High　mass　energy　density　coupled　with　high　power　density　is　highly　desired　for　electrical　double-layer　capacitors.　Usually　the　capacitive　performance　is　improved　by　optimizing　the　pore　size　and　volume　distribution.　Herein,　the　authors　report　an　efficient　approach　to　optimize　the　porous　structure　through　a　facile　ionothermal　carbonization　method.　A　series　of　hierarchical　porous　carbons　with　unique　sub-micrometer　sized　morphology,　high　surface　area　and　abundant　mesopores　(e.g.　SBET　=　2532　m2　g-1　and　Vmeso　=　1.077　cm3　g-1)　have　been　synthesized,　using　Jujun　grass　as　a　nitrogen-containing　precursor.　The　ionic　liquid　acts　not　only　as　a　reaction　medium　for　the　conversion　of　biomass　to　carbon　but　also　as　a　porogenic　agent　for　inducing　mesoporosity.　The　results　indicate　that　the　ionothermal　method　can　balance　the　micro-　and　mesoporosity　of　the　optimized　porous　carbon,　making　it　one　of　the　competent　alternatives　to　the　state-of-the-art　electrodes　for　ultra-high　energy　density　supercapacitors.　The　optimized　ionothermal　carbon　(ITC-JG-900)　shows　an　impressively　high　specific　capacitance　of　336　F　g-1　at　1　A　g-1　in　6　M　KOH,　and　even　retained　a　capacitance　of　222　F　g-1　at　10　A　g-1,　which　is　66%　of　the　initial　capacitance.　The　maximum　energy　density　of　ITC-JG-900　as　a　supercapacitor　is　found　to　be　over　72.7　W　h　kg-1　when　the　power　density　is　1204　W　kg-1,　which　is　higher　than　those　of　most　of　the　equivalent　benchmarks　tested　in　aqueous　electrolytes.　©　2017　The　Royal　Society　of　Chemistry.