Chun-Er Lin

Progress in polymeric separators for lithium ion batteries

This study reviews the recent developments and the characteristics of polymeric separators used for lithium ion batteries. According to the structure and composition of the separators, they are broadly divided into four types: (1) polyolefin microporous separators, (2) heterochain polymer microporous separators, (3) polymer electrolytes and (4) non-woven separators. In particular, polymer electrolytes were defined as one category of separators for convenient description in this review; these feature intermediates between the two electrodes and possess transport properties comparable with the separator in liquid LIBs. For each category, the structure, characteristics, modification, and performance of separators are described. Finally, guidelines for further improvements in this research are outlined.

Gel polymer electrolyte-based on PVDF/fluorinated amphiphilic copolymer blends for high performance lithium-ion batteries

This paper describes the preparation and properties of PVDF/P(hexafluorobutyl methacrylate-co-poly(ethyleneglycol)methacrylate)(P(HFBMA-co-PEGMA)) blend gel polymer electrolyte (GPE) for high-performance lithium-ion batteries. The fluorinated amphiphilic copolymer, P(HFBMA-co-PEGMA), was synthesized by simple radical polymerization and then blended into poly(vinylidene fluoride) (PVDF) matrix via immersion phase inversion process. The composition, morphologies, liquid electrolyte uptake of the blend membranes and electrochemical properties of the corresponding GPEs were systematically investigated. It is found that the introduction of P(HFBMA-co-PEGMA) results in a slight increase in porosity, a reduction in crystallinity and better affinity with liquid electrolyte, which consequently lead to a substantial increase in liquid electrolyte uptake and ion conductivity. For the membrane with P(HFBMA-co-PEGMA)/PVDF mass ratio in 1.7/10, the liquid electrolyte uptake and ionic conductivity reach to 387% and 3.19 mS cm−1, respectively. In addition, the resulting GPE is electrochemically stable up to about 4.5 V (vs. Li+/Li).

Carboxylated polyimide separator with excellent lithium ion transport properties for a high-power density lithium-ion battery

The improvement of lithium ion transport properties, along with the ever-increasing demand for high-power density, is key to boosting the development of lithium-ion batteries. Here, we present a new class of carboxylated polyimide (PI) separator, which can be fabricated via an alkali treatment-based surface modification. The –COOH groups with unshared electron pairs were proposed to contribute to the desolvation of lithium ions and an increase in the lithium ion transport rate. Notably, the modification did not destroy the microstructure of the PI separator, and thus the effect of –COOH groups on the lithium ion transport properties was clearly demonstrated in this work. The result showed that the carboxylated PI separator was conducive to improving the lithium ion transference number (up to 0.87), which is four times higher than that for the original PI separator. More importantly, for the first time, the –COOH group was calculated to increase the lithium ion transport rate by more than six times. Benefiting from its high lithium ion transference number and slightly increased ionic conductivity, the cell assembled with the carboxylated PI separator achieved a better cycle performance and higher rate capability than that with the original PI separator.

High permselectivity hyperbranched polyester/polyamide ultrathin films with nanoscale heterogeneity

Membranes with both high permeability and selectivity are desirable for practical separation. In this work, three hyperbranched polyesters (HPEs) with different numbers of hydroxyl-terminated groups and molecular structures were, respectively, incorporated into a polyamide film formed by the interfacial reaction between the mixtures of HPE/piperazine (PIP) and trimesoyl chloride (TMC) on PVC hollow fiber substrates to endow the corresponding thin film composite (TFC) membranes with different permselectivity performances. The successful incorporation of HPEs into the cross-linked polyamide matrix and their gradient distribution in the corresponding selective layer were confirmed by ATR-FTIR and XPS analyses. Moreover, the permeation experiments for the fabricated TFC membranes revealed that HPEs most likely existed within the network or/and aggregate pores of the polyamide matrix due to their nanometer sizes and flexible molecular structures. Both the changes in the pore structures and the increase in the hydrophilicity of the polyamide matrix with the introduction of abundant hydroxyl groups pending in the HPE molecules led to the permeate flux of the TFC membrane increasing significantly. Importantly, nearly spherical H40 HPEs with intramolecular cavities could act as a molecular sieve to endow the selective layer with a high rejection capability. Meanwhile, the H40/PIP selective layer with a more negatively charged surface exhibited a higher rejection for SO42− ions while maintaining a low rejection for Cl− ions. These findings encourage further exploration of a new alternative material with such structures like HPE by interfacial polymerization to construct an ultrathin barrier film with high permselectivity performance.