A NASICON‐Type Positive Electrode for Na Batteries with High Energy Density: Na₄MnV(PO₄)₃

Abstract

Among them, Na 3 V 2 (PO 4 ) 3 (NVP) has been identified so far as the most interesting one as it possesses satisfactory energy density and high power for extended cycle life. Its crystal structure can be described as a 3D framework of VO 6 octahedra and PO 4 tetrahedra connected to each other by common corners forming so-called "lantern units" along the c direction of the commonly used hexagonal cell. Sodium cations were described as randomly disordered over two sodium sites (Na(1), 6b and Na(2), 18e) [18] until Chotard et al. discovered that below 280 K, the so called α-NVP form crystallized in a monoclinic superstructure due to a fully ordered distribution of Na +[19] similar to the ordering previously reported in α-Na 3 Ti 2 (PO 4 ) 3 . [51] The synthesis of Na 3 V 2 (PO 4 ) 3 was first reported by Delmas. [20] Gopalakrishnan [21] later on reported on the possible extraction of three Na + toward the novel sodium-free V IV V V (PO 4 ) 3 composition. Afterward, the electrochemical extraction of Na + from Na 3 V 2 (PO 4 ) 3 to NaV 2 (PO 4 ) 3 (with a theoretical capacity of 117.6 mAh g −1 at 3.4 V vs Na/Na) was extensively investigated. [22][23][24][25][26] A large number of special treatments (e.g., carbon coating, particle shape controlling) was also proposed to improve battery performances. [27][28][29][30][31] It is important to note that only 2Na formula unit −1 have been completely removed from the structure during charging up to now. A possible activation of the V 4+/5+ redox couple at higher voltages may also contribute to the increasing of the energy density of NVP-based materials, as demonstrated in a series of works by using a metal substitution of a part of V 3+ in the structure of NVP. In recent years, several elements have been chosen for the partial substitution of V into the crystal structure of this promising material (such as Ni, [32][33][34] Al, [35,36] Fe 3+ , [34,37] Zr 4+ , [38] Mn 3+ , [39] Mn 2+ , [34,40] Cr 3+ , [41][42][43] Ti 4+ , [44][45][46] Mo 6+ , [47] and Mg 2+[48] ).In this work, Mn 2+ was used as a substituting ion to enhance the capacity of the Na 3 V 2 (PO 4 ) 3 cathode material. Inspired by the recent work of Zhou et al., [34] nearly single-phase Na 4 MnV(PO 4 ) 3 (98.5 wt%) powders were synthesized and studied structurally and electrochemically in details. In operando X-ray diffraction (XRD) studies during electrochemical operation show for the first time that Na 4 MnV(PO 4 ) 3 can deliver 156 mAh g −1 toward the new composition NaMnV(PO 4 ) 3 . Results and DiscussionThe crystal structure of Na 4 MnV(PO 4 ) 3 has been fully determined using high-resolution synchrotron powder XRD (SXRD) dataThe mixed Mn 2+ /V 3+ Na-super-ionic-conductor (NASICON) cathode material Na 4 MnV(PO 4 ) 3 is prepared by solid-state reaction at 800 °C under argon. When used as a positive electrode in Na batteries, this material can exchange three electrons for two transition metals, that is, yielding a high gravimetric capacity of 156 mAh g −1 on charge when the upper cutoff voltage is set to 4.3 V...

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 ) .…”

“…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 .…”

“…In the present work, we elucidate the role of Mn 2+ on the structural and electrochemical sodium (de)intercalation properties of the Na 3+ y V 2− y Mn y (PO 4 ) 3 (0 ≤ y ≤ 1) cathodes. The systematic introduction of Mn 2+ into the NVP lattice can alter the electronic structures of the cathodes which is expected to improve the cell voltages through the participation of different V‐ and Mn‐redox couples . Further it also increases the Na‐ion concentration in the framework which can affect its mobility .…”

“…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.…”