UC Santa Barbara

Sodium-ion batteries (SIBs) are attractive alternatives for electrochemical energy storagedue to sodium’s natural abundance and cost-effectiveness when compared to their lithium counterparts. Among the various Na-ion cathode classes, sodium superionic conductor (NASICON) structured Na3V2(PO4)3 (NVP) has emerged as a promising candidate for SIB applications owing to its high structural stability, 3−D Na+ diffusion network, and high operating voltage of 3.4 V vs. Na+/Na0. Although promising, the impact of temperature dependent Na+/ vacancy ordering transitions, potential cation migration, and influence of phase separation upon Na (de)intercalation on Na+ diffusion is still poorly understood.

Here, we focus on the effects of isovalent (Al3+) and aliovalent (Mg2+) substitutionfor V into the NVP framework and investigate a series of Na3+yV2−yMgy(PO4)3 (Mg- NVP) (y = 0−1.0) and Na3V2−yAly(PO4)3 (Al-NVP) (y = 0, 0.5) cathodes to better understand the impact of V by substitution on the Na (de)intercalation mechanics by complementing short and long range characterization techniques such as synchrotron X-ray diffraction, 23Na, 31P, 51V solid-state NMR, and scanning electron microscopy (SEM) with first principles calculations to unravel the complex electrochemical cycling behavior of these materials. We demonstrate the vastly different effect of each dopant on electrochemical performance with an emphasis on (de)intercalation mechanics and causes of structural degradation during high voltage cycling.

In the case of Mg-NVP, Na extraction and reinsertion results in a two-phase reactionmechanism when y < 0.5 and transitions to a solid-solution mechanism above y = 0.5, when cycled over a potential window of 3.8−2.75 V. Conversely, when Al3+ is introduced into the NVP framework, it exacerbates the formation of an intermediary Na2.24V1.5Al0.5(PO4)3 phase leading to successive biphasic reactions during Na (de)intercalation, in stark contrast to the solid-solution mechanism that emerges at higher Mg concentrations. It is found that the formation of the intermediary phase in Al-NVP helps reduce the propensity for particle cracking during long term cycling and lowers overpotentials associated with Na extraction during electrochemical cycling. Thus, we have highlighted the significant role of transition metal dopants to modify the reaction mechanisms associated with Na (de)intercalation in NASICON structured Na3V2(PO4)3.