Innovative High Performing Metal Organic Framework (MOF)-Laden Composite Polymer Electrolytes for All Solid State Lithium Batteries

The unique properties such as high single cell working potential, energy density, long cycle life, etc., have identified secondary Li–ion batteries as the ultimate power source for portable electronic devices such as laptop computers, cellular phones, digital cameras, etc. However, their possible widespread penetration into the large-scale hybrid electric vehicle (HEV) and plug-in HEV markets can be realized only when substantial improvements such as low cost, sustainability, safety, high rate capability and long calendar life are achieved.

The advancement in lithium battery technology relies mainly upon replacement of the conventional liquid electrolyte by an advanced solid polymer electrolyte. In order to achieve this goal, many lithium-conducting polymeric networks have been prepared and characterized. Among the polymer hosts explored so far, poly(ethylene oxide) - PEO has been indeed the most extensively studied system [1]. Unfortunately, solid polymer electrolytes comprising a polymer host and a lithium salt (e.g., PEO + LiClO₄) exhibit low ionic conductivity at ambient and sub-ambient temperatures. Numerous attempts have been made to enhance the ionic conductivity of PEO-based electrolytes; one of the most common ways is the addition of low molecular weight liquid plasticizers like ethylene carbonate, propylene carbonate, etc. The addition of plasticizers, despite ameliorating the ionic conductivity, it adversely deteriorates the mechanical integrity and safety; moreover, side reactions with lithium metal eventually occur.

Recently, new interesting structures have been described in the literature as metal organic frameworks (MOFs) [2]. Generally speaking, they are microporous solids consisting of an infinite network of metal centres (or inorganic clusters) bridged by simple organic linkers through metal–ligand coordination bonds. MOFs are widely used in catalysis, sensors, ion exchange, gas storage, purification, separation and sequestration; they are also used in optoelectronics to improve both electronic and proton conductivity. Nevertheless, to the best of our knowledge, so far no attempt has been made on the development of PEO-based composite polymer electrolytes (CPEs) encompassing an ad hoc synthesized Al-BTC (aluminium benzenetricarboxylate, see Fig. 1) MOF and a proper lithium salt showing an enhanced ionic conductivity of more than two order of magnitude at low temperature and excellent stability towards lithium metal even after a prolonged storage time. The obtained metal organic frameworks and macromolecular composite networks are thoroughly characterized from the structural, morphological and physico-chemical viewpoint of and, for the first time, an excellent long-term electrochemical behaviour in a lab-scale LiFePO4/CPE/Li cell is demonstrated (noteworthy stable even at low 50 °C), thus accounting for the development of high performing, safe all-solid-state lithium batteries. Even though lots of literature data are available on ceramic based fillers, the studies based on MOF encompassed polymer electrolytes are truly interesting due to its tailor-making capability and the broad spectrum of opportunities and possibilities it can bring. If very well-tuned, the MOF based fillers will be a true promising candidate and a vital ingredient which can enforce the intrusion of polymer electrolytes into the huge market of lithium-based batteries. 

[1] E. Quartarone, P. Mustarelli, Chem. Soc. Rev., 2011, 40, 2525.

[2] M. Eddaoudi, H. Li, O.M. Yaghi, J. Am. Chem. Soc., 2000, 122, 1391