Lithium-ion batteries, owing to their high power and energy densities, have become ubiquitous energy storage devices for portable electronic devices . A key metric in the design of Li-ion battery materials is rate capability for discharge , but there is a complex, materialdependent trade-off between increased cycling rates and reduced battery lifetime. Capacity fade and internal degradation resulting from long-term use of Li-ion batteries must be rapidly and accurately quantified in order to improve their performance, reliability, and safety and inform second-use and end-of-life decisions . Degradation of Li-ion batteries stems from a plethora of physical mechanisms, such as solid electrolyte interphase (SEI) formation and lithium metal plating at the anode and structural changes at the cathode, such as oxidation-induced cation disorder , rock-salt phase-transitions and densification , anti-site defects , and secondary particle cracking . These degradation mechanisms lead to losses in rate capability by lowering accessible capacity at the same current or power within the operating voltage range. When this measure of state of health (SoH) reaches a given threshold, such as 80% of the nominal capacity under slow discharge, the battery is considered to have reached the end of its useful life, but the internal state of degradation is usually unknown and difficult to assess.