In the recent developments for Na-ion batteries, red phosphorus electrode has been reported to present a reversible capacity for more than 2000 mAh/g.¹ However, the low electrical conductivity of red phosphorus becomes problematic, and thus limits its rate performance.

On the other hand, over the past decades, conversion materials have been extensively studied in the research field of Li battery as potential replacements of insertion materials. Compared to the conventional insertion mechanism, conversion reaction allows more electrons to participate in the electrochemical reaction, resulting in much higher reversible capacity. Among conversion materials, transition metal phosphides based on Co, Fe, Ni, and Sn^2-7 have been reported to electrochemically react with Li and Na, leading to the formation of Li₃P and Na₃P, together with nanosized metallic particles,⁸ where a network created by metal particles is expected to support good electrical conduction inside the electrode upon cycling.

Particularly, as iron is one of the most abundant elements in the Earth’s crust, the comparatively low-cost iron phosphides are highly desirable to meet the future request of large-scale production. Therefore, our major interest lies on the application of iron phosphides in Na-ion batteries. Indeed, intrinsic drawbacks of conversion materials should always be considered, such as severe volume change during sodiation/desodiation, large voltage hysteresis, and large irreversible capacity loss during the first cycles due to the reduction of electrolyte component.

In the present work, iron phosphides (FeP_x, x = 2, 4) were mechanochemically synthesized by high-energy ball milling directly from commercial iron and red phosphorus powder, as shown in Fig. a and b. FeP₂ shows no significant electrochemical reactivity towards Na. On the other hand, using acetylene black (AB) as carbon additive and sodium polyacrylate (PANa) as binder, composite electrodes based-on phosphorus-rich phase (FeP₄) is able to deliver a capacity of 1137 mAh/g with high Coulombic efficiency of 84% during the 1^st cycle, its reversible capacity of ca. 1000 mAh/g can be maintained for over 30 cycles, as shown in Fig. c. Moreover, with 20 wt% of AB, FeP₄ composite electrode presents an excellent rate capability up to 2C (3578 mA/g). As a result, the encouraging electrochemical performances suggest that FeP₄ is a highly promising negative electrode material for high energy Na-ion batteries.

Although considerable interest in transition metal phosphides has been revived, little attention was paid to the mechanism of their conversion reaction in Na-ion batteries. Our work also includes the investigation of the mechanism using X-ray powder diffraction and Mössbauer spectroscopy, the study of electrode morphology by applying scanning electron microscopy, as well as the analysis of solid electrolyte interphase on electrode surface, known as the SEI layer, by X-ray photoelectron spectroscopy.

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