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

doi:10.21203/rs.3.rs-2783165/v1

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

https://doi.org/10.21203/rs.3.rs-2783165/v1

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Aqueous sodium-ion batteries (ASIBs) are practically promising for large-scale energy storage, but their energy density and lifespan are hindered by water decomposition. Current strategies to enhance the water stability include using expensive fluorine-containing salts to create a solid electrolyte interface or adding potentially-flammable organic co-solvents in the electrolyte to reduce water activity. However, these methods have significantly increased cost and safety risk. Shifting electrolytes from near neutrality to alkalinity can fundamentally suppress hydrogen evolution, but trigger oxygen evolution and cathode dissolution. Here, we present an alkaline-type ASIB with Mn-based Prussian blue analogue cathode, which exhibits a record lifespan of 13,000 cycles at 10 C together with high energy density of 90 Wh kg−1 at 0.5 C. This is achieved by building a nickel/carbon layer to induce a H3O+-rich local environment near the cathode surface, thereby suppressing oxygen evolution and cathode dissolution. Simultaneously, Ni atoms can be in-situ embedded into the cathode to enable its durability. At an industry-level mass loading > 30 mg cm−1, the pouch cell exhibits excellent stability with a capacity retention of ~ 100% following 200 cycles at 300 mA g−1, outperforming previously reported aqueous batteries.

Physical sciences/Materials science/Materials for energy and catalysis/Batteries

Physical sciences/Energy science and technology/Energy storage/Batteries

The growing demand for large-scale energy storage has boosted the development of batteries that prioritize safety, low environmental impact, and cost-effectiveness^1-3. On account of abundant sodium and the compatibility with commercial industrial systems⁴, aqueous sodium-ion batteries (ASIBs) are practically promising for affordable, sustainable, and safe large-scale energy storage. However, their energy density and cycling stability are limited due to the narrow electrochemical stability window of 1.23 V for water. Additionally, the accumulation of flammable hydrogen (H₂) from water decomposition during cycling compromises battery safety and restricts the development of ASIBs. One common strategy for improving the performance of aqueous batteries is to use expensive fluorine-containing salts to create a solid-electrolyte interphase (SEI)⁵ that can suppress the hydrogen evolution reaction (HER) and increase the electrochemical window of the electrolyte. However, the high solubility of SEI components such as LiF, NaF, and Na₂CO₃ limits its durability⁶. Moreover, the high cost of fluorine-containing salts significantly compromises the cost-effectiveness of aqueous batteries. Another approach involves using co-solvents⁷, including polymers, to improve the water stability of the electrolyte. Unfortunately, these strategies significantly increase the viscosity of the electrolyte, making it challenging to match with high loading electrodes for commercial applications. The organic co-solvents' potential flammability also compromises the safety of aqueous batteries. Therefore, developing alternative strategies to enhance the water stability of aqueous batteries while maintaining their cost-effectiveness and safety is a critical research priority.

Compared to conventional aqueous electrolytes with a pH close to 7, alkaline electrolytes offer thermodynamic suppression of HER on the anode based on the Pourbaix diagram for water⁸. Whereas shifting the electrolyte from near neutrality to alkalinity aggravates the oxygen evolution reaction (OER) on the cathode⁹. Furthermore, the high concentration of OH⁻ in electrolytes limits the choice of cathodes as it interacts with transition metal-based electrodes, leading to the deterioration of electrode structures, especially for Prussian blue analogues (PBAs) cathodes¹⁰. As the commonly used Mn-based PBAs, they have been widely studied in traditional aqueous batteries benefiting from advantages of non-toxicity, low cost, and high energy density^5-7. Nevertheless, their potential use in alkaline electrolytes is constrained due to the strong Jahn-Teller effects in the redox couples of Mn²⁺/Mn³⁺ as well as the dissolution of Fe¹¹. Consequently, PBA-based alkaline ASIBs have not been reported to date.

Here, we report a hydrogen-free alkaline ASIB based on a Mn-based PBA cathode (Na₂MnFe(CN)_6,NMF), NaTi₂(PO₄)₃ (NTP) anode, and an affordable alkaline electrolyte of fluorine-free sodium perchlorate (NaClO₄) whose price is much cheaper than commonly-used sodium triflate and sodium bis(trifluoromethylsulfonyl)imide in highly concentrated electrolytes. As illustrated in Fig. 1a, the alkaline electrolyte effectively suppresses HER at the anode. Moreover, by coating a commercially available nickel/carbon (Ni/C) nanoparticle-based layer on the NMF cathode, a H₃O⁺-rich local environment forms near the cathode surface. This H₃O⁺-rich local environment results from the irreversible formation of Ni(OH)₂ and reversible Ni(OH)₂/NiOOH redox (confirmed by in-situ Attenuated Total Reflectance Infrared (ATR-IR), Raman and Operando synchrotron X-ray powder diffraction, XRPD), which significantly reduces OER and electrode dissolution. Additionally, partial Ni atoms in the coating are in-situ embedded in the cathode to stabilize the NMF structure in alkaline media, as confirmed by operando Raman and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM).

Electrochemical performance of alkaline NMF/NTP coin cells

At first, the prepared NMF, NTP, and the commercial Ni/C powders were studied by X-ray diffraction (XRD, Supplementary Figs. 1-3), indicating their good crystallinity for further applications in batteries. Then, the impact of salt concentration in electrolytes on HER was investigated by in-situ differential electrochemical mass spectrometry (DEMS) in NaClO₄electrolytes with selected salt concentrations. Results show that, without forming a reliable SEI, increasing the electrolyte concentration cannot change the onset potential of HER (Supplementary Fig. 4).

Notably, increasing the alkalinity of electrolyte suppresses HER (Supplementary Figs. 5a-c), which contributes to a better cycling stability of NTP anode (Supplementary Figs. 6a-c). However, the increased alkalinity of electrolyte propels OER (Supplementary Fig. 5d) and also increases the dissolution rate of Fe and Mn elements¹² of the NMF cathode, leading to the poor cycling stability even at a water-stable voltage range of 0-1.0 V (Supplementary Figs. 6d-e and h). In comparison, after coating Ni/C layer on the NMF electrode (thickness: ca. 1 µm, Supplementary Fig. 7), its cycling stability is significantly enhanced (Supplementary Figs. 6f), as confirmed by the unchanged electrolyte colour (Supplementary Figs. 6i) as well as the suppressed Fe dissolving concentration in the electrolyte verified by results of inductively coupled plasma mass spectrometry (ICP-MS) (Supplementary Fig. 8).

The performance of NMF/NTP full cells using neutral electrolyte or alkaline electrolyte with/without Ni/C coating, was evaluated in a wide charging voltage range of 0.5 to 2.2 V. The NMF/NTP full cell with Ni/C coating exhibits a highly boosted rate performance and higher average discharge voltage than those without Ni/C coating, contributing to the fast-charge ability and high energy density of the battery (Supplementary Figs. 9a-b). Fig. 1b compares the cycling performance of NMF/NTP batteries under three different conditions at 1 C. Batteries without Ni/C coating in both neutral and alkaline electrolytes show a rapid capacity decay with capacity retention of < 60% following 200 cycles, whereas the alkaline-based battery with Ni/C coating exhibits a higher retention of ~ 100%. Furthermore, the electrolyte exhibits a lower freezing point in comparison to previously reported highly concentrated electrolytes^{5, 11},, allowing the battery to function effectively at low temperatures. (Supplementary Fig. 10). Consequently, the battery with Ni/C coating displays a capacity retention of 91.3% after 200 cycles at 0.5 C under −30 °C (Fig. 1c). Importantly, this full cell features a record lifespan of > 13,000 cycles with a high capacity retention of 74.3% at 10 C (Fig. 1d), surpassing mostly previously reported aqueous batteries¹³.

Pouch cell’s performance and comparison

To meet commercial requirements for large-scale energy storage, a Ni/C coated NMF//alkaline electrolyte//NTP pouch cell was assembled with the electrode loading of ca. 20 mg cm⁻². This pouch cell exhibits a high capacity retention of 85% following 1,000 cycles at 500 mA g⁻¹ (Fig. 2a). Additionally, the mass loading of single electrode can be further increased to the industry level of > 30 mg cm⁻² benefiting from the low viscosity of the alkaline electrolyte (6.0 mPa s, Supplementary Table 1). Under such high loading, NMF/NTP pouch cell shows stable cycling life with a capacity retention of ~ 100% within 200 cycles at 300 mA g⁻¹ (Fig. 2b). This large-size pouch cell also exhibits an ultra-high stability under ‘harsh’ conditions of cutting and immersion in water (Supplementary Videos 1-2, Supplementary Figs. 11a-c and Fig. 2c). In addition, the cut pouch cell can continuously power a humidity clock in water for > 20 h (Supplementary Video 3 and Fig. 2d). This confirms that the battery is resistant to electrolyte leakage and can withstand significant damage in high-humidity environments. As a result, the battery offers high safety for practical applications in energy storage and underwater electrical equipment.

Significantly, in addition to high safety, our battery design reduces the cost of manufacture and fabrication via hiring low-cost fluorine-free salt. Compared to other electrolytes for reported aqueous Li/Na/K ion batteries with an energy density of > 80 Wh kg⁻¹ (based on total mass of anode and cathode), our electrolyte has a significantly lower unit-cost (the detailed comparison displays in Supplementary Fig. 12 and Supplementary Table 2), which guarantees the actual application of alkaline-based batteries. Furthermore, our batteries can achieve a high cycling stability and discharge capacity under ultra-low positive/negative capacity ratio of 1.06. Under such a low capacity ratio and 0.5 C charging rate, a greatest energy density amongst ASIBs (90 Wh kg⁻¹) can be obtained (Fig. 2e and Supplementary Table 3). Compared to reported electrochemical battery storage devices, our battery system possesses significant advantages including, use of abundant elements (such as Fe, Mn, and Ti), high safety (high toleration to high humidity environment), environment friendliness (non-poisonous electrolyte), and a record lifespan (Fig. 2f and Supplementary Table 4). Although its energy density is slightly lower than that of Li-ion and nickel-metal-hydride (Ni-MH) batteries, its cost-efficacy and safety, namely key requirements for large-scale energy storage, are superior.

The origin of H₃O⁺-rich local environment

To determine underlying factors for the high performance of the alkaline ASIB, the interface structure with Ni/C coating was assessed viain-situ ATR-IR spectroscopy. The carbon coating was taken as a ‘control’ to eliminate any influences of carbon support. For the electrode modified by pure carbon, the spectra exhibit no apparent change even after being charged to 1.3 V (vs. Ag/AgCl), evidencing that the carbon and binder do not change microenvironment of the cathode surface (Fig. 3a, the function of carbon in Ni/C coating is discussed in Supplementary Text S1 and Supplementary Fig. 13). In contrast, with Ni/C modification, new peaks at 1,798 and 2,032 cm⁻¹ appear when the potential is > 0.6 V, attributed to two

(Supplementary Fig. 14a). Operando DEMS was employed to determine the water decomposition in this alkaline battery system during battery cycling. The battery without Ni/C coating exhibits HER and OER concurrently at a low positive/negative capacity ratio in the neutral electrolyte (Supplementary Fig. 14b). However, after coating Ni/C on the NMF cathode in alkaline electrolyte, both HER and OER become inconspicuous, except for trace O₂ at the first cycle before activating the surface coating (Fig. 3b). It can be concluded that the H₃O⁺-rich local environment induced by Ni/C protective layer suppresses the deteriorative OER in the alkaline electrolyte, while the alkaline electrolyte effectively retards HER.

The H⁺ accumulation mechanism on the electrode surface with Ni/C is illustrated in Fig. 3c. The H₃O⁺ comes from water dissociation, inequivalent adsorption ability of H⁺ and OH⁻ on Ni, together with a reversible transformation between Ni(OH)₂ and NiOOH. Ni nanoparticles promote water dissociation, resulting in the production of H⁺ and OH⁻ around this coating¹⁶. The OH⁻ from bulk alkaline electrolyte and water dissociation on the surface of Ni can be confined due to the strong interaction between Ni and OH⁻, as evidenced by density functional theory (DFT) simulations (Supplementary Table 5). As the applied potential on Ni increases, the interaction between Ni and H⁺ becomes weaker whilst the interaction between Ni and OH⁻ remains stable, evidencing that H₃O⁺ is more likely to be generated at high voltages, which is confirmed by ATR-IR and Raman spectra mentioned before. Moreover, Ni is oxidized to Ni(OH)₂ at higher voltages to bond with OH⁻. Additional H⁺is generated when Ni(OH)₂ is oxidized to NiOOH, as confirmed by XRPD and soft X-ray absorption spectra (XAS, Supplementary Figs. 15-16 and Supplementary discussion Text S2). Thereby, H⁺ bonds with nearby water molecules rather than Ni nanoparticles to form H₃O⁺. These H₃O⁺ ions exposed to bulk alkaline electrolyte are readily neutralized by excess OH⁻. In contrast, because of physical blocking of the coating, abundant H₃O⁺ ions underneath the coating accumulate, leading to a H₃O⁺-rich local environment on the cathode surface that suppresses OER during battery operation.

Ni-substituted process

Besides the increased OER, the improved alkalinity of electrolyte also compromises the cycling stability of PBA-based cathode material (without Ni/C coating). During charging/discharging process, the lattice constants of NMF changes in limited ranges accompanied by redox reactions of Mn, which causes collapse of Fe–CN–Mn bridges and vacancy generation¹⁷, as confirmed by gradually missing of the first discharge plateau (Figs. 4a-b). The dissolution of NMF is aggravated in alkaline electrolyte (Supplementary Figs. 17-18), which further decreases the stability of NMF cathode¹². In contrast, the first discharge plateau for NMF/NTP cell and the structure of NMF cathode exhibit ultra-high stability after Ni/C coating even after 80 cycles (Fig. 4c). The stable discharge plateau for NMF/NTP after coating is mainly due to the in-situ substitution of Ni atoms to balance ‘tiny’ structural disturbances caused by redox reactions in Mn sites^{10, 18, 19} (Fig. 4d).

This hypothesis is confirmed by operando Raman spectra of Ni/C coated NMF cathodes cycled in the alkaline electrolyte (Fig. 4e). Peaks in the range of 2,050 to 2,200 cm⁻¹ are assigned to cyano (CN) groups, evidencing that CN⁻ groups bond with transition-metal ions with diverse valence states. Prior to cycling, two peaks at 2,089 and 2,124 cm⁻¹ are visible, corresponding to Fe²⁺−CN−Mn²⁺ and Fe²⁺−CN−Mn³⁺ vibrations, respectively²⁰. After charging to 1.89 V, both peaks disappear, evidencing transformation of Fe²⁺ to Fe³⁺ and Mn²⁺ to Mn³⁺. More importantly, after charging to 2.2 V, a new ‘weak’ peak appears at 2,195 cm⁻¹ corresponding to Fe³⁺−CN−Ni^{2+ 21}. This finding confirms the introduction of Ni atom in NMF particle following transformation of Mn²⁺ to Mn³⁺. After discharging to 0.5 V, peaks for Fe²⁺−CN−Mn²⁺ and Fe²⁺−CN−Mn³⁺ shift to 2,092 and 2128 cm⁻¹ and a new peak appears at 2,164 cm⁻¹that is assigned to Fe²⁺−CN−Ni^{2+ 21}. The introduction of Ni is also proved by the HAADF-STEM and energy-dispersive spectroscopy (EDS) mapping (Fig. 4f). Additionally, the EDS line-scan spectra for a single NMF particle confirms that Ni atoms are introduced into the edge of particles to suppress dissolution of inner Mn atoms (Fig. 4g). The EDS mappings for NMF cathode with Ni/C coating following 1^st, 5^th and 20^th cycles (Supplementary Table 6 and Supplementary Fig. 19) evidence that the content of Ni in NMF particles is stable after the first cycle, confirming that the introduction of Ni into NMF cathode reaches an equilibrium in the first cycle to guarantee a long-term stability of the cathode.

This structure stability of NMF cathode achieved by the introduction of ‘inert’ Ni atoms in alkaline electrolyte is confirmed via XRPD patterns during charge/discharge. The structural evolutions of NMF structure happened at a high voltage range over 1.7 V, ascribing to the Mn²⁺ to Mn³⁺ (Fig. 5a). Fig. 5b shows the 2D contour map of NMF reflection. The highly reversible structure evolution during charge and discharge process can be easily observed. Further, the Rietveld refinements of NMF with/without Ni/C coating after 1^st cycle show that both electrodes exhibit cubic phases with Fm-3m space group and a = b = c (Figs. 5c-d and Supplementary Table 7), however, the lattice parameters for Ni/C coated NMF (5.28161 Å) are greater than that for uncoated NMF (5.26358 Å). This finding is attributed to the introduction of Ni in Ni/C coated NMF upon cycling. An increased a (b, c) contributes to a boosted rate performance for the cathode, which is consistent with our results (Supplementary Fig. 9a). Importantly, compared to the deteriorated structure of uncoated NMF after 1^st and 3^rd cycle (Fig. 5e), the overlapping patterns for Ni/C coated NMF following 1^st and 3^rd cycle confirm the excellent stability of NMF and that Ni introduction occurs at 1^st cycle, because otherwise, continuous Ni introduction will change the XRPD pattern (Fig. 5f).

To assess the possible universality of the new electrode modification strategy in alkaline batteries, the Co/C nanoparticle was employed to build the cathode coating as well. Similar to Ni nanoparticle, Co can be oxidized to Co(OH)₂ in alkaline media and, it possesses reversible redox pair of Co(OH)₂/CoOOH, together with ability to in-situ substitute the Mn atom. As a result, the good stability of battery with Co/C coating is achieved (Supplementary Fig. 20). This discovery provides evidence for the universality of creating H₃O⁺-rich cathode surfaces and in-situ optimizing the NMF structure by building metal nanoparticle coating to enhance the performance of alkaline ASIBs.

A new aqueous battery system, differing from traditional ASIBs based on near neutral electrolyte, is presented with a fluorine-free alkaline electrolyte to suppress H₂ evolution on the anode and a Ni/C coating to alleviate both O₂ evolution and electrode dissolution on the cathode. This system achieves long cycling stability (13,000 cycles) and high energy density (90 Wh kg⁻¹) in the alkaline electrolyte through the Ni/C coating induced H₃O⁺-rich local environment and in-situ electrode Ni modification. Remarkably, the cost of this alkaline electrolyte is ca. 25 times cheaper than commonly used highly concentrated fluorine-containing electrolytes. The pouch cell, assembled with an ultra-high electrode loading of > 30 mg cm⁻², maintains a capacity retention of ~100% following 200 cycles, while demonstrating excellent safety even after being cut and immersed in water. This aqueous alkaline battery design strategy shows the universality by extending to Co/C and exhibits the prospect for achieving high energy density by coupling with other lower redox potential anodes (Supplementary Fig. 21). Importantly, this strategy can be expanded flexibly to various aqueous batteries and will boost the large-scale application of aqueous batteries.

Materials

The Na₂MnFe(CN)₆ (NMF) cathode and NaTi₂(PO₄)₃(NTP)/C anode were synthesized based on reported methods²². The Nafion-Na was prepared through the neutralization reaction of 10 mL Nafion (purchased from DuPont, D520, 5 wt%) with 0.01 M NaOH solution drop-by-drop. Product was collected following removal of the solvent at 60 °C. 20% Ni/C was purchased from Fuel Cell Store.

Electrode preparation

The NMF electrode was prepared via mechanically mixing 80wt.% NMF, 10wt.% Super-P and 10wt.% polytetrafluoroethylene (PTFE) binder dispersed in ethanol solvent (AR). The mixture was pressed on a Ti-mesh at a pressure of 6 MPa and dried at 70 °C for 2 h in a vacuum oven. The NTP electrode was prepared by the same procedure as for NMF electrode using 80 wt.% NTP/C (~ 5% C), 10 wt.% Super-P and 10wt.% PTFE. The mass loading for electrodes was ca. 10 mg cm⁻². The mass ratio of anode and cathode in coin cell was ~ 1/1.06. The electrodes in the pouch cell were prepared with the same method. The mass ratio of anode and cathode in pouch cell was ~ 1/1.15. The specific capacity computation was based on the mass of cathode.

Electrolyte preparation

41.6 g NaClO₄ was dissolved in 20 mL water to obtain near neutral electrolyte. Alkaline electrolytes were obtained via addition of, respectively, 0.1, 0.2, 0.4 and 0.8 mL 1 M NaOH solution into 30 mL of neutral electrolyte.

Coating preparation

The coating was prepared as follows: 0.1 g Nafion-Na was dissolved in mixed solution of 0.45 g N, N-Dimethylformamide and 0.45 g isopropanol at 60 °C. 0.025 g Ni/C was, magnetic stirred for 0.5 h, and ultrasounded for 0.5 h. Procedures were replicated three times to give an even mixture. 10 µL cm⁻² solution was sprayed on the surface of cathode discs. Following removal of the solvent at room temperature (RT) (~ 25 °C) in N₂-filled glove-box under vacuum over 24 h, the electrode discs were coated homogenously.

Electrochemical measurement

LSV was measured in a three-electrode cell at a scanning speed of 1 mV s⁻¹. In the three-electrode cell, glass carbon was used as the working electrode, Ti as the counter electrode, and an Ag/AgCl electrode as the reference electrode, respectively. The three-electrode cell for testing cathodes was assembled with NMF composite (20 mg cm⁻²) as the working electrode, activated carbon as the counter electrode, and Ag/AgCl as reference electrode. The three-electrode cell for testing anodes was assembled with the NTP composite (19 mg cm⁻²) as the working electrode, activated carbon as the counter electrode, and Ag/AgCl as reference electrode.

Characterizations

In situ Attenuated Total Reflectance Infrared (ATR-IR) spectroscopy was performed with a Thermo-Fisher Nicolet iS20 equipped with a liquid, nitrogen-cooled HgCdTe (MCT) detector using a VeeMax III ATR accessory (Pike Technologies). A germanium prism (60°, Pike Technologies) was mounted in a PIKE electrochemical three-electrode cell with an Ag/AgCl reference electrode (Pine Research) and a Pt- wire counter electrode. To obviate any significant influences from the electrolyte, nanocarbon particles and polymer support, the spectrum without applying voltage was taken as a background. Tests were undertaken using a three-electrode cell with Pt as the counter electrode and Ag/AgCl as reference. DEMS was used to monitor volatile gases of H₂ and O₂ produced during battery operation at RT (Hidden HPR40). In-situ Raman spectroscopy for the Ni/C coating layer was determined using a confocal Raman microscope (Horiba LabRAM HR Evolution) with a 60X (1.0 N.A) water-immersion objective (Olympus). Laser wavelength was 532 nm. A screen-printed electrode (Pine, RRPE1002C) was used as the electrode for tests with a CHI 760E work-station. The working electrode was coated with Au via magnetron sputtering. 15 mL, 17 m (molar/kg_solvent) NaClO₄ in D₂O with addition of 0.2 mL 1 M (molar/L) NaOH in D₂O was used as the isotopically labelled electrolyte. XRD patterns were determined using a Bruker-AXS Micro-diffractometer (D8 ADVANCE) with Cu-Kα1 radiation (λ = 1.5405 Å). HAADF-STEM, EDS mapping and line-scan spectra were used to confirm existence of Ni in cycled NMF particles (FEI Titan Themis 80-200). A cross-section of coated NMF cathode was determined via field emission (FE) focused ion-beam (FIB, Helios NanoLab 600), and data collected by SEM and EDS with a field emission scanning electron microscope (FEI Quanta 450). ICP-MS was collected using Agilent 8900. Soft XAS was tested in the Australian Synchrotron. Operando synchrotron X-ray powder diffraction was conducted in the Australian Synchrotron and, the battery tested using Neware battery test system (CT-3008-5V1mA-164, Shenzhen, China). Home-made 2032-coin cells were used for data collection. Both sides of the cell cases were ‘punched’ with a central, 5 mm diameter hole, and sealed with Kapton film as the beam entrance. XRD data from the synchrotron were refined via the Rietveld method using GSAS II software.

DFT simulation

Density functional theory (DFT) computations were performed by Vienna Ab Initio Simulation Package (VASP.5.4.4)^{23, 24}. Generalized gradient approximation (GGA) with Perdew-Burke-Ernzerhof (PBE) function was used for describing the exchange-correlation potential^{25, 26}. Ni(111) and Ni(200) slabs were modeled using 33 unit cells with four layers, whilst the two topmost layers were allowed to fully relax until the convergence criterion of 10⁻⁵ eV for energy and 0.02 eV/Å for final forces on atoms, whilst other layers were fixed. Energy cut-off was set as 600 eV. DFT-D3 correction method was used for describing the van der Waals interaction. A (33) Monkhorst-Pack k-point grid mesh was used. An implicit solvent model was used to simulate the solvent environment using Polarizable Continuum Model (PCM) provided by VASPSOL^{27, 28}, where 𝜀_𝑟 = 80 for the water system.

To satisfy Equation (1) a manipulation of surface charge prior to and following adsorption (Q₀ and Q₁) is needed to maintain a specific Fermi energy. For this, ΔG_ads under CP-DFT () can be rewritten as³⁰:

Acknowledgements: The authors gratefully acknowledge financial support from the Australian Research Council (DP220102596, LP210301397 and DE230100471). DFT computations were undertaken with the assistance of resources and services from the National Computational Infrastructure (NCI) and Phoenix High Performance Computing, supported by the Australian Government and, The University of Adelaide. This research was undertaken on the XAS and Soft X-ray beamlines at the Australian Synchrotron, part of ANSTO. The authors acknowledge Dr. Ashley for assistance with TEM testing, and TEM measurements were undertaken at Adelaide Microscopy, the Centre for Advanced Microscopy and Microanalysis. The authors thank Tingting Liu from the Qingdao Institute of Bioenergy and Bioprocess Technology for the assistance with of DEMS testing.

Author contributions: S.-Z. Qiao conceived and supervised the project. H. Wu and J. Hao designed research, conducted characterizations and electrochemical measurement. H. Wu, S.-Z. Qiao, K. Davey, C. Wang and J. Hao analysed data and wrote manuscript. H. Wu, J. Hao, Y. Jiang, J. Liu and X. Xu conducted characterizations of materials. Y. Jiao conducted DFT computations. H. Wu and J. Hao contribute equally to this work. All authors agreed on the manuscript and approved submission.

Competing interests: H. Wu and S.-Z. Qiao have filed a PCT provisional patent covering materials and sodium aqueous battery application(s) described in this manuscript.

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Yes there is potential Competing Interest. H. Wu and S.-Z. Qiao have filed a PCT provisional patent covering materials and sodium aqueous battery application(s) described in this manuscript.

SupplementaryVideo1.mp4
The cut pouch cell to power blue lights in water
SupplementaryVideo2.mp4
The cut pouch cell to power an electric fan in water
SupplementaryVideo3.mp4
The cut pouch cell to power a hygrometer in water
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