Enhancing the Structural Stability and Electrochemical Performance of High-Nickel Cathode Materials through Ti Doping with an Exothermic Non-oxide Precursor.

The foreseeable global cobalt (Co) crisis has driven the demand for cathode materials with less Co dependence, where high-nickel layered oxides are a promising solution due to their high energy density and low cost. However, these materials suffer from poor cycling stability and rapid voltage decay due to lattice displacement and nanostrain accumulation. Here, we introduced an exothermic TiN dopant via a scalable coating method to stabilize LiNi0.917Co0.056Mn0.026O2 (NCM92) materials. The exothermic reaction of TiN conversion generates extra heat during the calcination process on the cathode surface, promotes the lithiation process, and tunes the morphology of the cathode material, resulting in compact and conformal smaller particle sizes to provide better particle integration and lithium diffusion coefficient. Moreover, the Ti dopant substitutes the Ni3+ site to generate stronger Ti-O bonding, leading to higher structural stability and extended cycle life. The Ti-doped NCM (NCM92_TiN) shows a remarkable cycling stability of maintaining 80% capacity retention for 400 cycles, while bare NCM92 can only reach 88 cycles. Furthermore, the NCM92_TiN cathodes demonstrate an enhanced rate capability and achieve a discharge capacity of over 168 mAh g-1 at 5C.

Formation and Detriments of Residual Alkaline Compounds on High-Nickel Layered Oxide Cathodes.

High-nickel layered oxides LiNixM1-xO2 (x ≥ 0.9) have emerged as promising cathode materials for automotive batteries due to their high energy density and lower cost. However, the formation and accumulation of surface alkaline compounds during storage hinder their mass production and commercialization. Here, a validated chemical method is employed to deconvolute and quantify the evolution of each residual lithium compound in four representative cathodes during ambient-air storage, viz., LiNiO2 (LNO), LiNi0.95Co0.05O2 (NC), LiNi0.95Mn0.05O2 (NM), and LiNi0.95Al0.05O2 (NA). Furthermore, the activation energy of the reaction between water and the cathode is determined by measuring the leached LiOH concentration at various temperatures. While residual lithium and time-of-flight secondary-ion mass spectrometry measurements collectively reveal that the air stability overall follows the trend of NM > NA ≈ NC > LNO, the aged NM exhibits the highest charge-transfer resistance and the worst electrochemical performance among the cathodes. In situ, X-ray diffraction and scanning transmission electron microscopy unveil that the aged NM is plagued by a large area of resistive spinel-like M3-xLixO4 phases, leading to aggravated particle reaction heterogeneity. Finally, a one-step recalcination method is demonstrated effective in fully restoring the degraded cathodes. This work provides insights into overcoming air sensitivity issues of high-Ni cathodes.

Boron-Silicon Alloy Nanoparticles as a Promising New Material in Lithium-Ion Battery Anodes.

Silicon's potential as a lithium-ion battery (LIB) anode is hindered by the reactivity of the lithium silicide (Li x Si) interface. This study introduces an innovative approach by alloying silicon with boron, creating boron/silicon (BSi) nanoparticles synthesized via plasma-enhanced chemical vapor deposition. These nanoparticles exhibit altered electronic structures as evidenced by optical, structural, and chemical analysis. Integrated into LIB anodes, BSi demonstrates outstanding cycle stability, surpassing 1000 lithiation and delithiation cycles with minimal capacity fade or impedance growth. Detailed electrochemical and microscopic characterization reveal very little SEI growth through 1000 cycles, which suggests that electrolyte degradation is virtually nonexistent. This unconventional strategy offers a promising avenue for high-performance LIB anodes with the potential for rapid scale-up, marking a significant advancement in silicon anode technology.

Fluoro-Ethylene-Carbonate Plays a Double-Edged Role on the Stability of Si Anode-Based Rechargeable Batteries During Cycling and Calendar Aging.

The energy storage density of Li-ion batteries can be improved by replacing graphite anodes with high-capacity Si-based materials, though instabilities have limited their implementation. Performance degradation mechanisms that occur in Si anodes can be divided into cycling stability (capacity retention after repeated battery cycles) and calendar aging (shelf life). While cycling instabilities and improvement strategies have been researched intensively, there is little known about the underlying mechanisms that cause calendar aging. In this work, multiple electron microscope techniques are used to explore the mechanism that governs calendar aging from the sub-nanometer-to-electrode scale. Plasma focused ion beam tomography is used to create 3D reconstructions of calendar aged electrodes and revealed the growth of a LiF-rich layer at the interface between the copper current collector and the silicon material, which can lead to delamination and increased interfacial impendence. The LiF layer appeared to derive from the fluoro-ethylene-carbonate electrolyte additive, which is commonly used to improve cycling stability in Si-based systems. The results reveal that additives necessary to improve cycling stability can cause performance degradation over the long-term during calendar aging. The results show that high performing, stable systems require careful design to simultaneously mitigate both cycling and calendar aging instabilities.

Synergetic Dual-Additive Electrolyte Enables Highly Stable Performance in Sodium Metal Batteries.

Sodium (Na)-metal batteries (SMBs) are considered one of the most promising candidates for the large-scale energy storage market owing to their high theoretical capacity (1,166 mAh g-1) and the abundance of Na raw material. However, the limited stability of electrolytes still hindered the application of SMBs. Herein, sulfolane (Sul) and vinylene carbonate (VC) are identified as effective dual additives that can largely stabilize propylene carbonate (PC)-based electrolytes, prevent dendrite growth, and extend the cycle life of SMBs. The cycling stability of the Na/NaNi0.68Mn0.22Co0.1O2 (NaNMC) cell with this dual-additive electrolyte is remarkably enhanced, with a capacity retention of 94% and a Coulombic efficiency (CE) of 99.9% over 600 cycles at a 5 C (750 mA g-1) rate. The superior cycling performance of the cells can be attributed to the homogenous, dense, and thin hybrid solid electrolyte interphase consisting of F- and S-containing species on the surface of both the Na metal anode and the NaNMC cathode by adding dual additives. Such unique interphases can effectively facilitate Na-ion transport kinetics and avoid electrolyte depletion during repeated cycling at a very high rate of 5 C. This electrolyte design is believed to result in further improvements in the performance of SMBs.

In situ observation of the atomic shuffles during the { 11 2 ¯ 1 } twinning in hexagonal close-packed rhenium.

Twinning, on par with dislocations, is critically required in plastic deformation of hexagonal close-packed crystals at low temperatures. In contrast to that in cubic-structured crystals, twinning in hexagonal close-packed crystals requires atomic shuffles in addition to shear. Though the twinning shear that is carried by twinning dislocations has been captured for decades, direct experimental observation of the atomic shuffles, especially when the shuffling mode is not unique and does not confine to the plane of shear, remains a formidable challenge to date. Here, by using in-situ transmission electron microscopy, we directly capture the atomic mechanism of the 11 2 ¯ 1 twinning in hexagonal close packed rhenium nanocrystals. Results show that the 11 2 ¯ 1 twinning is dominated by the (b1/2, h1/2) twinning disconnections. In contrast to conventional expectations, the atomic shuffles accompanying the twinning disconnections proceed on alternative basal planes along 1/6 1 1 ¯ 00 , which may be attributed to the free surface in nanocrystal samples, leading to a lack of mirror symmetry across the 11 2 ¯ 1 twin boundary.

Surface and Bulk Stabilization of Silicon Anodes with Mixed-Multivalent Additives: Ca(TFSI)2 and Mg(TFSI)2.

Silicon is drawing attention as an emerging anode material for the next generation of lithium-ion batteries due to its higher capacity compared with commercial graphite. However, silicon anions formed during lithiation are highly reactive with binder and electrolyte components, creating an unstable SEI layer and limiting the calendar life of silicon anodes. The reactivity of lithium silicide and the formation of an unstable SEI layer are mitigated by utilizing a mixture of Ca and Mg multivalent cations as an electrolyte additive for Si anodes to improve their calendar life. The effect of mixed salts on the bulk and surface of the silicon anodes was studied by multiple structural characterization techniques. Ca and Mg ions in the electrolyte formed relatively thermodynamically stable quaternary Li-Ca-Mg-Si Zintl phases in an in situ fashion and a more stable and denser SEI layer on the Si particles. These in turn protect silicon particles against side reactions with electrolytes in a coin cell. The full cell with the mixed cation electrolyte demonstrates enhanced calendar life performance with lower measured current and current leakage in comparison to that of the baseline electrolyte due to reduced side reactions. Electron microscopy, HR-XRD, and solid-state NMR results showed that electrodes with mixed cations tended to have less cracking on the electrode surface, and the presence of mixed cations enhances cation migration and formation of quaternary Zintl phases stabilizing the bulk and forming a more stable SEI in comparison to baseline electrolyte and electrolyte with single multivalent cations.

Facet-Dependent Ni Segregation in a Micron-Sized Single-Crystal Li1.2Ni0.2Mn0.6O2 Cathode.

Elemental surface segregation in cathode materials is critical for determining the phase and interfacial reaction between the electrode and electrolyte, which consequently affects the electrochemical properties. Single-crystal cathodes of Li1.2Ni0.2Mn0.6O2 and Li1.2Ni0.2Mn0.6O1.95F0.05 with octahedral morphologies of (102)- and (003)-dominated facets have been manifested to show enhanced electrochemical properties. However, the surface structural features of such single crystals have not been investigated. Herein, using scanning transmission electron microscopy, energy dispersive X-ray spectroscopy, and electron energy loss spectroscopy, we probe the elemental surface segregation characteristics in these single-crystal cathodes. We reveal that Ni surface segregation shows dependence on the crystal facet such that it occurs on crystal facets with a mix of cations and anions but not on the facets with only cations or anions. Furthermore, facet-dependent surface reconstructions are observed, featuring a spinel-like structure at the Ni-rich facet but a rock-salt structure at the facet without Ni segregation. The commonly known Mn reduction appears at the single-crystal surfaces and is more pronounced at the facet without Ni segregation. We further reveal that fluorination leads to stabilization of surface oxygens. This study provides detailed structural and chemical information about the facet-dependent Ni surface segregation and the resulting phase formation in the rather less explored micron-sized octahedral Li1.2Ni0.2Mn0.6O2 and Li1.2Ni0.2Mn0.6O1.95F0.05 single crystals, which is key to further exploration of the electrochemical properties of the cathodes in the form of microsized single crystals.

Regenerative Solid Interfaces Enhance High-Performance All-Solid-State Lithium Batteries.

The performance of all-solid-state lithium batteries (ASSLBs) is significantly impacted by lithium interfacial instability, which originates from the dynamic chemical, morphological, and mechanical changes during deep Li plating and stripping. In this study, we introduce a facile approach to generate a conductive and regenerative solid interface, enhancing both the Li interfacial stability and overall cell performance. The regenerative interface is primarily composed of nanosized lithium iodide (nano-LiI), which originates in situ from the adopted solid-state electrolyte (SSE). During cell operation, the nano-LiI interfacial layer can reversibly diffuse back and forth in synchronization with Li plating and stripping. The interface and dynamic process improve the adhesion and Li+ transport between the Li anode and SSE, facilitating uniform Li plating and stripping. As a result, the metallic Li anode operates stably for over 1000 h at high current densities and even under elevated temperatures. By using metallic Li as the anode directly, we demonstrate stable cycling of all-solid-state Li-sulfur batteries for over 250 cycles at an areal capacity of >2 mA h cm-2 and room temperature. This study offers insights into the design of regenerative and Li+-conductive interfaces to tackle solid interfacial challenges for high-performance ASSLBs.

Fluorination Effect on Lithium- and Manganese-Rich Layered Oxide Cathodes.

Lithium- and manganese-rich (LMR) layered oxides are promising high-energy cathodes for next-generation lithium-ion batteries, yet their commercialization has been hindered by a number of performance issues. While fluorination has been explored as a mitigating approach, results from polycrystalline-particle-based studies are inconsistent and the mechanism for improvement in some reports remains unclear. In the present study, we develop an in situ fluorination method that leads to fluorinated LMR with no apparent impurities. Using well-defined single-crystal Li1.2Ni0.2Mn0.6O2 (LNMO) as a platform, we show that a high fluorination level leads to decreased oxygen activities, reduced side reactions at high voltages, and a broadly improved cathode performance. Detailed characterization reveals a particle-level Mn3+ concentration gradient from the surface to the bulk of fluorinated-LNMO crystals, ascribed to the formation of a Ni-rich LizNixMn2-xO4-yFy (x > 0.5) spinel phase on the surface and a "spinel-layered" coherent structure in the bulk where domains of a LiNi0.5Mn1.5O4 high-voltage spinel phase are integrated into the native layered framework. This work provides fundamental understanding of the fluorination effect on LMR and key insights for future development of high-energy Mn-based cathodes with an intergrown/composite crystal structure.

Unlocking Li superionic conductivity in face-centred cubic oxides via face-sharing configurations.

Oxides with a face-centred cubic (fcc) anion sublattice are generally not considered as solid-state electrolytes as the structural framework is thought to be unfavourable for lithium (Li) superionic conduction. Here we demonstrate Li superionic conductivity in fcc-type oxides in which face-sharing Li configurations have been created through cation over-stoichiometry in rocksalt-type lattices via excess Li. We find that the face-sharing Li configurations create a novel spinel with unconventional stoichiometry and raise the energy of Li, thereby promoting fast Li-ion conduction. The over-stoichiometric Li-In-Sn-O compound exhibits a total Li superionic conductivity of 3.38 × 10-4 S cm-1 at room temperature with a low migration barrier of 255 meV. Our work unlocks the potential of designing Li superionic conductors in a prototypical structural framework with vast chemical flexibility, providing fertile ground for discovering new solid-state electrolytes.

Microstructural Insights into Performance Loss of High-Voltage Spinel Cathodes for Lithium-ion Batteries.

Spinel-structured LiNix Mn2-x O4 (LNMO), with low-cost earth-abundant constituents, is a promising high-voltage cathode material for lithium-ion batteries. Even though extensive electrochemical investigations have been conducted on these materials, few studies have explored correlations between their loss in performance and associated changes in microstructure. Here, down to the atomic scale, the structural evolution of these materials is investigated upon the progressive cycling of lithium-ion cells. Transgranular cracking is revealed to be a key feature during cycling; this cracking is initiated at the particle surface and leads to the penetration of electrolytes along the crack path, thereby increasing particle exposure to the electrolyte. The lattice structure on the crack surface shows spatial variances, featuring a top layer of rock-salt, a sublayer of a Mn3 O4 -like arrangement, and then a mixed-cation region adjacent to the bulk lattice. The transgranular cracking, along with the emergence of local lattice distortion, becomes more evident with extended cycling. Further, phase transformation at primary particle surfaces and void formation through vacancy condensation is found in the cycled samples. All these features collectively contribute to the performance degradation of the battery cells during electrochemical cycling.

Atomic-scale probing of short-range order and its impact on electrochemical properties in cation-disordered oxide cathodes.

Chemical short-range-order has been widely noticed to dictate the electrochemical properties of Li-excess cation-disordered rocksalt oxides, a class of cathode based on earth abundant elements for next-generation high-energy-density batteries. Existence of short-range-order is normally evidenced by a diffused intensity pattern in reciprocal space, however, derivation of local atomic arrangements of short-range-order in real space is hardly possible. Here, by a combination of aberration-corrected scanning transmission electron microscopy, electron diffraction, and cluster-expansion Monte Carlo simulations, we reveal the short-range-order is a convolution of three basic types: tetrahedron, octahedron, and cube. We discover that short-range-order directly correlates with Li percolation channels, which correspondingly affects Li transport behavior. We further demonstrate that short-range-order can be effectively manipulated by anion doping or post-synthesis thermal treatment, creating new avenues for tailoring the electrochemical properties. Our results provide fundamental insights for decoding the complex relationship between local chemical ordering and properties of crystalline compounds.

Dynamic Molecular Investigation of the Solid-Electrolyte Interphase of an Anode-Free Lithium Metal Battery Using In Situ Liquid SIMS and Cryo-TEM.

We use in situ liquid secondary ion mass spectroscopy, cryogenic transmission electron microscopy, and density functional theory calculation to delineate the molecular process in the formation of the solid-electrolyte interphase (SEI) layer under the dynamic operating conditions. We discover that the onset potential for SEI layer formation and the thickness of the SEI show dependence on the solvation shell structure. On a Cu film anode, the SEI is noticed to start to form at around 2.0 V (nominal cell voltage) with a final thickness of about 40-50 nm in the 1.0 M LiPF6/EC-DMC electrolyte, while for the case of 1.0 M LiFSI/DME, the SEI starts to form at around 1.5 V with a final thickness of about 20 nm. Our observations clearly indicate the inner and outer SEI layer formation and dissipation upon charging and discharging, implying a continued evolution of electrolyte structure with extended cycling.

Guided anisotropic oxygen transport in vacancy ordered oxides.

Anisotropic and efficient transport of ions under external stimuli governs the operation and failure mechanisms of energy-conversion systems and microelectronics devices. However, fundamental understanding of ion hopping processes is impeded by the lack of atomically precise materials and probes that allow for the monitoring and control at the appropriate time- and length- scales. In this work, using in-situ transmission electron microscopy, we directly show that oxygen ion migration in vacancy ordered, semiconducting SrFeO2.5 epitaxial thin films can be guided to proceed through two distinctly different diffusion pathways, each resulting in different polymorphs of SrFeO2.75 with different ground electronic properties before reaching a fully oxidized, metallic SrFeO3 phase. The diffusion steps and reaction intermediates are revealed by means of ab-initio calculations. The principles of controlling oxygen diffusion pathways and reaction intermediates demonstrated here may advance the rational design of structurally ordered oxides for tailored applications and provide insights for developing devices with multiple states of regulation.

Enhanced Electrochemical Performance of Disordered Rocksalt Cathodes Enabled by a Graphite Conductive Additive.

Cobalt-free cation-disordered rocksalt (DRX) cathodes are a promising class of materials for next-generation Li-ion batteries. Although they have high theoretical specific capacities (>300 mA h/g) and moderate operating voltages (∼3.5 V vs Li/Li+), DRX cathodes typically require a high carbon content (up to 30 wt %) to fully utilize the active material which has a detrimental impact on cell-level energy density. To assess pathways to reduce the electrode's carbon content, the present study investigates how the carbon's microstructure and loading (10-20 wt %) influence the performance of DRX cathodes with the nominal composition Li1.2Mn0.5Ti0.3O1.9F0.1. While electrodes prepared with conventional disordered carbon additives (C65 and ketjenblack) exhibit rapid capacity fade due to an unstable cathode/electrolyte interface, DRX cathodes containing 10 wt % graphite show superior cycling performance (e.g., reversible capacities ∼260 mA h/g with 85% capacity retention after 50 cycles) and rate capability (∼135 mA h/g at 1000 mA/g). A suite of characterization tools was employed to evaluate the performance differences among these composite electrodes. Overall, these results indicate that the superior performance of the graphite-based cathodes is largely attributed to the: (i) formation of a uniform graphitic coating on DRX particles which protects the surface from parasitic reactions at high states of charge and (ii) homogeneous dispersion of the active material and carbon throughout the composite cathode which provides a robust electronically conductive network that can withstand repeated charge-discharge cycles. Overall, this study provides key scientific insights on how the carbon microstructure and electrode processing influence the performance of DRX cathodes. Based on these results, exploration of alternative routes to apply graphitic coatings is recommended to further optimize the material performance.

In Situ Visualization of the Pinning Effect of Planar Defects on Li Ion Insertion.

Longevity of Li ion batteries strongly depends on the interaction of transporting Li ions in electrode crystals with defects. However, detailed interactions between the Li ion flux and structural defects in the host crystal remain obscure due to the transient nature of such interactions. Here, by in situ transmission electron microscopy and density function theory calculations, we reveal how the diffusion pathways and transport kinetics of a Li ion can be affected by planar defects in a tungsten trioxide lattice. We uncover that changes in charge distribution and lattice spacing along the planar defects disrupt the continuity of ion conduction channels and dramatically increase the energy barrier of Li diffusion, thus, arresting Li ions at the defect sites and twisting the lithiation front. The atomic-scale understanding holds critical implications for rational interface design in solid-state batteries and solid oxide fuel cells.