Lithium reservoir system and method for rechargeable lithium ion batteries

A method to replenish lithium-ion capacity of a battery cell including two working electrodes and an auxiliary electrode, the method comprising:

• measuring an open circuit potential of each of the two working electrodes using the auxiliary electrode as a reference electrode during an equilibrium state of the working electrodes at open circuit;
• determining a state of charge of each of the two working electrodes based on the measured open circuit potential of each of the two working electrodes;
• and
• transferring lithium from the auxiliary electrode to at least one of the two working electrodes based on the determined state of charge,
• wherein:
• the battery cell includes:
• a first separator region arranged between the two working electrodes to separate the two working electrodes so that none of the two working electrodes are electronically connected within the cell;
• and
• a second separator region arranged between the auxiliary electrode and the two working electrodes to separate the auxiliary electrode from the two working electrodes so that none of the two working electrodes is electronically connected to the auxiliary electrode within the cell;
• the two working electrodes include a negative electrode and a positive electrode;
• each of the two working electrodes includes an active material, an inert material, an electrolyte, and a current collector;
• and
• the auxiliary electrode includes a lithium reservoir.