Suppressing Cation Migration and Reducing Particle Cracks in a Layered Fe‐Based Cathode for Advanced Sodium‐Ion Batteries

Abstract

thus designing high-performance cathodes is essential for the commercial application of SIBs. [7][8][9] So far, layered transition metal oxides, polyanions, and organic compounds are the main several kinds of cathodes to be studied for SIBs. [10][11][12] Among these materials, the layered oxides is one of the most promising candidates for their large theoretical capacities and simple analogies with LiTMO 2 . [13] Among the layered oxides, layered Fe-based oxides have attracted great interest due to their low cost and abundant resources. O3-type NaFeO 2 (NFO) has been extensively studied, [14][15][16] however, the reversible capacity is limited about 80 mA h g −1 at the narrow potential region, far below the theoretical capacity. One of the major hurdles is the Fe-ion migration to the Na layers when charged to high potential, thereby blocking the Na-ion diffusion path. [17] Doping with 3d transition-metal ions [18] in the slabs of O3-type NFO such as Ti, [19] Co, [20,21] and Ni [22] is an usual way, which indeed enhances the electrochemical performance. Through active Ni doping, O3-NaFe 0.3 Ni 0.7 O 2 could deliver a large capacity of 135 mA h g −1 , however, the capacity still rapidly drops within 30 cycles. Thus, to improve the structural stability remains a great challenge.Herein, we report a strategy of Ru substitution in NFO, which successfully suppresses the Fe-ion migration during cycling. We employed the advanced atomic-scale characterization to monitor the migration of Fe-ion.Moreover, the particle cracks is first discovered in cycling NFO which may be another major reason for the poor electrochemical performance. [23][24][25] Fe-ion migration has a direct correlation with the particle cracks generation. Results show that huge and intense internal strain which is the driving force for the particle cracks, arises in the distorted structure coming from Fe-ion migration. Besides, Fe-ion migration is part of the atomic mechanism at the initial stage of the particle cracks growth. [26] Correspondingly, Ru doping could also reduce the generation of particle cracks effectively.Comparatively, Ru doping NFO demonstrates the Fe-migration-free phenomena and accordingly suppresses the resulted side effect. Ru doping NFO could deliver a large Sodium-ion batteries have huge potential in large-scale energy storage applications. Layered Fe-based oxides are one of the desirable cathode materials due to abundance in the earth crust and high activity in electrochemical processes. However, Fe-ion migration to Na layers is one of the major hurdles leading to irreversible structural degradation. Herein, it is revealed that distinct Fe-ion migration in cycling NaFeO 2 (NFO) should be mainly responsible for the strong local lattice strain and resulting particle cracks, all of which results in the deterioration of electrochemical performance. More importantly, a strategy of Ru doping could effectively suppress the Fe-ion migration and then reduce the local lattice strain and the particle cracks, finally to greatly enhance the sod...

A Scalable Approach to Na2FeP2O7@Carbon/Expanded Graphite as a Low‐Cost and High‐Performance Cathode for Sodium‐Ion Batteries

“…[29,30] With a theoretical capacity of 97 mAh g À 1 , the pyrophosphate could provide a stable working voltage of 3.0 V. In addition, Fe-based materials are currently indeed attractive cathode for SIBs due to the advantage of low cost and abundance raw materials. [31,32] Despite the above advantages, the practical application of Na 2 FeP 2 O 7 is still limited by its low electronic conductivity and accompanying poor rate performance. [17,30,33,34] Actually, the poor electronic conductivity of this pyrophosphate could be ascribed to the discontinuity between the metal octahedrons in its crystal structure, which results in discontinuous electrons pathways and thus influencing the rate performance of Na 2 FeP 2 O 7 .…”

“…Specially, this pyrophosphate consists of FeO 6 octahedra and P 2 O 7 pyrophosphate groups which formed by corner‐sharing PO 4 tetrahedra . With a theoretical capacity of 97 mAh g −1 , the pyrophosphate could provide a stable working voltage of 3.0 V. In addition, Fe‐based materials are currently indeed attractive cathode for SIBs due to the advantage of low cost and abundance raw materials . Despite the above advantages, the practical application of Na 2 FeP 2 O 7 is still limited by its low electronic conductivity and accompanying poor rate performance .…”

Iron‐Based Layered Cathodes for Sodium‐Ion Batteries

“…Recently, Xu et al reported a Ru-doping strategy to suppress Fe-ion migration in NaFeO 2 . [78] In the obtained O3-type Na 4 FeRuO 6 (NFRO), partial Na + occupy at TM sites, while Na and [Na 1/3 Fe 1/3 Ru 1/3 ] slabs form alternate layers in the array of cubic close packed oxide-ions. Actually, this is a typical A 2 MO 3 type layered material, of which the most famous prototype is Li 2 MnO 3 .…”

“…[70] Then, by using aberration-corrected scanning transmission electron microscopy (STEM), Fe ion migration upon Na extraction in various Fe-based layered oxides was directly observed at the atomic scale. [73,74,78] In the case of NaFeO 2 , Fe ions occupied in sodium layer are clearly detected by STEM and electron energy loss spectroscopy (EELS) after charging the electrode to 4.4 V (Figure 6c-f). In the following discharge process, the NaFeO 2 phase fails to be fully recovered even after discharging to 1.5 V, indicating the irreversibility or partial irreversibility of Fe 3 + migration.…”

In-situ/operando characterization techniques in lithium-ion batteries and beyond

“…3(a, b)) [43][44][45][46]. Besides, a series of novel cathode materials for Na-ion batteries were developed and their structural evolutions were revealed by the combination of in-situ XRD and other characterization methods [47][48][49]. Xi and coworkers reported a novel Na 0.88 -Cr 0.88 Ru 0.12 O 2 cathode material for high-performance sodiumion batteries [47].…”