energy. However, the uneven distribution of lithium resources and ever-growing price make it hard to fully support the fields of large-scale energy storage and electric vehicles. [3,4] In this context, room temperature sodium-ion batteries (SIBs), which are similar to LIBs in electrochemical characteristics and working mechanisms, emerge as a promising technology for grid-scale energy storage because of the wide abundance in the earth's crust and low cost of sodium resources. [5][6][7][8][9][10] In the past decades, tremendous efforts have been made in exploring appropriate electrode materials for SIBs, such as transition metal oxides, [11][12][13][14][15] phosphates, [16][17][18][19] ferrocyanides, [9,20,21] metal alloys, [22][23][24][25] hard carbon, [26][27][28][29][30][31][32][33][34] and chalcogenides, [35][36][37][38][39][40][41] and some of them have shown acceptable electrochemical performance. What's more, a lot of reviews have summarized recent developments in electrode materials for SIBs. [42][43][44][45][46][47][48][49] Nevertheless, the larger ionic radius of sodium ion (1.02 Å) compared to lithium ion (0.76 Å) and the higher molar weight of the Na have been regarded as the greatest challenge to hinder the performance improvement of SIBs, resulting in lower energy densities of SIBs compared to the lithium counterpart. In this way, SIB technology is quite suitable for the gridscale facility, where cost and longevity are the more important parameters than the energy density of the device.In the quest to build structurally stable SIBs with long lifespans, various 2D layered transition metal oxides and 3D framework materials have been studied for a long time. [50][51][52] The layered transition metal oxides can deliver high theoretical and reversible capacities, but they suffer from the multiple-phase transition during sodium uptake and lower redox potential owing to their strong covalence nature. [53,54] In addition, they are very sensitive to a moist atmosphere and the adding proportion of sodium resources during synthesis plays a critical role in the composition of the final product. Another type of widely reported cathode material is the Prussian blue analogs which have an open framework to accommodate the large Na + ions and can reversibly extract/insert two Na + per unit at high rates. [55] However, the products synthesized through the traditional process always contain a large number of lattice defects and coordinated water, which have a great influence on the active sites for Na + ion storage. Additionally, the production rate of Prussian blue analogs is low and the structure is thermally unstable, which may arise concerns about cost and safety problems when putting them into practical application. Given this It has long been the goal to develop rechargeable batteries with low cost and long cycling life. Polyanionic compounds offer attractive advantages of robust frameworks, long-term stability, and cost-effectiveness, making them ideal candidates as electrode materials for grid-scale energy stor...

Although great achievements have been gained on a series of Na4Fe x P4O12+x (2 ≤ x ≤ 4) materials such as Na2FeP2O7 (NFPO), Na4Fe3(PO4)2P2O7 (NFPP), and NaFePO4 (NFP), the phase evolution characteristics on these Na4Fe x P4O12+x materials are still lacking. Herein, 17 Na4Fe x P4O12+x samples with varied x are investigated via both experimental and computational methods. It is revealed that only three phases of NFPO, NFPP, and NFP exist in the Na4Fe x P4O12+x system, and Fe-defects tend to form at Fe2 sites in NFPP, resulting in a highly pure phase of Fe-defective NFPP (x = 2.91). The NFPP (x = 2.91) exhibits the highest specific capacity (110.1 mAh g–1) among the 17 Na4Fe x P4O12+x samples. The pouch cells assembled with the NFPP (x = 2.91) cathode and hard carbon anode show a good comprehensive electrochemical performance. We believe that this work can serve as an indispensable reference to promote the practical application of sodium-ion batteries.

No abstract

No abstract

Fe-based polyanionic materials are one of the most promising cathode materials for practical sodium-ion batteries due to their rich-resource, low cost, and excellent electrochemical performance. Although great achievements have been gained on a series of Na4FexP4O12 + x (2 ≤ x ≤ 4) materials such as Na2FeP2O7 (NFPO), Na4Fe3(PO4)2P2O7 (NFPP) and NaFePO4 (NFP), the structure and phase evolution characteristics on these Na4FexP4O12 + x are still lacking, making it difficult to synthesize these materials with pure phase and optimal electrochemical performance. Herein, seventeen Na4FexP4O12 + x samples with varied x are investigated via both experimental and computational methods to disclose the phase evolution properties. It reveals that only three phases of NFPO, NFPP, and NFP exist in the Na4FexP4O12 + x system, and Fe-defects tend to form at Fe2 sites in NFPP, resulting in a highly pure phase of Fe-defective NFPP (x = 2.91). The NFPP (x = 2.91) exhibits the best electrochemical performance among the seventeen Na4FexP4O12 + x samples. The pouch cells assembled with the NFPP (x = 2.91) cathode and hard carbon anode show excellent rate capability, superior low-temperature performance, high over-discharge endurance, and decent cycling stability. We believe that this work not only clarifies some important issues regarding the phase evolution in Fe-based polyanionic materials, but also serves as an indispensable reference to promote the practical application of low-cost sodium-ion batteries.