Zhenlian Chen

A new method applicable to study solid compounds with multiple polyhedral structures.

A new direct summation method, named as polyhedron method, is proposed to calculate Madelung energy. This method calculates sums of electrostatic interactions over sets of neutral polyhedron unit pairs rather than conventional ion pairs; this gives Madelung constant in a matrix. With robustly rapid convergence, polyhedron method is generally applicable for complex compounds containing multiple polyhedral building‐blocks and numerical polyhedral connection modes. The matrical analysis suggests face‐sharing between octahedral pairs and edge‐sharing between tetrahedral pairs can be electrostatically stable, against Paulings third rule. Further, the matrical calculation of Madelung energies offers a unique advantage to evaluate enormous configurations of cation distributions in a given lattice in a high‐throughput manner. That is applicable to study solid solution composites, polymorphism, and defect structures, including but not limited to intermediate phase of delithiated cathode compounds, charge order or antisite defects, and extensively magnetic order.

Correlation of intercalation potential with d-electron configurations for cathode compounds of lithium-ion batteries

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.

Degradation Mechanism of Dimethyl Carbonate (DMC) Dissociation on the LiCoO2 Cathode Surface: A First-Principles Study

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.