In a battery system, an electrochemical cell has a first compartment containing a positive electrolyte with an electrode 2 at least partially immersed in the positive electrolyte containing the first redox active material, forming a positive half cell. A second compartment contains a negative electrolyte with an electrode 1 at least partially immersed in the negative electrolyte containing the second redox active material, forming a negative half -cell. A third electrolyte is substantially immiscible and in contact with the positive and the negative electrolyte. The third electrolyte contains a common ion with the positive electrolyte and another common ion with the negative electrolyte, establishing a polarisation between the positive and negative electrolytes and enhancing the potential difference between the positive and negative electrode.