“Sci. Rep.” reports an ultra-thin aluminum–air battery with a solid electrolyte thinned via aerosol jet printing.
Product Introduction
Equipment Parameters
I. Abstract of the Paper’s Core Research
This study formulated a neutral PVA–NaCl–glycerol gel ink suitable for aerosol atomization, employing aerosol‑layered printing to precisely control electrolyte thickness. The effects of membrane thickness and salt concentration on ionic conductivity and battery discharge performance were systematically investigated, and functional validation was carried out for both single cells and series‑connected flexible battery packs. The key findings are as follows:
Atomizable neutral gel ink system
Using polyvinyl alcohol as the film-forming matrix, sodium chloride as the ionic source, and glycerol as a plasticizer, the ink formulation exhibits a viscosity suitable for pneumatic atomization. After printing, allowing it to stand at room temperature for 12 hours enables spontaneous crosslinking, yielding a uniform, pore-free solid gel with no delamination or salt precipitation.
Layer-thickness–controlled, linear, and precise forming process
For every 10 printed layers, the corresponding film thickness is 18.3 μm; with 10–40 layers, the ultra-thin electrolyte can be precisely tuned to a thickness range of 18.3–74.5 μm. Layer-by-layer and continuous printing both exhibit excellent thickness uniformity, with no interfacial gaps in the cross-section and seamless adhesion to the electrodes.
The membrane thickness directly determines the ionic conductivity.
The thinner the membrane, the lower the impedance: at 18.3 μm, the ultra-thin electrolyte exhibits a conductivity of 5.2 × 10⁻⁴ S, with an internal resistance of only 12 Ω; in contrast, for a 74.5 μm‑thick membrane, the internal resistance increases to 63 Ω, and the peak power density drops from 1.98 to 0.98 mW·cm⁻².
There exists an optimal matching window for salt concentration.
The conductivity is optimal at an NaCl concentration of 1.8 mol/L; at higher concentrations, viscosity increases and ion migration is hindered, while at lower concentrations, insufficient charge carriers lead to a marked decline in the discharge plateau.
Flexible Battery Integration and Practical Validation
A single-cell battery assembled with an ultra-thin electrolyte, an aluminum‑foil anode, and a carbon‑cloth air cathode exhibits excellent intermittent discharge stability. A multi‑cell series‑connected flexible battery pack with an overall thickness of less than 1.3 mm is bendable and can stably power small fans and miniature electronic toys, making it well suited for powering wearable patches.
II. Complete Preparation Procedure for Ultrathin Aerosol Electrolytes
Ink formulation: Sodium chloride and glycerol are dissolved in deionized water, and PVA is stirred at 60°C until fully dissolved. The resulting mixture is homogenized to yield a low-viscosity, print‑able gel‑based ink precursor.
Optomec aerosol deposition system, 200 μm nozzle, fixed sheath/carrier gas ratio, printing speed of 10 mm/s;
Layered grid cross‑deposition, with 10/20/30/40 layers sequentially fabricated to produce electrolyte membranes of varying thicknesses.
The film was allowed to stand at room temperature for 12 hours to complete hydrogen-bond‑mediated crosslinking and curing; its uniformity was characterized by SEM and X-ray analysis.
A sandwich‑structured aluminum–air battery was assembled, and electrochemical impedance spectroscopy and galvanostatic discharge were performed using an electrochemical workstation.
A flexible wearable battery pack is fabricated via multi-cell series connection, and its power‑supply performance is validated under bending conditions.
III. Core Advantages of the Three- and Five‑Layer Aerosol Processes (Comparison with Casting and Spin Coating)
Precise linear control of micron-scale thickness
Traditional casting produces layers on the order of millimeters thick with uneven thickness, whereas aerosol jet printing can achieve resolutions down to tens of micrometers and allows on-demand customization of power and capacity requirements.
The membrane layer is dense and seamlessly adheres to the electrode.
At the microscale, droplets deposit uniformly, and the electrolyte forms a seamless interface with both the anode and cathode, significantly reducing interfacial contact resistance and enhancing discharge efficiency.
Digital graphic customization, compatible with irregular-shaped flexible substrates.
CAD allows customization of electrolyte shapes, enabling the printing of irregular wearable patches and curved battery films, whereas casting is limited to producing regular rectangular sheets.
Significant savings on ink consumables, with no large-scale waste.
Localized deposition prints only within the battery area, whereas casting an entire monolithic structure results in high edge‑trimming losses, while the cost of gel‑based raw materials has dropped significantly.
Flexible substrate formed at room temperature with no high-temperature damage.
The entire process involves room-temperature printing and curing, ensuring that heat-sensitive flexible substrates such as PI, textile fabrics, and medical patches remain free from thermal deformation.
IV. Five Core Technological Innovations of This Study
Innovation 1: Formulation of a Neutral PVA Gel Ink Compatible with Aerosol Printing
It addresses the challenges of traditional hydrogels—high viscosity and inability to be atomized—and enables spontaneous crosslinking at room temperature after printing, eliminating the need for high-temperature curing.
Innovation 2: A complete process for layer-by-layer cross-deposition of ultrathin electrolytes
A linear correlation standard between the number of printing layers and film thickness has been established, enabling the fabrication of solid-state gels with controllable thicknesses on the order of tens of micrometers, thereby filling a critical gap in the production of ultrathin metal–air electrolytes.
Innovative Three-Membrane Thickness – A Quantitative Theoretical Framework for Ionic Conductivity
The inverse relationship between thickness and both internal resistance and power density is clearly established, and criteria for selecting electrolyte thickness in lightweight, high‑power batteries are provided.
Innovative 4‑component neutral, non‑corrosive gel system
Unlike conventional strongly alkaline electrolytes, it poses no risk of skin corrosion and can be used directly to power wearable devices.
Innovative Integrated Solution for a Bendable Ultra-Thin Series-Connected Battery Pack
Micron-scale electrolyte paired with a flexible current collector, resulting in an ultra-thin, skin‑conformable design; multiple cells connected in series boost the output voltage, meeting the power requirements of miniature devices.
V. Current Industry Challenges and Corresponding Solutions for Overcoming Them
Challenge 1: Conventional cast gels have millimeter‑scale thickness, resulting in heavy batteries, long ion‑transport pathways, and low power output.
✅ Aerosol-layered printing achieves an ultra-thin film as low as 18.3 μm, significantly shortening ion migration distances and reducing internal resistance by more than 80%.
Challenge 2: The thickness of the cast film is uneven, resulting in poor batch-to-batch consistency in battery performance.
✅ Each layer exhibits uniform thickness, with precise control over the number of layers, and the discharge curves of batteries from the same batch show excellent overlap.
Challenge 3: The use of strong alkaline electrolytes poses safety risks due to corrosion and leakage.
✅ Neutral NaCl gel, non‑corrosive, and non‑irritating to the skin even if damaged, making it safer for medical use.
Challenge 4: Spin coating can only fabricate thin photoresist films over small areas, making it unsuitable for producing large‑area flexible batteries.
✅ Aerosol‑jet printing enables the continuous fabrication of electrolyte membranes on large‑area, irregularly shaped textile substrates.
Challenge 5: Thick gels are prone to cracking and delamination upon bending, resulting in poor wear durability.
✅ Ultra-thin, micrometer‑scale gel for flexible lifting; withstands repeated bending without cracking and conforms to the skin with no hard, protruding edges.
VI. Future Diversified Industrial Application Scenarios
1 Medical Wearable Physiological Sensor Patch
An ultra-thin, flexible aluminum–air battery that is skin‑friendly and non‑irritating, providing continuous low‑power energy to electrocardiogram and sweat‑monitoring sensors.
2 Flexible RFID, disposable electronic tags
A lightweight, ultra-thin power source that can be adhered to clothing or packaging, providing low-cost, single-use power.
3 Foldable mini smart toys, portable mini fans
Ultra-thin series‑connected battery pack, maintains shape under bending, and serves as a lightweight power source for consumer microelectronics.
4 Flexible Fabric-Integrated Energy Storage Devices
Direct printing of electrolytes onto fabric surfaces, enabling integrated power‑supply units for textile electronics and smart apparel.
5 Rapid Development Platform for Laboratory Metal–Air Batteries
Aerosol-based rapid fabrication of electrolytes with varying thicknesses enables high-throughput screening of battery performance, thereby accelerating the iteration cycle of new materials.
Image and Text Guide
a–c show cross-sectional SEM images of the electrolyte for 10 layers, 30 layers printed in non‑continuous segments, and 30 layers printed continuously, respectively. The 10‑layer stack has a thickness of approximately 18.3 μm, with virtually no voids at the electrolyte/electrode interface; AJP enables seamless, conformal deposition onto the substrate. In the non‑continuous segmented printing mode, the thickness of each 10‑layer segment is essentially uniform, and the total thickness is comparable to that of the continuous 30‑layer stack (51.8 μm).
d represents the relationship between electrolyte thickness and the number of printing layers: 10, 20, 30, and 40 layers correspond to 18.3, 37.8, 51.8, and 74.5 μm, respectively, exhibiting an approximately linear trend, thereby demonstrating the high‑precision controllability of AJP over membrane thickness.
e shows a photograph of the electrolyte as printed on the substrate by AJP; f presents the XRD patterns of NaCl powder, pure PVA, and the PVA–NaCl–glycerol electrolyte. The main peak for all three materials is located at 2θ ≈ 19.6°, while the diffraction peak of the composite electrolyte is significantly weakened, indicating that NaCl is fully dissolved in water/glycerol without crystallizing out, and that the polymer matrix retains its original crystalline structure, resulting in excellent electrolyte homogeneity.
a shows the relationship between the logarithm of electrical conductivity, log σ, and the logarithm of frequency, log ω, over the temperature range of 313–343 K: in the low-frequency region, there is a frequency‑independent DC conductivity plateau, while in the high-frequency region, conductivity increases with frequency, consistent with Jonscher’s power law. As temperature rises, the dispersion peak shifts to higher frequencies, reflecting an acceleration of polymer chain segment relaxation.
b represents the Arrhenius‑fitted straight line of log σ versus 1000/T; the slope yields an ion activation energy of approximately 0.38 eV, indicating that the energy barrier for ion hopping within the gel is relatively low and that ion transport is predominantly governed by the thermally driven motion of polymer chain segments.
a shows the Nyquist plots of NaCl electrolytes with different thicknesses (18.3/37.8/51.8/74.5 μm) at 1.0 mol: the thicker the electrolyte, the larger the intercept on the real axis and the higher the bulk resistance.
b represents thickness—according to the relationship G = σA/l, the values 18.3/37.8/51.8/74.5 μm correspond to conductivities of 5.2/3.6/2.5/1.1 × 10⁻⁴ S, respectively; reducing the thickness shortens the ion transport path and leads to a substantial increase in conductivity.
c shows the Nyquist plot of a 37.8 μm‑thick electrolyte at NaCl concentrations ranging from 0.5 to 2.1 mol; d presents the concentration–conductivity relationship: as the NaCl concentration increases from 0.5 to 1.8 mol, the conductivity rises from 2.1 × 10⁻⁴ S to 5.9 × 10⁻⁴ S, but further increasing it to 2.1 mol leads to a decrease, owing to the elevated viscosity and reduced ion mobility caused by the excess NaCl.
The galvanostatic discharge curves for a and b are at 0.1 and 0.5 mA·cm⁻², respectively: the discharge plateaus are approximately 0.78 V (at 0.1 mA·cm⁻²) and 0.75 V (at 0.5 mA·cm⁻²). As the electrolyte thickness decreases, the total Cl⁻ content diminishes, leading to shorter discharge durations.
The specific capacity, c, varies with thickness: an electrolyte 74.5 μm thick delivers a specific capacity of 2011 mAh·g⁻¹ at 1 mA·cm⁻², outperforming conventional alkaline Al–air batteries at the same current density, which suffer from severe aluminum self-corrosion.
d shows the polarization curve and the power density curve: as the thickness increases from 18.3 to 74.5 μm, the peak power density decreases from 1.98 to 0.98 mW·cm⁻², and the limiting current density drops from 10.2 to 4 mA·cm⁻².
e corresponds to single-electrode polarization: the cathode performance remains virtually unchanged with increasing thickness, and the additional polarization arising from increased thickness is primarily due to mass-transfer losses on the anode side; f represents EIS fitting: the ohmic resistance decreases sharply from 63 Ω at 74.5 μm to 12 Ω at 18.3 μm.
Figure a shows the schematic of the battery‑stack structure: an aluminum foil anode, an ultrathin gel electrolyte, and a carbon‑cloth air cathode are sequentially laminated onto a muscle sticker; a polyimide film serves as an interlayer to secure the electrolyte; copper‑foil tape connects the individual cells in series; and the muscle sticker is perforated to allow oxygen access.
b and c show physical demonstrations: a 4-cell series‑connected battery pack (cylindrical single cells, 1 cm in diameter) weighs approximately 115 g and has a thickness of less than 1.3 mm; when attached to the skin or clothing on the forearm, it can power a small fan. A 8-cell series‑connected battery pack, when bent, can light up an Iron Man toy.
d shows the polarization and power‑density curves for a single cell, a 4‑cell series‑connected stack, and an 8‑cell series‑connected stack: the 8‑cell battery pack achieves a peak power density of 37.5 mW·cm⁻², approximately seven times that of a single cell, with a packaging efficiency of about 88%, thereby meeting the power‑supply requirements of most low‑power flexible wearable electronics.
a The working principle of AJP: PVA–NaCl–glycerol ink is atomized by a carrier gas in an ultrasonic atomizer, then subjected to particle selection via virtual impactors and focused by a sheath gas. It is subsequently ejected through a 200 μm nozzle (with a stand-off distance of 10 cm) onto the substrate, under constant temperature conditions of 25 °C, at a printing speed of 10 mm·s⁻¹, with a sheath-to-carrier gas ratio of 3.
b shows the actual print head and the electrolyte membrane during the printing process; c illustrates the layer-by-layer grid‑printing procedure: each layer is laid out as a grid using zigzag paths, with the next layer printed in an offset grid pattern, stacked on top. Every 10 layers form a complete electrolyte membrane, and ultimately 10–40 layers are printed to control the thickness.
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