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

Lithium metal anodes are highly desirable for next-generation rechargeable batteries due to their ultrahigh theoretical energy density. However, uncontrolled lithium dendrite growth poses significant challenges, including reduced Coulombic efficiency, poor cycling life, and safety hazards. This research addresses these challenges by drawing inspiration from biomineralization, the process by which living organisms precisely control the growth of inorganic crystals. The study introduces a biomacromolecule matrix derived from eggshell membranes to regulate lithium deposition and suppress dendrite formation. The use of cryo-electron microscopy (cryo-TEM) is a key aspect, allowing for atomic-level investigation of lithium structure and growth mechanisms. This detailed characterization is crucial in understanding and mitigating the limitations of lithium metal anodes, paving the way for the development of safer and more efficient batteries. The high energy density potential of lithium-metal batteries (LMBs) compared to current lithium-ion batteries is a driving force behind this research. The superior specific capacity and lowest electrochemical potential of metallic lithium make it an attractive anode material, promising substantially increased energy density when paired with various cathode materials such as lithium transition-metal oxides, sulfur, or air. However, the pervasive challenge of dendrite formation necessitates innovative solutions like the bioinspired approach presented in this study, which seeks to overcome the limitations that have hindered widespread adoption of LMBs. The overarching goal is to enhance the cycling performance and longevity of lithium metal batteries by controlling the lithium deposition process at the atomic level. Previous efforts to address dendrite formation have explored strategies such as creating lithiophilic matrices to influence lithium nucleation and growth, introducing buffer layers to mitigate concentration gradients, and constructing artificial solid electrolyte interphases (SEIs) to enhance stability. This research builds upon this existing body of work by introducing a novel biomacromolecule-based approach inspired by nature's sophisticated biomineralization processes.

Extensive research has focused on improving lithium metal anodes. Strategies include creating lithiophilic matrices to control lithium nucleation and growth, employing buffer layers to reduce concentration polarization and inhibit dendrite formation, and constructing artificial SEIs to stabilize the anode. Examples include using polyethyleneimine sponges, glass fibers, and various electrolyte additives to enhance SEI properties and promote uniform lithium deposition. However, these methods often lack a deep understanding of dendrite growth from a crystallographic perspective, which is crucial for effective control. This study addresses this gap by utilizing cryo-TEM to visualize lithium at the atomic level, gaining insights into growth mechanisms and preferred crystallographic orientations. The inspiration for this work is drawn from nature's biomineralization processes, where biomolecules guide the controlled formation of inorganic crystals. The eggshell's calcified structure, regulated by membrane proteins, serves as a prime example, with uniform nucleation and oriented growth of CaCO3 on the eggshell membrane (ESM). This biomimetic approach offers a potentially more effective way to control lithium deposition, addressing inherent challenges in current strategies.

The research involved several key steps: 1. **Eggshell Membrane (ESM) and Trifluoroethanol-modified ESM (TESM) Preparation:** Eggshells were first collected and cleaned. The calcium carbonate (CaCO3) was removed by etching with acetic acid, leaving behind the ESM. This ESM was then modified through a solvothermal treatment with trifluoroethanol (TFEA) to yield TESM. This modification aimed to improve the mechanical properties and ionic conductivity of the ESM, as well as its affinity for lithium ions. Freeze-drying was utilized to purify and preserve the resulting TESM. 2. **Material Characterization:** A wide range of techniques was used to characterize the ESM and TESM. Scanning electron microscopy (SEM) imaged the morphology and microstructure of the membranes, also examining lithium deposition patterns on modified and unmodified copper electrodes. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) analyzed the chemical structure and conformational changes after TFEA modification. Nanoindentation determined the mechanical properties (modulus) of both ESM and TESM. Contact angle measurements assessed the wettability of the membranes with different electrolytes. Inductively coupled plasma mass spectrometry (ICP-MS) measured the amount of lithium ions adsorbed by TESM. X-ray photoelectron spectroscopy (XPS) investigated the composition of the solid electrolyte interphase (SEI). Cryo-TEM was crucial in visualizing the atomic structure of lithium deposits and the SEI, crucial for understanding the dendrite suppression mechanism. The Bicinchoninic acid (BCA) assay quantified protein content in the SEI. 3. **Density Functional Theory (DFT) Calculations:** DFT calculations were employed to investigate the interaction between lithium ions and key chemical species within the TESM, including peptide bonds, carboxyl groups, and amino groups. This computational approach helped to explain the strong affinity between lithium ions and the TESM. 4. **Electrochemical Measurements:** CR2032 coin cells were assembled using lithium foil as the counter electrode and copper foil (modified with TESM or unmodified) as the working electrode. Ether-based and carbonate-based electrolytes with selected additives were used to evaluate the performance of the Li anodes. Electrochemical tests were conducted using a Neware multichannel battery cycler, including cycling tests at different current densities and areal capacities. Cyclic voltammetry (CV) determined the reduction potential of TESM. Electrochemical impedance spectroscopy (EIS) measured the interfacial resistance. Symmetric coin cells with Li-Li configuration and full cells with LiFePO4 cathodes were tested to evaluate overall battery performance. 5. **Statistical Analysis:** Appropriate statistical analysis was applied to the electrochemical results to determine the significance of the observed changes in Coulombic efficiency, cycling stability, and voltage profiles.

The study's key findings revolve around the efficacy of the TESM in suppressing lithium dendrite growth and enhancing the performance of lithium metal anodes. * **Dendrite Suppression:** Cryo-TEM revealed that the TESM effectively inhibits the growth of lithium dendrites along the preferred <111> crystallographic orientation. In contrast, dendrites were prevalent on bare copper electrodes. The TESM promotes the formation of uniform, spherical lithium deposits instead of dendritic structures. * **SEI Modification:** The TESM actively participates in SEI formation, with soluble protein species from TESM incorporated into the SEI. This incorporation leads to a more homogeneous lithium-ion flux and contributes to the uniform lithium deposition. The SEI formed with TESM displayed lower resistance compared to bare Li electrodes. * **Enhanced Electrochemical Performance:** Li anodes modified with TESM demonstrated significantly improved electrochemical properties. At a low current density (1 mA cm⁻²), the TESM-modified electrode maintained a high Coulombic efficiency (>97%) over extended cycling (>320 cycles). Even at a higher current density (5 mA cm⁻²), the efficiency remained high (>96% over 140 cycles). Symmetric cells employing TESM/Li exhibited excellent stability and low overpotential for thousands of hours, even at high current densities and areal capacities. * **Full-Cell Performance:** Full cells with a LiFePO4 cathode and TESM/Li anodes displayed superior cycling stability, maintaining capacity and efficiency for over 160 cycles, even at high LiFePO4 loading and low N/P ratio, indicating excellent practical applicability. * **Mechanism:** The mechanism involves strong affinity between lithium ions and polar groups within the TESM, facilitated by a 3D network structure that homogenizes lithium-ion distribution, thereby reducing the concentration gradient and mitigating the formation of dendrites. The modified SEI also contributes by facilitating homogeneous lithium-ion transport.

The findings significantly advance our understanding of lithium dendrite formation and its control. The bioinspired approach, leveraging the TESM, provides a highly effective method to suppress dendrite growth. The atomic-level visualization via cryo-TEM reveals crucial mechanistic insights, explaining the observed improvements. The homogenization of lithium-ion flux and the modification of the SEI are key factors in achieving the observed enhanced performance. This study underscores the power of biomimicry in materials science, drawing inspiration from nature's precise control of crystal growth. The superior performance of TESM-modified anodes compared to ESM-modified anodes and glass fiber control samples highlights the effectiveness of the TFEA modification in enhancing affinity and performance. The success in full-cell tests, even under practical conditions of high cathode loading and low N/P ratio, demonstrates the significant potential of this approach for developing commercially viable high-energy-density lithium metal batteries. This work provides valuable guidance for designing effective strategies to regulate lithium deposition and mitigate dendrite formation, a critical step in advancing lithium metal battery technology.

This research successfully demonstrated a biomineralization-inspired design strategy for lithium metal anodes using trifluoroethanol-modified eggshell membrane (TESM). TESM effectively suppresses dendrite growth, resulting in significantly improved cycling stability and Coulombic efficiency. The superior performance was observed across various current densities and areal capacities, with promising results in full-cell tests. This bioinspired approach offers a new avenue for developing high-safety, long-cycling lithium metal batteries. Future research could explore other biomacromolecules with enhanced properties and investigate the scalability and cost-effectiveness of this method for large-scale battery production.

While the study demonstrates the effectiveness of TESM in improving lithium metal anode performance, some limitations exist. The long-term stability of TESM under continuous cycling needs further investigation. The study focused primarily on two specific electrolyte types; more comprehensive evaluations with a broader range of electrolytes are needed to assess its general applicability. Scalability and cost-effectiveness of the TESM preparation and integration into battery production also need further investigation to determine its commercial viability.