Unexpected discovery of low-cost maricite NaFePO₄as a high-performance electrode for Na-ion batteries

Abstract

Battery chemistry based on earth-abundant elements has great potential for the development of cost-effective, large-scale energy storage systems. Herein, we report, for the first time, that maricite NaFePO 4 can function as an excellent cathode material for Na ion batteries, an unexpected result since it has been regarded as an electrochemically inactive electrode for rechargeable batteries. Our investigation of the Na re-(de)intercalation mechanism reveals that all Na ions can be deintercalated from the nano-sized maricite NaFePO 4 with simultaneous transformation into amorphous FePO 4 . Our quantum mechanics calculations show that the underlying reason for the remarkable electrochemical activity of NaFePO 4 is the significantly enhanced Na mobility in the transformed phase, which is $one fourth of the hopping activation barrier. Maricite NaFePO 4 , fully sodiated amorphous FePO 4 , delivered a capacity of 142 mA h g À1 (92% of the theoretical value) at the first cycle, and showed outstanding cyclability with a negligible capacity fade after 200 cycles (95% retention of the initial cycle).The demand for large-scale energy storage systems (EESs) has prompted considerable effort in the development of new types of batteries with cost-effective and sustainable properties. While the high cost of current Li ion batteries (LIBs) remains one of the major hurdles towards large-scale energy storage applications, 1-12 battery chemistry based on earth-abundant elements offers a feasible solution. Recently, Na ion batteries (NIBs) have been considered as a promising alternative to LIBs since the underlying electrochemical reaction is similar to that of LIBs, but is based on the unlimited resources of Na from seawater. [13][14][15][16][17][18][19][20] The use of redox chemistry using earth abundant transition metals would provide the optimal combination with Na electrochemistry further highlighting the advantage of NIBs.In recent years, considerable research has been carried out on Fe-based electrode materials for use in NIBs. Broader contextWe report, for the rst time, that maricite NaFePO 4 can function as an excellent cathode material for Na ion batteries, an unexpected result since it has been regarded as an electrochemically inactive electrode for rechargeable batteries. Our investigation of the Na re-(de)intercalation mechanism reveals that all Na ions can be deintercalated from the nanosized maricite NaFePO 4 with simultaneous transformation into amorphous FePO 4 . Our quantum mechanics calculations show that the underlying reason for the remarkable electrochemical activity of NaFePO 4 is the signicantly enhanced Na mobility in the transformed phase, which is $one fourth of the hopping activation barrier. Maricite NaFePO 4 , fully sodiated amorphous FePO 4 , delivered a capacity of 142 mA h g À1 (92% of the theoretical value) at the rst cycle, and showed outstanding cyclability with a negligible capacity fade aer 200 cycles (95% retention of the initial cycle).540 | Energy Environ. Sci., 2015, 8, 540-545This jour...

Recent Progress in Electrode Materials for Sodium‐Ion Batteries

“…[ 110,117,118 ] Recently, Kim et al demonstrated that the maricite NaFePO 4 phase, which is a thermodynamically stable phase based on its stoichiometry but had been believed to be electrochemically inactive because of the absence of apparent Na diffusion paths in the structure, can become electrochemically active when prepared as nano-sized particles. [ 115 ] It was reported that a reversible capacity of ≈142 mA h g −1 can be achieved with promising rate performances and negligible capacity decay up to 200 cycles (95% retention), as observed in Figure 5 e. They found the origin of the electrochemical activity comes from a rapid phase transformation from the maricite FePO 4 into amorphous FePO 4 occurring during the fi rst Na deintercalation process. During the subsequent cycles, the amorphous FePO 4 phase becomes the main contributor of the electrochemical activity.…”

“…Reproduced with permission. [ 115 ] [ 111,114 ] The composition-temperature diagram of olivine Na x FePO 4 proposed by Lu et al revealed that the solid-solution phase is favorable for x > 2/3 at room temperature, whereas phase separation into FePO 4 /Na 2/3 FePO 4 occurs for x < 2/3. [ 111 ] More recently, Galceran et al reported a similar phase reaction upon the charge process using in situ XRD analysis, where the solid-solution reaction occurs until the formation of the intermediate phase of Na 2/3 FePO 4 , and further Na extraction induces a phase separation into Na 2/3 FePO 4 and FePO 4 .…”

Beyond Li-ion: electrode materials for sodium- and magnesium-ion batteries

“…Contrary to the LIB system, alkaliated olivine FePO 4 is not stable and cannot be synthesized by conventional synthetic routes for NIB application [63,64]; however, maricite NaFePO 4 is generally regarded as thermodynamically stable and does merit investigation [65]. Kim et al [65] combined computational and experimental findings to identify the mechanism responsible for the electrochemical activity of maricite NaFePO 4 .…”

Advanced Cathode Materials for Sodium‐Ion Batteries: What Determines Our Choices?

“…[121] A crystalline Na 0.62 FePO 4 powder cathode was prepared by a hydrothermal method from stoichiometric mixtures of FeSO 4 ·7H 2 O, C 6 H 9 Na 3 O 9 , and NH 4 H 2 PO 4 in a water/ethylene glycol solvent at 200 °C for 20 h. [115] For the synthesis of Na 3 V 2 (PO 4 ) 3, V 2 O 5 was first solved in H 2 O 2 to yield a yellow solution, and then NaC 2 H 3 O 2 , NH 4 H 2 PO 4 , and PVP were sequentially dissolved in stoichiometric amounts, and the obtained dried powder was heated at 700-800 °C for 8 h under a 5% H 2 /Ar atmosphere to yield Na 3 V 2 (PO 4 ) 3.…”

“…Interestingly, Kim et al found that 50 nm-sized maricite-type Na 1−x FePO 4 showed a capacity of 142 mA h g −1 (92% of the theoretical value) in the voltage range of 1.5-4.5 V, and delivered an excellent cyclability with 95% capacity retention after 200 cycles, due to the improved Na mobility by a phase transformation from maricite to the amorphous phase. [121] Amorphous iron phosphate has been also suggested as a promising cathode for SIBs, and can be obtained by facile lowtemperature methods. [122] Due to the singlephase sodium-ion insertion/extraction reaction, amorphous FePO 4 can undertake a monotonous potential change, highlighting its application in practical batteries.…”