Enhancing the long-term Na-storage cyclability of conversion- type iron selenide composite by construction of 3D inherited hyperbranched polymer buffering matrix

Electrochemical conversion reactions provide more selections for Na-storage materials, but the reaction suffers from low reversibility and poor cyclability. Introducing an electrochemically inactive component is a common strategy, but the effect is quite limited since it could not stabilize the structure during long-term cycling. In this study, a new approach is developed using an amino group-functioned hyperbranched polymer (AHP) as a template and electrode additive for the design of high-performance FeSe₂-AHP composite with chemical interaction. The assembled FeSe₂-AHP composite nanoneedles were prepared by the selenylation of FeS-AHP composite microflowers and entirely inherit the polymer network from the precursor. The amino groups of AHP in composite coordinate with iron cations to achieve uniform polymer dispersion in the composite, and maintain the molecular level mixed state during the long-term cycling. Moreover, the in-situ constructed uniform 3D elastic polymer network effectively accommodates volume expansion and alleviates nanoparticle aggregation during sodiation/de-sodiation. FeSe₂-AHP composite provides a superior rate capability (584.8 mAh·g^-1 at 20 A·g^-1) and a remarkable cyclability with a capacity retention rate of 93.3% after 2, 000 cycles. FeSe₂-AHP composite shows a high pseudocapacitive behavior for the abundant nanometer interface established by AHP, enhancing the solid-state Na⁺ diffusion. The FeSe₂-AHP anode is also compatible with Na₃V₂(PO₄)₃/C cathode in a full Na-ion battery, which provides a high-power performance (powering 51 LEDs). The work herein highlights an innovative and efficient strategy for conversion-type material design and demonstrates the function of chemical interaction of polymer additive in the enhancement of long-term cyclability for conversion electrode.