Amorphous Metal Fluorides Coatings By Atomic Layer Deposition for Stable Li-Ion Batteries

Metal fluorides pose interesting optical and chemical properties and thin films of these materials can be used in various applications to serve as optical coatings, protective layers, catalysts, solid state electrolytes, and barrier layers for thin film batteries. The properties of metal fluorides show broad trends based on the charge of the metal. Metals in an oxidation state of +3 or lower tend to form ionic, refractory fluorides. Metals of +5 oxidation or higher tend to form covalently bonded fluorides, polymers, and discrete molecules.  Consequently, bi-metallic fluorides comprised of high and low valence metals might possess novel properties. The development of thin film methods for preparing these materials may pave the way for new applications of nanophase materials.

In order to reduce the unfavorable side reactions in Li-ion batteries, ultrathin metal oxides such as Al₂O₃, LiAlO₂, and LiTaO₃, deposited by atomic layer deposition, have been demonstrated [1,2] to significantly enhance both stability and safety of electrodes. However, metal oxide coatings are likely to be susceptible to hydrofluoric acid (HF) attack upon electrochemical cycling. Metal fluorides may be suitable alternatives for electrode coating materials due to their stability against HF attack and the strong bonding between metals and fluorine.

Here we report the atomic layer deposition of complex fluorides such as Al_xW_yF_z and Mg_xW_yF_z utilizing WF₆ as the source of both fluorine and the high valence metal with Al(CH₃)₃ (TMA) and Mg(Cp)₂ [4]. In-situ quartz crystal microbalance studies were performed during the deposition of these chemistries and in both cases the QCM data showed self-limiting, linear growth. Films were prepared at 200^oC and the deposited layers of Al_xW_yF_z and Mg_xW_yF_z were found to be x-ray amorphous in nature.  SEM EDAX elemental mapping of these layers, deposited in high aspect ratio trenches, showed uniform and conformal distributions of the various elements within the films, Figure 1.   We have used ~1 nm layers of Al_xW_yF_z and Mg_xW_yF_z materials in an effort to improve the performance of a model Li-ion battery cathode, LiCoO₂ [5].  This presentation will present the results of ALD of these materials, characterization, and Li-ion battery stability tests.

References

(1) Jung, Y. S.; Cavanagh, A. S.; Dillon, A. C.; Groner, M. D.; George, S. M.; Lee, S.-H. Journal of The Electrochemical Society 2010, 157, A75.

(2) Park, J. S.; Meng, X.; Elam, J. W.; Hao, S.; Wolverton, C.; Kim, C.; Cabana, J. Chemistry of Materials 2014.

(3) Li, X.; Liu, J.; Banis, M. N.; Lushington, A.; Li, R.; Cai, M.; Sun, X. Energy & Environmental Science 2014, 7, 768.

(4) Anil U. Mane, and Jeffrey W. Elam, ALD (2014), Kyoto, Japan

(5) Park J. S, Mane A. U, Elam J. W., Croy J. R., Submitted to Chemistry of Materials