Enhanced Thermal Resistance and Electrochemical Performance of the Trilayered PP/PE/PP Separators Using Alumina Coating for Lithium-Ion Batteries
Authors: Liao Chun-Chien , Fu Chun-Chien , Juang Ruey-Shin 
Published: February 2020

Lithium-ion batteries (LIBs) have served as a potential power source for portable electric vehicles and energy storage devices due to their high energy capability, high power density, cost competitiveness, long cycle life and little memory effect. However, safety issue is still one of the most challenges in affecting widespread use of LIBs as power sources. As a key component of LIBs, the separator should maintain electronic insulation between cathode and anode, enabling free ionic transport and isolating electronic flow. To reach the demand of safety issue, the design of separator takes one task that is responsible for preventing an internal short circuit from thermal evolution. An ideal separator is therefore expected to possess high ionic conductivity, excellent thermal stability, and superior electrochemical performance. One strategy to improve the thermal stability and electrolyte wettability on the separators is introducing ceramic coating layers containing hydrophilic inorganic powders such as ZrO2, SiO2, TiO2, and Al2O3. The above achievements direct us at the development of facile methods to improve the polymeric separators to meet the requirements of high-performance LIBs.

Recently, polymeric membranes such as the tri-layered PP/PE/PP have been extensively used as separators for LIB applications. This work work adopts a chemical-wet impregnation method to prepare alumina-coated separator, using commercial Al2O3 nanoparticles and poly(vinylidene fluoride-hexafluoropropylene) (PVdF-HFP) binder. The PVdF-HFP binder contains a ternary mixture comprising the polymer (PVdF-HFP), solvent (acetone), and nonsolvent (water), which is appropriate to mix with ceramic nanoparticles. One potential electrode material, spinel Li4Ti5O12, is served as the anode for the analysis of electrochemical performance, using charge-discharge cycling and electrochemical impedance spectroscopy.

Three densities of Al2O3 nanoparticles are uniformly dispersed and coated onto the surface of tri-layered polymeric separators. The bare and Al2O3-coated separators were denoted to U, AS1, AS2, and AS3 according to the weight ratio of Al2O3 to PVdF-HFP solution of 0, 0.06, 0.12, and 0.25 wt.%, respectively. TGA curves reflect that the thermal resistance of tri-layered separators is significantly enhanced after the deposition of Al2O3 coating layers. The alumina deposits display high thermal resistive ability under O2-containing atmosphere. Moreover, FE-SEM images show that the alumina nanoparticles with an average particle size of 300 nm are uniformly dispersed on the surface of the separators. With increasing alumina loading, close-packed Al2O3 nanoparticles interconnected by PVdF-HFP binder are grouped and coated upon the surface of the separators. Typical XRD patterns reveal that bare separator (U) possesses its original reflection bands, while alumina-coated separators (AS1, AS2, and AS3) exhibit additional representative bends, confirming the presence of α-Al2O3 crystals. On the other hand, the mass uptake of the electrolyte (1.0 M LiPF6 in a mixture of ethylene carbonate, propylene carbonate, and dimethyl carbonate with a weight ratio of 1:1:1) on the bare separator (U) is 270 mg cm-2, whereas that on AS1 separator attains as high as 515 mg cm-2. Also, the mass uptake on AS3 separator is 2.2 times higher than that of the bare one. The extent of thermal shrinkage on AS3 sample is ~2.26 times higher than that on U sample. This confirms that an obvious enhancement on thermal stability of polymeric polymer can achieve after the introduction of alumina coating, also demonstrated by the TGA results.

With regard to electrochemical performance, the LIB using AS3 separator delivers the highest discharge capacity among these samples, i.e., 166 mAh g-1 (0.1C) and 160 mAh g-1 (1C). The enhanced rate capability is presumably due to high Li-ion storage capacity and excellent wettability, imparting low Li-ion diffusion resistance and then speeding up electronic conduction. With the aid of Al2O3 coating layer, the equivalent series resistance of electrode is significantly reduced and the ionic conductivity is strongly improved, according to analysis of ac impedance spectroscopy. In a word, the amount of Al2O3 nanoparticles is picked up as a key parameter in examining electrochemical performance and thermal stability of LIBs. The enhanced performance is ascribed to the fact that Al2O3 coating layer is served as a robust skeleton to stabilize the separators, imparting a superior insulation and mass transport barrier against volatile compounds formed during the thermal decomposition process.