Abstract
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
0.8
Co
0.1
Mn
0.1
O
2
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.
All-solid-state Li-ion batteries promise safer electrochemical energy storage with larger volumetric and gravimetric energy densities. A major concern is the limited electrochemical stability of ...solid electrolytes and related detrimental electrochemical reactions, especially because of our restricted understanding. Here we demonstrate for the argyrodite-, garnet- and NASICON-type solid electrolytes that the favourable decomposition pathway is indirect rather than direct, via (de)lithiated states of the solid electrolyte, into the thermodynamically stable decomposition products. The consequence is that the electrochemical stability window of the solid electrolyte is notably larger than predicted for direct decomposition, rationalizing the observed stability window. The observed argyrodite metastable (de)lithiated solid electrolyte phases contribute to the (ir)reversible cycling capacity of all-solid-state batteries, in addition to the contribution of the decomposition products, comprehensively explaining solid electrolyte redox activity. The fundamental nature of the proposed mechanism suggests this is a key aspect for solid electrolytes in general, guiding interface and material design for all-solid-state batteries.
Nanosizing is a frequently applied strategy in recent years to improve storage properties of Li-ion electrodes and facilitate novel storage mechanisms. Due to particle size reduction, surface effects ...increasingly dominate, which can drastically change the storage properties. Using density functional theory calculations we investigate the impact of the surface environment on the Li-ion insertion properties in defective spinel Li4+x Ti5O12, a highly promising negative electrode material. The calculations reveal that the storage properties strongly depend on the surface orientation. The lowest energy (1 1 0) surface is predicted to be energetically favorable for Li-ion insertion into the vacant 16c sites. The (1 1 1) surface allows capacities that significantly exceed the bulk capacity Li7Ti5O12 at voltages greater than 0 V by occupation of 8a sites in addition to the fully occupied 16c sites. One of the key findings is that the surface environment extends nanometers into the storage material, leading to a distribution of voltages responsible for the curved voltage profile commonly observed in nanosized insertion electrode materials. Both the calculated surface-specific voltage profiles and the calculated particle size dependent voltage profiles are in good agreement with the experimental voltage profiles reported in literature. These results give a unique insight into the impact of nanostructuring and further possibilities of tailoring the Li-ion voltage profiles and capacities in lithium insertion materials.
The substantial influence of crystallite size on the properties of Li-ion storage materials has spurred intensive research in the emerging area of nanoionics. The development of nanoscale storage ...materials offers a promising strategy to increase the energy storage capabilities of Li-ion batteries, potentially making them suitable for electric vehicles. Nanosizing, which increases surface area, enhances the importance of interfaces and surfaces on directly observable materials properties such as the voltage profile and the phase diagram. As a result, nanosized materials can show improved storage properties, and materials inactive at the micro size can become excellent storage materials. We suggest novel surface storage mechanisms to explain these phenomena. First-order phase transitions, which are responsible for the batteries’ constant voltage output, are partially suppressed at the nanoscale. So far the morphological changes during the phase transition remain unclear. A complete understanding of the equilibrium and non-equilibrium properties of a collection of nanosized electrode particles within an actual electrode remains a formidable challenge. In this Account, we describe the efforts toward understanding the effects of nanosizing and its applications in representative insertion materials. We are particularly interested in the mechanisms and properties that will help to increase the energy storage of Li-ion batteries. We review and discuss the nanosize properties of lithium insertion materials, olivine LiFePO4, and titanium oxides. Although nanosizing intrinsically destabilizes materials, which is potentially detrimental for battery performance, the relative stability of oxide and phosphate insertion compounds makes it possible to exploit the advantages of nanosizing in these materials. The larger capacities and typical voltage profiles in nanosized materials appear to be related to the surface and interface properties that become pronounced at the nanosize, providing a potential means of tailoring the material properties by particle size and shape. The large irreversible capacity at the surface of some materials such as titanium oxides represents a disadvantage of nanosizing, but research is suggesting ways to resolve this problem. The changes in the first-order phase transition upon (de)lithiation could be related to the interface between the coexisting phases. At these interfaces, concentration gradients and strain lead to energy penalties, which significantly influence the thermodynamics of nanomaterial grains. However, it is less clear what nanoscaling effects predominate in the large collection of particles in actual electrodes. The complexity of these materials at the nanoscale and the difficulty in observing them in situ pose additional challenges. Future demands for stored electricity will require significant research progress in both nanomaterials synthesis and in situ monitoring.
Sodium-ion batteries have captured widespread attention for grid-scale energy storage owing to the natural abundance of sodium. The performance of such batteries is limited by available electrode ...materials, especially for sodium-ion layered oxides, motivating the exploration of high compositional diversity. How the composition determines the structural chemistry is decisive for the electrochemical performance but very challenging to predict, especially for complex compositions. We introduce the "cationic potential" that captures the key interactions of layered materials and makes it possible to predict the stacking structures. This is demonstrated through the rational design and preparation of layered electrode materials with improved performance. As the stacking structure determines the functional properties, this methodology offers a solution toward the design of alkali metal layered oxides.
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.
Recent developments of the five important in situ/operando characterization categories for lithium battery research are summarized, including X‐ray, electron, neutron, optical, and scanning probe techniques. For each technique, the operating principles and in situ cell design are described in detail, including representative studies of typical electrode materials and related processes summarized in tables for easy comparison and cross reference.
Using density functional theory molecular dynamics simulations, the origin of the Li-ion conductivity in argyrodite solid electrolytes is investigated. The simulations show that besides Li-ion ...vacancies in Li6PS5Cl and Li6PS5Br, the influence of halogen atoms on their local surroundings also plays an important role in Li-ion diffusion. The difference in Li-ion conductivity between Li6PS5Cl and Li6PS5I, which is several orders of magnitude, is caused by the distribution of the halogen ions over the available crystallographic sites. This suggests that altering the halogen distribution in Li argyrodites during synthesis could increase the Li-ion conductivity of these materials. For Li6PS5Cl, the simulations predict an optimal Cl distribution of 1:3 over sites 4a and 4c, resulting in a Li-ion conductivity that is 2 times larger than that of the currently prepared materials. On the basis of these results, simulations were performed on Li5PS4X2 (X = Cl, Br, or I), which show Li-ion conductivities similar to those of Li6PS5Cl and Li6PS5Br, suggesting that the Li5PS4X2 compounds are interesting new compositions for solid state electrolytes.
Solid-state batteries potentially offer increased lithium-ion battery energy density and safety as required for large-scale production of electrical vehicles. One of the key challenges toward ...high-performance solid-state batteries is the large impedance posed by the electrode-electrolyte interface. However, direct assessment of the lithium-ion transport across realistic electrode-electrolyte interfaces is tedious. Here we report two-dimensional lithium-ion exchange NMR accessing the spontaneous lithium-ion transport, providing insight on the influence of electrode preparation and battery cycling on the lithium-ion transport over the interface between an argyrodite solid-electrolyte and a sulfide electrode. Interfacial conductivity is shown to depend strongly on the preparation method and demonstrated to drop dramatically after a few electrochemical (dis)charge cycles due to both losses in interfacial contact and increased diffusional barriers. The reported exchange NMR facilitates non-invasive and selective measurement of lithium-ion interfacial transport, providing insight that can guide the electrolyte-electrode interface design for future all-solid-state batteries.
Density functional theory (DFT) molecular dynamics (MD)-simulations were performed on cubic and tetragonal Na3PS4. The MD simulations show that the Na-conductivity based on the predicted ...self-diffusion is high in both the cubic and tetragonal phases. Higher Na-ion conductivity in Na3PS4 can be obtained by introducing Na-ion vacancies. Just 2% vacancies result in a conductivity of 0.2 S/cm, which is an order of magnitude larger than the calculated conductivity of the stoichiometric compound. MD simulations of halogen-doped cubic Na3PS4 suggest a practical route to introduce vacancies, where Br-doping is predicted to result in the highest bulk conductivity. Detailed investigation of the Na-ion transitions during the MD simulation reveals the role of vacancies and phonons in the diffusion mechanism. Furthermore, the orders of magnitude difference between the MD simulations and experiments suggest that macroscopic conductivity can be significantly increased by reducing the grain boundary resistance.
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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