Revisiting the Corrosion of the Aluminum Current Collector in Lithium-Ion Batteries

High-mass loaded redox-active lignin functionalized carbonized wood collector to construct sustainable and high-performance supercapacitors

Traditional electrode materials for supercapacitors often face issues like high toxicity, cost, and non-renewability. To address these drawbacks, biomass-based alternatives are being explored, aligning with green development trends. Herein, carbonized wood (CW) with rich pore structure and redox-active lignin are combined to fabricate an all-wood-based sustainable supercapacitor electrode material. Due to its inherent porous structure, CW provides a larger surface area for accommodating active materials ion, enabling the electrode to achieve a higher lignin loading capacity of 2.82-11.68 mg/cm2. Furthermore, the utilization of lignin as a substitute for conventional transition metal-based pseudocapacitor material functionalized CW endows the electrode with exemplary electrochemical performance while guaranteeing the comprehensive sustainability of the electrode. This synergy confers the electrode with exceptional electrical performance, yielding an areal capacitance of 960.7 mF/cm2 at a current density of 1 mA/cm2. The symmetric supercapacitors (SSC) manufactured by this composite electrode can achieve a notable areal energy density of 0.14 mWh/cm2 and a power density of 15.98 mW/cm2, while maintaining an outstanding capacitance retention rate of 81 % after 50,000 cycles at 20 mA/cm2. The manufacture of CW-lignin electrode underscores the potential of utilizing renewable biomass resources as alternatives for developing high-performance energy storage applications, thereby reducing negative environmental impacts.

Graphene Enables Aluminum Current Collectors of 5 V Class Battery

The five volt-class battery is one promising candidate of high energy density lithium-ion batteries. However, it suffers from limited electrochemical performance due to many problems, one of which is Al current collector corrosion. The corrosion greatly affects the electrochemical performance of batteries, so uncovering the Al corrosion mechanism and developing its protection strategy in the 5 V-class battery becomes important. Here, we experimentally realize a corrosion-resistant Al current collector via graphene protection. The experimental and theoretical calculation indicate that graphene can work as a physical barrier to inhibit direct contact between LiPF6-based electrolyte and an Al current collector, reducing the side reactions between Al current collector and HF originated from electrolyte. What is more, graphene increases the Al corrosion reaction potential, raising the difficulty of electrochemical corrosion. These effects improve the electrochemical performance of the 5 V-class battery, especially the rate performance and cycling stability. The work is beneficial for the development of a 5 V-class battery.

Study of the Corrosion Behavior of Cathode Current Collector in LiFSI Electrolyte

Cycling aging is the one of the main reasons affecting the lifetime of lithium-ion batteries and the contribution of aluminum current collector corrosion to the ageing is not fully recognized. In general, aluminum is corrosion resistant to electrolyte since a non-permeable surface film of alumina is naturally formed. However, corrosion of aluminum current collector can still occur under certain conditions such as lithium bis(fluorosulfonyl)imide (LiFSI)-based electrolyte or high voltage. Herein, we investigates the corrosion of aluminum current collector in the electrolyte of 1.2 M LiFSI in ethylene carbonate (EC) and ethyl methyl carbonate (EMC) mixed solvents. The electrochemical results shows that the corrosion current of aluminum is enhanced by cycling time and potential, which is correlated with the surface species and morphology. The formation of AlF3, which is induced by deep penetration of F- anions through surface passivation film, leads to internal volume change and the surface crack in the end. Our work will be inspiring for future development of high-energy-density and high-power-density lithium-ion batteries in which the LiFSI salt will be intensively used.

Side Reactions/Changes in Lithium-Ion Batteries: Mechanisms and Strategies for Creating Safer and Better Batteries

Lithium-ion batteries (LIBs), in which lithium ions function as charge carriers, are considered the most competitive energy storage devices due to their high energy and power density. However, battery materials, especially with high capacity undergo side reactions and changes that result in capacity decay and safety issues. A deep understanding of the reactions that cause changes in the battery's internal components and the mechanisms of those reactions is needed to build safer and better batteries. This review focuses on the processes of battery failures, with voltage and temperature as the underlying factors. Voltage-induced failures result from anode interfacial reactions, current collector corrosion, cathode interfacial reactions, overcharge, and over-discharge, while temperature-induced failure mechanisms include SEI decomposition, separator damage, and interfacial reactions between electrodes and electrolytes. The review also presents protective strategies for controlling these reactions. As a result, the reader is offered a comprehensive overview of the safety features and failure mechanisms of various LIB components.

Delineating the Impact of Transition-Metal Crossover on Solid-Electrolyte Interphase Formation with Ion Mass Spectrometry

Lithium-metal batteries (LMB) employing cobalt-free layered-oxide cathodes are a sustainable path forward to achieving high energy densities, but these cathodes exhibit substantial transition-metal dissolution during high-voltage cycling. While transition-metal crossover is recognized to disrupt solid-electrolyte interphase (SEI) formation on graphite anodes, experimental evidence is necessary to demonstrate this for lithium-metal anodes. In this work, advanced high-resolution 3D chemical analysis is conducted with time-of-flight secondary-ion mass spectrometry (TOF-SIMS) to establish spatial correlations between the transition metals and electrolyte decomposition products found on cycled lithium-metal anodes. Insights into the localization of various chemistries linked to crucial processes that define LMB performance, such as lithium deposition, SEI growth, and transition-metal deposition are deduced from a precise elemental and spatial analysis of the SEI. Heterogenous transition-metal deposition is found to perpetuate both heterogeneous SEI growth and lithium deposition on lithium-metal anodes. These correlations are confirmed across various lithium-metal anodes that are cycled with different cobalt-free cathodes and electrolytes. An advanced electrolyte that is stable to higher voltages is shown to minimize transition-metal crossover and its effects on lithium-metal anodes. Overall, these results highlight the importance of maintaining uniform SEI coverage on lithium-metal anodes, which is disrupted by transition-metal crossover during operation at high voltages.

Electrolyte Engineering Empowers Li||CFx Batteries to Achieve High Energy Density and Low Self-Discharge at Harsh Conditions

Given its exceptional theoretical energy density (over 2000 Wh kg-1), lithium||carbon fluoride (Li||CFx) battery has garnered global attention. N-methylpyrrolidone (NMP)-based electrolyte is regarded as one promising candidate for tremendously enhancing the energy density of Li||CFx battery, provided self-discharge challenges can be resolved. This study successfully achieves a low self-discharge (LSD) and desirable electrochemical performance in Li||CFx batteries at high temperatures by utilizing NMP as the solvent and incorporating additional ingredients, including vinylene carbonate additive, as well as the dual-salt systems formed by LiBF4 with three different Li salts, namely lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, and LiNO3. The experimental results unfold that the proposed methods not only minimize aluminum current collector corrosion, but also effectively passivate the Li metal anode. Among them, LiNO3 exhibits the most pronounced effect that achieves an energy density of ≈2400 Wh kg-1 at a current density of 10 mA g-1 at 30 °C, nearly 0% capacity-fade rate after 300 h of storage at 60 °C, and the capability to maintain a stable open-circuit voltage over 4000 h. This work provides a distinctive perspective on how to realize both high energy density and LSD rates at high temperature of Li||CFx battery.

Critical Contribution of Imbalanced Charge Loss to Performance Deterioration of Si-Based Lithium-Ion Cells during Calendar Aging

Increasing the energy density of lithium-ion batteries, and thereby reducing costs, is a major target for industry and academic research. One of the best opportunities is to replace the traditional graphite anode with a high-capacity anode material, such as silicon. However, Si-based lithium-ion batteries have been widely reported to suffer from a limited calendar life for automobile applications. Heretofore, there lacks a fundamental understanding of calendar aging for rationally developing mitigation strategies. Both open-circuit voltage and voltage-hold aging protocols were utilized to characterize the aging behavior of Si-based cells. Particularly, a high-precision leakage current measurement was applied to quantitatively measure the rate of parasitic reactions at the electrode/electrolyte interface. The rate of parasitic reactions at the Si anode was found 5 times and 15 times faster than those of LiNi0.8Mn0.1Co0.1O2 and LiFePO4 cathodes, respectively. The imbalanced charge loss from parasitic reactions plays a critical role in exacerbating performance deterioration. In addition, a linear relationship between capacity loss and charge consumption from parasitic reactions provides fundamental support to assess calendar life through voltage-hold tests. These new findings imply that longer calendar life can be achieved by suppressing parasitic reactions at the Si anode to balance charge consumption during calendar aging.

Hexacyclic Chelated Lithium Stable Solvates for Highly Reversible Cycling of High-Voltage Lithium Metal Battery

Ether-based electrolytes that are endowed with decent compatibility towards lithium anode have been regarded as promising candidates for constructing energy-dense lithium metal batteries (LMBs), but their applications are hindered by low oxidation stability in conventional salt concentration. Here, we reported that regulating the chelating power and coordination structure can remarkably increase the high-voltage stability of ether-based electrolytes and lifespan of LMBs. Two ether molecules of 1,3-dimethoxypropane (DMP) and 1,3-diethoxypropane (DEP) are designed and synthesized as solvents of electrolytes to replace the traditional ether solvent (1,2-dimethoxyethane, DME). Both computational and spectra reveal that the transition from five- to six-membered chelate solvation structure by adding one methylene on DME results in the formation of weak Li solvates, which increase the reversibility and high-voltage stability in LMBs. Even under lean electrolyte (5 mL Ah-1 ) and low anode to cathode ratio (2.6), the fabricated high-voltage Li||LiNi0.8 Co0.1 Mn0.1 O2 LMBs using electrolyte of 2.30 M Lithiumbisfluorosulfonimide (LiFSI)/DMP still show capacity retention over 90 % after 184 cycles. This work highlights the importance of designing the coordination structures in non-fluorine ether electrolytes for rechargeable batteries.

Systematic Study of Aluminum Corrosion in Ionic Liquid Electrolytes for Sodium-Ion Batteries: Impact of Temperature and Concentration

The development of sodium-ion batteries utilizing sulfonylamide-based electrolytes is significantly encumbered by the corrosion of the Al current collector, resulting in capacity loss and poor cycling stability. While ionic liquid electrolytes have been reported to suppress Al corrosion, a recent study found that pitting corrosion occurs even when ionic liquids are employed. This study investigates the effects of temperature and Na salt concentration on the Al corrosion behavior in different sulfonylamide-based ionic liquid electrolytes for sodium-ion batteries. In the present work, cyclic voltammetry measurements and scanning electron microscopy showed that severe Al corrosion occurred in ionic liquids at high temperatures and low salt concentrations. X-ray photoelectron spectroscopy was employed to identify the different elemental components and verify the thickness of the passivation layer formed under varied salt concentrations and temperatures. The differences in the corrosion behaviors observed under the various conditions are ascribed to the ratio of free [FSA]^- to Na⁺-coordinating [FSA]^- in the electrolyte and the stability of the newly formed passivation layer. This work aims at augmenting the understanding of Al corrosion behavior in ionic liquid electrolytes to develop advanced batteries.

A Review on Regulating Li+ Solvation Structures in Carbonate Electrolytes for Lithium Metal Batteries

Lithium metal batteries (LMBs) are considered promising candidates for next-generation battery systems due to their high energy density. However, commercialized carbonate electrolytes cannot be used in LMBs due to their poor compatibility with lithium metal anodes. While increasing cut-off voltage is an effective way to boost the energy density of LMBs, conventional ethylene carbonate-based electrolytes undergo a number of side reactions at high voltages. It is therefore critical to upgrade conventional carbonate electrolytes, the performance of which is highly influenced by the solvation structure of lithium ions (Li⁺ ). This review provides a comprehensive overview of the strategies to regulate the solvation structure of Li⁺ in carbonate electrolytes for LMBs by better understanding the science behind the Li⁺ solvation structure and Li⁺ behavior. Different strategies are systematically compared to help select better electrolytes for specific applications. The remaining scientific and technical problems are pointed out, and directions for future research on carbonate electrolytes for LMBs are proposed.

A Review of Nonaqueous Electrolytes, Binders, and Separators for Lithium-Ion Batteries

Lithium-ion batteries (LIBs) are the most important electrochemical energy storage devices due to their high energy density, long cycle life, and low cost. During the past decades, many review papers outlining the advantages of state-of-the-art LIBs have been published, and extensive efforts have been devoted to improving their specific energy density and cycle life performance. These papers are primarily focused on the design and development of various advanced cathode and anode electrode materials, with less attention given to the other important components of the battery. The “nonelectroconductive” components are of equal importance to electrode active materials and can significantly affect the performance of LIBs. They could directly impact the capacity, safety, charging time, and cycle life of batteries and thus affect their commercial application. This review summarizes the recent progress in the development of nonaqueous electrolytes, binders, and separators for LIBs and discusses their impact on the battery performance. In addition, the challenges and perspectives for future development of LIBs are discussed, and new avenues for state-of-the-art LIBs to reach their full potential for a wide range of practical applications are outlined.

Graphic Abstract

Chasing protons in lithium-ion batteries

Parasitic reactions between delithiated cathode materials and non-aqueous electrolytes have been a major barrier that limits the upper cutoff potential of cathode materials. It is of great importance to suppress such parasitic reactions to unleash the high-energy-density potential of high voltage cathode materials. Although major effort has been made to identify the chemical composition of the cathode electrolyte interface using various cutting edge characterization tools, the chemical nature of parasitic reactions remains a puzzle. This severely hinders the rational development of stable high voltage cathode/electrolyte pairs for high-energy density lithium-ion batteries. This feature article highlights our latest effort in understanding the chemical/electrochemical role of the cathode electrolyte interface using protons as a chemical tracer for parasitic reactions.

Direct-Chemical Vapor Deposition-Enabled Graphene for Emerging Energy Storage: Versatility, Essentiality, and Possibility

The direct chemical vapor deposition (CVD) technique has stimulated an enormous scientific and industrial interest to enable the conformal growth of graphene over multifarious substrates, which readily bypasses tedious transfer procedure and empowers innovative materials paradigm. Compared to the prevailing graphene materials (i.e., reduced graphene oxide and liquid-phase exfoliated graphene), the direct-CVD-enabled graphene harnesses appealing structural advantages and physicochemical properties, accordingly playing a pivotal role in the realm of electrochemical energy storage. Despite conspicuous progress achieved in this frontier, a comprehensive overview is still lacking by far and the synthesis-structure-property-application nexus of direct-CVD-enabled graphene remains elusive. In this topical review, rather than simply compiling the state-of-the-art advancements, the versatile roles of direct-CVD-enabled graphene are itemized as (i) modificator, (ii) cultivator, (iii) defender, and (iv) decider. Furthermore, essential effects on the performance optimization are elucidated, with an emphasis on fundamental properties and underlying mechanisms. At the end, perspectives with respect to the material production and device fabrication are sketched, aiming to navigate the future development of direct-CVD-enabled graphene en-route toward pragmatic energy applications and beyond.

Stable non-corrosive sulfonimide salt for 4-V-class lithium metal batteries

Rechargeable lithium metal (Li⁰) batteries (RLMBs) are considered attractive for improving Li-ion batteries. Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) has been extensively used as a conducting salt for RLMBs due to its advantageous stability and innocuity. However, LiTFSI-based electrolytes are corrosive towards aluminium (Al⁰) current collectors at low potentials (>3.8 V versus Li/Li⁺), thereby excluding their application in 4-V-class RLMBs. Herein, we report on a non-corrosive sulfonimide salt, lithium (difluoromethanesulfonyl)(trifluoromethanesulfonyl)imide (LiDFTFSI), that remarkably suppresses the anodic dissolution of the Al⁰ current collector at high potentials (>4.2 V versus Li/Li⁺) and significantly improves the cycling performance of Li(Ni_1/3Mn_1/3Co_1/3)O₂ (NMC111) cells. In addition, this sulfonimide salt results in the growth of an advantageous solid electrolyte interphase on the Li⁰ electrode. The replacement of either LiTFSI or LiPF₆ with LiDFTFSI endows a Li⁰||NMC111 cell with superior cycling stability and capacity retention (87% at cycle 200), demonstrating the decisive role of the salt anion in dictating the electrochemical performance of RLMBs.

Ionic liquid-containing cathodes empowering ceramic solid electrolytes

Although ceramic solid electrolytes, such as Li₇La₃Zr₂O₁₂ (LLZO), are promising candidates to replace conventional liquid electrolytes for developing safe and high-energy-density solid-state Li-metal batteries, the large interfacial resistance between cathodes and ceramic solid electrolytes severely limits their practical application. Here we developed an ionic liquid (IL)-containing while nonfluidic quasi-solid-state LiCoO₂ (LCO) composite cathode, which can maintain good contact with an Al-doped LLZO (Al-LLZO) ceramic electrolyte. Accordingly the interfacial resistance between LCO and Al-LLZO was significantly decreased. Quasi-solid-state LCO/Al-LLZO/Li cells demonstrated relatively high capacity retention of about 80% after 100 cycles at 60°C. The capacity decay was mainly because of the instability of the IL. Nevertheless, the IL-containing LCO cathode enabled the use of Al-LLZO as a solid electrolyte in a simple and practical way. Identifying a suitable IL is critical for the development of quasi-solid-state Li-metal batteries with a ceramic solid electrolyte.

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An ionic liquid (IL)-containing quasi-solid-state LiCoO₂ (LCO) cathode was developed

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Quasi-solid-state LCO/Al-doped Li₇La₃Zr₂O₁₂/Li cell showed high capacity retention

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Optimal IL content in the quasi-solid-state LCO cathode was about 11 wt%

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Battery capacity degradation was mainly due to the instability of the IL

Steric Effect Tuned Ion Solvation Enabling Stable Cycling of High-Voltage Lithium Metal Battery

1,2-Dimethoxyethane (DME) is a common electrolyte solvent for lithium metal batteries. Various DME-based electrolyte designs have improved long-term cyclability of high-voltage full cells. However, insufficient Coulombic efficiency at the Li anode and poor high-voltage stability remain a challenge for DME electrolytes. Here, we report a molecular design principle that utilizes a steric hindrance effect to tune the solvation structures of Li⁺ ions. We hypothesized that by substituting the methoxy groups on DME with larger-sized ethoxy groups, the resulting 1,2-diethoxyethane (DEE) should have a weaker solvation ability and consequently more anion-rich inner solvation shells, both of which enhance interfacial stability at the cathode and anode. Experimental and computational evidence indicates such steric-effect-based design leads to an appreciable improvement in electrochemical stability of lithium bis(fluorosulfonyl)imide (LiFSI)/DEE electrolytes. Under stringent full-cell conditions of 4.8 mAh cm^-2 NMC811, 50 μm thin Li, and high cutoff voltage at 4.4 V, 4 M LiFSI/DEE enabled 182 cycles until 80% capacity retention while 4 M LiFSI/DME only achieved 94 cycles. This work points out a promising path toward the molecular design of non-fluorinated ether-based electrolyte solvents for practical high-voltage Li metal batteries.

Complementary Electrolyte Design for Li Metal Batteries in Electric Vehicle Applications

A complementary electrolyte system with 0.8 M lithium bis(fluorosulfonylimide) (LiFSI) salt and 2 wt % lithium perchlorate (LiClO₄) additive in fluoroethylene carbonate (FEC)/ethyl methyl carbonate (EMC) solution enables not only stable cycling of lithium metal batteries (LMBs) with practical loading (<30 μm lithium anode, cathode loading > 4 mAh/cm²) but also outstanding degradation stability toward the end of cycle life when compared to the conventional electrolyte. Although the use of LiFSI salt can increase the electrolyte conductivity and lengthen the cycle life of LMBs, the aged lithium anode morphology formed by the sacrificial decomposition of LiFSI is highly porous, leading to an abrupt cell capacity drop toward the end of cycling. Moreover, the inability to stop aluminum corrosion by the LiFSI-based electrolyte also causes cracking of the cathode tab during prolonged cycling. It is observed that a highly porous aged lithium consumed electrolyte at a higher rate, leading to the dry-out of electrolyte solvents. On the contrary, dense aged lithium anode morphology increased the localized current applied on the lithium, causing the formation of lithium dendrite. Thus, porosity control is the key to enhance battery performance. In this complementary system, LiClO₄ was introduced as an advanced additive to not only improve the capacity retention rate but also mitigate the abrupt capacity drop toward the end of cycle life because LiClO₄ acted as a pore astringent reducing the porosity of the aged lithium metal anode to the desired level. Moreover, the addition of LiClO₄ can also suppress the Al corrosion, allowing stable high-voltage cycling of LMBs. The synergistic effect of combining LiFSI salt and a LiClO₄ additive leads to an electrolyte system that can facilitate the application of high-energy LMBs with practical electrode loading.

Atomic-scale constituting stable interface for improved LiNi0.6Mn0.2Co0.2O2 cathodes of lithium-ion batteries

Ascribed to their higher capacity and lower cost compared to conventional LiCoO₂, the Ni-rich layered LiNi_0.6Mn_0.2Co_0.2O₂ (NMC622) is now considered as one promising cathode for lithium-ion batteries (LIBs). However, it still suffers from some evident performance degradation, especially under high cutoff voltages (i.e., >4.3 V versus Li/Li⁺). The performance degradation typically is exhibited as capacity fading and voltage drop, mainly originating from an instable interface between the NMC622 and electrolyte as well as the evolution of the NMC structure. To improve the interfacial and structural stability of NMC cathodes, herein we deposited an ultrathin layer of Al₂O₃ coatings (<5 nm) conformally over NMC622 composite electrodes directly using atomic layer deposition (ALD). It was found that, under different upper cutoff voltages (4.3, 4.5, and 4.7 V), the ALD Al₂O₃ coatings enable enhanced performance of NMC622 cathodes with better cyclability and higher capacity. Particularly, the beneficial effects of the ALD Al₂O₃ coatings are more remarkable at higher upper cutoff voltages (4.5 and 4.7 V). Furthermore, the ALD coatings can significantly improve the rate capability of NMC622. To this end, we utilized a suite of characterization tools and performed a series of electrochemical tests to clarify the effects of the ALD Al₂O₃ coatings. This study revealed that the beneficial effects of the Al₂O₃ ALD coatings are multiple: (i) serving as an artificial layer of solid electrolyte interphase to mitigate undesirable interfacial reactions; (ii) acting as a physical barrier to inhibit metal dissolution of NMC; and (iii) forming a reinforced networked overcoating to boost the mechanical integrity of NMC cathodes. This study is favorable for designing high-performance NMC cathodes.

Nature of the Cathode-Electrolyte Interface in Highly Concentrated Electrolytes Used in Graphite Dual-Ion Batteries

Dual-ion batteries (DIBs) generally operate beyond 4.7 V vs Li⁺/Li⁰ and rely on the intercalation of both cations and anions in graphite electrodes. Major challenges facing the development of DIBs are linked to electrolyte decomposition at the cathode-electrolyte interface (CEI), graphite exfoliation, and corrosion of Al current collectors. In this work, X-ray photoelectron spectroscopy (XPS) is employed to gain a broad understanding of the nature and dynamics of the CEI built on anion-intercalated graphite cycled both in highly concentrated electrolytes (HCEs) of common lithium salts (LiPF₆, LiFSI, and LiTFSI) in carbonate solvents and in a typical ionic liquid. Though Al metal current collectors were adequately stable in all HCEs, the Coulombic efficiency was substantially higher for HCEs based on LiFSI and LiTFSI salts. Specific capacities ranging from 80 to 100 mAh g^-1 were achieved with a Coulombic efficiency above 90% over extended cycling, but cells with LiPF₆-based electrolytes were characterized by <70% Coulombic efficiency and specific capacities of merely ca. 60 mAh g^-1. The poor performance in LiPF₆-containing electrolytes is indicative of the continual buildup of decomposition products at the interface due to oxidation, forming a thick interfacial layer rich in Li_xPF_y, PO_xF_y, Li_xPO_yF_z, and organic carbonates as evidenced by XPS. In contrast, insights from XPS analyses suggested that anion intercalation and deintercalation processes in the range from 3 to 5.1 V give rise to scant or extremely thin surface layers on graphite electrodes cycled in LiFSI- and LiTFSI-containing HCEs, even allowing for probing anions intercalated in the near-surface bulk. In addition, ex situ Raman, SEM and TEM characterizations revealed the presence of a thick coating on graphite particles cycled in LiPF₆-based electrolytes regardless of salt concentration, while hardly any surface film was observed in the case of concentrated LiFSI and LiTFSI electrolytes.

Electrochemical stability of steel, Ti, and Cu current collectors in water-in-salt electrolyte for green batteries and supercapacitors

The electrochemical behaviour of steel, copper, and titanium current collectors was studied in aqueous solutions of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) at various concentrations, from 0.5 up to 20 m. As the concentration of the electrolyte increases, the electrochemical window of water stability widens according to the “water-in-salt” concept. The metal grids have been studied electrochemically, both under anodic and cathodic conditions, by means of cyclic voltammetry and chronoamperometry. Subsequently, a microscopic analysis with SEM and compositional analysis with XPS was carried out to evaluate the surface modifications following electrochemical stress. We found that copper is not very suitable for this kind of application, while titanium and steel showed interesting behaviour and large electrochemical window.

Electrosprayed Robust Graphene Layer Constructing Ultrastable Electrode Interface for High-Voltage Lithium-Ion Batteries

Aluminum (Al) foil serving as the most widely used cathode current collector for lithium-ion batteries (LIBs) is still not flawless to fulfill the increasing demand of rechargeable energy storage systems. The limited contact area and weak adhesion to cathode material as well as local corrosion during long-term operations could deteriorate the performance of LIBs with a higher working voltage. Herein, a reduced graphene oxide (RGO)-modified Al foil (RGO/Al) is developed via electrospraying to increase interfacial adhesion and inhibit anodic corrosion as a functional current collector. Valid corrosion resistance to electrolyte and strengthened adhesion of electrode particles to current collectors are beneficial to improve the interfacial reaction dynamics. The RGO/Al-based LiNi_0.5Mn_1.5O₄ cells (LNMO-RGO/Al) exhibit better electrochemical performances in terms of long-term cycling discharge capacity retention (90% after 840 cycles at 1 C), rate capability (101.8 mAh g^-1 at 5 C), and interfacial resistance, prominently superior to bare Al-based cells (LNMO-Al). This work not only contributes to long-term stable operation of high-voltage LIBs but also brings new opportunities for the development of next-generation 5 V LIBs.

Review of the Design of Current Collectors for Improving the Battery Performance in Lithium-Ion and Post-Lithium-Ion Batteries

Current collectors (CCs) are an important and indispensable constituent of lithium-ion batteries (LIBs) and other batteries. CCs serve a vital bridge function in supporting active materials such as cathode and anode materials, binders, and conductive additives, as well as electrochemically connecting the overall structure of anodes and cathodes with an external circuit. Recently, various factors of CCs such as the thickness, hardness, compositions, coating layers, and structures have been modified to improve aspects of battery performance such as the charge/discharge cyclability, energy density, and the rate performance of a cell. In this paper, the details of interesting and useful attempts of preparing CCs for high battery performance in lithium-ion and post-lithium-ion batteries are reviewed. The advantages and disadvantages of these attempts are discussed.

Interfacial Engineering of a Heteroatom-Doped Graphene Layer on Patterned Aluminum Foil for Ultrafast Lithium Storage Kinetics

The design of the interfacial architecture between the electrode and the current collector in lithium-ion batteries (LIB) plays a key role in achieving ultrafast lithium storage kinetics with respect to efficient charge transfer and cycle stability. However, in recent years, despite considerable efforts in the structural and chemical engineering of active materials (anode and cathode materials), interfacial architectures between the electrode and the current collector have received relatively insufficient attention in the case of ultrafast LIBs. Here, the interface architecture of a micropatterned Al current collector with a heteroatom-doped graphene interfacial layer is developed using roll pressing and dip coating processes. The cathode electrode fabricated with the resultant current collector offers increased contact area with enhanced interfacial stability between the electrode and the current collector because of micropatterns with heteroatom-doped graphene formed on the current collector, leading to outstanding ultrafast cycling capacity (105.8 mA h g^-1) at 20 C. Furthermore, at extremely high rate and long-term cycling performance, significant ultrafast cycling stability (specific capacity of 87.1 mA h g^-1 with capacity retention of 82.3% at 20 C after 1000 cycles) is noted. These improved ultrafast and ultra-stable performances are explained in terms of the increased electron collection/provision site with a high contact area between the electrode and the current collector for enhanced ultrafast cycling capacity and the effective corrosion prevention of the current collector with fast charge transfer for ultrafast cycling stability.

Toward Low-Cost All-Organic and Biodegradable Li-Ion Batteries

This work presents an alternative method for fabricating Li-ion electrodes in which the use of aluminum/copper current collectors and expensive binders is avoided. Low-cost natural cellulose fibers with a 2-mm length are employed as binder and support for the electrode. The objective of this method is to eliminate the use of heavy and inactive current collector foils as substrates and to replace conventional costly binders with cellulose fibers. Moreover, no harmful solvents, such as N-methylpyrrolidone, are employed for film fabrication. Water-soluble carbons are also utilized to reduce the preparation time and to achieve a better repartition of carbon in the electrode, thus improving the electrochemical performance. Flexible and resistant LiFePO4 (LFP), Li4Ti5O12 (LTO), organic 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA), and graphite electrodes are obtained with active mass loadings similar to those obtained by the current casting method. The initial discharge capacity of approximately 130 mAh·g-1 at 2 C is obtained for an LFP/LTO paper battery with an approximately 91.6% capacity retention after 1000 cycles. An all-organic prelithiated PTCDA/graphite cell without a transition metal is prepared and electrochemically tested. It is one of the first self-standing batteries that is composed of organic redox active molecules and biodegradable components reported in literature.

Rechargeable Lithium Metal Batteries with an In-Built Solid-State Polymer Electrolyte and a High Voltage/Loading Ni-Rich Layered Cathode

Solid-state batteries enabled by solid-state polymer electrolytes (SPEs) are under active consideration for their promise as cost-effective platforms that simultaneously support high-energy and safe electrochemical energy storage. The limited oxidative stability and poor interfacial charge transport in conventional polymer electrolytes are well known, but difficult challenges must be addressed if high-voltage intercalating cathodes are to be used in such batteries. Here, ether-based electrolytes are in situ polymerized by a ring-opening reaction in the presence of aluminum fluoride (AlF₃ ) to create SPEs inside LiNi_0.6 Co_0.2 Mn_0.2 O₂ (NCM) || Li batteries that are able to overcome both challenges. AlF₃ plays a dual role as a Lewis acid catalyst and for the building of fluoridized cathode-electrolyte interphases, protecting both the electrolyte and aluminum current collector from degradation reactions. The solid-state NCM || Li metal batteries exhibit enhanced specific capacity of 153 mAh g^-1 under high areal capacity of 3.0 mAh cm^-2 . This work offers an important pathway toward solid-state polymer electrolytes for high-voltage solid-state batteries.

Stretchable electrochemical energy storage devices

Deformable energy storage devices are needed to power the next generation of wearable electronics. This review highlights the most recent advances in stretchable energy storage devices with a focus on batteries and supercapacitors.

Building Better Batteries in the Solid State: A Review

Most of the current commercialized lithium batteries employ liquid electrolytes, despite their vulnerability to battery fire hazards, because they avoid the formation of dendrites on the anode side, which is commonly encountered in solid-state batteries. In a review two years ago, we focused on the challenges and issues facing lithium metal for solid-state rechargeable batteries, pointed to the progress made in addressing this drawback, and concluded that a situation could be envisioned where solid-state batteries would again win over liquid batteries for different applications in the near future. However, an additional drawback of solid-state batteries is the lower ionic conductivity of the electrolyte. Therefore, extensive research efforts have been invested in the last few years to overcome this problem, the reward of which has been significant progress. It is the purpose of this review to report these recent works and the state of the art on solid electrolytes. In addition to solid electrolytes stricto sensu, there are other electrolytes that are mainly solids, but with some added liquid. In some cases, the amount of liquid added is only on the microliter scale; the addition of liquid is aimed at only improving the contact between a solid-state electrolyte and an electrode, for instance. In some other cases, the amount of liquid is larger, as in the case of gel polymers. It is also an acceptable solution if the amount of liquid is small enough to maintain the safety of the cell; such cases are also considered in this review. Different chemistries are examined, including not only Li-air, Li-O2, and Li-S, but also sodium-ion batteries, which are also subject to intensive research. The challenges toward commercialization are also considered.

Suppressing Corrosion of Aluminum Foils via Highly Conductive Graphene-like Carbon Coating in High-Performance Lithium-Based Batteries

Aluminum foil is the predominant cathodic current collector in lithium-based batteries due to the high electronic conductivity, stable chemical/electrochemical properties, low density, and low cost. However, with the development of next-generation lithium batteries, Al current collectors face new challenges, such as the requirement of increased chemical stability at high voltage, long-cycle-life batteries with different electrolyte systems, as well as improved electronic conductivity and adhesion for new electrode materials. In this study, we demonstrate a novel graphene-like carbon (GLC) coating on the Al foil in lithium-based batteries. Various physical and electrochemical characterizations are conducted to reveal the electronic conductivity and electrochemical stability of the GLC-Al foil in both carbonate- and ether-based electrolytes. Full-cell tests, including Li-S batteries and high-voltage Li-ion batteries, are performed to demonstrate the significantly improved cycling and rate performance of batteries with the use of the GLC-Al foil as current collectors. The cell using the GLC-Al foil can greatly reduce the potential polarization in Li-S batteries and can obtain a reversible capacity of 750 mAh g^-1 over 100 cycles at 0.5C. Even with high-sulfur-loading cathodes, the Li-S battery at 1C still maintains over 500 mAh g^-1 after 100 cycles. In high-voltage Li-ion batteries, the GLC-Al foil significantly improves the high-rate performance, showing an increased retained capacity by over 100 mAh g^-1 after 450 cycles at 1C compared to the bare foil. It is believed that the developed GLC-Al foil brings new opportunities to enhance the battery life of lithium-based batteries.

Fluorine and Lithium: Ideal Partners for High-Performance Rechargeable Battery Electrolytes

Further enhancement in the energy densities of rechargeable lithium batteries calls for novel cell chemistry with advanced electrode materials that are compatible with suitable electrolytes without compromising the overall performance and safety, especially when considering high-voltage applications. Significant advancements in cell chemistry based on traditional organic carbonate-based electrolytes may be successfully achieved by introducing fluorine into the salt, solvent/cosolvent, or functional additive structure. The combination of the benefits from different constituents enables optimization of the electrolyte and battery chemistry toward specific, targeted applications. This Review aims to highlight key research activities and technical developments of fluorine-based materials for aprotic non-aqueous solvent-based electrolytes and their components along with the related ongoing scientific challenges and limitations. Ionic liquid-based electrolytes containing fluorine will not be considered in this Review.

Identifying Active Sites for Parasitic Reactions at the Cathode-Electrolyte Interface

Nickel-rich transition metal oxides are the most promising high-voltage and high-capacity cathode materials for high-energy-density lithium batteries. Improving the chemical/electrochemical stability of the cathode-electrolyte interface has been the major technical focus to enable this class of cathode materials. In this work, LiCoO₂ is adopted as the model cathode material to investigate the active sites for parasitic reactions between the delithiated cathode and the nonaqueous electrolyte. Both ab initio calculations and experimental results clearly show that the partially coordinated transition metal atoms at the surface are responsible for the parasitic reactions at the cathode-electrolyte interface. This finding lays out fundamental support for rational interfacial engineering to further improve the life and safety characteristics of nickel-rich cathode materials.

Electrostatic Self-Assembly Enabling Integrated Bulk and Interfacial Sodium Storage in 3D Titania-Graphene Hybrid

Room-temperature sodium-ion batteries have attracted increased attention for energy storage due to the natural abundance of sodium. However, it remains a huge challenge to develop versatile electrode materials with favorable properties, which requires smart structure design and good mechanistic understanding. Herein, we reported a general and scalable approach to synthesize three-dimensional (3D) titania-graphene hybrid via electrostatic-interaction-induced self-assembly. Synchrotron X-ray probe, transmission electron microscopy, and computational modeling revealed that the strong interaction between titania and graphene through comparably strong van der Waals forces not only facilitates bulk Na⁺ intercalation but also enhances the interfacial sodium storage. As a result, the titania-graphene hybrid exhibits exceptional long-term cycle stability up to 5000 cycles, and ultrahigh rate capability up to 20 C for sodium storage. Furthermore, density function theory calculation indicated that the interfacial Li⁺, K⁺, Mg^2+, and Al³⁺ storage can be enhanced as well. The proposed general strategy opens up new avenues to create versatile materials for advanced battery systems.

Curious Case of Positive Current Collectors: Corrosion and Passivation at High Temperature

In the evaluation of compatibility of different components of cell for high-energy and extreme-conditions applications, the highly focused are positive and negative electrodes and their interaction with electrolyte. However, for high-temperature application, the other components are also of significant influence and contribute toward the total health of battery. In present study, we have investigated the behavior of aluminum, the most common current collector for positive electrode materials for its electrochemical and temperature stability. For electrochemical stability, different electrolytes, organic and room temperature ionic liquids with varying Li salts (LiTFSI, LiFSI), are investigated. The combination of electrochemical and spectroscopic investigations reflects the varying mechanism of passivation at room and high temperature, as different compositions of decomposed complexes are found at the surface of metals.

Chemical versus Electrochemical Electrolyte Oxidation on NMC111, NMC622, NMC811, LNMO, and Conductive Carbon

We compare the stability of alkyl carbonate electrolyte on NMC111, -622, and -811, LNMO, and conductive carbon electrodes. We prove that CO₂ and CO evolution onset potentials depend on the electrode material and increase in the order NMC811 < NMC111 ≈ NMC622 < conductive carbon ≈ LNMO, which we rationalize by two fundamentally different oxidation mechanisms, the chemical and the electrochemical electrolyte oxidation. Additionally, in contrast to the widespread understanding that transition metals in cathode active materials catalyze the electrolyte oxidation, we will prove that such a catalytic effect on the electrochemical electrolyte oxidation does not exist.