  Lithium-ion technologies are increasingly employed to electrify
transportation and provide stationary energy storage for electrical grids, and
as such their development has garnered much attention. However, their
deployment is still relatively limited, and their broader adoption will depend
on their potential for cost reduction and performance improvement.
Understanding this potential can inform critical climate change mitigation
strategies, including public policies and technology development efforts.
However, many existing models of past cost decline, which often serve as
starting points for forecasting models, rely on limited data series and
measures of technological progress. Here we systematically collect, harmonize,
and combine various data series of price, market size, research and
development, and performance of lithium-ion technologies. We then develop
representative series for these measures and employ performance curve models to
estimate improvement rates. We also develop a method to incorporate additional
performance characteristics into these models, including energy density and
specific energy performance metrics. When energy density is incorporated into
the definition of service provided by a lithium-ion cell, estimated
technological improvement rates increase considerably, suggesting that
previously reported improvement rates might underestimate the rate of
lithium-ion technologies' change. Moreover, our estimates suggest the degree to
which lithium-ion technologies' price decline might have been limited by
performance requirements other than cost per energy capacity. These rates also
suggest that battery technologies developed for stationary applications, where
restrictions on volume and mass are relaxed, might achieve faster cost
declines, though engineering-based mechanistic cost modeling is required to
further characterize this potential.
