Arnaud Demortiere1,3,4,Sorina Cretu1,2,Nicolas Folastre1,3,4,David Traodec5,Rainer Straubinger6,Nynke Krans6,Martial Duchamp2
Laboratoire de Reactivite et Chimie des Solides1,Nanyang Technological University2,RS2E-French Research Network on Electrochemical Energy Storage3,ALISTORE- European Research Institute4,IEMN5,Protochips6
Arnaud Demortiere1,3,4,Sorina Cretu1,2,Nicolas Folastre1,3,4,David Traodec5,Rainer Straubinger6,Nynke Krans6,Martial Duchamp2
Laboratoire de Reactivite et Chimie des Solides1,Nanyang Technological University2,RS2E-French Research Network on Electrochemical Energy Storage3,ALISTORE- European Research Institute4,IEMN5,Protochips6
The increasing request for electric vehicles (EV) demands high energy density batteries, wide temperature compatibility and improved safety technology. Solid-state Li-ion batteries (SSB)<sup>1</sup> could represent a possible future solution for current battery technology as the liquid electrolyte containing flammable organic materials is replaced by a solid electrolyte (SE) removing the safety concerns. Moreover, SSB technology offers the possibility to use high voltage cathode materials as some SE present wide electrochemical window and also opens the possibility to use lithium metal as anode resulting in batteries with a higher energy density<sup>2</sup>.<br/>SSB performances improved over the last years, but there are still challenges as interfaces, grain boundaries formation, generation of cracks and dendrites during the cycling which needs to be solved in order to make them a sustainable future battery technology<sup>2–4</sup>. To get a better insight into the limiting parameters of SSB performances comprehensive information about the dynamic process occurring at the interfaces between the solid electrolyte and the electrodes during the electrochemical reaction at nanoscale are essential, thus <i>in-situ</i> experiments are being required. <i>in situ</i> Transmission Electron Microscopy (TEM)<sup>5,6</sup> represents the perfect tool for understanding the failure mechanism in ASSB as it allows us to visualize the interfaces, will enable us to follow the structural changes using electron diffraction and 4D-STEM as well as the chemical changes using EDX and EELS mapping.<br/>During this study, SSB containing Li<sub>1.5</sub>Al<sub>0.5</sub>Ge<sub>1.5</sub>(PO<sub>4</sub>)<sub>3 </sub>(LAGP) oxide solid electrolyte due to their high stability in the air, LiFePO4 (LFP) as positive electrode and Li<sub>3</sub>V<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub> (LVP) as negative electrode allowing us to obtain a full inorganic solid-state battery in one single shot using Spark Plasma Sintering (SPS)<sup>7</sup>.<br/><i>In situ</i> TEM experiment carried out using our microbattery obtained by Focused Ion Beam (FIB) and connected on Protochips chip revealed the simultaneous process of LAGP particle size reduction with the formation of grain boundaries areas between the solid electrolyte grains during the cycling. 4D-STEM analysis performed before and after the<i> in-situ </i>TEM cycling experiment revealed the presence of amorphous regions in the SE and electrode mixture after the cycling process suggesting that the grain boundary phase formed after the cycling process is amorphous. The presence of FePO<sub>4</sub> (FP) phase structure analyzed by 4D-STEM mapping is consistent with delithiation process occurred over the electrochemical cycling. The phase mapping also revealed a domino-cascasde lithiation process with a clear phase separated between LFP and FP grains. Moreover, the presence of Al-rich areas was spotted in the LAGP in the initial state and those areas represented the weak point in the SE as they were the initial point for crack propagation in the battery. STEM-EDX map analysis was performed before and after the cycling process displayed the unexpected presence of carbon in the pristine SE which possibly increased the electronic conductivity and contributed to the crack propagation. EDX map revealed that oxygen signal from the near vicinity of the solid electrolyte/ positive electrode was also diminished after the electrochemical reaction, most likely reacting with the carbon and leading to the formation of LiCO<sub>3 </sub> producing possible irreversible changes at the interface solid electrolyte/positive electrode.<br/>1. Janek, J. & Zeier, W. G. <i>Nature Energy</i> <b>1</b>, 1–4 (2016).<br/>2. Hatzell, K. B. <i>et al.</i> <i>ACS Energy Letters</i> <b>5</b>, 922–934 (2020).<br/>3. Lewis, J. A., Tippens, J., Cortes, F. J. Q. & McDowell, M. T. <b>1</b>, 845–857 (2019).<br/>4. Zhao, W., Yi, J., He, P. & Zhou, H. <b>2</b>, 574–605 (2019).<br/>5. Fawey, M. H. <i>et al.</i> <i>Journal of Power Sources</i> <b>466</b>, 228283 (2020).<br/>6. Meng, Y. <i>et al.</i> <i>Electrochemical Society Interface</i> 49–53 (2011).<br/>7. Aboulaich, A. <i>et al.</i> <i>Advanced Energy Materials</i> <b>1</b>, 179–183 (2011).