Spectrum-Dependent Spiro-OMeTAD Oxidization Mechanism in Perovskite Solar Cells

We propose a spectrum-dependent mechanism for the oxidation of 2,2′,7,7′-tetrakis­(<i>N</i>,<i>N</i>-di-<i>p</i>-methoxyphenylamine)-9,9′-spirobifluorene (Spiro-OMeTAD) with bis­(trifluoromethane)­sulfonimide lithium salt (LiTFSI), which is commonly used in perovskite solar cells as the hole transport layer. The perovskite layer plays different roles in the Spiro-OMeTAD oxidization for various spectral ranges. The effect of oxidized Spiro-OMeTAD on the solar cell performance was observed and characterized. With the initial long-wavelength illumination (>450 nm), the charge recombination at the TiO₂/Spiro-OMeTAD interface was increased due to the higher amount of the oxidized Spiro-OMeTAD. On the other hand, the increased conductivity of the Spiro-OMeTAD layer and enhanced charge transfer at the Au/Spiro-OMeTAD interface facilitated the solar cell performance

Engineering Three-Dimensionally Electrodeposited Si-on-Ni Inverse Opal Structure for High Volumetric Capacity Li-Ion Microbattery Anode

Aiming at improving the volumetric capacity of nanostructured Li-ion battery anode, an electrodeposited Si-on-Ni inverse opal structure has been proposed in the present work. This type of electrode provides three-dimensional bi-continuous pathways for ion/electron transport and high surface area-to-volume ratios, and thus exhibits lower interfacial resistance, but higher effective Li ions diffusion coefficients, when compared to the Si-on-Ni nanocable array electrode of the same active material mass. As a result, improved volumetric capacities and rate capabilities have been demonstrated in the Si-on-Ni inverse opal anode. We also show that optimization of the volumetric capacities and the rate performance of the inverse opal electrode can be realized by manipulating the pore size of the Ni scaffold and the thickness of the Si deposit

Improved Rate Capability for Dry Thick Electrodes through Finite Elements Method and Machine Learning Coupling

A coupled finite elements method (FEM) and machine learning (ML) workflow is presented to optimize the rate capability of thick positive electrodes (ca. 150 μm and 8 mAh/cm2). An ML model is trained based on the geometrical observables of individual LiNi0.8Mn0.1Co0.1O2 particles and their average state of discharge (SOD) predicted from FEM modeling. This model not only bypasses lengthy FEM simulations but also provides deeper insights on the importance of pore tortuosity and the active particle size, identified as the limiting phenomenon during the discharge. Based on these findings, a bilayer configuration is proposed to tackle the identified limiting factors for the rate capability. The benefits of this structured electrode are validated through FEM by comparing its performance to a pristine monolayer electrode. Finally, experimental validation using dry processing demonstrates a 40% higher volumetric capacity of the bilayer electrode when compared to the previously reported thick NMC electrodes

Modified Coprecipitation Synthesis of Mesostructure-Controlled Li-Rich Layered Oxides for Minimizing Voltage Degradation

Modified carbonate coprecipitation synthesis without addition of chelating agent is introduced to obtain mesostructure-controlled Li-rich layered oxides. The designed mesostructure for target material Li_1.2Ni_0.2Mn_0.6O₂ has uniformly dispersed spherical secondary particles with size around 3 μm. These micrometer-sized particles consist of self-assembled crystallites with size of ∼150 nm. This unique design not only decreases the surface area compared with the sample with dispersive particles, but also increases overall structural mechanical stability compared with the sample with larger dense secondary particles as observed by transmission X-ray microscope. As a result, the voltage decay and capacity loss during long-term cycling have been minimized to a large extent. Our findings clearly demonstrate that mesostructure design of Li-rich layered oxides play a key role in optimizing this class of cathode materials. Surprisingly, the voltage fading issue can be partially mitigated by such an approach

Effect of Surface Modification on Nano-Structured LiNi_0.5Mn_1.5O₄ Spinel Materials

Fine-tuning of particle size and morphology has been shown to result in differential material performance in the area of secondary lithium-ion batteries. For instance, reduction of particle size to the nanoregime typically leads to better transport of electrochemically active species by increasing the amount of reaction sites as a result of higher electrode surface area. The spinel-phase oxide LiNi_0.5Mn_1.5O₄ (LNMO), was prepared using a sol–gel based template synthesis to yield nanowire morphology without any additional binders or electronic conducting agents. Therefore, proper experimentation of the nanosize effect can be achieved in this study. The spinel phase LMNO is a high energy electrode material currently being explored for use in lithium-ion batteries, with a specific capacity of 146 mAh/g and high-voltage plateau at ∼4.7 V (vs Li/Li⁺). However, research has shown that extensive electrolyte decomposition and the formation of a surface passivation layer results when LMNO is implemented as a cathode in electrochemical cells. As a result of the high surface area associated with nanosized particles, manganese ion dissolution results in capacity fading over prolonged cycling. In order to prevent these detrimental effects without compromising electrochemical performance, various coating methods have been explored. In this work, TiO₂ and Al₂O₃ thin films were deposited using atomic layer deposition (ALD) on the surface of LNMO particles. This resulted in effective surface protection by prevention of electrolyte side reactions and a sharp reduction in resistance at the electrode/electrolyte interface region

Nanoscale Compositional Mapping of Commercial LiNi_0.8Co_0.15Al_0.05O₂ Cathodes Using Atom Probe Tomography

Nickel-rich cathodes provide improved specific capacity, which leads to higher gravimetric energy density, which, in turn, is critical for electric vehicles. However, poor long-term capacity retention at elevated temperatures/high C rates (the rate of charge and discharge on a battery) stems from material issues: surface phase changes, corrosive side reactions with the electrolyte, ion dissolution, and propagation of cracks. Introducing dopants, developing nanoscale surface coatings, and graded core–shell structures all improved the electrochemical performance of nickel-rich cathodes. However, material-level understanding of the effect of Li composition and distribution in Ni-rich cathodes is limited due to a lack of characterization methods available that can directly image Li at the nanoscale. Hence, it is critical to establish methods such as atom probe tomography (APT) that have both nanometer-scale spatial resolution and high compositional sensitivity to quantitatively analyze battery cathodes. To fully realize its potential as a method for quantitative compositional analysis of commercial Li-ion batteries, we provide a comprehensive description of the challenges in sample preparation and analyze the dependency of the analysis parameters, specifically laser pulse energy on the measured stoichiometry of elements in a high-Ni-content cathode material LiNi0.8Co0.15Al0.05O2 (NCA). Our findings show that the stoichiometry variations cannot be explained by charge–state ratios or Ga implantation damage alone during FIB preparation, indicating that additional factors such as crystallographic orientation may need to be considered to achieve quantitative nanoscale compositional analysis of such battery cathodes using APT

Key Parameters in Determining the Reactivity of Lithium Metal Battery

Lithium metal anodes are crucial for high-energy-density batteries, but concerns regarding their safety remain. Limited investigations have evaluated the reactivity of Li metal anodes in full cell configurations. In this study, differential scanning calorimetry (DSC) and in situ Fourier-transform infrared spectroscopy (FTIR) were employed to quantitatively examine the Li metal reactivity. Lithiated graphite (Li-Gr) and lithiated silicon (Li-Si) were also compared. The reactivity of plated Li was systematically investigated when combined with different electrolyte compositions, morphologies, atmospheres, and various cathode materials (NMC622, LFP, and LNMO). It was discovered that all cell components, such as electrolyte composition, Li morphology, control of inactive Li accumulation, and cathode stability, play essential roles in regulating the reactivity of the plated Li. By optimizing these factors, the Li metal full cell exhibited no significant thermal reaction up to 400 °C. This research identifies key parameters for controlling Li metal reactivity, potentially advancing lithium metal battery design and manufacturing

Deciphering the Impact of the Active Lithium Reservoir in Anode-Free Pouch Cells

Anode-free batteries, which revolutionize energy storage by discarding traditional anodes in favor of copper foil to plate lithium directly from the cathode, offer increased energy densities and better safety than conventional lithium metal cells. However, their advantage is tempered by a significantly reduced cycle life, attributed to lithium loss through parasitic reactions. Comprehending the lithium inventory evolution under diverse conditions is vital for developing strategies to improve their performance. Herein, using coulometry, titration gas chromatography, and cryogenic scanning electron microscopy, we analyze the evolution of active and inactive lithium in NMC622||Cu pouch cells. Our results reveal the mechanism of lithium reservoir formation due to cathode irreversibility and its subsequent impact on cell performance across various electrolytes. Our findings not only highlight the significance of the lithium reservoir for anode-free cells’ cycle life but also explore its modulation through charge/discharge rates, offering new opportunities to understand and enhance anode-free cells’ cycle life

Lithium Lanthanum Titanium Oxides: A Fast Ionic Conductive Coating for Lithium-Ion Battery Cathodes

This work introduces Li–La–Ti–O (LLTO), which is a fast lithium-ion conductor, as an effective coating material for cathode materials used in rechargeable lithium-ion batteries. This fast Li-ion conductor is characterized by first-principles calculations showing low activation barrier for lithium diffusion at various different lithium concentrations. The morphology and the microstructure of the pristine electrode and coated electrode materials are characterized systematically, and we show clear evidence of the presence of the coating after electrochemical cycling. The coated electrodes show significantly improved rate capabilities and cycling performance, compared to the pristine electrodes. The possible reasons for such enhancements are explored experimentally using potentiostatic intermittent titration technique (PITT), electrochemical impedance spectroscopy (EIS). Because of the high lithium conductivity in the LLTO coating material, the chemical Li⁺ diffusion coefficient is one magnitude of order higher in the coated samples than that in the uncoated samples. In addition, the impedances of both interfacial charge transfer and Li⁺ transportation in the solid-electrolyte-interphase (SEI) layer are reduced up to 50% in the coated samples. Our findings provide significant insights into the role of coating regarding the improvements of electrochemical properties, as well as the potential use of solid electrolyte as an effective coating material

Role of LiCoO₂ Surface Terminations in Oxygen Reduction and Evolution Kinetics

Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activities of LiCoO₂ nanorods with sizes in the range from 9 to 40 nm were studied in alkaline solution. The sides of these nanorods were terminated with low-index surfaces such as (003), while the tips were terminated largely with high-index surfaces such as (104), as revealed by high-resolution transmission electron microscopy. Electron energy loss spectroscopy demonstrated that low-spin Co³⁺ prevailed on the sides, while the tips exhibited predominantly high- or intermediate-spin Co³⁺. We correlated the electronic and atomic structure to higher specific ORR and OER activities at the tips as compared to the sides, which was accompanied by more facile redox of Co^2+/3+ and higher charge transferred per unit area. These findings highlight the critical role of surface terminations and electronic structures of transition-metal oxides on the ORR and OER activity