Critical bimetallic phosphide layer enables fast electron transfer and extra energy supply for flexible quasi‑solid‑state zinc batteries

A breakthrough study published in Nano-Micro Letters reveals how a critical bimetallic phosphide layer (CBPL) can simultaneously accelerate electron transfer and deliver extra energy in nickel-zinc batteries, shattering the traditional ceiling on energy and power density. Led by Liang He from Sichuan University, the work establishes a scalable, bifunctional surface-modification route that turns NiCo-layered double hydroxide (NiCo-LDH) cathodes into record performers for flexible quasi-solid-state zinc batteries.

Why This Research Matters

• Overcoming Nickel-Cathode Limitations: Conventional nickel-based cathodes suffer from sluggish kinetics, low active-material utilization and structural collapse, restricting aqueous zinc batteries to modest energy density. The CBPL strategy directly tackles these bottlenecks by introducing a highly conductive heterostructure that both transports electrons and participates in redox reactions.

• Enabling Flexible, Fast-Charging Devices: Beyond grid storage, wearable electronics, IoT sensors and soft robotics demand compact, safe and deformable power sources. The reported flexible pouch cells maintain stable output under repeated bending, validating real-world viability.

Innovative Design and Mechanisms

• Gradient Phosphidizing & Heterostructure Engineering: A precisely controlled low-temperature phosphidizing treatment converts the outer surface of NiCo-LDH into a nanoscale Ni₂P/Co₂P layer while preserving an inner NiCo-LDH core. The resulting core–shell architecture forms abundant heterointerfaces that redistribute charge density and lower the OH^- adsorption energy (−1.31 eV versus −0.77 eV for bare LDH).

• Dual-Function CBPL: Unlike conventional surface coatings that merely protect, the CBPL is electrochemically active. It provides an additional low-voltage redox plateau (≈0.35 V versus Hg/HgO) that supplements the high-voltage Ni(OH)₂/Co(OH)₂ ↔ NiOOH/CoOOH reaction, effectively doubling the depth-of-discharge without sacrificing rate capability.

• Fast Electron/Ion Transport: The metallic conductivity of Ni₂P/Co₂P, combined with the intimate heterojunction, cuts charge-transfer resistance to <1 Ω and enables 72 % capacity retention at 40 C. Density-functional theory confirms a ten-fold increase in electronic states near the Fermi level compared with pristine LDH.

Applications and Future Outlook

• Record Energy/Power Metrics: Assembled NiCo-P_1.0//Zn full cells deliver 503.62 Wh kg^-1 at 493 W kg^-1 and 18.62 kW kg^-1 at 336 Wh kg^-1—outperforming state-of-the-art nickel-zinc, zinc-ion and alkaline chemistries.

• Flexible Quasi-Solid-State Pouch Cells: A 2 cm × 2 cm hydrogel-based pouch sustains 150 cycles at 1 C with 71 % retention while powering a timer during 6 h of flat-bend-flat deformation. Three cells in series light a 34-red/46-yellow LED neon sign, demonstrating modular scalability.

• Scalable Manufacturing: The method employs earth-abundant Ni, Co and NaH₂PO₂, uses open-air phosphidizing at 350 °C and is directly compatible with roll-to-roll coating onto carbon cloth, promising low-cost mass production.

• Future Research Directions: Next steps include optimizing phosphorus dosage to suppress phosphate by-products, integrating CBPL with 3D-printed current collectors and coupling the cathode with dendrite-free zinc anodes for 10 000-cycle grid modules.

Conclusions
This study demonstrates that a critical bimetallic phosphide layer can simultaneously act as a high-conductivity highway and an extra energy reservoir, pushing aqueous nickel-zinc batteries well beyond their traditional limits. With unmatched energy/power density and mechanical flexibility, the CBPL-modified NiCo-LDH platform paves the way for next-generation safe, fast-charging and bendable energy-storage systems across consumer electronics and large-scale storage markets.