Lithium ion secondary battery, assembled battery, vehicle, battery-equipped device, battery system, and method for detecting deterioration of lithium ion secondary battery

A lithium ion secondary battery comprising:

• a power generating element having a positive electrode plate and a negative electrode plate;
• a battery case housing the power generating element;
• and
• an electrolyte containing lithium ions, the electrolyte being retained in the battery case,
• wherein the electrolyte includes:
• a retained electrolyte as a part retained between the positive electrode plate and the negative electrode plate in the power generating element;
• and
• a stored electrolyte as another part stored between the power generating element and the battery case so that the stored electrolyte and the retained electrolyte are allowed to communicate with each other,
• the lithium ion secondary battery comprises stored-electrolyte physical quantity measuring means capable of measuring a concentration correlated physical quantity having a correlation with a concentration of the lithium ions in the stored electrolyte
• wherein the stored-electrolyte physical quantity measuring means comprises:
• a first measurement electrode including a first electrode main part contacting the stored electrolyte and a first conductive part exposed to the outside of the battery case and electrically connected to the first electrode main part;
• a reference electrolyte containing lithium ions of a reference concentration;
• a reference electrolyte container containing the reference electrolyte;
• a second measurement electrode including a second electrode main part contacting the reference electrolyte and a second conductive part exposed to the outside of the reference electrolyte container and electrically connected to the second electrode main part;
• and
• a separating member having a first surface that contacts with the stored electrolyte and a second surface that contacts with the reference electrolyte to separate the stored electrolyte and the reference electrolyte from each other, the separating member being arranged to prevent ion migration between the first and second surfaces resulting from a concentration difference between the stored electrolyte and the reference electrolyte and to enable measuring of a voltage generated between the reference electrolyte and the stored electrolyte by the first and second measurement electrodes.