  The useful life of electrochemical energy storage (EES) is a critical factor
to EES planning, operation, and economic assessment. Today, systems commonly
assume a physical end-of-life criterion, retiring EES when the remaining
capacity reaches a threshold below which the EES is of little use because of
functionality degradation. Here, we propose an economic end of life criterion,
where EES is retired when it cannot earn positive net economic benefit in its
intended application. This criterion depends on the use case and degradation
characteristics of the EES, but is independent of initial capital cost. Using
an intertemporal operational framework to consider functionality and
profitability degradation, our case study shows that the economic end of life
could occur significantly faster than the physical end of life. We argue that
both criteria should be applied in EES system planning and assessment. We also
analyze how R&D efforts should consider cycling capability and calendar
degradation rate when considering the economic end-of-life of EES.
