To investigate how phosphorus-doped
carbon micropores can significantly
enhance the capacitance of electrochemical double-layer capacitors
(EDLCs), we performed first-principles calculations on phosphorus-doped
monolayer (MGP) and bilayer graphene (BGP-x, x = 4–8), simulating macropores or mesopores and
micropores, respectively. In addition, we studied the stable structure,
relative capacitance, and reaction characteristics of Na+, Li+, and their hydrated ion structures {[Na(H2O)
n
]+, [Li(H2O)
n
]+, n = 1–4}
on MGP, as well as their insertion behavior in BGP-x. Surprisingly, we observed a desolvation phenomenon within the phosphorus-doped
carbon micropores. The calculation results suggest that the adsorption
of [Na(H2O)4]+ and [Li(H2O)4]+ on MGP is primarily attributed to the
stronger OH–π interaction compared to the Na+–π and Li+–π interactions. On
the other hand, the stability of the structure after embedding [Na(H2O)
n
]+ and [Li(H2O)
n
]+ in BGP-x can be explained by the opposite scenario. The equilibrium
between these two interactions and the continuous reduction in interlayer
spacing facilitate the desolvation of [Na(H2O)
n
]+ and [Li(H2O)
n
]+. A decrease in the number of water
molecules bound to Na+ and Li+ ions leads to
a maximum increase in relative capacitance of 2.7 and 3.0 times, respectively.
This indicates that desolvation can significantly enhance the capacitance
of EDLCs. The complete desolvation size for Na+ is less
than 4 Å, while for Li+, it is 4.3 Å. Based on
these calculations, when using phosphorus-doped carbon materials as
electrode materials, selecting an electrolyte containing Li+ rather than Na+ would result in a higher capacitance.
The calculation results reveal the mysterious mechanism behind the
significant increase in capacitance caused by phosphorus-doped carbon
micropores. In addition to this, they also provide us with a magical
key to optimize the perfect combination of electrolyte composition
and carbon material pore size/type.