Hexagonal BC₃: A Robust Electrode Material for Li, Na, and K Ion Batteries

We have investigated the stability, maximum intercalation capacity, and voltage profile of alkali metal intercalated hexagonal BC₃ (M_<i>x</i>BC₃), for 0 < <i>x</i> ≤ 2 and M = Li, Na, and K. Our calculations, based on dispersion-corrected density functional theory, show that these intercalation compounds are stable with respect to BC₃ and their bulk metal counterparts. Moreover, we found that among all M_<i>x</i>BC₃ considered, the maximum stable capacity corresponds to an <i>x</i> value of 1.5, 1, and 1.5 for Li, Na, and K, respectively. These values are associated with large gravimetric capacities of 572 mA h/g for Na and 858 mA h/g for Li and K. Importantly, we show that metal intercalated hexagonal BC₃ has the advantage of a small open-circuit voltage variation of approximately 0.49, 0.12, and 0.16 V for Li, Na, and K, respectively. Our results suggest that BC₃ can become a robust alternative to graphitic electrodes in metal ion batteries, thus encouraging further experimental work

Potassium Ion Batteries with Graphitic Materials

Graphite intercalation compounds (GICs) have attracted tremendous attention due to their exceptional properties that can be finely tuned by controlling the intercalation species and concentrations. Here, we report for the first time that potassium (K) ions can electrochemically intercalate into graphitic materials, such as graphite and reduced graphene oxide (RGO) at ambient temperature and pressure. Our experiments reveal that graphite can deliver a reversible capacity of 207 mAh/g. Combining experiments with <i>ab initio</i> calculations, we propose a three-step staging process during the intercalation of K ions into graphite: C → KC₂₄ (Stage III) → KC₁₆ (Stage II) → KC₈ (Stage I). Moreover, we find that K ions can also intercalate into RGO film with even higher reversible capacity (222 mAh/g). We also show that K ions intercalation can effectively increase the optical transparence of the RGO film from 29.0% to 84.3%. First-principles calculations suggest that this trend is attributed to a decreased absorbance produced by K ions intercalation. Our results open opportunities for novel nonaqueous K-ion based electrochemical battery technologies and optical applications

Tunable Broadband Nanocarbon Transparent Conductor by Electrochemical Intercalation

Optical transparent and electrical conducting materials with broadband transmission are important for many applications in optoelectronic, telecommunications, and military devices. However, studies of broadband transparent conductors and their controlled modulation are scarce. In this study, we report that reversible transmittance modulation has been achieved with sandwiched nanocarbon thin films (containing carbon nanotubes (CNTs) and reduced graphene oxide (rGO)) <i>via</i> electrochemical alkali-ion intercalation/deintercalation. The transmittance modulation covers a broad range from the visible (450 nm) to the infrared (5 μm), which can be achieved only by rGO rather than pristine graphene films. The large broadband transmittance modulation is understood with DFT calculations, which suggest a decrease in interband transitions in the visible range as well as a reduced reflection in the IR range upon intercalation. We find that a larger interlayer distance in few-layer rGO results in a significant increase in transparency in the infrared region of the spectrum, in agreement with experimental results. Furthermore, a reduced plasma frequency in rGO compared to few-layer graphene is also important to understand the experimental results for broadband transparency in rGO. The broadband transmittance modulation of the CNT/rGO/CNT systems can potentially lead to electrochromic and thermal camouflage applications

Scalable Holey Graphene Synthesis and Dense Electrode Fabrication toward High-Performance Ultracapacitors

Graphene has attracted a lot of attention for ultracapacitor electrodes because of its high electrical conductivity, high surface area, and superb chemical stability. However, poor volumetric capacitive performance of typical graphene-based electrodes has hindered their practical applications because of the extremely low density. Herein we report a scalable synthesis method of holey graphene (h-Graphene) in a single step without using any catalysts or special chemicals. The film made of the as-synthesized h-Graphene exhibited relatively strong mechanical strength, 2D hole morphology, high density, and facile processability. This scalable one-step synthesis method for h-Graphene is time-efficient, cost-efficient, environmentally friendly, and generally applicable to other two-dimensional materials. The ultracapacitor electrodes based on the h-Graphene show a remarkably improved volumetric capacitance with about 700% increase compared to that of regular graphene electrodes. Modeling on individual h-Graphene was carried out to understand the excellent processability and improved ultracapacitor performance