Brief introduction of the results
Aqueous sodium-ion batteries (ASIBs) have great prospects for large-scale energy storage, but their energy density and service life are limited by water splitting. Current methods to improve water stability include the use of expensive fluoride-containing salts to form a solid electrolyte interface and the addition of potentially flammable co-solvents to the electrolyte to reduce water activity. However, these methods significantly increase cost and security. Changing the electrolyte from near-neutral to alkaline not only inhibits the hydrogen evolution reaction (HER), but also initiates the oxygen evolution reaction (OER) and cathode dissolution.
Based on this, Professor Qiao Shizhang et al. from the University of Adelaide, Australia, reported an alkaline aqueous sodium-ion battery (ASIBs), which is composed of Mn-Kiprussian blue analogues (PBAs), a positive electrode (Na2MnFe(CN)6, NMF), a NaTi2(PO4)3(NTP) negative electrode, and a fluorine-free sodium perchlorate (NaClO4) alkaline electrolyte, which is significantly lower than that of sodium trifluoride and bis(trifluoromethylsulfonyl) commonly used in high-concentration electrolytes Sodium imide, an alkaline electrolyte, inhibits HER at the negative electrode.
Tests found that the battery had a lifetime of 10 cycles at 13,000 C and a high energy density of 0.5 Wh kg-88.9 at 1 C. In-situ attenuated total reflection infrared (ATR-IR) and operando synchrotron X-ray powder diffraction (XRPD) confirmed that coating the NMF cathode with a layer of commercial nickel/carbon (Ni/C) nanoparticles formed an H3O+-rich local environment near the cathode surface, and the H3O+-rich local environment was due to the irreversible formation of Ni(OH)2 and Ni(OH)2/ The reversible redox of NiOOH significantly reduces OER and electrode dissolution, improving the durability of the battery. In addition, it was confirmed by operando Raman spectroscopy and high-angle annular darkfield scanning transmission electron microscopy (HAADF-STEM) that some Ni atoms in the coating were embedded in situ in the positive electrode to stabilize the NMF structure in the alkaline medium.
Background:
Due to the abundant Na resources and compatibility with commercial/industrial systems, aqueous sodium-ion batteries (ASIBs) have great promise in large-scale energy storage. However, due to the narrow electrochemical stabilization window of water, its energy density and cycling stability are limited. In addition, the accumulation of hydrogen (H2) produced by water splitting during the cycle endangers the safety of batteries and limits the development of ASIBs. Improving aqueous battery performance is often done by using expensive fluoride salts to construct a solid electrolyte interphase (SEI) to inhibit HER and increase the electrochemical window of the electrolyte, but the high solubility of the SEI component limits durability. The use of co-solvents such as polymers can improve the water stability of the electrolyte, but increasing the viscosity of the electrolyte makes it difficult to match the commercially available high-load electrodes.
Changing the electrolyte from near-neutral to basic, enhancing OER on the cathode, inhibiting HER, and high concentrations of OH- in the electrolyte, limit the choice of the cathode, as the transition metal-based electrode interacts with OH-, resulting in a deterioration of the electrode structure, especially the Mn-based PBAs cathode. PBAs have the advantages of non-toxicity, low cost, and high energy density, and are widely used cathode materials in traditional aqueous batteries. However, the application of Fe in alkaline electrolytes is limited due to the redox coupling of Mn2+/Mn3+ and the strong Jahn-Teller effect caused by the dissolution of Fe(CN)63/4-complex. Therefore, basic ASIBs based on PBAs have not yet been developed.
Illustrated reading
Over a wide range of charge voltages of 0.5-2.2 V, the authors tested the performance of NMF//NTP whole cells with neutral electrolyte and alkaline electrolyte (with or without Ni/C coating). At 1 C, cells without Ni/C coating exhibited rapid capacity decay in both neutral and alkaline electrolytes, with a capacity retention rate of 60% after 200 cycles, while alkaline batteries with Ni/C coating exhibited a capacity retention rate of about 100%. In addition, the capacity retention rate of the Ni/C coating battery after 200 cycles at -30 °C and 0.5 C was 91.3%. What's more, the full cell showed a record life of 13,000 cycles in the alkaline electrolyte, with a high capacity retention rate of 74.3% at 10 C, outperforming the performance of many aqueous batteries that have been reported.
Figure 1. Electrochemical performance of NMF//NTP coin batteries within 0.5-2.2 V
The authors assembled a Ni/C-coated alkaline electrolyte NMF//NTP bag battery with an electrode load of approximately 20 mg cm-2. The volume retention rate was approximately 85% after 1000 cycles at 500 mA g-1 and 100% after 200 cycles at 300 mA g-1. It also shows high stability under "harsh" cutting and immersion in water. The cut pouch battery continuously powered the digital hygrometer thermometer in water for 20 h, confirming that the battery is resistant to electrolyte leakage and can withstand significant damage in high humidity environments. At the charging rates of 1.06 and 0.5 C at the positive and negative electrode capacity ratios, the energy density reaches 88.9 Wh kg-1.
Figure 2. Electrochemical performance of NMF//NTP bag batteries
Using in-situ ATR-IR spectroscopy, the authors analyzed the interfacial structure of the Ni/C coating. For pure carbon-modified electrodes, there is no significant change in the spectrum even when charged to 1.3 V, indicating that the carbon and support do not alter the local environment on the cathode surface. Under Ni/C modification, when the potential exceeds 0.6 V, new peaks appear at 1798 and 2032 cm-1, attributed to two asymmetric O-H stretch modes of H3O+. In the alkaline electrolyte, after Ni/C coating on the NMF cathode, neither HER nor OER was noticeable, except for the trace amount of O2 in the first cycle before activating the surface coating. Therefore, the H3O+-rich local environment induced by the Ni/C protective layer inhibits OER in alkaline electrolytes, while alkaline electrolytes retards HER.
Figure 3. Generate an H3O+-rich microenvironment
During the charging process, Mn dissolution occurs on the surface of the Mn-based PBA cathode, resulting in the generation of Mn vacancies. In the unprotected system, the continuous dissolution of Mn ions leads to structural collapse, which adversely affects the cycling stability of the battery. In the Ni/C protection system, the in-situ substituted Ni atoms equilibrium Mn dissolution caused by small structural perturbations.
During the discharge process, Ni oxidation produces Ni2+, which gradually enters the crystal skeleton and fills the Mn vacancy by forming Ni-N bonds. The operando Raman spectrum confirmed that there were two distinct peaks at 2089 and 2124 cm-1 before cycling, corresponding to Fe2+-CN-Mn2+ and Fe2+-CN-Mn3+ vibrations, respectively, and the two peaks disappeared after charging to 1.89 V, indicating that Fe2+ was transformed into Fe3+ and Mn2+ was transformed into Mn3+. The results show that the introduction of Ni atoms in NMF particles occurs with the transition from Mn2+ to Mn3+. EDS line-scan spectroscopy of a single NMF particle confirmed that the introduction of Ni atoms into the edge of the particles could inhibit the dissolution of internal Mn atoms.
Figure 4. Determination of the reaction mechanism and in situ Ni substitution
Figure 5. operando structure analysis of NMF cathode in 0.5-2.2 V cycles
Bibliographic information
Alkaline-based aqueous sodium-ion batteries for large-scale energy storage. Nat. Commun., 2024.