Towards Solid Batteries Operating at Ambient Temperature: Safe, Highly Conducting Polymer Electrolytes Based on Cross-Linked Polymer Matrixes

Standard commercial rechargeable batteries use liquid electrolytes as ion transport media, which are based on toxic and volatile organic carbonate solvents, and their flammability clearly raises safety concerns. The most striking solution at present is to switch on all solid-state designs exploiting polymer materials, films, ceramics, etc., particularly in Li-ion battery (LiB) sector. To enhance low ionic conductivity, low cation transport properties and stringent processing conditions (use of organic solvents), researchers worldwide have proposed several approaches including the most promising in situ preparation of thermoset polymer electrolytes using solvent-free processes. Ionic conductivity is directly linked to polymer chain mobility. It can be controlled through crystallinity reduction, which in turn increases the amorphous character of the polymer matrix, by means of addition of plasticisers, ceramic fillers, and proper crosslinking. In the present talk, an overview will be offered of the recent developments on innovative polymer electrolytes for Li/Na-based batteries by means of UV-induced photopolymerization (UV-curing), which is easily scalable to an industrial level due to its easiness and rapidity in processing, high efficiency and eco-friendliness as the use of solvent is avoided. Crosslinking produced during curing allows the incorporation of high amount of RTIL (e.g., imidazolium, pyrrolidinium) or tetraglyme and lithium salt (TFSI– anion), leading to polymer electrolytes with remarkable homogeneity and robustness. Samples are thermally stable up to > 300 °C, which is particularly interesting for application in LIBs with increased safety. Excellent ionic conductivity (> 0.1 mS cm–1 at 25 °C), high transport number, wide electrochemical stability (> 5 V vs. Li) and stable interfacial properties are obtained. To increase even more the cycling ability at (sub-) ambient temperatures, recent efforts have been dedicated to the formulation of composite hybrid polymer electrolytes (CPEs), where the ceramic superionic conducting material is homogeneously embedded in the polymeric matrix. As compared to the pristine components, CPEs are stiff while preserving flexibility, are easily processed, and can be conceived to attain improved ionic transport and interfacial contact with the electrodes. Electrochemical performances in lab-scale devices are evaluated by means of cyclic voltammetry and galvanostatic charge/discharge cycling, exploiting different electrode materials prepared by water-based procedures with green cellulosic binders. The lab-scale Li-polymer cells assembled with the different electrode materials (e.g., LiFePO4, Li-rich NMC, LiNiMnO2, TiO2) show highly reversible charge/discharge characteristics with full specific capacities at 0.1 mA cm-2, limited capacity fading upon very long-term reversible cycling and stable operation for hundreds of cycles at ambient temperature. Significantly, the feasibility of using novel electrolytes in real cell configuration at ambient temperature is established by suitably adopting direct in situ polymerization directly over the electrode films, thus obtaining an intimate electrode/electrolyte interface and a full active material utilisation. The obstacles related to hazardous reactivity towards Li-metal are nullified by the proposed approach, leading to the assembly of superior Li-/Na-based cells conceived for applications that demand high energy and/or power, including smart-grid storage and electric-/hybrid-electric vehicles.