On the Capacity Losses Seen for Optimized Nano‐Si Composite Electrodes in Li‐Metal Half‐Cells

While the use of silicon‐based electrodes can increase the capacity of Li‐ion batteries considerably, their application is associated with significant capacity losses. In this work, the influences of solid electrolyte interphase (SEI) formation, volume expansion, and lithium trapping are evaluated for two different electrochemical cycling schemes using lithium‐metal half‐cells containing silicon nanoparticle–based composite electrodes. Lithium trapping, caused by incomplete delithiation, is demonstrated to be the main reason for the capacity loss while SEI formation and dissolution affect the accumulated capacity loss due to a decreased coulombic efficiency. The capacity losses can be explained by the increasing lithium concentration in the electrode causing a decreasing lithiation potential and the lithiation cut‐off limit being reached faster. A lithium‐to‐silicon atomic ratio of 3.28 is found for a silicon electrode after 650 cycles using 1200 mAhg^?1 capacity limited cycling. The results further show that the lithiation step is the capacity‐limiting step and that the capacity losses can be minimized by increasing the efficiency of the delithiation step via the inclusion of constant voltage delithiation steps. Lithium trapping due to incomplete delithiation consequently constitutes a very important capacity loss phenomenon for silicon composite electrodes.     

Electrochemical Reaction of Lithium with Nanostructured Silicon Anodes: A Study by In‐Situ Synchrotron X‐Ray Diffraction and Electron Energy‐Loss Spectroscopy

Silicon‐based anodes are an appealing alternative to graphite for lithium‐ion batteries because of their extremely high capacity. However, poor cycling stability and slow kinetics continue to limit the widespread use of silicon in commercial batteries. Performance improvement has been often demonstrated in nanostructured silicon electrodes, but the reaction mechanisms involved in the electrochemical lithiation of nanoscale silicon are not well understood. Here, in‐situ synchrotron X‐ray diffraction is used to monitor the subtle structural changes occurring in Si nanoparticles in a Si‐C composite electrode during lithiation. Local analysis by electron energy‐loss spectroscopy and transmission electron microscopy is performed to interrogate the nanoscale morphological changes and phase evolution of Si particles at different depths of discharge. It is shown that upon lithiation, Si nanoparticles behave quite differently than their micrometer‐sized counterparts. Although both undergo an electrochemical amorphization, the micrometer‐sized silicon exhibits a linear transformation during lithiation, while a two‐step process occurs in the nanoscale Si. In the first half of the discharge, lithium reacts with surfaces, grain boundaries and planar defects. As the reaction proceeds and the cell voltage drops, lithium consumes the crystalline core transforming it into amorphous Li_xSi with a primary particle size of just a few nanometers. Unlike the bulk silicon electrode, no Li₁₅Si₄ or other crystalline Li_xSi phases were formed in nanoscale Si at the fully‐lithiated state.     

Electrochemical Stiffness Changes in Lithium Manganese Oxide Electrodes

In situ strain and stress measurements are performed on composite electrodes to monitor potential‐dependent stiffness changes in lithium manganese oxide (LiMn₂O₄). Lithium insertion and removal results in asynchronous strain and stress generation in the electrode. Electrochemical stiffness changes are calculated by combining coordinated stress and strain measurements. The electrode experiences dramatic changes in electrochemical stiffness due to potential‐dependent Li⁺ ion intercalation mechanisms. The development of stress in the early stages of delithiation (at ≈3.95 V) due to a kinetic barrier at the electrode surface gives rise to stiffness changes in the electrode. Strain generation due to phase transformations reduces stiffness in the electrode at 4.17 V during delithiation and at 4.11 V during lithiation. During lithiation, stress generation due to Coulombic repulsions between occupied and incoming Li⁺ ions leads to stiffening of the electrode at 3.96 V. The electrode also experiences greater changes in stiffness during delithiation compared to lithiation. These changes in electrochemical stiffness provide insight into the interplay between mechanical and electrochemical properties which control electrode response to lithiation and delithiation.     

In Situ Measurement of Solid Electrolyte Interphase Evolution on Silicon Anodes Using Atomic Force Microscopy

In situ measurements of the growth of solid electrolyte interphase (SEI) layer on silicon and the lithiation‐induced volume changes in silicon in lithium ion half‐cells are reported. Thin film amorphous silicon electrodes are fabricated in a configuration that allows unambiguous separation of the total thickness change into contribution from SEI thickness and silicon volume change. Electrodes are assembled into a custom‐designed electrochemical cell, which is integrated with an atomic force microscope. The electrodes are subjected to constant potential lithiation/delithiation at a sequence of potential values and the thickness measurements are made at each potential after equilibrium is reached. Experiments are carried out with two electrolytes—1.2 m lithium hexafluoro‐phosphate (LiPF₆) in ethylene carbonate (EC) and 1.2 m LiPF₆ in propylene carbonate (PC)—to investigate the influence of electrolyte composition on SEI evolution. It is observed that SEI formation occurs predominantly during the first lithiation and the maximum SEI thickness is ≈17 and 10 nm respectively for EC and PC electrolytes. This study also presents the measured Si expansion ratio versus equilibrium potential and charge capacity versus equilibrium potential; both relationships display hysteresis, which is explained in terms of the stress–potential coupling in silicon.     

Influence of Oxides on the Stress Evolution and Reversibility during SnOx Conversion and Li‐Sn Alloying Reactions

The effect of varying the oxygen content in Sn and SnO_x films during potential dependent SnO_x conversion reactions and Li_ySn alloying relevant to Li ion battery anodes is examined. For metallic Sn films, the stresses and stability of the films are controlled by Li alloying reactions. Small, non‐contacting separated Sn particles exhibit higher electrochemical stability relative to more continuous polycrystalline films with larger particles. Metallic Sn particles develop tensile stress during Li_ySn de‐alloying as porous structures are formed. The amount of stress associated with lithiation and delithiation of well‐separated metallic particles decreases as a porous, easy to lithiate, material forms with cycling. During the lithiation of oxides, conversion reactions (SnO_x → Sn) and the lithiation of the metallic Sn control the stress responses of the films, leading to highly potential‐dependent stress development. In particular, evidence for a multistep electrochemical mechanism, in which partially reversible lithiation of the oxygen‐containing phases is conjoined with a fully reversible lithiation of the metallic phases of the Sn, is found. The electrochemical stress analysis provides new insight into these mechanisms and delineates the extent of the reversibility of lithiation and conversion reactions of oxides.     

Atomic Structure of Li2MnO3 after Partial Delithiation and Re‐Lithiation

Li₂MnO₃ is the parent compound of the well‐studied Li‐rich Mn‐based cathode materials xLi₂MnO₃·(1‐x)LiMO₂ for high‐energy‐density Li‐ion batteries. Li₂MnO₃ has a very high theoretical capacity of 458 mA h g^?1 for extracting 2 Li. However, the delithiation and lithiation behaviors and the corresponding structure evolution mechanism in both Li₂MnO₃ and Li‐rich Mn‐based cathode materials are still not very clear. In this research, the atomic structures of Li₂MnO₃ before and after partial delithiation and re‐lithiation are observed with spherical aberration‐corrected scanning transmission electron microscopy (STEM). All atoms in Li₂MnO₃ can be visualized directly in annular bright‐field images. It is confirmed accordingly that the lithium can be extracted from the LiMn₂ planes and some manganese atoms can migrate into the Li layer after electrochemical delithiation. In addition, the manganese atoms can move reversibly in the (001) plane when ca. 18.6% lithium is extracted and 12.4% lithium is re‐inserted. LiMnO₂ domains are also observed in some areas in Li_1.63MnO₃ at the first cycle. As for the position and occupancy of oxygen, no significant difference is found between Li_1.63MnO₃ and Li₂MnO₃.     

Spectroscopic Insight into Li‐Ion Batteries during Operation: An Alternative Infrared Approach

Multiple‐internal‐reflection infrared spectroscopy allows for the study of thin‐film amorphous silicon electrodes in situ and in operando, in conditions typical of those used in Li‐ion batteries. It brings an enhanced sensitivity, and the attenuated‐total‐reflection geometry allows for the extraction of quantitative information. When electrodes are cycled in representative electrolytes, the simultaneously recorded infrared spectra give an insight into the solid/electrolyte interphase (SEI) composition. They also unravel the dynamic behavior of this SEI layer by quantitatively assessing its thickness, which increases during silicon lithiation and partially decreases during delithiation. Li‐ion solvation effects in the vicinity of the electrode indicate that lithium incorporation in the solid phase is the rate‐determining step of the electrochemical processes during lithiation. The lithiation of the active material also results in the irreversible consumption of a large quantity of hydrogen in the pristine material. Finally, the evolution of the electronic absorption of the electrode material suggests that lithium diffusion is much easier after the first lithiation than in the pristine material. Therefore, in situ Fourier‐transform infrared spectroscopy performed in a well‐suited configuration efficiently extracts original and quantitative pieces of information on the surface and bulk phenomena affecting Li‐ion electrodes during their operation in realistic conditions.     

The Impact of Initial SEI Formation Conditions on Strain‐Induced Capacity Losses in Silicon Electrodes

The solid electrolyte interphase (SEI) that passivates silicon surfaces in Li ion batteries is subjected to extremely large mechanical strains during electrochemical cycling. The resulting degradation of these SEI films is a critical problem that limits the cycle life of silicon‐based electrodes. With the complex multiphase microstructure in conventional porous electrodes, it is not possible to directly measure the impact of these strains on SEI formation and capacity loss. To overcome this limitation a new in situ method is presented for applying controlled mechanical strains to SEI during electrochemical cycling. This approach uses patterned silicon films with different sized islands that act as model electrode particles. During lithiation/delithiation, the lateral expansion/contraction of the island edges applies in plane strains to the SEI. Detailed analysis of the island size effect then provides quantitative measurements of the impact of strain on the excess capacity losses that occur in different potential ranges. One key finding is that the applied strains lead to large capacity losses during lithiation only (during all cycles). Also, employing fast and slow SEI formation (first cycle) leads to large differences in the strain‐induced losses that occur during subsequent cycling.     

Solid‐State Prelithiation Enables High‐Performance Li‐Al‐H Anode for Solid‐State Batteries

Lithium alanates exhibit high theoretical specific capacities and appropriate lithiation/delithiation potentials, but suffer from poor reversibility, cycling stability, and rate capability due to their sluggish kinetics and extensive side reactions. Herein, a novel and facile solid‐state prelithiation approach is proposed to in situ prepare a Li₃AlH₆‐Al nanocomposite from a short‐circuited electrochemical reaction between LiAlH₄ and Li with the help of fast electron and Li‐ion conductors (C and P6₃mc LiBH₄). This nanocomposite consists of dispersive Al nanograins and an amorphous Li₃AlH₆ matrix, which enables superior electrochemical performance in solid‐state cells, as much higher specific capacity (2266 mAh g^?1), Coulombic efficiency (88%), cycling stability (71% retention in the 100th cycle), and rate capability (1429 mAh g^?1 at 1 A g^?1) are achieved. In addition, this nanocomposite works well in the solid‐state full cell with LiCoO₂ cathode, demonstrating its promising application prospects. Mechanism analysis reveals that the dispersive Al nanograins and amorphous Li₃AlH₆ matrix can dramatically enhance the lithiation and delithiation kinetics without side reactions, which is mainly responsible for the excellent overall performance. Moreover, this solid‐state prelithiation approach is general and can also be applied to other Li‐poor electrode materials for further modification of their electrochemical behavior.     

Microstructure Study of Electrochemically Driven LixSi

We report the direct observation of microstructural changes of Li_xSi electrode with lithium insertion. HRTEM experiments confirm that lithiated amorphous silicon forms a shell around a core made up of the unlithiated silicon and that fully lithiated silicon contains a large number of pores of which concentration increases toward the center of the particle. Chemomechanical modeling is employed in order to explain this mechanical degradation resulting from stresses in the Li_xSi particles with lithium insertion. Because lithiation‐induced volume expansion and pulverization are the key mechanical effects that plague the performance and lifetime of high‐capacity Si anodes in lithium‐ion batteries, our observations and chemomechanical simulation provide important mechanistic insight for the design of advanced battery materials.     

High‐Performance All‐Solid‐State Lithium–Sulfur Batteries Enabled by Amorphous Sulfur‐Coated Reduced Graphene Oxide Cathodes

Safety and the polysulfide shuttle reaction are two major challenges for liquid electrolyte lithium–sulfur (Li–S) batteries. Although use of solid‐state electrolytes can overcome these two challenges, it also brings new challenges by increasing the interface resistance and stress/strain. In this work, the interface resistance and stress/strain of sulfur cathodes are significantly reduced by conformal coating ≈2 nm sulfur (S) onto reduced graphene oxide (rGO). An Li–S full cell consisting of an rGO@S‐Li₁₀GeP₂S₁₂‐acetylene black (AB) composite cathode is evaluated. At 60 °C, the all‐solid‐state Li–S cell demonstrates a similar electrochemical performance as in liquid organic electrolyte, with high rate capacities of 1525.6, 1384.5, 1336.3, 903.2, 502.6, and 204.7 mA h g^?1 at 0.05, 0.1, 0.5, 1.0, 2.0, and 5.0 C, respectively. It can maintain a high and reversible capacity of 830 mA h g^?1 at 1.0 C for 750 cycles. The uniform distribution of the rGO@S nanocomposite in the Li₁₀GeP₂S₁₂‐AB matrix generates uniform volume changes during lithiation/delithiation, significantly reducing the stress/strain, thus extending the cycle life. Minimization of the stress/strain of solid cells is the key for a long cycle life of all‐solid‐state Li–S batteries.     

Li–Ti Cation Mixing Enhanced Structural and Performance Stability of Li‐Rich Layered Oxide

Li‐rich layered metal oxides are one type of the most promising cathode materials in lithium‐ion batteries but suffer from severe voltage decay during cycling because of the continuous transition metal (TM) migration into the Li layers. A Li‐rich layered metal oxide Li_1.2Ti_0.26Ni_0.18Co_0.18Mn_0.18O₂ (LTR) is hereby designed, in which some of the Ti⁴⁺ cations are intrinsically present in the Li layers. The native Li–Ti cation mixing structure enhances the tolerance for structural distortion and inhibits the migration of the TM ions in the TMO₂ slabs during (de)lithiation. Consequently, LTR exhibits a remarkable cycling stability of 97% capacity retention after 182 cycles, and the average discharge potential drops only 90 mV in 100 cycles. In‐depth studies by electron energy loss spectroscopy and aberration‐corrected scanning transmission electron microscopy demonstrate the Li–Ti mixing structure. The charge compensation mechanism is uncovered with X‐ray absorption spectroscopy and explained with the density function theory calculations. These results show the superiority of introducing transition metal ions into the Li layers in reinforcing the structural stability of the Li‐rich layered metal oxides. These findings shed light on a possible path to the development of Li‐rich materials with better potential retention and a longer lifespan.     

In Operando Strain Measurement of Bicontinuous Silicon‐Coated Nickel Inverse Opal Anodes for Li‐Ion Batteries

Elastic strains are measured in operando in a nanostructured silicon‐coated nickel inverse opal scaffold anode, using X‐ray diffraction to study the Si (de)lithiation‐induced Ni strains. The volume expansion upon lithiation of the Si in the anode is constrained by the surrounding Ni scaffold, causing mismatch stresses and strains in the Si and Ni phases during cycling. The Ni strains are measured in operando during (dis)charge cycles, using diffraction peak position and peak broadness to describe the distribution of strain in the Ni. During lithiation, compressive strains in the Ni first increase linearly with charge, after which a gradually decreasing strain rate is observed as the maximum lithiation state is approached; upon delithiation a similar process occurs. In‐plane average compressive strains on the order of 990 ± 40 με are measured in the Ni scaffold during lithiation, corresponding to compressive stresses of 215 ± 9 MPa. The decreasing strain rates and decreasing maximum and recovered strains suggest that plasticity in Ni and/or Si, as well as delamination between Ni and Si, may occur during cycling. Rate sensitivity in capacity is correlated with strain and a maximum Ni compressive stress of 230 ± 40 MPa is measured at the maximum state of lithiation.     

Cu3P Binary Phosphide: Synthesis via a Wet Mechanochemical Method and Electrochemical Behavior as Negative Electrode Material for Lithium‐Ion Batteries

Mechanochemical synthesis of Cu₃P in the presence of n‐dodecane results in a material with a secondary particle size distribution of 10 μm, secondary particles which consist of homogeneously agglomerated 20 nm primary particles. The electrochemical performance of Cu₃P with lithium is influenced by the reaction depth, in other words by the lower potential cut‐off. During the electrochemical reaction, the displacement of copper by lithium from the Cu₃P structure until the formation of Li₃P and Cu deteriorates the capacity retention. Improved performance was obtained when the charge potential was limited to 0.50 V (vs. Li/Li⁺) and the formation of the Li_xCu_3‐xP phase (0 ≤ × ≤ 2). In this case, when the potential is limited to 0.5 V, the capacity is stable for more than 50 cycles. Acceptable electrochemical performances in Li‐ion cells within the voltage range 0.50–2.0 V (vs. Li/Li⁺) were shown when Cu₃P was used as an anode and Li_1.2(Ni_0.13Mn_0.54Co_0.13)O₂ and LiNi_0.5Mn_1.5O₄ as positive electrode materials.     

3D Porous Cu–Zn Alloys as Alternative Anode Materials for Li‐Ion Batteries with Superior Low T Performance

Zinc is recently gaining interest in the battery community as potential alternative anode material, because of its large natural abundance and potentially larger volumetric density than graphite. Nevertheless, pure Zn anodes have shown so far very poor cycling performance. Here, the electrochemical performance of Zn‐rich porous Cu–Zn alloys electrodeposited by an environmentally friendly (aqueous) dynamic hydrogen bubble template method is reported. The lithiation/delithiation mechanism is studied in detail by both in situ and ex situ X‐ray diffraction, indicating the reversible displacement of Zn from the Cu–Zn alloy upon reaction with Li. The influence of the alloy composition on the performance of carbon‐ and binder‐free electrodes is also investigated. The optimal Cu:Zn atomic ratio is found to be 18:82, which provides impressive rate capability up to 10 A g^?1 (≈30C), and promising capacity retention upon more than 500 cycles. The high electronic conductivity provided by Cu, and the porous electrode morphology also enable superior lithium storage capability at low temperature. Cu₁₈Zn₈₂ can indeed steadily deliver ≈200 mAh g^?1 at ?20 °C, whereas an analogous commercial graphite electrode rapidly fades to only 12 mAh g^?1.     

Enabling High‐Energy Solid‐State Batteries with Stable Anode Interphase by the Use of Columnar Silicon Anodes

All‐solid‐state batteries (ASSBs) with silicon anodes are promising candidates to overcome energy limitations of conventional lithium‐ion batteries. However, silicon undergoes severe volume changes during cycling leading to rapid degradation. In this study, a columnar silicon anode (col‐Si) fabricated by a scalable physical vapor deposition process (PVD) is integrated in all‐solid‐state batteries based on argyrodite‐type electrolyte (Li₆PS₅Cl, 3 mS cm^?1) and Ni‐rich layered oxide cathodes (LiNi_0.9Co_0.05Mn_0.05O₂, NCM) with a high specific capacity (210 mAh g^?1). The column structure exhibits a 1D breathing mechanism similar to lithium, which preserves the interface toward the electrolyte. Stable cycling is demonstrated for more than 100 cycles with a high coulombic efficiency (CE) of 99.7–99.9% in full cells with industrially relevant areal loadings of 3.5 mAh cm^?2, which is the highest value reported so far for ASSB full cells with silicon anodes. Impedance spectroscopy revealed that anode resistance is drastically reduced after first lithiation, which allows high charging currents of 0.9 mA cm^?2 at room temperature without the occurrence of dendrites and short circuits. Finally, in‐operando monitoring of pouch cells gave valuable insights into the breathing behavior of the solid‐state cell.     

Lithium Intercalation Behavior in Multilayer Silicon Electrodes

Next generation lithium battery materials will require a fundamental shift from those based on intercalation to elements or compounds that alloy directly with lithium. Intermetallics, for instance, can electrochemically alloy to Li_4.4M (M = Si, Ge, Sn, etc.), providing order‐of‐magnitude increases in energy density. Unlike the stable crystal structure of intercalation materials, intermetallic‐based electrodes undergo dramatic volume changes that rapidly degrade the performance of the battery. Here, the energy density of silicon is combined with the structural reversibility of an intercalation material using a silicon/metal‐silicide multilayer. In operando X‐ray reflectivity confirms the multilayer's structural reversibility during lithium insertion and extraction, despite an overall 3.3‐fold vertical expansion. The multilayer electrodes also show enhanced long‐term cyclability and rate capabilities relative to a comparable silicon thin film electrode. This intercalation behavior found by dimensionally constraining silicon's lithiation promises applicability to a wide range of conversion reactions.     

Quantification of Pseudocapacitive Contribution in Nanocage‐Shaped Silicon–Carbon Composite Anode

Pseudocapacitive materials have been highlighted as promising electrode materials to overcome slow diffusion‐limited redox mechanism in active materials, which impedes fast charging/discharging in energy storage devices. However, previously reported pseudocapacitive properties have been rarely used in lithium‐ion batteries (LIBs) and evaluation methods have been limited to those focused on thin‐film‐type electrodes. Hence, a nanocage‐shaped silicon–carbon composite anode is proposed with excellent pseudocapacitive qualities for LIB applications. This composite anode exhibits a superior rate capability compared to other Si‐based anodes, including commercial silicon nanoparticles, because of the higher pseudocapacitive contribution coming from ultrathin Si layer. Furthermore, unprecedent 3D pore design in cage shape, which prevents the particle scale expansion even after full lithiation demonstrates the high cycling stability. This concept can potentially be used to realize high‐power and high‐energy LIB anode materials.     

Stress‐Relieved Nanowires by Silicon Substitution for High‐Capacity and Stable Lithium Storage

Silicon is promising as a high energy anode for next‐generation lithium‐ion batteries. However, severe capacity fading upon cycling associated with huge volume change is still an obstacle for silicon toward practical applications. Herein, the authors report that Si‐substituted Zn₂(GeO₄)_0.8(SiO₄)_0.2 nanowires can effectively suppress volume expansion effect, exhibiting high specific capacity (1274 mA h g^?1 at 0.2 A g^?1 after 700 cycles) and ultralong cycling stability (2000 cycles at 5 A g^?1 with a capacity decay rate of 0.008% per cycle), which represents outstanding comprehensive performance. The superior performance is ascribed to the substitution of Si atom that imparts to the nanowires not only high reactivity and reversibility, but also the unique stress‐relieved property upon lithiation which is further confirmed by detailed density‐functional theory computation. This work provides a new guideline for designing high‐performance Si‐based materials toward practical energy storage applications.     

Galvanic Corrosion of Lithium‐Powder‐Based Electrodes

Lithium metal is considered to be the most promising anode for the next generation of batteries if the issues related to safety and low coulombic efficiency can be overcome. It is known that the initial morphology of the lithium metal anode has a great influence on the cycling characteristics of a lithium metal battery (LMB). Lithium‐powder‐based electrodes (Li_p‐electrodes) are reported to diminish the occurrence of high surface area lithium deposits. Usually, ultra‐thin lithium foils (<50 µm) and Li_p‐electrodes are prepared on a copper substrate, thus a metal–metal contact area is generated. The combination of these two metals in the presence of an electrolyte, however, can lead to galvanic corrosion. Herein, the corrosion behavior of Li_p‐electrodes is studied. The porosity of such electrodes leads to a high amount of accessible Cu surface in contact with electrolyte. As a consequence, Li_p‐electrodes aged for 1 week in the electrolyte show spontaneous lithium dissolution near the junction to copper and void formation on the lithium‐powder particles. This corrosion process affects the delivered capacity of Li_p‐electrodes and increases the overvoltage of the lithium electrodissolution process. The occurrence of corrosion at the Cu|Li_p interface raises concerns about the practicality of multi‐metallic component systems for LMBs.