High refractive index polymers (HRIPs) have recently emerged as an important class of materials for use in a variety of optoelectronic devices including image sensors, lithography, and light-emitting diodes. However, achieving polymers having refractive index exceeding 1.8 while maintaining full transparency in the visible range still remains formidably challenging. Here, we present a unique one-step vapor-phase process, termed sulfur chemical vapor deposition, to generate highly stable, ultrahigh refractive index (n > 1.9) polymers directly from elemental sulfur. The deposition process involved vapor-phase radical polymerization between elemental sulfur and vinyl monomers to provide polymer films with controlled thickness and sulfur content, along with the refractive index as high as 1.91. Notably, the HRIP thin film showed unprecedented optical transparency throughout the visible range, attributed to the absence of long polysulfide segments within the polymer, which will serve as a key component in a wide range of optical devices.

High refractive index (RI) thin films are of critical importance for advanced optical devices, and the high refractive index polymers (HRIPs) constitute an interesting class of materials for high RI thin films due to low cost, good processability, light weight, and high flexibility. However, HRIPs have yet to realize their full potential in high RI thin film applications due to their relatively low RI, strong absorption in the blue light region, and limited film formation methods such as rapid vitrification. Herein, we report a development of a new HRIP thin film generated through a one-step vapor-phase process, termed sulfur chemical vapor deposition (sCVD), using elemental sulfur and divinyl benzene. The developed poly­(sulfur-co-divinyl benzene) (pSDVB-sCVD) film exhibited RI (measured at 632.8 nm) exceeding 1.97, one of the highest RIs among polymers without metallic elements reported to date. Because the sCVD utilized vaporized sulfur with a unique sulfur-cracking step, formation of long polysulfide chains was suppressed efficiently, while high sulfur content as high as 85 wt % could be achieved with no apparent phase separation. Unlike most of inorganic high RI materials, pSDVB-sCVD was highly transparent in the entire visible range and showed extremely low birefringence of 10 × 10–4. The HRIP thin film with unprecedentedly high RI, together with outstanding transparency and low birefringence, will serve as a key component in a wide range of high-end optical device applications.

We herein report the preparation of high refractive index polymers (HRIPs) with enhanced thermal stability from the copolymerization of S 8 and divinylbenzene (DVB) in a single step through sulfur chemical vapor deposition (sCVD). Varying the process temperature in sCVD allowed for the preparation of a series of high sulfur content polymers which, in comparison to previously reported sulfur-derived polymers with similar compositions, displayed significantly enhanced solvent resistance, glass transition temperature (T g ), and thermal stability attributed to the absence of long polysulfide chains in the polymer matrix. The resulting HRIP films displayed high transmittance over the entire visible range while showing an unprecedentedly high T g of 110 °C, which is one of the highest to date among HRIPs with refractive index (RI) exceeding 1.8 reported to date. With the combination of ultrahigh (>1.8) RI, thermal stability, and high T g , the HRIPs can serve as compelling materials for advanced optical applications.

The commercial use of lithium−sulfur (Li−S) batteries is hampered by the shuttle phenomenon in cathodes and the uncontrolled growth of Li dendrites in anodes. Designing functional material-coated separators is gaining importance in the effort to tackle these issues in both of the electrodes simultaneously. Here, an initiated chemical vapor deposition (iCVD) technique is introduced in Li−S batteries for the first time to homogeneously deposit a polyvinylimidazole (pVIDZ) nanolayer on the separator. An ultrathin and ultralight-weight polymer coating nanolayer with a thickness of 70−100 nm and a weight of 0.055 mg cm −2 was achieved. Furthermore, the pVIDZ layer on the separator was observed to perform a bifunctional role in stabilizing the anode by alleviating Li dendrite growth and also to improve the cycle stability of the cathode by inhibiting the shuttle phenomenon. Consequently, even with high sulfur loading electrodes of 4 mg cm −2 , the use of iCVD-derived bifunctional separators exhibits an initial discharge capacity of 881 mA h g −1 at a rate of 1.0 C while maintaining 84.5% of its initial capacity after 300 cycles corresponding to a capacity decay of only 0.051% per cycle.

Li-battery development, Li metal anodes have yet been applied to practical battery systems because of their unstable surface chemistry, which induces dendrite formation during the charge/discharge processes. Metallic Li itself is a strong reducing agent due to its high hydration enthalpy (−520 kJ mol −1 ), [2] so it readily reacts with electrolytes to form solidelectrolyte interphase (SEI) layers even without applying an electrical potential. [3] However, these layers are mechanically weak due to heterogeneous multigrain boundaries that easily crack upon the volume change of the Li metal anode. [4] The cracks generated in these layers allow Li to grow through them, exposing the fresh Li to electrolytes. As additional SEI layers spontaneously build upon the surface, the remaining liquid electrolytes are consumed to depletion (Figure S1, Supporting Information). Since the Li dendrites grow over the subsequent cycles by consuming Li metal and electrolytes into unavailing dead Li, Li metal batteries show poor coulombic efficiencies over a long time, short battery cycle life, and safety issues with the explosive dangers of lithium metal. [1a,5] One notable strategy that has been proposed to address these problems is the "artificial SEI layer" method, which involves modifying the Li surface with functional foreign layers to