An ion conductive polyimide encapsulation: New insight and significant performance enhancement of sodium based P2 layered cathodes

It is essential to stabilize the surface of P2 layered cathode materials at high cut-off voltages (>4.3 V) in order to construct high-energy sodium ion batteries (SIB) that are promising for commercial application. When the voltage exceeds 4.3 V, large volume changes due to phase transitions and active species dissolution affect the structural stability of high voltage cathodes. In this study, we report a novel method of enhancing the electrochemical cycling performance of P2-type Na-2/3(Mn0.54Ni0.13Co0.13)O-2 (NNMC) materials through ion-conductive polyimide (PI) encapsulation. The electrochemical performance of ultrathin PI coated NNMC (PI-NNMC) is one of the best reported in the literature among layered cathodes in terms of cyclic stability (82% after 100 cycles at 0.16 A g(-1)) at a high voltage range between 2 and 4.5 V, compared to the pristine (46%) and Al2O3-coated NNMC (70%). At high current (5C), the NNMC-PI electrode demonstrates superior cyclability by retaining 70% of its capacity after 500 cycles. The ultrathin PI layer possesses excellent surface protection, high ionic conductivity (vs Al2O3 coating) and facile ion transport, thus enabling a fast and durable redox electrochemistry in NNMC materials for high-performance sodium storage above 4.3 V.