Role of intermediate phase for stable cycling of Na ₇ V ₄ (P ₂ O ₇ ) ₄ PO ₄ in sodium ion battery

Abstract

Sodium ion batteries offer promising opportunities in emerging utility grid applications because of the low cost of raw materials, yet low energy density and limited cycle life remain critical drawbacks in their electrochemical operations. Herein, we report a vanadium-based ortho-diphosphate, Na 7 V 4 (P 2 O 7 ) 4 PO 4 , or VODP, that significantly reduces all these drawbacks. Indeed, VODP exhibits single-valued voltage plateaus at 3.88 V vs. Na/Na + while retaining substantial capacity (>78%) over 1,000 cycles. Electronic structure calculations reveal that the remarkable single plateau and cycle life originate from an intermediate phase (a very shallow voltage step) that is similar both in the energy level and lattice parameters to those of fully intercalated and deintercalated states. We propose a theoretical scheme in which the reaction barrier that arises from lattice mismatches can be evaluated by using a simple energetic consideration, suggesting that the presence of intermediate phases is beneficial for cell kinetics by buffering the differences in lattice parameters between initial and final phases. We expect these insights into the role of intermediate phases found for VODP hold in general and thus provide a helpful guideline in the further understanding and design of battery materials.cathode | single voltage | atomic reorganization | ab initio calculation S odium ion batteries (SIBs) provide advantages of unlimited resource, low material cost, and easy worldwide accessibility that could make them the material of choice for grid-scale energy storage systems (1, 2). Unfortunately, the electrochemical performance of current SIBs remains inferior to that of lithium ion batteries (LIBs). In particular, we require SIB cathode materials that give a long cycle life with large capacities and high voltages under fast operating conditions, comparable to the Li counterparts.Numerous layered Na x MO 2 [M = Co, Mn, Fe, Ni, Cr, and multicomponent transition metals (TMs)] have been intensively studied as candidate SIB cathodes because such layer-structured materials typically exhibit higher capacities than other classes of materials (3-10). However, they suffer from a substantial capacity decay with cycling, owing to crystal structure collapse and/or unstable electrode-electrolyte interfaces (10). For more stable host frameworks, various polyanion structures have also been studied as SIB cathodes as a natural extension of the success shown in the LIB materials of the same classes, including TM fluorophosphates (11-15), pyrophosphates (16-18) and phosphates (19)(20)(21)(22). However, most of these materials still exhibited insufficient cycle lifetimes and large voltage steps that limit their practical capacity. The Prussian blue family has also been extensively investigated for both organic (23, 24) and aqueous electrolytes (25) owing to its unique advantages related to low material cost, low temperature synthesis, and decent electrochemical performance from well-developed ionic channel structures. However, the as-syn...

Boron Substituted Na3V2(P1−xBxO4)3 Cathode Materials with Enhanced Performance for Sodium‐Ion Batteries

“…In the past few years, many compounds including layer‐structured oxide and polyanionic phosphates have been investigated to find a suitable cathode material for reversible and rapid intercalation/deintercalation of Na + 7, 8, 9, 10, 11, 12, 13, 14. Among them, sodium super ion conductor (NASICON) structured Na 3 V 2 (PO 4 ) 3 with 3D open framework and large tunnels for Na + migration has aroused an extensive interest as cathode materials for SIBs owing to its superiority of good stability, moderate potential plateau, high energy density, and so on 14, 15, 16, 17, 18.…”

Polyanion‐Type Electrode Materials for Sodium‐Ion Batteries

“…Two vanadium‐mixed polyanion electrodes, Na 7 V 4 (P 2 O 7 ) 4 PO 4 130 and Na 7 V 3 (P 2 O 7 ) 4 131 have also attracted significant attention. These two composites are considered promising cathode candidates due to their high operating potentials: near 4.0 V vs. Na + /Na and superior high rate long cycling capability (even at a high rate of 20 C and 3 C after 800 cycles, approximately 94% and 91% of the capacity, respectively, was maintained).…”

Phosphate Framework Electrode Materials for Sodium Ion Batteries

“…The Na 7 V 4 (P 2 O 7 ) 4 (PO 4 ) holds exceptional electrochemical properties represented by well‐defined high voltage profiles at 3.88 V (Figure 13d) and substantial capacity retention over 1000 cycles 235. The material showed a V 3+ /V 4+ redox reaction with Na 5 V 3.5+ 4 (P 2 O 7 ) 4 (PO 4 ) as intermediate phase, resulting in two plateaus in charge/discharge curves 236.…”