Dual‑ion co‑storage via solvation structure tuning toward ultrafast and durable zinc‑organic batteries

As the demand for safe, low-cost, and fast-charging energy-storage technologies continues to grow, aqueous zinc-ion batteries have emerged as a promising alternative to conventional lithium-ion batteries. Their water-based electrolytes offer high ionic conductivity and eliminate the flammability concerns associated with organic electrolytes. However, the practical development of zinc-organic batteries remains limited by the dissolution of organic cathodes, sluggish ion transport, and structural degradation during repeated cycling.

Now, researchers have developed a simple electrolyte-engineering strategy that simultaneously regulates Zn ²⁺ solvation, suppresses cathode dissolution, and enables dual-ion storage. By introducing potassium chloride (KCl) as a co-solute into a ZnCl ₂ aqueous electrolyte, the researchers significantly improve the kinetics and durability of Zn//PTCDA batteries, while revealing a previously overlooked reversible Zn ²⁺ /K⁺ co-storage mechanism.

Why This Electrolyte Matters

3,4,9,10-Perylenetetracarboxylic dianhydride (PTCDA) is an attractive organic cathode because of its layered molecular structure and abundant carbonyl groups, which provide reversible redox-active sites for metal-ion storage. However, like many small organic molecules, PTCDA tends to dissolve in aqueous electrolytes, resulting in rapid capacity fading.

Instead of modifying the electrode through complicated polymerization or structural reconstruction, the researchers take a different approach: tuning the electrolyte itself.

KCl is introduced into a 2 M ZnCl ₂ aqueous electrolyte. The high concentration of KCl not only provides additional Cl⁻ ions but also fundamentally reorganizes the Zn ²⁺ solvation environment and the hydrogen-bonding network of water.

Reconstructing the Zn ²⁺ Solvation Structure

Spectroscopic measurements and molecular dynamics simulations reveal a striking change in the Zn ²⁺ coordination environment.

In the conventional ZnCl ₂ electrolyte, Zn ²⁺ is predominantly surrounded by approximately six water molecules, forming a strongly hydrated solvation shell. With saturated KCl addition, four of these water molecules are replaced by Cl⁻ ions, leading to the formation of [Zn(H ₂ O) ₂ Cl ₄ ] ^2- -rich solvation structures.

This transformation substantially weakens the interaction between Zn ²⁺ and its hydration shell. As a result, the Zn ²⁺ desolvation energy decreases from 66.51 to 38.14 kJ mol ^-1 , greatly facilitating Zn ²⁺ transport and insertion into the cathode.

At the same time, the electrolyte conductivity almost doubles, increasing from 53.6 to 109 mS cm ^-1 . Even at −20 °C, the modified electrolyte maintains an ionic conductivity of approximately 13.3 mS cm ^-1 , demonstrating remarkable low-temperature transport capability.

KCl Does More Than Improve Ion Transport

The role of KCl extends beyond Zn ² ⁺ solvation.

The high ionic strength of KCl induces a “salting-out” effect, while the reorganized water network reduces water activity. Together, these effects significantly suppress the dissolution of PTCDA molecules into the aqueous electrolyte.

After a seven-day soaking test, the KCl-containing electrolyte shows almost no visible coloration, whereas the conventional ZnCl ₂ electrolyte becomes distinctly yellow because of PTCDA dissolution. Quantitative analysis further reveals that the carbon content dissolved in the optimized electrolyte is approximately half that of the conventional ZnCl ₂ electrolyte. Meanwhile, KCl also makes the electrolyte less acidic, suppressing hydrogen evolution and zinc corrosion. The improved wettability of the PTCDA electrode further promotes a more uniform Zn ² ⁺ flux and contributes to enhanced reaction stability.

Unlocking Dual-Ion Co-Storage

The most intriguing discovery is that KCl does not simply modify the electrolyte—it introduces a second charge-storage ion into the PTCDA cathode. Ex situ FT-IR, XPS, ICP, and elemental mapping confirm that both Zn ² ⁺ and K⁺ reversibly intercalate into the PTCDA structure.

During discharge, the carbonyl groups of PTCDA undergo a reversible transformation associated with Zn ² ⁺ insertion, while K⁺ content simultaneously increases within the bulk electrode. The K⁺ signal remains after surface sputtering and is uniformly distributed throughout the active material, providing strong evidence for genuine K⁺ bulk intercalation rather than simple surface adsorption.This establishes a reversible Zn ² ⁺/K⁺ dual-ion co-storage mechanism.

K⁺ Acts as Both a Charge Shield and Structural Pillar

The researchers propose that K⁺ plays two crucial roles inside the PTCDA framework. First, K⁺ acts as a charge shield. By partially compensating the negative charge associated with carbonyl groups, K⁺ reduces electrostatic repulsion and facilitates subsequent Zn ² ⁺ insertion.

Second, K⁺ serves as a structural pillar within the layered PTCDA framework. Its presence helps buffer the lattice strain caused by Zn²⁺ insertion, reducing structural distortion and preventing progressive cathode degradation. This cooperative mechanism provides a powerful explanation for why introducing KCl can simultaneously accelerate ion transport and dramatically improve cycling stability.

Outstanding Rate Performance

The optimized Zn//PTCDA battery achieves a high capacity of 124.7 mAh g ^-1 at 1 A g ^-1 , corresponding to approximately 91.7% of the theoretical capacity, with an average discharge voltage of 0.65 V. More remarkably, the battery retains 70 mAh g ^-1 at 30 A g ^-1 , corresponding to 56% capacity retention. At this ultrahigh current density, the battery reaches a power density of approximately 22.8 kW kg ^-1 .

The voltage hysteresis is also substantially reduced, decreasing from 320 to 240 mV at 1 A g ^-1 and from 627 to 235 mV at 30 A g ^-1 . These results demonstrate that electrolyte engineering can dramatically reduce the kinetic barriers associated with ion insertion and extraction.

Pseudocapacitive Storage Enables Ultrafast Kinetics

Electrochemical analysis reveals that the Zn ²⁺ /K⁺ co-storage process is not purely diffusion-controlled.

The b-values of the four redox peaks reach 0.83, 0.85, 0.86, and 0.94, indicating a strong capacitive contribution. At 0.5 mV s⁻¹, the pseudocapacitive contribution reaches 92.8%, increasing further to 98.3% at 10 mV s ^-1 . The optimized electrolyte also dramatically improves Zn ²⁺ diffusion. The calculated Zn ²⁺ diffusion coefficient reaches 8.44 × 10 ^-10 cm ² s ^-1 , approximately two orders of magnitude higher than that in the conventional ZnCl ₂ electrolyte.

DFT calculations provide further insight: the Zn ²⁺ migration energy barrier inside PTCDA decreases from 1.24 to 0.76 eV in the presence of the KCl-modified electrolyte. The larger K⁺ ions help alleviate steric hindrance within the PTCDA structure, facilitating subsequent Zn ²⁺ transport.

Exceptional Long-Term and Low-Temperature Stability

The benefits of the modified electrolyte become particularly clear during long-term cycling.

At 10 A g ^-1 , the optimized Zn//PTCDA battery retains 90.9% of its capacity after 10,000 cycles. Even at the extremely high current density of 30 A g ^-1 , it maintains 93.6% capacity retention after 10,000 cycles.

By comparison, the battery using conventional ZnCl ₂ retains only 51.2% of its capacity after 2,500 cycles.

The electrolyte also provides impressive low-temperature adaptability. Because KCl lowers the freezing point of the electrolyte, the modified system remains operational at −20 °C. At this temperature, the battery retains 96% of its capacity, compared with 78% for the conventional electrolyte, and can still deliver approximately 40 mAh g ^-1 at 30 A g ^-1 .

A New Strategy for Durable Zinc-Organic Batteries

Overall, this work demonstrates that electrolyte solvation engineering can simultaneously solve several fundamental problems in aqueous zinc-organic batteries.

KCl addition reorganizes the Zn ²⁺ solvation structure, nearly doubles ionic conductivity, lowers the Zn ²⁺ desolvation barrier, suppresses PTCDA dissolution, and improves low-temperature performance. More importantly, the electrolyte enables reversible Zn ²⁺ /K⁺ co-storage, with K⁺ functioning as both a charge shield and structural pillar to stabilize the organic cathode.

This work establishes a simple and effective multi-salt electrolyte strategy that connects solvation-structure regulation with dual-ion storage chemistry, offering a promising pathway toward ultrafast, durable, high-power, and environmentally adaptable zinc-organic batteries.

Nano-Micro Letters

10.1007/s40820-026-02304-7

News article

Dual‑Ion Co‑Storage via Solvation Structure Tuning Toward Ultrafast and Durable Zinc‑Organic Batteries

28-Jul-2026