Approaching the theoretical capacity limit of Na₂FeSiO₄-based cathodes with fully reversible two-electron redox reaction for sodium-ion battery

Abstract

Orthosilicate compounds are emerging as a promising class of low-cost and intrinsically safe cathodes due to the strong inductive effects of polyanion groups for rechargeable sodium-ion batteries. However, enabling two-electron redox reaction and actualizing the appealing high theoretical capacity of ∼270 mAh g⁻¹ for orthosilicates remain challenging. Here, fully reversible two-electron redox reaction in sodium iron orthosilicate cathodes by fluorine doping are reported. Owing to the unlocking of the Fe³⁺/Fe⁴⁺ redox couple, F-doped Na₂FeSiO₄ displays exceptionally high capacity of 271 mAh g⁻¹ that has never been reported for polyanionic cathodes. Based on the newly built crystal structure model of triclinic phase, fluorine doping is demonstrated to greatly promote charge redistribution and accelerate the electron exchange, hosting more sodium ions in the framework and stabilizing Fe⁴⁺ containing intermediate phases thermodynamically. The zero-strain characteristics of fluorine-doped orthosilicate ensure its excellent cycling stability with 93.7% capacity retention over 200 cycles. The successful unlocking of the trapped sodium in orthosilicates provides valuable insights and opens up a new avenue for the development of high capacity cathode materials for rechargeable batteries.

Introduction

The landscape of energy storage, especially in large-scale grids, has been extended beyond lithium-ion batteries (LIBs) to other electrochemical systems because of the lithium resource shortage [[1], [2], [3]]. Owing to the abundance of sodium sources and the similar energy storage principle to that of LIBs, advances in sodium-ion batteries (SIBs) will usher in a new stage of low-cost, sustainable, and large-scale energy storage systems [[4], [5], [6], [7], [8], [9]]. The large molar mass and ionic radius of sodium, however, aggravates the difficulty in the reversible sodiation/desodiation of electrodes and, correspondingly, the final electrochemical performance [[10], [11], [12]]. To improve sodium storage performance, there has been an intensive search for new cathode materials, which are generally classified as transition metal oxides, Prussian blue analogs, and polyanion compounds. Considering the severity of the safety issue for rechargeable batteries, particularly in large-scale applications, polyanion compounds have drawn widespread attention owing to their intrinsic safe characteristics during cycling, which originate from the strong inductive effects of polyanion groups [[13], [14], [15], [16]].

More than a dozen polyanionic systems have been explored as cathodes for SIBs, as shown in Fig. 1 and Table S1, although their relatively low capacity has become the main obstacle for achieving high energy density in SIBs [[17], [18], [19], [20]]. For the polyanion compounds with a single-electron redox reaction, for example, NaFePO₄, there is a theoretical capacity limit of ∼140 mAh g⁻¹ [[21], [22], [23], [24], [25], [26], [27]]. On the other hand, for the complex polyanion compounds with multipolyanion groups, such as Na₃V₂(PO₄)₃ and Na₂Fe₂(SO₄)₃, the theoretical capacities are generally lower than 130 mAh g⁻¹, although the two-electron redox reaction might occur [[28], [29], [30], [31], [32]]. The only opportunity to achieve high capacity lies in the polyanion compounds that possess a single polyanion group in their formulas and might afford a two-electron redox reaction, for example, Na₂FeSiO₄ with a theoretical capacity of 276 mAh g⁻¹. Unfortunately, preliminary work on Na₂FeSiO₄-based cathodes has failed to realize the full potential of their theoretical capacity [[33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43]]. The main reason for this is that the high redox potential of the polyanion group, exceeding the voltage range of the state-of-the-art electrolyte (<4.5 V vs. Na/Na⁺), impedes further redox reaction once one Na⁺ is already involved [[44], [45], [46], [47], [48]]. In addition, low electronic conductivity of Na₂FeSiO₄-based cathodes leads to lower capacity and rate capability compared with other cathodes [49,50]. From this point of view, it is of great significance, but very challenging, to unlock the trapped reaction in such polyanion compounds, for the sake of pursuing higher capacity cathodes for SIBs.

Herein, we have achieved the high theoretical capacity of Na₂FeSiO₄-based cathodes through unlocking the trapped Na⁺ to realize the complete two-electron redox reaction. Upon introducing fluorine, 25% F-doped Na₂FeSiO₄ (NFSF) exhibits an exceptionally high capacity of 271 mAh g⁻¹ when charged up to the safe voltage limit of organic electrolyte (<4.5 V), which approaches the theoretical capacity limit and is the highest value so far among all polyanionic cathodes. The crystal structure model of Na₂FeSiO₄ with triclinic phase was firstly constructed to reveal the origin of the high capacity based on theoretical calculations. The Fe³⁺/Fe⁴⁺ redox couple was verified to be completely activated owing to the acceleration of electron exchange by fluorine and the formation of stable intermediate phases. Benefiting from its good structural stability and the protection of a thin carbon layer, F-doped Na₂FeSiO₄ cathodes delivered excellent cycling performance and superior rate capability.