Composite solid electrolytes toward safer Lithium-metal cells

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 so-called “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, replacement of liquid electrolytes with a solid electrolyte has been recognized as a fundamental approach to effectively address the above problems. 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. For example, the preparation of methacrylate-based polymer matrix, in a one pot, solvent free, thermally induced radical polymerization, allows the addition of a wide range of organic and inorganic additives up to really high ratio. Meanwhile, eventual activation with small amount of liquid electrolyte allowed to obtain SCEs 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. In particular, the use of inorganic additive permitted to enhance mechanical properties thus avoiding lithium dendrites nucleation and stabilizing Li stripping/platting in Li metal cells, while addition of dextrin based nanosponges limiting oxygen cross-over phenomenon in Li-O2 cells allowed to stabilize SEI layer, hence greatly improving cells safety in both cases.