Thermodynamic Insights into Polymorphism-Driven Lithium-Ion Storage in Monoelemental 2D Materials

Abstract

Monoelemental two-dimensional materials (borophene, silicene, etc.) are exciting candidates for electrodes in lithium-ion batteries because of their ultralight molar mass. However, these materials' lithium-ion binding mechanism can be complex as the inherited polymorphism may induce phase changes during the charge−discharge cycles. Here, we combine geneticalgorithm-based bottom-up and stochastic top-down structure searching techniques to conduct thermodynamic scrutiny of the lithiated compounds of 2D allotropes of four elements: B, Al, Si, and P. Our first-principles-based high-throughput computations unveil polymorphism-driven lithium-ion binding process and other nonidealities (e.g., bond cleavage, adsorbent phase change, and electroplating), which lacks mention in earlier works. While monolayer B (2479 mAh/g), Al (993 mAh/g), and Si (954 mAh/g) have been demonstrated here as excellent candidates for Li-ion storage, P falls short of the expectation. Our well-designed computational framework, which always searches for lithiated structures at global minima, provides convincing thermodynamical insights and realistic reversible specific-capacity values. This will expectedly open up future experimental efforts to design monoelemental two-dimensional material-based anodes with specific polymorphic structures.

Global Minima Search for Sodium- and Magnesium-Adsorbed Polymorphic Borophene

“…21,22 We assess the thermodynamic suitability of cationborophene compounds by combining a bottom-up evolutionary structure-searching and a top-down random structure-searching algorithm. While a monolayer of boron has been demonstrated as an excellent candidate for lithium-ion storage, 9 we find that the results are less promising for sodium-ion storage due to the relatively high formation energies and phase deformation upon adsorption in contrast to previous studies. 23 However, the monolayer honeycomb phase of borophene was found to be a highly reliable anode material for magnesium-ion storage with good formation energy (−0.14 eV/atom) and high specific capacity (3648.54 mAh/g) at the composition ratio of 0.5.…”

“…This is based on an evolutionary algorithm featuring local optimization, real-space representation, and flexible physically motivated variation operators. 9 The number of atoms in each structure is kept between 2 and 12 to limit the computational budget as many structures need to be explored for each configuration. After relaxation, only structures with thickness less than 6.5 Å are considered; the thicker structures are assigned high positive energy, and their generation is contained.…”

“…Bottom-up structure searching for the most stable structures in the composition space of Na x B y and Mg x B y is performed using the Universal Structure Predictor: Evolutionary Xtallography (USPEX) − code interfaced with VASP. This is based on an evolutionary algorithm featuring local optimization, real-space representation, and flexible physically motivated variation operators . The number of atoms in each structure is kept between 2 and 12 to limit the computational budget as many structures need to be explored for each configuration.…”

“…The most stable structures are in the range of x = 0.1–0.15, making it possible to synthesize mixed-phase samples instead of single-phase ones for better performance . Theoretical studies , tout borophene as an exceptional candidate for Li-ion battery anodes because of its unique morphology and metallic characteristics. However, due to the extremely high demand, Li, dubbed “the new oil,” is becoming increasingly costly .…”

“…However, most of these efforts rely on a computationally economic “uniform-adsorption” model, which most of the time is not guaranteed to result in the ionic ground state. − More rigorous approaches , involving “global minima search” techniques have demonstrated that the uniform-adsorption model predicts nonrealistic values of the specific capacity of any anode material since it cannot capture the cleavage and restoration of chemical bonds during the lithiation and delithiation process. A recent study also demonstrated the necessity of the global minima search technique for polymorphism-dominated materials (like borophene) to capture the adsorbate-induced phase changes.…”