High-voltage Positive Electrode Research Articles

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.

Decomposition of Electrolyte Solutions at Positive Electrodes

Preventing the decomposition reactions of electrolyte solutions is essential for extending the lifetime of lithium-ion batteries. However, the high voltage (>4.5 V) reactivity of commonly used electrolyte solutions is still poorly understood; electrolyte decomposition at the positive electrode is generally believed to proceed via electrochemical oxidation of the organic carbonate solvent,1 while more recent work has revealed an alternative pathway involving reactive lattice oxygen species originating from the positive electrode (i.e., singlet oxygen).2 Unfortunately, we still have a limited understanding of these reactions, the products that form, and which chemical structural units in the electrolyte solution make them reactive - all of which make it challenging to assess and mitigate the impact of electrolyte decomposition on the battery’s lifetime.In this work, operando gas measurements and solution NMR were combined to study the electrochemical and chemical decomposition of widely used electrolyte salts, solvents, and additives. First, the electrochemical stability of the electrolyte components was reviewed against C65 electrodes in a standard H-cell setup: all solvents reveal little decomposition up to 5.5 V (vs Li/Li+), consistent with previous electrochemical measurements.3,4 Based on the experimentally detected soluble and gaseous products, the electrochemical decomposition mechanism is inferred. Subsequently, the chemical stability against reactive oxygen species (i.e., singlet oxygen; 1O2, peroxide anion; O2 2 −, and superoxide radical anion, O2 −) was explored. All carbonate solvents demonstrated instability against reactive oxygen species. As before, the experimentally detected gaseous and soluble products were used to infer the decomposition mechanisms. The proposed mechanisms were verified using 17O and 18O isotopic labeling for NMR spectroscopy and mass spectrometry measurements, respectively. The findings on the reactivity between the electrolyte components and reactive oxygen species provide a deeper understanding of electrolyte decomposition processes occurring at high voltage positive electrodes (e.g., Li-rich layered and disordered rocksalt oxides, high-voltage spinels, and polyanionic materials). Moreover, these insights will support the development of more stable electrolyte formulations, further extending the lifetime of lithium-ion batteries.(1) Imhof, R.; Novak, P. Oxidative Electrolyte Solvent Degradation in Lithium-Ion Batteries: An In Situ Differential Electrochemical Mass Spectrometry Investigation. J Electrochem Soc 1999, 146 (5), 1702–1706.(2) Jung, R.; Metzger, M.; Maglia, F.; Stinner, C.; Gasteiger, H. A. Oxygen Release and Its Effect on the Cycling Stability of LiNi x Mn y Co z O 2 (NMC) Cathode Materials for Li-Ion Batteries. J Electrochem Soc 2017, 164 (7), A1361–A1377. https://doi.org/10.1149/2.0021707jes.(3) Zhang, X.; Soc, J. E.; Zhang, X.; Kostecki, R.; Richardson, T. J.; Pugh, J. K.; Ross, P. N. Electrochemical and Infrared Studies of the Reduction of Organic Carbonates Electrochemical and Infrared Studies of the Reduction of Organic Carbonates. Journal of The Electrochemical Society 2001, 148 (12), A1341–A1345. https://doi.org/10.1149/1.1415547.(4) Moshkovich, M.; Cojocaru, M.; Gottlieb, H. E.; Aurbach, D. The Study of the Anodic Stability of Alkyl Carbonate Solutions by in Situ FTIR Spectroscopy, EQCM, NMR and MS. Journal of Electroanalytical Chemistry 2001, 497, 84–96. https://doi.org/http://dx.doi.org/10.1016/S0022-0728(00)00457-5.

Impact of synergistic interfacial modification on the electrochemical performance of LiNi0.5Mn1.5O4 cathode materials

Developing sophisticated lithium-ion batteries with high energy and power density requires using high-voltage positive electrodes. Due to its three-dimensional lithium-ion diffusion and greater nominal operating voltage, spinel LiNi0.5Mn1.5O4 has emerged as one of lithium-ion batteries' most viable cathode materials. Electrolyte breakdown, Mn dissolution, and rapid cathode-electrolyte interface (CEI) degradation in lithium-ion cells are exacerbated by the high operating voltage of LNMO. Consequently, the long-term cycling of LNMO is hampered by such adverse side effects, making the commercialization of such a battery impractical. Here, we document the enhancement in the electrochemical performance of LNMO by surface modification utilizing a combination of Al2O3 coating and Graphene enveloping employing a facile wet synthesis technique. The presence of highly crystalline spherical secondary microspheres consisting of primary nanoparticles of disordered LiNi0.5Mn1.5O4, the surface modification with Al2O3, and the subsequent graphene wrapping were all confirmed by structural and surface analysis techniques. The fabricated cells containing the enhanced cathode material (LNMO-Al-GO) were cycled at a C/10 rate for 100 cycles in a voltage window of 3.5–4.9 V, providing a specific discharge capacity of 134.7 ± 3.8 mAhg−1. Delivering a capacity retention of 97.7 ± 3.9% compared to the unmodified LNMO sample (84.7 ± 5.3%). Ex-situ XRD, Electrochemical Impedance Spectroscopy (EIS), and Differential Scanning Calorimetry (DSC) investigations reveal that the alumina coating protects the cathode by acting as a hydrogen fluoride (H.F.) scavenger and minimizes unfavorable phase formations at the CEI, inhibiting Mn3+ dissolution and enhancing cyclability.

Development of a Polymer-Based Silicon-NMC Solid-State Cell

Solid-state batteries are seen as the next generation of battery technology with the promise of high energy density and improved safety as compared to conventional lithium-ion batteries. To achieve these goals, high-capacity negative electrodes, e.g., silicon or lithium, need to be combined with high capacity and high voltage positive electrodes, e.g., Ni-rich NMC. This combination of active materials provides a number of significant challenges for the solid-state electrolyte. If silicon is used as the anode active material, significant volume changes during lithiation/delithiation occur. These volume changes lead to a variety of problems including irreversible loss of lithium and eventual disintegration of the electrodes, resulting in capacity fade. Therefore, the electrolyte must be sufficiently elastic to buffer these changes. If Ni-rich NMC is used as a cathode active material, then the electrolyte must be stable at voltages up to at least 4.2 V. There are currently few, if any, electrolyte solutions that can address these challenges simultaneously.In the ASTRABAT project, a silicon-NMC solid-state cell has been developed based on two tailored polymer electrolytes, which allows the specific challenges of each cell compartment to be addressed separately. A vinylidene fluoride copolymer-based electrolyte has been developed for use as a catholyte and a hybrid inorganic-organic polymer electrolyte as the anolyte. This work will report a characterization of both electrolytes, along with their electrochemical performance in solid-state half-cells and full-cells.

(Invited) SiO and LiNi0.5Mn1.5O4 Composite Electrodes with Functional Polymer Binders

This study focuses on the optimization of polymer binders in lithium-ion batteries (LIBs) to facilitate the development of high-energy-density storage devices. Despite 30 years of commercialization, the demand for high energy density storage devices has continued to increase, making LIBs a topic of significant research interest. One major avenue of research has focused on high-voltage positive electrode materials such as LiNi0.5Mn1.5O4 (LNMO) and high-capacity negative electrode materials like SiO. However, the practical use of these materials is hindered by issues associated with electrolyte decomposition and large volume changes during the charge-discharge process.To address these challenges, we explored the chemistry of polymer binders blended in composite electrodes to maintain the structure of the electrode and form a passivation/protective layer on the active material surface to suppress side reactions.(1, 2) In particular, we investigated the use of "functional binders" to improve the performance of high-voltage LNMO composite electrodes. We synthesized sulfated alginate binders, which are significantly cheaper and stiffer than commonly used poly(vinylidene fluoride) (PVdF) and uniformly cover the surface of LNMO.(3)The high polarity of the sulfate group also increased the binder's affinity for the electrolyte. During the initial charge-discharge cycle, the electrolyte decomposition products generated on the alginate-covered active materials participated in the formation of a protective passivation layer that suppressed further decomposition during subsequent cycles, resulting in enhanced cycling and rate performances.We also explored the use of partially-neutralized crosslinked sodium polyacrylate (CLPA) binders with flexible monomers to mitigate the effects of volume changes in SiO electrodes.(4)Additionally, we investigated the “maturation” effects on battery performances by storing the composite electrodes with CLPA-based binders in a humid atmosphere for a few days before drying. Our results show that the binder with 20% flexible monomer (CLPA-20) exhibited the highest peel strength and long-term cyclability, and the maturation treatment further improved these properties. Cross-sectional SEM images revealed that the binder and conductive carbon were uniformly dispersed in the composite layer in the matured electrodes, indicating the migration of active materials and binders during the maturation process. Furthermore, surface analysis of the cycled electrode showed that the superior mechanical properties of the electrode endowed by the maturation treatment helped in maintaining the passivation layer on the active material surface, resulting in improved cycling performance.Overall, our findings suggest that optimizing polymer binders can enhance the performance of LIBs by controlling the electrode reaction via binder chemistry. The strategies developed in this study could facilitate the future development of high-energy-density LIBs, making them more accessible and commercially viable for various applications.References (1) R. Tatara, S. Komaba et al., ChemElectroChem, 8, 4345 (2021).(2) H. Isozumi, R. Tatara, S. Komaba et al., ACS Appl. Energy Mater., 3, 7978 (2020).(3) A. Oishi, R. Tatara, S. Komaba et al., ACS Appl. Mater. Interfaces, 14, 51808 (2022).(4) S. Yamazaki, R. Tatara, S. Komaba et al., Mater. Adv., 4, 1637 (2023).

A Hexafluorophosphate‐Based Ionic Liquid as Multifunctional Interfacial Layer between High Voltage Positive Electrode and Solid‐State Electrolyte for Sodium Secondary Batteries

AbstractSodium secondary batteries have gained accolades as future energy storage devices due to their low costs and environmental benignity, but are heavily impeded by the poor anodic stabilities of most electrolytes, including solid‐state electrolytes (SSE), that render them incompatible with high‐voltage positive materials. This study reports, for the first time, a new synthesis technique using a fluorohydrogenate ionic liquid (IL)precursor to prepare a [DEME][PF6] ([DEME]+: N,N‐diethyl‐N‐methyl‐N‐(2‐methoxyethyl) ammonium) with high yield and high purity. Herein, a Na[PF6]‐[DEME][PF6] IL is formulated and subjected to a series of electrochemical tests to validate its performance in battery applications. The present IL harbors a strong oxidative stability (up to 5.2 V on Pt and >4.5 V on conductive carbon electrodes) that aids in the suppression of oxidative decompositions of one typical SSE, for example, beta alumina solid electrolyte (BASE), thereby extending their electrochemical window in hybrid SSE systems. A hybrid solid‐state Na secondary battery employing a high voltage positive electrode, Na3V2(PO4)2F3, is assembled using the BASE/IL configuration, and features energy density and superior cycling performance. This work demonstrates that sandwiching an SSE between the oxidatively stable [PF6]− IL can be an effective design for high voltage operation Na secondary batteries.

Low-Concentration Electrolyte Enables High-Voltage Positive Electrode Na4Co3(PO4)2P2O7 with Good Cycle Stability

Low energy density has been a limiting factor for further large-scale commercialization of sodium-ion batteries (SIBs). In this work, we prepare a high-voltage cathode material Na4Co3(PO4)2P2O7 with a low carbon content by a simple solid-state method. In order to counter the poor oxidative stability of the conventional electrolyte, the highly fluorinated co-solvent 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) is introduced to adjust the Na+ solvation sheath structure. Consequently, we design a low-concentration SIB electrolyte (0.3 M NaPF6 in PC/FEC/TTE = 1/1/4, v/v/v). The oxidation stability of this electrolyte is greatly improved due to the special Na+ solvation sheath structure. Under high-voltage cycling, it can form a dense and thin cathode electrolyte interphase film on the cathode surface, which improves the interfacial stability between the electrolyte and the cathode material. Thus, the Na||NCPP cells in the 0.3 M-114 electrolytes maintain good cycle stability even when charged to a high voltage of 4.7 V. Meanwhile, it achieves a reversible capacity of 95.7 mA h g–1 in the initial cycle and a capacity retention of 94.25% after 150 cycles at 1 C. Moreover, it has an energy density of up to 420 W h kg–1 at 1 C. This study provides a practical way to establish a stable interface between the high-voltage positive electrode and the electrolyte in SIBs.

A monofluoride ether-based electrolyte solution for fast-charging and low-temperature non-aqueous lithium metal batteries

The electrochemical stability window of the electrolyte solution limits the energy content of non-aqueous lithium metal batteries. In particular, although electrolytes comprising fluorinated solvents show good oxidation stability against high-voltage positive electrode active materials such as LiNi0.8Co0.1Mn0.1O2 (NCM811), the ionic conductivity is adversely affected and, thus, the battery cycling performance at high current rates and low temperatures. To address these issues, here we report the design and synthesis of a monofluoride ether as an electrolyte solvent with Li-F and Li-O tridentate coordination chemistries. The monofluoro substituent (-CH2F) in the solvent molecule, differently from the difluoro (-CHF2) and trifluoro (-CF3) counterparts, improves the electrolyte ionic conductivity without narrowing the oxidation stability. Indeed, the electrolyte solution with the monofluoride ether solvent demonstrates good compatibility with positive and negative electrodes in a wide range of temperatures (i.e., from −60 °C to +60 °C) and at high charge/discharge rates (e.g., at 17.5 mA cm−2). Using this electrolyte solution, we assemble and test a 320 mAh Li||NCM811 multi-layer pouch cell, which delivers a specific energy of 426 Wh kg−1 (based on the weight of the entire cell) and capacity retention of 80% after 200 cycles at 0.8/8 mA cm−2 charge/discharge rate and 30 °C.

Surface engineering of inorganic solid-state electrolytes via interlayers strategy for developing long-cycling quasi-all-solid-state lithium batteries

Lithium metal batteries (LMBs) with inorganic solid-state electrolytes are considered promising secondary battery systems because of their higher energy content than their Li-ion counterpart. However, the LMB performance remains unsatisfactory for commercialization, primarily owing to the inability of the inorganic solid-state electrolytes to hinder lithium dendrite propagation. Here, using an Ag-coated Li6.4La3Zr1.7Ta0.3O12 (LLZTO) inorganic solid electrolyte in combination with a silver-carbon interlayer, we demonstrate the production of stable interfacially engineered lab-scale LMBs. Via experimental measurements and computational modelling, we prove that the interlayers strategy effectively regulates lithium stripping/plating and prevents dendrite penetration in the solid-state electrolyte pellet. By coupling the surface-engineered LLZTO with a lithium metal negative electrode, a high-voltage positive electrode with an ionic liquid-based liquid electrolyte solution in pouch cell configuration, we report 800 cycles at 1.6 mA/cm2 and 25 °C without applying external pressure. This cell enables an initial discharge capacity of about 3 mAh/cm2 and a discharge capacity retention of about 85%.

Mg-Doped KFeSO4F as a High-Performance Cathode Material for Potassium-Ion Batteries

Potassium iron sulfate fluoride (KFeSO4F) is a high-voltage positive electrode material for potassium-ion batteries, but its practical performance remains limited due to its moderate electronic conductivity. In this study, we employed Mg ion doping in the Fe site of KFeSO4F to tune the crystallinity and ionic/electronic conductivity. Furthermore, we made a composite with carbon nanotubes to enhance the electrode’s electronic conductivity. In a K-ion cell filled with concentrated potassium bis(fluorosulfonyl)amide/triglyme electrolyte, the KFe0.95Mg0.05SO4F/CNT composite electrode delivers discharge (potassiation) capacities of 107 and 100 mAh g–1 in the first and 100th cycles at a rate of 0.1C with reasonable rate capability. UV–vis absorption spectroscopy studies of Mg-doped KFeSO4F powder samples showed a narrower band gap compared to KFeSO4F. Furthermore, from the combined results of X-ray diffraction and absorption spectroscopy studies, we found that multiple two-phase reaction mechanisms through the Fe3+/Fe2+ redox occurred during the reversible depotassiation/potassiation cycle.

Ionic Liquid-Based Electrolytes for the Long-Term Cycling of High-Voltage Lithium-Based Cathode Materials

High-voltage cathode materials, granting high-energy density lithium batteries, face the challenge of developing functional electrolytes enabling their stable operation at voltages higher than 4 V (vs. Li+/Li). Non-volatile and poorly flammable ionic liquid electrolytes (ILEs) are successfully applied to overcome this challenge, even with low-Co or Co-free positive electrode materials. With the ionic liquid-based electrolytes, the cathodes exhibit remarkable electrochemical performance, achieving initial specific capacity above 200 mAh g-1 and outstanding capacity retention (up to 88% over 1000 cycles). More importantly, these electrolytes enable average Coulombic efficiency as high as 99.94%. The excellent compatibility of the ILEs with the high-voltage positive electrode materials enables the realization of cells achieving rather high specific energies.

Development of vanadium-based polyanion positive electrode active materials for high-voltage sodium-based batteries

Polyanion compounds offer a playground for designing prospective electrode active materials for sodium-ion storage due to their structural diversity and chemical variety. Here, by combining a NaVPO4F composition and KTiOPO4-type framework via a low-temperature (e.g., 190 °C) ion-exchange synthesis approach, we develop a high-capacity and high-voltage positive electrode active material. When tested in a coin cell configuration in combination with a Na metal negative electrode and a NaPF6-based non-aqueous electrolyte solution, this cathode active material enables a discharge capacity of 136 mAh g−1 at 14.3 mA g−1 with an average cell discharge voltage of about 4.0 V. Furthermore, a specific discharge capacity of 123 mAh g−1 at 5.7 A g−1 is also reported for the same cell configuration. Through ex situ and operando structural characterizations, we also demonstrate that the reversible Na-ion storage at the positive electrode occurs mostly via a solid-solution de/insertion mechanism.

Non-aqueous rechargeable calcium-ion batteries based on high voltage zirconium-doped ammonium vanadium oxide cathode

Calcium-ion batteries heed the research attention as a energy storage system due to the merits of natural abundance, low-cost, reduction potential (−2.87 V vs. SHE) similar to lithium (−3.04 V vs. SHE) and high theoretical working potential of calcium. However, the practical realization experiences a barrier due to the lack of suitable high-voltage positive electrodes to accommodate the large Ca 2+ with sufficiently fast ionic conduction. Herein, we investigate the zirconium-doped ammonium vanadium oxide (Zr–NH 4 V 4 O 10) as a high-voltage cathode for the rechargeable calcium-ion batteries for the first time at room temperature. It exhibited an initial discharge capacity of 78 mAh g −1 with an average discharge voltage of ∼3.0 V vs. Ca 2+ |Ca and delivered an initial energy density of 231 W h kg −1 in calcium triflate [Ca(CF 3 SO 3) 2 ] added in propylene carbonate electrolyte. Moreover, the Zr–NH 4 V 4 O 10 host presents a high-rate capability, excellent cycling stability (0.021% capacity decay per cycle) for 500 cycles with ∼89% capacity retention, maintaining ∼100% Coulombic efficiency. This is one of the first demonstrations of reversible Ca-ion storage with a high voltage (∼3.0 V) using a conventional electrolyte.

Amorphous Positive Electrode Materials Prepared Using LiMn1.5Ni0.5O4 and Lithium Oxyacid Salts

LiNi0.5Mn1.5O4 is an attractive choice for a high-voltage positive electrode material for all-solid-state batteries. However, its use in all-solid-state batteries is challenging because of low ionic conductivity. In this study, we synthesized new amorphous electrode materials from LiMn1.5Ni0.5O4 and lithium oxyacid salts. The synthesized amorphous electrodes exhibited higher ionic conductivity and capacity than a LiMn1.5Ni0.5O4 crystal in both liquid- and solid-type cells. Among these electrodes, amorphous 80LiMn1.5Ni0.5O4·20Li3PO4 (mol %) demonstrated a high reversible capacity of 220 mAh g−1 in a liquid-type cell.

Methodology and Electrochemical Techniques for Development of All-Solid-State Batteries

Despite some progress performed, state-of-the-art lithium ion batteries still require improvements in energy and power to extend the range of electric vehicles and reduce charging time. In this domain, all-solid-state batteries are viable alternatives to conventional batteries employing organic electrolytes because of their benefits, i.e., high power density, high energy density, long-life operation and safety. These advantages stem from the great features of inorganic solid electrolytes, which are single ion conductor, so a high lithium ion transport number, and no-liquid nature. In particular, the sulfide-based solid electrolytes possess favorable mechanical properties, allowing all-solid-state batteries to be easily prepared via simple mixing and cold-pressing processes, facilitating the scale-up.Sulfide-based solid-electrolytes can potentially be employed in conjunction with a lithium metal negative electrode and 5V-class high voltage positive electrode material. Indeed, lithium metal is believed to be the most promising negative electrode due to its specific large capacity (3862 mAh.g-1), low volumetric density (0.534 g.cm-3 at 20°C) and the lowest electrochemical potential (-3.03V vs ENH). Nevertheless, like most metal, lithium metal is morphologically dynamic. Its surface morphology is modified, since during the electrochemical cycling, a part of lithium migrates to the other electrode to react and is then plating on its surface heterogeneously, leading to a volume change and sometimes dendrite growth with potential internal short circuit and life-threatening accidents. Ceramic solid electrolytes have been considered to be the ideal solution to prevent dendrite growth because of their high shear modulus and high lithium transference number. In the same time, the chemical nature and composition of ceramic solid electrolyte can affect its reactivity with lithium metal. In the same time, the electrochemical stability of solid electrolyte depending of its nature and composition is a key parameter to mix it with the different components of positive electrode as active materials and electronic conductors to ensure the better percolation and performances. The oxide–based ceramic solid electrolytes can be shaped by sintering at high temperature, leading to a grain and –grain boundary microstructure with some porosity, facilitating the lithium dendrite growth through grain boundaries. Due to the low density and plasticity of sulfide based inorganic solid electrolyte, the dendrite growth through the particle-particle contact can be reduced but still present.Hence different parameters can influence the performances and aging of all solid state ceramic battery. To explore them, various electrochemical technics and associated specific treatments can be developed to extract and identify each phenomena and ensure the better development of this technology. A specific sequential methodology will be presented with different examples from the materials, interfaces with lithium metal, pseudo-composites and temperature effects up to aging of total all-solid–state ceramic battery based on sulfide technology. The extracted different parameters and the relationship between them will be presented.

Dual-anion ionic liquid electrolyte enables stable Ni-rich cathodes in lithium-metal batteries

<h2>Summary</h2> High-energy-density lithium-metal batteries face the challenge of developing functional electrolytes enabling both the stabilization of the lithium-metal negative electrode and high-voltage positive electrodes (> 4 V versus Li⁺/Li). Herein, a low-volatility and non-flammable ionic liquid electrolyte (ILE) incorporating two anions, bis(fluorosulfonyl) imide (FSI) and bis(trifluoromethanesulfonyl)imide (TFSI), is successfully applied to overcome this challenge, employing the high-energy, low-Co, and Ni-rich positive-electrode material, LiNi_0.88Co_0.09Mn_0.03O₂ (NCM88), in Li-metal batteries. With this specific electrolyte, the cathode exhibits remarkable electrochemical performance, achieving an initial specific capacity of 214 mAh g⁻¹ and outstanding capacity retention of 88% over 1,000 cycles. More importantly, this electrolyte enables an average Coulombic efficiency of 99.94%. The excellent compatibility of the dual-anion ILE with both the lithium metal (50 μm) and the high-voltage positive-electrode material enables the realization of Li-metal cells achieving specific energies of more than 560 Wh kg⁻¹ based on their combined active material masses.

Toward the Development of a High-Voltage Mg Cathode Using a Chromium Sulfide Host

Author(s): Blanc, L; Bartel, CJ; Kim, H; Tian, Y; Kim, H; Miura, A; Ceder, G; Nazar, LF | Abstract: Development of Mg-ion batteries as advanced electrochemical energy storage systems relies on the design and discovery of high-voltage positive electrode (cathode) materials. To date, a variety of sulfide cathodes have been reported (e.g., MgxMo6S8, MgxTi2S4, etc.), but the voltages of these materials are too low to prepare a high energy density Mg cell. Theoretical computations predicted that MgxCr2S4 operating with the high-voltage Cr3+/4+ redox couple would serve as a suitable cathode candidate, but experimental attempts to extract Mg2+ from the lattice have been largely unsuccessful. We show that reversible electrochemical activity within a thiospinel framework (AB2S4) relies on a redox-active transition metal present in the B site; otherwise, anionic redox activity triggers decomposition of the spinel structure. Since Cr and S states are highly coupled in MgCr2S4, the Cr3+/4+ redox couple is inaccessible so that reversible (de)intercalation of Mg2+ cannot occur and charging leads to dissolution of the active material. These findings point to an insufficiency in the screening criteria previously used to identify MgCr2S4 as a promising Mg cathode. Thus, a computable descriptor based on the electronic structure of the discharged material is proposed to predict the prevalence of cation vs anion redox and improve future surveys of potential candidates. It is unlikely that the high-voltage Cr redox couple will be accessible to oxidation in the presence of sulfur within the restrictions of a spinel framework; however, it is possible that a suitable layered MgxCrS2 structure could serve as a reversible high-voltage Mg cathode.

Dissolution of cathode–electrolyte interphase deposited on LiNi0.5Mn1.5O4 for lithium-ion batteries

Adopting high-voltage positive electrodes is essential for realizing advanced lithium-ion batteries with high energy and power density. However, the operating potentials of electrodes are beyond the electrochemical stability window of commercial electrolytes, leading to a low coulombic efficiency and poor cycle life. Although the decomposition products of an electrolyte form a cathode–electrolyte interphase (CEI), which helps passivate the electrode surface and suppress further decomposition of the electrolyte in subsequent cycles, the CEI is associated with passivation failure, the mechanisms of which have rarely been investigated. This work demonstrates the local dissolution of CEI and the detrimental effects of dissolution on the electrochemical performance of a high-voltage spinel electrode. The corrosive acidic species, namely HF, is generated during electrolyte oxidation and damages the CEI by leaching inorganic components such as LiF and LixPOyFz. However, in the presence of Al2O3, which is an HF scavenger, the CEI is maintained relatively stable, and the coulombic efficiency is improved significantly. The results highlight the HF effect on the partial dissolution of the CEI and the additional role of Al2O3 in assisting CEI deposition.

Proton-assisted calcium-ion storage in aromatic organic molecular crystal with coplanar stacked structure

Rechargeable calcium-ion batteries are intriguing alternatives for use as post-lithium-ion batteries. However, the high charge density of divalent Ca2+ establishes a strong electrostatic interaction with the hosting lattice, which results in low-capacity Ca-ion storage. The ionic radius of Ca2+ further leads to sluggish ionic diffusion, hindering high-rate capability performances. Here, we report 5,7,12,14-pentacenetetrone (PT) as an organic crystal electrode active material for aqueous Ca-ion storage. The weak π-π stacked layers of the PT molecules render a flexible and robust structure suitable for Ca-ion storage. In addition, the channels within the PT crystal provide efficient pathways for fast ionic diffusion. The PT anode exhibits large specific capacity (150.5 mAh g-1 at 5 A g-1), high-rate capability (86.1 mAh g-1 at 100 A g-1) and favorable low-temperature performances. A mechanistic study identifies proton-assisted uptake/removal of Ca2+ in PT during cycling. First principle calculations suggest that the Ca ions tend to stay in the interstitial space of the PT channels and are stabilized by carbonyls from adjacent PT molecules. Finally, pairing with a high-voltage positive electrode, a full aqueous Ca-ion cell is assembled and tested.

An overlooked issue for high-voltage Li-ion batteries: Suppressing the intercalation of anions into conductive carbon

Summary High-voltage Li-ion batteries have been extensively studied to increase energy density of batteries. However, their cycling stability has remained poor, despite various strategies being proposed to overcome the issues of high-potential cathodes, such as electrolyte oxidation and transition metal dissolution. Herein, we report anion intercalation into the cathode conductive carbon as an overlooked yet critical issue. We propose a concentrated sulfolane (SL)-based electrolyte that prevents anion intercalation via two mechanisms: (1) by offering a high activation barrier to intercalation via its strong anion-Li+ interaction and (2) by forming a sulfur-containing, anion-blocking SL-derived interphase. This electrolyte, used with graphitized acetylene black, which is oxidatively stable but usually susceptible to anion intercalation, enables the stable operation of a Li2CoPO4F/graphite full cell (cut-off voltage = 5.2 V), with 93% capacity retention after 1,000 cycles and an average Coulombic efficiency of ≥99.9%. This is a pivotal strategy to enhance the reversibility of >5 V Li-ion batteries on a commercial level.