  Accurate tracking of the internal electrochemical states of lithium-ion
battery during cycling enables advanced battery management systems to operate
the battery safely and maintain high performance while minimizing battery
degradation. To this end, techniques based on voltage measurement have shown
promise for estimating the lithium surface concentration of active material
particles, which is an important state for avoiding aging mechanisms such as
lithium plating. However, methods relying on voltage often lead to large
estimation errors when the model parameters change during aging. In this paper,
we utilize the in-situ measurement of the battery expansion to augment the
voltage and develop an observer to estimate the lithium surface concentration
distribution in each electrode particle. We demonstrate that the addition of
the expansion signal enables us to correct the negative electrode concentration
states in addition to the positive electrode. As a result, compared to a
voltage only observer, the proposed observer can successfully recover the
surface concentration when the electrodes' stoichiometric window changes, which
is a common occurrence under aging by loss of lithium inventory. With a 5%
shift in the electrodes' stoichiometric window, the results indicate a
reduction in state estimation error for the negative electrode surface
concentration. Under this simulated aged condition, the voltage based observer
had 9.3% error as compared to the proposed voltage and expansion observer which
had 0.1% error in negative electrode surface concentration.
