In-Situ TEM Studies of Fluoride Based Solid State Batteries

The increasing demand for energy storage resulted in improving the performance of Lithium ion batteries (LIB) and a continuing search for alternative battery technologies. Reversible batteries based on a fluoride anion shuttle (fluoride ion battery) are an interesting alternative to LIBs [1]. Fluoride ion batteries can theoretically achieve a high-energy density above 5000 Wh.L^-1 , which is e.g. 50% above the theoretical capacity of a Li air cell [2]. However, research in the field of fluoride ion batteries is at an early stage of development, needing large improvements to meet the requirements for application. Understanding the electrochemical reactions occurring in the battery electrodes during cycling is essential to improve the performance and cyclic stability of fluoride ion batteries.

Battery research groups worldwide, are try to directly observe the structural and chemical evolution of battery components in real space and to correlate this with the corresponding ex-situ cyclic behavior. In-situ analysis comprises complex sample environment systems, requiring careful development of experimental aspects to enable correlation with the true operating conditions. In-situ TEM is one of the few techniques that can provide direct structural and compositional information of micron-sized batteries during cycling. These in-situ studies often necessitate unique sample preparation techniques. An additional aspect is electron beam damage of battery materials, which modify the system and complicate the data interpretation. Here, beam damage challenges and sample preparation possibilities along with our strategies to identify an optimum system for in-situ TEM studies will be presented.

We selected a fluoride ion battery system for this study.Ball milling of a mixture of (1−y)LaF₃ and yBaF₂ was employed to prepare a La_0.9Ba_0.1F_2.9 solid electrolyte. This electrolyte was initially studied for its structure, composition and stability towards the electron beam. A mixture of Cu (90%), as an active material, and C (10%) was used as a cathode. The anode, in case of a half-discharged-state, was prepared from a mixture of Mg, MgF₂, La_0.9Ba_0.1F_2.9 (for ionic conductivity), and C (for electronic conductivity). Cathode, anode and electrolyte were pressed together to form a pellet. A focused ion beam system was used to prepare a thin cross-section of the complete battery and electrically contacted on an MEMS based device at the edge of the electrodes (Fig. 1a). For electrochemical measurements, an Aduro sample holder was used in the Titan 80-300 TEM. 

Variations in morphology, structure and composition of the electrodes, electrolyte and their interfaces were characterized using TEM, STEM, and SAED during electrochemical cycling (Fig. 1b). The HRTEM images, SAED studies and STEM-Map of the cathode at the interface indicate the formation of CuF₂ phase after charging (Fig. 1c,d), which was not present in the as-prepared state. The sample preparation, and the changes in the morphology, structure and composition of the La, Ba/ La_0.9Ba_0.1F_2.9/BiF₃ and the Mg/ La_0.9Ba_0.1F_2.9/CuF₂ systems will be presented and discussed.

References

[1] M. Anji Reddy & M. Fichtner, J. Mater. Chem., 2011, 43, 17059.

[2] C. Rongeat, M. Anji Reddy, R. Witter and M. Fichtner, J. Phys. Chem. C, 2013, 117 4943.