Facilitating the redox conversion of CoSe2 nanorods by Ti3C2Tx to improve the electrode durability as anodes for sodium-ion batteries

Nanostructured metal selenides based on conversion reactions are promising anode materials for sodium-ion batteries (SIBs). However, the repeat structural degradation accompanied by the detrimental intermediate of sodium selenides (NaxSe) leads them to suffer from continuous capacity decaying and under-voltage failure. In this work, CoSe2 is chosen as a representative by the introduction of a polar Ti3C2Tx matrix to alleviate the performance deterioration caused by the crystal structure evolution. CoSe2 nanorods are in situ grown on the Ti3C2Tx (CoSe2/Ti3C2Tx) surface by a simple one-step hydrothermal reaction during the reduction environment. A tight chemical bonding is formed between CoSe2 and Ti3C2Tx by oxygen bridging, which maintains the stable charge and ion transport during cycling. Meanwhile, Ti3C2Tx facilitates copper coming from the collector being diffused in the electrode, and is involved in the electrochemical reaction in an ether-based electrolyte, resulting in CoSe2 being partially converted to Cu2−xSe after long cycles. Synchronously, Ti3C2Tx improves the mutual transformation of NaxSe ↔ Na2Se during the intercalation/de-intercalation of metal selenides. Therefore, via optimizing the content of Ti3C2Tx, the CoSe2/Ti3C2Tx-10 composite obtains excellent long cycle stability, delivering a specific capacity of 343 mA h g−1 at 0.3 A g−1 after 1200 cycles with a capacity retention of 98%. Even at 10.0 A g−1, CoSe2/Ti3C2Tx-10 exerts about 200 mA h g−1 initial capacity, and remains at 140 mA h g−1 after 1400 cycles. The strategy of introducing a polar matrix to improve the long cycling stability of the metal selenide provides a new opportunity for the development of new anode materials for SIBs.