Meaning
Internal voids within the structure of hard carbon or other anode materials that are smaller than one nanometer and inaccessible to external gases. Sub-nanometer closed pores provide storage sites for lithium ions that contribute to the overall capacity of the battery without being affected by the electrolyte. These features are created during the high-temperature carbonization of precursor materials.
Capacity Benefit
Small voids allow for the insertion of lithium at low potentials, which increases the energy density of the resulting cell. Because sub-nanometer closed pores do not come into contact with the solvent, they reduce the amount of lithium lost to the formation of the solid electrolyte interphase layer. This mechanism enhances the first-cycle efficiency and long-term stability of the anode during extended cycling.
Manufacturing Control
Temperature profiles during the pyrolysis process determine the size and distribution of these internal gaps. Achieving a high volume of sub-nanometer closed pores requires precise control over the heating rate and the chemical composition of the organic starting material. Producers must balance the creation of these pores with the overall conductivity and density of the carbon particles.
Analytical Measurement
Characterization of these hidden structures often requires small-angle X-ray scattering. If sub-nanometer closed pores are too numerous, they may lead to mechanical instability.
