Nanoparticle-coated separators for lithium-ion batteries with advanced electrochemical performance

Abstract

We report a simple, scalable approach to improve the interfacial characteristics and, thereby, the performance of commonly used polyolefin based battery separators. The nanoparticle-coated separators are synthesized by first plasma treating the membrane in oxygen to create surface anchoring groups followed by immersion into a dispersion of positively charged SiO 2 nanoparticles. The process leads to nanoparticles electrostatically adsorbed not only onto the exterior of the surface but also inside the pores of the membrane. The thickness and depth of the coatings can be fine-tuned by controlling the f-potential of the nanoparticles. The membranes show improved wetting to common battery electrolytes such as propylene carbonate. Cells based on the nanoparticle-coated membranes are operable even in a simple mixture of EC/PC. In contrast, an identical cell based on the pristine, untreated membrane fails to be charged even after addition of a surfactant to improve electrolyte wetting. When evaluated in a Li-ion cell using an EC/PC/DEC/VC electrolyte mixture, the nanoparticle-coated separator retains 92% of its charge capacity after 100 cycles compared to 80 and 77% for the plasma only treated and pristine membrane, respectively.The rapid emergence and popularity of portable electronics has motivated extensive research efforts to meet the ever increasing demands for mobile power sources in terms of energy storage capacity, form factor, weight, and lifetime with minimal safety risk and environmental impact. Currently, Li-ion rechargeable batteries are the benchmark of this technology and are the subject of intense research and development.1-3 Certain operational limitations of the Li-ion batteries are directly or indirectly related to the performance characteristics of the separator that is a key component of each electrochemical converter. 4 The separator is essentially a diaphragm, whose function is to prevent physical and electrical contact between the electrodes, while allowing ionic current flow. [5][6][7] Ideally, the separators for Li-ion batteries should be porous and thin but mechanically robust. In addition, they should exhibit chemical and dimensional stability, low thermal shrinkage, and good wetting to common liquid electrolytes. Microporous membranes based on polyethylene (PE) or polypropylene (PP), their blends, their copolymers and their laminates have found widespread application as separators for Li-ion batteries, since they adequately fulfill most of the requirements presented above. The main drawback of the polyolefin separators is their poor wetting to cyclic electrolytes with high dielectric constant such as ethylene carbonate (EC) and propylene carbonate (PC), an effect that results in increased internal resistance of the cells. Modifications of the polyolefin surface can improve the wetting characteristics of the separators but the chemistry is rather challenging and the performance depends critically on the nature and the grafting density of the functional groups. [8][9][10][11][12][13][14][15] ...

Lithium ion battery separators: Development and performance characterization of a composite membrane

“…Heat exposure to these stretched separators initiates re-coiling of the elongated polymer chains, observed as shrinkage or the shape memory effect, causing stress to build at the fixed points of the separator. Creating non-woven mat separators and coating commercial separators with fine ceramic particles have been evaluated to mitigate or avoid the thermal shrinkage [4][5][6][7][8][9][10][11][12][13][14][15][16].…”

“…However, these separators alone do not provide the sufficient flexibility, especially when they are thin. Therefore, one route is to use them as coating layers on commercial separators or even on electrodes directly [13][14][15][16]. Particle coated separators have exhibited an improved thermal stability at high temperatures, without significant thermal shrinkage, due to a specified loading of micro or nano particles [13][14][15].…”

Manufacture and Surface Modification of Polyolefin Separator

“…This treatment increased the surface energy of the PP separator from 37.8 to 47.7 m N/m, and accordingly led to about nine times increase in the alkali absorption ratio (AAR) without loss over 90 days storage at ambient temperature. Plasma grafting has also been used as the modifying carrier to coat the polyolefin separator with nanoparticles by first plasma treating the membrane in oxygen to create surface anchoring groups and then immersing the membrane into a dispersion of positively charged SiO 2 nanoparticles [53]. This treatment led to the nanoparticles electrostatically adsorbed onto the separator, which resulted in immediate wetting of the polyolefin separator by the highly polar ethylene carbonate and propylene carbonate that otherwise are unable to wet the pristine polyolefin separator.…”

Functional Nanomaterials For Energy And Sustainability

“…A novel coating approach has been recently developed that focuses on the electrostatic immobilization of positively charged nanoparticles to the plasma treated substrates in order to create a uniform and persistent surface coverage [37]. The advantage of the method is that it allows the deposition of nanoparticles not only to the exterior of the membrane, but also to the internal pores at a depth of several microns.…”

“…At the same time, the electrolyte uptake is 4 times higher for the coated membrane and the ionic conductivity of the wet membranes exhibit a 5-fold increase. As a result, cells equipped with the nanocoated separators exhibit superior electrochemical performance [37]. …”