Unlocking Enhanced Capacitive Deionization of NaTi(PO)/Carbon Materials by the Yolk-Shell Design.

The low salt adsorption capacities (SACs) of benchmark carbon materials (usually below 20 mg g) are one of the most challenging issues limiting further commercial development of capacitive deionization (CDI), an energetically favorable method for sustainable water desalination. Sodium superionic conductor (NASICON)-structured NaTi(PO) (NTP) materials, especially used in combination with carbon to prepare NTP/C materials, provide emerging options for higher CDI performance but face the problems of poor cycling stability and dissolution of active materials. In this study, we report the development of the yolk-shell nanoarchitecture of NASICON-structured NTP/C materials (denoted as -NTP@C) using a metal-organic framework@covalent organic polymer (MOF@COP) as a sacrificial template and space-confined nanoreactor. As expected, -NTP@C exhibits good CDI performance, including exemplary SACs with a maximum SAC of 124.72 mg g at 1.8 V in the constant-voltage mode and 202.76 mg g at 100 mA g in the constant-current mode, and good cycling stability without obvious performance degradation or energy consumption increase over 100 cycles. Furthermore, X-ray diffraction used to study CDI cycling clearly exhibits the good structural stability of -NTP@C during repeated ion intercalation/deintercalation processes, and the finite element simulation shows why yolk-shell nanostructures exhibit better performance than other materials. This study provides a new synthetic paradigm for preparing yolk-shell structured materials from MOF@COP and highlights the potential use of yolk-shell nanoarchitectures for electrochemical desalination.