Although layered lithium oxides have become the cathode of choice for state‐of‐the‐art Li‐ion batteries, substantial gaps remain between the practical and theoretical energy densities. With the aim of supporting efforts to close this gap, this work reviews the fundamental operating mechanisms and challenges of Li intercalation in layered oxides, contrasts how these challenges play out differently for different materials (with emphasis on Ni–Co–Al (NCA) and Ni–Mn–Co (NMC) alloys), and summarizes the extensive corpus of modifications and extensions to the layered lithium oxides. Particular emphasis is given to the fundamental mechanisms behind the operation and degradation of layered intercalation electrode materials as well as novel modifications and extensions, including Na‐ion and cation‐disordered materials.

Lithium metal has been considered the "holy grail" anode material for rechargeable batteries despite the fact that its dendritic growth and low Coulombic efficiency (CE) have crippled its practical use for decades. Its high chemical reactivity and low stability make it difficult to explore the intrinsic chemical and physical properties of the electrochemically deposited lithium (EDLi) and its accompanying solid electrolyte interphase (SEI). To prevent the dendritic growth and enhance the electrochemical reversibility, it is crucial to understand the nano- and mesostructures of EDLi. However, Li metal is very sensitive to beam damage and has low contrast for commonly used characterization techniques such as electron microscopy. Inspired by biological imaging techniques, this work demonstrates the power of cryogenic (cryo)-electron microscopy to reveal the detailed structure of EDLi and the SEI composition at the nanoscale while minimizing beam damage during imaging. Surprisingly, the results show that the nucleation-dominated EDLi (5 min at 0.5 mA cm) is amorphous, while there is some crystalline LiF present in the SEI. The EDLi grown from various electrolytes with different additives exhibits distinctive surface properties. Consequently, these results highlight the importance of the SEI and its relationship with the CE. Our findings not only illustrate the capabilities of cryogenic microscopy for beam (thermal)-sensitive materials but also yield crucial structural information on the EDLi evolution with and without electrolyte additives.

Hybrid organic-inorganic materials for high-efficiency, low-cost photovoltaic devices have seen rapid progress since the introduction of lead based perovskites and solid-state hole transport layers. Although majority of the materials used for perovskite solar cells (PSC) are introduced from dye-sensitized solar cells (DSSCs), the presence of a perovskite capping layer as opposed to a single dye molecule (in DSSCs) changes the interactions between the various layers in perovskite solar cells. 4-tert-Butylpyridine (tBP), commonly used in PSCs, is assumed to function as a charge recombination inhibitor, similar to DSSCs. However, the presence of a perovskite capping layer calls for a re-evaluation of its function in PSCs. Using TEM (transmission electron microscopy), we first confirm the role of tBP as a HTL morphology controller in PSCs. Our observations suggest that tBP significantly improves the uniformity of the HTL and avoids accumulation of Li salt. We also study degradation pathways by using FTIR (Fourier transform infrared spectroscopy) and APT (atom probe tomography) to investigate and visualize in 3-dimensions the moisture content associated with the Li salt. Long-term effects, over 1000 h, due to evaporation of tBP have also been studied. Based on our findings, a PSC failure mechanism associated with the morphological change of the HTL is proposed. tBP, the morphology controller in HTL, plays a key role in this process, and thus this study highlights the need for additive materials with higher boiling points for consistent long-term performance of PSCs.

To obtain high-energy density Li-ion batteries for the nextgeneration storage devices, silicon anodes provide a viable option because of their high theoretical capacity, low operating potential versus lithium (Li), and environmental abundance. However, the silicon electrode suffers from large volume expansion (∼300%) that leads to mechanical failure, cracks in the SEI (solid electrolyte interphase), and loss of contact with the current collector, all of which severely impede the capacity retention. In this respect, the choice of binders, carbon, electrolyte, and the morphology of the silicon itself plays a critical role in improving capacity retention. Of specific mention is the role of binders where a carboxylic acid-heavy group, PAA (polyacrylic acid), has been demonstrated to have better cycling capacity retention as compared to CMC (carboxy methyl cellulose). Traditionally, the role of binders has been proposed as a soft matrix backbone that allows volume expansion of the anode while preserving its morphology. However, the effect of the binder on both the rate of formation of SEI species across cycles and its distribution around the silicon nanoparticles has not been completely investigated. Herein, we use two different binders (PAA and CMC) coupled with LiFSI (lithium bis(fluorosulfonyl)imide)/EMI-FSI (1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide) ionic liquid as the electrolyte to understand the effect of binder on the SEI. Using STEM-EDX (scanning transmission electron microscopy− energy-dispersive X-ray spectroscopy), EELS (electron energy loss spectroscopy), and XPS (X-ray photoelectron spectroscopy), we discuss the evolution of the SEI on the Si electrode for both binders. Our results indicate that a faster decomposition of FSI − with a PAA binder leads to LiF (lithium fluoride) formation, making F − unavailable for subsequent SEI formation cycles. This allows further decomposition of the LiFSI salt to sulfates and sulfides which form a crucial component of the SEI around silicon nanoparticles after 100 cycles in the PAA binder-based system. The dual effects of faster consumption of F − to form LiF together with the distribution of passivating sulfides in the SEI could allow for better capacity retention in the PAA binder system as compared to that with CMC.

Li-ion cathodes based on conversion reactions such as iron fluoride (FeF 2 ) can achieve in principle high specific capacity. However, significant capacity fading is observed upon cycling. This has been attributed in part to the formation and continuous growth of a solid electrolyte interphase (SEI) layer at the cathode/electrolyte interface. In this work, scanning transmission electron microscopy, electron energy loss spectroscopy, selected area electron diffraction, and X-ray photoelectron spectroscopy were used in combination to study both the structural changes of the FeF 2 /C active material and the growth and evolution of the SEI layer upon cycling. Two main sources of capacity loss have been found. An increasing amount of Fe 2+ appeared trapped inside the SEI layer with increasing cycle number, thus resulting in the loss of active material. In addition, reconversion is strongly impeded with increasing cycle number, leaving untransformed LiF and Fe 0 upon delithiation. This correlates with the irreversible growth of a SEI layer that limits electronic and ionic transport.