With the advent of the microelectronics and the "smart-devices", the need for micro-sources of energy has risen. To meet these needs, a massive production of microbatteries is required in the next decades. Physical vapor deposition (PVD) is the process commonly used to fabricate thin film all solid state microbatteries. Commercially, LiCoO₂ and lithium are used respectively as positive and negative electrodes. LiPON is commonly used as solid electrolyte because of its good ionic conductivity, good thermal and electrochemical stability and also for its good interfacial stability with the electrodes. However, the PVD process requires very expensive deposition machineries, with a high energy cost, which impact consequently the price and the development of the microbatteries. Processes involving liquid step, such as ink jet printing or screen printing propose cheaper machinery and much higher production rate. Moreover, these techniques are precise enough to deposit any pattern either by located drop jetting or by use of specific mask fitting the desired shape.

In this communication, we will report our efforts to develop all or partially printed microbatteries. We have worked on replacing LiPON by another solid electrolyte which could be deposited by printing. Ionogels are solid electrolyte in which an ionic liquid (IL) is confined within a solid host network. Although solid like electrolytes, ionogels are formed from a liquid precursor, and are thus appropriate materials for the purpose. The liquid precursor is obtained by mixing the IL with organic, inorganic or hybrid monomers that are then polymerized to build in-situ the confining network. Furthermore, ionogels show good ionic conductivity and also good thermal and electrochemical stability, thus making them attractive solid electrolyte for lithium batteries. The time required to form a fully solidified material can vary depending on the nature of the confining network and the polymerization process. By using photo-polymerization, which is very fast, the manufacturing time is lesser than an hour, while for LiPON it can last more than ten hours.

We have identified a promising printable photo-polymerizable ionogel composition with high ionic conductivity, pressure-sensitive-adhesive character, low interface resistance with lithium electrode, which allows stable cycling of LiFePO₄/lithium cells, at least up to 1200 cycles at ambient temperature, C/5 rate and for surface capacities typical of microbatteries, e.g. 0.15 mAh cm^-2. This ionogel is based on short molecular weight monomers with high content of polymerizable functionalities (triacrylate and diacrylate) and ILs composed of fluorinated sulfonylimide anions combined with alkylated pyrrolidinium cations [1].

We have evaluated the feasibility of making all or partially printed microbatteries with this printable solid electrolyte, not starting from the ground but trying to fit in an existing industrial process that uses PVD for manufacturing microbatteries. Microbatteries assembled from a dense PVD made LiCoO₂ layer, a printed photo-polymerized ionogel, and a lithium layer exhibit good performance. However, the photo-polymerized ionogel showed very low compatibility when the lithium electrode is directly deposited by PVD on the top of it. Thus the lithium electrode must be mechanically assembled with the ionogel electrolyte. Microbatteries assembled from a printed porous LiFePO₄ composite layer [2], a printed photo-polymerized ionogel, and lithium layer (an already made foil or evaporated layer and mechanically pressed on the ionogel surface) also demonstrated good performance.

Acknowledgements

Financial funding from the project TOURS 2015 n° O12590-418393 (Programme d’Investissement d’Avenir – FSN – AAP Nanoélectronique n°1) is acknowledged.

References

[1] D. Aidoud, A. Etiemble, D. Guy-Bouyssou, E. Maire, J. Le Bideau, D. Guyomard, B. Lestriez, "Interfacial stability and electrochemical behavior of Li/LiFePO₄ microbatteries using novel soft and weakly adhesive photo-ionogel electrolytes”, J. Power Sources, 330, 2016, 92-103.

[2] P.-E. Delannoy, B. Riou, T. Brousse, J. Le Bideau, D. Guyomard, B. Lestriez, “Ink-jet printed porous composite LiFePO₄ electrode from aqueous suspension for microbatteries”, J. Power Sources, 2015, 287, 261-268.