Naming phosphonium cations in ionic liquids
Skepticism is an integral part of a good scientist. Remember the motto &quot;Paper never refused ink&quot; when reading papers and its Supporting Information. You will find incorrect references (very frequent), occasionally wrong calculations, wrong formulae, or wrong names as you read more and more papers just because scientists, like normal human beings, make mistakes.
The strategy in such cases is not to stop at two papers, because the odds are 50:50, but find at least 6-7 papers to confirm the truth.
Your notation is correct for $\ce{P2225}$.

Reference: Another paper from RSC Two phosphonium ionic liquids with high Li+ transport number, Phys. Chem. Chem. Phys., 2015,17, 23041-23051
The odds are in your favor now.