Synergistic Role of Transition Metals and Polyanionic Frameworks in Phosphate-Based Cathode Materials for Sodium-Ion Batteries

TL;DR

This review addresses the challenge of developing high-voltage, cost-effective cathodes for sodium-ion batteries by surveying phosphate-based polyanionic frameworks. It dissects six framework families—PO$_4$, PO$_4$F, P$_2$O$_7$, mix-ortho/pyrophosphate, NASICON-type, and oxyfluorophosphates—focusing on how local TM environments, MO$_6$/XO$_4$ connectivity, and inductive effects tune redox potentials and sodium-storage mechanisms. A key finding is that multi-electron redox in NASICON and mix-phosphate/pyrophosphate hybrids, together with high-voltage fluorophosphate motifs, can deliver higher energy densities, while practical deployment hinges on scalable, green synthesis and electrolyte compatibility; Fe- and Mn-based systems emerge as cost-effective options, with high-entropy and Ni/Mn co-designs offering avenues to improve performance. The authors emphasize extending the electrolyte window beyond 5 V and leveraging structural-engineering strategies (high-entropy NASICON, mix-phosphate hybrids) to unlock multi-electron, high-voltage cathodes, aiming to achieve 160–220 Wh kg$^{-1}$ in practical cells at acceptable costs. Collectively, the work guides material design toward scalable, high-voltage phosphate-based cathodes that could make sodium-ion batteries competitive with lithium-ion counterparts for stationary and mobile energy storage.

Abstract

Ongoing research in the area of advanced cathode materials for sodium-ion batteries (SIBs) is expected to reduce reliance on lithium-ion batteries (LIBs), providing more affordable and sustainable energy storage solutions. Polyanionic compounds have emerged as promising options due to their stable structure and ability to withstand high-voltage conditions as well as fast charging capabilities. This review offers a thorough discussion of phosphate-based polyanionic cathodes for SIBs, exploring their structure, electrochemical performance with various transition metals, and existing challenges. We discuss different polyanionic frameworks, such as ortho-phosphates, fluoro-phosphates, pyro-phosphates, mix pyro-phosphates, and NASICON-based phosphates, highlighting their unique structural characteristics and ability to perform well across a wide potential range. Further, we delve into the mechanisms governing sodium storage and tunability of redox potentials in polyanionic materials, providing insights into the factors that affect their electrochemical performance. Finally, we outline future research directions and potential avenues for the practical applications of polyanionic high-voltage cathodes in sodium-ion battery technologies.

Figures (13)

• Figure 1: (a) A comparison between Li$^+$ and Na$^+$ as charge carriers, including global market prices (cost conversion 07-Apr-2025, 1 USD = 7.28 CNY) TE of key battery elements and their corresponding elemental abundance (wt%) in the Earth's crust PT. (b) The schematic illustration of the variation in redox voltage with changes in MO$_6$--PO$_4$ connectivity. (c) The influence of different ligands on redox voltage. (d) The strategic advantages of mixed-phosphate polyanionic cathodes compared to ortho- and pyro-phosphate frameworks. (e) An overview of the individual advantages and limitations of Fe and Mn, and how their appropriate combination can overcome respective challenges in sodium-ion batteries.
• Figure 2: A schematic comparison of experimental operating voltages of Fe$^{2+}$/Fe$^{3+}$ redox in various phosphate-based anionic environments and of different transition metals in NASICON (Na$_x$MM$^\prime$(PO$_4$)$_3$) structured materials. The depiction of crystal field splitting for M$^{2+}$ (M=Mn, Fe, Co, Ni) cations in an octahedral coordination environment is also presented.
• Figure 3: (a) The appropriate representation for the minimum and maximum potential thresholds concerning electrolyte stability, as well as the energy levels associated with the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), adapted from Peljo_EES_18. (b) The attraction of electrons towards PO$_4$ tetrahedra with high electronegativity and the electronegativity trend of elements F, S, P, and Si, as per the Pauling scale. (c) The visual representation of PO$_4$ tetrahedra, MO$_6$ octahedra, and P$_2$O$_7$ unit. (d) The illustration of the primary pathway for sodium diffusion in NASICON structure (Na$_3$Zr$_2$Si$_2$PO$_{12}$). Here, blue spheres represent the sodium ions' positions within the crystal structure, and blue triangles indicate key structural points labeled T$_1$ and T$_2$, which act as bottlenecks for sodium movement, specifically from Na(1) to the midpoint and from the midpoint to Na(2), respectively Haarmann_SSI_21. (e) A schematic representation of the difference between single-ion migration and multi-ion concerted migration and their related energy barrier He_NatCom_17.
• Figure 4: The crystal structure of the NaFePO$_4$ along the $b$ direction and arrows indicating the plausible sodium ion diffusion pathways in (a) Olivine, and (b) Maricite phase, respectively, adapted from Zhu_JPS_19. The illustration of migration energy of sodium ions along different paths in (c) Olivine and (d) Maricite phase, respectively Zhu_JPS_19. The GCD profiles of (e) Olivine NaFePO$_4$ after 10$^{th}$, 30$^{th}$, 60$^{th}$, and 100$^{th}$ cycle Zhu_NS_13, (f) Olivine NaMnPO$_4$ (against lithium) Boyadzhieva_RSCA_15, (g) Maricite NaFePO$_4$Kim_EES_15_Fe, and (h) Maricite NaMnPO$_4$ pristine and milled-coated Mohsin_IJE_23. (i) The crystal structure of the red phase NaCoPO$_4$, and (j) the corresponding GCD discharging profile along with the charging profile and capacity retention shown in the inset Gutierrez_ACSAMI_17. The CIF file to construct (a) is taken from (mp-746030), (b) from (mp-19226), (i) from (mp-562436) Jain_APLM_13.
• Figure 5: The crystal structure of NaVOPO$_4$ in (a) tetragonal (Na$_{0.8}$VOPO$_4$), (b) monoclinic, (c) triclinic phase, (d) $\beta$-orthorhombic (space group $Pnma$), (e) orthorhombic ($Pna2_1$) and the corresponding GCD profile of (f) tetragonal phase (cyclic voltametery is in inset) He_CM_16_T, (g) the monoclinic phase for 1$^{st}$--3$^{rd}$ cycles Song_CC_13, (h) triclinic phase for five cycles Fang_C_18, (i) $\beta$-orthorhombic ($Pnma$) for 1$^{st}$-50$^{th}$ cycles and cycling stability in inset He_CM_16_O, (j) orthorhombic ($Pna2_1$) at C/40 current rate (dQ/dE shown in inset) Shraer_ESM_24, and (k) amorphous phase for first five cycles Fang_CCSC_21. The CIF file to construct (a) is taken from He_CM_16_T, (b) from Song_CC_13, (c) from Fang_C_18, (d) from He_CM_16_O, and (e) from Shraer_ESM_24.