Na₂FeP₂O₇ as a Promising Iron‐Based Pyrophosphate Cathode for Sodium Rechargeable Batteries: A Combined Experimental and Theoretical Study

Abstract

Considering the promising electrochemical performance of the recently reported pyrophosphate family in lithium ion batteries as well as the increasing importance of sodium ion batteries (SIBs) for emerging large‐scale applications, here, the crystal structure, electrochemical properties, and thermal stability of Na2FeP2O7, the first example ever reported in the pyrophosphate family for SIBs, are investigated. Na2FeP2O7 maintains well‐defined channel structures (triclinic framework under the P1 space group) and exhibits a reversible capacity of ≈90 mAh g−1 with good cycling performance. Both quasi‐equilibrium measurements and first‐principles calculations consistently indicate that Na2FeP2O7 undergoes two kinds of reactions over the entire voltage range of 2.0–4.5 V (vs Na/Na+): a single‐phase reaction around 2.5 V and a series of two‐phase reactions in the voltage range of 3.0–3.25 V. Na2FeP2O7 shows excellent thermal stability up to 500 °C, even in the partially desodiated state (NaFeP2O7), which suggests its safe character, a property that is very critical for large‐scale battery applications.

Exploring the Ni redox activity in polyanionic compounds as conceivable high potential cathodes for Na rechargeable batteries

“…8 Unfortunately, several studies have demonstrated that the implementation of this material is restricted by several drawbacks such as the intrinsic sluggish kinetics attributable to the low electronic conductivity, the poor lithium transport in commonly used electrolyte systems and the structural instability of the delithiated phases upon cycling that require further studies to obtain high-performance Ni-based phosphates as high voltage cathodes for battery application. [8][9][10][11][12] With regard to Na-based systems, considerable research has also been devoted to NASICON (sodium (Na) Super Ionic CONductor)-type polyanionic materials, such as NaMPO 4 , 13,14 Na 2 MP 2 O 7 (where M is a transition metal) 15,16 and Na 3 V 2 (PO 4 ) 3 , 17,18 as cathode materials for room temperature Na-ion batteries. However, the low energy densities resulting from the low theoretical capacity and, generally, cell voltage, as well as the poor rate capability compared with their lithium counterparts, have inhibited the widespread development of Na-ion batteries.…”

“…Indeed, first principle calculations confirmed a relatively low migration barrier for diffusion of Na ions. 15 In this context, a new polyanionic compound of the general formula Na 4 M 3 (PO 4 ) 2 (P 2 O 7 ) with the Fe 2+/3+ redox couple has been proposed by Kang and colleagues 22,23 as a promising cathode in terms of its superior Na mobility and thermal stability. The material has a theoretical capacity of 129 mAh g − 1 and an operating potential of ∼ 3.2 V vs Na + /Na, higher than the potential exhibited by NaFePO 4 and Na 2 FeP 2 O 7 .…”

Full Utilization of Superior Charge-Discharge Characteristics of Na1.56Fe1.22P2O7Positive Electrode by Using Ionic Liquid Electrolyte

“…8 In order to overcome such limitations in electrode performance, phosphate-based framework materials have been proposed as positive electrode materials in Na secondary batteries. [9][10][11][12][13][14][15][16] The robust framework undergoes a topotactic Na insertion/extraction reaction with a small volume change upon electrochemical cycling. Among them, pyrophosphates (Na 2 MP 2 O 7 , where M = Fe, Mn, or Co) have attracted interest owing to their favorable electrochemical activity and good thermal stability.…”

“…Among them, pyrophosphates (Na 2 MP 2 O 7 , where M = Fe, Mn, or Co) have attracted interest owing to their favorable electrochemical activity and good thermal stability. [10][11][12][13][14][15][16] However, they are less appealing in terms of theoretical capacity (ca. 97 mAh g −1 , 1 Na per formula unit) as compared to layered oxides (ca.…”

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

“…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)(12)(13)(14)(15), pyrophosphates (16)(17)(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.…”