Synchrotron Near-Field Infrared Nanospectroscopy and Nanoimaging of Lithium Fluoride in Solid Electrolyte Interphases in Li-Ion Battery Anodes.

Lithium fluoride (LiF) is a ubiquitous component in the solid electrolyte interphase (SEI) layer in Li-ion batteries. However, its nanoscale structure, morphology, and topology, important factors for understanding LiF and SEI film functionality, including electrode passivity, are often unknown due to limitations in spatial resolution of common characterization techniques. Ultrabroadband near-field synchrotron infrared nanospectroscopy (SINS) enables such detection and mapping of LiF in SEI layers in the far-infrared region down to ca. 322 cm-1 with a nanoscale spatial resolution of ca. 20 nm. The surface sensitivity of SINS and the large infrared absorption cross section of LiF, which can support local surface phonons under certain circumstances, enabled characterization of model LiF samples of varying structure, thickness, surface roughness, and degree of crystallinity, as confirmed by atomic force microscopy, attenuated total reflectance FTIR, SINS, X-ray photoelectron spectroscopy, high-angle annular dark-field, and scanning transmission electron microscopy. Enabled by this approach, LiF within SEI films formed on Cu, Si, and metallic glass Si40Al50Fe10 electrodes was detected and characterized. The nanoscale morphologies and topologies of LiF in these SEI layers were evaluated to gain insights into LiF nucleation, growth, and the resulting nuances in the electrode surface passivity.

Lattice-Oxygen-Stabilized Li- and Mn-Rich Cathodes with Sub-Micrometer Particles by Modifying the Excess-Li Distribution.

In recent years, Li- and Mn-rich layered oxides (LMRs) have been vigorously explored as promising cathodes for next-generation, Li-ion batteries due to their high specific energy. Nevertheless, their actual implementation is still far from a reality since the trade-off relationship between the particle size and chemical reversibility prevents LMRs from achieving a satisfactory, industrial energy density. To solve this material dilemma, herein, a novel morphological and structural design is introduced to Li1.11 Mn0.49 Ni0.29 Co0.11 O2 , reporting a sub-micrometer-level LMR with a relatively delocalized, excess-Li system. This system exhibits an ultrahigh energy density of 2880 Wh L-1 and a long-lasting cycle retention of 83.1% after the 100th cycle for 45 °C full-cell cycling, despite its practical electrode conditions. This outstanding electrochemical performance is a result of greater lattice-oxygen stability in the delocalized excess-Li system because of the low amount of highly oxidized oxygen ions. Geometric dispersion of the labile oxygen ions effectively suppresses oxygen evolution from the lattice when delithiated, eradicating the rapid energy degradation in a practical cell system.

Metal-Ion Chelating Gel Polymer Electrolyte for Ni-Rich Layered Cathode Materials at a High Voltage and an Elevated Temperature.

Nickel-rich layered oxides (LiNi1-x-yCoxMnyO2; (1 - x - y) ≥ 0.6), the high-energy-density cathode materials of lithium-ion batteries (LIBs), are seriously unstable at voltages higher than 4.5 V versus Li/Li+ and temperatures higher than 50 °C. Herein, we demonstrated that the failure mechanism of a nickel-rich layered oxide (LiNi0.6Co0.2Mn0.2O2) behind the instability was successfully suppressed by employing cyanoethyl poly(vinyl alcohol) having pyrrolidone moieties (Pyrd-PVA-CN) as a metal-ion-chelating gel polymer electrolyte (GPE). The metal-ion-chelating GPE blocked the plating of transition-metal ions dissolved from the cathode by capturing the ions (anode protection). High-concentration metal-ion environments developed around the cathode surface by the GPE suppressed the irreversible phase transition of the cathode material from the layered structure to the rock-salt structure (cathode protection). Resultantly, the capacity retention was significantly improved at a high voltage and a high temperature. Capacity retention and coulombic efficiency of a full-cell configuration of a nickel-rich layered oxide with graphite were significantly improved in the presence of the GPE especially at a high cutoff voltage (4.4 V) and an elevated temperature (55 °C).

Scalable Synthesis of Hollow ß-SiC/Si Anodes via Selective Thermal Oxidation for Lithium-Ion Batteries.

Silicon for anodes in lithium-ion batteries has received much attention owing to its superior specific capacity. There has been a rapid increase of research related to void engineering to address the silicon failure mechanism stemming from the massive volume change during (dis)charging in the past decade. Nevertheless, conventional synthetic methods require complex synthetic procedures and toxic reagents to form a void space, so they have an obvious limitation to reach practical application. Here, we introduce SiCx consisting of nanocrystallite Si embedded in the inactive matrix of ß-SiC to fabricate various types of void structures using thermal etching with a scalable one-pot CVD method. The structural features of SiCx make the carbonaceous template possible to be etched selectively without Si oxidation at high temperature with an air atmosphere. Furthermore, bottom-up gas phase synthesis of SiCx ensures atomically identical structural features (e.g., homogeneously distributed Si and ß-SiC) regardless of different types of sacrificial templates. For these reasons, various types of SiCx hollow structures having shells, tubes, and sheets can be synthesized by simply employing different morphologies of the carbon template. As a result, the morphological effect of different hollow structures can be deeply investigated as well as the free volume effect originating from void engineering from both a electrochemical and computational point of view. In terms of selective thermal oxidation, the SiCx hollow shell achieves a much higher initial Coulombic efficiency (>89%) than that of the Si hollow shell (65%) because of its nonoxidative property originating from structural characteristics of SiCx during thermal etching. Moreover, the findings based on the clearly observed different electrochemical features between half-cell and full-cell configuration give insight into further Si anode research.

Calendering-Compatible Macroporous Architecture for Silicon-Graphite Composite toward High-Energy Lithium-Ion Batteries.

Porous strategies based on nanoengineering successfully mitigate several problems related to volume expansion of alloying anodes. However, practical application of porous alloying anodes is challenging because of limitations such as calendering incompatibility, low mass loading, and excessive usage of nonactive materials, all of which cause a lower volumetric energy density in comparison with conventional graphite anodes. In particular, during calendering, porous structures in alloying-based composites easily collapse under high pressure, attenuating the porous characteristics. Herein, this work proposes a calendering-compatible macroporous architecture for a Si-graphite anode to maximize the volumetric energy density. The anode is composed of an elastic outermost carbon covering, a nonfilling porous structure, and a graphite core. Owing to the lubricative properties of the elastic carbon covering, the macroporous structure coated by the brittle Si nanolayer can withstand high pressure and maintain its porous architecture during electrode calendering. Scalable methods using mechanical agitation and chemical vapor deposition are adopted. The as-prepared composite exhibits excellent electrochemical stability of >3.6 mAh cm-2 , with mitigated electrode expansion. Furthermore, full-cell evaluation shows that the composite achieves higher energy density (932 Wh L-1 ) and higher specific energy (333 Wh kg-1 ) with stable cycling than has been reported in previous studies.

Boosting Reaction Homogeneity in High-Energy Lithium-Ion Battery Cathode Materials.

Conventional nickel-rich cathode materials suffer from reaction heterogeneity during electrochemical cycling particularly at high temperature, because of their polycrystalline properties and secondary particle morphology. Despite intensive research on the morphological evolution of polycrystalline nickel-rich materials, its practical investigation at the electrode and cell levels is still rarely discussed. Herein, an intrinsic limitation of polycrystalline nickel-rich cathode materials in high-energy full-cells is discovered under industrial electrode-fabrication conditions. Owing to their highly unstable chemo-mechanical properties, even after the first cycle, nickel-rich materials are degraded in the longitudinal direction of the high-energy electrode. This inhomogeneous degradation behavior of nickel-rich materials at the electrode level originates from the overutilization of active materials on the surface side, causing a severe non-uniform potential distribution during long-term cycling. In addition, this phenomenon continuously lowers the reversibility of lithium ions. Consequently, considering the degradation of polycrystalline nickel-rich materials, this study suggests the adoption of a robust single-crystalline LiNi0.8 Co0.1 Mn0.1 O2 as a feasible alternative, to effectively suppress the localized overutilization of active materials. Such an adoption can stabilize the electrochemical performance of high-energy lithium-ion cells, in which superior capacity retention above ≈80% after 1000 cycles at 45 °C is demonstrated.

Building High-Rate Nickel-Rich Cathodes by Self-Organization of Structurally Stable Macrovoid.

Nickel-rich materials, as a front-running cathode for lithium-ion batteries suffer from inherent degradation issues such as inter/intragranular cracks and phase transition under the high-current density condition. Although vigorous efforts have mitigated these current issues, the practical applications are not successfully achieved due to the material instability and complex synthesis process. Herein, a structurally stable, macrovoid-containing, nickel-rich material is developed using an affordable, scalable, and one-pot coprecipitation method without using surfactants/etching agents/complex-ion forming agents. The strategically developed macrovoid-induced cathode via a self-organization process exhibits excellent full-cell rate capability, cycle life at discharge rate of 5 C, and structural stability even at the industrial electrode conditions, owing to the fast Li-ion diffusion, the internal macrovoid acting as "buffer zones" for stress relief, and highly stable nanostructure around the void during cycling. This strategy for nickel-rich cathodes can be viable for industries in the preparation of high-performance lithium-ion cells.

Advances and Prospects of Sulfide All-Solid-State Lithium Batteries via One-to-One Comparison with Conventional Liquid Lithium Ion Batteries.

Owing to the safety issue of lithium ion batteries (LIBs) under the harsh operating conditions of electric vehicles and mobile devices, all-solid-state lithium batteries (ASSLBs) that utilize inorganic solid electrolytes are regarded as a secure next-generation battery system. Significant efforts are devoted to developing each component of ASSLBs, such as the solid electrolyte and the active materials, which have led to considerable improvements in their electrochemical properties. Among the various solid electrolytes such as sulfide, polymer, and oxide, the sulfide solid electrolyte is considered as the most promising candidate for commercialization because of its high lithium ion conductivity and mechanical properties. However, the disparity in energy and power density between the current sulfide ASSLBs and conventional LIBs is still wide, owing to a lack of understanding of the battery electrode system. Representative developments of ASSLBs in terms of the sulfide solid electrolyte, active materials, and electrode engineering are presented with emphasis on the current status of their electrochemical performances, compared to those of LIBs. As a rational method to realizing high energy sulfide ASSLBs, the requirements for the sulfide solid electrolytes and active materials are provided along through simple experimental demonstrations. Potential future research directions in the development of commercially viable sulfide ASSLBs are suggested.

Controllable Solid Electrolyte Interphase in Nickel-Rich Cathodes by an Electrochemical Rearrangement for Stable Lithium-Ion Batteries.

The layered nickel-rich materials have attracted extensive attention as a promising cathode candidate for high-energy density lithium-ion batteries (LIBs). However, they have been suffering from inherent structural and electrochemical degradation including severe capacity loss at high electrode loading density (>3.0 g cm-3 ) and high temperature cycling (>60 °C). In this study, an effective and viable way of creating an artificial solid-electrolyte interphase (SEI) layer on the cathode surface by a simple, one-step approach is reported. It is found that the initial artificial SEI compounds on the cathode surface can electrochemically grow along grain boundaries by reacting with the by-products during battery cycling. The developed nickel-rich cathode demonstrates exceptional capacity retention and structural integrity under industrial electrode fabricating conditions with the electrode loading level of ≈12 mg cm-2 and density of ≈3.3 g cm-3 . This finding could be a breakthrough for the LIB technology, providing a rational approach for the development of advanced cathode materials.

Issues and Challenges Facing Flexible Lithium-Ion Batteries for Practical Application.

With the advent of flexible electronics, lithium-ion batteries have become a key component of high performance energy storage systems. Thus, considerable effort is made to keep up with the development of flexible lithium-ion batteries. To date, many researchers have studied newly designed batteries with flexibility, however, there are several significant challenges that need to be overcome, such as degradation of electrodes under external load, poor battery performance, and complicated cell preparation procedures. In addition, an in-depth understanding of the current challenges for flexible batteries is rarely addressed in a systematical and practical way. Herein, recent progress and current issues of flexible lithium-ion batteries in terms of battery materials and cell designs are reviewed. A critical overview of important issues and challenges for the practical application of flexible lithium-ion batteries is also provided. Finally, the strategies are discussed to overcome current limitations of the practical use of flexible lithium-based batteries, providing a direction for future research.

Postpatterned Electrodes for Flexible Node-Type Lithium-Ion Batteries.

A flexible node-type lithium-ion battery (LIB) with novel postpatterned electrodes is developed via a simple, one-step process involving an imprinting step of a conventional electrode in the presence of a flattened mesh template. The node-type LIBs containing segmented parts for energy storage and mechanical deformation demonstrate a good cycle stability.