A high-throughput assessment of the adsorption capacity and Li-ion diffusion dynamics in Mo-based ordered double-transition-metal MXenes as anode materials for fast-charging LIBs

Unusual thermo-mechanical properties of the Janus Mo2ScC2OH MXene monolayer

In this study, we investigated the structural, electronic, mechanical, dynamical, and thermal properties of the Janus Mo2ScC2OH MXene monolayer using ab initio calculations based on density functional theory. The results showed that Janus Mo2ScC2OH is metallic and mechanically stable. We found that Mo2ScC2OH has high Poisson's ratio. Phonon dispersion calculations revealed that there are no imaginary frequencies, suggesting that the Janus Mo2ScC2OH monolayer is dynamically stable. Debye temperature, Grüneisen parameters, thermodynamic properties, and lattice thermal conductivity have also been calculated. The results revealed that Mo2ScC2OH has high negative Grüneisen parameters, indicating that it could be a negative thermal expansion material. Additionally, we noted that the Janus Mo2ScC2OH monolayer exhibits a relatively low lattice thermal conductivity, with a notable contribution from the ZA mode.

Investigation on electrochemical performance of striped, β12 and χ3 Borophene as anode materials for lithium-ion batteries

By developing next-generation lithium-ion batteries (LIBS), demand for exploring novel anode materials with exclusive electrochemical features and ultra-high capacity is increasing. In the current research, first-principles theory, and density functional theory (DFT) calculations were conducted to extensively investigate and compare the capability of three different borophene nanolayers, including striped, β12, and χ3 borophene, as a novel candidate for anode electrode in LIBs. We first predicted the most preferential Li atom adsorption sites on the three borophene structures. The predicted average formation energies for striped, β12, and χ3 borophene were obtained 3.123, 3.184, and 3.216 eV, respectively. The positive value of formation energy exhibits the sufficient stability of the structures. Moreover, the negative adsorption energy proved that Li atom insertion on all borophene monolayers is thermodynamically favorable. In order to simulate the lithiation process, we gradually increased the concentration of Li atoms. We found that the fully lithiated striped, β12 and χ3 borophenes could provide ultra-high specific capacities of 1700, 1983, and 1859 mAh/g, respectively. Structural analysis proved that the surface area expansion rate of the striped borophene in a fully lithiated state was 1%, which was lower than those of β12 and χ3 borophene with 3.33% and 2.63%, respectively. The analyses of electronic properties confirmed that borophenes were inherently metallic and superior ion conductive agents, even after fully lithiated state. Ion diffusion was studied using Nudged elastic band method and the value of diffusion energy barrier ranged from 0.03 to 0.36 eV which was lower than other promising 2D anode materials. Furthermore, open-circuit voltage results demonstrated that the electronic potential of modeled borophenes was low enough to be in the acceptable range of under 1.2V. All these reports exhibited that borophene nanolayers with excellent specific capacity and superior conductivity were desired candidates for anode materials of next generation LIBs.

Prediction of boridenes as high-performance anodes for alkaline metal and alkaline Earth metal ion batteries

We conducted a comprehensive density functional theory investigation using the r²SCAN-rVV10 functional on the structural stability and electrochemical properties of boridenes for their use as anode materials in rechargeable alkaline (earth) metal-ion batteries (Li⁺, Na⁺, K⁺, Mg²⁺ and Ca²⁺). According to first-principles molecular dynamics simulations and reaction thermodynamic calculations, Mo_4/3B₂(OH)₂ and Mo_4/3B₂F₂ are unstable in the presence of alkaline (earth) metal ions due to the surface-conversion reactions between the surface terminations and adsorbates. Meanwhile, the bare Mo_4/3B₂ and Mo_4/3B₂O₂ monolayers not only can accommodate alkaline (earth) metal ions, but also form stable multi-layer adsorption structures for most of the studied metal ions (Li⁺, Na⁺, K⁺, Mg²⁺ and Ca²⁺). The predicted gravimetric capacities of the bare Mo_4/3B₂ monolayer (Mo_4/3B₂O₂) are 625.9 mA h g^-1 (357.3 mA h g^-1), 247.20 mA h g^-1 (392.1 mA h g^-1), 101.8 mA h g^-1 (206.4 mA h g^-1), 667.0 mA h g^-1, and 413.0 mA h g^-1 (485.4 mA h g^-1) for Li⁺, Na⁺, K⁺, Mg²⁺ and Ca²⁺ ions, respectively. The bare Mo_4/3B₂ exhibits lower onset charging open circuit voltages for alkaline (earth) metal ions than that of Mo_4/3B₂O₂. The diffusivities of the metal ions were revealed to be high on the boridene monolayer especially for the outer fully stable adsorption layers, where the migration energy barriers were found to be less than 0.10 eV. Similar to that of MXenes, the negative electron cloud (NEC) also plays a vital role in stabilizing the observed multi-layer adsorption structures for various metal ions on either the bare Mo_4/3B₂ or Mo_4/3B₂O₂ monolayer.

Regulation of Dendrite-Free Li Plating via Lithiophilic Sites on Lithium-Alloy Surface

Lithium (Li) deposition behavior plays an important role in dendrite formation and the subsequent performance of lithium metal batteries. This work reveals the impact of the lithiophilic sites of lithium-alloy on the Li plating process via the first-principles calculations. We find that the Li deposition mechanisms on the Li metal and Li22Sn5 surface are different due to the lithiophilic sites. We first propose that Li plating on the Li metal surface goes through the "adsorption-reduction-desorption-heterogeneous nucleation-cluster drop" process, while it undergoes the "adsorption-reduction-growth" process on the Li22Sn5 surface. The lower adsorption energy contributes to the easy adsorption of Li on the lithiophilic sites of the Li22Sn5 surface. The lower Li reduction energy on the Li metal surface indicates that it is easy for Li to be reduced on the Li metal surface, attributed to its higher Fermi energy level. Furthermore, the faster Li diffusion on the Li22Sn5 surface results in smooth Li deposition, which is based on a "two-Li synergy diffusion" mechanism. However, Li diffuses more slowly on the Li metal surface than on the Li22Sn5 surface due to the "single Li diffusion" mechanism. This work provides a fundamental understanding on the impact of lithiophilic sites of Li alloy on the Li plating process and points out that the future design of 3D Li-alloy substrates decorated with multilithiophilic sites can prevent dendrite formation on the lithium-alloy substrate by guiding uniform Li deposition.

Hierarchical MXene/transition metal chalcogenide heterostructures for electrochemical energy storage and conversion

MXenes have gained rapidly increasing attention owing to their two-dimensional (2D) layered structures and unique mechanical and physicochemical properties. However, MXenes have some intrinsic limitations (e.g., the restacking tendency of the 2D structure) that hinder their practical applications. Transition metal chalcogenide (TMC) materials such as SnS, NiS, MoS₂, FeS₂, and NiSe₂ have attracted much interest for energy storage and conversion by virture of their earth-abundance, low costs, moderate overpotentials, and unique layered structures. Nonetheless, the intrinsic poor electronic conductivity and huge volume change of TMC materials during the alkali metal-ion intercalation/deintercalation process cause fast capacity fading and poor-rate and poor-cycling performances. Constructing heterostructures based on metallic conductive MXenes and highly electrochemically active TMCs is a promising and effective strategy to solve these problems and enhance the electrochemical performances. This review highlights and discusses the recent research development of MXenes and hierarchical MXene/TMC heterostructures, with a focus on the synthesis strategies, surface/heterointerface engineering, and potential applications for lithium-ion batteries, sodium-ion batteries, lithium-sulfur batteries, supercapacitors, electrocatalysis, and photocatalysis. The critical challenges and perspectives of the future development of MXenes and hierarchical MXene/TMC heterostructures for electrochemical energy storage and conversion are forecasted.