LiFePO4 as cathode, Li4Ti5O12 as anode，and celgard 2320 as separator, were assembled to the whole battery. I prepared 20 of them, and the open circuit voltages of them all are zero. Their electrochemical performance was shown as below. 18 of them behaves like fig.1, only two cells have the circles like fig.2 and 3.
The OCV of 0 means something is wrong with your preparation. For LTO//LFP full cell, the OCV should be around 1.3 V. Did you check the GDC of each half cell?
An OCP of zero (or anything in the mV range) usually means the cell is shorting within. This is much more likely with only one layer of celguard as separator, which is why many use glass fibre separators in coin cells (evidence here: Article A Guide to Full Coin Cell Making for Academic Researchers
)Alternatively, you could cut the electrodes to a smaller diameter than the separator and ensure very accurate alignment of the layers, but that's much more challenging than just using glass fibre.
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