Degradation determination device and degradation determination method for lithium ion secondary battery

Abstract

A degradation determination device includes: a measuring unit measuring an open-circuit voltage characteristic indicating an open-circuit voltage variation with respect to a lithium ion secondary battery capacity variation; and a determining unit determining a degradation state due to wear and precipitation of lithium using a parameter for identifying the open-circuit voltage characteristic that substantially coincides with the measured open-circuit voltage characteristic. The parameter includes single electrode capacity retention rates expressed by the expressions (I) positive electrode capacity retention rate=degraded positive electrode capacity/initial positive electrode capacity and (II) negative electrode capacity retention rate=degraded negative electrode capacity/initial negative electrode capacity, and a battery capacity variation expressed by the expression (III) battery capacity variation=degraded negative electrode capacity×shift of a negative electrode composition axis with respect to a positive electrode composition axis.