Metallic lithium is a promising anode to increase the energy density of rechargeable lithium batteries. Despite extensive efforts, detrimental reactivity of lithium metal with electrolytes and uncontrolled dendrite growth remain challenging interconnected issues hindering highly reversible Li-metal batteries. Herein, we report a rationally designed amide-based electrolyte based on the desired interface products. This amide electrolyte achieves a high average Coulombic efficiency during cycling, resulting in an outstanding capacity retention with a 3.5 mAh cm⁻² high-mass-loaded LiNi_0.8Co_0.1Mn_0.1O₂ cathode. The interface reactions with the amide electrolyte lead to the predicted solid electrolyte interface species, having favorable properties such as high ionic conductivity and high stability. Operando monitoring the lithium spatial distribution reveals that the highly reversible behavior is related to denser deposition as well as top-down stripping, which decreases the formation of porous deposits and inactive lithium, providing new insights for the development of interface chemistries for metal batteries.

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The attention towards lithium (Li) metal anodes has been rekindled in recent years as it would boost the energy-density of Li batteries. However, notorious safety issues and cycling instability severely hinder their commercialization, especially when cycled in traditional carbonic ester electrolytes that exhibit a wide voltage window and are compatible with most of the cathode materials. Herein, lithium difluorophosphate (LiDFP) and vinylene carbonate (VC) are combined, and demonstrated to be synergistic in constructing in situ a mechanically stable and highly Li-ion conducting surface film on the Li metal anode. This results in uniform and compact Li deposition largely suppressing the formation of Li dendrites, dead lithium and irreversible Li-species as revealed by operando neutron depth profiling (NDP). This enables long-term cycling stability and enhancement of the Coulombic efficiency for rechargeable Li metal anodes. By combining solid state nuclear magnetic resonance (SSNMR) and spectroscopic studies, it is demonstrated that VC slows down the LiDFP reduction, yet promoting the breaking of the P–F bonds, which leads to a protective film. This film is rich in LiF–Li₃PO₄ inorganic compounds, distributed homogeneously, that is embedded in a matrix of P–O–C species and macromolecular organic compounds like lithium ethylene dicarbonate. This composition is responsible for the improved ionic conductivity and mechanical stability of the protective film during extended cycles. The detailed insight in the additives interaction provides new opportunities for the design of rational surface films necessary for realizing high-performance lithium metal batteries.

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The sustainable energy transition relies on energy storage technology. Crucial to meet the ever increasing energy storage demand is profound understanding of the governing processes. Yet, due to the inherent difficulty to study light ions with conventional techniques, limited methodology is available for operando monitoring of lithium ion batteries. A non-invasive and versatile alternative is Neutron Depth Profiling(NDP). This technique provides information on the spatial and temporal lithium concentration during (dis)charge. In this work NDP is used to shed light on key challenges for lithium ion batteries. The results provide detailed understanding of electrode parameters such as tortuosity and Li-ion transport. This allows to reduce the battery internal resistance or increase charging current, thereby reducing charge times. Post lithium ion battery technology relies on a reversible lithium-metal anode, this would enable batteries based on the conversion reaction of lithium with oxygen or sulfur. Furthermore, a lithium metal anode can double pack level energy density when employing current cathodes. Using NDP, we can monitor the lithium concentration profile as the material is plated. This allows to study the dependency with respect to current density, electrolyte composition and cycling history. Moreover NDP allows to follow lithium independent of oxidation state. Hence enabling to monitor battery failure originating from lithium polysulfide dissolution in the liquid electrolyte. The findings as presented rationalize electrode design towards high energy-dense, safe and low-cost Li-S batteries. The thesis concludes with a revolutionary concept based on a gas filled gridpix time projection chamber. A gridpix detector allows a 3D particle trace reconstruction. Hence a 3D spatial isotope specific, lithium-6, distribution is obtained. This technique would cater for a whole new range of topics to study.@en

Despite the lower gravimetric capacity, Li ₄ Ti ₅ O ₁₂ is an important alternative to graphite anodes, owing to its excellent high temperature stability, high rate capability, and negligible volume change. Although surfaces with lithium compositions exceeding Li ₇ Ti ₅ O ₁₂ were observed previously during the first charge-discharge cycles, no stable reversible capacities were achieved during prolonged cycling. Here, structural engineering has been applied to enhance the electrochemical performance of epitaxial Li ₄ Ti ₅ O ₁₂ thin films as compared to polycrystalline samples. Variation in the crystal orientation of the Li ₄ Ti ₅ O ₁₂ thin films led to distinct differences in surface morphology with pyramidal, rooftop, or flat nanostructures for respectively (100), (110), and (111) orientations. High discharge capacities of 280-310 mAh·g ^-1 were achieved due to significant surface contributions in lithium storage. The lithiation mechanism of bulk Li ₄ Ti ₅ O ₁₂ thin films was analyzed by a phase-field model, which indicated the lithiation wave to be moving faster along the grain boundaries before moving inward to the bulk of the grains. The (100)-oriented Li ₄ Ti ₅ O ₁₂ films exhibited the highest capacities, the best rate performance up to 30C, and good cyclability, demonstrating enhanced cycle life and doubling of reversible capacities in contrast to previous polycrystalline studies.

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The development of safe and high-performance Li-metal anodes is crucial to meet the demanded increase in energy density of batteries. However, severe reactivity of Li metal with typical electrolytes and dendrite formation leads to a poor cycle life and safety concerns. Therefore, it is essential to develop electrolytes that passivate the reactivity toward Li metal and suppress dendrite formation. Carbonate electrolytes display severe reactivity toward Li metal; however, they are preferred above the more volatile ether-based electrolytes. Here, a carbonate electrolyte gel polymer approach is combined with LiNO₃ as an additive to stabilize Li-metal plating. This electrolyte design strategy is systematically monitored by operando neutron depth profiling (NDP) to follow the evolution of the plated Li-metal density and the inactive lithium in the solid electrolyte interface (SEI) during cycling. Individually, the application of the LiNO₃ electrolyte additive and the gel polymer approach are shown to be effective. Moreover, when used in conjunction, the effects are complementary in increasing the plated Li density, reducing inactive Li species, and reducing the overpotentials. The LiNO₃ additive leads to more compact plating; however, it results in a significant buildup of inactive Li species in a double-layer SEI structure, which challenges the cell performance over longer cycling. In contrast, the gel polymer strongly suppresses the buildup of inactive Li species by immobilizing the carbonate electrolyte species; however, the plating is less dense and occurs with a significant overpotential. Combining the LiNO₃ additive with the gel polymer approach results in a thin and homogeneous SEI with a high conductivity through the presence of Li₃N and a limited buildup of inactive Li species over cycling. Through this approach, even high plating capacities, reaching 7 mAh/cm², can be maintained at a high efficiency. The rational design strategy, empowered by monitoring the Li-density evolution, demonstrates the possibilities of achieving stable operation of Li metal in carbonate-based electrolytes.

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The commercial uptake of lithium-sulfur (Li-S) batteries is undermined by their rapid performance decay and short cycle life. These problems originate from the dissolution of lithium polysulfide in liquid electrolytes, causing charge and active material to shuttle between electrodes. The dynamics of intractable polysulfide migration at different length scales often tend to escape the probing ability of many analytical techniques. Spatial and temporal visualization of Li in Li-S electrodes and direct mechanistic understanding of how polysulfides are regulated across Li-S batteries starting from current collector and active layer coating to electrode-electrolyte interface are still lacking. To address this we employ neutron depth profiling across Li-S electrodes using the naturally occurring isotope, 6Li, which yields direct spatial information on Li-S electrochemistry. Using three types of Li-S electrodes, namely, carbon-sulfur, carbon-sulfur with 10% lithium titanium oxide (LTO), and carbon-sulfur with LTO membrane, we provide direct evidence for the migration, adsorption, and confinement of polysulfides in Li-S cells at work. Our findings further provide insights into the dynamics of polysulfide dissolution and re-utilization in relation to Li-S battery capacity and longevity to aid rational electrode designs toward high-energy, safe, and low-cost batteries.

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Accurate modeling of the internal battery resistance is imperative in predicting the state of charge and state of health. A mathematical model has been developed that, in addition to the ionic transport, introduces an accurate description of the electronic transport in the porous semiconducting LiFePO₄ electrodes. The model is based on the fundamental principles of electrochemistry, electrochemical kinetics, and semiconductor physics, combining them in an efficient model. This framework provides for the non-ohmic nature of semiconductor electrode materials and their current dependent conductivity. The model is validated by comparison with experimental data of Li-ion concentration profiles. It is demonstrated that the mass transport of the electrons, typically simplified or considered negligible in calculation models, have a significant influence on the electrode kinetics and therefore on the current dependent internal resistance of the battery. The accurate description of the internal resistance and the related heat production under various cycling conditions allows the design of safer battery electrode architectures. Additionally, the model allows optimization of the electrode components for various loading regime, increasing the effective energy density leading to decreasing demand for materials and costs. The present model, its principles, and methods are generally applicable and can be used for the description of the wide range of energy storage materials and systems where combined ion and electron transport takes place.

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Neutron Depth Profiling (NDP) allows determination of the spatial distribution of specific isotopes, via neutron capture reactions. In a capture reaction charged particles with fixed kinetic energy are formed, where their energy loss through the material of interest can be used to provide the depth of the original isotope. As lithium-6 has a relatively large probability for such a capture reaction, it can be used by battery scientists to study the lithium concentration in the electrodes even during battery operation. The selective measurement of the 6Li isotope makes it a direct and sensitive technique, whereas the penetrative character of the neutrons allows practical battery pouch cells to be studied. Using NDP lithium diffusion and reaction rates can be studied operando as a function of depth, opening a large range of opportunities including the study of alloying reactions, metal plating, and (de) intercalation in insertion hosts. In the study of high rate cycling of intercalation materials the relatively low Li density challenges counting statistics while the limited change in electrode density due to the Li-ion insertion and extraction allows straightforward determination of the Li density as a function of electrode depth. If an electrode can be (dis)charged reversibly, data can be acquired and accumulated over multiple cycles to increase the time resolution. For Li metal plating and alloying reactions, the large lithium density allows good time resolution, however the large change of the electrode composition and density makes extracting the Li-density as a function of depth more challenging. Here an effective method is presented, using calibration measurements of the individual components, based on which the ratio of the components as a function of depth can be determined as well as the total Li-density. The same principles can be applied to insertion host materials, where the differences in density due to electrolyte infiltration yield the electrode porosity as a function of depth. This is of particular importance for battery electrodes where porosity has a direct influence on the energy density and charge transport.@en

Electrical mobility demands an increase of battery energy density beyond current lithium-ion technology. A crucial bottleneck is the development of safe and reversible lithium-metal anodes, which is challenged by short circuits caused by lithium-metal dendrites and a short cycle life owing to the reactivity with electrolytes. The evolution of the lithium-metal-film morphology is relatively poorly understood because it is difficult to monitor lithium, in particular during battery operation. Here we employ operando neutron depth profiling as a noninvasive and versatile technique, complementary to microscopic techniques, providing the spatial distribution/density of lithium during plating and stripping. The evolution of the lithium-metal-density-profile is shown to depend on the current density, electrolyte composition and cycling history, and allows monitoring the amount and distribution of inactive lithium over cycling. A small amount of reversible lithium uptake in the copper current collector during plating and stripping is revealed, providing insights towards improved lithium-metal anodes.

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The original HTML version of this Article omitted to list Zhengcao Li as a corresponding author. Correspondingly, the original PDF version of this Article incorrectly stated that ‘Correspondence and requests for materials should be addressed to M.W. (email: m.wagemaker@tudelft.nl)’, instead of the correct ‘Correspondence and requests for materials should be addressed to Z.L. (email: zcli@tsinghua.edu.cn) or to M.W. (email: m.wagemaker@tudelft.nl)’. This has been corrected in both the PDF and HTML versions of the Article.

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One of the main goals in lithium ion battery electrode design is to increase the power density. This requires insight in the relation between the complex heterogeneous microstructure existing of active material, conductive additive and electrolyte providing the required electronic and Li-ion transport. FIB-SEM is used to determine the three phase 3D morphology, and Li-ion concentration profiles obtained with Neutron Depth Profiling (NDP) are compared for two cases, conventional LiFePO₄ electrodes and better performing carbonate templated LiFePO₄ electrodes. This provides detailed understanding of the impact of key parameters such as the tortuosity for electron and Li-ion transport though the electrodes. The created hierarchical pore network of the templated electrodes, containing micron sized pores, appears to be effective only at high rate charge where electrolyte depletion is hindering fast discharge. Surprisingly the carbonate templating method results in a better electronic conductive CB network, enhancing the activity of LiFePO₄ near the electrolyte-electrode interface as directly observed with NDP, which in a large part is responsible for the improved rate performance both during charge and discharge. The results demonstrate that standard electrodes have a far from optimal charge transport network and that significantly improved electrode performance should be possible by engineering the microstructure.

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