Guided Lithium Metal Deposition and Improved Lithium Coulombic Efficiency through Synergistic Effects of LiAsF₆ and Cyclic Carbonate Additives

Spatial and morphology control over lithium (Li) metal nucleation and growth, as well as improving Li Coulombic efficiency (CE), are among the most challenging issues for rechargeable Li metal batteries. Here, we report that LiAsF₆ and cyclic carbonate additives such as vinylene carbonate (VC) or fluoroethylene carbonate (FEC) can work synergistically to address these challenges. It is revealed that LiAsF₆ can be reduced to Li_<i>x</i>As alloy and LiF, which act as nanosized seeds for Li growth and form a robust solid electrolyte interface layer. The addition of VC or FEC not only enables the uniform distribution of Li_<i>x</i>As seeds but also improves the flexibility of the solid electrolyte interface layer. As a result, highly compact, uniform, and dendrite-free Li film with vertically aligned column structure can be obtained with increased Li CE, and the Li metal batteries using the electrolyte with both LiAsF₆ and cyclic carbonate additives can have improved cycle life

Improving Cycling Performance of Anode-Free Lithium Batteries by Pressure and Voltage Control

Anode-free lithium batteries (AFLBs) have great potential to provide a higher energy density than most other batteries. However, the performance of AFLBs is very sensitive to pressure and other operating parameters. In this work, the operating voltage range and the internal pressures applied to AFLBs using a localized high-concentration electrolyte have been optimized for the coin cells widely used in the early stage research of AFLBs. With an optimized cycling protocol, a thin layer of uniform nucleation sites can be formed during the initial cycle, which will facilitate smooth Li deposition/stripping in the subsequent cycles of AFLBs. The solid–electrolyte interphase layer formed under optimized pressure and uniform pressure distribution exhibits good stability for long-term cycling. The internal pressure in the coin cells has been optimized to improve the cycling performance of AFLBs (Cu||NMC811) with 72% capacity retention in 100 cycles

Corrosion/Fragmentation of Layered Composite Cathode and Related Capacity/Voltage Fading during Cycling Process

The Li-rich, Mn-rich (LMR) layered structure materials exhibit very high discharge capacities exceeding 250 mAh g^–1 and are very promising cathodes to be used in lithium ion batteries. However, significant barriers, such as voltage fade and low rate capability, still need to be overcome before the practical applications of these materials. A detailed study of the voltage/capacity fading mechanism will be beneficial for further tailoring the electrode structure and thus improving the electrochemical performances of these layered cathodes. Here, we report detailed studies of structural changes of LMR layered cathode Li­[Li_0.2Ni_0.2Mn_0.6]­O₂ after long-term cycling by aberration-corrected scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). The fundamental findings provide new insights into capacity/voltage fading mechanism of Li­[Li_0.2Ni_0.2Mn_0.6]­O₂. Sponge-like structure and fragmented pieces were found on the surface of cathode after extended cycling. Formation of Mn²⁺ species and reduced Li content in the fragments leads to the significant capacity loss during cycling. These results also imply the functional mechanism of surface coatings, for example, AlF₃, which can protect the electrode from etching by acidic species in the electrolyte, suppress cathode corrosion/fragmentation, and thus improve long-term cycling stability

Bending-Induced Symmetry Breaking of Lithiation in Germanium Nanowires

From signal transduction of living cells to oxidation and corrosion of metals, mechanical stress intimately couples with chemical reactions, regulating these biological and physiochemical processes. The coupled effect is particularly evident in the electrochemical lithiation/delithiation cycling of high-capacity electrodes, such as silicon (Si), where on the one hand lithiation-generated stress mediates lithiation kinetics and on the other the electrochemical reaction rate regulates stress generation and mechanical failure of the electrodes. Here we report for the first time the evidence on the controlled lithiation in germanium nanowires (GeNWs) through external bending. Contrary to the symmetric core–shell lithiation in free-standing GeNWs, we show bending the GeNWs breaks the lithiation symmetry, speeding up lithaition at the tensile side while slowing down at the compressive side of the GeNWs. The bending-induced symmetry breaking of lithiation in GeNWs is further corroborated by chemomechanical modeling. In the light of the coupled effect between lithiation kinetics and mechanical stress in the electrochemical cycling, our findings shed light on strain/stress engineering of durable high-rate electrodes and energy harvesting through mechanical motion

Discharge Performance of Li–O₂ Batteries Using a Multiscale Modeling Approach

To study the discharge performance of Li–O₂ batteries, we propose a multiscale modeling framework that links models in an upscaling fashion from the nanoscale to mesoscale and finally to the device scale. We have effectively reconstructed the microstructure of a Li–O₂ air electrode <i>in silico</i>, conserving the porosity, surface-to-volume ratio, and pore size distribution of the real air electrode structure. The mechanism of rate-dependent morphology of Li₂O₂ growth is incorporated into the mesoscale model. The correlation between the active-surface-to-volume ratio and averaged Li₂O₂ concentration is derived to link different scales. The proposed approach’s accuracy is first demonstrated by comparing the predicted discharge curves of Li–O₂ batteries with experimental results at the high current density. Next, the validated modeling approach effectively captures the significant improvement in discharge capacity due to the formation of Li₂O₂ particles. Finally, it predicts the discharge capacities of Li–O₂ batteries with different air electrode microstructure designs and operating conditions

Observation of Electron-Beam-Induced Phase Evolution Mimicking the Effect of the Charge–Discharge Cycle in Li-Rich Layered Cathode Materials Used for Li Ion Batteries

Capacity loss and voltage fade upon electrochemical charge–discharge cycling observed in lithium-rich layered cathode oxides (Li­[Li_<i>x</i>Mn_<i>y</i>TM_{1–<i>x</i>–<i>y</i>}]­O₂, where TM = Ni, Co, or Fe) have recently been correlated with a gradual phase transformation featuring the formation of a surface reconstructed layer (SRL) that evolves from a thin (<2 nm), defect spinel layer upon the first charge to a relatively thick (∼5 nm), spinel or rock-salt layer upon continuous charge–discharge cycling. Here we report observations of an SRL and structural evolution of the SRL on the Li­[Li_0.2Ni_0.2Mn_0.6]­O₂ (LNMO) particles, which are identical to those reported due to the charge–discharge cycle but are a result of electron-beam irradiation during scanning transmission electron microscopy (STEM) imaging. Sensitivity of the lithium-rich layered oxides to high-energy electrons leads to the formation of a thin, defect spinel layer on surfaces of the particles upon exposure to a 200 kV electron beam for as little as 30 s under normal high-resolution STEM imaging conditions. Further electron irradiation produces a thicker layer of the spinel phase, ultimately producing a rock-salt layer at a higher electron exposure. Atomic-scale chemical mapping by energy dispersive X-ray spectroscopy in STEM indicates the electron-beam-induced SRL formation on LNMO is accomplished by migration of the transition metal ions to the Li sites without deconstruction of the lattice. This study provides insight into understanding the mechanism of forming the SRL and also possibly a means of studying structural evolution in the Li-rich layered oxides without involving electrochemistry

Extremely Stable Sodium Metal Batteries Enabled by Localized High-Concentration Electrolytes

Sodium (Na) metal is a promising anode for Na-ion batteries. However, the high reactivity of Na metal with electrolytes and the low Na metal cycling efficiency have limited its practical application in rechargeable Na metal batteries. High-concentration electrolytes (HCE, ≥4 M) consisting of sodium bis­(fluorosulfonyl)­imide (NaFSI) and ether solvent could ensure the stable cycling of Na metal with high Coulombic efficiency but at the cost of high viscosity, poor wettability, and high salt cost. Here, we report that the salt concentration could be significantly reduced (≤1.5 M) by a hydrofluoroether as an “inert” diluent, which maintains the solvation structures of HCE, thereby forming a localized high-concentration electrolyte (LHCE). A LHCE [2.1 M NaFSI/1,2-dimethoxyethane (DME)–bis­(2,2,2-trifluoroethyl) ether (BTFE) (solvent molar ratio 1:2)] enables dendrite-free Na deposition with a high Coulombic efficiency of >99%, fast charging (20C), and stable cycling (90.8% retention after 40 000 cycles) of Na∥Na₃V₂(PO₄)₃ batteries

Probing the Failure Mechanism of SnO₂ Nanowires for Sodium-Ion Batteries

Nonlithium metals such as sodium have attracted wide attention as a potential charge carrying ion for rechargeable batteries. Using in situ transmission electron microscopy in combination with density functional theory calculations, we probed the structural and chemical evolution of SnO₂ nanowire anodes in Na-ion batteries and compared them quantitatively with results from Li-ion batteries (Huang, J. Y.; et al. Science 2010, 330, 1515−1520). Upon Na insertion into SnO₂, a displacement reaction occurs, leading to the formation of amorphous Na_<i>x</i>Sn nanoparticles dispersed in Na₂O matrix. With further Na insertion, the Na_<i>x</i>Sn crystallized into Na₁₅Sn₄ (<i>x</i> = 3.75). Upon extraction of Na (desodiation), the Na_<i>x</i>Sn transforms to Sn nanoparticles. Associated with the dealloying, pores are found to form, leading to a structure of Sn particles confined in a hollow matrix of Na₂O. These pores greatly increase electrical impedance, therefore accounting for the poor cyclability of SnO₂. DFT calculations indicate that Na⁺ diffuses 30 times slower than Li⁺ in SnO₂, in agreement with in situ TEM measurement. Insertion of Na can chemomechanically soften the reaction product to a greater extent than in lithiation. Therefore, in contrast to the lithiation of SnO₂ significantly less dislocation plasticity was seen ahead of the sodiation front. This direct comparison of the results from Na and Li highlights the critical role of ionic size and electronic structure of different ionic species on the charge/discharge rate and failure mechanisms in these batteries

Dendrites and Pits: Untangling the Complex Behavior of Lithium Metal Anodes through Operando Video Microscopy

Enabling ultra-high energy density rechargeable Li batteries would have widespread impact on society. However the critical challenges of Li metal anodes (most notably cycle life and safety) remain unsolved. This is attributed to the evolution of Li metal morphology during cycling, which leads to dendrite growth and surface pitting. Herein, we present a comprehensive understanding of the voltage variations observed during Li metal cycling, which is directly correlated to morphology evolution through the use of operando video microscopy. A custom-designed visualization cell was developed to enable operando synchronized observation of Li metal electrode morphology and electrochemical behavior during cycling. A mechanistic understanding of the complex behavior of these electrodes is gained through correlation with continuum-scale modeling, which provides insight into the dominant surface kinetics. This work provides a detailed explanation of (1) when dendrite nucleation occurs, (2) how those dendrites evolve as a function of time, (3) when surface pitting occurs during Li electrodissolution, (4) kinetic parameters that dictate overpotential as the electrode morphology evolves, and (5) how this understanding can be applied to evaluate electrode performance in a variety of electrolytes. The results provide detailed insight into the interplay between morphology and the dominant electrochemical processes occurring on the Li electrode surface through an improved understanding of changes in cell voltage, which represents a powerful new platform for analysis