Stable Sodium-Based Batteries with Advanced Electrolytes and Layered-Oxide Cathodes
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Tuning local chemistry of P2 layered-oxide cathode for high energy and long cycles of sodium-ion battery
Layered transition-metal oxides have attracted intensive interest for cathode materials of sodium-ion batteries. However, they are hindered by the limited capacity and inferior phase transition due to the gliding of transition-metal layers upon Na + extraction and insertion in the cathode materials. Here, we report that the large-sized K + is riveted in the prismatic Na + sites of P2-Na 0.612 K 0.056 MnO 2 to enable more thermodynamically favorable Na + vacancies. The Mn-O bonds are reinforced to reduce phase transition during charge and discharge. 0.901 Na + per formula are reversibly extracted and inserted, in which only the two-phase transition of P2 ↔ P’2 occurs at low voltages. It exhibits the highest specific capacity of 240.5 mAh g -1 and energy density of 654 Wh kg -1 based on the redox of Mn 3+ /Mn 4+ , and a capacity retention of 98.2% after 100 cycles. This investigation will shed lights on the tuneable chemical environments of transition-metal oxides for advanced cathode materials and promote the development of sodium-ion batteries.
25 ENERGY STORAGE↗
Crossover Effects in Batteries with High–Nickel Cathodes and Lithium–Metal Anodes
It is well understood that cathode-to-anode crossover, especially of transition-metal ions, can significantly impact the long-term cycling of lithium-ion batteries. The dissolved transition-metal ions in lithium-ion cells deposit on the graphite anode, disrupt the solid-electrolyte interphase (SEI), and catalyze further side reactions. Meanwhile, crossover effects in lithium-metal batteries have rarely been studied. This study is the first to investigate crossover effects in lithium-metal batteries with high-nickel layered-oxide cathodes. It is shown that the crossover of transition-metal ions from LiNi 0.9 Mn 0.05 Co 0.05 O 2 has minimal effect on the lithium-metal anode (LMA) due to the following reasons. The catalytic transition metals 1) have less effect on an inherently reactive LMA, 2) are diluted in a thicker SEI, and 3) are produced in overall lower quantity due to the limited cycle life of the LMA. Conversely, the LMA generates soluble decomposition products that cross over to the cathode even during early cycling. This crossover accelerates impedance growth and capacity fade at the cathode and is partially responsible for the mismatch between the performance of half and full-cells with layered-oxide cathodes. Finally, this study highlights the need for better battery design with LMA, potentially including electrolyte or cell modifications.
25 ENERGY STORAGE↗
A perspective on single-crystal layered oxide cathodes for lithium-ion batteries
As the demand for lithium-ion batteries grows exponentially to feed the nascent electric-vehicle and grid-storage markets, the need for higher energy density and longer cycle life becomes more apparent. Increasing the nickel content in the layered-oxide cathodes has been a dominant strategy to increase energy density, but this has exacerbated the surface reactivity concerns. Furthermore, the high states of charge associated with increased capacity lead to cracking of the polycrystalline cathode particles, which exposes fresh surfaces and accelerates capacity fade. Single-crystal cathode particles with low susceptibility to cracking have recently taken on intense interest due to their remarkable cyclability. Many different approaches to single-crystal synthesis have been explored, but there has been no systematic analysis of the different techniques to date. Additionally, many reports have taken on a trial-and-error approach, ignoring well established literature on grain growth outside the battery field. This perspective reviews the literature on single-crystal cathode synthesis through the lens of classic grain growth theories, providing insight and hopefully accelerating the adoption of the promising single-crystal cathode morphology.
25 ENERGY STORAGE↗
A High Energy–Density, Cobalt–Free, Low–Nickel LiNi 0.7 Mn 0.25 Al 0.05 O 2 Cathode with a High–Voltage Electrolyte for Lithium–Metal Batteries
Cobalt-free cathode materials have garnered increased attention for applications in next-generation batteries for electric vehicles, as cobalt is considered to have at a high supply chain risk. Here, the use of a localized saturated electrolyte (LSE) to enable stable cycling of a cobalt-free, low-nickel layered-oxide cathode LiNi 0.7 Mn 0.25 Al 0.05 O 2 (NMA-70) to higher voltages (4.6 V) in a lithium-metal battery is demonstrated. Compared to the baseline LP57 electrolyte, the LSE extends the cycle life from ≈100 cycles to ≈400 cycles before reaching 80% capacity retention. Visual indicators of cell degradation, such as product deposition, are observed on electrodes cycled in LP57. It is shown that cycling NMA-70 in LSE reduces the overall active material loss and overpotential growth during extended cycling. This is attributed to the formation of a beneficial fluorinated interphase layer, a lower degree of rock-salt phase formation, and a reduction in the gas evolution from the cathode surface. The decrease in gas evolution from the cathode cycled in LSE reflects a lower degree of electrolyte reactivity and an overall improvement in the safety characteristics of the cell. Furthermore, this study highlights the importance of a stable electrolyte to enable the high-voltage cycling of alternative, lower nickel, and cobalt-free cathodes.
25 ENERGY STORAGE↗
Design of an Online Electrochemical Mass Spectrometry System to Study Gas Evolution from Cells with Lean and Volatile Electrolytes
Gas evolution in high-energy Li-ion batteries remains a pervasive problem for a multitude of chemistries, jeopardizing the electrochemical performance and safety for consumers of electric vehicles. Many electrode–electrolyte degradation processes evolve gasses that may be detected in-situ with online electrochemical mass spectrometry (OEMS). In this work, details are provided for the setup and validation of an OEMS system that operates well under lean and volatile electrolyte conditions. Quite notably, the OEMS cells with only 40 µL of electrolyte and intermittent headspace sampling exhibit comparable electrochemical performance to flooded coin-cells. It is demonstrated that the onset time, shape, and magnitude of the gas evolution profiles calculated from mass spectrometer measurements match well to a known pressure reference through the use of an empirically determined fraction of removal. The off-gassing characteristics from a set of layered-oxide materials, NMC532, NMC811, and LNO, are used to further validate the OEMS setup against the literature. It is shown that many of the features present in the OEMS curves for equivalent systems from other groups are captured by this OEMS system. Finally, at an upper cut-off voltage of 4.4 V, LNO exhibits an intense release of CO 2 , O 2 , and CO gas relative to NMC532 and NMC811.
25 ENERGY STORAGE↗
Nanometer-Thick Sr 2 IrO 4 Freestanding Films for Flexible Electronics
In this article, we report the structural and optical properties of nanoscale Sr 2 IrO 4 freestanding thin-films fabricated using a water-soluble Sr 3 Al 2 O 6 layer. The coherent lattice structure, phonon modes, two-magnon Raman scattering, and optical absorption spectra of the Sr 2 IrO 4 nanomembrane are analogous to those of the layered iridate epitaxial thin-films and single crystals. Remarkably, the formation of 3-unit-cell-thick SrIrO 3 and interfacial composite layers alleviates the antiphase boundaries at the Sr 2 IrO 4 /Sr 3 Al 2 O 6 interface, resulting in structurally-robust nanomembranes. Our experimental results show that this freestanding thin-film approach of layered oxides can provide techniques for tuning or realizing unprecedented states beyond conventional thin-film methods, suggesting a pathway in achieving flexible layered-oxide electronics.
36 MATERIALS SCIENCE↗
Gas Generation in Lithium Cells with High-Nickel Cathodes and Localized High-Concentration Electrolytes
High-nickel layered-oxide cathodes (LiNi x Mn y Co 1-x-y O 2 , x ≥ 0.8) exhibit high capacities, but also experience rapid capacity fade during cycling, and are susceptible to heat generation and gas release. Advanced electrolytes, such as localized high-concentration electrolytes (LHCE), substantially stabilize the cathode during cycling and have lower flammability than conventional electrolytes, but gas generation with these electrolytes is yet to be assessed. We demonstrate here that gas generation from a high-nickel cathode in an LHCE is half as much as in a conventional electrolyte at 4.4 V. The gas generation in LHCE is further reduced at 4.3 V, but the LHCE generates a similar amount of gas as the conventional electrolyte at 4.6 V. Neither electrolyte can prevent gas generation after cycling; cathodes after 200 cycles generate similar amounts of gas as pristine cathodes during high-voltage hold. Finally, it is shown that in both electrolytes, oxygen from the cathode lattice plays a critical role in gas generation.
25 ENERGY STORAGE↗
Impact of Dopants on Suppressing Gas Evolution from High-Nickel Layered Oxide Cathodes
Gas release from high-Ni layered-oxide cathodes (LiNi x Mn 1-x-y-z Co y Al z O 2 ; x > 0.8) can jeopardize the overall performance and safety characteristics of the cell. A comprehensive assessment of rational cathode design with common dopants, such as Ni, Co, Al, and Mn, to suppress gas evolution is crucial for battery safety, yet it remains to be conducted. Furthermore, we present an in-situ gas analysis on nine high-Ni cathode materials with online electrochemical mass spectrometry (OEMS). We show that regardless of the dopant, reactive oxygen release from the cathode lattice remains a critical process for gas evolution. A series of comparisons reveals that the intensity and onset point of gas release is strongly dependent on the cathode composition. Notably, Al and Mn are the most effective dopants to suppress gas evolution from the cathode at 4.4 V. We further highlight lattice stability limits, across these nine compositions, between 85 % to 93 % state of charge.
25 ENERGY STORAGE↗
Suppressing strain propagation in ultrahigh-Ni cathodes during fast charging via epitaxial entropy-assisted coating
Surface reconstruction and the associated severe strain propagation have long been reported as the major cause of cathode failure during fast charging and long-term cycling. Despite tremendous attempts, no known strategies can simultaneously address the electro-chemomechanical instability without sacrificing energy and power density. Here we report an epitaxial entropy-assisted coating strategy for ultrahigh-Ni LiNi x Co y Mn 1-x-y O 2 (x ≥ 0.9) cathodes via an oriented attachment-driven reaction between Wadsley-Roth phase-based oxides and the layered-oxide cathodes. Further, the high anti-cracking and anti-corrosion tolerances as well as the fast ionic transport of the entropy-assisted surface effectively improved the fast charging/discharging capability, wide temperature tolerance and thermal stability of the ultrahigh-Ni cathodes. Comprehensive analysis from the primary and secondary particle level to the electrode level using multi-scale in situ synchrotron X-ray probes reveals greatly reduced lattice dislocations, anisotropic lattice strain and oxygen release as well as improved bulk/local structural stability, even when charging beyond the threshold state of charge (75%) of layered cathodes. Layered Ni-rich oxide cathodes are susceptible to challenges with surface reconstruction and strain propagation, limiting their cyclability. The authors propose a solution involving oriented attachment-driven reactions, utilizing Wadsley-Roth nanocrystals and layered oxide to induce an epitaxial entropy-assisted coating, effectively addressing these issues.
25 ENERGY STORAGE↗
Li-rich cathodes for rechargeable Li-based batteries: reaction mechanisms and advanced characterization techniques
Due to their high specific capacities beyond 250 mA h g -1 , lithium-rich oxides have been considered as promising cathodes for the next generation power batteries, bridging the capacity gap between traditional layered-oxide based lithium-ion batteries and future lithium metal batteries such as lithium sulfur and lithium air batteries. However, the practical application of Li-rich oxides has been hindered by formidable challenges. To address these challenges, the understanding of their electrochemical behaviors becomes critical and is expected to offer effective guidance for both materials and cell development. This work aims to provide fundamental insights into the reaction mechanisms, electrochemical challenges and modification strategies of lithium-rich oxides. We first summarize the research history, the pristine structures, and the classification of lithium-rich oxides. Then we review the critical reaction mechanisms that are closely related to their electrochemical features and performances, such as lattice oxygen oxidation, oxygen vacancy formation, transition-metal migration, layered to spinel transitions, ‘two-phase mechanism’, and lattice evolution. These discussions are coupled with state-of-the-art characterization techniques. As a comparison, the anionic redox reactions of layered sodium transition metal oxides are also discussed. Finally, after a brief overview of the correlation among the aforementioned mechanisms, we provide perspectives on the rational design of lithium-rich oxides with high energy densities and long-term cycling stability.
25 ENERGY STORAGE↗
Examining Performance Loss Mechanisms in Lithium-Ion Batteries with the High-Voltage Mn-Rich Spinel Positive Electrodes
The high-voltage spinel, with the nominal composition of LiNi 0.5-x Mn 1.5+x O 4 (LNMO), could be a sustainable alternative to the layered-oxide positive electrodes used in lithium-ion batteries. However, commercial acceptance has been limited as LNMO cells display rapid performance loss during cycling. To examine reasons for this loss we prepared cells with LNMO-based positive and either graphite or lithium titanate (LTO) based negative electrodes. Our initial cells displayed high impedance and rapid impedance rise during cycling. Adding single walled carbon nanotubes in the positive lowered initial cell impedance but impedance increased during cycling because of coating delamination from the Al foil. Using a primed Al current collector solved the delamination problem: cells with this current collector showed only a small impedance rise. Regarding capacity, cells with the LTO negative showed higher initial capacities and smaller fade. Our data indicate that cell capacity retention is determined by reduction reactions at the negative electrode that lower capacity and oxidation reactions at the positive electrode that increase capacity: both types of reactions deplete the cell electrolyte during extended cycling. All things considered, species generated at high voltages, either in the electrolyte or at the LNMO electrode, drive cell capacity fade.
25 ENERGY STORAGE↗
Consequences of long-term water exposure for bulk crystal structure and surface composition/chemistry of nickel-rich layered oxide materials for Li-ion batteries
Water exposure of layered nickel-rich transition metal oxide electrodes, widely used in high-energy lithium-ion batteries, has detrimental effects on the electrochemical performance, which complicates electrode handling and prevents implementation of environmentally benign aqueous processing procedures. Elucidating the degradation mechanisms in play may help rationally mitigate/circumvent key challenges. Here, the bulk structural consequences of long-term (>2.5 years) deuterated water (D2O) exposure of intercalation materials with compositions Li x Ni 0.5 Co 0.2 Mn 0.3 O 2 (NCM523) and Li x Ni 0.8 Co 0.1 Mn 0.1 O 2 (NCM811) are studied by neutron powder diffraction (NPD). Detailed inspection of the NPD data reveals gradual formation of a secondary crystalline phase in all exposed samples, not previously reported for this system. This unknown phase forms faster in liquid- compared to vapor-exposed compounds. Structural modelling of the NPD data shows a stable level of Li/Ni anti-site defects and does not indicate any significant changes in lattice parameters or hydrogen-lithium (D + /Li + ) exchange in the structure. Consequently, the secondary phase formation must take place via transformation rather than modification of the parent material. X-ray photoelectron spectroscopy data indicate formation of LiHCO 3 /Li 2 CO 3 at the surface and a Li-deficient oxide in the sub-surface region of the pristine compounds, and the presence of adsorbed water and transition metal hydroxides at the exposed sample surfaces.
25 ENERGY STORAGE↗
Spatially-resolved lithiation dynamics from operando X-ray diffraction and electrochemical modeling of lithium-ion cells
In this study, energy dispersive X-ray diffraction is used to profile the time evolution of ordered Li x C 6 phases in solid electrodes of lithium-ion cells charged at rates between 0.2 and 4.7C (where 1C corresponds to full discharge in 1 h). The methods for quantifying lithium concentration in these phases from the acquired diffraction patterns are described. Compact expressions for time-dependent concentration gradients in the solid electrodes using orthogonal polynomial expansions are presented. Experimentally, these gradients persisted in lithiated graphite electrodes even after the cells rested at open-circuit for over 9 h. A multiphase electrochemical model of graphite intercalation captured many of the observed behaviors, including the progression of phase transitions and the persistent gradients at zero current. However, the magnitude of concentration gradients in both the oxide cathode and graphite anode is underestimated by the model, even at moderate currents.
25 ENERGY STORAGE↗