Lithium metal with its high theoretical capacity and low negative potential is considered one of the most important candidates to raise the energy density of all-solid-state batteries. However, lithium filament growth and its induced solid electrolyte decomposition pose severe challenges to realize a long cycle life. Here, dendrite growth in solid-state Li metal batteries is alleviated by introducing a high dielectric material, barium titanate, as a filler that removes the electric field gradients that catalyze dendrite formation. In symmetrical Li-metal cells, this results in a very small over-potential of only 48 mV at a relatively high current density of 1 mA cm⁻², when cycling a capacity of 2 mA h cm⁻² during 1700 h. The high dielectric filler improves the Coulombic efficiency and cycle life of full cells and suppresses electrolyte decomposition as indicated by solid-state nuclear magnetic resonance (NMR) and X-ray photoelectron spectroscopy (XPS) measurements. This indicates that the high dielectric filler can suppress dendrite formation, thereby reducing solid electrolyte decomposition reactions, resulting in the observed low overpotentials and improved cycling efficiency.

The original version of this article contained errors in Figure 3a and Figure 3f. In Figure 3a, the activation energies (Ea) were calculated using a log scale instead of a logarithm ln scale. In Figure 3f, the y-axis interval was not properly selected. The correct y-axis interval in Figure 3f and the numerical values of the activation energy are now provided in Figure 3a and the main text. These errors have been corrected in the HTML and PDF versions of the article.

Improving the reversibility of lithium metal batteries is one of the challenges in current battery research. This requires better fundamental understanding of the evolution of the lithium deposition morphology, which is very complex due to the various parameters involved in different systems. Here, we clarify the fundamental origins of lithium deposition coverage in achieving highly reversible and compact lithium deposits, providing a comprehensive picture in the relationship between the lithium microstructure and solid electrolyte interphase (SEI) for lithium metal batteries. Systematic variation of the salt concentration offers a framework that brings forward the different aspects that play a role in cycling reversibility. Higher nucleation densities are formed in lower concentration electrolytes, which have the advantage of higher lithium deposition coverage; however, it goes along with the formation of an organic-rich instable SEI which is unfavorable for the reversibility during (dis)charging. On the other hand, the growth of large deposits benefiting from the formation of an inorganic-rich stable SEI is observed in higher concentration electrolytes, but the initial small nucleation density prevents full coverage of the current collector, thus compromising the plated lithium metal density. Taking advantages of the paradox, a nanostructured substrate is rationally applied, which increases the nucleation density realizing a higher deposition coverage and thus more compact plating at intermediate concentration (∼1.0 M) electrolytes, leading to extended reversible cycling of batteries.

The development of commercial solid-state batteries has to date been hindered by the individual limitations of inorganic and organic solid electrolytes, motivating hybrid concepts. However, the room-temperature conductivity of hybrid solid electrolytes is still insufficient to support the required battery performance. A key challenge is to assess the Li-ion transport over the inorganic and organic interfaces and relate this to surface chemistry. Here we study the interphase structure and the Li-ion transport across the interface of hybrid solid electrolytes using solid-state nuclear magnetic resonance spectroscopy. In a hybrid solid polyethylene oxide polymer–inorganic electrolyte, we introduce two representative types of ionic liquid that have different miscibilities with the polymer. The poorly miscible ionic liquid wets the polymer–inorganic interface and increases the local polarizability. This lowers the diffusional barrier, resulting in an overall room-temperature conductivity of 2.47 × 10⁻⁴ S cm⁻¹. A critical current density of 0.25 mA cm⁻² versus a Li-metal anode shows improved stability, allowing cycling of a LiFePO₄–Li-metal solid-state cell at room temperature with a Coulombic efficiency of 99.9%. Tailoring the local interface environment between the inorganic and organic solid electrolyte components in hybrid solid electrolytes seems to be a viable route towards designing highly conducting hybrid solid electrolytes.

ConspectusDriven by the intrinsic safety and potential to achieve higher energy densities, solid-state Li-metal batteries are intensively researched. The ideal solid electrolyte should possess a high conductivity, should have electrochemical stability both toward the Li-metal anode and to high voltage cathodes, should suppress dendrites, should provide flexibility to deal with the volumetric changes of the electrodes, and should be easy to process. This challenging combination is to date not fulfilled by any solid electrolyte, be it organic, inorganic, or even a hybrid of the two. Pushing the development of solid electrolytes toward reversible room temperature operation when used in tandem with Li-metal anodes demands an understanding of critical processes that determine the properties of the solid electrolyte. These include the complex Li-ion transport as well as the Li-metal plating processes. This already presents the first experimental hurdle as the ability to directly and noninvasively monitor the Li-ion kinetics, Li densities, and Li chemistries, under in/situ or operando, is not trivial.The scope of this Account is the investigation and improvement of solid electrolytes, with the emphasis on the possibilities offered by solid-state NMR and neutron depth profiling as direct probes for the study of critical processes that involve Li ions and Li metal. Solid-state NMR allows us to unravel the complex interface chemical environment and the diffusion processes both in the bulk solid electrolyte and in the interface environment. These studies shed light on the role of interface composition, wetting and space-charge layers, on the macroscopic battery performance. Another technique that enables probing Li directly is operando neutron depth profiling, which allows us to determine the Li density as a function of depth. It provides a noninvasive and effectively nondestructive tool to examine delamination, irreversible reactions and dendrite formation during plating/stripping. Results demonstrate that it is very challenging to maintain the contact between Li metal and the SE during cycling, especially for the "anode-less"or "anode-free"configuration under low-pressure conditions. A perspective is provided on the potential improvement of the Li-ion transport, dendrite suppression, and preventing Li-metal-solid-electrolyte delamination as well as on the potential role of solid-state NMR and NDP techniques to guide these developments.

A key challenge for solid-state-batteries development is to design electrode-electrolyte interfaces that combine (electro)chemical and mechanical stability with facile Li-ion transport. However, while the solid-electrolyte/electrode interfacial area should be maximized to facilitate the transport of high electrical currents on the one hand, on the other hand, this area should be minimized to reduce the parasitic interfacial reactions and promote the overall cell stability. To improve these aspects simultaneously, we report the use of an interfacial inorganic coating and the study of its impact on the local Li-ion transport over the grain boundaries. Via exchange-NMR measurements, we quantify the equilibrium between the various phases present at the interface between an S-based positive electrode and an inorganic solid-electrolyte. We also demonstrate the beneficial effect of the LiI coating on the all-solid-state cell performances, which leads to efficient sulfur activation and prevention of solid-electrolyte decomposition. Finally, we report 200 cycles with a stable capacity of around 600 mAh g⁻¹ at 0.264 mA cm⁻² for a full lab-scale cell comprising of LiI-coated Li₂S-based cathode, Li-In alloy anode and Li₆PS₅Cl solid electrolyte.

Solid-state lithium-metal batteries are considered to be promising candidates for next-generation high-energy density storage devices to power electrical vehicles. Critical challenges for solid-state lithium-metal batteries include the large morphological changes associated with the plating and stripping of lithium metal and decomposition of the solid electrolyte, because of the reductive nature of the lithium metal, both increasing the lithium metal-solid electrolyte interface resistance. This is especially challenging when starting in the discharged state with a bare anode or "anode-less"current collector facing the solid electrolyte. To overcome this, a 100-nm thin layer of ZnO is deposited on the copper current collector with atomic layer deposition (ALD). During the first charge, this results in more homogeneous lithium-metal growth, rationalized by the formation of a Zn-Li alloy that acts as seed crystals for the lithium metal. The resulting more homogeneous lithium-metal growth maintains better contact with the solid electrolyte, leading to more reversible cycling of lithium metal. Minor prelithiating of the ZnO/Cu anode with 1 mAh/cm2 further improves the cycling performance, as demonstrated in a full all-solid-state cell using LiFePO4 as a cathode, resulting in an average Coulombic efficiency of >95%. These findings mark the first steps in an interface strategy to overcome the challenges at the solid electrolyte/lithium-metal interface in solid-state lithium-metal batteries.

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.

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.

The influence of space-charge layers on the ionic charge transport over cathode-solid electrolyte interfaces in all-solid-state batteries remains unclear because of the difficulty to unravel it from other contributions to the ion transport over the interfaces. Here, we reveal the effect of the space-charge layers by systematically tuning the space-charge layer on and off between Li _xV ₂O ₅ and Li _1.5Al _0.5Ge _1.5(PO ₃) ₄ (LAGP), by changing the Li _xV ₂O ₅ potential and selectively measuring the ion transport over the interface by two-dimensional (2D) NMR exchange. The activation energy is demonstrated to be 0.315 eV for lithium-ion exchange over the space-charge-free interface, which increases dramatically to 0.515 eV for the interface with a space-charge layer. Comparison with a space-charge model indicates that the charge distribution due to the space-charge layer is responsible for the increased interface resistance. Thereby, the present work provides selective and quantitative insight into the effect of space-charge layers over electrode-electrolyte interfaces on ionic transport.

Layered Na-based oxides with the general composition of NaxTMO2 (TM: transition metal) have attracted significant attention for their high compositional diversity that provides tunable electrochemical performance for electrodes in sodium-ion batteries. The various compositions bring forward complex structural chemistry that is decisive for the layered stacking structure, Na-ion conductivity, and the redox activity, potentially promising new avenues in functional material properties. In this work, we have explored the maximum Na content in P2-type layered oxides and discovered that the high-content Na in the host enhances the structural stability; moreover, it promotes the oxidation of low-valent cations to their high oxidation states (in this case Ni2+). This can be rationalized by the increased hybridization of the O(2p)-TM(3d-eg*) states, affecting both the local TM environment as well as the interactions between the NaO2 and TMO2 layers. These properties are highly beneficial for the Na storage capabilities as required for cathode materials in sodium-ion batteries. It leads to excellent Na-ion mobility, a large storage capacity (>100 mAh g-1 between 2.0-4.0 V), yet preventing the detrimental sliding of the TMO2 layers (P2-O2 structural transition), as reflected by the ultralong cycle life (3000 (dis)charge cycles demonstrated). These findings expand the horizons of high Na-content P2-type materials, providing new insights of the electronic and structural chemistry for advanced cathode materials.

Suppressing the dendrite formation and managing the volume change of lithium (Li) metal anode have been global challenges in the lithium batteries community. Herein, a duplex copper (Cu) foil with an ant-nest-like network and a dense substrate is reported for an ultrastable Li metal anode. The duplex Cu is fabricated by sulfurization of thick Cu foil with a subsequent skeleton self-welding procedure. Uniform Li deposition is achieved by the 3D interconnected architecture and lithiophilic surface of self-welded Cu skeleton. The sufficient space in the porous layer enables a large areal capacity for Li and significantly improves the electrode–electrolyte interface. Simulations reveal that the structure allows proper electric field penetration into the connected tunnels. The assembled Li anodes exhibit high coulombic efficiency (97.3% over 300 cycles) and long lifespan (>880 h) at a current density of 1 mA cm⁻² with a capacity of 1 mAh cm⁻². Stable and deep cycling can be maintained up to 50 times at a high capacity of 10 mAh cm⁻².

Surface degradation is a common challenge for many electrode materials. The active surface usually reacts with the molecules in the surrounding environment to form byproducts that hinder the diffusion channels for Li ions and electrons, increase the energy barrier for (de)lithiation reactions, and ultimately shorten the cycle life. Herein, the growth of surface Li ₂ CO ₃ on LiNi _x Co _y Mn _y O ₂ (x = 0.33, 0.6, 0.7, 0.8, x + 2y = 1) cathodes upon storage has been systematically investigated. Ni-rich surfaces are found to result in more Li ₂ CO ₃ growth, based on which three discrete degradation models for layered oxides are proposed. The increase and discretization of the energy barrier for individual particles also explain the State-of-Charge heterogeneity phenomena observed by in situ XRD and the change of cyclic voltammetry curves. By providing a comprehensive picture of surface deterioration of the NCM cathode family, this study enhances the understanding of the degradation mechanism that determines the cycle life of electrode materials.

The increasing demands of energy storage require the significant improvement of current Li-ion battery electrode materials and the development of advanced electrode materials. Thus, it is necessary to gain an in-depth understanding of the reaction processes, degradation mechanism, and thermal decomposition mechanisms under realistic operation conditions. This understanding can be obtained by in situ/operando characterization techniques, which provide information on the structure evolution, redox mechanism, solid-electrolyte interphase (SEI) formation, side reactions, and Li-ion transport properties under operating conditions. Here, the recent developments in the in situ/operando techniques employed for the investigation of the structural stability, dynamic properties, chemical environment changes, and morphological evolution are described and summarized. The experimental approaches reviewed here include X-ray, electron, neutron, optical, and scanning probes. The experimental methods and operating principles, especially the in situ cell designs, are described in detail. Representative studies of the in situ/operando techniques are summarized, and finally the major current challenges and future opportunities are discussed. Several important battery challenges are likely to benefit from these in situ/operando techniques, including the inhomogeneous reactions of high-energy-density cathodes, the development of safe and reversible Li metal plating, and the development of stable SEI.

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.

In common hybrid solid electrolytes (HSEs), either the ionic conductivity of the polymer electrolyte is enhanced by the presence of a nanosized inorganic filler, which effectively decrease the glass-transition temperature, or the polymer solid electrolyte acts mostly as a flexible host for the inorganic solid electrolyte, the latter providing the conductivity. Here a true HSE is developed that makes optimal use of the high conductivity of the inorganic solid electrolyte and the flexibility of the polymer matrix. It is demonstrated that the LAGP (Li_1.5Al_0.5Ge_1.5(PO₄)₃) participates in the overall conductivity and that the interface environment between the poly(ethylene oxide) (PEO) and LAGP plays a key role in utilizing the high conductivity of the LAGP. This HSE demonstrates promising cycling versus Li-metal anodes and in a full Li-metal solid-state battery. This strategy offers a promising route for the development of Li-metal solid-state batteries, aiming for safe and reversible high-energy-density batteries.