Amorphous iron phosphate: potential host for various charge carrier ions

Abstract

In response to the ever-increasing global demand for viable energy-storage systems, sodium and potassium batteries appear to be promising alternatives to lithium ion batteries because of the abundance, low cost and environmental benignity of sodium/potassium. Electrical energy storage via ion-intercalation reactions in crystalline electrodes is critically dependent on the sizes of the guest ions. Herein, we report on the use of a porous amorphous iron phosphate synthesized using ambient temperature strategies as a potential host that stores electrical energy through the feasible insertion of mono-/di-/tri-valent ions. A combination of ex situ studies reveals the existence of a reversible amorphous-to-crystalline transition in this versatile electrode during electrochemical reactions with monovalent sodium, potassium and lithium. This reconstitutive reaction contributes to realizing specific capacities of 179 and 156 mAhg⁻¹ versus sodium and potassium at current densities of 10 and 5 mAg⁻¹, respectively. This finding facilitates the feasible development of several amorphous electrodes with similar phase behavior for energy-storage applications.

Introduction

Since 1990, the global demand for electricity has increased twice as much as the demand for energy overall, and the demand for electricity is expected to further increase by more than two-thirds over the next 20 years. Energy storage/conversion technologies have therefore become a crucial research topic as we seek to make society sustainable. In particular, electrical energy storage is critical not only for supporting electronic, vehicular and load-leveling applications but also for efficiently commercializing renewable solar and wind power. Rechargeable Li-ion batteries with an output energy exceeding 90% have emerged as one of the most effective electrochemical energy-storage technologies, and these batteries power most modern-day electronic devices.¹ Despite substantial research to enhance Li-ion batteries for high-power applications, aspects such as their availability, cost and safety still remain to be fully addressed.² The controversies surrounding the accessible global lithium reserves and the anticipated energy demand may greatly impact the cost of Li-ion batteries in the long term.³ Although advancing Li-ion battery technologies for electric vehicle applications is attractive, the quest for alternative energy sources for smart grid-scale storage applications has recently gained significant momentum. Rechargeable sodium and potassium batteries offer tremendous potential because they utilize inexpensive, abundant and environmentally benign sodium/potassium elements.^{4, 5,}

Figure 7

Electrochemical studies of Zn/Mg insertion in amorphous FePO₄. The initial voltage-capacity curves of (a) zinc and (b) magnesium cells cycled between 1.8–0.5 and 1.7–0.4 V versus zinc and magnesium under current densities of 10 mA g⁻¹ and 5 mA g⁻¹, respectively. Active material loading for the cells was 2.5 mg cm⁻².

Discussion

The impressive Na/K-insertion capabilities of the amorphous FePO₄ achieved under a wide potential range offer potential solutions for alternative batteries. Although sodium battery cathodes are increasingly reported, the demonstration of a prospective potassium battery cathode operating at 2.5 V is unique. In fact, the possibility of K insertion at apparently low potentials in crystalline metal-organic-framework cathodes with three-dimensional porous features in an aqueous electrolyte medium was attributed to the average ion channel size being comparable with that of the solvated potassium ions. This size match facilitates rapid ion diffusion in the lattice and contributes to electrochemical reactivity over extended cycling.⁸ The present study, however, showcases the possibility of using amorphous FePO₄ with porous morphologies formed from particle aggregates and inter-cluster voids as prospective host towards the reversible insertion of K, Na and Li ions. Furthermore, the nano-scale particles may be advantageous for insertion/de-insertion because the high surface-to-volume ratio of nanomaterials facilitates rapid ion diffusion. The amorphous characteristics of FePO₄ are represented by short-range ordering of constituents at atomic-scale lengths, and this material undergoes a transformation to a crystalline phase with a long-range ordered structure, as deduced from the high-resolution SXRD and TEM results. The electrochemically induced reversible disorder-to-order phase transition appears to contribute to the electrode performance of amorphous FePO₄, and the identification of such phase transitions is familiar.⁴⁰ Although the present study details the experimental observation of a unique electrochemical mechanism, the underlying factors that are responsible for this finding still require theoretical thermodynamic approaches at the nano-scale level. More importantly, the present study demonstrates the feasibility of using amorphous-based cathodes as plausible insertion hosts for divalent and trivalent ions. Furthermore, the inexpensive and simple ambient temperature strategy used to obtain this environmentally benign amorphous cathode with porous properties not only tends to simplify large-scale commercialization but also provides opportunities to develop similar transition metal-phosphate electrodes that undergo electrochemically induced reversible reconstitutive reactions for energy-storage applications.