Chen Ling

We discovered that non-transition metal clusters have great potential as rechargeable Mg battery cathodes. Fullerene (C₆₀), one of the prototype materials, was discharged and recharged with a remarkable rate capability. This unique rate performance is attributed to its capability to delocalize electrons on the entire cluster rather than to individual atoms.

Cobalt tungstate (CoWO₄) was recently reported as a catalyst that drives the oxygen evolution reaction. Interestingly, the catalytic activity of crystalline CoWO₄ is orders of magnitude lower than its amorphous counterpart. In order to better understand the mechanical origin of these observations, herein we study the oxygen evolution on the crystalline CoWO₄ surface using first-principles calculations. By comparing the calculated data with experimental measurements, we demonstrate that GGA + U calculations with U = 4.5 effectively predict the structural and electronic properties of crystalline CoWO₄. With this U value the overpotential for oxygen evolution on the crystalline CoWO₄ surface is 0.97 V. This prediction agrees fairly well with experimental estimation, and is consistent with the low catalytic activity of crystalline CoWO₄. By a comprehensive analysis of the reaction process, such high overpotential is attributed to the instability of oxygen radicals formed during the oxygen evolution. Our results highlight the importance to stabilize oxygen radicals during oxygen evolution reaction. It also paves the road for future optimization of CoWO₄ as oxygen evolution catalyst.

Vanadyl phosphate (VOPO₄) is an attractive candidate as a Li-ion battery cathode with potential to reach higher energy density than that of olivine LiFePO₄. However, the limited knowledge of the complex polymorphism of VOPO₄ and LiVOPO₄ prevents any systematic optimization. Here we present a comprehensive first-principles study on the phase stability of VOPO₄ and LiVOPO₄ and its impact on the electrochemical properties. βVOPO₄ and αLiVOPO₄ are predicted to be the most stable phases of VOPO₄ and LiVOPO₄, respectively. The crucial factor that determines the phase stability is the connection between cation polyhedra including the tilting of VO₆ octahedra and the distance between cations, but not the bonding between cations and anions. The calculated data well explain the experimentally observed transition from the α_II- to the α_I-phase and from the ε- to the α-phase induced by lithiation. The transition from the β- to the α-phase is also possible but is associated with much larger structural deformation. The voltages calculated using possible phase transitions taken into account are in excellent agreement with experimental measurements. The diffusivity in βLiVOPO₄ and αLiVOPO₄ differs by five orders of magnitude at 300 K. These results elucidate the importance of the phase stability of VOPO₄ and LiVOPO₄ in the application of Li-ion battery cathodes and pave the way for their future improvements.

The Na-ion battery has recently gained a lot of interest as a low-cost alternative to the current Li-ion battery technology. Its feasibility strongly depends on the development of suitable electrode materials. In the present work we propose a novel anode candidate, boron-doped graphene, for the Na-ion battery. Our first-principles calculations demonstrate that the sodiation of boron-doped graphene well preserves its structural integrity. The 2D-BC₃ anode has the average sodiation voltage of 0.44 V in an appropriate range to avoid the safety concerns caused by the formation of dendritic deposits. The capacity of the 2D-BC₃ anode reaches ∼2.04 times that of the graphite anode in a Li-ion battery and ∼2.52 times that of hard carbon in a Na-ion battery. The high electronic mobility and Na mobility on boron-doped graphene indicates that it has a high potential to reach good rate performance. These suggest the promising potential of boron-doped graphene to serve as an anode for a rechargeable Na-ion battery.

We report anisotropic expansion of self-assembled colloidal polystyrene–poly(dimethylsiloxane) crystals and its impact on the phonon band structure at hypersonic frequencies. The structural expansion was achieved by a multistep infiltration–polymerization process. Such a process expands the interplanar lattice distance 17% after 8 cycles whereas the in-plane distance remains unaffected. The variation of hypersonic phonon band structure induced by the anisotropic lattice expansion was recorded by Brillouin measurements. In the sample before expansion, a phononic band gap between 3.7 and 4.4 GHz is observed; after 17% structural expansion, the gap is shifted to a lower frequency between 3.5 and 4.0 GHz. This study offers a facile approach to control the macroscopic structure of colloidal crystals with great potential in designing tunable phononic devices.

KFeSO₄F was recently reported as a cathode candidate for the removal and insertion of Li⁺, Na⁺ and K⁺. Here the mechanism for the electrochemical extraction of K⁺ from KFeSO₄F is unveiled using density functional calculations. Consistent with experimental observations, first-principles calculations predict a thermodynamically favourable phase transition when the potassium content in K_xFeSO₄F is below 50% (x ≤0.5). The mechanism that drives this phase transition is carefully investigated. It is shown that the extraction of K⁺ can be expressed as a sequential removal of K⁺ and selective oxidation of Fe²⁺ ions. Before the composition reaches K_0.5FeSO₄F the selective oxidation only produces neighbouring Fe²⁺–Fe³⁺ pairs. The crystal structure is stable in this stage. Further removal of K⁺ after the composition reaches K_0.5FeSO₄F generates Fe³⁺–Fe³⁺ pairs. It triggers the phase transition in order to minimize the strong electrostatic repulsion between highly charged Fe³⁺ ions. The voltage profile for the removal of K⁺ from KFeSO₄F is also affected by this special mechanism. These results not only provide a mechanistic explanation of the experimental observations, but also suggest the cation repulsion is a key factor that affects the structural stability and the voltage profile of the cathode material.