By comparison, the polar molecule water, which has extraordinarily good heat capacity, due in part to hydrogen bonding, is 75 J/molK at room temperature.
So water has almost 31x the heat capacity of tungsten by weight.
You list many solids as having a molar heat capacity of "25 J/molK", water as having a molar heat capacity of "75 J/molK", and conclude that water has almost 31x the heat capacity, but naively comparing the numbers cited only gets you 3x. Where does the other 10x factor come from?
The parent post happened to pick a solid material for a nice round molar mass ratio.
Focused sunlight is certainly capable of boiling water.
It's my understanding that nitrates are potentially explosive, but I haven't heard of any other anions being used except for fluoride (which has its own corrosion/toxicity issues).
Nitrates can of course be an oxidizing agent, so you'd like to keep them away from being e.g. carbothermically or aluminothermically reduced.
I know the lithium salt was of interest as a more thermally stable battery electrolyte (relative to LiPF6), although interest eventually shifted to lithium difluorooxalatoborate (two fluorines and one oxalate attached to boron tetrahedrally) which is still stable enough for batteries but not as stable as LiBOB. LiBOB is reported to decompose at about 290 C ( http://ma.ecsdl.org/content/MA2010-02/9/597.full.pdf ),
so I would expect the heavier analogs to go somewhat higher, but I've never heard of these salts being considered in the case of thermal storage. I do not know the thermal limit of trisoxalatophosphate salts, only that they look like oxalatoborates and share with nitrates the property of being "non-coordinating".
What about the use of fiberglass and carbon fiber tanks? Is heat tolerance a problem with those?
Materials that are stable liquids at relatively high temperatures and that exhibit low vapor pressure have been investigated for quite some time as heat transfer fluids. See, for instance, the Molten Salt Reactor Experiment at Oak Ridge where a lithium fluoride-beryllium fluoride mixture was used.
The size of the phase diagram space to investigate comes from the phase rule, and the conditions for equilibrium from the Gibb's criteria. The classic materials science approach (how I learned) is to try and map the phase diagram space experimentally. This can become unwieldy for many component systems.
Common chloride salts and/or nitrate salts have been investigated as potential heat transfer/storage fluids, usually in eutectic compositions to lower the melting point. The actual ionic compositions of the molten fluids can be rather involved.
The widely used austenitic stainless steels (e.g. 304, 316) are susceptible to corrosion attack from chloride ions, so understanding of coordination chemistry around chlorine is important to determine the likely stability of a containment vessel.
It would be ideal to find salts comprised of common materials that formed moderate melting temperature eutectics, have no free chlorides (or no chlorine at all), low vapor pressure, and long term stability in the presence of common stainless (or even mild) steel. Potential candidates could be nitrates or the so called deep eutectic solvents.
Hopefully this is helpful detail,