Densified vertically lamellar electrode architectures for compact energy storage.

As one of the most compact electrochemical energy storage systems, lithium-ion batteries (LIBs) are playing an indispensable role in the process of vehicle electrification to accelerate the shift to sustainable mobility. Making battery electrodes thicker is a promising strategy for improving the energy density of LIBs which is essential for applications with weight or volume constraints, such as electric-powered transportation; however, their power densities are often significantly restricted due to elongated and tortuous charge traveling distances. Here, we propose an effective methodology that couples bidirectional freeze-casting and compression-induced densification to create densified vertically lamellar electrode architectures for compact energy storage. The vertically lamellar architectures not only overcome the critical thickness limit for conventional electrodes but also facilitate and redistribute the lithium-ion flux enabling both high rate capability and stable cyclability. Furthermore, this proposed methodology is universal as demonstrated in various electrochemical active material systems. This study offers a facile approach that realizes simultaneous high energy and high power in high-loading battery electrodes and provides useful rationales in designing electrode architectures for scalable energy storage systems.

Electron and Ion Transport in Lithium and Lithium-Ion Battery Negative and Positive Composite Electrodes.

Electrochemical energy storage systems, specifically lithium and lithium-ion batteries, are ubiquitous in contemporary society with the widespread deployment of portable electronic devices. Emerging storage applications such as integration of renewable energy generation and expanded adoption of electric vehicles present an array of functional demands. Critical to battery function are electron and ion transport as they determine the energy output of the battery under application conditions and what portion of the total energy contained in the battery can be utilized. This review considers electron and ion transport processes for active materials as well as positive and negative composite electrodes. Length and time scales over many orders of magnitude are relevant ranging from atomic arrangements of materials and short times for electron conduction to large format batteries and many years of operation. Characterization over this diversity of scales demands multiple methods to obtain a complete view of the transport processes involved. In addition, we offer a perspective on strategies for enabling rational design of electrodes, the role of continuum modeling, and the fundamental science needed for continued advancement of electrochemical energy storage systems with improved energy density, power, and lifetime.

Vertically assembled nanosheet networks for high-density thick battery electrodes.

As one of the prevailing energy storage systems, lithium-ion batteries (LIBs) have become an essential pillar in electric vehicles (EVs) during the past decade, contributing significantly to a carbon-neutral future. However, the complete transition to electric vehicles requires LIBs with yet higher energy and power densities. Here, we propose an effective methodology via controlled nanosheet self-assembly to prepare low-tortuosity yet high-density and high-toughness thick electrodes. By introducing a delicate densification in a three-dimensionally interconnected nanosheet network to maintain its vertical architecture, facile electron and ion transports are enabled despite their high packing density. This dense and thick electrode is capable of delivering a high volumetric capacity >1,600 mAh cm-3, with an areal capacity up to 32 mAh cm-2, which is among the best reported in the literature. The high-performance electrodes with superior mechanical and electrochemical properties demonstrated in this work provide a potentially universal methodology in designing advanced battery electrodes with versatile anisotropic properties.

Fast-Charging, Binder-Free Lithium Battery Cathodes Enabled via Multidimensional Conductive Networks.

To meet the growing demands in both energy and power densities of lithium ion batteries, electrode structures must be capable of facile electron and ion transport while minimizing the content of electrochemically inactive components. Herein, binder-free LiFePO4 (LFP) cathodes are fabricated with a multidimensional conductive architecture that allows for fast-charging capability, reaching a specific capacity of 94 mAh g-1 at 4 C. Such multidimensional networks consist of active material particles wrapped by 1D single-walled carbon nanotubes (CNTs) and bound together using 2D MXene (Ti3C2Tx) nanosheets. The CNTs form a porous coating layer and improve local electron transport across the LFP surface, while the Ti3C2Tx nanosheets provide simultaneously high electrode integrity and conductive pathways through the bulk of the electrode. This work highlights the ability of multidimensional conductive fillers to realize simultaneously superior electrochemical and mechanical properties, providing useful insights into future fast-charging electrode designs for scalable electrochemical systems.

Assembled MXene-Liquid Metal Cages on Silicon Microparticles as Self-Healing Battery Anodes.

In pursuit of higher energy density in lithium-ion batteries, silicon (Si) has been recognized as a promising candidate to replace commercial graphite due to its high theoretical capacity. However, the pulverization issue of Si microparticles during lithiation/delithiation results in electrical contact loss and increased side reactions, significantly limiting its practical applications. Herein, we propose to utilize liquid metal (LM) particles as the bridging agent, which assemble conductive MXene (Ti3C2Tx) sheets via coordination chemistry, forming cage-like structures encapsulating mSi particles as self-healing high-energy anodes. Due to the integration of robust Ti3C2Tx sheets and deformable LM particles as conductive buffering cages, simultaneously high-rate capability and cyclability can be realized. Post-mortem analysis revealed the cage structural integrity and the maintained electrical percolating network after cycling. This work introduces an effective approach to accommodate structural change via a resilient encapsulating cage and offers useful interface design considerations for versatile battery electrodes.

Defect-Driven Configurational Entropy in the High-Entropy Oxide Li1.5MO3-δ.

Layered lithiated oxides are promising materials for next generation Li-ion battery cathode materials; however, instability during cycling results in poor performance over time compared to the high capacities theoretically possible with these materials. Here we report the characterizations of a Li1.47Mn0.57Al0.13Fe0.095Co0.105Ni0.095O2.49 high-entropy layered oxide (HELO) with the Li2MO3 structure where M = Mn, Al, Fe, Co, and Ni. Using electron microscopy and X-ray spectroscopy, we identify a homogeneous Li2MO3 structure stabilized by the entropic contribution of oxygen vacancies. This defect-driven entropy would not be attainable in the LiMO2 structure sometimes observed in similar materials as a secondary phase owing to the presence of fewer O sites and a 3+ oxidation state for the metal site; instead, a Li2-γMO3-δ is produced. Beyond Li2MO3, this defect-driven entropy approach to stabilizing novel compositions and phases can be applied to a wide array of future cathode materials including spinel and rock salt structures.

Mechanism of Chalcophanite Nucleation in Zinc Hydroxide Sulfate Cathodes in Aqueous Zinc Batteries.

Aqueous Zn-ion batteries with MnO2-based cathodes have seen significant attention owing to their high theoretical capacities, safety, and low cost; however, much debate remains regarding the reaction mechanism that dominates energy storage. In this work, we report our electron microscopy study of cathodes containing zinc hydroxide sulfate (Zn4SO4(OH)6·xH2O, ZHS) together with carbon nanotubes cycled in electrolytes containing ZnSO4 with varied amounts of MnSO4 incorporated. The primary Mn-containing phase is formed in situ in the cathode during cycling, where a dissolution-deposition reaction is identified between ZHS and chalcophanite (ZnMn3O7·3H2O). Mechanistic details of this reaction, in which the chalcophanite nucleates then separates from the ZHS flakes as the ZHS dissolves while acting as the primary Zn source for the reaction, are revealed using surface sensitive methods. These findings indicate the reaction is local to the ZHS flakes, providing new insight toward the importance of ZHS and the cathode microstructure.

Gradient Architecture Design in Scalable Porous Battery Electrodes.

Because it has been demonstrated to be effective toward faster ion diffusion inside the pore space, low-tortuosity porous architecture has become the focus in thick electrode designs, and other possibilities are rarely investigated. To advance current understanding in the structure-affected electrochemistry and to broaden horizons for thick electrode designs, we present a gradient electrode design, where porous channels are vertically aligned with smaller openings on one end and larger openings on the other. With its 3D morphology carefully visualized by Raman mapping, the electrochemical properties between opposite orientations of the gradient electrodes are compared, and faster energy storage kinetics is found in larger openings and more concentrated active material near the separator. As further verified by simulation, this study on gradient electrode design deepens the knowledge of structure-related electrochemistry and brings perspectives in high-energy battery electrode designs.

Tortuosity Engineering for Improved Charge Storage Kinetics in High-Areal-Capacity Battery Electrodes.

The increasing demands of electronic devices and electric transportation necessitate lithium-ion batteries with simultaneous high energy and power capabilities. However, rate capabilities are often limited in high-loading electrodes due to the lengthy and tortuous ion transport paths with their electrochemical behaviors governed by complicated electrode architectures still elusive. Here, we report the electrode-level tortuosity engineering design enabling improved charge storage kinetics in high-energy electrodes. Both high areal capacity and high-rate capability can be achieved beyond the practical level of mass loadings in electrodes with vertically oriented architectures. The electrochemical properties in electrodes with various architectures were quantitatively investigated through correlating the characteristic time with tortuosity. The lithium-ion transport kinetics regulated by electrode architectures was further studied via combining the three-dimensional electrode architecture visualization and simulation. The tortuosity-controlled charge storage kinetics revealed in this study can be extended to general electrode systems and provide useful design consideration for next-generation high-energy/power batteries.

Simultaneous Elucidation of Solid and Solution Manganese Environments via Multiphase Operando Extended X-ray Absorption Fine Structure Spectroscopy in Aqueous Zn/MnO2 Batteries.

Aqueous Zn/MnO2 batteries (AZMOB) with mildly acidic electrolytes hold promise as potential green grid-level energy storage solutions for clean power generation. Mechanistic understanding is critical to advance capacity retention needed by the application but is complex due to the evolution of the cathode solid phases and the presence of dissolved manganese in the electrolyte due to a dissolution-deposition redox process. This work introduces operando multiphase extended X-ray absorption fine structure (EXAFS) analysis enabling simultaneous characterization of both aqueous and solid phases involved in the Mn redox reactions. The methodology was successfully conducted in multiple electrolytes (ZnSO4, Zn(CF3SO3)2, and Zn(CH3COO)2) revealing similar manganese coordination environments but quantitative differences in distribution of Mnn+ species in the solid and solution phases. Complementary Raman spectroscopy was utilized to identify the less crystalline Mn-containing products formed under charge at the cathodes. This was further augmented by transmission electron microscopy (TEM) to reveal the morphology and surface condition of the deposited solids. The results demonstrate an effective approach for bulk-level characterization of poorly crystalline multiphase solids while simultaneously gaining insight into the dissolved transition-metal species in solution. This work provides demonstration of a useful approach toward gaining insight into complex electrochemical mechanisms where both solid state and dissolved active materials are important contributors to redox activity.

Multimodal electrochemistry coupled microcalorimetric and X-ray probing of the capacity fade mechanisms of Nickel rich NMC - progress and outlook.

Lithium nickel manganese cobalt oxide (NMC) is a commercially successful Li-ion battery cathode due to its high energy density; however, its delivered capacity must be intentionally limited to achieve capacity retention over extended cycling. To design next-generation NMC batteries with longer life and higher capacity the origins of high potential capacity fade must be understood. Operando hard X-ray characterization techniques are critical for this endeavor as they allow the acquisition of information about the evolution of structure, oxidation state, and coordination environment of NMC as the material (de)lithiates in a functional battery. This perspective outlines recent developments in the elucidation of capacity fade mechanisms in NMC through hard X-ray probes, surface sensitive soft X-ray characterization, and isothermal microcalorimetry. A case study on the effect of charging potential on NMC811 over extended cycling is presented to illustrate the benefits of these approaches. The results showed that charging to 4.7 V leads to higher delivered capacity, but much greater fade as compared to charging to 4.3 V. Operando XRD and SEM results indicated that particle fracture from increased structural distortions at >4.3 V was a contributor to capacity fade. Operando hard XAS revealed significant Ni and Co redox during cycling as well as a Jahn-Teller distortion at the discharged state (Ni3+); however, minimal differences were observed between the cells charged to 4.3 and 4.7 V. Additional XAS analyses using soft X-rays revealed significant surface reconstruction after cycling to 4.7 V, revealing another contribution to fade. Operando isothermal microcalorimetry (IMC) indicated that the high voltage charge to 4.7 V resulted in a doubling of the heat dissipation when compared to charging to 4.3 V. A lowered chemical-to-electrical energy conversion efficiency due to thermal energy waste was observed, providing a complementary characterization of electrochemical degradation. The work demonstrates the utility of multi-modal X-ray and microcalorimetric approaches to understand the causes of capacity fade in lithium-ion batteries with Ni-rich NMC.

Building Efficient Ion Pathway in Highly Densified Thick Electrodes with High Gravimetric and Volumetric Energy Densities.

A common practice in thick electrode design is to increase porosity to boost charge transport kinetics. However, a high porosity offsets the advantages of thick electrodes in both gravimetric and volumetric energy densities. Here we design a freestanding thick electrode composed of highly densified active material regions connected by continuous electrolyte-buffering voids. By wet calendering of the phase-inversion electrode, the continuous compact active material region and continuous ion transport network are controllably formed. Rate capabilities and cycling stability at high LiFePO4 loading of 126 mg cm-2 were achieved for the densified cathode with porosity as low as 38%. The decreased porosity and efficient void utilization enable high gravimetric/volumetric energy densities of 330 Wh kg-1 and 614 Wh L-1, as well as improved power densities. The versatility of this method and the industrial compatible "roll-to-roll" fabrication demonstrate an important step toward the practical application of thick electrodes.

Tunable Porous Electrode Architectures for Enhanced Li-Ion Storage Kinetics in Thick Electrodes.

Thick electrodes, although promising toward high-energy battery systems, suffer from restricted lithium-ion transport kinetics due to prolonged diffusion lengths and tortuous transport pathways. Despite the emerging low-tortuosity designs, capacity retention under higher current densities is still limited. Herein, we employ a modified ice-templating method to fabricate low-tortuosity porous electrodes with tunable wall thickness and channel width and systematically investigate the critical impacts of the fine structural parameters on the thick electrode electrochemistry. While the porous electrodes with thick walls show diminished capability under a C-rate larger than 1.5 C, those with thinner walls could maintain ∼70% capacity under 2.5 C. The superior capacity retention is ascribed to the fast diffusion into the thin lamellar walls compared with their thicker counterparts. This study provides deeper insights into structure-affected electrochemistry and opens up new perspective of 3D porous architectural designs for high-energy and high-power electrodes.

Local and Bulk Probe of Vanadium-Substituted α-Manganese Oxide (α-KxVyMn8-yO16) Lithium Electrochemistry.

A series of V-substituted α-MnO2 (KxMn8-yVyO16·nH2O, y = 0, 0.2, 0.34, 0.75) samples were successfully synthesized without crystalline or amorphous impurities, as evidenced by X-ray diffraction (XRD) and Raman spectroscopy. Transmission electron microscopy (TEM) revealed a morphological evolution from nanorods to nanoplatelets as V-substitution increased, while electron-energy loss spectroscopy (EELS) confirmed uniform distribution of vanadium within the materials. Rietveld refinement of synchrotron XRD showed an increase in bond lengths and a larger range of bond angles with increasing V-substitution. X-ray absorption spectroscopy (XAS) of the as-prepared materials revealed the V valence to be >4+ and the Mn valence to decrease with increasing V content. Upon electrochemical lithiation, increasing amounts of V were found to preserve the Mn-Mnedge relationship at higher depths of discharge, indicating enhanced structural stability. Electrochemical testing showed the y = 0.75 V-substituted sample to deliver the highest capacity and capacity retention after 50 cycles. The experimental findings were consistent with the predictions of density functional theory (DFT), where the V centers impart structural stability to the manganese oxide framework upon lithiation. The enhanced electrochemistry of the y = 0.75 V-substituted sample is also attributed to its smaller crystallite size in the form of a nanoplatelet morphology, which promotes facile ion access via reduced Li-ion diffusion path lengths.

Structural and electrochemical investigation of crystallite size controlled zinc ferrite (ZnFe2O4).

Zinc ferrite, ZnFe2O4(ZFO), is a promising electrode material for next generation Li-ion batteries because of its high theoretical capacity and low environmental impact. In this report, synthetic control of crystallite size from the nanometer to submicron scale enabled probing of the relationships between ZFO size and electrochemical behavior. A facile two-step coprecipitation and annealing preparation method was used to prepare ZFO with controlled sizes ranging ∼9 to >200 nm. Complementary synchrotron and electron microscopy techniques were used to characterize the series of materials. Increasing the annealing temperature increased crystallinity and decreased microstrain, while local structural ordering was maintained independent of crystallite size. Electrochemical characterization revealed that the smaller sized materials delivered higher capacities during initial lithiation. Larger sized particles exhibited a lack of distinct electrochemical signatures above 1.0 V, suggesting that the longer diffusion length associated with greater crystallite size causes the lithiation process to proceed via non discrete lithium insertion, cation migration, and conversion processes. Notably, larger particles exhibited enhanced electrochemical reversibility over 50 cycles, with capacity retention improving from <20% to >40% at C/2 cycling rate. This intriguing result was probed through x-ray absorption spectroscopy (XAS) and x-ray photoelectron spectroscopy (XPS) measurements of the cycled electrodes. XAS revealed that the larger crystallite size materials do not completely convert to Fe0during the first lithiation and that independent of size, delithiation results in the formation of nanocrystalline FeO and ZnO phases rather than ZnFe2O4. After 20 cycles, the larger crystallites showed reversibility between partially oxidized FeO in the charged state and Fe0in the discharged state, while the smaller crystallite size material was electrochemically inactive as Fe0. XPS analysis revealed more significant solid electrolyte interphase (SEI) formation on the cycled electrodes utilizing ZFO with smaller crystallite size. This finding suggests that excessive SEI buildup on the smaller sized, higher surface area ZFO particles contributes to their reduced electrochemical reversibility relative to the larger crystallite size materials.

Impact of sodium vanadium oxide (NaV3O8, NVO) material synthesis conditions on charge storage mechanism in Zn-ion aqueous batteries.

The electrochemical charge storage of sodium vanadate (NaV3O8 or NVO) cathodes in aqueous Zn-ion batteries has been hypothesized to be influenced by the inclusion of structural water for facilitating ion transfer in the material. Materials properties considered important (morphology, crystallite and particle size, surface area) are systematically studied herein through investigation of two NVO materials, NaV3O8·0.34H2O [NVO(300)] and NaV3O8·0.05H2O [NVO(500)], with different water content, acicular morphologies with different size and surface area achieved via post-synthesis heat treatment. The electrochemistry of the two materials was evaluated in aqueous Zn-ion cells with 2 M ZnSO4 electrolyte using cyclic voltammetry, galvanostatic cycling, and rate capability testing. The thinner NVO(300) nanobelts (0.13 µm) demonstrate greater specific capacities and higher effective diffusion coefficients relative to the thicker NVO(500) nanorods. Notably however, while cells containing NVO(500) deliver lower specific capacity, they demonstrate enhanced capacity retention with cycling. The structural changes accompanying oxidation and reduction are elucidated via ex situ X-ray diffraction, transmission electron microscopy, and operando V K-edge X-ray absorption spectroscopy (XAS), where NVO material properties are shown to influence the ion insertion. Operando XAS verified that electron transfer corresponds directly to change in vanadium oxidation state, affirming vanadium redox as the governing electrochemical process.

Lithium vanadium oxide (Li1.1V3O8) thick porous electrodes with high rate capacity: utilization and evolution upon extended cycling elucidated via operando energy dispersive X-ray diffraction and continuum simulation.

The phase distribution of lithiated LVO in thick (∼500 µm) porous electrodes (TPEs) designed to facilitate both ion and electron transport was determined using synchrotron-based operando energy dispersive X-ray diffraction (EDXRD). Probing 3 positions in the TPE while cycling at a 1C rate revealed a homogeneous phase transition across the thickness of the electrode at the 1st and 95th cycles. Continuum modelling indicated uniform lithiation across the TPE in agreement with the EDXRD results and ascribed decreasing accessible active material to be the cause of loss in delivered capacity between the 1st and 95th cycles. The model was supported by the observation of significant particle fracture by SEM consistent with loss of electrical contact. Overall, the combination of operando EDXRD, continuum modeling, and ex situ measurements enabled a deeper understanding of lithium vanadium oxide transport properties under high rate extended cycling within a thick highly porous electrode architecture.

Self-healing, Improved Efficiency Solid State Rechargeable Li/I2 Based Battery.

Solid state electrolytes are receiving significant interest due to the prospect of improved safety, however, addressing the incidence and consequence of internal short circuits remains an important issue. Herein, a battery based on a LiI-LiI(HPN)2 solid state electrolyte demonstrated self-healing after internal shorting where the cells recovered and continued to cycle effectively. The functional rechargeable electrochemistry of the self-forming Li/I2-based battery was investigated through interfacial modification by inclusion of Li metal (at the negative interface), and/or fabricated carbon nanotube substrates at the positive interface. A cell design with lithium metal at the negative and a carbon substrate at the positive interface produced Coulombic efficiencies > 90% over 60 cycles. Finally, the beneficial effects of moderately elevated temperature were established where a 10°C temperature increase led to ~5X lower resistance.

Unraveling the Dissolution-Mediated Reaction Mechanism of α-MnO2 Cathodes for Aqueous Zn-Ion Batteries.

Aqueous Zn/α-MnO2 batteries have attracted immense interest owing to their high energy density, low cost, and safety, making them desirable for future large-scale energy application. Despite these merits, the comprehensive understanding of their reaction mechanism has been elusive due to the limitations of standard bulk characterization. Here, via transmission electron microscopy, the dissolution-mediated reaction mechanism of a Zn/α-MnO2 system is discovered and explored in full scope to involve reversible formation of Zn4 SO4 (OH)6 ·xH2 O and "birnessite-like" Zn-MnOx phase upon cycling. Overall, α-MnO2 acts primarily as a source for cell activation through dissolution and thus is not directly involved in the Zn redox chemistry. This microscopic study offers a unique knowledge on the unconventional reaction chemistry of Zn/α-MnO2 batteries.

Solution-Based, Anion-Doping of Li4 Ti5 O12 Nanoflowers for Lithium-Ion Battery Applications.

Solution-based, anionic doping represents a convenient strategy with which to improve upon the conductivity of candidate anode materials such as Li4 Ti5 O12 (LTO). As such, novel synthetic hydrothermally-inspired protocols have primarily been devised herein, aimed at the large-scale production of unique halogen-doped, micron-scale, three-dimensional, hierarchical LTO flower-like motifs. Although fluorine (F) doping has been explored, the use of chlorine (Cl) dopants is the primary focus here. Several experimental variables, such as dopant amount, lithium hydroxide concentration, and titanium butoxide purity, were probed and perfected. Furthermore, the Cl doping process did not damage the intrinsic LTO morphology. The analysis, based on interpreting a compilation of SEM, XRD, XPS, and TEM-EDS results, was used to determine an optimized dopant concentration of Cl. Electrochemical tests demonstrated an increased capacity via cycling of 12 % for a Cl-doped sample as compared with pristine LTO. Moreover, the Cl-doped LTO sample described in this study exhibited the highest discharge capacity yet reported at an observed rate of 2C for this material at 143mAh g-1 . Overall, these data suggest that the Cl dopant likely enhances not only the ion transport capabilities, but also the overall electrical conductivity of our as-prepared structures. To help explain these favorable findings, theoretical DFT calculations were used to postulate that the electronic conductivity and Li diffusion were likely improved by the presence of increased Ti3+ ion concentration coupled with widening of the Li migration channel.