Biomacromolecules enabled dendrite-free lithium metal battery and its origin revealed by cryo-electron microscopy

Lithium metal, with the highest specific capacity and the lowest electrochemical potential, is a leading candidate anode for next-generation high-energy batteries. Yet its commercialization is hindered by uncontrolled dendritic growth that causes low Coulombic efficiency, poor cyclability, and safety risks. Prior approaches include creating lithiophilic hosts to regulate nucleation and growth, inserting buffer layers to mitigate ion concentration gradients and current hotspots, and engineering artificial solid-electrolyte interphases (SEIs) via electrolyte additives or coatings. However, a crystallography-centered understanding and control of lithium growth has remained limited due to Li’s characterization challenges. Inspired by biomineralization—where biomolecules regulate nucleation and oriented growth of inorganic crystals such as CaCO3 in eggshells—this study hypothesizes that a biomacromolecular matrix can regulate Li nucleation and growth orientations, suppress dendritic pathways (particularly preferred <111>), and improve SEI composition to enable dendrite-free Li deposition. The authors propose using a trifluoroethanol-modified eggshell membrane (TESM) to guide Li deposition and employ cryo-TEM to reveal the atomic-scale mechanism.

The paper surveys strategies to suppress lithium dendrites: (1) lithiophilic matrices and seeded growth to lower nucleation overpotential and guide uniform Li deposition; (2) lithiophilic buffer layers and porous hosts to reduce concentration polarization and homogenize Li+ flux; (3) artificial SEIs via electrolyte additives or coatings to enhance mechanical robustness and ionic transport. High local current densities and Li+ concentration gradients are known to promote dendrite proliferation. Prior cryo-EM work established preferred Li dendrite growth along <111>. Additives like nitrate can lower SEI resistance and alter deposition morphology toward bulk rather than tip growth. Despite progress, a crystallographic regulation of Li growth via biomolecule-inspired matrices has been scarcely explored due to Li’s sensitivity and characterization difficulty.

- Materials preparation: Eggshells were rinsed and decalcified in 25% acetic acid overnight to obtain eggshell membrane (ESM). ESM was immersed in 90% 2,2,2-trifluoroethanol (TFEA) at −80 °C overnight to yield TESM, rinsed with deionized water, and freeze-dried for 24 h. - Characterization: SEM examined morphology and pore/fiber structure. ATR-FTIR assessed protein secondary structure changes (β-sheet to α-helix). Nanoindentation measured mechanical modulus. Contact angle tests evaluated wettability; thermal stability was assessed by heat deterioration. Cryo-TEM (with cryo-transfer holder) characterized Li nuclei/deposits at atomic resolution; samples were prepared by depositing Li on TEM grids at 1 mA cm−2 to 0.5 mAh cm−2 in ether electrolyte. XPS, FTIR, and a BCA protein assay probed SEI composition and TESM incorporation. ICP-MS quantified Li+ adsorption vs time. - DFT calculations: VASP with GGA-PBE and PAW potentials, 500+ eV cutoff, forces converged to 0.02 eV/Å, 15 Å vacuum. Binding energies of Li+ with TESM peptide moieties (peptide bond, carboxyl, amino) were computed and compared with PP separator. - Electrochemistry: CR2032 coin cells assembled in Ar glove box. Electrolytes: ether-based 1.0 M LiTFSI in DOL/DME (1:1) with 1 wt% LiNO3; carbonate-based 1.0 M LiPF6 in EC/DEC/EMC (1:1:1) with 1 wt% FEC. Half-cells (Li-Cu; Cu modified with TESM or ESM) underwent formation (0.01–1.0 V, 0.05 mA cm−2, 3 cycles), then Li plating (1 or 2 mAh cm−2) and stripping to 1.5 V for CE evaluation across current densities (1–5 mA cm−2) and areal capacities (1–2 mAh cm−2). Symmetric Li–Li cells were tested at 1, 3, 5, and 10 mA cm−2 with capacities 1–5 mAh cm−2 for long-term stability and overpotential. EIS measured interfacial resistance (1 MHz–1 mHz). Full cells paired TESM/Li or bare Li with LiFePO4 (≈3.0 mAh cm−2, N/P ≈ 3.3), using 60 µL electrolyte, cycled between 2.5–4.2 V at 1C.

- TESM structure and properties: FTIR confirms protein conformational change from β-sheet to α-helix after TFEA treatment. TESM shows higher modulus (~200 MPa) vs ESM (~100 MPa). Ionic conductivity increases from 2.1 to 4.9 mS cm−1 (ESM to TESM). TESM exhibits near-0° contact angle with ether electrolytes (vs PP ~44.2°) and superior thermal stability up to 200 °C. - Li affinity and ion redistribution: DFT binding energies for Li+ with TESM moieties: peptide bond 0.74 eV, carboxyl 0.38 eV, amino 0.55 eV; PP separator exhibits repulsive interaction (−0.29 eV). ICP-MS shows increasing Li+ uptake by TESM over time, indicating strong Li+ capture. - Morphology control: SEM shows dendrite-free, spherical Li deposition on TESM-modified Cu; bare Cu exhibits long dendrites. Cryo-TEM reveals Li microspheres in presence of TESM, with lattice spacings 2.48 Å ({110}) and 1.44 Å ({211}), indicating growth along <110> and <211>. Without TESM, dendrites grow along <110>, <211>, and preferred <111>. With TESM, growth along <111> is rarely observed, indicating suppression of the dendritic pathway. Similar trends occur in carbonate-based electrolyte. - SEI participation: XPS/cryo-TEM indicate elevated N and S in Li deposits on TESM/Cu. BCA assay and FTIR confirm TESM-derived protein species in the SEI. CV shows a reduction peak ~1.4 V attributable to TESM reduction; TESM is naturally soluble and contributes to SEI formation, especially near −0.05 V, improving ion transport and uniform deposition. - Half-cell CE and polarization: At 1 mA cm−2, TESM-modified Cu achieves 98% CE over 200 cycles and 97% over 320 cycles; at 5 mA cm−2, average CE ~96% over 140 cycles. Voltage hysteresis at 3 mA cm−2: TESM 97 mV vs ESM 203 mV and bare Cu 123 mV; TESM maintains the lowest hysteresis over cycling. - Symmetric cells: At 1 mA cm−2 (1 mAh cm−2), TESM/Li shows ~12 mV overpotential and ~2000 h stability; ESM/Li higher overpotential; bare Li shows increasing overpotential due to dendrites and SEI repair. At 5 mA cm−2 (1 mAh cm−2), bare Li fails ~180 h; TESM/Li sustains >1200 h (~3000 cycles) with ~45 mV overpotential. At higher areal capacities, TESM/Li maintains stability: 3 mAh cm−2 at 5 mA cm−2 stable >600 h with ~15 mV; 5 mAh cm−2 at 10 mA cm−2 stable >600 h with ~40 mV. EIS shows TESM/Li has lower and more stable interfacial resistance. - Full cells: TESM/Li || LiFePO4 (≈3.0 mAh cm−2, N/P ≈ 3.3) retains >150 mAh g−1 for >160 cycles with near-100% CE. Bare Li full cells show rapid capacity fade (<50% retention after 20 cycles).

The findings support the hypothesis that a biomacromolecule matrix can regulate Li crystallization and deposition. TESM’s polar functional groups and 3D porous network adsorb Li+ and homogenize ion flux, reducing local current density spikes and concentration gradients that drive dendrite formation. Cryo-TEM at atomic resolution shows that TESM suppresses the preferred dendritic growth along <111>, steering growth toward orientations associated with spherical deposits (<110>, <211>). TESM-derived soluble protein species reduce at low potentials and integrate into the SEI, lowering interfacial resistance and enhancing Li+ transport, which promotes uniform deposition on bulk surfaces rather than at protruding tips. Consequently, TESM enables higher CE, lower overpotentials, and long-term cycling stability under high current densities and areal capacities. These mechanisms align with biomineralization principles where organic matrices direct crystallization orientation and morphology.

This work introduces a biomineralization-inspired strategy using TFEA-modified eggshell membrane (TESM) to achieve dendrite-free Li metal anodes. TESM enhances mechanical and ionic properties, strongly binds Li+, and integrates into the SEI, leading to homogenized ion flux and suppression of dendritic growth, particularly along the preferred <111> direction as revealed by cryo-TEM. Electrochemically, TESM delivers high Coulombic efficiencies, low voltage hysteresis, ultralong symmetric-cell lifetimes at high current densities, and improved full-cell performance with LiFePO4 under practical loading and low N/P. The approach provides atomic-level insight into dendrite suppression and offers a bioinspired route toward safer, long-life Li metal batteries. Potential future directions include optimizing biomacromolecule chemistry and structure, evaluating broader electrolyte systems and cathode pairings, and scaling and durability assessments under practical operating conditions.

The authors do not explicitly state limitations. Most experiments are demonstrated in coin-cell configurations and specific electrolytes; while carbonate electrolyte behavior is briefly examined, broader practical conditions and long-term stability in diverse environments are not extensively discussed.