The faster kinetics of nanostructured charge-storage materials are often accompanied by accelerated degradation and failure, compromising the potential advantages of nanostructure and nanoporosity.  Understanding and controlling the exact mechanisms of this degradation is a complex materials characterization challenge.  The recent availability of in situ and in operando techniques allows new fundamental studies of these issues, including the failure of nanoscale porosity to fully accommodate the mechanical strains of charge insertion.

Here, we use atomic force microscopy in liquid electrolytes to experimentally measure the mechanical expansion of porous, nanostructured Li-ion cathodes during charge cycling.  Using thin films of Li_xMnO₂ on Pt collectors, we analyze expansion as a function of scan rate, voltage window, and porosity.  Comparative measurements in aqueous and nonaqueous electrolytes confirm the role of Li-ion species in the results.  The rate dependent measurements of expansion can be separated into diffusion-limited, bulk components driven by Mn^3+/4+ charging and faster, surface processes, in the same way that these contributions to capacitance are normally separated.  Consequently, we have extracted the electrochemical-mechanical coupling coefficients ∂V/∂Q for surface and bulk processes separately.  While the bulk coupling coefficient provides an excellent match to values predicted from first-principles theory, the surface coupling is surprisingly large.  In fact, surface capacitance is responsible for a majority of the mechanical expansion at high rates, even when most of this capacitance would normally be attributed to double-layer charging mechanisms.  The result demonstrates the importance of Mn^3+/4+ species at the electrolyte interface and the inability of these species to take advantage of the film’s nanoporosity for accommodating expansion.  In other words, fast surface charging processes do not involve mechanical relaxation of the material into adjacent pores, but rather drive expansion of the solid matrix.