What is the difference between a lithium-ion battery's internal resistance and internal impedance? Are both the same, and if not, which is greater? How can these values be measured, and how can they help analyzing battery degradation?
Resistance usually means resistance at DC meaning constant (over time) voltages and currents.
Impedance is usually a term used for the resistance of resistors including the effect (the impedance) of capacitors and inductors (reactive elements).
At DC (constant voltages and currents) you can assume that resistance and impedance mean the same thing. For a battery usually only DC is considered so you can just assume resistance and impedance are the same thing.
When that's not the case (impedance is different from the resistance) then a frequency should be specified. The frequency is needed to make the inductors and capacitors have an effect. At DC inductors and capacitors basically do nothing.
The resistance of a Li-ion cell is at DC and varies with the state of charge and temperature.
The impedance of a Li-ion cell is at AC and varies with the frequency.
Cell manufacturers almost always specify the maximum impedance at 1 kHz. That is useless to you the user because you use the cell at DC. Often, cell manufacturers misrepresent the impedance at 1 kHz as "resistance ".
Occasionally, cell manufacturers specify the maximum resistance (DC). You can extract the resistance from the discharge curves that the cell manufacturer often publishes.
The DC resistance and AC impedance of a Li-ion cell are unrelated. You cannot deduce one from the other.
The resistance of a Li-ion cell is measured by applying a DC load to it, noting the voltage drop and the load current, and calculating the resistance as the ratio of the two.
The impedance of a Li-ion cell is measured with a battery impedance meter. It applies a 1 kHz current to the cell, measures the magnitude (not the phase) of the resulting 1 kHz voltage, and calculates the impedance as the ratio of the two.