On the Relevance of the Polar β-Phase of Poly(vinylidene fluoride) for High Performance Lithium-Ion Battery Separators

Abstract

Separator membranes based on poly(vinylidene fluoride), PVDF, poly(vinylidene fluoride-co-trifluoroethylene), PVDF-TrFE, poly(vinylidene fluorideco-hexafluropropylene), PVDF-HFP and poly(vinylidene fluoride-cochlorotrifluoroethylene), PVDF-CTFE were prepared by solvent casting method using N,N-dimethylformamide (DMF) as solvent. In all cases, the same polymer/solvent ratio and solvent evaporation temperature were used. For all membranes, porous microstructure is achieved with a degree of porosity larger than 50%. The β-phase content as well as degree of crystallinity were different for each membrane, which were lower for the co-polymer membranes when compared with PVDF. On the other hand, the observed ionic conductivity values, electrolyte uptake, tortuosity and MacMullin number were similar for all membranes. The electrochemical performance of the separator membranes was evaluated in Li/C-LiFePO4 half-cell configuration showing good cyclability and rate capability for all membranes. Among the all separator membranes, PVDF-TrFE demonstrate the best electrochemical performance, with a discharge capacity value of 87 mAh.g-1 after 50 cycles with a capacity retention of 78 % at 2C. Finally, the correlation between the β-phase content in the membranes and the cycling performance was demonstrated (which was significant at high-C rates): larger β-phase contents, leading higher polarity, facilitates faster lithium ion migration within the separator for similar microstructures.

Carbon Nanolights in Piezopolymers are Self‐Organizing Toward Color Tunable Luminous Hybrids for Kinetic Energy Harvesting

“…Similar to PVDF, PVDF‐TrFE is a semicrystalline polymer with at least four polymorphs (α, β, γ, and δ) . The content and structure of crystalline β phase is responsible for its piezoresponse magnitude . However, homopolymer PVDF generally crystallizes in nonpolar α phase, whereas copolymer PVDF‐TrFE readily crystallizes into polar β phase regardless of processing method .…”

“…The content and structure of crystalline β phase is responsible for its piezoresponse magnitude . However, homopolymer PVDF generally crystallizes in nonpolar α phase, whereas copolymer PVDF‐TrFE readily crystallizes into polar β phase regardless of processing method . The introduction of TrFE co‐monomer into VDF sterically hinders the formation of a trans‐gauche‐trans‐gauche ( tg + tg − ) configuration .…”

“…However, homopolymer PVDF generally crystallizes in nonpolar α phase, whereas copolymer PVDF‐TrFE readily crystallizes into polar β phase regardless of processing method . The introduction of TrFE co‐monomer into VDF sterically hinders the formation of a trans‐gauche‐trans‐gauche ( tg + tg − ) configuration . It forces the molecular chains into an all‐trans ( tttt ) conformation (β phase) .…”

“…[29][30][31][32][33][34][35][36][37] Diversified approaches have been developed via the mechanisms of elec tromagnetic induction, electrostatic generation, or piezo electric effect. [29][30][31][32][33][34][35][36][37] Especially, piezoelectric nanogenerators (PNGs) show more advantages in capturing the dynamic mechanical energy with variable amplitudes and frequen cies. [29][30][31][32][33][34][35][36][37] Recent advances in PNGs focus on boosting power generation/conversion efficiency and developing hybridized piezoelectric materials for multiple adaptive electronics and optoelectronics.…”

Physico‐Chemical Characterizations of Poly(vinylidene fluoride)/Cu3(BTC)2 Composite Membranes Prepared by In Situ Crystal Growth

“…The choice of PVDF was encouraged by its ability to form homogenous solution with metallic salts. In addition, thanks to the outstanding electroactive properties of its β ‐phase, PVDF and its membranes are important materials with extensively technological applications such as membrane distillation sensors , lithium‐ion battery , energy harvesting , and actuators .…”