Solid state Lithium-Ion cells have a very thin solid electrolyte. The electrodes need to be in contact with this solid electrolyte. In case of expansion, the solid electrolyte will not be able to settle down like a liquid electrolyte and therefore the electrodes may lose contact with the electrolyte. This leads to a reduction in capacity. Compression is required in order to prevent this expansion, the ThunderSky cells PP case is not strong enough to provide this compression by itself, so that the pack requires external compression. So the answer is NO - do not use the cells without compression.