Pinning-controlled　Sm2(Co,　M)17　(M　=　Fe,　Cu,　and　Zr)　magnets　with　cellular　nanostructures　are　the　strongest　high-temperature　permanent　magnets.　The　squareness　factor　of　such　magnets　is　smaller　than　those　of　nucleation-controlled　permanent　magnets,　leading　to　a　lower-than-ideal　maximum　energy　product.　One　of　the　main　reasons　for　this　poor　squareness　is　that　the　pinning　strength　is　weaker　at　cell　edges　than　at　1:5H　cell　boundaries.　However,　the　structure　of　these　edges　remains　a　topic　of　debate.　To　identify　the　microstructure　of　cell　edges,　electron　diffraction,　TEM　bright/dark　field　imaging,　and　HRTEM　imaging　on　a　model　magnet　Sm25Co50.2Fe16.2Cu5.6Zr3.0　(mass　fraction,　%)　were　performed　using　both　[100]2:17R　and　[101]2:17R　zone　axes.　The　results　revealed　a　rhombohedral　2:17R＇　phase　at　some　of　the　edges,　with　one　faulting　basal　layer　in　the　2:17R　lattice.　Further　comparative　investigations　revealed　that　all　the　extra　superlattice　reflections　result　from　the　2:17R＇　phase,　excluding　the　previously　identified　2:17H　or　Smn　+　1Co5n　-　1　or　their　mixture　that　can　only　produce　a　part　of　such　superlattice　reflections.　Owing　to　the　2:17R＇　phase　with　a　faulted　basal　plane,　the　free　energy　at　the　cell　edges　is　higher　than　that　of　the　2:17R　cell　interiors,　leading　to　repulsive　domain-wall-pinning　unfavorable　for　the　squareness　factor.　This　study　provides　important　evidence　for　understanding　the　microstructural　origin　of　the　poor　squareness　factor　obtained　for　Sm2(Co,　M)17　permanent　magnets.　©　2021,　Science　Press.　All　right　reserved.