Cheap and easily processable polymer electrolytes for sodium-based batteries

The importance of clean and efficient energy production and storage has enormously grown over the past decade, primarily driven by concerns over global warming, dwindling fossil-fuel reserves and increasing demand for portable electronics, electric mobility and grid storage systems. Renewables are the fastest-growing source of energy for electricity generation; in particular, non-hydropower renewables accounted for 5% of total world electricity generation in 2012 and their share in 2040 is around 14%, with much of the expected growth coming from wind power and photovoltaics. Wide-scale implementation of renewable energy will demand a sudden growth of inexpensive, safe and efficient energy storage systems. Batteries that could function at the power grid level would help to stabilize electricity pricing and balancing the peak demands by ensuring a steady supply of energy. Nowadays, Li-ion batteries (LiBs) represent the most widely used technology in this respect. One of the arduous challenges in this field is the substitution of conventional liquid electrolytes based on organic solvents, which are volatile and hazardous. Solid polymer electrolytes (SPEs) exhibit appealing properties to replace liquid electrolytes. Moreover, research efforts are directed towards alternative systems to LIBs, because lithium is expensive and its resources are geographically constrained. Sodium exhibits suitable electrochemical properties, close to those of lithium, and it is very abundant. These features make Na-based batteries proficient candidates to replace LiBs, particularly when large-scale energy storage is envisaged.Here, we offer an overview of our recent developments on polymer electrolytes for Na-ion batteries. Polymer electrolytes were prepared through different techniques, exploiting both simple casting and UV-curing and using various additives to improve specific characteristics (e.g., RTILs, cellulose derivatives, glymes). In particular, UV-curing allowed obtaining self-standing polymer electrolytes with desirable properties of flexibility, shape retention upon thermal stress, improved interfacial contact with the electrodes and ionic conductivity suitable for practical application. Thermal, mechanical, morphological and electrochemical properties of the resulting polymer electrolytes were thoroughly investigated. They exhibited excellent ionic conductivity and wide electrochemical stability window, which ensured safe operation at ambient conditions. Electrochemical performances in lab-scale devices are presented, evaluated by means of cycling voltammetry and galvanostatic charge/discharge cycling exploiting different electrode materials (prepared by water-based procedures with green cellulosic binders). The highly ionic conducting (>1 mS cm–1 at 25 °C) polymer electrolytes were used in a lab-scale sodium cell with nanostructured working electrodes (e.g., hard carbons, TiO2, high voltage phosphates). The obtained results in terms of ambient temperature cycling behaviour (stable specific capacity of about 250 mAh g–11 at 0.1 mA cm–2 and overall remarkable stability, for a quasi-solid state Na polymer cell, upon very long term cycling exceeding 1000 reversible cycles at 0.5 mA cm–2 corresponding to > 5000 h of continuous operation) demonstrate the promising prospects of this novel XPE to be implemented in the next-generation of sodium-based batteries conceived for large-scale energy storage systems, such as those connected to photovoltaic and wind factories. Work on Na-ion polymer batteries for moderate temperature application is at an early stage, only lab-scale cells were demonstrated so far. Nevertheless, with the appropriate choice and optimization of electrode/electrolyte materials (and successful combination thereof), the intriguing characteristics of the newly developed polymer electrolyte systems here presented postulate the possibility of their effective implementation in safe, durable and high energy density secondary Na-based polymer devices conceived for green-grid storage and operating at ambient temperatures.