Methyl Methacrylate (MMA)-Grafted Polypropylene (PP) Separator for Rechargeable Lithium Metal Battery

Abstract

In this article the polypropylene (PP) separator grafted by methyl methacrylate (MMA) functional groups with plasma technique has been primarily investigated. The MMA groups are expected to form a stable interface with lithium metal electrode. In addition, the effect of the surface functional group MMA on the graphite and the LiFePO4 electrode has been also primarily examined. The cyclic voltammetry (CV) tests have exhibited faster deposition/dissolution reaction kinetics of lithium on stainless steel electrode. The impedance measurements have shown a smaller interface resistance between the electrolyte (PP-MMA separator) and the lithium metal electrode. The interface reaction kinetics of the electrolyte (separator)/lithium electrode can be effectively enhanced by the MMA surface functional groups. It is also found that the existence of the MMA-groups at the separator surface can increase the reversible capacity of both LiFePO4 and graphite electrode. Rechargeable secondary batteries with lithium metal as the negative electrode have a relatively high energy density in comparison with those with carbon-based materials as the negative electrode due to the high theoretical capacity of 3860 mAh/g. In the early stage of the researches on lithium battery systems, many attempts had been made to improve the cycle life of the lithium metal electrode. Later researchers paid their attention on the splendid appearance of the graphite-based carbon materials. As well-known, the carbon-based materials have been successfully used as the negative electrode in the commercial rechargeable Li-ion batteries nowadays. In fact, lithium metal electrode still has potential in the rechargeable lithium-based micro-batteries and the all-solid-state lithium batteries, etc. 1–6 For some devices such as the implantable medicine devices and microchips, the output power of the batteries is relatively small, but the cycle life must be long enough. Safety is the most important factor to the commercial applications; the solution leakage is absolutely not allowed in such systems. This type of battery is named as energy-type battery. The issue that the lithium metal anode faces is that the formation of the lithium dendrites leads to two main consequences. Firstly, the electrically isolated lithium is not to be used and will shed. Secondly, the penetration of fine lithium particles into the separator will cause the internal short. There are three main strategies that are used to improve the cyclic performance of the lithium metal electrode. The first strategy has been extensively investigated by researchers and is just on Li-alloys and Li-intermetallics. 2 The improvement of the cycle stability of the lithium electrode is expected by modifying the solid electrolyte interface (SEI) or by buffering the volume expansion. The second one is focused on the electrolyte additives, such as HF, CO2, sulfides and surfactants. 7 The additives play the role of decreasing the content of the deleterious impurities in SEI layers, stabilizing the SEI, modifying the morphology of SEI and weakening or delaying the formation of the lithium dendrites by the specific adsorption of these surfactants on the convex points of the surface that has a relatively high surface energy. Ishikawa and Morita 8 have compiled the related results. We thought that the first strategy came from the lithium metal itself; the second one originated from the electrolyte. There also exists the third strategy which is on the separator. After being assembled, the lithium metal electrode is in close contact with the separator. A more stable SEI layer can be constructed by modifying the surface of the separator and by grafting some functional groups into the separator surface. These functional groups are expected to improve the stability of the interface between the liquid electrolyte (separator) and the lithium metal electrode. In commercial lithium ion batteries, the separator is also in contact with the positive