Lithium Recovery from Geologic Brines by Advanced Cation-Exchange Ionomers

Abstract

As of today a massive restructuring of the energy system is taking place at the global level to curtail the use of fossil fuels and foster the large-scale implementation of renewable sources such as the sun and the wind. This is a crucial stepping stone to minimize the emissions of the greenhouse gases associated with the combustion of fossil fuels, with the ultimate goal to slow down global warming and ensure that the development of humankind could become compatible with the conservation of the natural environment.A major drawback of most renewable sources (e.g., the sun and the wind) is that they are intermittent and non-programmable. Hence, energy storage and distribution technologies are necessary to close the spatial and temporal gaps between the production of energy and its exploitation by end users. In this regard, lithium batteries (LiBs) are becoming one of the most promising tools to profitably use the energy obtained from renewable sources to power to a broad variety of applications, from portable electronics to light-duty (LD) vehicles.After decades of R&D efforts a large-scale rollout of LD vehicles powered by LiBs is currently taking place worldwide. Consequently, the production of massive amounts of LiBs is underway, straining the traditional supply chains of lithium and leading to a huge increase in the cost of this element, now listed by the European Union as a Critical Raw Material [1]. Accordingly, major efforts are dedicated at the global level to identify new and unexploited sources of lithium. In recent years it was acknowledged the potential of geologic brines to supply lithium. Geologic brines are widely available and abundant worldwide. However, an effective and inexpensive technology to selectively extract Li from a brine (that is typically much richer in other cations such as Na+, and Ca2+) is still not available.In this contribution a family of very inexpensive and chemically-stable cation-exchange ionomers is proposed. The physicochemical features of the ionomers are elucidated, with a particular reference to the cation-exchange capacity, the morphology (probed by scanning electron microscopy) and the porosimetric features (investigated by nitrogen physisorption techniques). In a second step it is studied the kinetics of cation binding upon immersing the ionomers into a suitable synthetic brine closely mimicking a natural specimen and including the following cations: Li+, Na+, K+, Ca2+ and Mg2+. The kinetic information is then used in experiments meant to determine the binding features at equilibrium of the various cations present in the synthetic brine to the ionomers.The following binding features of the ionomers are determined: (i) number of distinct binding sites; (ii) number of cations that can be coordinated to each binding site; and (iii) maximum theoretical binding of each cation to the ionomer, with a particular reference to Li+ [2]. The physicochemical properties of the ionomers are finally correlated to the binding parameters, allowing to suggest new strategies to devise enhanced ionomers able to extract selectively and cheaply Li+ from the brines.