Lithium-ion battery performance is governed by ionic transport mechanisms over a wide range of size scales.

The optimization of ionic conductivity and lithium-ion battery stability can be achieved by independently tuning the ion transport and mechanical robustness of block polymer (BP) electrolytes. However, the ionic conductivity of BP electrolytes is inherently limited by the covalent attachment of the ionically conductive block to the mechanically robust block, among other factors. Herein, the BP electrolyte polystyrene-block-poly(oligo-oxyethylene methacrylate) [PS-b-POEM] was blended with POEM homopolymers of varying molecular weights. The incorporation of a higher molecular weight homopolymer additive (α > 1 state) promoted a "dry brush-like" homopolymer distribution within the BP self-assembly and led to higher lithium salt concentrations in the more mobile homopolymer-rich region, increasing overall ionic conductivity relative to the "wet brush-like" (α < 1 state) and unblended composites, where α is the molecular weight ratio between the POEM homopolymer and the POEM block in the copolymer. Neutron and X-ray reflectometry (NR and XRR, respectively) provided additional details on the lithium salt and polymer distributions. From XRR, the α > 1 blends showed increased interfacial widths in comparison to their BP (unblended) or α < 1 counterparts because of the more central distribution of the homopolymer. This result, paired with NR data that suggested even salt concentrations across the POEM domains, implied that there was a higher salt concentration in the homopolymer POEM-rich regions in the dry brush blend than in the wet brush blend. Furthermore, using 7 Li solid-state nuclear magnetic resonance spectroscopy, we found a temperature corresponding to a transition in lithium mobility (T Li mobility ) that was a function of blend type. T Li mobility was found to be 39 °C above T g in all cases. Interestingly, the ionic conductivity of the blended BPs was highest in the α > 1 composites, even though these composites had higher T g s than the α < 1 composites, demonstrating that homopolymer-rich conducting pathways formed in the α > 1 assemblies had a larger influence on conductivity than the greater lithium ion mobility in the α < 1 blends.

Annealing colloidal nanocrystal coatings in a selenium-containing environment to form polycrystalline thin films of the earth-abundant solar absorber copper zinc tin sulfoselenide (CZTSSe) is an attractive approach for making solar cells. We used a closed selenization system to investigate how coatings comprising copper zinc tin sulfide (CZTS) nanocrystals evolve into polycrystalline CZTSSe thin films and studied the effects of selenium vapor pressure, annealing temperature, and heating rate. These studies revealed two different types of microstructures and two different grain growth mechanisms depending on whether the CZTS nanocrystals are exposed to selenium vapor only or to both selenium vapor and liquid selenium. Coatings annealed in the presence of selenium vapor form a microstructure comprising micron-size CZTSSe grains on top of a nanocrystalline, carbon-rich, CZTSSe layer. The film microstructure is controlled by concurrent normal and abnormal grain growth, and the grain size distribution is bimodal, similar to that observed when CZTS nanocrystal coatings are annealed in sulfur vapor. The size of the abnormal crystals increases with selenium pressure and temperature to as large as 4 μm after annealing at 700 °C in 450 Torr of selenium. Carbon, initially present on nanocrystals as dispersion stabilizing ligands, segregates to the region between the CZTSSe grains and the substrate instead of desorbing from the coating as volatile reaction products such as CSe 2 . Experiments suggest that carbon segregation occurs due to the tendency for CSe 2 to polymerize and form (CSe 2-x ) n . Coatings annealed in the presence of liquid selenium exhibit neither the bimodal grain size distribution nor the carbon-rich layer between CZTSSe grains and the substrate. In the presence of liquid selenium, the CZTS nanocrystals selenize, grow, and coarsen to ∼1 μm in size, forming compact CZTSSe films through liquid phase sintering, a mechanism wherein both grain size coarsening and film densification are mediated by the presence of a liquid phase.