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

The study addresses the central challenge in aqueous sodium-ion batteries (ASIBs): low energy density and poor cycling stability caused by the narrow electrochemical stability window of water and parasitic hydrogen/oxygen evolution. Conventional improvements rely on expensive fluorinated salts to form SEI layers or adding organic co-solvents/polymers to depress water activity, both of which raise costs, viscosity, and safety risks. The authors propose shifting from near-neutral to alkaline electrolytes to thermodynamically suppress hydrogen evolution at the anode. However, alkalinity intensifies oxygen evolution and transition-metal dissolution at the cathode, particularly for Mn-based Prussian blue analogues (PBAs), which suffer from Jahn–Teller effects and Fe(CN)6 dissolution under high OH− concentration. The research aims to realize a safe, low-cost, fluorine-free alkaline ASIB with high energy density and long lifespan by mitigating cathode-side OER and dissolution while leveraging anode-side HER suppression.

Prior work has explored: (i) highly concentrated or fluorinated-salt electrolytes that enlarge the electrochemical stability window via SEI formation, yet suffer from solubility of SEI components (LiF, NaF, Na2CO3) and high cost; (ii) hybrid or polymeric co-solvents to reduce water activity, which increase viscosity and introduce flammability; and (iii) traditional near-neutral aqueous electrolytes for PBAs offering safety and cost advantages but limited by water’s 1.23 V stability window and gas evolution. Alkaline electrolytes suppress HER per water Pourbaix behavior, but they exacerbate OER and structural degradation/dissolution of Mn-based PBAs (Jahn–Teller effects and Fe(CN)6 4− dissolution), limiting their practicality. This work situates itself by avoiding fluorinated salts and flammable co-solvents, instead engineering the cathode–electrolyte interface in alkaline media to control OER and dissolution while preserving low viscosity and cost.

Materials and electrodes: Na2MnFe(CN)6 (NMF) cathode synthesized by co-precipitation of Na4Fe(CN)6 and MnCl2 in aqueous NaCl, followed by washing, drying, and vacuum treatment. NaTi2(PO4)3/C (NTP/C) anode prepared via solution mixing of sodium acetate, NH4H2PO4, PVP, Ti(OBu)4, followed by calcination at 800 °C under Ar to yield ~5 wt% carbon. Commercial 20% Ni/C and Co/C were used. Nafion-Na obtained by neutralizing Nafion with NaOH. Electrode fabrication: Slurries of 80 wt% active (NMF or NTP/C), 10 wt% Super P, 10 wt% PTFE in ethanol were pressed onto Ti mesh (6 MPa) and dried. Mass loading ~10 mg cm−2. Coin-cell anode/cathode mass ratio ~1/1.06; pouch cell ~1/1.15. Specific capacities referenced to cathode mass. Electrolytes: Neutral electrolyte prepared by dissolving NaClO4 to 17 m. Alkaline electrolytes formed by adding 0.1–0.8 mL of 1 M NaOH to 30 mL neutral electrolyte; optimized condition used 0.4 mL 1 M NaOH per 30 mL. Ni/C coating: 0.1 g Nafion-Na in DMF/isopropanol (0.45 g/0.45 g) at 60 °C combined with 0.025 g Ni/C, stirred and ultrasonicated, repeated three times; 10 µL cm−2 sprayed onto cathode and dried under N2 at room temperature (vacuum, 24 h) to form ~1 µm coating. Electrochemical testing: LSV at 1 mV s−1 in three-electrode setup (glassy carbon or composite electrodes as working, Ag/AgCl reference, Ti or Pt counter). Full cells (NMF//NTP) tested from 0.5–2.2 V at specified C-rates and temperatures. DEMS to monitor H2/O2 evolution during cycling. Characterization: In-situ ATR-IR to detect interfacial species; operando synchrotron XRPD with Kapton windows for structural evolution and Rietveld refinements; HAADF-STEM, SEM/FIB, EDS mapping/line scans for elemental distributions; ICP-MS for dissolved metals; soft XAS for Ni redox; viscosity measured (6.0 mPa s for alkaline electrolyte). Pouch cells assembled with high loading (~20–30+ mg cm−2) and evaluated for cycling and safety (cutting/immersion tests). DFT computations: VASP with GGA-PBE, D3 dispersion, implicit solvation (VASPSOL, εr=80). Ni slab models for adsorption energetics under charge-neutral and constant-potential frameworks to assess OH− vs H+ adsorption and potential effects. Substitution energetics in NMF with Hubbard U (Mn 4.0 eV, Ni 5.5 eV, Fe 4.0 eV) in 2×2×2 supercells to evaluate spontaneous Ni-for-Mn substitution (ΔE).

- Interface engineering: A Ni/C nanoparticle plus Nafion-Na coating on NMF creates a H3O+-rich local environment during charging through Ni→Ni(OH)2 formation and reversible Ni(OH)2/NiOOH redox, preferential OH− adsorption on Ni, and OH− transport inhibition by Nafion, suppressing cathode OER in alkaline electrolyte (supported by in-situ ATR-IR, operando XRPD, DEMS). - Anode protection: Alkalinity suppresses HER at the NTP anode, reducing over-discharge; salt concentration increase alone does not shift HER onset. - Structural stabilization: In-situ Ni2+ incorporation into NMF fills Mn vacancies during cycling, stabilizing the PBA framework. Evidence: operando Raman (appearance of Fe–CN–Ni bands at 2195 and 2164 cm−1), STEM-EDS mapping/line scans (Ni at particle edges), and Rietveld refinements showing larger lattice parameter with coating (a=b=c=5.28161 Å vs 5.26358 Å uncoated). - Thermodynamics: DFT shows OH− prefers adsorption on Ni surfaces versus H+, leading to local OH− depletion and pH reduction; Ni-for-Mn substitution is spontaneous with ΔE = −8.06 eV. - Coin-cell performance (0.5–2.2 V): With Ni/C coating in alkaline electrolyte, near-100% capacity retention after 200 cycles at 1 C, markedly better than <60% without coating. At −30 °C, 91.3% retention after 200 cycles at 0.5 C. Record long life >13,000 cycles at 10 C with 74.3% capacity retention. Higher average discharge voltage and rate performance versus uncoated. - Pouch-cell performance: ~20 mg cm−2 loading shows 85% capacity retention after 1,000 cycles at 500 mA g−1. With >30 mg cm−2 loading, ~100% retention over 200 cycles at 300 mA g−1. Demonstrated safety: continuous operation while cut and immersed in water for >20 h; minimal volume change and negligible gas evolution. - Energy density and electrolyte properties: Cathode-based energy density 88.9 Wh kg−1 at 0.5 C with low P/N ratio (1.06). Predicted practical pouch-cell energy density ~61 Wh kg−1 using realistic cell components. Alkaline electrolyte viscosity low (6.0 mPa s), enabling high mass loading and low-temperature operation. Co/C coating shows similar benefits, suggesting method universality.

The work tackles the dual challenge in ASIBs: suppressing HER to widen the anodic stability and mitigating OER and cathode dissolution at the cathode in alkaline media. By using a fluorine-free alkaline NaClO4-based electrolyte, HER is thermodynamically suppressed at the NTP anode. The Ni/C–Nafion coating establishes a H3O+-rich interfacial microenvironment that curtails OER and protects the Mn-based PBA cathode from structural degradation. Concurrent in-situ Ni substitution of Mn vacancies further stabilizes the PBA lattice, increasing lattice parameters and improving rate capability. Together, these mechanisms deliver long cycle life (>13,000 cycles at 10 C), robust low-temperature operation, and high energy density (88.9 Wh kg−1) without resorting to expensive fluorinated salts or flammable co-solvents. The pouch-cell demonstrations under high loading and harsh conditions corroborate practical safety and scalability. Extension to Co/C coatings indicates generalizability of the interfacial strategy.

The study introduces a hydrogen-free alkaline ASIB architecture using a fluorine-free NaClO4-based alkaline electrolyte and a Ni/C–Nafion coating on an NMF cathode to simultaneously suppress HER and OER while preventing cathode dissolution. Key outcomes include record-long lifespan (>13,000 cycles at 10 C with 74.3% retention), high energy density (88.9 Wh kg−1 at 0.5 C), excellent low-temperature performance, and safe, high-loading pouch-cell operation with minimal gas evolution. Mechanistically, a H3O+-rich interfacial environment and in-situ Ni substitution stabilize the Mn-based PBA cathode in alkaline conditions. The approach appears universal (validated with Co/C) and holds promise for higher energy densities by pairing with lower-potential anodes. Future research may explore optimizing coating compositions, alternative metal nanoparticles, broader cathode chemistries, and full cell engineering toward higher cell-level energy density and scalability.