Protection of lithium metal is of significant interest to the battery community: as the lightest and most electropositive alkali metal, lithium has a low density, resulting in an extremely high specific capacity value of 3.86Ah/g and energy density of 11,701Wh/kg, making it the holy grail of battery anodes.  However, due to the extreme reactivity of lithium metal surfaces, controlled protection and characterization are extremely difficult.

We have developed a bespoke integrated deposition, transfer, and characterization system capable of coating lithium metal anodes using Atomic layer deposition (ALD), characterizing the surface chemistry using XPS/SEM/UPS/Auger spectroscopy, and transfer directly from UHV into an Ar glovebox for battery assembly, disassembly, and electrochemical testing.  After testing, the metal anodes can be transferred directly back to the XPS/SEM/UPS/Auger system for characterization.

Protection coatings for lithium metal require the coatings be conformal, easily deposited at low temperature, and stable against organic electrolytes.  Preventing direct contact of the lithium metal surface with the electrolyte and any dissolved species that would ordinarily react will ensure that the lithium atoms are oxidized in a reversible manner, rather than losing ions/electrons to highly stable, low ionic conductivity surface species such as Li₂CO₃.  Lithium metal protection may allow the relaxation of purity requirements for both electrolytes and, in the case of Li-O₂ batteries, cathode gas purity.

To date, Li metal protection has been demonstrated with coatings of chlorosilanes[1] and other polymeric[2], organic[3], and inorganic coatings, but ALD has not been applied directly to lithium metal anodes for the purposes of protection.  Known for conformality, low temperature deposition, excellent thickness control, ALD serves as an excellent permeation barrier, suggesting that it can be pinhole-free a promising technique for this application.

We have developed ALD processes suitable for the protection of Li metal, and we assess the behavior of these functional ALD coatings electrochemically using over-potential studies and electrochemical impedance spectroscopy measurements.

 Surface characterization by semi in-situ XPS of protected and pristine lithium metal subjected to both gaseous environments and electrolytes with controlled/measured H₂O content will be discussed, where the surface characterization is accomplished without ambient air exposure.

We show that protection of lithium metal is not only possible with ALD, but that it is effective at preventing the tarnishing of the lithium metal surface due to reaction with atmospheric H₂O and CO₂.  Using our unique capabilities, we are able to probe the chemistry of both protected and unprotected lithium metal, while using in-situ gas dosing we can decouple and characterize the interface chemistry and reactions.

We demonstrate as a proof of concept that a 14 nm thick, ALD Al₂O₃ layer can protect the Li surface from corrosion due to atmosphere, sulfur, and electrolyte exposure. 

Furthermore, using Li-S battery cells as a test system, we demonstrate an improved capacity retention using ALD protected anodes over cells assembled with bare Li metal anodes for up to 100 cycles.

This work has implications and impact on the metallic lithium anode battery systems Li-O₂, Li-S and Li-NMC. Additionally, it is promising to pursue corresponding protection layers for Mg ad Na, where similar deleterious surface reactions may apply.  Fundamental understanding of the controlled surface protection of lithium by ALD coatings could be extended to these other next-generation metal anode battery systems, and indeed could have a beneficial impact on the cyclability and storage lifetimes of lithium metal anode batteries.

References:

[1]         F. Marchioni, K. Star, E. Menke, T. Buffeteau, L. Servant, B. Dunn, and F. Wudl, “Protection of Lithium Metal Surfaces Using Chlorosilanes,” Langmuir, vol. 23, no. 23, pp. 11597–11602, Nov. 2007.

[2]         S. M. Choi, I. S. Kang, Y.-K. Sun, J.-H. Song, S.-M. Chung, and D.-W. Kim, “Cycling Characteristics of Lithium Metal Batteries Assembled with a Surface Modified Lithium Electrode,” J Power Sources, vol. 244, no. c, pp. 363–368, Dec. 2013.

[3]         J. Heine, S. Krüger, C. Hartnig, U. Wietelmann, M. Winter, and P. Bieker, “Coated Lithium Powder (CLiP) Electrodes for Lithium-Metal Batteries,” Adv. Energy Mater., pp. 1–7, Nov. 2013.