Bipolar solid state battery cells

A bipolar solid state battery cell comprising a plurality of electrochemical units arranged in a stack so that adjacent electrochemical units share an electronic conductor, the bipolar solid state battery cell, comprising:

• a cathode current collector;
• a first electrochemical unit comprising a first catholyte layer comprising a first active material, a first solid state electrolyte, and a first electronic conductor;
• x second electrochemical units, wherein each second electrochemical unit individually comprises a second catholyte layer comprising a second active material, a second solid state electrolyte, and a second electronic conductor;
• a third electrochemical unit comprising a third catholyte layer comprising a third active material, and a third solid state electrolyte;
• and
• an anode current collector;
• wherein the plurality of stacked electrochemical units are arranged in series, wherein the number x of second electrochemical units is between 0 and 8,
• characterised in that the bipolar solid state battery cell further comprises an electrically insulating layer provided at the exterior surface of the cathode current collector and/or at the exterior surface of the anode current collector, and comprising a ceramic material.