Layered Sodium Transition Metal Oxides Research Articles

Impact of Potassium in Layered Cobalt Oxide Cathodes on Electrochemical Performance in Sodium‐Ion Batteries

AbstractRechargeable sodium‐ion batteries are promising sustainable energy storage systems, owing to their low cost and high energy density. However, the existing cathode chemistries, particularly layered sodium transition metal oxides, exhibit restricted electrochemical performance, which hinders their extensive applications. To overcome challenges, originating from kinetically limited structural transformations, various cation substitutions are reported in the literature. Especially the partial Na–K replacement is discussed recently, which can increase bulk Na‐diffusivity and suppress undesirable phase transitions. Here, a critical study of potassium doping in different layered materials P3‐K0.5Co0.33Mn0.67O2 and P3‐K0.5Co0.33Mn0.67O2*0.5H2O with redox‐active Co (and Mn), and P2‐Na0.7Co0.8Ti0.2O2 and P2‐K0.1Na0.7Co0.8Ti0.2O2 with Co activity is presented. It is found that the presence of K significantly enhances the moisture sensitivity: P3‐K0.5Co0.33Mn0.67O2*0.5H2O is quickly formed in the air from K0.5Co0.33Mn0.67O2, while Na0.7Co0.8Ti0.2O2 remains stable under the same conditions. In electrochemical Na‐cells, a partial K–Na cation exchange in P3‐K0.5Co0.33Mn0.67O2 and P3‐K0.5Co0.33Mn0.67O2*0.5H2O occurs without applying current. The electrochemical performance of P3‐K0.5Co0.33Mn0.67O2 is slightly increased in comparison to K‐free P2‐Na0.67Co0.33MnO.67O2 from the literature, staying, however, behind the performance of Na0.7Co0.8Ti0.2O2. K0.1Na0.7Co0.8Ti0.2O2 shows weaker rate capability than isostructural Na0.7Co0.8Ti0.2O2, reflecting a negative impact of K on Na‐diffusivity. Therefore, effect of K‐incorporation in layered structures on the battery performance strongly depends on the cathode composition.

Revealing the Nature of Binary-Phase on Structural Stability of Sodium Layered Oxide Cathodes.

The emergence of layered sodium transition metal oxides featuring a multiphase structure presents a promising approach for cathode materials in sodium-ion batteries, showcasing notably improved energy storage capacity. However, the advancement of cathodes with multiphase structures faces obstacles due to the limited understanding of the integrated structural effects. Herein, the integrated structural effects by an in-depth structure-chemistry analysis in the developed layered cathode system NaxCu0.1Co0.1Ni0.25Mn0.4Ti0.15O2 with purposely designed P2/O3 phase integration, are comprehended. The results affirm that integrated phase ratio plays a pivotal role in electrochemical/structural stability, particularly at high voltage and with the incorporation of anionic redox. In contrast to previous reports advocating solely for the enhanced electrochemical performance in biphasic structures, it is demonstrated that an inappropriate composite structure is more destructive than a single-phase design. The in situ X-ray diffraction results, coupled with density functional theory computations further confirm that the biphasic structure with P2:O3 = 4:6 shows suppressed irreversible phase transition at high desodiated states and thus exhibits optimized electrochemical performance. These fundamental discoveries provide clues to the design of high-performance layered oxide cathodes for next-generation SIBs.

Layer-Type AxRhO2 (A = Li, Na, K, Rb, Cs) and KxMO2 (M = Co, Rh, Ir) Electrodes: Structure and Electrochemistry

Layered lithium transition-metal oxides have remained at the forefront since the first commercialization of layered LiCoO2 in lithium-ion batteries in 1991. The scientific interest in layered sodium transition-metal oxides as well as potassium transition-metal oxides has been considerably growing for application to sodium- and potassium-ion batteries. We extend our systematic studies on the series of layered oxides to AxRhO2 (A = Li, Na, K, Rb, Cs) and KxMO2 (M = Co, Rh, Ir) electrode system focusing on their crystal structure and electrochemical behaviors in non-aqueous cells as a function of A+ ion. Further studies on P2-type KxRhO2 in comparison to P2-type KxCoO2 show that larger Rh(III/IV) ion than Co(III/IV) is believed to be beneficial for reversibility and kinetic of redox process, accompanied with reversible structural evolution upon oxidation/reduction maintaining the P2 framework.

Competition between dual alkali metal ions fuels high-performance sodium-ion batteries

Dual alkali metal ion battery is expected to apply in future high-efficiency energy storage applications as it can skillfully combine the advantages of Li+ and Na+. However, the specific insertion and extraction processes of the two ions are not well explained. Herein, a layered sodium transition metal oxide electrode with ordered mixing of Na/Li ions is prepared by ion exchange method. The ion behavior is explained in terms of activation energy of chemical reaction and Gibbs free energy. Based on thermodynamic analysis and material characterizations, the priorities of Na+ and Li+ deblock reactions are determined. The competition mechanism of alkali metal ions and the double gain effect of Li are revealed by electrochemical experiments. This work is expected to provide a positive impact on the future design of multifunctional cathodes for advanced rechargeable batteries.

Stabilizing lattice oxygen redox in layered sodium transition metal oxide through spin singlet state

Reversible lattice oxygen redox reactions offer the potential to enhance energy density and lower battery cathode costs. However, their widespread adoption faces obstacles like substantial voltage hysteresis and poor stability. The current research addresses these challenges by achieving a non-hysteresis, long-term stable oxygen redox reaction in the P3-type Na2/3Cu1/3Mn2/3O2. Here we show this is accomplished by forming spin singlet states during charge and discharge. Detailed analysis, including in-situ X-ray diffraction, shows highly reversible structural changes during cycling. In addition, local CuO6 Jahn-Teller distortions persist throughout, with dynamic Cu-O bond length variations. In-situ hard X-ray absorption and ex-situ soft X-ray absorption study, along with density function theory calculations, reveal two distinct charge compensation mechanisms at approximately 3.66 V and 3.99 V plateaus. Notably, we observe a Zhang-Rice-like singlet state during 3.99 V charging, offering an alternative charge compensation mechanism to stabilize the active oxygen redox reaction.

Electronic States Tailoring and Pinning Effect Boost High-Power Sodium-Ion Storage of Oriented Hollow P2-Type Cathode Materials.

Fierce phase transformation and limited sodium ion diffusion dynamics are critical obstacles that hinder the practical energy storage applications of P2-type layered sodium transition metal oxides (NaxTMO2). Herein, a synergistic strategy of electronic state tailoring and pillar effect was carefully implemented by substituting divalent Mg2+ into Na0.67Ni0.33Mn0.67O2 material with unique oriented hollow rodlike structures. Mg2+substitution can not only facilitate the anionic oxygen redox reactions and electronic conductivity through increasing the electronic states at Femi energy but also act as pillars within TMO2 layers to alleviate the severe phase transformation to improve structure stability. Moreover, the oriented hollow structure incorporating sufficient buffer spaces and rationally exposed electrochemically active facets effectively alleviates the stresses induced by low volume changes of 8% and provides more open channels for Na+ ion diffusion without crossing multiple grain boundaries. Hence, the Na0.67Mg0.08Ni0.25Mn0.67O2 cathode showed a superior rate capability with high energy density and cycling stability for sodium-ion storage. The underlying mechanisms of these achievements were deciphered through diversified dynamic analysis and the first principle calculations, providing new insights into P2-type NaxTMO2 cathodes for the infinite prospect as an alternative to lithium-ion batteries.

High-Performance Layered Oxides for Sodium-Ion Batteries Achieved through Combined Aluminum Substitution and Surface Treatment

Layered sodium transition metal oxides belong to electrode materials for sodium-ion batteries that combine, in a better way, high performance with environmental requirements. However, their cycling stability is still far from desirable. Herein, we demonstrate a rational approach to control the cycling stability of sodium-deficient nickel manganese oxides, Na2/3Ni1/2Mn1/2O2, with two- and three-layer stacking through Al substitution and Al2O3 treatment. Layered Na2/3Ni1/2Mn1/2O2 oxide displays a limited ability to accommodate aluminum in its structure (i.e., up to 8 at. %). The substitution of Ni ions with electrochemically inactive Al3+ ions and keeping the amount of Mn ions in Na2/3Ni1/2−xAlxMn1/2O2 leads to the stabilization of the two-layer stacking and favors the participation of lattice oxygen in the electrochemical reaction in addition to Ni ions. This results in an increase in the specific capacity of the Al-substituted oxides. Furthermore, the kinetics of the cationic migration between layers occurring during oxide cycling was manipulated by oxide morphology. The best cycling stability is observed for Na2/3Ni0.42Al0.08Mn1/2O2 having a column-like morphology of stacked plate-like particles along the common faces. The treatment of the layered oxides with Al2O3 mitigates the Mn dissolution reaction during electrode cycling in the NaPF6-based electrolyte, thus contributing to a high cycling stability.

The Air Stability of Sodium Layered Oxide Cathodes

Sodium-ion batteries (NIBs) are listed as one of the ideal alternatives for lithium-ion batteries (LIBs), due to the abundant sodium resources, cost-effective electrode materials of NIBs, and same architecture of NIBs to LIBs. To enable the practical implementation of NIBs, advanced cathodes with higher energy/power densities, better safety and cycle life, as well as lower cost are required. Layered lithium transition metal oxides (LiTMO2) are one of the most successful cathode materials for commercial LIBs. Similarly, layered sodium transition metal oxides (NaxTMO2, also termed as sodium layered oxides) are of particular interest for commercial NIBs owing to their high specific capacity, a wide variety of redox-active elements, and the possibility for the manufacturers to employ established synthesis processes as their lithium counterparts.Sodium layered oxides are built up by ordered stacking of alternate alkali-metal (Na+) layers and transition metal layers (TmO2). The two-dimensional structure makes them the natural hosts for alkali-metal ions and other ions or small molecules, such as H2O. Therefore, when exposed to moist atmospheres, layered oxide materials tend to react with H2O which adsorbed on their surface and thus deteriorate their structure and electrochemical performances. Accordingly, the air-sensitive sodium layered oxides should be well protected from the moist atmospheres, rendering a higher manufacturing and preservation cost.Here, based on the reaction mechanisms, critical influencing factors, and modification methods of layered oxides in moisture, we try to reach a comprehensive understanding of the air-stability of sodium layered oxides. Moreover, future efforts to resolve the air-stability of sodium layered oxides from Argonne National Laboratory will be also presented.

Insights into the Na-Alloying Mechanism and Zintl Phase Transition of Lead-Based Anodes

Sodium-ion batteries (SIBs) has been received growing attention in the electrical energy storage fields due to their low cost and earth-abundant sodium. [1] However, there has been a lack of new discoveries, growth directions, and real advancement with respect to Na-storage anodes. Despite the chemical similarities between sodium and lithium as alkali elements, the larger Na ion than Li ions (ionic radii of 0.98 Å and 0.68 Å, respectively) is limited to insertion into host materials and results in different phase transition behavior. [2] Among the available anode candidate materials for SIBs, lead (Pb), which has a large atomic size than other elements (e.g. Si, Sn), provides a big interstitial space to accommodate large Na ions by fast ionic diffusion, enabling reversible Na alloying/dealloying and exhibiting high volumetric capacity. [3] Furthermore, when Pb is used as anode with layered sodium transition metal oxide as cathode, the energy density of the pouch-type cell is estimated to be 549 Wh/L and the cost is lower than 63.5 USD/kWh according to the Argonne BatPac model. [4] Therefore, Pb-based materials have competitive potential as promising anodes and it is crucial to understand the electrochemical process from a fundamental perspective.Here, we investigate a unique Na storage mechanism using a novel Pb-based carbon nanocomposite anode synthesized by a simple high-energy milling method. The electrochemical data show a decent cycle performance with a reversible capacity of 381 mAh/g. Nevertheless, the Na-storage performance of the Pb-based anode was not attractive compared to Li cells. In-situ X-ray diffraction and ex-situ X-ray absorption spectroscopy reveal the reaction mechanism and Zintl-phase formation that limits the Na storage, unlike the Li reaction. We expect these findings provide fundamental knowledge of Na-alloying reaction and guidance for designing anode materials for high-performance SIBs.[1] K. Kubota and S. Komaba Journal of The Electrochemical Society, 2015, 162 (14) A2538-A2550[2] M. Lao, Y. Zhang, W. Luo, Q. Yan, W. Sun, and S. X. Dou Adv. Mater. 2017, 29 , 1700622[3] Chia-Yun Chou, Myungsuk Lee, and Gyeong S. Hwang, J. Phys. Chem. C 2015, 119, 27, 14843–14850[4] P. Nelson, K. Gallagher, I. Bloom, Dennis Dees, and Shabbir Ahmed, BatPaC, Argonne National Laboratory. http://www.cse.anl.gov/batpac.

NaxMnyNi1-YO2 Cathode Materials for Sodium-Ion Batteries: Structure, Synthesis, Electrochemistry and Influence of Ambient Storage

Expanding energy generation from renewables is inevitable to reduce the impact of man-made climate change. With that, the need for intermediate energy storage is gaining in significance. Today, lithium-ion batteries (LIBs) are dominating mobile drive-trains and also play a key role in stationary energy storage. LIBs are incorporating critical raw materials in view of availability and economic importance, such as cobalt, lithium, copper and graphite.Sodium-based batteries, in which sodium replaces lithium as ionic charge-carrier, utilize the same working principles but substitute critical raw materials for abundant and cost-effective alternatives. Hard carbon replaces graphite as anode active material, copper foils are substituted by aluminum current collectors and manganese-based cobalt-free layered host lattices offer promising performance as cathode active material 1,2. By applying established production processes, investment costs are reduced and a rapid scale-up is enabled (Drop-In technology) 3, making SIBs a sustainable, efficient and cost-effective complementary technology to LIBs 4.Among various known cathode active materials for SIBs, the family of layered sodium transition metal oxides (NaxMO2, 1>x>0) offers promising electrochemical performance 2,5–7. These compounds show a wide structural variety (O3, P3, P2) due to the ionic radius of sodium and the tendency for Na+/vacancy ordering 8.The scope of our presentation will be low-cost manganese-based, cobalt-free layered NaxMnyNi1-yO2 cathode active materials for SIBs. We will discuss the influence of the transition metal stoichiometry y on the structure based on Neutron and X-ray diffraction experiments. Using advanced electrochemical methods and diffraction experiments, these structural models are then correlated with physical and electrochemical properties such as Na+/vacancy orderings, solid diffusion coefficients and potential profiles. For y = 3/4, a synthesis phase diagram will be presented covering a broad range of sodium content x and calcination temperature. For phase-pure P2-NaxMn3/4Ni1/4O2, we will present the influence of the calcination process on the structure and discuss the electrochemical properties in half-cells in-depth. For optimized materials, attractive initial specific discharge capacities beyond 220 mAh g-1 are obtained in sodium half-cells between 1.5 – 4.3 V. A capacity decay occurs during electrochemical cycling within this full voltage window. The origin of the capacity decay will be discussed based on electrochemical studies and ex-situ investigations of the morphology with SEM and local structure with HRTEM. Finally, we will present the influence of storage in ambient air to gain insights on the large-scale processability of the materials.The chosen synthesis route adapts industrially established processes for NCM production for SIB cathode materials, enables to tune powder properties to technical specifications and is highly scalable. The broad scope of this work addresses raw material questions, fundamental investigations and industrially relevant production processes.ACKNOWLEDMENTS:The German Federal Ministry of Education and Research (BMBF) supported this work within the project TRANSITION (03XP0186C) and ExcellBattMat (03XP0257A and 03XP0257C).REFERENCES Larcher, D. & Tarascon, J.-M. Towards greener and more sustainable batteries for electrical energy storage. Nature Chem 7, 19–29; 10.1038/nchem.2085 (2015).Hasa, I. et al. Challenges of today for Na-based batteries of the future: From materials to cell metrics. Journal of Power Sources 482, 228872; 10.1016/j.jpowsour.2020.228872 (2021).Tarascon, J.-M. Na-ion versus Li-ion Batteries: Complementarity Rather than Competitiveness. Joule 4, 1616–1620; 10.1016/j.joule.2020.06.003 (2020).Vaalma, C., Buchholz, D., Weil, M. & Passerini, S. A cost and resource analysis of sodium-ion batteries. Nat Rev Mater 3, 1–11; 10.1038/natrevmats.2018.13 (2018).Nagore Ortiz-Vitoriano, Nicholas E. Drewett, Elena Gonzalo & Teófilo Rojo. High performance manganese-based layered oxide cathodes: overcoming the challenges of sodium ion batteries. Energy Environ. Sci. 10, 1051–1074; 10.1039/C7EE00566K (2017).Nuria Tapia-Ruiz et al. 2021 roadmap for sodium-ion batteries. J. Phys. Energy 3, 31503; 10.1088/2515-7655/ac01ef (2021).Gonzalo, E., Zarrabeitia, M., Drewett, N. E., López del Amo, Juan Miguel & Rojo, T. Sodium manganese-rich layered oxides: Potential candidates as positive electrode for Sodium-ion batteries. Energy Storage Materials 34, 682–707; 10.1016/j.ensm.2020.10.010 (2021).Kubota, K., Kumakura, S., Yoda, Y., Kuroki, K. & Komaba, S. Electrochemistry and Solid‐State Chemistry of NaMeO 2 (Me = 3d Transition Metals). Adv. Energy Mater. 8, 1703415; 10.1002/aenm.201703415 (2018).

Stable Electrochemical Properties of Titanium Doped Layered P2-type Na0.67Ni0.15 Mn0.85O2 Cathode Material for Sodium Ion Batteries

P2-Na0.67Ni0.15Mn0.85O2 layered sodium transition metal oxides have drawn much attention as a promising candidate cathode due to its high specific capacity and high working voltage. However, undesired cyclability and poor rate performance originated from the serve Jahn-Teller effect and mixed Na+/vacancy ordering during the cycle process. To address this issue, we designed a series P2-type Na0.67Ni0.15Mn0.85−xTixO2 (x = 0, 0.05, 0.10, 0.15) cathode by using a modified sol-gel method, and comprehensively investigated the influence of Ti doping Mn sites with the structural and electrochemical properties. It was found that Ti-substitution not only increases the interlayer distance to suppress Na+/vacancy ordering, but also reduces Mn3+ content to alleviate the Jahn-Teller effect. Notable, the Na0.67Ni0.15Mn0.80Ti0.05O2 electrode exhibited a remarkable cycling stability (85.36% capacity retention after 200 cycles at 1 C). Ti-substitution would greatly increase charge/discharge reversibility and boosts Na+ diffusion mobility kinetics, which were clearly elucidated by Ex-situ X-ray Diffraction (XRD), cyclic voltammetry (CV) and Galvanostatic Intermittent Titration Technique (GITT), respectively. This simple and effective work provides a novel strategy for developing high-performance cathode materials for practical SIBs.

Copper Substitution in P2-Type Sodium Layered Oxide To Mitigate Phase Transition and Enhance Cyclability of Sodium-Ion Batteries.

Development of high-performance cathode materials is one of the key challenges in the practical application of sodium-ion batteries. Among all the cathode materials, layered sodium transition-metal oxides are particularly attractive. However, undesired phase transitions are often reported and have detrimental effects on the structure stability and electrochemical performance. Cu substitution of zinc in the P2-type Na0.6Mn0.7Ni0.15Zn0.15-xCuxO2 (x = 0, 0.075, and 0.15) composites was investigated in this study for mitigating the biphase transition and enhancing the electrochemical performance of sodium-ion batteries. The coupling effect of Zn and Cu enables an excellent capacity retention of 96.4% of the initial discharge capacity after 150 cycles at 0.1 C in the Na/Na0.6Mn0.7Ni0.15Zn0.075Cu0.075O2 cell. The biphase transition that occurred in the high voltage range has been significantly suppressed after the incorporation of Cu in Na0.6Mn0.7Ni0.15Zn0.15O2, which was confirmed by in situ X-ray diffraction studies. Moreover, the substitution of the inert element Zn with electrochemically active Cu leads to the suppression of anionic redox and the occurrence of Cu2+/3+ redox reaction, and the electrolyte decomposition is impeded after the introduction of electrochemically active Cu in the Na0.6Mn0.7Ni0.15Zn0.15-xCuxO2 composite cathode. The enhanced electrochemical performance in the Na0.6Mn0.7Ni0.15Zn0.075Cu0.075O2 electrode can be ascribed to the coexistence of Zn and Cu and alleviated volumetric change as well as suppressed electrode/electrolyte side reaction after Cu substitution.

Unravelling the Nature of the Intrinsic Complex Structure of Binary-Phase Na-Layered Oxides.

The layered sodium transition metal oxide, NaTMO2 (TM = transition metal), with a binary or ternary phases has displayed outstanding electrochemical performance as a new class of strategy cathode materials for sodium-ion batteries (SIBs). Herein, an in-depth phase analysis of developed Na1-x TMO2 cathode materials, Na0.76 Ni0.20 Fe0.40 Mn0.40 O2 with P2- and O3-type phases (NFMO-P2/O3) is offered. Structural visualization on an atomic scale is also provided and the following findings are unveiled: i) the existence of a mixed-phase intergrowth layer distribution and unequal distribution of P2 and O3 phases along two different crystal plane indices and ii) a complete reversible charge/discharge process for the initial two cycles that displays a simple phase transformation, which is unprecedented. Moreover, first-principles calculations support the evidence of the formation of a binary NFMO-P2/O3 compound, over the proposed hypothetical monophasic structures (O3, P3, O'3, and P2 phases). As a result, the synergetic effect of the simultaneous existence of P- and O-type phases with their unique structures allows an extraordinary level of capacity retention in a wide range of voltage (1.5-4.5 V). It is believed that the insightful understanding of the proposed materials can introduce new perspectives for the development of high-voltage cathode materials for SIBs.

Long-enduring oxygen redox enabling robust layered cathodes for sodium-ion batteries

Anionic redox chemistry has emerged as a promising pathway attracted extensively attention to meet the demands of high energy sodium-ion batteries. Nonetheless, suchlike cathode materials suffer from poor reversible oxygen redox reactions leading to severe capacity degradation along with sluggish kinetics and voltage decay. In this work, layered sodium transition metal oxides Na0.67Mg0.28Mn0.72-xRuxO2 (x = 0–0.6) are presented, where the optimized sample (x = 0.12) offers an attractively sustainable capacity of 222 mAh g−1 and a distinguished rate capability of 101.7 mAh g−1 at 50C, based on the anionic and cationic activity. The results reveal that the oxygen redox retention is endured at 94.1% in the optimized sample (x = 0.12) after 55 cycles, in contrast to 48.9% of the pristine sample (x = 0), which is the maximum value reported up to now. Meanwhile the lattice parameters show almost no variation, according to the in situ/ex situ X-ray diffraction experiments. Ru-substitution significantly improves the anionic redox reversibility, the structural stability and sodium diffusion, resulting in the noteworthy electrochemical performance advancement. The research performs an effective method to moderate the problems existing in anionic redox reactions and makes a reference for subsequent materials design.

Shear-resistant interface of layered oxide cathodes for sodium ion batteries

Layered sodium transition metal (TM) oxides exhibit great potential as high energy density cathode materials for sodium-ion batteries (SIBs). The large Na ions, nevertheless, adopts various coordination environments that are dependent of the sodium concentration, giving rise to cyclical gliding of TM layers and P-O phase transitions upon Na extraction/insertion process. The detrimental interlayer-gliding induced phase transformations lead to deteriorated round-trip energy efficiency, rate capability and cycling stability of electrodes. Herein, we demonstrate a shear-resistant interface via the supersaturation of lithium to overcome the interlayer-gliding behavior and inhibit the multiple P-O phase transitions in P2-type Na0.67Mn0.67Ni0.33O2. The results indicate that the nanoscale interface is composed of lithium-enriched O3 nanodomains in the P2 phase matrix, resulting in smooth charge/discharge profiles and superior cycling stability of P2-type Na0.67Mn0.67Ni0.33O2 cathode under a high cut-off voltage of 4.5 V. This work highlights the concept of modulating the interfacial shear stress for improving the long-term cycling stability of high-voltage layered cathode materials that suffer from severe phase transformations.

Thermal and Electrochemical Interface Compatibility of a Hydroborate Solid Electrolyte with 3 V-Class Cathodes for All-Solid-State Sodium Batteries.

Thermal stability of solid electrolytes and their compatibility with battery electrodes are key factors to ensure stable cycling and high operational safety of all-solid-state batteries. Here, we study the compatibility of a hydroborate solid electrolyte Na4(B12H12)(B10H10) with 3 V-class cathode active materials: NaCrO2, NaMnO2, and NaFeO2. Among these layered sodium transition metal oxide cathodes, NaCrO2 shows the highest thermal compatibility in contact with the hydroborate solid electrolyte up to 525 °C in the discharged state. Furthermore, the electrolyte remains intact upon the internal thermal decomposition of the charged, that is, desodiated, cathode (Na0.5CrO2) above 250 °C, demonstrating the potential for highly safe hydroborate-based all-solid-state batteries with a wide operating temperature range. The experimentally determined onset temperatures of thermal decomposition of Na4(B12H12)(B10H10) in contact with 3 V-class cathodes surpass those of sulfide and selenide solid electrolytes, exceeding previous thermodynamic calculations. Our results also highlight the need to identify relevant decomposition pathways of hydroborates to enable more valid theoretical predictions.

(Invited) Role of Lithium Doping in Layered Transition Metal Oxide Electrodes for Sodium Ion Batteries

Large-scale energy storage systems are urgently needed to enable the success of renewable but intermittent energy sources such as solar and wind power. Sodium ion batteries are a promising technology to address the need for large-scale energy storage systems, but their performance needs improvement to be competitive for this application. Dopants, especially lithium, have been shown to improve the performance of layered sodium transition metal oxide positive electrodes significantly. This talk will discuss the nature in which lithium is incorporated into the layered structures has a distinct effect on the modification of the electrodes’ properties, enhancing capacity and/or stability. A thorough understanding of the effect of lithium dopant location can inform the design of superior sodium ion battery positive electrode materials. High-temperature processing to tailor the positive electrode structures will also be discussed. High performance cathodes with lithium dopants may help address the growing challenge of large-scale energy storage to support renewable energy technology.

(Invited) Oxide- and Polyanion- based Cathode Materials for Li-ion and Na-ion Batteries

Na- ion layered oxide as cathodes have attracted both the scientific community and industry. They offer high gravimetric energy density, tunability of properties and ease of synthesis. This leads to both interesting science and low cost applied materials.Layered oxides are generally classified by the nomenclature introduced by Delmas et. al.1 which depends on the alkali environment and stack of layers in the unit cell. The same team also used layered oxides for the first-time as potential positive electrode for reversible sodium storage.2 In this talk we present our contributions on (i) O3, (ii) P3 and (iii) modified P3 type layered oxides as cathode materials for Na-ion batteries.O3-type layered sodium transition metal oxides (NaxMO2, M = transition metal ions) are one of the most attractive cathode materials considering their capacity. However, the use of O3 phases is limited due to their low redox voltage and several associated phase changes which are unfavorable for long cycling. 3-5 In this presentation, we show the effect of Cu, Ni and Ti doping in the O3 structure and relate the observed local environments to their storage performances. Cu2+ Jahn-Teller distortion leads to irreversibility and dictates the amount of Cu2+ that can be doped in the layer. Our results show that 10-12% of Cu-doping is optimal. In order to further examine the effect of local structures, we doped Ni2+ and achieved excellent structural reversibility which favors high-rate performance and cycling stability. 6 Na-sufficient Fe and Mn based P3 layered oxide is able to accommodate higher than 0.67 moles of Na-ions. 7,8 We present the effects of voltage limits on the storage performances and highlight associated phase changes during electrochemical cycling. XANES data confirm the charge compensation mechanism during electrochemical cycling. Full cell data further elucidate the sufficiency of Na-ions in the P3 structure. 8 Lastly, the storage performances of P3-Na0.9Fe0.5Mn0.5O2 are compared with those for modified P3 phase, P3 (major)/ O3 (minor) layered oxide. The phase changes during cycling and comprehension of charge compensation mechanism from XANES of this modified P3 structure are reported.

Electrodeposition of atmosphere-sensitive ternary sodium transition metal oxide films for sodium-based electrochemical energy storage

We introduce an intermediate-temperature (350 °C) dry molten sodium hydroxide-mediated binder-free electrodeposition process to grow the previously electrochemically inaccessible air- and moisture-sensitive layered sodium transition metal oxides, NaxMO2 (M = Co, Mn, Ni, Fe), in both thin and thick film form, compounds which are conventionally synthesized in powder form by solid-state reactions at temperatures ≥700 °C. As a key motivation for this work, several of these oxides are of interest as cathode materials for emerging sodium-ion-based electrochemical energy storage systems. Despite the low synthesis temperature and short reaction times, our electrodeposited oxides retain the key structural and electrochemical performance observed in high-temperature bulk synthesized materials. We demonstrate that tens of micrometers thick >75% dense NaxCoO2 and NaxMnO2 can be deposited in under 1 h. When used as cathodes for sodium-ion batteries, these materials exhibit near theoretical gravimetric capacities, chemical diffusion coefficients of Na+ ions (∼10-12 cm2⋅s-1), and high reversible areal capacities in the range ∼0.25 to 0.76 mA⋅h⋅cm-2, values significantly higher than those reported for binder-free sodium cathodes deposited by other techniques. The method described here resolves longstanding intrinsic challenges associated with traditional aqueous solution-based electrodeposition of ceramic oxides and opens a general solution chemistry approach for electrochemical processing of hitherto unexplored air- and moisture-sensitive high valent multinary structures with extended frameworks.

Unlocking the potential of P3 structure for practical Sodium-ion batteries by fabricating zero strain framework for Na+ intercalation

Layered sodium transition metal oxides (NaTMO2) have garned considerate attention as candidates for cathode materials in practical Na-ion batteries because of their potentially high energy density and operating voltage. Among the several categories (P2, P3, O3) of layered NaTMO2 materials, P3 type cathodes are least studied due to the fast capacity/voltage fading induced by P3-O3 phase transition, as shown by the very few compositions visited in previous reports. It is highly desirable to stabilize the P3 phase throughout the sodium extraction/insertion process as P polytype allows facile migration of Na+-ions between two adjacent prismatic sites. In this work, we synthesized a phase pure P3-Na0.75Mg0.08Co0.10Ni0.2Mn0.60O2 (P3-MNCM) via the incorporation of Mg, while successfully maintains the P3 structure during the charging/discharging process. This together with good ionic conductivity, endows good structural reversibility, and consequently leads to superior cycling performance and rate performance than undoped composition. In situ X-ray diffraction (XRD) further verifies that P3-MNCM cathode demonstrates a near zero strain feature. Of special importance is that the P3-O3 phase transition is effectively suppressed with the incoporation of Mg into the transition metal layer. These findings demonstrate the potential of P3 structure as a promising choice for Na ion cathode materials, which open a feritile avenue and will hopefully inspire more endeavors in the exploration of high performance sodium ion batteries.