Multinuclear Solid-State\nNMR Study of the Aqueous\nSolid-Electrolyte Interphase Formation in NaTi<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub>

Solid-electrolyte interphases enable stable operation\nof non-aqueous\nLi-ion batteries, but their formation, especially in aqueous systems,\nis not well understood. NASICON-structured NaTi<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub> is a widely studied ion insertion negative electrode\nmaterial for various aqueous electrochemical applications. This study\nuses multinuclear <sup>31</sup>P, <sup>23</sup>Na, <sup>13</sup>C, <sup>47,49</sup>Ti solid-state NMR to examine NaTi<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub> aqueous degradation and solid-electrolyte interphase\nformation. The results indicate that interphase consists of amorphous\nphases similar to TiO(OH)(H<sub>2</sub>PO<sub>4</sub>)·<i>n</i>H<sub>2</sub>O and carboxylic groups on carbonaceous phases,\nformed through electrochemical degradation. The formation of the aqueous\nsolid-electrolyte interphase results in low Coulombic efficiency,\npronounced self-discharge, and capacity loss of NaTi<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub> during charge–discharge cycling.