New　tin-based　metal-phosphine　complexes　of　[Sn(OH)4(PPh3)2]　and　[Sn(OH)2(PPh3)2]　have　been　successfully　synthesized　and　used　as　supercapacitor　electrodes　for　the　first　time,　exhibiting　a　high　specific　capacitance,　a　good　rate　capability,　and　an　excellent　cycling　stability.　The　specific　capacitances　(highest　specific　capacitance　for　tin-based　materials)　of　1204F　g−1　and　764F　g−1　for　two　samples　at　a　current　density　of　1　A　g−1　in　6　M　KOH　can　respectively　be　achieved,　and　their　capacitance　retention　remained　at　95.1%　and　89.2%　even　after　15,000　cycles　at　a　current　density　of　10　A　g−1.　Furthermore,　a　flexible　quasi-solid-state　asymmetric　supercapacitor　composed　of　Sn(OH)2(PPh3)2　and　activated　carbon　was　assembled　and　exhibited　a　specific　capacitance　of　290.6　mF　cm−2　at　a　current　density　of　1　mA　cm−2.　More　importantly,　this　device　also　displayed　excellent　cyclic　stability　of　∼100%　for　1800　cycles　during　the　galvanostatic　charge/discharge　process　at　5　mF　cm−2.　©　2021