The LTLE synthesis process of few-layer borophene sheets is schematically illustrated in Fig. 1a. By optimizing the sonication power and solvent concentration, massive production of 2D borophene sheets has been successfully achieved. N-methyl pyrrolidone (NMP) was found to be the most effective among a series of solvents adopted in our experiment, as seen in Supplementary Figs. 1 and 2. The color of the product solution is dark-brown or dark-black in Fig. 1b, varying with the sheet concentration. Moreover, the mass of 2D sheets reaches as high as 10 mg and their yield is over 20 %, evidently increased compared with previous reports (≤ 10 %48,49). The low-temperature approach is thus believed to improve the exfoliation efficiency of non-layered bulk materials, because of which significantly enhances the anisotropy discrepancy between the in-plane and out-of-plane covalence bonds50. Scanning electron microscope (SEM) and atomic force microscope (AFM) images of the as-grown products are respectively in Fig. 1c, d, where ultrathin 2D borophene sheets are observed to have an edge length of 2 ~ 5 µm and exhibit uniform and smooth appearance. The thickness of 2D sheet is observed to be only 1.32 ~ 2.32 nm, suggesting its ultra-thin nature.
X-ray diffraction (XRD) pattern of 2D borophene sheets (Supplementary Fig. 3) shows the same characteristic diffraction pattern as that of the theoretically calculated β12-borophene using density functional theory (DFT), clearly different from that of the bulk β-rhombohedral boron powder (JCPDS No. 00-031-0207). As seen in Fig. 1e, the X-ray photoelectron spectrum (XPS) of B 1s core level is consisted of two characteristic components, attributed to the B-B species at 187.5 eV51 and the B-O species at 189.1 eV52, respectively. And the molar ratio of the B-B to B-O species is estimated to be more than 94 %, suggesting a majority of pure boron composition in 2D borophene sheets. The minor B-O species are supposed to originate from the edge oxidation of borophene sheets during the short exposure to the air after being taken out for XPS measurements (Supplementary Fig. 4)52.
Raman spectroscopy was employed to better differentiate the 2D borophene sheets from the bulk boron powders53,54. Four Raman peaks of 2D borophene sheets are clearly identified (Fig. 1f) as the fingerprints of β12 phase55, differing from those of bulk boron with β-rhombohedral phase. Accordingly, the strong peak at ~ 268 cm− 1 is ascribed to the out-of-plane bending vibration mode \(\left({B}_{1u}^{1}\right(X\left)\right)\) of β12 phase55. And the other peaks at ~ 423, ~901 and ~ 1017 cm− 1 are respectively indexed as the \({B}_{1g}^{2}\), \({A}_{g}^{2}\left(S\right)\) and \({B}_{1g}^{1}\) modes, resulting from the in-plane stretching modes of β12 phase55.
Transmission electron microscopy (TEM) was performed to determine the surface configuration of 2D borophene sheets. A typical TEM image in Fig. 2a exhibits a similar planar morphology in line with the aforementioned SEM and AFM results (Fig. 1c, d). Close examination (Fig. 2a inset) reveals an ultrathin thickness of only 6 atomic layers with an adjacent planar distance of 5.1 Å. The high-resolution TEM (HRTEM) image further verifies high-quality single crystal nature of 2D borophene sheets. As shown in Fig. 2b, the 2D borophene sheets are found to have a hexagonal honeycomb lattice with a perfect planar periodicity of a = b ≈ 2.76 Å and the intersection angle θ of about 120° in the unit cell. Based on the DFT calculations, we thus propose a novel allotrope with \(P\stackrel{-}{6}m2\) symmetry (referred to as β12-B5) for few-layer β12-borophene sheets, where there are 5 boron atoms in a unit cell (Fig. 2c). In this model, both of the lattice constants (a and b) of few-layer borophene are 2.83 Å and the angle θ between a and b vectors is equal to 120°, which are in good agreement with the HRTEM results (Fig. 2b). Based on the theoretical model, the bright contrasts in the HRTEM image (Fig. 2b) thus correspond to the six-member rings of the boron honeycomb lattice (Fig. 2c). Besides, the layer distance of adjacent (001) planes is theoretically calculated to be about 5.0 Å for β12-B5 borophene, nearly identical to the experimental results (5.1 Å) measured by TEM. Statistically, the thickness of most of the 2D sheets is less than 5 nm. (Supplementary Fig. 5a). Therefore, the atomic layer numbers of the as-synthesized borophene sheets should be less than 10, unveiling the ultrathin nature of few-layer borophene. In addition, the 2D β12-borophene sheets are thermodynamically stable as evidenced by the absence of any negative frequency in the entire Brillouin zone (Fig. 2d) according to the density functional perturbation theory (DFPT). Similar calculations are carried out on the (104) plane (Supplementary Fig. 5), which are also in good agreement with our experimental results. High-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) and energy dispersive X-ray spectroscopy (EDX) mapping (Fig. 2e-h) images also reveal a uniform distribution of boron element across the 2D sheet with a pure boron content over 98 %, which is in good consistent with the electron energy loss spectrum (EELS) (Supplementary Fig. 5b).
Based on all characterizations mentioned above, we can thus conclude that single crystalline few-layer borophene sheets with β12-B5 phase were successfully synthesized using LTLE. In comparison with other synthetic methods summarized in Supplementary Table 1, our route is thus low-cost, facile and high-efficient for scalable production of single crystalline β12-borophene sheets towards practical applications such as Li-S battery.
To demonstrate the catalytic activity of few-layer β12-borophene sheets for Li-S battery, the potentiostatic experiments were carried out to monitor the liquid-solid conversion in the nucleation and growth of Li2S from polysulfides. The galvanostatic discharge was respectively performed on CNT/β12-borophene (CNT/B) and bare CNT hosts at 2.05 V, in which 0.01 V overpotential was used to induce the generation of Li2S. All the cells reached the highest potentiostatic current after about 1000 s, but the nucleation abilities of Li2S were found to be completely different, causing a capacity of 193 mAh g− 1 and 72 mAh g− 1 for CNT/B and CNT electrodes, respectively (Fig. 3a). Besides, the dissolution ability of solid Li2S was also remarkably promoted by the implantation of few-layer β12-borophene. After the fully conversion of sulfurs into Li2S, the Li2S dissolution was kinetically evaluated by using a potentiostatic charge process. Clearly, a larger oxidation current was detected on CNT/B (0.13 mA cm− 2) enabled cell in comparison with bare CNT electrode (0.11 mA cm− 2), unveiling the excellent electrocatalysis behaviors of 2D β12-borophene in enhancing the dissolution of Li2S (Fig. 3b).
To gain insight into the enhancement effect of β12-borophene sheets on the liquid-liquid conversion process (Li2Sy to Li2Sx, 8 ≥ x ༞2, 8 ≥ y ༞2), Li2S8 symmetric cells were employed for the cyclic voltammetry (CV) measurements. The CNT/B-based cell yielded a higher redox current than the bare CNT-based cell, suggesting enhanced reactivity of the polysulfide on β12-borophene interface (Fig. 3c). The kinetic-regulating role of β12-borophene was subsequently demonstrated in actual Li-S batteries. The CV curves of the as-assembled Li-S batteries exhibited two typical redox peaks, corresponding to the formation of soluble polysulfides (2.2–2.4 V) and solid Li2S (2.0-2.1 V), respectively. And the two overlapped anodic peaks (2.4–2.6 V) were attributed to the sequential oxidation of Li2S and polysulfides29. In contrast with the Li-S battery using non-undecorated CNT electrode, the Li-S battery using CNT/B electrode possessed higher current density (Fig. 3d). As shown in Fig. 3e, the Tafel plots of the first oxidation process of the cells using CNT and CNT/B were respectively 57 and 29 mV dec− 1, where the smaller Tafel slope of the CNT/B-based cell suggests that the β12-borophene induces higher surface reaction rates. In addition, the simulated interfacial impedance of the Li-S cells sharply decreased from 41.2 Ω to 24.9 Ω when the electrodes changed from CNT to CNT/B (Fig. 3f), reflecting the β12-borophene was more favorable for the interface electrochemical reactions8.
Considering the distinguished electrocatalytic reactivity and polysulfide interactions of the CNT/B-based Li-S battery in the sulfur redox reactions, their actual working performances were further evaluated by regarding the bare CNT-based Li-S battery as a reference. In our experiments, the same amount of polysulfide (Li2S8) solution was added as active material (Supplementary Fig. 6). As seen in Fig. 4a, the CV profiles of CNT/B-based battery overlap each other and exhibit excellent reversibility in the redox process, revealing the high-efficiency utilization of sulfur. The galvanostatic charge/discharge profiles are shown in Fig. 4b. The high reversible specific capacities of 1329, 1236, 1159, 1057, 993, and 919 mAh g− 1 were obtained at 0.3, 0.5, 1, 2, 3, and 5 C rates (1 C = 1675 mAh g− 1), respectively. Even if the current density increased to 8 C, the CNT/B-based Li-S battery still remained an ultrahigh capacity of 721 mAh g− 1. More significantly, after returning current density back to 0.3 C, a reversible capacity of 1216 mAh g− 1 recovered immediately with a columbic efficiency of nearly 100 % (Fig. 4c). By contrast, the battery using bare CNT electrode exhibited inferior rate performances, such as a lower initial capacity of 981 mAh g− 1 at 0.3 C and a rapider degradation into 394 mAh g− 1 with the increase of capacity to 8 C as well as unsatisfactory capacity restoration after high-rate test (Fig. 4c). As shown in Fig. 4d, the CNT/B-based battery possessed a much lower polarization voltage of 188 mV than the CNT-based battery (217 mV), further revealing the outstanding catalytic property of β12-borophene sheets for polysulfide conversion.
The CNT/B cathode also exhibited excellent cycling stability at current density of 0.5 C, as found in Fig. 4e. The capacity fading rate was only 0.003% per cycle and kept nearly unvaried after 300 cycles when the initial capacity of the CNT/B-based Li-S battery was 1110 mAh g− 1. Moreover, the CNT/B-based cell maintained a high coulombic efficiency of ~ 100% in continuous 300 cycle measurements. On the contrary, the bare CNT-based cell delivered a low capacity of 918 mAh g− 1 and sharply decreased to 394 mAh g− 1 after 300 cycles, resulting in a fast-fading rate of 0.2 % per cycle (Fig. 4e and Supplementary Fig. 7). In addition, both of the high- and low-plateau capacities of CNT/B electrode were much better than bare CNT electrode, demonstrating few-layer β12-borophene sheets can effectively suppress the polysulfide diffusion and improve the polysulfide immobilization (Supplementary Fig. 9)56. Moreover, high areal sulfur loadings of 5.3 mg cm− 2 and 7.8 mg cm− 2 with low E/S ratios of 9.8 and 6.8 ml g− 1 were respectively performed on the Li-S batteries to test the high-energy density behaviors. It was noted that the areal capacities of the CNT/B-based Li-S cells can reach up to 4.6 and 5.2 mAh cm− 2 when the capacities respectively adopted 871 and 661 mAh g− 1 (Fig. 4f), which were much higher than those of 4.0 mAh cm− 2 for commercial Li-ion batteries57. Impressively, the CNT/B-based battery could preserve an enough high reversible capacity of 572 mAh g− 1 with an extremely-low capacity decay rate of 0.039 % per cycle after 1000 long-term cycles, reflecting excellent cycling stability (Fig. 4g and Supplementary Fig. 8). Notably, the ultralow decay rate and ultrahigh rate performance of 2D β12-borophene sheets are superior to most of other 2D material-based Li-S batteries (Supplementary Table 2), such as phosphorene (785 mAh g− 1 at 3 C, decay rate of 0.053% for 1000 cycles)30, C3N4 (340 mAh g− 1 at 4 C, decay rate of 0.5% for 200 cycles)58, and graphene (700 mAh g− 1 at 2 C, decay rate of 0.5% for 70 cycles)59.
Finally, we calculated the adsorption energy of soluble polysulfides on a monolayer β12-borophene using DFT calculation to comprehend the improvement mechanism of β12-borophene sheets on Li-S batteries, as observed in Supplementary Fig. 10. Figure 5a gives the optimized configurations of S8 and Li2Sn on monolayer β12-borophene sheet. Based on the DFT calculations, S8 has the weakest adsorption energy on borophene of only 1.23 eV among all configurations, and the adsorption energy gradually increases with the progression of the polysulfides’ lithiation and eventually arrives at 3.8 eV for the fully-lithiated Li2S (Fig. 5b). The adsorption energy of polysulfides on β12-borophene is far higher than that on CNT (below 1 eV), unveiling that β12-borophene can anchor polysulfide and inhibit the shuttle of lithium polysulfide more effectively than CNT. Figure 5c shows the typical partial density of states (PDOS) of Li2S4 on monolayer β12-borophene, and more details can be seen in Supplementary Fig. 11. The 2p orbital electrons of Li2S4 and β12-borophene were found to overlap near the Fermi level, suggesting the formation of a strong chemical bonding between β12-borophene and Li2S4 cluster. This is probably originated from a strong charge transfer of 0.22 e from β12-borophene to Li2S4 cluster (Fig. 5d) based on the charge density difference and bader charge analysis. The strong chemical interaction between β12-borophene and Li2S4 cluster can be also ascertained because the dark-yellow color of Li2S4 solution will gradually attenuate with the increase of the mixing time with β12-borophene (Supplementary Fig. 12). Furthermore, the diffusion barrier of Li+ on β12-borophene was deduced to be only 0.10 eV (Fig. 5e), much lower than that (0.28 eV) on CNT (Supplementary Fig. 13). The enhanced surface migration of Li+ on borophene would further accelerate the nucleation and decomposition of Li2Sn and thus improves the capacity and charge-discharge rate of Li-S battery60.
In summary, we have developed a novel, facile and high-yield LTLE strategy to produce single crystalline few-layer β12-borophene sheets. As promising 2D electrode materials, the β12-borophene sheets were firstly used as efficient polysulfide-conversion electrocatalysts for Li-S batteries. Due to the usage of few-layer β12-borophene sheets, the CNT/B-based Li-S batteries exhibited a high areal sulfur loading of 5.2 mgh cm− 2 at 7.8 mg cm− 2 under a low E/S ratio of 6.8 ml g− 1 at 0.3 C. Compared with the CNT-based Li-S cell, the CNT/B-based Li-S cell exhibited a better rate performance of as high as 721 mAh g− 1 at 8 C and a much lower decay rate of only 0.039 % in 1000 cycles. By DFT calculations, β12-borophene had a lower surface diffusion barrier of Li ion and a stronger adsorption for Li2Sn clusters than CNT, which can effectively inhibit the shuttle effect of polysulfides and accelerate their decomposition at the same time. These should be responsible for the extraordinary catalytic activity of β12-borophene towards polysulfides in the CNT/B-based Li-S cell. Therefore, our strategy will pave a new way for the design of high-energy rechargeable batteries through the exploration of 2D boron-based nanomaterials.