Aqueous　zinc–iodine　batteries　hold　immense　potential　for　grid-scale　storage　owing　to　their　inherent　safety,　cost-effectiveness,　and　eco-friendliness.　However,　their　commercialization　is　hindered　by　limited　cycle　life　caused　by　irreversible　Zn　plating/stripping　and　I2/I−　redox　reaction.　Herein,　we　propose　1-(2-hydroxyethyl)-1-methylguanidine　dihydrogen　phosphate　(HMDP)　as　an　effective　bifunctional　additive　that　not　only　stabilizes　Zn　anode　by　forming　a　respirable-interphase　but　also　suppresses　the　generation　of　polyiodide　ion　at　the　cathode　by　separating　I2　and　I−.　The　Zn//Cu　cells　with　HMDP　achieve　a　Coulombic　efficiency　of　99.63　%　over　650　cycles　(2　mA　cm−2,　1　mAh　cm−2)　and　retain　85.83　%　initial　efficiency　after　7-day　calendar　aging.　Surprisingly,　the　Zn//Zn　symmetric　cells　with　HMDP　deliver　an　ultra-long　cycle-life　of　5150　h　(0.5　mA　cm−2,　0.5　mAh　cm−2)　thanks　to　the　formation　of　respirable-interphase.　Meanwhile,　the　Zn//I2　full　cells　retain　81.04　%　capacity　at　2　A　g−1　after　＞3000　cycles　due　to　the　suppression　of　polyiodide　ion.　This　work　establishes　strategies　of　I2–I−　separator　and　respirable-interphase　to　design　advanced　zinc-iodine　batteries.　©　2025　Elsevier　B.V.