Counter‐Intuitive Structural Instability Aroused by Transition Metal Migration in Polyanionic Sodium Ion Host

Abstract

The structure of polyanionic materials is conventionally known to be free of transition metal migration and structurally stable when storing/releasing sodium ions. Herein, the observation of enhanced cycling stability of a typical polyanionic cathode, Na3VCr(PO4)3 (NVCP) at lower temperature (−15 °C vs 30 °C), triggers the exploration of its structural origins with a surprising finding that the migratable nature of vanadium in NVCP leads to detrimental structural degradation of the polyanionic host upon cycling. The correlation between long range and short range structural change associated with this atomic migration is established via a strong combination of various in situ/ex situ characterization tools, revealing the essential V–to–Na1 site migration. Such transition metal migration is effectively suppressed when V atoms are pinned to their original position in the lattice by lowering the temperature. More importantly and practically, a room temperature‐based deep sodiation strategy is further developed to recover the structure. This work challenges the long‐standing assumption of the stability of the polyanionic framework structure and calls for urgent attention to the structural understanding of the NVCP system as well as strategy development for property enhancement.

A Novel NASICON‐Typed Na4VMn0.5Fe0.5(PO4)3 Cathode for High‐Performance Na‐Ion Batteries

“…Upon subsequent sodiation/desodiation, ≈2.18 Na + reversibly inserted into and extracted from the VMF-0.5 framework. Such a reversible structure evolution involved in both solid-solution and biphase electrochemical reactions has also been demonstrated in other similar NASICON cathodes, such as typical Na 2 VTi(PO 4 ) 3 , [10] Na 3 TiMn(PO 4 ) 3 , [22] Na 4 VMn(PO 4 ) 3 , [23] Na 3 VFe(PO 4 ) 3 , [23] Na 3 V 2 (PO 4 ) 3 , [34] Na 3 VCr(PO 4 ) 3 , [59,60] etc. The total unit volume variation (Figure S21, Supporting Information) during Na + insertion and extraction processes was calculated as 8.41% (ΔV/V pristine ), which is close to that of NVP cathode (8.26%) [61] and smaller than that of VM reported (9.28%).…”

Improved Performance of Na3TiMn(PO4)3 Using a Non-stoichiometric Synthesis Strategy

“…This kind of cation migration from an M1 site to a TM site has been observed for Mn 3+ in the isostructural Mn-doped LiTi 2 (PO 4 ) 3 materials . The tendency for Mn 4+ to occupy the TM site in the isostructural Mn-doped Mn 0.5 Ti 2– x Mn y (PO 4 ) 3 can also well support the speculation. , A recent study on NASICON-structured Na 3 CrV­(PO 4 ) 3 confirmed that V can migrate reversibly between M1 and TM sites during the charge/discharge process, and an obvious voltage hysteresis was also observed . The present work confirms that transition metal cations can migrate in NASICON-structured compounds, leading to voltage hysteresis.…”

Reversible Activation of V4+/V5+ Redox Couples in NASICON Phosphate Cathodes

“…In the cases of x values more than 0.4 (0.6 ≤ x ≤ 1), the discharge capacity instead decreases with increasing Fe/Na content due to the poor reversibility of V 4+ /V 5+ redox. Similar to the Mn‐rich Na 4 VMn(PO 4 ) 3 [39 ] and Na 3.75 V 1.25 Mn 0.75 (PO 4 ) 3 , [ 17 ] the poor reversibility of V 4+ /V 5+ transition in the Fe‐rich Na 3+ x V 2− x Fe x (PO 4 ) 3 (0.6 ≤ x ≤ 1) cathodes is possibly linked to extraction of Na1 causing the rearrangement of structure, which is not in favor the migration of Na1 back to its initial position during the sequent discharge. [ 40 ] In addition to the content of dopants, to explore the possible rule on the reversible activation of the V 4+ /V 5+ redox by the various metal doping strategy, the representative Na 3 V 1.6 Al 0.4 (PO 4 ) 3 (denoted as NVAP‐0.4, doped by trivalent‐metal dopants of Al 3+ ) and Na 3.4 V 1.6 Mg 0.4 (PO 4 ) 3 (denoted as NVMP‐0.4, doped by bivalent‐metal dopants of Mg 2+ ) were also investigated.…”