Electrochemical energy storage systems and methods featuring optimal membrane systems

A flow battery comprising:

• a first half-cell containing a first aqueous solution comprising a first active material containing at least one mobile ion;
• a second half-cell containing a second aqueous solution comprising a second active material containing at least one mobile ion;
• wherein the mobile ions comprise Li+, K+, Na+, Ca2+, Sr2+, or a combination thereof;
• a separator disposed between the first half-cell and the second half-cell;
• wherein the separator has a thickness of about 100 microns or less and comprises a porous membrane;
• and
• wherein the flow battery is configured to operate with a current efficiency of at least 85% at a current density of at least 100 mA/cm2.

A flow battery comprising:

• a first half-cell containing a first aqueous solution comprising a first active material containing at least one mobile ion;
• a second half-cell containing a second aqueous solution comprising a second active material containing at least one mobile ion;
• wherein the mobile ions comprise Li+, K+, Na+, Ca2+, Sr2+, or a combination thereof;
• a separator disposed between the first half-cell and the second half-cell;
• wherein the separator has a thickness of about 100 microns or less and comprises a plurality of layers, a first layer being capable of ionic conduction and a second layer being capable of selective ion transport;
• and
• wherein the flow battery is configured to operate with a current efficiency of at least 85% at a current density of at least 100 mA/cm2.