Mechanistic Understanding of Additive Reductive Degradation and SEI Formation in High‐Voltage NMC811||SiO_x‐Containing Cells via Operando ATR‐FTIR Spectroscopy

Abstract

The implementation of silicon (Si)‐containing negative electrodes is widely discussed as an approach to increase the specific capacity of lithium‐ion batteries. However, challenges caused by severe volume changes and continuous (re‐)formation of the solid‐electrolyte interphase (SEI) on Si need to be overcome. The volume changes lead to electrolyte consumption and active lithium loss, decaying the cell performance and cycle life. Herein, the additive 2‐sulfobenzoic acid anhydride (2‐SBA) is utilized as an SEI‐forming electrolyte additive for SiOx‐containing anodes. The addition of 2‐SBA to a state‐of‐the‐art carbonate‐based electrolyte in high‐voltage LiNi0.8Mn0.1Co0.1O2, NMC811||artificial graphite +20% SiOx pouch cells leads to improved electrochemical performance, resulting in a doubled cell cycle life. The origin of the enhanced cell performance is mechanistically investigated by developing an advanced experimental technique based on operando attenuated total reflection Fourier‐transform infrared (ATR‐FTIR) spectroscopy. The operando ATR‐FTIR spectroscopy results elucidate the degradation mechanism via anhydride ring‐opening reactions after electrochemical reduction on the anode surface. Additionally, ion chromatography conductivity detection mass spectrometry, scanning electron microscopy, energy dispersive X‐ray analysis, and quantum chemistry calculations are employed to further elucidate the working mechanisms of the additive and its degradation products.

Lithium-ion battery cell formation: status and future directions towards a knowledge-based process design

“…Silicon-based active materials, such as Si or silicon dioxide (SiO x ), offer higher energy density potential, which can be up to ten times the theoretical capacity of Gr. 197–199 However, large volume changes of up to 300% place significant demands and can lead to pulverization, lithiation retardation, and unstable SEI growth, as can be seen in Fig. 10B.…”

“…201 However, the formation of a stable layer is critical to prevent thick SEI and continuous capacity loss. This challenge can be addressed by the development of new electrolytes 202,203 or additives, 197,200,204 material design strategies, 205 the cycling strategy 206 and the surface properties. 207–209 In addition, Si electrodes undergo significant morphological changes, including crystalline-to-amorphous phase transitions, especially during initial charge–discharge cycles.…”