Towards Aging Resistant Lithium Polymer Batteries for Wide Temperature Applications

A wide interest is mounting in the field of polymer electrolytes, due to their application in energy efficient devices such as rechargeable batteries, photo-electrochemical cells, electrochromic devices, fuel cells and super capacitors. Polymer electrolytes exhibit unique advantages such as mechanical integrity, wide variety of fabrication methods in desirable size and shape, possibility to fabricate an intimate electrode/electrolyte interface and adapt to a lightweight, leak proof construction, safety and economic packaging structure. Free radical photo-polymerization (UV-curing) can be an interesting alternative process to produce polymer electrolytes for Li-ion batteries. It takes place at ambient temperature: a liquid polyfunctional monomer, containing a proper photo-initiator, forms a cross-linked film upon UV irradiation. It appears highly advantageous, due to its easiness and rapidity in processing, very short time with high efficiency and eco-friendliness as the use of solvent is avoided.

In the present work, profoundly ion conducting, self-standing and tack-free ethylene oxide based polymer electrolytes encompassing a room temperature ionic liquid (RTIL) with specific amounts of lithium salt are successfully prepared via a rapid and easily up-scalable UV curing process. All the prepared materials are thoroughly characterized in terms of their physical, chemical and morphological properties, and eventually galvanostatically cycled in lab-scale lithium batteries (LIBs).

The appearance of the cross-linked polymer electrolyte is shown in Figure 1A, where the remarkable mechanical abusability is demonstrated. The crosslinking produced by UV irradiation allows the incorporation of higher amount of RTIL (imidazolium, pyrrolidinium etc.) and/or tetraethylene glycol dimethyl ether (tetraglyme) with lithium salt (based on TFSI^{- ­}anion), leading to a material with remarkable morphological characteristics in terms of homogeneity and robustness. FESEM analysis was conducted to characterize the morphology of the photo cured polymer films. A representative top-view is shown in Figure 1B, where the typically wrinkled texture characteristics of highly amorphous, cross-linked polymer electrolytes is evidenced. The bright and dark areas in the image belong to amorphous PEO domains alternated to some residual ordered (semi crystalline) domains, respectively; the wrinkled texture derives from the formation of crosslinking domains between the polymer chains. Similar kind of textures are also present in polymer films kept at temperature under high stress (50 bar for 15 mins at 90 °C) during the preparation process. The UV-cured PEO-based polymer network is able to efficiently hold the RTIL without any leakage. Tensile analysis confirmed that the UV-irradiated membranes showed an average Young’s modulus Eof 0.2 ± 0.05 MPa. If one considers that more than 50 wt. % of ionic liquid is generally incorporated, these are highly satisfying values. The SPE showed the thermal stability up to 375 °C under inert conditions, and such a remarkable result is particularly interesting for application in Li-ion batteries with increased safety. X-ray diffraction analysis (XRD) was used to get further information on the fundamental role of photo curing step in reducing the overall crystallinity of the SPE to nearly fully amorphous state.

The SPE exhibited excellent ionic conductivity (>10^–4 Scm^–1 at room temperature, Figure 1C), electrochemical stability (>5V vs. Li⁺/Li), and interfacial stability. At 20°C the conductivity value is equal to 2.5×10^-4 Scm^-1. This result is encouraging as it is sufficiently high to allow ambient temperature operation of Li-ion cells. It exceeds 10^-3 Scm^-1 already at 50°C. The Vogel-Tamman-Fulcher (VTF) behavior of the SPE was verified fitting the conductivity data with respect to temperature. The value of activation energy was found to be 8.23 kJ mol^-1. The ability to resist the lithium dendrite nucleation and growth was tested by galvanostatic polarization studies. They showed resistance at current intensity >0.1mAcm^-2. The lab-scale Li-polymer cell assembled showed stable charge/discharge characteristics without any capacity (135 mAh g^-1) fading at C/5 current regime (Figure 1D). The overall performance of the SPEs postulates the possibility of effective implementation in the next generation of safe, durable and high energy density secondary all-solid Li-metal polymer batteries working at ambient and/or sub-ambient temperatures.

Acknowledgement: MARS-EV project has received funding from the European Union Seventh Framework Program (FP7/2007-2013) under grant agreement n° 609201. Lithops batteries S.r.l. provided the LiFePO₄electrode.

Figure 1. A) Appearance of the light-cured cross-linked polymer electrolyte, B) FESEM analysis at high magnification of the sample surface; C)Arrhenius plot for ionic conductivity as a function of temperature of the polymer electrolyte membrane (in the inset, the fitting by means of the VTF equation), D) the graph showing the specific capacity vs cycle number, at 50°C and coulombic efficiency.