Naming phosphonium cations in ionic liquids In Rennie et al., J. Phys. Chem. Lett., 2013, the following cation and anion structures are given in Figure 1.  Figure 1(f) is captioned as butyltrimethylphosphonium, which seems to match the given structure.  However, in the caption, the abbreviation $\text{P}_{2225}$ is given, which seems strange:

Rennie et al. repeat this in their Supporting Information:

But, other authors, such as Tsuchida et al., J. Mol. Liq., 2018, use $\text{P}_{2225}$ as an abbreviation for triethyl-n-pentylphosphonium, which makes sense if the subscript numbers represent the number of carbon atoms in the substituent alkyl groups:

Does it seem to you that there is a typo in the Rennie et al. paper? Or, what are the standard abbreviations for phosphonium cations?
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.