Symmetric molecular design: Stable lithium metal batteries

The relentless pursuit of high-energy-density batteries has reignited global interest in lithium metal batteries (LMBs). However, the commercialization of LMBs has faced insurmountable hurdles during their initial exploration, primarily due to uncontrolled lithium dendrite growth, electrolyte decomposition, and catastrophic safety risks such as thermal runaway. These challenges led to the abandonment of lithium metal anodes in favor of graphite-based systems, which dominated the market due to their inherent stability, albeit at the cost of significantly lower energy density.

Advancements in materials science, interfacial engineering, and a deepening understanding of failure mechanisms have driven the resurgence of LMBs in recent years. Modern applications such as electric aviation, portable electronics, and grid-scale energy storage demand energy densities exceeding the practical limits of lithium-ion batteries. Lithium metal, when paired with high-voltage cathodes like nickel-rich layered oxides or lithium cobalt oxide, offers a theoretical pathway to cell-level energy density exceeding 500 Wh kg−1. Furthermore, constructing energy storage devices with lithium metal anodes can unlock even greater advantages. For instance, coupling a lithium metal anode with a sulfur cathode achieves a theoretical energy density of up to 2,600 Wh kg−1. Despite this potential, the Achilles’ heel of LMBs persists: the unstable solid-electrolyte interphase (SEI) and inhomogeneous lithium deposition. The SEI is inherently fragile in conventional carbonate-based electrolytes, leading to continuous electrolyte consumption, rapid capacity fade, and dendritic morphologies that puncture separators, causing internal short circuits. Addressing these issues necessitates a paradigm shift in electrolyte design, where molecular-level engineering of solvents, salts, and additives plays a pivotal role.

Electrolytes and lithium salts are the lifeblood of batteries, dictating ion transport kinetics, SEI composition, and safety. Key requirements for LMB electrolytes include (1) a high Li+ transference number to mitigate concentration polarization, (2) an anion-derived inorganic-rich SEI to suppress side reactions, and (3) non-flammability to prevent thermal runaway. Emerging strategies focus on tailoring solvation structures to regulate Li+ flux and SEI composition. Innovations such as localized high-concentration electrolytes (LHCEs), fluorinated solvents, and ionic liquid (IL) additives demonstrate improved interfacial stability by promoting anion-derived inorganic SEI components. These approaches highlight the critical balance required between ionic conductivity, desolvation energy, and interfacial compatibility, which is a triad that defines the viability of next-generation electrolytes.

By regulating the geometric symmetry and electronic distribution of molecular structures, symmetric molecular design optimizes ion transport pathways, interfacial stability, and material reversibility, emerging as a critical strategy to address challenges such as lithium dendrite growth and electrolyte decomposition in LMBs. The symmetric molecular design introduces organic salt cations and anions with symmetric structures, which form strong ion-pair interactions through close packing. This minimizes steric hindrance and disordered intermolecular stacking, facilitating homogeneous ion diffusion. The symmetric arrangement of ion pairs in liquid electrolytes suppresses excessive anion clustering in solvated Li+ structures, thereby promoting efficient Li+ desolvation. Furthermore, by reducing the coordination number of Li+ with anions through robust ion-pair interactions, symmetric molecules generate a densely packed stationary layer at the electrode surface. This layer uniformly attracts anions to the Li-metal interface by electrostatic interactions, fostering the formation of a homogeneous, inorganic-rich SEI, effectively suppressing side reactions. Additionally, symmetric molecules enhance ion transference numbers by balancing anion/cation migration rates, mitigating concentration polarization, and extending the cycling lifespan. Notably, many commercially available lithium salts and solvents inherently exhibit symmetric molecular structures. As evidenced by comparative analyses, symmetric and asymmetric molecules exhibit distinct structural disparities. These differences significantly affect ion solvation behavior and modify ion migration kinetics, directly impacting battery rate capability. Furthermore, molecular configuration governs SEI formation and interfacial dynamics, ultimately dictating divergent outcomes of safety and cycling stability in LMBs.