Towards safer post Li-ion technologies

Introduction Nowadays, electrical energy storage is one of the most critical issue to answer global warming by effectively replacing fossil energies by renewable ones. The Li-ion technology, widely studied and available on the market for multiple application is now reaching its limits and does not represent alone a viable option toward energetic transition. Therefore, one option currently under study is the use of metallic lithium as anode both in Li-ion cells and in the socalled “post Li-ion technologies”. In this perspective, lithium metal anode represents the “holy grail” of battery research for its extremely high theoretical specific capacity (3860 mA h g-1), the lowest redox potential (-3.040 V vs the standard hydrogen electrode) and a low gravimetric density (0.534 g cm-3). However, metallic Li also presents many challenges derived primarily from dendrite formation upon cycling causing both safety issues and poor cycling performance. In addition, liquid electrolytes contain combustible organic solvents that can cause leakage and fire risks during overcharge or abused operations, especially in large-scale operation. Therefore, evolution from liquid electrolytes with towards innovative composite solutions has been recognized as a fundamental approach to effectively address the above problems. Materials and methods The preparation of methacrylate-based polymer matrix, in a one pot, solvent free, either thermally or UV induced, radical polymerization, allows the addition of a wide range of organic and inorganic additives up to really high ratio to new composite electrolytes. Meanwhile, eventual activation with small amount of liquid electrolyte results in solid composite electrolytes with outstanding room-temperature conductivities for metallic lithium batteries. The simplicity of the formulation and the preparation method open the road to highly versatile electrolytes, adaptable in function of the final application. Results and Discussion Generally, all solid-state electrolytes can be classified into 3 categories: solid polymer electrolytes (SPE), inorganic ceramic electrolytes (ICE) and solid composite electrolytes (SCE). While SPEs suffer from poor ionic conductivity at room temperature and low thermal and electrochemical stability, and ICEs from poor interfacial contact with electrodes, SCEs solve these issues and benefit from both their advantages such as good ionic conductivity, good flexibility and intimate contact with the electrodes. In particular, the use of inorganic additive in high concentration permitted to enhance mechanical properties thus avoiding lithium dendrites nucleation and stabilizing Li stripping/platting in Li metal cells. In parallel, the introduction in the precursor formulation, of the dynamic quadruple hydrogen-bonding ureidopyrimidinone (UPy) group allowed the fabrication of self-healing polymer electrolytes, beneficial in the performances of the cells. Meanwhile, addition of dextrin based nanosponges limiting oxygen cross-over phenomenon in Li-O2 cells allowed to stabilize SEI layer and improve the cycling. Last but not least, introduction of sensors directly inside the cells is an ulterior tool to monitor and enhance cells safety.