Herein,　a　one-step　route　has　been　developed　for　synthesizing　terbium-phosphine　(Tb-(Ph)3P)　compound,　which　exhibited　an　excellent　rate　capability　and　an　outstanding　long　cycling　stability.　The　highest　specific　capacitance　of　1866　F　g−1　has　been　obtained　at　2　A/g　in　6　M　KOH,　and　the　capacitance　retention　of　81.02%　has　also　been　achieved　at　10　A/g　after　30,000　cycles,　resulted　in　a　maximum　energy　density　of　93.3　W　h　kg−1　and　a　maximum　power　density　of　6.04　kW　kg−1.　Such　an　excellent　performance　might　be　attributed　to　the　insolvability　and　intrinsic　characteristics　of　the　Tb-based　phosphine　complexes,　which　provided　abundant　active　sites　for　the　charge　storage　and　conductive　networks　for　electron　transport.　Moreover,　a　flexible　quasi-solid-state　asymmetric　supercapacitor　composed　of　Tb-(Ph)3P　and　active　carbon　has　been　fabricated,　and　a　specific　capacitance　of　275　mF　cm−2　at　a　current　density　of　2　mA　cm−2,　and　a　capacitance　retention　of　100%　after　10,000　cycles　at　10　mA　cm−2　have　been　respectively　achieved,　exhibiting　an　excellent　electrochemical　property.　Thus,　the　present　work　might　provide　a　new　strategy　for　designing　new　metal–organic　complex　materials　and　their　applications　in　energy　storage　devices.　©　2022　Elsevier　B.V.