A promising cathode material for rechargeable batteries is LiMn2O4, which exhibits higher operating voltage, reduced toxicity and lower costs as compared to commonly used LiCoO2 cathodes. However, LiMn2O4 suffers from limited cycle life, as excessive capacity fading occurs during battery cycling due to dissolution of Mn into the acidic electrolyte. Here, we show that by structural engineering of stable, epitaxial LiMn2O4 thin films the electrochemical properties can be enhanced as compared to polycrystalline samples. Control of the specific crystal orientation of the LiMn2O4 thin films resulted in dramatic differences in surface morphology with pyramidal, rooftop or flat features for respectively (100), (110), and (111) orientations. All three types of LiMn2O4 films expose predominantly ⟨111⟩ crystal facets, which is the lowest energy state surface for this spinel structure. The (100)-oriented LiMn2O4 films exhibited the highest capacities and (dis)charging rates up to 33C, and good cyclability over a thousand cycles, demonstrating enhanced cycle life without excessive capacity fading as compared to previous polycrystalline studies.
Despite the lower gravimetric capacity, Li4Ti5O12 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 Li7Ti5O12 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 Li4Ti5O12 thin films as compared to polycrystalline samples. Variation in the crystal orientation of the Li4Ti5O12 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 Li4Ti5O12 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 Li4Ti5O12 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.
Stability issues in thermoelectric Na x CoO 2 thin films have been solved by the addition of an in situ amorphous AlO x capping layer, which prevents previously reported degradation when exposed to air. These chemically stable thin films enable detailed analysis of the intrinsic thermoelectric properties and form a significant progress towards applications. Single phase Na x CoO 2 thin films with a low surface roughness are grown by pulsed laser deposition with either an epitaxial or textured crystal structure on Al 2 O 3 (001) or LaAlO 3 (001) single crystal substrates, respectively. For textured thin films a resistivity and thermopower of 0.99 mV cm and 69 mV K 21 are observed at room temperature, respectively. Based on an estimated thermal conductivity for thin films, the dimensionless figure of merit is very comparable to Na x CoO 2 single crystals, demonstrating the effectiveness of the developed capping layer.
Halide‐based solid electrolytes are currently growing in interest in solid‐state batteries due to their high electrochemical stability window compared to sulfide electrolytes. However, often a bilayer separator of a sulfide and a halide is used and it is unclear why such setup is necessary, besides the instability of the halides against lithium metal. It is shown that an electrolyte bilayer improves the capacity retention as it suppresses interfacial resistance growth monitored by impedance spectroscopy. By using in‐depth analytical characterization of buried interphases by time‐of‐flight secondary ion mass spectrometry and focused ion beam scanning electron microscopy analyses, an indium‐sulfide rich region is detected at the halide and sulfide contact area, visualizing the chemical incompatibility of these two electrolytes. The results highlight the need to consider more than just the electrochemical stability of electrolyte materials, showing that chemical compatibility of all components may be paramount when using halide‐based solid electrolytes in solid‐state batteries.
Layered lithium transition-metal oxides, such as LiCoO 2 and its doped and lithium-rich analogues, have become the most attractive cathode material for current lithium-ion batteries due to their excellent power and energy densities. However, parasitic reactions at the cathode–electrolyte interface, such as metal-ion dissolution and electrolyte degradation, instigate major safety and performance issues. Although metal oxide coatings can enhance the chemical and structural stability, their insulating nature and lattice mismatch with the adjacent cathode material can act as a physical barrier for ion transport, which increases the charge-transfer resistance across the interface and impedes cell performance at high rates. Here, epitaxial engineering is applied to stabilize a cubic (100)-oriented TiO layer on top of single (104)-oriented LiCoO 2 thin films to study the effect of a conductive coating on the electrochemical performance. Lattice matching between the (104) LiCoO 2 surface facets and the (100) TiO plane enables the formation of the titanium mono-oxide phase, which dramatically enhances the cycling stability as well as the rate capability of LiCoO 2 . This cubic TiO coating enhances the preservation of the phase and structural stability across the (104) LiCoO 2 surface. The results suggest a more stable Co 3+ oxidation state, which not only limits the cobalt-ion dissolution into the electrolyte but also suppresses the catalytic degradation of the liquid electrolyte. Furthermore, the high c-rate performance combined with high Columbic efficiency indicates that interstitial sites in the cubic TiO lattice offer facile pathways for fast lithium-ion transport.
Their suggested stability towards high-voltage cathode materials makes halide-based solid electrolytes currently an interesting class of ionic conductors for solid-state batteries. Especially the LiMn 2 O 4 spinel cathode active material is of interest due to its slightly higher nominal voltage and more resilience to overcharging compared to LiCoO 2 and LiNi x Mn y Co z O 2 cathodes. Typically, a standard ratio of active material to solid electrolyte is used in composites for solid-state batteries. However, for ideal transport properties, and thus to achieve balanced and optimal partial-conductivities, this ratio needs to be reoptimized each time the material basis is changed. In this work, we show transport in the composite measured through both DC polarization as well as transmission line modeling of the impedance spectra. By balancing the partial transport parameters of the composite, an optimum capacity of the solid-state batteries is achieved. This work shows characterization and optimization of transport is required for unlocking the full potential of solid-state batteries.
Ceramic-based nanocomposites are a rapidly evolving research area as they are currently being used in a wide range of applications. Epitaxial vertically aligned nanocomposites (VANs) offer promising advantages over conventional planar multilayers as key functionalities are tailored by the strong coupling at their vertical interfaces. However, limited knowledge exists of which material systems are compatible in composite films and which types of structures are optimal for a given functionality. No lithium-based VANs have yet been explored for energy storage, while 3D solid-state batteries offer great promise for enhanced energy and power densities. Although solid-on-solid kinetic Monte Carlo simulation (KMCS) models of VAN growth have previously been developed, phase separation was forced into the systems by limiting hopping directions and/or tuning the activation energies for hopping. Here, we study the influence of the temperature and deposition rate on the morphology evolution of lithium-based VANs, consisting of a promising LiMn2O4 cathode and a Li0.5La0.5TiO3 electrolyte, by applying a KMCS model with activation energies for hopping obtained experimentally and with minimum restrictions for hopping directions. Although the model considers only the kinetic processes away from thermodynamic equilibrium, which would determine the final shape of the pillars within the matrix, the trends in pillar size and distribution within the simulated VANs are in good agreement with experiments. This provides an elegant tool to predict the growth of VAN materials as the experimental activation energies and higher degrees of freedom for hopping result in a more realistic and low computational cost model to obtain accurate simulations of VAN materials.
Since their introduction in the 1990s, lithium ion (Li-ion) batteries have become the main power source for portable electronics and power tools applications. As society transitions towards electric and zero emission mobility, next generation electric cars require lithium batteries with superior energy and power density (respectively ℎ • −3 and • −3), without compromising safety and environmental concerns [1,2]. Also for stationary applications (such as grid stabilization and uninterruptable power supplies) lithium batteries become more popular due to their high energy and power density [3]. Energy density is the amount of energy (usually Wh) a certain volume or weight contains. Power density is the rate of energy flow a volume is capable of. An example of power density differences can be given by how fast a battery can be charged or discharged. At high power densities a car or electric motorcycle can be quickly charged or can have high power outputs and quickly accelerate. Two examples are shown in figure 1. Figure 1, A popular electric car brand concept car and an electric motorcycle both exceeding speeds above 200 km/h. These high speeds are possible through the high power density and therefore output of their respective batteries.
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