Two-dimensional tetragonal transition-metal carbide anodes for non-lithium-ion batteries.

Searching for high-performance anode materials with high energy-density, fast kinetics, and good stability is a key challenge for non-lithium-ion batteries (NLIBs), such as Na+, K+, Mg2+, Ca2+, Zn2+ and Al3+ ion batteries. Here, we systematically investigated the performance of a new class of two-dimensional tetragonal transition-metal carbides (tetr-MCs) using first-principles calculations, as anodes for NLIBs. The results show that tetr-MCs are ideal anode materials with good stabilities, favorable mechanical properties, intrinsic metallic properties, high theoretical capacities, and fast ion diffusion rate for NLIBs. Among all tetr-MCs, we found that the energy barrier of Mg atoms on tetr-TiC is only 54 meV and that of Al atoms on tetr-VC is 101 meV, which are lower than the energy barriers of 230-500 meV of the well-studied MXenes, indicating that tetr-VC and tetr-TiC monolayers are promising anodes for NLIBs. Therefore, compared to MXenes, tetr-MCs show many advantages for NLIB applications, such as a lower diffusion barrier (minimum 54 meV), a high theoretical capacity (up to 1450 mA h g-1), and a lower average open circuit voltage (0.05-0.77 V). The results are of great significance for the experimental preparation of excellent anode materials for NLIBs.

Monodispersed Transition Metals Induced Ordinary-Pressure Phase Transformation from Graphite to Diamond: A First-Principles Calculation.

High pressure and high temperature are normally required for the transformation of graphite to diamond; thus, finding a method that allows the transformation to occur under ordinary pressure will be extremely promising for diamond synthesis. Here, it is found that graphite spontaneously transforms into diamond without any pressure by adding monodispersed transition metals, and the universal rules that will help predict the role of certain elements in the phase transition were studied. The results show that the favorable transition metals possess an atomic radius of 0.136-0.160 nm and an unfilled d-orbital of d2s2-d7s2, which allow more charge transfer and accumulation at the proper position between the metal and dangling C atoms, leading to stronger metal-C bonds and a lower energy barrier for the transition. This provides a universal method to prepare diamond from graphite under ordinary pressure and also provides a way for the synthesis from sp2 to sp3 bonded materials.