Cathode For Na-ion Batteries Research Articles

Elucidating the Structural Evolution of O3-type NaNi1/3Fe1/3Mn1/3O2: A Prototype Cathode for Na-Ion Battery.

Extending the depth-of-charge (DoC) of the layered oxide cathode presents an essential route to improve the competitiveness of the Na-ion battery versus the commercialized LiFePO4-based Li-ion battery (0.8 CNY/Wh). However, the DoC-dependent boundary between detrimental/irreversible structural distortion and neutral/reversible structure interconversion cannot be clearly distinguished, which is attributed to the ambiguous recognition of correlation among the complex phase transition, local covalent environment evolution, and charge compensation. Herein, to bridge the above gap, we employed O3-NaNi1/3Fe1/3Mn1/3O2 as the prototype cathode and extended the target DoC from typical Na0.4 (∼125 mAh/g, 4.0 V cutoff) to Na0.2 (∼180 mAh/g, 4.3 V cutoff). Regarding phase transition and charge compensation, the O3-to-P3 phase transition occurs before moderate Na0.4-DoC (Fe/Mn redox silence, Ni oxidation dominated), while further desodiation (start from Na0.4) induces a P-to-O slab transition, resulting in the coexistence of P3 and OP2 phases and subsequent OP2/O3 intergrowth phases at higher DoC (Na0.2), upon which the Fe3+-to-Fe4+ oxidation is activated for capacity contribution. The local covalent environment presents severer deviation at high DoC (merely 0.2 mol desodiation from Na0.4 to Na0.2), which can be attributed not only to the slab gliding induced by the P-to-O slab transition but also to the further aggravation caused by the Jahn-Teller distortion of the FeO6 octahedron. Such irreversible distortion of the local covalent environment would be accumulated and evolved/deteriorated into structural degradation during long-term cycling. Furthermore, the rate-dependent artificial regulation of redox process has been demonstrated and the doping strategy toward structural stabilization has been proposed.

Unveiling the Influence of Water Molecules on the Structural Dynamics of Prussian Blue Analogues.

3D-framework Prussian blue analogues (PBAs) are appealing as a cost-effective, sustainable cathodes for Na-ion batteries. However, the aqueous-based synthesis of PBAs inherently introduces three different forms of water molecules (surface, interstitial and crystal) into the structure. Removal of water molecules causes phase transformation from monoclinic (M) to rhombohedral (R). This work presents the effects of water molecules on the structure before the phase transformation temperature, employing two promising PBA cathodes, Na2Fe[Fe(CN)6]·1.69H2O and Na2Mn[Fe(CN)6]·1.76H2O. Specifically, the water molecules impact the molecular interactions at the local structure and the electrochemical properties. This work has performed calculations on low-vacancy Na2M[Fe(CN)6] PBAs (where M = Mn, Fe, Co, Ni and Cu) to understand the dehydration energy. Employing in situ high-temperature X-ray diffraction and Raman spectroscopy, this work observes that water removal induces negative thermal expansion and stronger interactions between C≡N and Na ions, resulting in biphasic reactions with sluggish kinetics. Additionally, water molecules play a role in maintaining the open 3D tunnels and facilitating a solid-solution like insertion of Na ions. Calculated phonon-Raman spectra provide insights into cyanide group deformations, revealing the interactions between water molecules, alkali-ions, and transition-metal ions. This study enhances the understanding of the relationship among electronic, vibrational, and electrochemical properties.

Designing high-performance phosphate cathode toward Ah-level Na-ion batteries

The phosphate material Na3V2(PO4)3 is one of promising cathodes for Na-ion batteries owing to its superior electrochemical reversibility. However, the high-potential V4+/V5+ redox couple (4.0 V vs. Na+/Na) in pure Na3V2(PO4)3 cathode is not activated, resulting in a limited energy density. Although conventional single-metallic substitutions can activate V4+/V5+ redox plateau, the energy density has no significant improvement due to the limited capacity of V4+/V5+ redox plateau. Here, we propose a bi-metallic (Al3+ and Fe3+) substitution strategy, which can increase the reversible capacity of V4+/V5+ couple via lowering the energy needed during desodiation process, thus realizing a remarkable improvement of energy density. Moreover, we reveal that Fe3+substitution can improve the rate performance of cathode by reducing band gap and Na-ion diffusion activation energy. As a result, the designed Na3V1.5Al0.3Fe0.2(PO4)3 cathode delivers a high energy density of 399.7 Wh kg-1 (capacity 116.2 mAh g−1, average voltage 3.44 V) and a high rate capability (91.2% capacity retention from 0.1 C to 20 C), as well as superior cycling stability (89.5% capacity retention after 8000 cycles at 10 C). In addition, we demonstrate a 0.8 Ah 18,650-type Na3V1.5Al0.3Fe0.2(PO4)3//hard carbon full cell with 92.4% capacity retention over 400 cycles at 0.5 C. This work presents the design of high-performance phosphate cathode via bi-metallic substitution, boosting the development of practical Na-ion batteries.

Tailoring monoclinic Na3V2(PO4)3-based cathode via bimetallic substitution for high-energy and long-lifespan Na-ion batteries

Na3V2(PO4)3 is a promising cathode for Na-ion batteries (NIBs) owing to the high electrochemical reversibility. The Na3V2(PO4)3 has two typical polymorphs including rhombohedral and monoclinic phases; the former has been extensively studied, whereas the latter is rarely reported. Here, we successfully designed monoclinic Na3V2(PO4)-based cathode via Ga and Fe substitutions owing to the lowered lattice energy. In addition, we revealed that Ga substitution improves average voltage owing to the activation of the V4+/V5+ redox couple and the Fe substitution enhances rate capability due to the decreased band gap and Na-ion diffusion activation energy. As a result, the designed monoclinic Na3V1.6Ga0.2Fe0.2(PO4)3 cathode exhibits high voltage plateaus (3.4 V and 4 V) and high-rate capability (from 116.8 mA h/g at 0.2 C to 103 mA h/g at 20 C) as well as superior cycling stability (99.9% capacity retention over 4,500 cycles at 5 C). Moreover, the assembled Na3V1.6Ga0.2Fe0.2(PO4)3//hard carbon full cell delivers a high-energy density of 313.8 Wh/kg with 86.4% capacity retention after 100 cycles at 1 C. This work demonstrates the design of monoclinic Na3V2(PO4)3-based cathode via bimetallic substitution, providing a new route for development of high-energy and long-lifespan NIBs.

Crystal water-capturing and film-forming bifunctional electrolyte additive for stabilizing sodium iron hexacyanoferrate cathode for Na-ion batteries

Due to its affordability, strong structural stability, and high rate capability, rhombohedral sodium ferrous hexacyanoferrate (FeHCF) has been identified as a potential cathode material for sodium ion batteries. Nevertheless, the presence of inherent crystal water and the susceptibility of its interface stability significantly hinder its capacity and cycling performance. In this study, we propose a novel modification approach involving the addition of ZnCl2 to effectively capture crystal water and facilitate the in-situ formation of a ZnO coating on the FeHCF surface. Due to the decreased crystal water content and the presence of stable ZnO interphases, the occurrence of side reactions between the electrode and electrolyte, as well as crystal particle cracks in FeHCF during repeated sodiation/desodiation processes, is effectively reduced. Consequently, the modified FeHCF demonstrates an extended operational lifespan of 1000 cycles, with an initial discharge capacity of 112.0 mAh/g and a capacity retention rate of 77.8 % at a current rate of 1C within the voltage range of 2.0–4.2 V. Our research offers valuable insights into the removal of crystal water and the engineering of incident interfaces to enhance the cycle life of FeHCF cathodes in sodium-ion batteries.

Optimizing O3-Type Fe-Mn-Based Cathodes for Na-Ion Batteries: Unveiling the Synergistic Impact of Chemical Co-Substitution

Layered transition metal oxide-Na x MO2 (M = transition metal) is the most widely explored class of cathode materials in lithium and sodium-ion batteries (LIBs and SIBs).1 The replacement of expensive Co and Ni with cheaper elements like Fe and Mn is appealing, which can reduce the overall cost of the battery. Despite the higher operating voltage of Fe3+/Fe4+ and the large specific capacity of Mn3+/Mn4+, issues like irreversible Fe-migration and J. T. distortion of Mn3+/Mn4+ are yet to be overcome.2,3 To improve the electrochemical performances of layered oxide cathodes, various cationic substitutions (e.g., Li+, Cu2+, Mg2+, Zn2+, and Ti4+) are partially substituted in the place of M.4–6 Along this line, in search of better Fe/Mn-based cathodes, we selected an O3-type Na[Fe0.50Mn0.50]O2 (NFMO) material and co-substituted Li+ and Cu2+ in the transition metal layer to create a composition of Na[LixCuyFe0.50-x-yMn0.50]O2 (NCLFMO) and compared their electrochemical performances.. Both materials crystallize in a rhombohedral structure and Rietveld refinement and TEM analyses suggest an enhanced O-Na-O spacing after substitution. The NFMO and NCLFMO cathodes display a capacity of 135 and 121 mAh g-1 in the potential window of 4.0–2.0 V at 0.1 C rate, with an average voltage (Vavg) of 2.63 V and 3.31 V respectively, which leads to an increased energy density of 400 Wh kg-1 for NCLFMO.7 Following substitution, the capacity contributions of Mn and Fe become 20.2% and 79.8%, respectively, instead of 49.8% and 50.2% in NFMO. Clearly, it indicates a pronounced Fe4+/Fe3+ redox activity with a suppressed J. T. distortion for Mn4+/Mn3+ redox. Chemical co-substitution significantly improves capacity retention, with NCLFMO retaining 98% of its initial discharge capacity after 500 cycles, compared to 52% for NFMO. Better rate capability, faster diffusion coefficient of Na+ ions from GITT, and reduced cell resistance from impedance spectroscopy support the enhanced performances of NCLFMO. In-situ XRD measurement reveals a reversible O3-P3-O’3-P3-O3 phase transition and ex-situ x-ray absorption spectroscopy analysis confirms a prominent Fe4+/Fe3+ redox with a suppressed Mn4+/Mn3+ redox during charge-discharge cycling. More significantly, NCLFMO remains air-stable, retaining its structure and electrochemical properties after exposure to air for 30 days. Finally, a full cell with precycled HC is constructed, delivering a capacity of 105 mAh g-1 with a Vavg of 3.15 V in the potential window of 4.0-1.5 V at a 0.1 C rate. In conclusion, the (Li + Cu)-co-substitution strategy improves the overall performance of the Fe-Mn-based layered oxide cathode, paving the way for developing better cathodes for commercialized SIBs. References J. Y. Hwang, S. T. Myung, and Y. K. Sun, Chem. Soc. Rev., 46, 3529–3614 (2017).S. Komaba, N. Yabuuchi, T. Nakayama, A. Ogata, T. Ishikawa and I. Nakai, Inorg. Chem., 51, 6211–6220 (2012).B. Mortemard De Boisse, J. H. Cheng, D. Carlier, M. Guignard, C. J. Pan, S. Bordère, D. Filimonov, C. Drathen, E. Suard, B. J. Hwang, A. Wattiaux and C. Delmas, J. Mater. Chem. A, 3, 10976–10989 (2015).L. Yang, X. Li, J. Liu, S. Xiong, X. Ma, P. Liu, J. Bai, W. Xu, Y. Tang, Y. Y. Hu, M. Liu and H. Chen, J. Am. Chem. Soc., 141, 6680–6689 (2019).G. Singh, N. Tapia-Ruiz, J. M. Lopez Del Amo, U. Maitra, J. W. Somerville, A. R. Armstrong, J. Martinez De Ilarduya, T. Rojo and P. G. Bruce, Chem. Mater., 28, 5087–5094 (2016).L. Wang, Y. G. Sun, L. L. Hu, J. Y. Piao, J. Guo, A. Manthiram, J. Ma and A. M. Cao J. Mater. Chem. A, 5, 8752–8761 (2017).A. Ghosh, R. Hegde, K. Kumar and P. Senguttuvan, J. Am. Chem. Soc., submitted(2023).

Induced Anion Redox before Full Oxidation of Ru4+/Ru5+ through Local Configuration Modulation in P2-Na0.67[Mg0.33Ru0.67]O2

The study of anionic redox in layered-oxide cathodes for Na-ion batteries (NIBs) has been extensive, aiming to attain high-energy density. The involvement of Mn4+ or Ru5+ oxidation states is known to be crucial for the anionic O2 −/O− reaction during Na+ de/intercalation. Nevertheless, our findings indicate that anionic redox can activate in Ru-based layered-oxide cathodes prior to complete Ru4+/Ru5+ oxidation. By employing first-principles calculation and experimental techniques, we disclose that additional Na+ deintercalation from P2-Na0.33[Mg0.33Ru0.67]O2 relies on anionic oxidation after extracting 0.33 mol Na+ from P2-Na0.67[Mg0.33Ru0.67]O2, concomitant with cationic oxidation of Ru4+/Ru4.5+. Specifically, only the oxygen adjacent to 2Mg/1Ru is implicated in anionic redox during Na+ de/intercalation, indicating that the Na–O–Mg arrangement in the P2-Na0.33[Mg0.33Ru0.67]O2 structure significantly reduces the thermodynamic stability of anionic redox compared to cationic redox. Through O anionic and Ru cationic reactions, P2-Na0.67[Mg0.33Ru0.67]O2 demonstrates a substantial specific capacity of ∼172 mA h g− 1 and exceptional power capability, facilitated by facile Na+ diffusion and reversible structural changes during charge/discharge. These findings propose an innovative strategy to enhance anionic redox activity by modifying the local oxygen environment, paving the way for high-energy-density cathode materials in NIBs.

P2-Type Na0.84Li0.1Ni0.27Mn0.63O2-Layered Oxide Na-Ion Battery Cathode: Structural Insights and Electrochemical Compatibility with Room-Temperature Ionic Liquids.

Modern technologies that can replace state-of-the-art Li-ion batteries (LIBs), such as Na-ion batteries (NIBs), are currently driving new advancements in energy storage research. Developing functional active materials having sustainable features and enhanced performances able to assess their exploitation in the large-scale market represents a major challenge. Rationally designed P2-type layered transition metal (TM) oxides can enable high-energy NIB cathodes, where the tailored composition directly tunes the electrochemical and structural properties. Such positive electrodes need stable electrolytes, and exploration of unconventional room-temperature ionic liquid (RTIL)-based formulations paves the route toward safer options to flammable organic solvents. Notwithstanding the fact that Li+ doping in these materials has been proposed as a viable strategy to improve structural issues, an in-depth understanding of structure-property relationship as well as electrochemical testing with innovative RTIL-based electrolytes is still missing. Herein, we propose the solid-state synthesis of P2-Na0.84Li0.1Ni0.27Mn0.63O2 (NLNMO) cathode material, which exhibits promising structural reversibility and superior capacity retention upon cycling when tested in combination with RTIL-based electrolytes (EMI-, PYR14-, and N1114-FSI) compared to the standard NaClO4/PC. As unveiled from DFT calculations, lattice integrity is ensured by the reduced Jahn-Teller distortion upon Na removal exerted by Mn4+ and Li+ sublattices, while the good redox reversibility is mainly associated with the electrochemically active Ni2+/Ni3+/Ni4+ series burdening the charge compensation upon desodiation. By declaring the electrochemical compatibility of the P2-NLNMO cathode with three RTIL-based electrolytes and dissecting the role of Li/Ni/Mn sublattices in determining the electrochemical behavior, our comprehensive study enlightens the potential application of this electrode/electrolyte setup for future high-energy NIB prototype cells.

Mild and Fast Chemical Presodiation of Na0.44MnO2

This work presents a mild, fast, and scalable approach for chemical presodiation of Na-ion battery cathodes employing a tunnel-type Na0.44MnO2 (NMO) as a model material to demonstrate its sodiation with sodium-phanazine solutions. After presodiation using this approach, there is an 80% increase in specific capacity and a 66% increase in specific energy of NMO in full cells with hard carbon anodes.

Mastering the synthesis of high Na-content, moisture-stable layered oxide cathode for Na-ion batteries

Sodium layered oxides NaxMO2 (x ≤ 1 and M = transition metal) are of great interest for sodium-ion batteries due to their high energy density and cost-effectiveness. However, these materials, whether they are stoichiometric (Na/M ≈ 1 as in O3 NaMO2) or not (Na/M ≈ 0.7 as in P3/P2 NaxMO2), have certain disadvantages, namely sensitivity to humidity or inadequate capacity, respectively. Herein, we propose an intermediate composition Na0.85Ni0.38Zn0.04Mn0.48Ti0.1O2 that we succeed to stabilize in either O3 or a nanoscale mixture of O3–P3 or O3–P2 phases as proven by X-ray diffraction and transmission electron microscopy, through complex synthesis approaches including quenching, slow cooling and annealing in different atmospheres (Ar, air, O2 etc). We rationalize the stabilization of different phases and microstructure as a function of synthesis conditions and show how it influences the electrochemical performance. Through this study we identified a single phase O3 Na0.85Ni0.38Zn0.04Mn0.48Ti0.1O2 synthesized at 1000 °C in air, which exhibits a high capacity of ∼170 mAh/g and good moisture stability. Furthermore, thanks to the synthesis-structure- electrochemical performance relationship identified here, we believe that this study will provide a reliable basis for optimizing the synthesis for best performing sodium layered oxides for commercialization.

Status and prospects of O3-type layered oxide cathodes for Na-ion batteries: stability, reversibility, and reaction mechanism

Status and prospects of O3-type layered oxide cathodes for Na-ion batteries: stability, reversibility, and reaction mechanism

Distinct electrochemical behavior of P3 and P2 polytypes of Mn/Ni-based Na-ion battery cathode

A layered sodium-ion battery cathode Na34Mn1116Ni316Al216O2 (NMNA) prepared in P3 and P2 polytype structures by varying the calcination temperature is systematically investigated for its structural and electrochemical behavior. P3-phase prepared at 650 °C delivered a higher initial capacity of 164 mAh/g (∼148 mAh/g for P2-phase calcined at 850 °C) at 0.1C. In contrast, the P2-phase sample exhibited better rate performance (87 mAh/g vs. 70 mAh/g for P3-phase at 4C) and capacity retention of ∼47 % after 200 cycles (only ∼26 % for P2-phase). Electrochemical impedance measurements before and after cycling confirmed that the increase in charge transfer resistance is responsible for the poor rate performance of the P3 phase.

Understanding the Correlation between Electrochemical Performance and Operating Mechanism of a Co-free Layered-Spinel Composite Cathode for Na-Ion Batteries.

Compositing different crystal structures of layered transition metal oxides (LTMOs) is an emerging strategy to improve the electrochemical performance of LTMOs in sodium-ion batteries. Herein, a cobalt-free P2/P3-layered spinel composite, P2/P3-LS-Na1/2Mn2/3Ni1/6Fe1/6O2 (LS-NMNF), is synthesized, and the synergistic effects from the P2/P3 and spinel phases were investigated. The material delivers an initial discharge capacity of 143 mAh g-1 in the voltage range of 1.5-4.0 V and displays a capacity retention of 73% at the 50th cycle. The material shows a discharge capacity of 72 mAh g-1 at 5C. This superior rate performance by the material could be by virtue of the increased electronic conductivity contribution of the incorporated spinel phase. The charge compensation mechanism of the material is investigated by in operando X-ray absorption spectroscopy (in a voltage range of 1.5-4.5 V vs Na+/Na), which revealed the contribution of all transition metals toward the generated capacity. The crystal structure evolution of each phase during electrochemical cycling was analyzed by in operando X-ray diffraction. Unlike in the case of many reported P2/P3 composite cathode materials and spinel-incorporated cobalt-containing P2/P3 composites, the formation of a P'2 phase at the end of discharge is absent here.

Rational design of an optimal Al-substituted layered oxide cathode for Na-ion batteries

Layered oxide cathodes, a promising avenue for Na-ion batteries, hold the highest potential for commercialization. Herein, we delve into the structural and electrochemical properties of Al-substituted layered oxides in our quest to pinpoint the optimal cathode composition in the Na3/4(Mn-Al-Ni)O2 pseudo-ternary system. The cathode materials investigated were synthesized in three distinct phase configurations, which include two monophasic configurations with P3 and P2-type structures and a third biphasic cathode with equal proportions of P3 and P2 phases. The fractions of the P3 and P2 type phases in the cathode materials were manipulated by adjusting the calcination temperature. The varying concentration of Mn3+ and Mn4+, confirmed by X-ray photoelectron spectroscopy, was found to impact the cyclic stability of these materials significantly. During electrochemical testing, the P3 cathodes showed impressive rate performance and exhibited an excellent specific capacity of 195 mAh g−1 at 0.1C. Regarding cyclic performance, the biphasic cathodes consistently outperformed their monophasic counterparts, with P3/P2-Na0.75Mn0.50Ni0.25Al0.25O2 exhibiting 82 % capacity retention after 300 cycles. Analysis of operando Synchrotron XRD data revealed an absence of P3 to O3 type phase transition in the cathodes even at low voltages where large structural variations to the unit cell structure were observed. The absence of P3 to O3 transformations and the superior electrochemical performance of Na0.75Mn0.50Ni0.25Al0.25O2 underlines the importance of Al substitution and P3/P2 biphasic structure in enhancing the electrochemical performance of layered oxides.

Modulating Valence Electrons and Na Occupancy in Layered Cathodes for High-Performance Na-Ion Batteries.

Na-ion batteries (NIBs) hold promise as a leading option for large-scale energy storage. However, their development faces challenges due to the lack of high-performance cathode materials. P2-type layered oxides are seen as potential cathode materials for NIBs due to higher structure stability, yet their commercialization is hindered by limited capacity and subpar phase transitions during Na extraction and insertion at high voltages. In this study, we introduce a new P2-type cathode material, Na0.76Ni0.23Li0.1Ti0.02Mn0.65O1.998F0.02 (NLTMOF), synthesized with ternary Li/Ti/F substitution. This modification of ternary Li/Ti/F substitution significantly tailors the electronic structures, increasing the number of valence electrons near the Fermi energy level. This facilitates the electronic conductivity and their involvement in charge compensation, thereby enhancing reversible capacity. Additionally, ternary doping synergistically adjusts the Na occupancy at the Na layer for favorable Na extraction without P2-O2 phase transitions even under a high voltage of 4.4 V, boosting cycling stability. As a result, NLTMOF demonstrates a reversible capacity of 110.0 and 132.2 mAh g-1 at 2-4.2 and 2-4.4 V, respectively, and maintains greatly enhanced cycling stability over long cycles. This study sheds light on the design of transition metal oxides for advanced cathode materials through the modulation of electronic structure and Na occupancy in cathode materials, thus promoting the development of NIBs.

Zinc-substituted Fe-based Prussian blue analogues induce a weak Jahn-Teller effect to enhance stability of sodium ion batteries

Prussian blue analogues (PBAs) have attracted wide attention as cathode materials for sodium-ion batteries. Nevertheless, the poor cycle performance caused by the [Fe(CN)6]4− defects and Jahn-Teller (J-T) effect hinders its practical application. Here, Zn-substituted PBAs were synthesized through co-precipitation method and studied as cathodes for Na-ion batteries. The synthesized Na1.14Zn0.39Fe0.61[Fe(CN)₆]·4.14H2O (NZFC) and Na1.18Zn0.42Fe0.58[Fe(CN)₆]·4.96H2O (NZFE) showed outstanding exceptional electrochemical properties, including high capacity (first capacity of 114.56 and 97.76 mAh·g−1 at 10 mA·g−1, respectively), excellent rate property (71.81 and 68.69 mAh·g−1 at 500 mA·g−1, respectively), and cycling stability (64.72 % and 80.27 %, respectively) after 350 cycles at 200 mA g−1, which is attribute to introduction of Zn element induced a weak Jahn-Teller effect and a low-spins and a high-spins FeII/FeIII redox sites coordinated by C and N atoms prompt reversible diffusion of sodium ions and without structural transformation in the Na-ion storage. Using combined experiment and theory via ex situ X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations, it is confirmed that the weak J-T distortion contributes significantly to fast-overall kinetics, structural stability, and high electronic conductivity of the electrode. These findings provide new insights into comprehending for the future development of high-performance and stable sodium ion batteries.

P2-type low-cost and moisture-stable cathode for sodium-ion batteries

Mn-based P2-type oxides are considered as promising cathodes for Na-ion batteries; however, they face significant challenges, including structural degradation when charged at high cutoff voltages and structural changes upon storing in a humid atmosphere. In response to these issues, we have designed an oxide with co-doping of Cu and Al which can balance both cost and structural stability. The redox reaction of Cu2+/3+ can provide certain charge compensation, and the introduction of Al can further suppress the Jahn-Teller effect of Mn, thereby achieving superior long-term cycling performance. The ex-situ XRD testing indicates that Cu/Al co-doping can effectively suppress the phase transition of P2-O2 at high voltage, thereby explaining the improvement in electrochemical performance. DFT calculations reveal a high chemical tolerance to moisture, with lower adsorption energy for H2O compared to pure Na0.67Cu0.25Mn0.75O2. A representative Na0.67Cu0.20Al0.05Mn0.75O2 cathode demonstrates impressive reversible capacities of 148.7 mAh/g at 0.2 C, along with a remarkable capacity retention of 79.1% (2 C, 500 cycles).

Unravelling the ion transport and the interphase properties of a mixed olivine cathode for Na-ion battery

The replacement of Li by Na in an analogue battery to the commercial Li-ion one appears a sustainable strategy to overcome the several concerns triggered by the increased demand for the electrochemical energy storage. However, the apparently simple change of the alkali metal represents a challenging step which requires notable and dedicated studies. Therefore, we investigate herein the features of a NaFe0.6Mn0.4PO4 (NFMP) cathode with triphylite structure achieved from the conversion of a LiFe0.6Mn0.4PO4 (LFMP) olivine for application in Na-ion battery. The work initially characterizes the structure, morphology and performances in sodium cell of NFMP, achieving a maximum capacity exceeding 100 mAh g−1 at a temperature of 55 °C, adequate rate capability, and suitable retention confirmed by ex-situ measurements. Subsequently, the study compares in parallel key parameters of the NFMP and LFMP such as Na+/Li+ ions diffusion, interfacial characteristics, and reaction mechanism in Na/Li cells using various electrochemical techniques. The data reveal that relatively limited modifications of NFMP chemistry, structure and morphology compared to LFMP greatly impact the reaction mechanism, kinetics and electrochemical features. These changes are ascribed to the different physical and chemical features of the two compounds, the slower mobility of Na+ with respect to Li+, and a more resistive electrode/electrolyte interphase of sodium compared with lithium. Relevantly, the study reveals analogue trends of the charge transfer resistance and the ion diffusion coefficient in NFMP and LFMP during the electrochemical process in half-cell. Hence, the NFMP achieved herein is suggested as a possible candidate for application in a low-cost, efficient, and environmentally friendly Na-ion battery.

Identification of optimal composition with superior electrochemical properties along the zero Mn3+ line in Na0.75(Mn-Al-Ni)O2 pseudo ternary system

Biphasic layered oxide cathodes, known for their superior electrochemical performance, are prime candidates for commercializing in Na-ion batteries. Herein, we unveil a series of P3/P2 monophasic and biphasic Al-substituted Na34Mn5-x8Al2x8Ni3-x8O2 layered oxide cathodes that lie along the ‘zero Mn3+ line’ in the Na3/4(Mn-Al-Ni)O2 pseudo-ternary system. The structural analysis showed a larger Na+ conduction bottleneck area in both P3 and P2 structures with a higher Al3+ content, which enhanced their rate performance. In each composition, the P3/P2 biphasic compound with nearly equal fractions of P3 and P2 phases outperformed their monophasic counterparts in almost all electrochemical performance parameters. Operando synchrotron XRD measurements obtained for the monophasic P3 and biphasic P2/P3 samples revealed the absence of the O3 phase during cycling. The high structure stability and faster Na+ transport kinetics in the biphasic samples underpins the enhancement of electrochemical properties in the Al-substituted P3/P2 cathodes. These results highlight fixed oxidation state lines as a novel tool to identify and design layered oxide cathodes for Na-ion batteries in pseudo-ternary diagrams involving Jahn-Teller active cations.

Development of High-Performance Iron-Based Phosphate Cathodes toward Practical Na-Ion Batteries.

Iron-based phosphate cathode of Na4Fe3(PO4)2(P2O7) has been regarded as a low-cost and structurally stable cathode material for Na-ion batteries (NIBs). However, their practical application is greatly hindered by the insufficient electrochemical performance and limited energy density. Here, we report a new iron-based phosphate cathode of Na4.5Fe3.5(PO4)2.5(P2O7) with the intergrown heterostructure of the maricite-type NaFePO4 and orthorhombic Na4Fe3(PO4)2(P2O7) phases at a mole ratio of 0.5:1. Benefited from the increased composition ratio and the spontaneous activation of the maricite-type NaFePO4 phase, the as-prepared Na4.5Fe3.5(PO4)2.5(P2O7) composites deliver a reversible capacity over 130 mA h g-1 and energy density close to 400 W h kg-1, which is far beyond that of the single-phase Na4Fe3(PO4)2(P2O7) cathode (∼120 mA h g-1 and ∼350 W h kg-1). Moreover, the kg-level products from the scale-up synthesis demonstrate a stable cycling performance over 2000 times at 3 C in pouch cells. We believe that our findings could show the way forward the practical application of the iron-based phosphate cathodes for NIBs.