In the search for new cathode materials for sodium ion batteries, the exploration of polyanionic compounds has led to attractive candidates in terms of high redox potential and cycling stability. Herein we report the synthesis of two new sodium transition metal pentaborates Na 3 MB 5 O 10 (M = Fe, Co), where Na 3 FeB 5 O 10 represents the first sodium iron borate reported at present. By means of synchrotron X-ray diffraction, we reveal a layered structure consisting of pentaborate B 5 O 10 groups connected through M 2+ in tetrahedral coordination, providing possible three-dimensional Na-ion migration pathways. Inspired by these structural features we examined the electrochemical performances versus sodium and show that Na 3 FeB 5 O 10 is active at an average potential of 2.5 V vs. Na + /Na 0 , correlated to the Fe 3+ /Fe 2+ redox couple as deduced from ex situ Mössbauer measurements. This contrasts with Na 3 CoB 5 O 10 which is electrochemical inactive. Moreover, we show that their electrochemical performances are kinetically limited as deduced by complementary AC/DC conductivity measurements, hence confirming once again the complexity in designing highly performant borate-based electrodes.