Enhancing Na⁺ Extraction Limit through High Voltage Activation of the NASICON-Type Na₄MnV(PO₄)₃ Cathode

Abstract

Sodium (de)intercalation in the Na 4 MnV-(PO 4 ) 3 NASICON-type cathode has been studied with operando synchrotron X-ray powder diffraction and galvanostatic cycling up to 3.8 and 4.0 V cutoff voltages. Symmetry reduction from rhombohedral to monoclinic was observed immediately after the start of the electrochemical desodiation, with restoring of the rhombohedral phase at 3.8 V. Cycling within the 2.5−3.8 V potential range proceeds through both solid-solution and two-phase processes. An additional voltage plateau at ∼3.9 V is observed, associated with "unlocking" of the Na1 site in the rhombohedral phase. Reverse insertion of Na + cations proceeds via the entire solid-solution region. The experimentally observed discharge capacity increases by ≈14% after raising cutoff voltage. KEYWORDS: sodium-ion battery, NASICON, Na 4 MnV(PO 4 ) 3 , phase transitions, operando XRD

High Capacity and High‐Rate NASICON‐Na3.75V1.25Mn0.75(PO4)3 Cathode for Na‐Ion Batteries via Modulating Electronic and Crystal Structures

“…Further upon charging to 3.8 V versus Na + /Na 0 , a new NASICON phase (with R c space group) appears with a composition of Na 1.75 V 1.25 Mn 0.75 (PO 4 ) (i.e., after the removal of additional 0.75 moles of sodium ions pfu). The refinements on the corresponding XRD patterns have shown a strong and slight decrease of a ‐ and c ‐parameters, respectively ( Table 1 ), implying the progressive removal of Na‐ions from Na(2) while the occupancy of Na(1) remains unchanged . The change in the unit cell volume (from Na 3.75 V 1.25 Mn 0.75 (PO 4 ) to Na 1.75 V 1.25 Mn 0.75 (PO 4 )) is found to be 8.4% and is comparable with the volume changes reported for the end members (Na 3 V 2 (PO 4 ) 3 ) & Na 4 VMn(PO 4 ) 3 ) .…”

“…The refinements on the corresponding XRD patterns have shown a strong and slight decrease of a ‐ and c ‐parameters, respectively ( Table 1 ), implying the progressive removal of Na‐ions from Na(2) while the occupancy of Na(1) remains unchanged . The change in the unit cell volume (from Na 3.75 V 1.25 Mn 0.75 (PO 4 ) to Na 1.75 V 1.25 Mn 0.75 (PO 4 )) is found to be 8.4% and is comparable with the volume changes reported for the end members (Na 3 V 2 (PO 4 ) 3 ) & Na 4 VMn(PO 4 ) 3 ) . After the discharge to 2.75 V versus Na + /Na 0 , the structure of the electrode is fully restored which demonstrated the high reversibility of the electrochemical reactions when the upper cut‐off voltage is set to 3.8 V. To seek further details on the contribution of vanadium and manganese cations during this redox process, ex situ XAS are collected on Na 3.75 V 1.25 Mn 0.75 (PO 4 ) cathodes with different state of charges and the normalized absorption spectra at V‐ and Mn‐K edges are shown in Figure b and Figure e, respectively.…”

“…An amount of 0.65 moles of sodium ion pfu is lost during the first cycle. Similar trends in the voltage profiles are observed for the Na 4 VMn(PO 4 ) 3 cathode when it is charged to 4.2 V versus Na + /Na 0 with the electrochemical exchanges of ≈2.29 and 1.6 moles of sodium ions pfu during the first charge and discharge, respectively . The pronounced changes in the voltage profile of the last two cathodes can be better viewed in their corresponding d Q /d V curves (Figure d,e).…”

“…To shed more insights on the structural evolution and the corresponding redox processes of Na 3+ y V 2− y Mn y (PO 4 ) 3 cathodes, two representative samples, that is, y = 0.25 and 0.75, from the series are further subjected for ex situ XRD and XAS studies. Note that the detailed studies on the (de)intercalation mechanism of the end members were previously reported . The ex situ XRD patterns of Na 3.25 V 1.75 Mn 0.25 (PO 4 ) 3 cathode with different state of charge are displayed in Figure S5a,d, Supporting Information, and the corresponding refined lattice parameters are shown in Table 1 .…”

“…When the same cathode is made of small particles (<200 nm) with conductive carbon network (8.6 wt%), it shows stellar electrochemical performances (109 mA h g ‐1 at 0.5C and 87% of capacity retention after 4000 cycles at 20C rate). Further, the Na 4 VMn(PO 4 ) 3 cathode delivers higher first charge capacity (≈150 mA h g −1 for an extraction of 3 mol of Na) upon high voltage oxidation, thanks to the activation of V 5+ /V 4+ couple at ≈4.0 V versus Na + /Na 0 . However, it irreversibly transforms into another NASICON phase at higher voltages which leads to its rapid capacity decay on the subsequent cycles.…”