Electrochemical Flash Sintering: A New Tool to Obtain All Solid-State Batteries in Few Seconds
Authors: Lachal Marie , El Khal Hana , Bouvard Didier , Chaix Jean-Marc , Bouchet Renaud , Steil Marlu Cesar 
Published: May 2020

Nowadays, inorganic All Solid-State Batteries (ASSBs) encounter a strong resurgence of scientific and technological interest stimulated by the discovery of new high conductivity solid electrolytes at room temperature (≥ 1 mS), as well as the development of fast Electrical Current Activated Sintering (ECAS) processes. One of the advantages of ASSBs is that they guarantee a unique safety due to the chemical and thermal stability of the materials. However, manufacturing dense ceramics requires sintering temperatures generally higher than 700 °C to lead to optimal mechanical and conductive properties. At these temperatures, the occurrence of undesired chemical reactions at the interfaces is an issue for electrochemical applications due to the formation of interdiffusion layers that block the charge transfer.
For this purpose, Flash Sintering (FS) is very effective to densify conductive ceramics in a few seconds at significantly lower temperatures compared to conventional sintering.1,2 FS consists in making the current directly flows through the sample without neither the use of a conductive die nor the application of a charge. However, this technique requires a reversible electrochemical reaction to allow the charge transfer between the electronic conductor (i.e. the current collectors) and the ionic conductor (i.e. ceramic electrolyte).3 In the case of pure cationic conductors (Li+, Na+, K+, etc.), Pt electrodes which are commonly used for FS, are herein blocking electrodes preventing the current from flowing. A mixed cationic electronic conductor is then required to ensure the charge transfer reaction.
Through this work, we demonstrate the feasibility of FS on Li+ ionic conductors such as Li1.3Al0.3Ti1.7(PO4)3 (LATP) or Li1.5Al0.3Ge1.5(PO4)3 (LAGP), using LiCoO2 (LCO) or LiNi1/3Mn1/3Co1/3O2 (NMC) mixed Li+/e- conductors as electrode materials. Using the concept of Electrochemical Flash Sintering, the “flash” event was obtained on two multi-component systems: LATP sandwiched between two pure LCO layers as electrodes and LATP sandwiched between two composite electrodes (LCO + LATP). Microstructural and chemical analyses were employed to characterise the densification at the vicinity of the interfaces. It is shown that a composite electrode both allows the flash to occur and prevents the delamination observed with pure LCO by lowering interfacial strains.4 This multi-material architecture turns out to be the one of an All-Solid-State Li-ion Battery, opening the path of using FS as a promising fast process to build functional multi-materials for energy storage devices.
1. Cologna, M.; Rashkova, B.; Raj, R. Flash sintering of nanograin zirconia in < 5 s at 850°C. J. Am. Ceram. Soc. 2010, 93, 3556–3559.
2. Steil, M. C.; Marinha, D.; Aman, Y.; Gomes, J. R. C.; Kleitz, M. From conventional ac flash-sintering of YSZ to hyper-flash and double flash. J. Eur. Ceram. Soc. 2013, 33, 2093–
3. Caliman, L.B.; Bouchet, R.; Gouvea, D.; Soudant, P.; Steil, M.C. Flash sintering of ionic conductors: the need of a reversible electrochemical reaction, J. Eur. Ceram. Soc. 2016, 36, 1253–1260.
4. Lachal, M.; El. Khal, H.; Bouvard, D.; Chaix, J-M.; Bouchet, R.; Steil, M.C. Electrochemical Flash Sintering of advanced multi-materials for energy storage devices, J. Eur. Ceram. Soc. submitted.

Figure 1