Alkaline-based aqueous sodium-ion batteries for large-scale energy storage

The escalating demand for large-scale energy storage necessitates the development of safer, more environmentally friendly, and cost-effective battery technologies. Aqueous sodium-ion batteries (ASIBs) are attractive candidates due to the abundance of sodium resources and their compatibility with existing industrial infrastructure. However, their practical application is hindered by the limited electrochemical stability window of water (1.23 V), leading to water decomposition and the formation of flammable hydrogen gas, compromising both safety and performance. Current strategies to enhance water stability, such as using expensive fluorine-containing salts to form a solid electrolyte interphase (SEI) or incorporating flammable co-solvents, significantly increase costs and safety risks. This research explores an alternative approach: utilizing alkaline electrolytes. While alkaline electrolytes inherently suppress hydrogen evolution reaction (HER) at the anode, they can intensify oxygen evolution reaction (OER) at the cathode and promote cathode dissolution, particularly in Mn-based Prussian blue analogues (PBAs) – commonly used cathodes known for their low cost, non-toxicity, and high energy density. The challenge lies in developing an alkaline ASIB that overcomes these limitations, achieving both high energy density and cycling stability.

Previous studies have explored various methods to improve the performance of aqueous batteries. The use of fluorine-containing salts to create a protective SEI layer is effective in suppressing HER but suffers from high cost and limited durability due to the solubility of SEI components. The addition of co-solvents improves water stability but increases electrolyte viscosity, hindering commercial applications with high-loading electrodes, and introduces flammability concerns. Alkaline electrolytes have been shown to thermodynamically favor HER suppression but can accelerate OER and cathode degradation, particularly in Mn-based PBA cathodes. The existing literature lacks successful implementations of high-performance alkaline ASIBs based on PBA cathodes due to the challenges posed by Mn and Fe dissolution in alkaline environments.

This study employs a Mn-based PBA cathode (Na₂MnFe(CN)₆, NMF), a NaTi₂(PO₄)₃ (NTP) anode, and a cost-effective, fluorine-free alkaline electrolyte of sodium perchlorate (NaClO₄). To overcome the limitations of alkaline electrolytes, a nickel/carbon (Ni/C) nanoparticle layer is coated onto the NMF cathode. The electrochemical performance is evaluated using coin cells and pouch cells. Characterization techniques include X-ray diffraction (XRD), in-situ differential electrochemical mass spectrometry (DEMS), in-situ Attenuated Total Reflectance Infrared (ATR-IR) spectroscopy, operando synchrotron X-ray powder diffraction (XRPD), operando Raman spectroscopy, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), inductively coupled plasma mass spectrometry (ICP-MS), and soft X-ray absorption spectroscopy (XAS). Density functional theory (DFT) calculations are performed to investigate the adsorption energies of OH⁻ and H⁺ on Ni surfaces and the energy of Ni substitution in the NMF structure. The coin cell tests explored the impact of salt concentration and alkalinity on HER and OER, while pouch cell tests simulated commercial scenarios with high electrode loading. Various techniques were used to analyze the structural and compositional changes in the cathode before, during, and after cycling, including in-situ and operando measurements to monitor electrochemical processes and their impact on the battery materials.

The alkaline electrolyte effectively suppresses HER at the anode. The Ni/C coating on the NMF cathode creates a H₃O⁺-rich local environment near the cathode surface, significantly reducing OER and electrode dissolution. This H₃O⁺-rich environment is attributed to the irreversible formation of Ni(OH)₂ and the reversible Ni(OH)₂/NiOOH redox reactions, confirmed by in-situ ATR-IR and operando XRPD. Simultaneously, Ni atoms are in-situ embedded into the cathode structure, stabilizing the NMF framework and further enhancing battery durability, as evidenced by operando Raman and HAADF-STEM analyses. The NMF//NTP full cell with the Ni/C coating demonstrates remarkably improved cycling stability, achieving over 13,000 cycles at 10 C with 74.3% capacity retention and a high energy density of 88.9 Wh kg⁻¹ at 0.5 C. A pouch cell with high electrode loading (>30 mg cm²) maintains nearly 100% capacity retention after 200 cycles and exhibits exceptional safety, maintaining functionality even after being cut and submerged in water. DFT calculations support the observed mechanisms, showing that Ni substitution is energetically favorable and that Ni surfaces have a greater affinity for OH⁻ compared to H⁺. The method demonstrates universality by successfully applying it to a Co/C coating, showing promise for wider applicability.

The findings address the critical challenges of developing high-performance alkaline ASIBs. The novel approach of using a Ni/C coating to create a H₃O⁺-rich microenvironment near the cathode effectively mitigates the detrimental effects of alkalinity on the cathode, allowing for both high energy density and exceptional cycling life. The in-situ Ni substitution further stabilizes the PBA structure, contributing to the long-term stability. The success with both Ni/C and Co/C coatings suggests this approach may be adaptable to other metal-based aqueous batteries. The impressive performance of the pouch cell under extreme conditions (cut and submerged in water) highlights the enhanced safety of this alkaline ASIB design, making it a strong candidate for large-scale energy storage applications, particularly those requiring high safety.

This study successfully demonstrates a high-performance alkaline ASIB employing a novel Ni/C cathode coating strategy. This approach addresses the limitations of existing aqueous battery technologies by suppressing both HER and OER, achieving unprecedented cycling stability and high energy density. The universality of the method is also demonstrated, opening possibilities for wider application across various aqueous battery systems. Future research could explore other transition metals for coating, optimization of coating thickness and composition for different cathode materials, and investigation of alternative anodes to further enhance battery performance.

The study focuses primarily on the NMF//NTP system, and further research is needed to evaluate the generalizability of the Ni/C coating strategy across different cathode and anode materials. While the pouch cell demonstrates high safety, long-term stability under real-world conditions requires further investigation. The DFT calculations, while supporting the experimental observations, are simplifications of complex electrochemical processes and further computational studies could refine the understanding of the underlying mechanisms.