Designer Lithium Reservoirs for Ultralong Life Lithium Batteries for Grid Storage.

The minimization of irreversible active lithium loss stands as a pivotal concern in rechargeable lithium batteries, particularly in the context of grid-storage applications, where achieving the utmost energy density over prolonged cycling is imperative to meet stringent demands, notably in terms of life cost. Departing from conventional methodologies advocating electrode prelithiation and/or electrolyte additives, a new paradigm is proposed here: the integration of a designer lithium reservoir (DLR) featuring lithium orthosilicate (Li4SiO4) and elemental sulfur. This approach concurrently addresses active lithium consumption through solid electrolyte interphase (SEI) formation and mitigates minor yet continuous parasitic reactions at the electrode/electrolyte interface during extended cycling. The remarkable synergy between the Li-ion conductive Li4SiO4 and the SEI-favorable elemental sulfur enables customizable compensation kinetics for active lithium loss throughout continuous cycling. The introduction of a minute quantity of DLR (3 wt% Li4SiO4@S) yields outstanding cycling stability in a prototype pouch cell (graphite||LiFePO4) with an ampere-hour-level capacity (≈2.3 Ah), demonstrating remarkable capacity retention (≈95%) even after 3000 cycles. This utilization of a DLR is poised to expedite the development of enduring lithium batteries for grid-storage applications and stimulate the design of practical, implantable rechargeable batteries based on related cell chemistries.

Domino Reactions Enabling Sulfur-Mediated Gradient Interphases for High-Energy Lithium Batteries.

Silicon (Si)-based anodes are currently considered a feasible solution to improve the energy density of lithium-ion batteries owing to their sufficient specific capacity and natural abundance. However, Si-based anodes exhibit low electric conductivities and large volume changes during cycling, which could easily trigger continuous breakdown/reparation of the as-formed solid-electrolyte-interphase (SEI) layer, seriously hampering their practical application in current battery technology. To control the chemoelectrochemical instability of the conventional SEI layer, we herein propose the introduction of elemental sulfur into nonaqueous electrolytes, aiming to build a sulfur-mediated gradient interphase (SMGI) layer on Si-based anodes. The SMGI layer is generated through the domino reactions (i.e., electrochemical cascade reactions) involving the electrochemical reductions of elemental sulfur followed by nucleophilic substitutions of fluoroethylene carbonate, which endows the corresponding SEI layer with strong elasticity and chemomechanical stability and enables rapid transportation of Li+ ions. Consequently, the prototype Si||LiNi0.8Co0.1Mn0.1O2 cells attain a high-energy density of 622.2 W h kg-1 and a capacity retention of 88.8% after 100 cycles. Unlike previous attempts based on sophisticated chemical modifications of electrolyte components, this study opens a new avenue in interphase design for long-lived and high-energy rechargeable batteries.

Electrolysis Process-Facilitated Engineering of Primary Particles of Cobalt-Free LiNiO2 for Improved Electrochemical Performance.

The particle morphology of LiNiO2 (LNO), the final product of Co-free high-Ni layered oxide cathode materials, must be engineered to prevent the degradation of electrochemical performance caused by the H2-H3 phase transition. Introducing a small amount of dopant oxides (Nb2O5 as an example) during the electrolysis synthesis of the Ni(OH)2 precursor facilitates the engineering of the primary particles of LNO, which is quick, simple, and inexpensive. In addition to the low concentration of Nb that entered the lattice structure, a combination of advanced characterizations indicates that the obtained LNO cathode material contains a high concentration of Nb in the primary particle boundaries in the form of lithium niobium oxide. This electrolysis method facilitated LNO (EMF-LNO) engineering successfully, reducing primary particle size and increasing particle packing density. Therefore, the EMF-LNO cathode material with engineered morphology exhibited increased mechanical strength and electrical contact, blocked electrolyte penetration during cycling, and reduced the H2-H3 phase transition effects.

Designer Cathode Additive for Stable Interphases on High-Energy Anodes.

Rechargeable lithium-based batteries built with high-energy anode materials (e.g., silicon-based and silicon-derivative materials) are considered a feasible solution to satisfy the stringent requirements imposed by emerging markets, including electric vehicles and grid storage, due to their higher energy density compared to contemporary lithium-ion batteries. The robustness of the solid electrolyte interphase (SEI) layer on high-energy anodes is critical to achieve long-term and stable cycling performances of the batteries. Herein, we propose a new type of designer cathode additive (DCA), i.e., an ultrathin coating layer of elemental sulfur on the cathode, for the in situ formation of a thin and robust SEI layer on various types of high-energy anodes. The DCA elemental sulfur undergoes simultaneous oxidation and reduction paths, forming lithium alkyl sulfate (R-OSO2OLi) and poly(ethylene oxide) (PEO)-like polymers on the anode surface. The as-formed R-OSO2OLi/PEO-modified SEI layer has good lithium cation (Li+) permeability to facilitate fast ion transportation across the interphases and superior elasticity to adapt to large volume changes, which is particularly effective for improving the cycling efficiency of high-energy anodes (e.g., ca. 14-35% increase in capacity retention for the silicon-carbon composite (SiC) or silicon-tin alloy (Si-Sn)||LiFePO4 cells). The present work opens a new avenue toward the practical deployment of high-energy rechargeable lithium-based batteries.

Electrolyzed Ni(OH)2 Precursor Sintered with LiOH/LiNiO3 Mixed Salt for Structurally and Electrochemically Stable Cobalt-Free LiNiO2 Cathode Materials.

Cobalt-free LiNiO2 cathode materials offer a higher energy density at a lower cost than high Co-containing cathode materials. However, Ni(OH)2 precursors for LiNiO2 cathodes are traditionally prepared by the coprecipitation method, which is expensive, complex, and time-consuming. Herein, we report a fast, facile, and inexpensive electrolysis process to prepare a Ni(OH)2 precursor, which was mixed with LiOH/LiNO3 salts to obtain a LiNiO2 cathode material. A combination of advanced characterization techniques revealed that the LiNiO2 cathode material prepared in this way exhibited an excellent layered structure with negligible Li/Ni site mixing and surface structural distortion. Electrochemical cycling of the LiNiO2 cathode material showed an initial discharge capacity of 235.2 mA h/g and a capacity retention of 80.2% after 100 cycles (at 1 C) between 2.75 and 4.3 V. The degradation of the cycling performance of the LiNiO2 cathode material was mainly attributed to the formation of a surface solid-electrolyte interface and a ∼5 nm rock salt-like structure, while the bulk structure of the cathode after cycling was generally stable.

Understanding the Effect of Atomic-Scale Surface Migration of Bridging Ions in Binding Li3PO4 to the Surface of Spinel Cathode Materials.

Spinel cathode materials (e.g., LiMn2O4 and LiNi0.5Mn1.5O4) with strongly bonded surface coatings are desirable for delivering improved electrochemical performance in long-term cycling. Here, we report that the introduction of bridging ions such as Fe and Co, which can diffuse into both the spinel cathode materials and Li3PO4, the latter is found to cover the spinel surface in the form of dense and uniform particles (∼2-3 nm). Detailed structural analysis of the surface reveals that the bridging ions diffuse into the 16c site of the spinel structure to form ion-doped spinel cathode materials, which contribute to the formation of strong bonds between the surface and Li3PO4, possibly via spinel-(surface bridging ions)-Li3PO4 bonds. The critical role of the surface bridging ions is further investigated by heating the as-formed Li3PO4-coated spinel cathode materials (with bridging ions) to high temperatures, resulting in further diffusion of bringing ions from the surface to the interior of the spinel materials and consequently depletion of the surface spinel-(surface bridging ions)-Li3PO4 bonds. This leads to the gradual growth of surface Li3PO4 particles (∼20 nm) and the exposure of the spinel surface.

Understanding Surface Structural Stabilization of the High-Temperature and High-Voltage Cycling Performance of Al3+-Modified LiMn2O4 Cathode Material.

Stabilization of the atomic-level surface structure of LiMn2O4 with Al3+ ions is shown to be significant in the improvement of cycling performance, particularly at a high temperature (55 °C) and high voltage (5.1 V). Detailed analysis by X-ray photoelectron spectroscopy, secondary ion mass spectrometry, scanning transmission electron microscopy-energy-dispersive X-ray spectroscopy, etc. reveals that Al3+ ions diffuse into the spinel to form a layered Li(Alx,Mny)O2 structure in the outmost surface where Al3+ concentration is the highest. Other Al3+ ions diffuse into the 8a sites of spinel to form a (Mn3-xAlx)O4 structure and the 16d sites of spinel to form Li(Mn2-xAlx)O4. These complicated surface structures, in particular the layered Li(Alx,Mny)O2, are present at the surface throughout cycling and effectively stabilize the surface structure by preventing dissolution of Mn ions and mitigating cathode-electrolyte reactions. With the Al3+ ions surface modification, a stable cycle performance (∼78% capacity retention after 150 cycles) and high Coulombic efficiency (∼99%) are achieved at 55 °C. More surprisingly, the surface-stabilized LiMn2O4 can be cycled up to 5.1 V without significant degradation, in contrast to the fast capacity degradation found in the unmodified case. Our findings demonstrate the critical role of ions coated on the surface in modifying the structural evolution of the surface of spinel electrode particles and thus will stimulate future efforts to optimize the surface properties of battery electrodes.

Unusual Spinel-to-Layered Transformation in LiMn2O4 Cathode Explained by Electrochemical and Thermal Stability Investigation.

Distorted surface regions (5-6 nm) with an unusual layered-like structure on LiMn2O4 cathode material were directly observed after it was cycled (3-4.9 V), indicating a possible spinel-to-layered structural transformation. Formation of these distorted regions severely degrades LiMn2O4 cathode capacity. As we attempt to get a better understanding of the exact crystal structure of the distorted regions, the structural transformation pathways and the origins of the distortion are made difficult by the regions' nanoscopic size. Inspired by the reduction of Mn4+ to Mn3+ in surface electronic structures that might be associated with oxygen loss during cycling, we further investigated the atomic-level surface structure of LiMn2O4 by heat-treatments between 600 and 900 °C in various atmospheres, finding similar surface spinel-to-layered structural transformation only for LiMn2O4 heat-treated in argon atmosphere for a few minutes (or more). Controllable and measurable oxygen loss during heat-treatments result in Mn3+ for charge compensation. The ions then undergo a disproportionation reaction, driving the spinel-to-layered transformation by way of an intermediate LiMn3O4-like structure. The distortion of the surface regions can be extended to the whole bulk by heat-treatment for 300-600 min, ultimately enabling us to identify the bulk-level structure as layered Li2MnO3 (C2/m). This work demonstrates the critical role of Mn3+ in controlling the kinetics of the structural transformation in spinel LiMn2O4 and suggests heat-treatment in argon as a convenient method to control the surface oxygen loss and consequently reconstruct the atomic-level surface structure.

3D visualization of inhomogeneous multi-layered structure and Young's modulus of the solid electrolyte interphase (SEI) on silicon anodes for lithium ion batteries.

The microstructure and mechanical properties of the solid electrolyte interphase (SEI) in non-aqueous lithium ion batteries are key issues for understanding and optimizing the electrochemical performance of lithium batteries. In this report, the three-dimensional (3D) multi-layered structures and the mechanical properties of the SEI formed on a silicon anode material for next generation lithium ion batteries have been visualized directly for the first time, through a scanning force spectroscopy method. The coverage of the SEI on silicon anodes is also obtained through 2D projection plots. The effects of temperature and the function of additives in the electrolyte on the SEI can be understood accordingly. A modified model about dynamic evolution of the SEI on the silicon anode material is also proposed, which aims to explain why the SEI is very thick and how the multi-layered structure is formed and decomposed dynamically.