathode materials for lithium-ion batteries.Stable dual-redox pairs and thin-layer liked kinetics benefit NCM cathode for 5 V lithium chemistry.Download high-res image (115KB)Download full-size image

Electrochemical cycling stabilities were compared for undoped and Al/Co dual-doped spinel LiMn2O4 synthesized by solid state reactions. We observed the suppression of particle fracture in Al/Co dual-doped LiMn2O4 during charge/discharge cycling and its distinguishable particle morphology with respect to the undoped material. Systematic first-principles calculations were performed on undoped, Al or Co single-doped, and Al/Co dual-doped LiMn2O4 to investigate their structural differences at the atomistic level. We reveal that while Jahn–Teller distortion associated with the Mn3+O6 octahedron is the origin of the lattice strain, the networking — i.e. the distribution of mixed valence Mn ions — is much more important to release the lattice strain, and thus to alleviating particle cracking. The calculations showed that the lattice mismatching between Li+ intercalation and deintercalation of LiMn2O4 can be significantly reduced by dual-doping, and therefore also the volumetric shrinkage during delithiation. This may account for the near disappearance of cracks on the surface of Al/Co–LiMn2O4 after 350 cycles, while some obvious cracks have developed in undoped LiMn2O4 at similar particle size even after 50 cycles. Correspondingly, Al/Co dual-doped LiMn2O4 showed a good cycling stability with a capacity retention of 84.1% after 350 cycles at a rate of 1C, 8% higher than the undoped phase.

LiNi1/3Co1/3Mn1/3O2, LiMn2O4 and LiCoO2 are paired to make the blended materials for the cathode of lithium-ion batteries. The factors impacting on the characteristics of blended materials are studied using constant current charge/discharge measurement and electrochemical impedance spectroscopy. The results show that the three pairs of blended materials exhibit very different synergetic effects in high C-rate discharging. The mechanism of particle synergetic effect has a physical root on the compensating material property of blending components, which fundamentally correlates with their similarity and difference in crystalline and electronic structures. The AC impedance show the obvious changes that alternate the high C-rate performance, due to reduced particle impedance in blended materials. The pairs of LiNi1/3Co1/3Mn1/3O2-LiMn2O and LiCoO2-LiMn2O4 present obvious increases in high C-rate reversible capacities than does the pair LiCoO2-LiNi1/3Co1/3Mn1/3O2.

The d-electron localization is widely recognized as important to transport properties of transition metal compounds, but its role in the energy conversion of intercalation reactions of cathode compounds is still not fully explored. In this work, the correlation of intercalation potential with electron affinity, a key energy term controlling electron intercalation, then with d-electron configuration, is investigated. Firstly, we find that the change of the intercalation potential with respect to the transition metal cations within the same structure class is correlated in an approximately mirror relationship with the electron affinity, based on first-principles calculations on three typical categories of cathode compounds including layered oxides and polyoxyanions Then, by using a new model Hamiltonian based on the crystal-field theory, we reveal that the evolution is governed by the combination of the crystal-field splitting and the on-site d–d exchange interactions. Further, we show that the charge order in solid-solution composites and the compatibility of multi-electron redox steps could be inferred from the energy terms with the d-electron configuration alternations. These findings may be applied to rationally designing new chemistry for the lithium-ion batteries and other metal-ion batteries.

First-principles calculations combined with XRD simulations are performed to systematically study crystal structures, bonding characteristics and electronic structures of LixCoSiO4 (x = 2.0, 1.5, 1.0) polymorphs with symmetries Pmn21-DP and Pbn21. The calculated average voltages by lithium extraction agree well with available experiments. CoO4 tetrahedron is the key structural unit to track the process of delithiation. The oxidation of CoO4 tetrahedron results in a special pattern of bonding characteristic, which corresponds to spin ordering and may be observable in XRD spectra according to simulation. We find delithiated phases are intrinsic Mott insulators, electronic band gaps change from Mott–Hubbard-type to charge-transfer-type during lithium removing. The swapping of near-gap states is associated with the contraction of the oxidized CoO4 units, indicating Peierls distortions that may be the physical origin of capacity degrading of Co–silicate chemistry.

We report on first-principles studies of lithium-intercalation-induced structural phase transitions in molybdenum disulphide (MoS2), a promising material for energy storage in lithium ion batteries. It is demonstrated that the inversion-symmetry-related Mo-S p-d covalence interaction and the anisotropy of d-band hybridization are the critical factors influencing the structural phase transitions upon Li ion intercalation. Li ion intercalation in 2H-MoS2 leads to two competing effects, i.e. the 2H-to-1T transition due to the weakening of Mo-S p-d interaction and the D6h crystal field, and the charge-density-wave transition due to the Peierls instability in Li-intercalated 2H phase. The stabilization of charge density wave in Li-intercalated MoS2 originates from the enhanced electron correlation due to nearest-neighbor Mo-Mo d-d covalence interaction, conforming to the extended Hubbard model. The magnitude of charge density wave is affected by Mo-S p-d covalence interaction and the anisotropy of d-band hybridization. In 1T phase of Li-intercalated MoS2, the strong anisotropy of d-band hybridization contributes to the strong Fermi surface nesting while the d-band nonbonding with S-p facilities effective electron injection.

A first-principles investigation on changes of the crystal structure and electronic structure by lithium intercalation was performed for LiNiVO4 and LiCoVO4 with inverse spinel structures. The spin states were found to change from low spin states in the de-lithiated phases to high spin states in the lithiated phases, correlated with the decrease in the octahedral crystal field splitting caused by lithium intercalation. Atomic basin population analysis combined with differential charge density analysis showed that the charge transfer caused by lithium intercalation reduces both the transition metal ions and the oxygen ions, simultaneously weakening the covalent-bonding between them. To obtain a deeper insight into the energy transfer process, the lithiation process was analyzed by considering contributions from the electronic part and ionic part, defined as electron affinity and lithium ion affinity, which costs and releases energy in the lithiation process, respectively. The electron affinity of the host was found to be characteristic of the redox potential while the lithium ion affinity is almost the same for these two cathodes. The 3d electron configuration of the transition metal ions is the dominant factor determining the redox potential although the transition metal ions and the oxygen ions are both reduced. For the host material with d5, d6, and d7 configurations, our results reveal a Λ-shape for the electron affinity, with the maximum at the d6 configuration, and correspondingly a V-shape for the reduction potential, with the minimum at the d6 configuration.

The d-electron localization is widely recognized as important to transport properties of transition metal compounds, but its role in the energy conversion of intercalation reactions of cathode compounds is still not fully explored. In this work, the correlation of intercalation potential with electron affinity, a key energy term controlling electron intercalation, then with d-electron configuration, is investigated. Firstly, we find that the change of the intercalation potential with respect to the transition metal cations within the same structure class is correlated in an approximately mirror relationship with the electron affinity, based on first-principles calculations on three typical categories of cathode compounds including layered oxides and polyoxyanions Then, by using a new model Hamiltonian based on the crystal-field theory, we reveal that the evolution is governed by the combination of the crystal-field splitting and the on-site d–d exchange interactions. Further, we show that the charge order in solid-solution composites and the compatibility of multi-electron redox steps could be inferred from the energy terms with the d-electron configuration alternations. These findings may be applied to rationally designing new chemistry for the lithium-ion batteries and other metal-ion batteries.

A first-principles investigation on changes of the crystal structure and electronic structure by lithium intercalation was performed for LiNiVO4 and LiCoVO4 with inverse spinel structures. The spin states were found to change from low spin states in the de-lithiated phases to high spin states in the lithiated phases, correlated with the decrease in the octahedral crystal field splitting caused by lithium intercalation. Atomic basin population analysis combined with differential charge density analysis showed that the charge transfer caused by lithium intercalation reduces both the transition metal ions and the oxygen ions, simultaneously weakening the covalent-bonding between them. To obtain a deeper insight into the energy transfer process, the lithiation process was analyzed by considering contributions from the electronic part and ionic part, defined as electron affinity and lithium ion affinity, which costs and releases energy in the lithiation process, respectively. The electron affinity of the host was found to be characteristic of the redox potential while the lithium ion affinity is almost the same for these two cathodes. The 3d electron configuration of the transition metal ions is the dominant factor determining the redox potential although the transition metal ions and the oxygen ions are both reduced. For the host material with d5, d6, and d7 configurations, our results reveal a Λ-shape for the electron affinity, with the maximum at the d6 configuration, and correspondingly a V-shape for the reduction potential, with the minimum at the d6 configuration.

Electrochemical cycling stabilities were compared for undoped and Al/Co dual-doped spinel LiMn2O4 synthesized by solid state reactions. We observed the suppression of particle fracture in Al/Co dual-doped LiMn2O4 during charge/discharge cycling and its distinguishable particle morphology with respect to the undoped material. Systematic first-principles calculations were performed on undoped, Al or Co single-doped, and Al/Co dual-doped LiMn2O4 to investigate their structural differences at the atomistic level. We reveal that while Jahn–Teller distortion associated with the Mn3+O6 octahedron is the origin of the lattice strain, the networking — i.e. the distribution of mixed valence Mn ions — is much more important to release the lattice strain, and thus to alleviating particle cracking. The calculations showed that the lattice mismatching between Li+ intercalation and deintercalation of LiMn2O4 can be significantly reduced by dual-doping, and therefore also the volumetric shrinkage during delithiation. This may account for the near disappearance of cracks on the surface of Al/Co–LiMn2O4 after 350 cycles, while some obvious cracks have developed in undoped LiMn2O4 at similar particle size even after 50 cycles. Correspondingly, Al/Co dual-doped LiMn2O4 showed a good cycling stability with a capacity retention of 84.1% after 350 cycles at a rate of 1C, 8% higher than the undoped phase.