The
discharged state affects the charge transfer resistance of
lithium-ion secondary batteries (LIBs), which is referred to as the
depth of discharge (DOD). To understand the intrinsic charge/discharge
property of LIBs, the DOD-dependent charge transfer resistance at
the solid–liquid interface is required. However, in a general
composite electrode, the conductive additive and organic polymeric
binder are unevenly distributed, resulting in a complicated electron
conduction/ion conduction path. As a result, estimating the DOD-dependent
rate-determining factor of LIBs is difficult. In contrast, in micro/nanoscale
electrochemical measurements, the primary or secondary particle is
evaluated without using a conductive additive and providing an ideal
mass transport condition. To control the DOD state of a single LiFePO4 active material and evaluate the DOD-dependent charge transfer
kinetic parameters, we use scanning electrochemical cell microscopy
(SECCM), which uses a micropipette to form an electrochemical cell
on a sample surface. The difference in charge transfer resistance
at the solid–liquid interface depending on the DOD state and
electrolyte solution could be confirmed using SECCM.