Meaning
Concentration of unpaired electron spins associated with structural irregularities or vacancies within an active material lattice. Higher paramagnetic defect density generally correlates with increased electrical conductivity but lower structural stability in anode carbons. This concentration is measured using electron spin resonance spectroscopy.
Lattice Imperfection
Missing atoms or disordered graphene sheets create localized electron spins that do not pair up with neighboring atoms. This paramagnetic defect density is highly sensitive to the temperature at which the carbon precursor is heated. Lower heat treatment temperatures leave more of these defects intact.
Electrochemical Activity
These localized spin states can act as active sites for ion adsorption during the initial cycles of the battery. An optimal paramagnetic defect density enhances the specific capacity of the hard carbon anode. However, if the concentration is too high, the material undergoes excessive side reactions with the electrolyte, reducing the initial coulombic efficiency.
This reduction increases the consumption of lithium, which decreases the energy density of the fully assembled cell.
Measurement Boundary
Measurements of these states must be conducted under controlled low-temperature conditions to avoid thermal excitation. High conductivity can shield the microwave signals used to detect paramagnetic defect density. This restriction makes the technique most suitable for dry, uncycled powders rather than complete electrode assemblies.
