The Zaghib Group at Hydro-Québec has used in situ SEM to see SiOx particles grow and shrink during cycling. SiOx is a promising anode material for Li-ion batteries due to a high theoretical specific capacity of 1338 mAh/g and less volume change than Si upon charge-discharge. Analysis of the morphology changes in SiOx particles provides insight into the failure mode associated with capacity fade on cycling.
In order to observe individual SiOx particles using in situ SEM, the SiOx-graphite electrode needs to be coated with a solid polymer electrolyte to prevent solvent evaporation under vacuum. Graphite is added to the electrode because its high conductivity and ability to form a stable solid electrolyte interface improves the rate capability and cyclability. After lithium is laminated to form the counter electrode, a cross section of the cell is exposed for SEM imaging since previous analysis of the electrode surface suggested that no volume change had occurred.
Using this cell design, in situ SEM revealed that bigger particles (ca. 13 μm) start to crack around 0.1 V during the lithiation process: fissures were observed as well as delamination from the current collector. The fissures showed a larger volume change when the electrode is discharged deeply. Figure 1 shows that a 13.02-μm particle expanded to 14.33 μm during lithiation at 0.03 V and contracted to 12.49 μm during delithiation at 1.85 V. Moreover, all of the cracks remained during the charging process. In general, it appeared that the smaller particles (< 2 μm) did not crack. This SiOx-graphite electrode with a polyimide binder exhibited a stable capacity of 600 mAh/g during charge-discharge at C/4 to 1C rates. These results indicate that smaller particles and elastic binders improve the cycle life of SiOx-graphite electrodes.
The Zaghib Group plans to use this in situ SEM tool to study the cycling behavior of Si nanoparticles next. Si is an attractive alternative material due to its higher theoretical specific capacity of 4200 mAh/g. Electrodes with different compositions of C-SiOx/Si/graphite will be evaluated to increase the capacity and the first cycle coulombic efficiency.