The Cathode-Electrolyte Interface: Insight and Functionality from Solid State NMR Spectroscopy

A lithium-ion battery cell consists of an anode and a cathode separated by a liquid electrolyte. During electrochemical cycling, electrons flow through an external circuit and lithium ions are extracted from/intercalated into the electrodes across their interface with the electrolyte. The performance of the cell, namely its capacity, power and lifetime, depends on the formation of a stable layer at the electrode-electrolyte interface. Li ion transport via this interface enables stable cycling of the battery. In addition, this layer prevents other non-reversible reactions from taking place at the interface that can impede Li ion transport and reduce the cell’s capacity.

The chemical composition and structure of this layer are the building blocks for understanding the functionality of the electrode-electrolyte interface. Nuclear Magnetic Resonance (NMR) spectroscopy, which is a non-destructive characterization technique, enables us to gain information about the detected nuclei. In addition to providing information about the chemical composition and structure of the material, NMR is unique in its ability to directly follow ion dynamics at a wide range of timescales.

Here I will present results from composition NMR studies of the LiNi_0.5Mn_1.5O₄ cathode interface. Combined with electrochemical analysis of the battery performance we observe reversible accumulation of organic and inorganic species on the surface of the electrode.

In addition, I will describe new NMR methodology for probing Li ion dynamics across the interface. We utilize chemical exchange saturation transfer (CEST) for detecting small populations of Li ions at the electrode interface which undergo chemical exchange with the large pool of Li ions in the electrolyte. Preliminary examination of the CEST effect on a model system of partially delithiated LiCoO₂ immersed in electrolyte reveal new Li environment of exchanging ions.