Metal Negative Electrode Research Articles

Interface engineering enabling thin lithium metal electrodes down to 0.78 μm for garnet-type solid-state batteries.

Controllable engineering of thin lithium (Li) metal is essential for increasing the energy density of solid-state batteries and clarifying the interfacial evolution mechanisms of a lithium metal negative electrode. However, fabricating a thin lithium electrode faces significant challenges due to the fragility and high viscosity of Li metal. Herein, through facile treatment of Ta-doped Li7La3Zr2O12 (LLZTO) with trifluoromethanesulfonic acid, its surface Li2CO3 species is converted into a lithiophilic layer with LiCF3SO3 and LiF components. It enables the thickness control of Li metal negative electrodes, ranging from 0.78 μm to 30 μm. Quasi-solid-state lithium-metal battery with an optimized 7.54 μm-thick lithium metal negative electrode, a commercial LiNi0.83Co0.11Mn0.06O2 positive electrode, and a negative/positive electrode capacity ratio of 1.1 shows a 500 cycles lifespan with a final discharge specific capacity of 99 mAh g-1 at 2.35 mA cm-2 and 25 °C. Through multi-scale characterizations of the thin lithium negative electrode, we clarify the multi-dimensional compositional evolution and failure mechanisms of lithium-deficient and -rich regions (0.78 μm and 7.54 μm), on its surface, inside it, or at the Li/LLZTO interface.

Entropy-Driven 60mol% Li Electrolyte for Li Metal-Free Batteries.

Highly Li-concentrated electrolytes are acknowledged for their compatibility with Li metal negative electrodes and high voltage positive electrodes to achieve high-energy Li metal batteries, showcasing stable and facile interfaces for Li deposition/dissolution and high anodic stability. This study aims to explore a highly concentrated electrolyte by adopting entropy-driven chemistry for Li metal-free (so-called anode-free) batteries. The combination of lithium bis(fluorosulfonyl)amide (LiFSA) and lithium trifluoromethanesulfonate (LiOTf)salts in a pyrrolidinium-based ionic liquid is found to significantly modify the coordination structure, resulting in an unprecedented 60mol% Li concentration and a low solvent-to-salt ratio of 0.67:1 in the electrolyte system. This novel 60mol% Li electrolyte demonstrates unique coordination stricture, featuring a high ratio of monodentate-anion structures and aggregates, which facilitates an enhanced Li+ transference number and improved anodic stability. Moreover, the developed electrolyte provides a facile de-coordination process and leads to the formation of an anion-based solid electrolyte interface, which enables stable Li deposition/dissolution properties and demonstrates excellent cycling stability in the Li metal-free full cell with a Li[Ni0.8Co0.1Mn0.1]O2 (NCM811) positive electrode.

Strategies for constructing stable solid electrolyte interface on the surface of lithium metal negative electrodes

Abstract With metallic lithium serving as the negative electrode material, lithium metal batteries are seen as the most promising next generation of high-specific energy batteries. However, the poor stability of the solid-state electrolyte interface (SEI) caused by the spontaneous interaction between metal lithium and electrolyte during battery cycling exacerbates several problems such lithium dendrite formation, lower Coulomb efficiency, and capacity degradation. The practical use of secondary lithium metal batteries is therefore constrained. Consequently, enhancing SEI stability is a focus of extensive study in the realm of lithium metal batteries. The ideal surface electrode should have strong surface electrical insulation, good lithium-ion conductivity, and high mechanical strength in order to manage the even deposition of lithium ions on the surface, facilitate ion transport, and inhibit dendritic growth. Therefore, constructing a functional SEI film is an effective strategy to solve the challenges faced by lithium metal negative electrodes. This article first introduces the formation mechanism and structural composition of SEI. Then, the failure reasons of SEI were analyzed. Finally, the modification strategy of SEI was emphasized, aiming to provide a reference for the research of metal lithium negative electrodes.

Stabilization of lithium metal in concentrated electrolytes: effects of electrode potential and solid electrolyte interphase formation.

Lithium (Li) metal negative electrodes have attracted wide attention for high-energy-density batteries. However, their low coulombic efficiency (CE) due to parasitic electrolyte reduction has been an alarming concern. Concentrated electrolytes are one of the promising concepts that can stabilize the Li metal/electrolyte interface, thus increasing the CE; however, its mechanism has remained controversial. In this work, we used a combination of LiN(SO2F)2 (LiFSI) and weakly solvating 1,2-diethoxyethane (DEE) as a model electrolyte to study how its liquid structure changes upon increasing salt concentration and how it is linked to the Li plating/stripping CE. Based on previous works, we focused on the Li electrode potential (ELi with reference to the redox potential of ferrocene) and solid-electrolyte-interphase (SEI) formation. Although ELi shows a different trend with DEE compared to conventional 1,2-dimethoxyethane (DME), which is accounted for by different ion-pair states of Li+ and FSI-, the ELi-CE plots overlap for both electrolytes, suggesting that ELi is one of the dominant factors of the CE. On the other hand, the extensive ion pairing results in the upward shift of the FSI- reduction potential, as demonstrated both experimentally and theoretically, which promotes the FSI--derived inorganic SEI. Both ELi and SEI contribute to increasing the Li plating/stripping CE.

The Mg Electrode Cycling Mechanism in Simple Salt Glyme Electrolytes

The lithium-ion battery (LIB) has been at the forefront of innovations in energy storage since its commercialisation over 30 years ago. As we reach the practical specific capacity limit of current state-of-the-art materials, novel and innovative battery technologies are required to satiate the requirements of energy intensive applications such as electric vehicles. 1 The use of a lithium metal negative electrode has been proposed to increase the energy density, however the high reactivity causes poor cycling performance due to electrolyte degradation, dendrite formation and safety issues. 2 The magnesium electrode offers a solution that has double the theoretical volumetric capacity of the lithium electrode while simultaneously mitigating dendritic growth, reducing raw material costs and greatly increasing sustainability. 3,4 Previous work has shown that traditional LIB electrolytes are not compatible with magnesium metal, and the plating and stripping mechanism that underpins how magnesium electrodes cycle is poorly understood. 5 Here we explored the mechanism of magnesium plating and stripping in glyme-based electrolytes with a range of additional additives. We show that in the pure electrolyte solution, interphase formation is critical to stable cycling but that this is accompanied by significant degradation and accumulation of inactive Mg.6 We establish the three-dimensional chemical composition of Mg deposits in the different electrolyte formulations using a combination of focused ion beam-scanning electron microscopy (FIB-SEM), energy dispersive x-ray spectroscopy (EDX) and transmission electron microscopy (TEM). We use these findings to develop a mechanistic understanding that explains the electrochemical cycling performance of magnesium metal negative electrodes. In situ electrochemical quartz crystal microbalance (EQCM) measurements are used to evaluate the plating and stripping efficiency of the electrodes and to identify non-electrochemical degradation reactions between Mg and the electrolyte solutions.

(Invited) Progress in Polymer Electrolytes for Li° Electrode

The incentives for the use of lithium metal negative electrode are easily justified considering the gain in energy density. Polymer electrolytes disclosed almost fifty years ago are still considered as an option for the most thought-for all solid-state systems. In fact, the only solid-state batteries commercialised are those from Blue Solutions® powering busses in Europe, using a Li° negative, a PEO [poly(ethylene oxide)] matrix with a LiTFSI solute and an LiFePO4 cathode materials and operating at 70°C. 1500 -2000 cycles are routinely obtained. The main drawbacks of PEO, which is stable to lithium metal is a low conductivity at room temperature (10-5 - 10-6 Ohm-1.cm-1 at 25°C) while it reaches easily 10-3 - 10-4 Ohm-1.cm-1 at 70°C. The other impedimenta of PEO is the relatively low transference/transport number of the Li+ cation (T+ ≈ 0.2), resulting in the formation of a salt concentration gradient in the electrolyte upon operation, with concomitant polarisation and the triggering of dendrites when the Sand time is reached, shortening the cycle life-time. Other polymer matrices like poly(ε-caprolactone) PCL and poly(propylene carbonate) PPC show slightly better conductivity and higher transference number (T+ ≈ 0.5), but are not stable in contact with lithium metal.Our strategy to increase the conductivity at lower temperature consisted in modifying the polymer architecture, from linear (PEO) to comb polymer in which medium length PEG segments (Mw 1000 - 2000) are attached to the main backbone via a flexible non-solvating tether (PPO). This optimises the speed of re-orientation of the solvating units, speed needed to favour the ionic motion. With these polymers and delocalised salts like LiTFSI or LiFSI, conductivities close to 10-4 Ohm-1.cm-1 at 25°C are obtained though with the same transference number as PEO.In order to improve the selectivity of the cation transport in PEs, it is necessary to immobilise the negative charges of the salt by attaching them to a polymer or nano-particles. The other option consists in modifying the salt architecture to slow down the diffusion/migration of the anions by increasing the interactions of the anions with themselves or with the polymer backbone.We have found that, starting for a simple cellulose derivative, the action of FSO2N=C=O results in the attachment of anions to the backbone and that the poly(salts) are now able to form an alloy with PEO or other poly(ethers), with excellent mechanical properties imparted by the rigid cellulose matrix, yet giving Li-only conductivities allowing battery operation. Alternatively, SiO2 or Al2O3 nano-particles can be grafted with delocalised anionic moieties, which result, when dispersed into PEO, in conducting composites with again excellent electrochemical and mechanical properties.Modification of the well-known anion TFSI to increase its interactions and slow it down have been undertaken: either hydrogen bonds, dipole interactions, chain entanglement, or π - π interactions have been introduced in the RSO2N(-)SO2CF3 anion with R equal respectively CF2H—, (CH3)2N—, (CH3OC2H4)2N—, C6H5—. In all cases, higher T+ were obtained as compared with TFSI-, though the total conductivity was lower, but resulting still in σLi+ that translates into better lithium plating efficiencies as seen in the classical Li°/PE/Li° cycling tests and in batteries lifecycle.A final drawback of PEO is its inability to withstand voltages above 3.9 V vs. Li+:Li° while PPC for instance, stable to 4.5V, does decompose rapidly on the negative Li°. The obvious idea to combine a PEO based anolyte and a PPC catholyte stumbles on the consideration that PEO is far more solvating than PPC and deplete totally the polyester compartment, with no conductivity remaining. We solved this problem with the use of an immobilised polyanion in both the anolyte and catholyte, resulting in the stable operation of a Li°/NMC622 battery.All these research results will be discussed in details.

The Effects of Small Amounts of Cobalt in LiNi1-XCoxO2 on Lithium Diffusion

Lithium-ion batteries with Co-free Ni-rich cathodes have attracted interest due to lower cost and supply chain issues compared to cobalt-containing cathodes1. However, cobalt is thought to provide some benefits in cycling stability and rate performance. In this work, cobalt substitution for nickel in the positive electrode material LiNi1-xCoxO2 at 0 ≤ x ≤ 0.10 is systematically investigated to determine the impact of Co and material synthesis conditions on Li diffusivity, measured using the Atlung Method for Intercalant Diffusion (AMID)2,3 in coin cells vs Li metal negative electrodes. Cobalt was found to have no impact on Li diffusivity in the intermediate voltage range (4.2 V to 3.7 V), while cation mixing (%Ni in Li layer) was found to have a strong correlation to slow Li diffusivity. At high voltage (4.3 V to 4.2 V), 0 to 10% cobalt incrementally suppresses the H2-H3 phase transition4,5 enabling significantly faster lithium diffusion. Cation mixing can be minimized through synthesis conditions, improving Li diffusivity for the low and intermediate voltage regions, without using Co. In summary, with optimal positive electrode material synthesis conditions and limiting cell upper cutoff voltage to 4.2 V, Co-free Ni-rich materials can be made with similar rate performance as 10% Cobalt-containing materials. Additionally, cobalt was found to have minimal impact on the following material properties: crystallinity of LiNi1-xCoxO2, surface impurities, particle size, and electronic conductivity. Cobalt substituted for nickel from 0% to 10% was found to decrease first cycle discharge capacity in the voltage range between 3.0 V and 4.3 V and improve capacity retention in coin cell cycling vs Li metal negative electrodes. The capacity retention improvement is most likely due to the suppression of the H2-H3 phase transition as Co is added5. Figure 1. (a), (b) shows %Ni in Li layer vs Cobalt content, obtained from powder XRD Rietveld refinement from a series of LiNi1-xCoxO2 materials. (a), (b) were synthesized under two different O2 flow velocities, both at 700oC for 20 hours, with a Li:TM ratio of 1.02:1. (c), (d) Show the lithium chemical diffusion coefficient vs cell voltage for the materials from (a), (b), respectively, measured using the Atlung Method for Intercalant Diffusion (AMID), in coin cells at 30oC. The protocol consists of 2C to C/160-rate discharge steps per voltage interval (0.1V), with OCV relaxation in between discharge steps. Figure 1

Degradation Control of All-Solid-State Na Batteries by Inorganic Oxide Coated Positive Electrode Active Material

All-solid-state Na batteries (ASSBs) are attracted attention owing to their high safety and resource properties for next-generation batteries. Na has the advantage of abundant resources compared to Li. In addition, polymer electrolyte is expected to improve the safety of the battery by suppressing the risk of ignition with liquid leakage. In general, the degradation of batteries using polymer electrolyte mainly occurs at the positive electrode (PE)/electrolyte interface during charge process. In case of batteries using active material having layered structures such as NaCoO2 and polyether electrolyte, the degradation of CoO2 structure and decomposition of polyether during the deintercalation of Na+ in the active material. These PE / electrolyte interface degradations contribute to increasing resistance of the battery. To suppress interfacial degradation, inorganic oxides coating for active materials were reported in the Li-based battery[1]. In this study, Na3PO4 or Na3Zr2Si2PO12 (NZSP) coated PEs were applied for ASSBs to suppress degradation. The degradation behavior of ASSBs was investigated by charge / discharge test and XRD analysis using extracted PE sheet from cell. Furthermore, Raman spectroscopy was applied on the cell before and after charge / discharge test to investigate structural changes in active material and polymer electrolyte.NaTFSA as Na salt, P(EO/PO) as polyether-based macromonomer, and DMPA as photoinitiator were mixed into Ar-filled glovebox. NaTFSA was added by ratio of [Na]/[O]=0.1 per molar of oxygen units of P(EO/PO). Polymer electrolyte films were prepared by UV irradiation for obtained homogeneous solution. NaCoO2 (NCO) as active material, acetylene black (AB) as conductive additive, and P(EO/MEEGE)-NaTFSA as binder were mixed by 82:5:13 weight ratio. The Na3PO4 and NZSP as inorganic oxides were added 5 wt.% and 10 wt.% for NCO, respectively. Applied slurry (thick. 50 µm) onto Al foil was cut to Φ16 mm after vacuum dry. The PE sheet, Na metal negative electrode, and polymer electrolyte were assembled into coin-cell to fabricate ASSBs. The charge / discharge tests for more than 10 cycles were performed at 333K after heat-treated at 363K for 48h. After charge / discharge test, XRD analysis was applied to extracted PE sheet from the cell under a low dew point for structural evaluation. Moreover, PE, polymer electrolyte, and Na metal were initially cut to 9 mm × 9 mm and introduced into the Raman cell with observation window to observe the cross-section of the cell. Raman spectroscopy was performed to the cross-section of the cell before and after charge / discharge test in order to investigate the degradation behavior of PE and polymer electrolyte.Fig. 1 shows charge / discharge profiles of non-coated and 10 wt.% NZSP coated PE in the cells. The 1st discharge capacity was 36 mAhg-1 in the non-coated system, and was also the insufficient capacity of about 30% from the theoretical capacity (ca. 118 mAhg-1) of NCO. In contrast, the NZSP-coated PE improved the 1st discharge capacity to 60 mAhg-1, and clear plateau was observed owing to their crystal structure change of NCO. These results suggest that inorganic oxide served as sufficient ionic conduction path between PE and electrolyte, reducing the interfacial resistance. Fig. 2 shows the cycle number dependencies of capacity and Coulombic efficiency. ASSB with inorganic oxides coated PE showed high discharge capacity retention and Coulombic efficiency in comparison with non-coated system. These results are suggesting that inorganic oxides coating active material surface suppress increase of interfacial resistance and degradation of the PE/electrolyte by prevent the formation of side products such as Co3O4 [2]. NZSP has higher Na ionic conductivity than Na3PO4, and can cover more apparent area of NCO surface than Na3PO4 because of their smaller particle size. Therefore, from comparing Na3PO4 and NZSP coating, NZSP more contributes to suppressing degradation of the cell. Fig. 3 shows the results of XRD analysis for PEs after charge / discharge test. Peaks attributed to NCO were confirmed in the PE before degradation, however, peak intensity decreased after degradation, especially for (003) and (006) of NCO. The peak derived from Co3O4 was confirmed only after degradation. These results suggest that structural degradation of NCO occurs during charging and discharging of the cell, resulting in the formation of Co3O4. In addition, the inorganic oxide coating should be contributed to maintaining the structure of the active material. In this presentation, the analysis results of structural changes of PE and electrolyte before and after charge / discharge test by Raman spectroscopic measurement will be also reported.

Rechargeable Li/Cl2 Battery Down to -80°C.

Low temperature rechargeable batteries are important to life in cold climates, polar/deep-sea expeditions, and space explorations. Here, this work reports 3.5-4V rechargeable lithium/chlorine (Li/Cl2 ) batteries operating down to -80 °C, employing Li metal negative electrode, a novel carbon dioxide (CO2 ) activated porous carbon (KJCO2 ) as the positive electrode, and a high ionic conductivity (≈5-20 mS cm-1 from -80 °C to room-temperature) electrolyte comprised of aluminum chloride (AlCl3 ), lithium chloride (LiCl), and lithium bis(fluorosulfonyl)imide (LiFSI) in low-melting-point (-104.5 °C) thionyl chloride (SOCl2 ). Between room-temperature and -80 °C, the Li/Cl2 battery delivers up to ≈29100-4500 mAh g-1 first discharge capacity (based on carbon mass) and a 1200-5000 mAh g-1 reversible capacity over up to 130 charge-discharge cycles. Mass spectrometry and X-ray photoelectron spectroscopy probe Cl2 trapped in the porous carbon upon LiCl electro-oxidation during charging. At -80 °C, Cl2 /SCl2 /S2 Cl2 generated by electro-oxidation in the charging step are trapped in porous KJCO2 carbon, allowing for reversible reduction to afford a high discharge voltage plateau near ≈4V with up to ≈1000 mAh g-1 capacity for SCl2 /S2 Cl2 reduction and up to ≈4000 mAh g-1 capacity at ≈3.1V plateau for Cl2 reduction.

Diffusion and Creep in Lithium Metal Anodes Induced by Plating and Stripping Reactions

Electrodeposition of lithium on lithium metal negative electrodes (anodes) of liquid cells produces filamentary deposits, resulting in poor cycling efficiency. Experiments suggest that filaments grow by extrusion of metal from the substrate, due to compressive stress introduced during deposition. A model is presented that explores the origin of stress generated in the anode during plating-stripping cycles. According to the model, plating or stripping reactions insert or remove lithium atoms at the interface between the metal and the solid electrolyte interphase (SEI) layer. Stress is induced by the resulting diffusion processes, with diffusion-induced strain accommodated by inelastic creep. Stress distributions during cycling are calculated, and are used in turn to predict curvature transients that would be measured in beam-deflection experiments. The calculations account for the three-dimensional surface morphology, since surface features that protrude above the surface plane do not contribute to curvature changes. Comparisons with recent curvature measurements demonstrate detailed agreement, with diffusion and creep parameters close to literature values. The results support the hypothesis that electrochemical reactions intrinsically generate stress. The model can serve as a framework for analysis of morphogical instability of the lithium anode interface in both liquid and solid-state cells.

Towards Understanding the Variation of Electrode Design Parameters on the Electrochemical Performance of Aluminum Graphite Batteries: An Experimental and Simulation Study

AbstractDue to their high power density and availability, aluminum batteries consisting of graphite positive electrode and ionic liquid electrolytes are promising candidates for post‐lithium‐ion batteries. However, the effect of the various electrode design parameters on their electrochemical performance is not well understood. Herein, a high‐fidelity physics‐based model validated with experimental data obtained from a Swagelok cell consisting of an aluminum metal negative electrode, imidazolium ionic liquid electrolyte, and graphite positive electrode is used to study the effects of various electrode design parameters on the discharge capacity. The model is used to optimize the design of the electrodes by generating several Ragone plots, estimating the optimum current density for a given cell design, and explaining the limitations of the cells based on the transport of the electroactive species. An optimum graphite thickness of 50 μm is obtained for all the discharge times considered in this study. Determining the ideal electrode configuration for Al‐graphite batteries using different ionic liquid electrolytes and considering various discharge durations could provide a reference point for evaluating the suitability of a specific ionic liquid electrolyte in a particular use case.

Shedding light on rechargeable Na/Cl2 battery

Advancing new ideas of rechargeable batteries represents an important path to meeting the ever-increasing energy storage needs. Recently, we showed rechargeable sodium/chlorine (Na/Cl2) (or lithium/chlorine Li/Cl2) batteries that used a Na (or Li) metal negative electrode, a microporous amorphous carbon nanosphere (aCNS) positive electrode, and an electrolyte containing dissolved aluminum chloride and fluoride additives in thionyl chloride [G. Zhu et al., Nature 596, 525-530 (2021) and G. Zhu et al., J. Am. Chem. Soc. 144, 22505-22513 (2022)]. The main battery redox reaction involved conversion between NaCl and Cl2 trapped in the carbon positive electrode, delivering a cyclable capacity of up to 1,200 mAh g-1 (based on positive electrode mass) at a ~3.5 V discharge voltage [G. Zhu et al., Nature 596, 525-530 (2021) and G. Zhu et al., J. Am. Chem. Soc. 144, 22505-22513 (2022)]. Here, we identified by X-ray photoelectron spectroscopy (XPS) that upon charging a Na/Cl2 battery, chlorination of carbon in the positive electrode occurred to form carbon-chlorine (C-Cl) accompanied by molecular Cl2 infiltrating the porous aCNS, consistent with Cl2 probed by mass spectrometry. Synchrotron X-ray diffraction observed the development of graphitic ordering in the initially amorphous aCNS under battery charging when the carbon matrix was oxidized/chlorinated and infiltrated with Cl2. The C-Cl, Cl2 species and graphitic ordering were reversible upon discharge, accompanied by NaCl formation. The results revealed redox conversion between NaCl and Cl2, reversible graphitic ordering/amorphourization of carbon through battery charge/discharge, and probed trapped Cl2 in porous carbon by XPS.

Microstructure and the Plating/Stripping Behavior of Magnesium Metal Negative Electrode for Rechargeable Magnesium Batteries

Recently, rechargeable magnesium (Mg) battery systems attract lots of attention owing to their promising theoretical energy density, low cost from the natural abundance of Mg, and less dendritic growth of Mg during charging. Nevertheless, rechargeable Mg battery systems suffer from sluggish reaction kinetics on the electrodes and poor cycle life, which greatly limit their practical competitiveness. Most studies focus on the development of novel electrolytes and their feasibility for rechargeable Mg batteries, while the plating/stripping behavior of Mg metal negative electrode during the charge/discharge cycle is relatively less studied.Therefore, in this research, the platting/stripping behavior of the Mg metal electrode and the resultant surface microstructure after cycling were investigated in an all phenyl complex (APC) (PhMgCl + AlCl3 in THF) electrolyte. A Swagelok-type Mg||Mg symmetric cell was used in this study with an air-free transfer system to prevent the charged/discharged electrode from oxidation when moving to a scanning electron microscope (SEM) to observe the surface microstructure.When discharging the pristine pure Mg electrode, the stripping was found to be highly localized with numerous discharge holes on the electrode surface. Interestingly, in the subsequent charging stage, the platted Mg preferentially deposited along the circumferences of the discharge holes, which might be related to the distribution of Mg species right after discharge and the surface conditions of the discharged surface. Later discharge was found to preferentially strip the platted Mg and leave marks around the discharge holes. In addition to cycling on a pure Mg electrode, the platting/stripping behavior and the resultant microstructure of Mg metal deposited on a copper (Cu) current collector were also studied and will be discussed for potential anode-free Mg battery applications. Regardless of the platting/stripping conditions, no dendritic Mg was found during the charge/discharge cycles. Figure 1

Non-Fluorinated Diluent in Localized High Concentration Electrolytes Enabling Superior Performance of Lithium Metal Negative Electrode Battery

Localized high-concentration electrolytes (LHCEs) are very promising strategies for the high-energy-density lithium (Li) metal batteries (LMBs). Nonsolvating diluents introduced in the LHCEs plays a critical role in physicochemical properties of LHCE and the overall LMB performance. However, there is a lack of design strategies for ideal nonsolvating diluents, and the reported cases are limited to fluorinated nonsolvating diluents (FNDs). FNDs suffer from accelerated decomposition at lithium metal, leading to electrolyte dry-up and ultimately battery failure. Furthermore, the high cost and potential environmental hazards of FNDs necessitate the development of non-fluorinated nonsolvating diluents (NFNDs). Here, we present a design rule for the ideal NFNDs by spectroscopically characterizing the Li+ solvation ability and miscibility. Our design rule identifies the ideal NFNDs, based on the superior cycling performane of candidate diluents over 350 cycles (99.0% ethoxybezene), 500 cycles (98.5% anisole), and 1400 cycles (99.0%, furan). NMR spectra revealed that the designed NFNDs were highly stable in electrolytes during extended cycles. Raman spectroscopy and theoretical calculation reveal that resonance of an electron pair on the oxygen atom of NFND molecules decreases the lithium-ion solvation ability, thereby achieving desirable non-solvating characteristics while maintaining good miscibility, superior cathodic stability, and low prices. Figure 1

A Conversion Type Iron Anode for High-Areal-Capacity Hybrid Redox Flow Batteries

Redox flow battery (RFB) is one of the most promising technologies for energy storage owing to its advantages of high safety, high efficiency, and low cost.1 Hybrid RFBs such as Zn-based RFBs offer a much higher energy density compared to all liquid RFBs. However, they rely on metal dissolution/deposition electrodes, which suffer from severe dendrite issues and poor reversibility, especially at high areal capacity and high current density. These inherent challenges of metal electrodes preclude their practical deployment for large-scale and long-duration energy storage applications.To break from this intrinsic limitation, in this work, we exploited a high-loading conversion electrodes to replace metal deposition electrodes to achieve high cycling stability at high areal capacity/current density for hybrid RFBs. With this strategy, using Fe3O4/Fe(OH)2 conversion negative electrode as an example coupling with ferricyanide/ferrocyanide positive electrode, we demonstrated conversion type all-iron hybrid RFBs with an unprecedented high cycling areal capacity of 126.6 mAh cm–2 at 50 mA cm–2 for 200 cycles (1000 hours) and 215 mAh cm–2 at 60 mA cm–2 for 100 cycles (700 hours) without capacity decay.2 The conversion electrode eliminates the dendrite issues and limitations on metal areal capacity, demonstrating unprecedented high cycling areal capacity and current density compared to conventional dissolution/deposition zinc metal negative electrode. We will discuss the compositional/morphological stability of the Fe3O4/Fe(OH)2 conversion iron negative electrode during the hybrid RFB operation and the techno-economic analysis of developed conversion type all-iron hybrid RFB for practical applications. Acknowledgment: The work described herein was supported by a grant from the Research Grant Council (RGC) of the Hong Kong Special Administrative Region, China (project no. C1002-21G). Reference: (1) Yao, Y. X.; Lei, J. F.; Shi, Y.; Ai, F.; Lu, Y. C. Assessment methods and performance metrics for redox flow batteries. Nat. Energy 2021, 6 (6), 582.(2) Shi, Y.; Wang, Z.; Yao, Y.; Wang, W.; Lu, Y.-C. High-Areal-Capacity Conversion Type Iron-Based Hybrid Redox Flow Batteries. Energy Environ. Sci. 2021, 10.1039/D1EE02258J.

Flexible and Stable N-Isopropylacrylamide/Sodium Alginate Gel Electrolytes for Aqueous Zn-MNO2 Batteries

Rechargeable aqueous Zn-ion batteries (ZIBs) have attracted considerable attention owing to their high theoretical capacity of 820 mA h g−1, low cost and intrinsic safety. However, the electrolyte leakage and the instability issues of Zn negative electrodes originating from side reactions between the aqueous electrolyte and Zn negative electrode not only restrict the battery stability, but also result in the short circuit of aqueous ZIBs. Herein, we report a flexible and stable N-isopropylacrylamide/sodium alginate (N-SA) gel electrolyte, which possesses high mechanical strength and high ionic conductivity of 2.96 × 10−2 S cm−1, and enables the Zn metal negative electrode and MnO2 positive electrode to reversibly and stably cycle. Compared to the liquid electrolyte, the N-SA hydrogel electrolyte can effectively form a uniform Zn deposition and suppress the generation of irreversible by-products. The assembled symmetric Zn/Zn cells at a current density of 1 mA cm−2 (capacity: 1 mAh cm−2) show a stable voltage profile, which maintains a low level of about 100 mV over 2600 h without an obvious short circuit or any overpotential increasing. Specially, the assembled Zn/N-SA/MnO2 batteries can deliver a high specific capacity of 182 mAh g−1 and maintain 98% capacity retention after 650 cycles at 0.5 A g−1. This work provides a simple method to fabricate high-performance SA-based hydrogel electrolytes, which illustrates their potential for flexible batteries for wearable electronics.

Aluminum foil negative electrodes with multiphase microstructure for all-solid-state Li-ion batteries

Metal negative electrodes that alloy with lithium have high theoretical charge storage capacity and are ideal candidates for developing high-energy rechargeable batteries. However, such electrode materials show limited reversibility in Li-ion batteries with standard non-aqueous liquid electrolyte solutions. To circumvent this issue, here we report the use of non-pre-lithiated aluminum-foil-based negative electrodes with engineered microstructures in an all-solid-state Li-ion cell configuration. When a 30-μm-thick Al94.5In5.5 negative electrode is combined with a Li6PS5Cl solid-state electrolyte and a LiNi0.6Mn0.2Co0.2O2-based positive electrode, lab-scale cells deliver hundreds of stable cycles with practically relevant areal capacities at high current densities (6.5 mA cm−2). We also demonstrate that the multiphase Al-In microstructure enables improved rate behavior and enhanced reversibility due to the distributed LiIn network within the aluminum matrix. These results demonstrate the possibility of improved all-solid-state batteries via metallurgical design of negative electrodes while simplifying manufacturing processes.

The Effects of Small Amounts of Cobalt in LiNi1−xCoxO2 on Lithium-ion Diffusion

Cobalt substitution for nickel in the positive electrode material LiNi1-xCoxO2 at 0 ≤ x ≤ 0.10 is investigated to determine the impact of cobalt on Li diffusivity, measured using the Atlung Method for Intercalant Diffusion (AMID) in coin cells. Cobalt was found to have little to no impact on Li diffusivity in the intermediate voltage range (4.2 V to 3.7 V). At high voltage (4.3 V to 4.2 V), 0 to 10% cobalt incrementally suppresses the H2–H3 phase transition and enables faster lithium diffusion. Additionally, at low voltage in the kinetic hindrance region (3.7 V to 3.0 V) cobalt can improve lithium diffusion by reducing cation mixing (nickel in the lithium layer). However, cation mixing can also be minimized through synthesis conditions, improving diffusivity without using cobalt. Cobalt was found to have minimal impact on the following material properties of LiNi1-xCoxO2: crystallinity, surface impurities, particle size, and electronic conductivity. Cobalt substituted for nickel from 0% to 10% was found to decrease first cycle discharge capacity in the voltage range between 4.3 V to 3.0 V and improve capacity retention in coin cell cycling vs Li metal negative electrodes. The latter impact is most likely due to the suppression of the H2–H3 phase transition as Co is added.

Production of high-energy 6-Ah-level Li | |LiNi0.83Co0.11Mn0.06O2 multi-layer pouch cells via negative electrode protective layer coating strategy

Stable lithium metal negative electrodes are desirable to produce high-energy batteries. However, when practical testing conditions are applied, lithium metal is unstable during battery cycling. Here, we propose poly(2-hydroxyethyl acrylate-co-sodium benzenesulfonate) (PHS) as negative electrode protective layer. The PHS contains soft poly (2-hydroxyethyl acrylate) and poly(sodium p-styrene sulfonate), which improve electrode flexibility, connection with the Cu current collector and transport of Li ions. Transmission electron cryomicroscopy measurements reveal that PHS induces the formation of a solid electrolyte interphase with a fluorinated rigid and crystalline internal structure. Furthermore, theoretical calculations suggest that the -SO3- group of poly(sodium p-styrene sulfonate) promotes Li-ion motion towards interchain migration through cation-dipole interaction, thus, enabling uniform Li-ion diffusion. Electrochemical measurements of Li | |PHS-coated-Cu coin cells demonstrate an average Coulombic efficiency of 99.46% at 1 mA/cm2, 6 mAh/cm2 and 25 °C. Moreover, when the PHS-coated Li metal negative electrode is paired with a high-areal-capacity LiNi0.83Co0.11Mn0.06O2-based positive electrode in multi-layer pouch cell configuration, the battery delivers an initial capacity of 6.86 Ah (corresponding to a specific energy of 489.7 Wh/kg) and, a 91.1% discharge capacity retention after 150 cycles at 2.5 mA/cm2, 25 °C and 172 kPa.

Cross-linked polyaniline for production of long lifespan aqueous iron||organic batteries with electrochromic properties

Aqueous iron batteries are appealing candidates for large-scale energy storage due to their safety and low-cost aspects. However, the development of aqueous Fe batteries is hindered by their inadequate long-term cycling stability. Here, we propose the synthesis and application as positive electrode active material of cross-linked polyaniline (C-PANI). We use melamine as the crosslinker to improve the electronical conductivity and electrochemical stability of the C-PANI. Indeed, when the C-PANI is tested in combination with a Fe metal negative electrode and 1 M iron trifluoromethanesulfonate (Fe(TOF)2) electrolyte solution, the coin cell can deliver a specific capacity of about 110 mAh g−1 and an average discharge voltage of 0.55 V after 39,000 cycles at 25 A g−1 with a test temperature of 28 °C ± 1 °C. Furthermore, mechanistic studies suggest that Fe2+ ions are bonded to TOF− anions to form positively charged complexes Fe(TOF)+, which are stored with protons in the C-PANI electrode structures. Finally, we also demonstrate the use of C-PANI in combination with a polymeric hydrogel electrolyte to produce a flexible reflective electrochromic lab-scale iron battery prototype.