Insight into the Vibrational and Thermodynamic Properties of Layered Lithium Transition-Metal Oxides LiMO₂ (M = Co, Ni, Mn): A First-Principles Study

Evaluation of the finite-temperature thermodynamic properties of the electrode materials generally helps to accurately describe the performance of Li-ion battery (LIBs). To know the characteristics of the layered lithium transition-metal oxides LiMO₂ (M = Co, Ni, Mn) comprehensively, herein, the vibrational and related thermodynamic quantities of these electrode materials are investigated by using density functional perturbation theory (DFPT). Local density approximation (LDA) and generalized gradient approximation with the Hubbard model correction (GGA+<i>U</i>) yield similar results, either for the phonon dispersion or for the thermodynamic functions. Among the three layered lithium transition-metal oxides, the vibrational and thermodynamic properties of LiNiO₂ is more close to that of LiMnO₂, while relatively far away from that of LiCoO₂, due to the same crystal structure of LiNiO₂ and LiMnO₂, which is different from that of LiCoO₂. In addition, the corrections of average intercalation voltage as a function of temperature for Li_0.75CoO₂ and Li_0.5CoO₂ are evaluated when considering the contribution of vibrational entropy. Since our theoretical results for LiCoO₂ agree well with those from experiments, we can provide the reliable thermodynamic data for the layered lithium transition-metal oxides

Nitrogen- and Phosphorus-Doped Biocarbon with Enhanced Electrocatalytic Activity for Oxygen Reduction

The oxygen reduction reaction (ORR) at the cathode of fuel cells and metal–air batteries requires efficient electrocatalysts to accelerate its reaction rate due to its sluggish kinetics. Nitrogen- and phosphorus-doped biocarbon has been fabricated via a simple and low-cost biosynthesis method using yeast cells as a precursor. The as-prepared biocarbon exhibits excellent electrocatalytic activity for the ORR. An onset potential of −0.076 V (vs Ag/AgCl) and a negative shift of only about 29 mV in the half-wave potential of the biocarbon as compared to commercial Pt/C (20 wt % Pt on Vulcan XC-72, Johnson Matthey) is achieved. The biocarbon possesses enhanced electron poverty in carbon atoms and a decreasing amount of less electroactive nitrogen and phosphorus dopants due to the biomineralization during the synthesis. The surface gap layer along with the mesopores in the biocarbon increases accessible active sites and facilitates the mass transfer during the ORR. These factors correlate with the high ORR activity of the biocarbon. The results demonstrate that biomineralization plays a critical role in tailoring the structure and the electrocatalytic activity of the biocarbon for ORR

2D Electrides as Promising Anode Materials for Na-Ion Batteries from First-Principles Study

Searching for suitable anodes with good performance is a key challenge for rechargeable Na-ion batteries (NIBs). Using the first-principles method, we predict that 2D nitrogen electride materials can be served as anode materials for NIBs. Particularly, we show that Ca₂N meets almost all the requirements of a good NIB anode. Each formula unit of a monolayer Ca₂N sheet can absorb up to four Na atoms, corresponding to a theoretical specific capacity of 1138 mAh·g^–1. The metallic character for both pristine Ca₂N and its Na intercalated state Na_<i>x</i>Ca₂N ensures good electronic conduction. Na diffusion along the 2D monolayer plane can be very fast even at room temperature, with a Na migration energy barrier as small as 0.084 eV. These properties are key to the excellent rate performance of an anode material. The average open-circuit voltage is calculated to be 0.18 V vs Na/Na⁺ for the chemical stoichiometry of Na₂Ca₂N and 0.09 V for Na₄Ca₂N. The relatively low average open-circuit voltage is beneficial to the overall voltage of the cell. In addition, the 2D monolayers have very small lattice change upon Na intercalation, which ensures a good cycling stability. All these results demonstrate that the Ca₂N monolayer could be an excellent anode material for NIBs

Synthesis and Lithium Storage Mechanism of Ultrafine MoO₂ Nanorods

Ultrafine MoO₂ nanorods with a diameter of ∼5 nm were successfully synthesized by a nanocasting method using mesoporous silica SBA-15 as hard template. This material demonstrates high reversible capacity, excellent cycling performance, and good rate capacity as an anode electrode material for Li ion batteries. The significant enhancement in the electrochemical Li storage performance in ultrafine MoO₂ nanorods is attributed to the nanorod structure with small diameter and efficient one-dimensional electron transport pathways. Moreover, density functional theory calculations were performed to elucidate the Li uptake/removal mechanism in the MoO₂ electrodes, which can help us understand the unique cycling behavior of MoO₂ material