The open-circuit voltage with respect to a Ca metal anode for an occupation from 0 to 6 Ca per rhombohedral unit cell was checked for possible use as an electrode material. The trends are the same for both phases and the exact voltages are shown in table 2: The first intercalation voltage is very high with 16.4 V for the Ti phase and 7.9 V for the Zr. Note that these high voltages are simply due to the fact that the NASICON material is extremely unstable in the absence of any inserted ion so that the numbers are of no practical relevance.