Recovery of isolated lithium through discharged state calendar ageing

Rechargeable Li-metal batteries have the potential to more than double the specific energy of the state-of-the-art rechargeable Li-ion batteries, making Li-metal batteries a prime candidate for next-generation high-energy battery technology1-3. However, current Li-metal batteries suffer from fast cycle degradation compared with their Li-ion battery counterparts2,3, preventing their practical adoption. A main contributor to capacity degradation is the disconnection of Li from the electrochemical circuit, forming isolated Li4-8. Calendar ageing studies have shown that resting in the charged state promotes further reaction of active Li with the surrounding electrolyte9-12. Here we discover that calendar ageing in the discharged state improves capacity retention through isolated Li recovery, which is in contrast with the well-known phenomenon of capacity degradation observed during the charged state calendar ageing. Inactive capacity recovery is verified through observation of Coulombic efficiency greater than 100% on both Li||Cu half-cells and anode-free cells using a hybrid continuous-resting cycling protocol and with titration gas chromatography. An operando optical setup further confirms excess isolated Li reactivation as the predominant contributor to the increased capacity recovery. These insights into a previously unknown pathway for capacity recovery through discharged state resting emphasize the marked impact of cycling strategies on Li-metal battery performance.

Proximity Matters: Interfacial Solvation Dictates Solid Electrolyte Interphase Composition

The composition of the solid electrolyte interphase (SEI) plays an important role in controlling Li-electrolyte reactions, but the underlying cause of SEI composition differences between electrolytes remains unclear. Many studies correlate SEI composition with the bulk solvation of Li ions in the electrolyte, but this correlation does not fully capture the interfacial phenomenon of SEI formation. Here, we provide a direct connection between SEI composition and Li-ion solvation by forming SEIs using polar substrates that modify interfacial solvation structures. We circumvent the deposition of Li metal by forming the SEI above Li+/Li redox potential. Using theory, we show that an increase in the probability density of anions near a polar substrate increases anion incorporation within the SEI, providing a direct correlation between interfacial solvation and SEI composition. Finally, we use this concept to form stable anion-rich SEIs, resulting in high performance lithium metal batteries.

Ultrahigh-Loading Manganese-Based Electrodes for Aqueous Batteries via Polymorph Tuning

Manganese-based aqueous batteries utilizing Mn2+ /MnO2 redox reactions are promising choices for grid-scale energy storage due to their high theoretical specific capacity, high power capability, low-cost, and intrinsic safety with water-based electrolytes. However, the application of such systems is hindered by the insulating nature of deposited MnO2 , resulting in low normalized areal loading (0.005-0.05 mAh cm-2 ) during the charge/discharge cycle. In this work, the electrochemical performance of various MnO2 polymorphs in Mn2+ /MnO2 redox reactions is investigated, and ɛ-MnO2 with low conductivity is determined to be the primary electrochemically deposited phase in normal acidic aqueous electrolyte. It is found that increasing the temperature can change the deposited phase from ɛ-MnO2 with low conductivity to γ-MnO2 with two order of magnitude increase in conductivity. It is demonstrated that the highly conductive γ-MnO2 can be effectively exploited for ultrahigh areal loading electrode, and a normalized areal loading of 33 mAh cm-2 is achieved. At a mild temperature of 50 °C, cells are cycled with an ultrahigh areal loading of 20 mAh cm-2 (1-2 orders of magnitude higher than previous studies) for over 200 cycles with only 13% capacity loss.

Autonomous alignment and healing in multilayer soft electronics using immiscible dynamic polymers

Self-healing soft electronic and robotic devices can, like human skin, recover autonomously from damage. While current devices use a single type of dynamic polymer for all functional layers to ensure strong interlayer adhesion, this approach requires manual layer alignment. In this study, we used two dynamic polymers, which have immiscible backbones but identical dynamic bonds, to maintain interlayer adhesion while enabling autonomous realignment during healing. These dynamic polymers exhibit a weakly interpenetrating and adhesive interface, whose width is tunable. When multilayered polymer films are misaligned after damage, these structures autonomously realign during healing to minimize interfacial free energy. We fabricated devices with conductive, dielectric, and magnetic particles that functionally heal after damage, enabling thin-film pressure sensors, magnetically assembled soft robots, and underwater circuit assembly.

Dissolution of the Solid Electrolyte Interphase and Its Effects on Lithium Metal Anode Cyclability

At >95% Coulombic efficiencies, most of the capacity loss for Li metal anodes (LMAs) is through the formation and growth of the solid electrolyte interphase (SEI). However, the mechanism through which this happens remains unclear. One property of the SEI that directly affects its formation and growth is the SEI's solubility in the electrolyte. Here, we systematically quantify and compare the solubility of SEIs derived from ether-based electrolytes optimized for LMAs using in-operando electrochemical quartz crystal microbalance (EQCM). A correlation among solubility, passivity, and cyclability established in this work reveals that SEI dissolution is a major contributor to the differences in passivity and electrochemical performance among battery electrolytes. Together with our EQCM, X-ray photoelectron spectroscopy (XPS), and nuclear magnetic resonance (NMR) spectroscopy results, we show that solubility depends on not only the SEI's composition but also the properties of the electrolyte. This provides a crucial piece of information that could help minimize capacity loss due to SEI formation and growth during battery cycling and aging.

Data-driven electrolyte design for lithium metal anodes

Improving Coulombic efficiency (CE) is key to the adoption of high energy density lithium metal batteries. Liquid electrolyte engineering has emerged as a promising strategy for improving the CE of lithium metal batteries, but its complexity renders the performance prediction and design of electrolytes challenging. Here, we develop machine learning (ML) models that assist and accelerate the design of high-performance electrolytes. Using the elemental composition of electrolytes as the features of our models, we apply linear regression, random forest, and bagging models to identify the critical features for predicting CE. Our models reveal that a reduction in the solvent oxygen content is critical for superior CE. We use the ML models to design electrolyte formulations with fluorine-free solvents that achieve a high CE of 99.70%. This work highlights the promise of data-driven approaches that can accelerate the design of high-performance electrolytes for lithium metal batteries.

Revealing the Multifunctions of Li3N in the Suspension Electrolyte for Lithium Metal Batteries

Inorganic-rich solid-electrolyte interphases (SEIs) on Li metal anodes improve the electrochemical performance of Li metal batteries (LMBs). Therefore, a fundamental understanding of the roles played by essential inorganic compounds in SEIs is critical to realizing and developing high-performance LMBs. Among the prevalent SEI inorganic compounds observed for Li metal anodes, Li₃N is often found in the SEIs of high-performance LMBs. Herein, we elucidate new features of Li₃N by utilizing a suspension electrolyte design that contributes to the improved electrochemical performance of the Li metal anode. Through empirical and computational studies, we show that Li₃N guides Li electrodeposition along its surface, creates a weakly solvating environment by decreasing Li⁺-solvent coordination, induces organic-poor SEI on the Li metal anode, and facilitates Li⁺ transport in the electrolyte. Importantly, recognizing specific roles of SEI inorganics for Li metal anodes can serve as one of the rational guidelines to design and optimize SEIs through electrolyte engineering for LMBs.

Investigating the Cyclability and Stability at the Interfaces of Composite Solid Electrolytes in Li Metal Batteries

Despite the fact that much work has been dedicated to finding the ideal additive for composite solid electrolytes (CSEs) for lithium-based solid-state batteries, little is known about the properties of a CSE that enable stable cycling with a lithium metal anode. In this work, we use three CSEs based on lithium nitride (Li₃N), a fast lithium-ion conductor, and lithium hydroxide (LiOH) to investigate the properties and interfacial interactions that impact the cyclability of CSEs. We present a method for stabilizing Li₃N with a shell of LiOH, and we incorporate Li₃N, core-shell particles, and LiOH into CSEs using polyethylene oxide (PEO) and lithium bis(trifluoromethanesulfonyl)imide. Through improved interfacial chemistry, CSEs with core-shell particles have superior electrochemical cycling performance compared to those with unprotected Li₃N in symmetric Li-Li cells. This CSE features a high ionic conductivity of 0.66 mS cm^-1 at 60 °C, a high critical current density of 1.2 mA cm^-2, and a wide voltage window of 0-5.1 V. Full cells with the core-shell CSE and lithium iron phosphate cathodes exhibit stable cycling and high reversible specific capacities in cells as high as 2.5 mAh cm^-2. We report that the improved ionic conductivity and amorphous PEO content have a limited effect on the solid-state electrolyte performance, while improving the electrolyte-Li metal anode interface is key to cycling longevity.

Suspension electrolyte with modified Li+ solvation environment for lithium metal batteries

Designing a stable solid-electrolyte interphase on a Li anode is imperative to developing reliable Li metal batteries. Herein, we report a suspension electrolyte design that modifies the Li⁺ solvation environment in liquid electrolytes and creates inorganic-rich solid-electrolyte interphases on Li. Li₂O nanoparticles suspended in liquid electrolytes were investigated as a proof of concept. Through theoretical and empirical analyses of Li₂O suspension electrolytes, the roles played by Li₂O in the liquid electrolyte and solid-electrolyte interphases of the Li anode are elucidated. Also, the suspension electrolyte design is applied in conventional and state-of-the-art high-performance electrolytes to demonstrate its applicability. Based on electrochemical analyses, improved Coulombic efficiency (up to ~99.7%), reduced Li nucleation overpotential, stabilized Li interphases and prolonged cycle life of anode-free cells (~70 cycles at 80% of initial capacity) were achieved with the suspension electrolytes. We expect this design principle and our findings to be expanded into developing electrolytes and solid-electrolyte interphases for Li metal batteries.

Capturing the swelling of solid-electrolyte interphase in lithium metal batteries

Although liquid-solid interfaces are foundational in broad areas of science, characterizing this delicate interface remains inherently difficult because of shortcomings in existing tools to access liquid and solid phases simultaneously at the nanoscale. This leads to substantial gaps in our understanding of the structure and chemistry of key interfaces in battery systems. We adopt and modify a thin film vitrification method to preserve the sensitive yet critical interfaces in batteries at native liquid electrolyte environments to enable cryo–electron microscopy and spectroscopy. We report substantial swelling of the solid-electrolyte interphase (SEI) on lithium metal anode in various electrolytes. The swelling behavior is dependent on electrolyte chemistry and is highly correlated to battery performance. Higher degrees of SEI swelling tend to exhibit poor electrochemical cycling.