Recently, the research group led by Associate Professor Sun Hao from the Centre for Frontier Science in Transformative Molecules at Shanghai Jiao Tong University, a resident scientist at the Zhangjiang Advanced Research Institute, and the School of Chemistry and Chemical Engineering, in collaboration with Academician Peng Huisheng from the Institute of Fibre Electronic Materials and Devices, the Department of Polymer Science, and the Advanced Materials Laboratory at Fudan University, proposed a multi-level assembly strategy based on two-dimensional polyamide (2DPA), termed the ‘molecule-layer-material’ approach. The research findings were published in Nature Materials under the title ‘Molecular engineering of two-dimensional polyamide interphase layers for anode-free lithium metal batteries’.

Research Background:

With the rapid advancement of consumer electronics, the Internet of Things, electric vehicles, and large-scale energy storage technologies, market demands for rechargeable battery performance continue to escalate. Batteries must deliver both high energy density for extended endurance and high power density to support rapid charging. Anode-free lithium metal batteries, which eliminate the need for anode active material during fabrication by retaining only the current collector (e.g., copper foil), significantly reduce battery volume and weight. They are regarded as a key direction for overcoming existing energy density limitations. Nevertheless, these batteries face a core challenge: the high nucleation energy barrier for lithium ions on the lithium-repellent copper foil surface slows the kinetics of lithium metal deposition/stripping reactions. This limits the battery's rate performance, making it difficult to meet the demands of fast-charging applications. Addressing this issue, extensive research has explored multiple avenues including optimising electrolyte composition, modifying the current collector interface, and introducing sacrificial agents at the cathode. Nevertheless, this bottleneck remains largely unresolved.

Polymeric materials, owing to their excellent film-forming properties and processability, have been employed to construct interfacial layers. These layers aim to enhance lithium affinity at the copper foil-electrolyte interface, thereby improving reaction kinetics and battery power density. Nevertheless, conventional polymers typically exhibit weak interactions between their functional groups and lithium ions. Moreover, their chain-entangled structures result in excessively thick interface layers (ranging from several micrometres to tens of micrometres), prolonging lithium ion transport pathways and significantly increasing interfacial impedance, thereby hindering the realisation of fast-charging capabilities. Consequently, the key to breakthroughs in power density for anode-less batteries lies in whether molecular structural design can synthesise two-dimensional polymer molecules capable of efficiently adsorbing lithium ions, which can then be further assembled into ultra-thin interfacial layers with special aggregated structures.

Addressing these challenges, the research group led by Associate Professor Sun Hao from the Centre for Frontier Science in Transformative Molecules at Shanghai Jiao Tong University, a resident scientist at the Zhangjiang Advanced Research Institute, and the School of Chemistry and Chemical Engineering, in collaboration with Academician Peng Huisheng from the Institute of Fibre Electronic Materials and Devices, the Department of Polymer Science, and the Advanced Materials Laboratory at Fudan University, proposed a multi-level assembly strategy based on two-dimensional polyamide (2DPA), termed the ‘molecule-to-sheet-to-material’ approach. Through precise chemical design of conjugated building blocks, polyamide molecules with a two-dimensional configuration were successfully synthesised. Utilising intermolecular hydrogen bonding and conjugation interactions, an ordered layered structure was constructed. This was further combined with lithiated perfluorosulfonic acid polymer (Lithiated Nafion, LN) to develop a 2DPA/LN composite interfacial layer just 50 nm thick. Copper foil modified with this composite interlayer achieved a critical current density of 30 mA cm−2 for lithium metal deposition and stripping, enabling stable lithium metal deposition at 10 mAh cm−2. Leveraging the exceptional lithium-ion affinity and film-forming properties of the 2DPA/LN composite interface layer, an 8 Ah anode-less pouch cell was successfully fabricated, exhibiting both high energy density (471 Wh kg−1) and high power density (622 W kg−1). This achievement provides crucial material and theoretical support for the practical development of next-generation anode-less lithium metal batteries.

Figure 1. Schematic illustration of the ‘molecule-layer-material’ multi-level assembly based on 2DPA, alongside morphological and structural characterisation of the 2DPA/LN interface layer.

The research findings were published in Nature Materials under the title ‘Molecular engineering of two-dimensional polyamide interphase layers for anode-free lithium metal batteries’. The first authors are Dr Wang Shuo (Postdoctoral Fellow), Dr Wang Yan (Assistant Researcher), and PhD candidate Ouyang Zhaofeng from the Centre for Frontier Molecular Science at Shanghai Jiao Tong University. Corresponding authors are Associate Professor Sun Hao from Shanghai Jiao Tong University and Academician Peng Huisheng from Fudan University. The primary corresponding institution is the Centre for Frontier Molecular Science at Shanghai Jiao Tong University. This work received substantial support from the National Natural Science Foundation of China, the Special Fund for Basic Research of Central Universities, the Centre for Transformative Molecular Frontier Science at Shanghai Jiao Tong University, the State Key Laboratory of Chemical Biology and Collaborative Material Innovation, the Zhangjiang Advanced Research Institute, and the State Key Laboratory of Polymer Molecular Engineering at Fudan University.

Research content:

Figure 2. Homogeneity analysis of the large-area 2DPA/LN interface layer

The research team synthesised 2DPA via a liquid-phase polycondensation reaction between melamine and 1,3,5-benzenetricarboxylic acid trichloride. Atomic force microscopy, transmission electron microscopy, and grazing-incidence wide-angle X-ray scattering results indicate that 2DPA exhibits a two-dimensional lamellar structure (Figure 1b, c), which assembles into a layered-stacked aggregate state (Figure 1e). Introduction of LN (mass ratio 5:1) into 2DPA followed by further assembly yielded a 2DPA/LN composite interfacial layer with an average thickness of 50.2 ± 0.3 nm (Figure 1d). The incorporation of LN significantly enhanced the ionic conductivity of the composite interface layer (Figure 1g) without disrupting the original layered assembly structure of 2DPA (Figure 1f). A 2DPA/LN-modified copper foil (2DPA/LN-Cu) with an area of 100 × 20 cm² was successfully fabricated via a spin-coating process (Figure 2a), demonstrating the material's excellent scalability. Further characterisation through scanning electron microscopy images from 15 distinct locations, Fourier transform infrared spectroscopy, and two-probe digital source table measurements confirmed the uniform distribution of the 2DPA/LN interfacial layer on the copper foil, exhibiting excellent film-forming properties and large-area consistency (Fig. 2b–e).

Figure 3. Adsorption-Conjugation Synergy Modulating the Internal Helmholtz Layer at the Copper-Electrolyte Interface

Density functional theory investigations revealed interactions between 2DPA and lithium ions. Results indicate lithium ions exhibit ion-dipole interactions (Figure 3b) with local dipole moments induced by the C=O groups of amide moieties and the C=N bonds of triazine rings in 2DPA (blue regions in Figure 3a), thereby achieving an ‘adsorption’ effect. X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy reveal that lithium ions undergo reversible adsorption and desorption at the 2DPA/LN interfacial layer (Fig. 3c–f). The π-π and p-π conjugated structures present in 2DPA facilitate effective electron delocalisation, resulting in a uniform electrostatic potential distribution (Fig. 3a). Consequently, lithium ions at the copper-electrolyte interface interact ionically with delocalised electrons in 2DPA, further promoting uniform lithium ion distribution and realising a ‘conjugated’ effect. In situ Raman spectroscopy results further validate the existence of the ‘adsorption-conjugation’ synergistic effect within the 2DPA/LN interface layer. During lithium metal deposition, lithium ions consistently maintain uniform distribution across the 2DPA/LN-Cu surface (Fig. 3g, h). Chronoamperometric testing revealed that the surface capacitance (C_d) of the 2DPA/LN-Cu interface was 0.24 F cm⁻², markedly higher than that of Cu (0.09 F cm⁻²) and CHPA/LN-Cu (0.11 F cm⁻²) (Fig. 3i–k). This indicates that the 2DPA/LN interfacial layer adsorbs additional lithium and anion ions into the inner Helmholtz layer, enabling uniform lithium ion distribution and promoting anion dissociation to form an electrochemically stable solid-state electrolyte intermediate phase (Figure 3l).

The synergistic ‘adsorption-conjugation’ effect effectively regulates the nucleation morphology of lithium metal. For instance, at a current density of 5 mA cm⁻², lithium metal on 2DPA/LN-Cu deposits with horizontal expansion around nucleation sites, forming larger-diameter bulk structures; whereas lithium nuclei on pure copper exhibit essentially unchanged diameters (Fig. 4a–c). The Li||2DPA/LN-Cu battery stably cycled 550 cycles at 1 mA cm⁻² and 1 mAh cm⁻², achieving an average coulombic efficiency of 98.8% (Figure 4d). In contrast, Li||Cu, Li||PA/LN-Cu, and Li||CHPA/LN-Cu batteries cycled only 200, 243, and 273 cycles respectively. The Li||2DPA/LN-Cu battery maintained stable cycling for 40 cycles even at a high current density of 20 mA cm⁻² (Figure 4e), with voltage hysteresis consistently below 0.35 V (Figure 4f). These results demonstrate that the ‘adsorption-conjugation’ synergistic effect significantly enhances the reversibility of lithium metal deposition/stripping.

Furthermore, the Li||2DPA/LN-Cu battery exhibits a critical current density of 30 mA cm⁻², compared to only 20 mA cm⁻² for the Li||CHPA/LN-Cu battery (Fig. 4g, h). The Li||2DPA/LN-Cu battery exhibited lower overpotentials across all current densities (Figure 4i). Even at a high areal capacity of 5 mAh cm⁻² and a current density of 20 mA cm⁻², the battery maintained stable cycling performance over 15 cycles (Figure 4j), demonstrating that the 2DPA/LN interfacial layer accommodates lithium metal deposition/stripping under high-capacity and high-current conditions. The Li||2DPA/LN-Cu battery achieves significantly higher maximum reversible deposition/stripping surface currents and surface capacities than previously reported polymer-based Li||Cu batteries (Figure 4h).

Figure 4. Electrochemical performance and lithium deposition morphology of lithium||copper batteries based on the 2DPA/LN interfacial layer

The chemical composition and spatial distribution of the mesophase in the solid-state electrolyte were investigated in depth using time-of-flight secondary ion mass spectrometry, X-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy. Results indicate that 2DPA predominantly distributes on the outer surface of lithium deposits (Fig. 5a, b). The 2DPA/LN interface layer induces the formation of an interfacial passivation layer enriched with lithium nitride and lithium fluoride (Fig. 5c-e). Atomic force microscopy revealed that the introduction of the 2DPA/LN interfacial layer substantially enhanced the mechanical strength of the solid electrolyte interphase, exhibiting excellent mechanical stability during long-term cycling (Fig. 5f, g). Pre- and post-cycling X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy results indicated that the chemical structure of the 2DPA/LN interfacial layer remained largely intact, demonstrating good electrochemical stability (Fig. 5h, i). Consequently, the 2DPA/LN interfacial layer synergistically suppresses side reactions between the electrolyte and deposited lithium in conjunction with the in situ-formed lithium nitride and lithium fluoride passivation layers, thereby substantially enhancing the stability of the lithium metal deposition/stripping process.

Figure 5. Performance analysis of the mesophase solid electrolyte derived from the 2DPA/LN interfacial layer

An anode-less lithium iron phosphate full cell constructed based on the 2DPA/LN interface layer (with a cathode areal capacity of 3 mAh cm⁻²) demonstrated superior rate performance and cycling stability. The cell maintained stable charge-discharge behaviour at a high rate of 5 C, exhibiting a capacity retention rate of 63.4% relative to the initial capacity (0.1 C) (Figure 6a). After 200 cycles at 0.3 C charge/0.5 C discharge conditions, the capacity retention reached 48.0% (Figure 6b). In contrast, anode-less lithium metal batteries based on Cu, PA/LN-Cu, and CHPA/LN-Cu achieved only 87, 98, and 115 cycles respectively. Furthermore, an 8 Ah anode-less nickel-cobalt-manganese ternary lithium metal pouch battery was successfully fabricated based on the 2DPA/LN interfacial layer. Its battery-grade energy density and power density reached 471 Wh kg⁻¹ and 622 W kg⁻¹, respectively, representing improvements of 88–125% and 490–710% over previous studies. achieving a synergistic breakthrough in both battery endurance and rapid charge-discharge capability (Figure 6d–h).

Figure 6. Electrochemical performance study of an anode-less lithium metal battery based on a 2DPA/LN interfacial layer

Summary:

This work proposes an ultra-thin 2DPA/LN composite interfacial layer constructed using two-dimensional polymers as assembly units, employing a multi-level ‘molecule-layer-material’ assembly strategy. This effectively improves the deposition/stripping kinetics and reversibility of lithium metal under high current density and areal capacity conditions. An 8 Ah anode-less pouch cell fabricated with this composite interlayer achieved an energy density of 471 Wh kg^(−1) and a power density of 622 W kg^(−1), significantly overcoming the rate performance limitations of anode-less lithium metal batteries. This demonstrates the efficacy and immense potential of molecular engineering strategies in creating and optimising high-performance battery systems. We believe that precise control over polymer material sequences and dimensions can further advance the development of high-performance battery systems, thereby supporting critical national demands.

Original article link:

https://doi.org/10.1038/s41563-025-02339-y