Aramid and zirconia coated separator for enhanced electrochemical performance of lithium-ion batteries

In recent years, the demand for hybrid electric vehicles (HEVs) and plug-in hybrid electric vehicles (PHEVs) has increased due to the desire to reduce increased emission of greenhouse gases and consumption of fossil fuels. Lithium ion batteries (LIBs) are regarded as suitable power sources for EV applications because of their high energy and power. However, safety is still an important and serious concern in the development of LIBs for these applications. To improve the safety of these batteries, many researchers have focused on the improvement of the thermal and mechanical properties of separators. The separator plays a key role in ensuring the safety of LIBs by preventing short circuit between the cathode and anode.This implies that the separator must be a good electrical insulator and must possess good mechanical and dimensional stability as well as electrochemical stability toward electrolyte and electrode materials. Polyethylene (PE) separators have been widely used in commercial LIBs and have many advantages such as good electrochemical stability, high mechanical strength, small pore size, and thin frames.Despites these properties, the poor thermal stability of PE separators is a disadvantage that needs to be resolved. When exposed to high-temperature environments, PE separators exhibit extensive thermal shrinkage and significant structural degradation, which may trigger internal short circuits in the LIBs. Consequently, the thermal stability of PE separators needs to be further improved to meet the rigorous safety standards for LIBs for EV applications. Since the introduction of separators such as safety-reinforced separators (SRS) by LG Chem., various ceramic composite separators have been proposed for practical use in LIBs. For example, Takemura et al. proposed a ceramic composite separator that was made of Al2O3 ceramic powder and a polymer binder on a PE framework and that exhibited good thermal stability. Zhang et al. also reported a CaCO3-based composite separator with excellent thermal stability, in which CaCO3 ceramic particles coating the separator acted as a heating-resistant material suppressing the thermal shrinkage of the separator.Previous works have largely focused on the improvement of thermal properties of separators by incorporating various ceramic particles. However, the effect of the ceramic particles on the electrochemical performance of LIBs has not yet been intensively studied. We herein propose a ceramic composite separator that is coated by a polymeric binder [poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-12wt%HFP) copolymer] and moisturized ZrO2 nanoparticles. We demonstrate the structural changes of the polymer coating layer induced by the moisture adsorbed on ZrO2 nanoparticles, and discuss the correlation between the microstructure of the ZrO2-composite separator and its electrochemical and thermal properties. Our findings can lead to robust ceramic composite separator designs that will improve thermal stability of LIBs for EV applications. Fig . 1. FE-SEM images of the composite separators with different coatings at different magnifications. (a) A separator coated with only PVDF-12wt%HFP copolymer. (b) The same separator at 1µm. (c) A composite separator coated in PVDF-12wt%HFP copolymer and dried ZrO2 nanoparticles (moisture content : 100 ppm). (d) The same separator at 1µm. (e) A composite separator coated in PVDF-12wt%HFP copolymer and moisturized ZrO2 nanoparticles (moisture content : 3000 ppm). (f) The same separator at 1µm. Figure 1

The commercial microporous polyethylene (PE) separators generally exhibit low thermal stability, poor electrolyte wettability, and significant safety concerns. Drawing inspiration from the formation process of biominerals, a collagen layer is first self-assembled on the PE separator surface to form a cross-linked network coating. Subsequently, the collagen matrix is mineralized with calcium carbonate nanocrystals, which undergo oriented growth within the collagen fibrils, thereby generating a stable inorganic mineral layer on the PE substrate (Mc@CaCO3-PE). The organized structure markedly enhances the thermal stability and mechanical strength of the composite separator. Moreover, compared with conventional PE separators, it demonstrates superior electrolyte wettability, achieving an electrolyte absorption rate as high as 161.6%. Notably, the mineralized layer facilitates the sustained release of Ca2+ ions, which facilitates the desolvation process of Li+ ions. It not only increases the lithium-ion transference number (0.82) but also promotes the formation of a stable solid-electrolyte interphase (SEI). At a current density of 0.5 mA cm-2, Li||Li symmetric cells with a Mc@CaCO3-PE separator can be stably cycled for more than 1200 h. This composite separator shows great potential as a high-performance separator for lithium metal batteries, and this strategy provides valuable guidance for the development of other high-performance composite separators.

A simple layer-by-layer (LbL) self-assembly process of poly(acrylic acid) (PAA) and ZrO2 was applied to construct functional ultrathin multilayers on polyethylene (PE) separators without sacrificing the excellent porous structure of separators. Such PAA/ZrO2 LbL-modified PE separators possess good electrolyte wettability, excellent electrolyte uptake, high ionic conductivity and large Li(+) transference number. More importantly, the top layer of LbL self-assembly would affect the dissociation of electrolyte and the formation of solid electrolyte interphase (SEI) layer in half-cells. Compared with the pristine and (PAA/ZrO2)1PAA-modified PE separators, (PAA/ZrO2)3-modified PE separator shows a larger Li(+) transference number (0.6) and a faster tendency to form a stable SEI layer, endowing half-cells with excellent capacity retention at high C-rates and superior cycling performance. These fascinating characteristics will provide the LbL self-assembly with a promising method to improve the surface property of PE separators for high performance lithium-ion batteries.

Enhanced thermal stability and lithium ion conductivity of polyethylene separator by coating colloidal SiO2 nanoparticles with porous shell

Novel ZrO2@Polyimde nano-microspheres-coated polyethylene separators for high energy density and high safety Li-ion battery

To overcome the shortcomings in the thermal stability and electrolyte wettability when a commercial polyethylene (PE) separator is used alone, a PE/glass fiber (GF)−Mg(OH)2/PE composite (PGMP) separator is developed, and the electrochemical and safety performance of lithium‐ion batteries is effectively enhanced. The PGMP separator is prepared by soaking a mixed dispersion solution of polyacrylate and Mg(OH)2 into the GF fabric substrate and subsequently bonding the PE microporous film on both substrate sides. Compared with the PE separator, PGMP separator exhibits enhanced mechanical strength (≥250 MPa), ionic conductivity, and electrolyte wettability. Furthermore, there is almost no shrinkage when this PGMP is annealed at 350 °C for 30 min. The nail penetration, impact, overcharge, and adiabatic rate calorimeter tests of LiNi0.5Co0.2Mn0.3O2/graphite pouch cell with a nominal capacity of 2500 mAh show that the PGMP separator effectively improves the safety performance. The thermal runaway temperature of the pouch cells is increased from about 120 to 146 °C, and the electrolyte wettability ability of the PGMP separator gives the cell a capacity retention of 85% after 500 cycles at 1.0 C. Combined with the advantages, it is indicated that this PGMP separator has great potential in commercial applications.

In this study, a novel composite separator based on polytetrafluoroethylene (PTFE) coating layers and a commercial polyethylene (PE) separator is developed for high performance Li-ion batteries. This composite separator is prepared by immersing a PE separator directly into a commercial PTFE suspension to obtain a self-binding PTFE/PE/PTFE tri-layered structure. Then, the as-prepared composite separator is further treated with a H2O2/H2SO4 solution to enhance its electrolyte affinity. The results show that the coating layer, consisting of close-packed PTFE particles, possesses a highly ordered nano-porous structure and an excellent electrolyte wettability property, which significantly enhance the ionic conductivity of the composite separator. Due to the presence of the PTFE-based coating layer, the composite separator exhibits better thermal stability compared with the PE separator, reaching the thermal-resistant grade of commercial ceramic-coated separators. By using different separators, CR2032-type unit half-cells composed of a Li anode and a LiFePO4 cathode were assembled, and their C-rate and cycling performances were evaluated. The cell assembled with the composite separator was proven to have better C-rate capability and cycling capacity retention than the cell with the polyethylene separator. It is expected that the composite separator can be a potential candidate as a coating-type separator for high-performance rechargeable Li-ion batteries.

New flame-retardant composite separators based on metal hydroxides for lithium-ion batteries

Development of plasma-treated polypropylene nonwoven-based composites for high-performance lithium-ion battery separators

In-depth correlation of separator pore structure and electrochemical performance in lithium-ion batteries

Preparation and electrochemical performance of ZrO2 nanoparticle-embedded nonwoven composite separator for lithium-ion batteries

Thermally stable poly-aromatic solid electrolyte coated polyethylene membrane as high-performance lithium-ion battery separator

ABSTRACTNovel composite polyethylene (PE) separators were prepared via thermally induced phase separation using SiO2 as the inorganic dopant and dioctyl phthalate as solvent. Through the control of silica content, the microstructure, thermal and crystalline properties, electrolyte uptake, thermal stability, and mechanical properties of the as‐prepared separators were investigated. The results showed that the doped silica particles favored the formation of the large pore size. The crystalline degree of PE was enhanced when doping silica in the ternary system below 3 wt %. The liquid electrolyte uptake was increased from 30.2% to 63.2% with doping silica content at 5 wt % in the mixed system, which benefits from the large pore size structure and the hydrophilicity of silica. The thermal decomposition temperature of the composite PE separators is 40°C higher than the pure PE separator due to the steric stabilization and the more stable space structure induced by the doped silica. The tensile strength was increased from 12 MPa to 13.3 MPa when doping 1 wt % silica, but decreased with further silica addition. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40724.

To improve the safety of lithium-ion batteries, poly(phenylene oxide)/mesoporous silica composite separator is facilely prepared by non-solvent induced phase separation wet-process and investigated in lithium-ion batteries (LIBs). Systematical investigations including morphology, porosity, electrolyte contact angle testing, electrolyte uptake/retention, and thermal shrinkage testing are carried out. The results demonstrate that the composite separator possesses notable features, such as uniform and porous surface morphology, symmetric interconnected porous structure all through the separator thickness. And owing to the relatively thermal-resistance constituents and well-developed microstructure, this separator shows superior thermal stability under 180 °C with little area shrinkage, higher electrolyte uptake/retention and ionic conductivity than the common polyethylene (PE) separator. Based on the above advantages, a cell with the as-prepared composite separator exhibits higher discharge capacity and better cycle property than that with a porous polyethylene separator. These results suggest that poly(phenylene oxide)/mesoporous silica composite separator is an effective separator for high performance LiBs.

Waterborne spontaneous and robust coating of POSS nanoparticles on hydrophobic surfaces for application as an advanced separator in lithium ion batteries