  Electronic structure calculations carried out to estimate the free energies
of Na(aq)+ exchange for lithium in LiM2(PO4)3 (LMP, M=Si4+,Ge4+,Ti4+,Sn4+,Zr4+)
and Li1+xAlxM2-x(PO4)3 (LAMP; M=Ge4+,Ti4+, between 0 and 0.5) compounds in
seawater. The calculations show that resistance to sodium-ion exchange
increases with decreasing cell volume. For the pure LMP compounds, only the
hypothetical LiSi2(PO4)3 is predicted to be stable against sodium ion exchange
in aqueous solution. The calculations indicate that increasing the extent of
Al3+ substitution for M4+ in the LAMP compounds increases the resistance to
exchange, and that both LAGP and LATP can be stabilized against sodium exchange
for x greater than or equal to approximately 0.5 Li1+xAlxM2-x(PO4)3.
