  When two electrode open-circuit potentials form a full-cell OCV (open-circuit
voltage) model, cell-level SOH (state of health) parameters related to LLI
(loss of lithium inventory) and LAM (loss of active materials) naturally
appear. Such models have been used to interpret experimental OCV measurements
and infer these SOH parameters associated with capacity fade. In this work, we
first re-parametrize a popular OCV model formulation by the N/P
(negative-to-positive) ratio and Li/P (lithium-to-positive) ratio, which have
more symmetric and intuitive physical meaning, and are also
pristine-condition-agnostic and cutoff-voltage-independent. We then study the
modal identifiability of capacity fade by mathematically deriving the gradients
of electrode slippage and cell OCV with respect to these SOH parameters, where
the electrode differential voltage fractions, which characterize each
electrode's relative contribution to the OCV slope, play a key role in passing
the influence of a fixed cutoff voltage to the parameter sensitivity. The
sensitivity gradients of the total capacity also reveal four characteristic
regimes regarding how much lithium inventory and active materials are limiting
the apparent capacity. We show the usefulness of these sensitivity gradients
with an application regarding degradation mode identifiability from OCV
measurements at different SOC (state of charge) windows.
