In Situ Raman Mapping of Si Island Electrodes and Stress Modeling as a Function of Lithiation and Size

Si is known for cracking and delamination during electrochemicalcycling of a battery due to the large volume change associated withLi insertion and extraction. However, it has been found experimentallythat patterned Si island electrodes that are 200 nm thick and lessthan 7 & mu;m wide can deform in a purely elastic manner. Inspiredby this, we performed in situ Raman stress characterization of modelpoly-crystalline Si island electrodes using an electrochemical cellcoupled with an immersion objective lens and designed for a shortworking distance. A 5 & mu;m wide Si island electrode showed a parabolicstress profile during lithiation, while for a 15 & mu;m Si islandelectrode, a stress plateau in the center of the electrode was observed.A continuum model with coupled electro-chemo-mechanical (ECM) physicswas established to understand the stress measurement. A qualitativeagreement was reached between modeling and experimental data, andthe critical size effect could be explained by the Li diffusive fluxas governed by competition between the Li concentration and hydrostaticstress gradients. Below the critical size, the stress gradient drivesLi toward the edges, where the electrode volume is free to expand,while above the critical size, the stress plateau inhibits Li diffusionto the edge and forces destructive stress relief by cracking. Thiswork represents a promising methodology for in situ characterizationof ECM coupling in battery electrodes, with suggestions provided forfurther improvement.