Lowering the Interfacial Resistance in LLZTO:PEO Electrolytes By Covalent Surface Modifications

Abstract

Reducing the high interfacial resistance across the cubic Li7La3Zr2O12 (LLZO) | poly(ethylene oxide) (PEO) interface in LLZO:PEO membranes is one of the key challenges to realize future solid-state lithium batteries.1,2 Recently, several groups presented various approaches to enhance the ionic charge transport across the interface.3,4 As an example, Gupta and Sakamoto showed that heat treatment to remove surface impurities and that an increase in the Li-salt concentration in PEO to match the chemical potential between LLZO and PEO lead to significantly lower interfacial resistance.3 Herein, we present an alternative but complimentary approach to improve the interfacial kinetics by an advance surface modification strategy. For this purpose, we activated the surface terminated oxygen of Li6.4La3Zr1.4Ta0.6O12 (LLZTO) particles by plasma etching and functionalized the activated oxygen by immersing the LLZTO particles in a (3-glycidyloxypropyl)trimethoxysilane solution to form covalently bonded self-assembled monolayers (SAMs). The SAMs are terminated by an epoxy group that react with the hydroxyl group of the PEO’s according to a ring-opening reaction. This reaction may lead to an ordered arrangement of covalently bonded PEO segments around the LLZO particles enabling the Li ions to easily cross the LLZTO | PEO interface. Indeed, we managed to lower the interfacial resistance by more than four orders of magnitude. Finally, with our approach we realized an area specific resistance of 500 Ω cm2 at 20 °C; to our knowledge, this value is the lowest one reported so far. Reference s : [1] J. Schwenzel, K. Kun, F. Langer in: R. Murugan, W. Weppner (Eds.), Solid Electrolytes, Springer, Swizerland, 2019, pp. 281–304. [2] X. Judez, G.G. Eshetu, C. Li, L.M. Rodriguez-Martinez, H. Zhang, M. Armand, Joule 2 (2018) 2208–2224. [3] A. Gupta, J. Sakamoto, Electrochem. Soc. Interface 28 (2019) 63–69. [4] Z. Huang, W. Pang, P. Liang, Z. Jin, N. Grundish, Y. Li, C.A. Wang, C.A., J. Mater. Chem. A, 7 (2019) 16425–16436.