Towards solid batteries operating at ambient temperature: (composite) polymer electrolytes based on cross-linked PEO matrix

Standard commercial batteries use liquid electrolytes as ion transport media; these 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. Polymer electrolytes are proposed as a safe alternative and, particularly in lithium-ion battery (LiB) sector, the transformation from liquid-state to solid-state construction is expected to improve safety, fast and reliable fabrication, and temperature stability. However, several constraints have impeded their intrusion into the mainstream such as low ionic conductivity, low cation transport properties and stringent processing conditions (use of organic solvents). Thus, researchers worldwide have proposed several approaches including the most promising in situ preparation of thermoset polymer electrolytes using the well-established solvent-free processes. Ionic conductivity can be controlled by reducing the crystallinity, which in turn increases the amorphous character of the polymer matrix. Indeed, lithium ion conduction is directly linked to polymer chain mobility. Several approaches have been adopted to reduce the crystalline behaviour of the polymer matrix such as addition of plasticisers, ceramic fillers and proper crosslinking. In the first part of the talk, an overview will be offered of the recent developments on innovative polymer electrolytes for alkali metal, particularly lithium-based, batteries. In our Labs, we develop different kind of polymer electrolytes by means of different techniques, including simple solvent casting and free radical UV-induced photopolymerization (UV-curing). UV-curing takes place at RT: a liquid poly-functional 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, thus easily scalable to an industrial level. All samples are thoroughly characterized in the physico-chemical and electrochemical viewpoint. They exhibit excellent ionic conductivity and wide electrochemical stability window, which ensure safe operation even at ambient conditions. Electrochemical performances in lab-scale devices are evaluated by means of cyclic voltammetry and galvanostatic charge/discharge cycling exploiting different electrode materials (eventually prepared by water-based procedures exploiting green carboxymethylcellulose as binder) as well as direct polymerization procedures on top of the electrode films in order to obtain an intimate electrode/electrolyte interface and a full active material utilisation. Nowadays, possible concerns about the safety of rechargeable lithium metal batteries has postponed their introduction into the smart electronics or automotive industries and have promoted advances in the field of non-flammable solid electrolytes. Among the oxide ceramic super lithium ion conductors, garnet-type Li7La3Zr2O12 (LLZO) has recently attracted much attention because of its relatively high ionic conductivity at room temperature (>10-4 S cm–1), negligible electronic conductivity and absence of harmful decomposition products upon contact with atmospheric moisture. Anyway, processing LLZO in pellets by sintering, results in brittle and more or less porous electrolytes, which often display poor interfacial contact with Li metal electrodes. Moreover, there are some reports of lithium dendrite growth and instability towards the cathode material - especially while processing of the electrode at high temperature - referred to cells assembled with this electrolyte family. To circumvent these problems, recent efforts have been dedicated to the formulation of composite hybrid polymer electrolytes (CPEs), where the ceramic material is embedded in a 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 conductivity and interfacial contact with the electrodes. As a result, in the second part of the talk, the results will be given about the development of a polymer based matrix containing poly(ethylene oxide) (PEO), lithium bis (trifluoromethylsulphonyl)imide (LiTFSI), tetra(ethylene glycol dimethyl ether) (G4) and a photoinitiator, which was added with LLZO particles, thoroughly mixed, formed into a film and cross-linked under UV radiation to obtain a composite hybrid electrolyte. This easy procedure allows obtaining self-standing CPEs with desirable properties of flexibility, shape retention upon thermal stress, improved interfacial contact with the electrodes and ionic conductivity suitable for practical application. Lab-scale lithium metal cells assembled with the CPEs and lithium iron phosphate (LFP) cathodes demonstrated specific capacities approaching 150 mAh g-1 at 0.1 mA cm-2 and could work for hundreds of cycles at ambient temperature. A multidisciplinary approach is adopted in our Labs to understand the role of photopolymerisation in tailor making the integral and requisite properties of the resulting polymer electrolyte to achieve acceptable conductivity, ionic mobility and resilience towards dendrite-induced short circuit reactions. Significantly, the feasibility of using novel electrolytes in real cell configuration at ambient temperature with various nanostructured electrodes is established by suitably adopting in situ polymerization directly over the electrode films. The obstacles related to hazardous dendrites and reactivity towards Li-metal are nullified, leading to the assembly of superior Li-ion and Li-metal cells conceived for applications that demand high energy and/or power, including smart-grid storage and electric-/hybrid-electric vehicles. We anticipate that the proposed approach would lead to a rational designing to address the significant challenges of alkali metal ion polymer batteries.