Different Effects of Al Substitution for Mn or Fe on the Structure and Electrochemical Properties of Na0.67Mn0.5Fe0.5O2 as a Sodium Ion Battery Cathode Material

Sodium layered oxide cathodes: properties, practicality and prospects

Rechargeable sodium-ion batteries (SIBs) have emerged as an advanced electrochemical energy storage technology with potential to alleviate the dependence on lithium resources. Similar to Li-ion batteries, the cathode materials play a decisive role in the cost and energy output of SIBs. Among various cathode materials, Na layered transition-metal (TM) oxides have become an appealing choice owing to their facile synthesis, high Na storage capacity/voltage that are suitable for use in high-energy SIBs, and high adaptivity to the large-scale manufacture of Li layered oxide analogues. However, going from the lab to the market, the practical use of Na layered oxide cathodes is limited by the ambiguous understanding of the fundamental structure-performance correlation of cathode materials and lack of customized material design strategies to meet the diverse demands in practical storage applications. In this review, we attempt to clarify the fundamental misunderstandings by elaborating the correlations between the electron configuration of the critical capacity-contributing elements (e.g., TM cations and oxygen anion) in oxides and their influence on the Na (de)intercalation (electro)chemistry and storage properties of the cathode. Subsequently, we discuss the issues that hinder the practical use of layered oxide cathodes, their origins and the corresponding strategies to address their issues and accelerate the target-oriented research and development of cathode materials. Finally, we discuss several new Na layered cathode materials that show prospects for next-generation SIBs, including layered oxides with anion redox and high entropy and highlight the use of layered oxides as cathodes for solid-state SIBs with higher energy and safety. In summary, we aim to offer insights into the rational design of high-performance Na layered oxide cathode materials towards the practical realization of sustainable electrochemical energy storage at a low cost.

Achieving High Initial Coulombic Efficiency and Capacity in a Surface Chemical Grafting Layer of Plateau-type Sodium Titanate

The plateau-type sodium titanate with suitable sodiation potential is a promising anode candidate for high safe and high energy density of sodium-ion batteries (SIBs). However, the poor initial Coulombic efficiency (ICE) and cyclic instability of sodium titanate are attributed to the unstable interfacial structure along with the decomposition of electrolytes, resulting in the continuous formation of solid electrolyte interface (SEI) film. To address this issue, a chemical grafting method is developed to fabricate a highly stable interface layer of inert Al2O3 on the sodium titanate anode, rendering the high ICE and excellent cycling stability. Based on theoretical calculations, NaPF6 are more likely adsorption on the Al2O3 surface and produce sodium fluoride. The formation of a thin and dense SEI film with rich sodium fluoride achieves the low interfacial resistances and charge-transfer resistances. Benefitting from our design, the obtained sodium titanate exhibits a high ICE from 67.7 % to 79.4 % and an enhanced reversible capacity from 151 mAh g-1 to 181 mAh g-1 at 20 mA g-1, along with an increase in capacity retention from 56.5 % to 80.6 % after 500 cycles. This work heralds a promising paradigm for rational regulation of interfacial stability to achieve high-performance anodes for SIBs.

Single-Crystal P2-Na0.67Mn0.67Ni0.33O2 Cathode Material with Improved Cycling Stability for Sodium-Ion Batteries

Layered oxides constitute one of the most promising cathode materials classes for large-scale sodium-ion batteries because of their high specific capacity, scalable synthesis, and low cost. However, their practical use is limited by their low energy density, physicochemical instability, and poor cycling stability. Aiming to mitigate these shortcomings, in this work, we synthesized polycrystalline (PC) and single-crystal (SC) P2-type Na0.67-δMn0.67Ni0.33O2 (NMNO) cathode materials through a solid-state route and evaluated their physicochemical and electrochemical performance. The SC-NMNO cathode with a large mean primary particle size (D50) of 12.7 μm was found to exhibit high cycling stability leading to 47% higher capacity retention than PC-NMNO after 175 cycles at 1C rate in the potential window 4.2-1.5 V. This could be attributed to the effective mitigation of parasitic side reactions at the electrode-electrolyte interface and suppressed intergranular cracking induced by anisotropic volume changes. This is confirmed by the lower volume variation of SC-NMNO (ΔV ∼ 1.0%) compared to PC-NMNO (ΔV ∼ 1.4%) upon charging to 4.2 V. Additionally, the SC-NMNO cathode displayed slightly higher thermal stability compared to PC-NMNO. Both cathodes exhibited good chemical stability against air and water exposure, thus enabling material storage/handling in the ambient atmosphere as well as making them suitable for aqueous processing. In this regard, PC-NMNO was investigated with two low-cost aqueous binders, carboxymethyl cellulose, and sodium trimetaphosphate, which exhibited higher binding strength and displayed excellent electrochemical performance compared to PVDF, which could potentially lead to significant cost reduction in electrode manufacturing.

Critical Review of Emerging Pre-metallization Technologies for Rechargeable Metal-Ion Batteries

Low Coulombic efficiency, low-capacity retention, and short cycle life are the primary challenges faced by various metal-ion batteries due to the loss of corresponding active metal. Practically, these issues can be significantly ameliorated by compensating for the loss of active metals using pre-metallization techniques. Herein, the state-of-the-art development in various pr-emetallization techniques is summarized. First, the origin of pre-metallization is elaborated and the Coulombic efficiency of different battery materials is compared. Second, different pre-metallization strategies, including direct physical contact, chemical strategies, electrochemical method, overmetallized approach, and the use of electrode additives are summarized. Third, the impact of pre-metallization on batteries, along with its role in improving Coulombic efficiency is discussed. Fourth, the various characterization techniques required for mechanistic studies in this field are outlined, from laboratory-level experiments to large scientific device. Finally, the current challenges and future opportunities of pre-metallization technology in improving Coulombic efficiency and cycle stability for various metal-ion batteries are discussed. In particular, the positive influence of pre-metallization reagents is emphasized in the anode-free battery systems. It is envisioned that this review will inspire the development of high-performance energy storage systems via the effective pre-metallization technologies.

Cu/Ti co-doping boosting P2-type Fe/Mn-based layered oxide cathodes for high-performance sodium storage

Environmentally friendly P2-type layered iron manganese oxides appear to be one of the most potential cathode materials for sodium-ion batteries (SIBs). However, their practical application is hindered by the unfavorable phase transitions, dissolution of transition metals, and poor air stability. One effective strategy by either single-cation doping or high-cost Li involved co-doping is used to alleviate the problems. Here, low-cost Cu/Ti co-doping is introduced to boost P2-Na0.7Cu0.2Fe0.2Mn0.5Ti0.1O2 as an air and electrochemical stable cathode material for SIBs. The resulting electrode delivers an initial capacity of 130 mAh g-1 at 0.1C within 2.0-4.2 V, a reversible discharge capacity of 61.0 mAh g-1 at a high rate of 5C and a capacity retention ratio exceeding 71.1% after 300 cycles. In particular, the co-doped crystal structure is well-maintained after 1 month of exposure to air, and even 3 days of soaking in water. Furthermore, the enhancement is elucidated by the effectively mitigated P2-Z and the completely suppressed P2-P'2 phase transitions, the decreased volume variation proved by in-situ X-ray diffraction (XRD), as well as the lowered transition-metal dissolution evidenced by inductively coupled plasma optical emission spectrometer (ICP-OES) and X-ray photoelectron spectroscopy (XPS). The low-lost Cu/Ti doping strategy could thus be effective for designing and preparing environmentally friendly and high-performance cathode materials for SIBs.

Constructing a multidimensional porous structure of K/Co co-substituted Na3V2(PO4)3/C attached on the lamellar Ti3C2Tx MXene substrate for superior sodium storage property

Na₃V₂(PO₄)₃ (NVP) materials have emerged as prospective cathodes for sodium-ion batteries (SIBs). However, its weak intrinsic conductivity has limited deeper research. Herein, we adopt the strategy of simultaneous K/Co co-substitution and Ti₃C₂T_x MXene (MX) introduction to optimize NVP. The K/Co co-substitution brings about the synergetic effect of NVP framework stabilization. Doping Co²⁺ generates beneficial holes and accelerating electronic conductivity. The MX plates are stacked at random to form a porous construction, increasing the contact areas to provide more active sites for Na⁺ shuttling and buffering the volume change. Furthermore, the lamellar MX and the carbon layers form efficient conductive networks that increase electron migration. Notably, K_0.1Na_2.95V_1.95Co_0.05(PO₄)₃@MX (KC05@MX) exhibited an initial capacity of 116 mA h g^-1 under 1 C with an extraordinary retention of 86.8% at the 400^th cycle. It realized high performance under 20 C and 50 C, and the outputs were 93.5 and 82.4 mA h g^-1 at the 1^st cycle and 66.6 and 53.4 mA h g^-1 at the 1000^th cycle, respectively, with slight capacity loss at 0.028% and 0.035%. Furthermore, the Bi₂Se₃//KC05@MX asymmetric full cell expressed great electrochemical properties, indicating the superior practical application prospect of KC05@MX.

In Operando Neutron Scattering Multiple-Scale Studies of Lithium-Ion Batteries

Real-time observation of the electrochemical mechanistic behavior at various scales offers new insightful information to improve the performance of lithium-ion batteries (LIBs). As complementary to the X-ray-based techniques and electron microscopy-based methodologies, neutron scattering provides additional and unique advantages in materials research, owing to the different interactions with atomic nuclei. The non-Z-dependent elemental contrast, in addition to the high penetration ability and weak interaction with matters, makes neutron scattering an advanced probing tool for the in operando mechanistic studies of LIBs. The neutron-based techniques, such as neutron powder diffraction, small-angle neutron scattering, neutron reflectometry, and neutron imaging, have their distinct functionalities and characteristics regimes. These result in their scopes of application distributed in different battery components and covering the full spectrum of all aspects of LIBs. The review surveys the state-of-the-art developments of real-time investigation of the dynamic evolutions of electrochemically active compounds at various scales using neutron techniques. The atomic-scale, the mesoscopic-scale, and at the macroscopic-scale within LIBs during electrochemical functioning provide insightful information to battery researchers. The authors envision that this review will popularize the applications of neutron-based techniques in LIB studies and furnish important inspirations to battery researchers for the rational design of the new generation of LIBs.

Completely suppressed high-voltage phase transition of P2/O3-Na0.7Li0.1Ni0.1Fe0.2Mn0.6O2via Li/Ni co-doping for sodium storage

A novel P2/O3-Na0.7Li0.1Ni0.1Fe0.2Mn0.6O2 cathode is prepared via Li/Ni co-doping, and delivers attractive cycling and rate performances due to the high Na+ diffusion coefficient and the complete suppression of the high-voltage P2–Z phase transition.

A green process for recycling and synthesis of cathode materials LiMn2O4 from spent lithium-ion batteries using citric acid

In the process of recycling spent lithium-ion batteries, citric acid is only used as a chelating agent to resynthesize new cathode materials by a sol-gel process or as a precipitant to separate Mn ions and Li ions.

Role of Lithium Doping in P2-Na0.67Ni0.33Mn0.67O2 for Sodium-Ion Batteries

P2-structured Na_0.67Ni_0.33Mn_0.67O₂ (PNNMO) is a promising Na-ion battery cathode material, but its rapid capacity decay during cycling remains a hurdle. Li doping in layered transition-metal oxide (TMO) cathode materials is known to enhance their electrochemical properties. Nevertheless, the influence of Li at different locations in the structure has not been investigated. Here, the crystallographic role and electrochemical impact of lithium on different sites in PNNMO is investigated in Li _x Na_0.67-y Ni_0.33Mn_0.67O_2+δ (0.00 ≤ x ≤ 0.2, y = 0, 0.1). Lithium occupancy on prismatic Na sites is promoted in Na-deficient (Na < 0.67) PNNMO, evidenced by ex situ and operando synchrotron X-ray diffraction, X-ray absorption spectroscopy, and ⁷Li solid-state nuclear magnetic resonance. Partial substitution of Na with Li leads to enhanced stability and slightly increased specific capacity compared to PNNMO. In contrast, when lithium is located primarily on octahedral TM sites, capacity is increased but at the cost of stability.

Understanding the Effect of Zn Doping on Stability of Cobalt-Free P2-Na0.60Fe0.5Mn0.5O2 Cathode for Sodium Ion Batteries

In this work, we report a sol-gel synthesis-based Zn-doped Na0.6Fe0.5Mn0.5O2 (NFM) cathode and understand the effect of Zn doping on the crystal structure and electrochemical performances such as discharge capacity and rate capability. Detailed X-Ray diffraction (XRD) pattern analysis indicated a decrease in the Na-layer thickness with Zn doping. Small amount of Zn2+ dopant (i.e., 2 at.%) slightly improved cycling stability, reversibility, and rate performances at higher discharge current rates. For example, at 1 C-rate (1 C = 260 mAh/g), the Zn2+-doped cathode retained a stable reversible capacity of 72 mAh/g, which was ~16% greater than that of NFM (62 mAh/g) and showed a minor improvement in the capacity retention of 60% compared to 55% for the pristine NFM after 65 cycles. Slight improvement in the electrochemical performance for the Zn-doped cathode can be attributed to a better structural stability, which prevented the initial phase transition and showed the presence of electrochemical active Fe3+/4+ even after 10 cycles compared to NFM.

Enhanced electrochemical properties of Ni-rich layered cathode materials via Mg2+ and Ti4+co-doping for lithium-ion batteries

To optimize the electrochemical performance of Ni-rich cathode materials, the 0.005 mol of Mg²⁺ and 0.005 mol of Ti⁴⁺co-doping LiNi_0.83Co_0.11Mn_0.06O₂ composite powders, labeled as NCM-11, are successfully prepared by being calcinated at 750 °C for 15 h following by an appropriate post-treatment, which are confirmed by XRD, EDS and XPS. The results suggest that NCM-11 presents a well-ordered layered structure with a low Li⁺/Ni²⁺ mixing degree of 1.46% and Mg²⁺ and Ti⁴⁺ ions are uniformly distributed across the lattice. The cell assembled with NCM-11 can deliver an initial discharge specific capacity of 194.2 mAh g^-1 and retain a discharge specific capacity of 163.0 mAh g^-1 after 100cycles at 2.0C at 25 °C. Furthermore, it still maintains a discharge specific capacity of 166.7 mAh g^-1 after 100cycles at 2.0C at 60 °C. More importantly, it also exhibits a higher discharge specific capacity of about 150.7 mAh g^-1 even at 5.0C. Those superior electrochemical performance can be mainly ascribed to the synergistic effect of Mg²⁺ and Ti⁴⁺co-doping, in which Mg²⁺ ions can occupy the Li⁺ layer to act as pillar ions and Ti⁴⁺ ions can occupy the transition metal ions layer to enlarge the interplane spacing. Thus, the heterovalent cations co-doping strategy can be considered as a simple and practical method to improve the electrochemical performance of Ni-rich layered cathode materials for lithium-ion batteries.

Whole-Voltage-Range Oxygen Redox in P2-Layered Cathode Materials for Sodium-Ion Batteries

Oxygen-redox of layer-structured metal-oxide cathodes has drawn great attention as an effective approach to break through the bottleneck of their capacity limit. However, reversible oxygen-redox can only be obtained in the high-voltage region (usually over 3.5 V) in current metal-oxide cathodes. Here, we realize reversible oxygen-redox in a wide voltage range of 1.5-4.5 V in a P2-layered Na_0.7 Mg_0.2 [Fe_0.2 Mn_0.6 □_0.2 ]O₂ cathode material, where intrinsic vacancies are located in transition-metal (TM) sites and Mg-ions are located in Na sites. Mg-ions in the Na layer serve as "pillars" to stabilize the layered structure during electrochemical cycling, especially in the high-voltage region. Intrinsic vacancies in the TM layer create the local configurations of "□-O-□", "Na-O-□" and "Mg-O-□" to trigger oxygen-redox in the whole voltage range of charge-discharge. Time-resolved techniques demonstrate that the P2 phase is well maintained in a wide potential window range of 1.5-4.5 V even at 10 C. It is revealed that charge compensation from Mn- and O-ions contributes to the whole voltage range of 1.5-4.5 V, while the redox of Fe-ions only contributes to the high-voltage region of 3.0-4.5 V. The orphaned electrons in the nonbonding 2p orbitals of O that point toward TM-vacancy sites are responsible for reversible oxygen-redox, and Mg-ions in Na sites suppress oxygen release effectively.

Ni-Doped Layered Manganese Oxide as a Stable Cathode for Potassium-Ion Batteries

Potassium-ion batteries (PIBs) are one of the promising alternatives to lithium-ion batteries (LIBs). Layered potassium manganese oxides are more attractive as cathodes for PIBs due to their high capacity, low cost, and simple synthesis method but suffer from the Jahn-Teller effect of Mn³⁺ in material synthesis. Here, a layered P3-type K_0.67Mn_0.83Ni_0.17O₂ material with a suppressed Jahn-Teller effect was successfully synthesized. K_0.67Mn_0.83Ni_0.17O₂ delivers a specific capacity of 122 mAh g^-1 at 20 mA g^-1 in the first discharge, superior rate performance, and good cycling stability (75% capacity retention cycled at a high rate of 500 mA g^-1 after 200 cycles). Besides, the K ion diffusion coefficient of the K_0.67Mn_0.83Ni_0.17O₂ electrode can reach 10^-11 cm² s^-1, which are larger than the Ni-free electrode. The X-ray diffraction and electron diffraction analyses demonstrate that appropriate nickel could suppress the Jahn-Teller effect and reduce the structural deterioration, resulting in more migration pathways for K ions, thus enhancing the rate capability and cycling performance. These results provide a strategy to develop high-performance cathode materials for PIBs and deepen the understanding of structural deterioration in layered manganese-based oxides.

Ionic Liquid-Based Electrolytes for Sodium-Ion Batteries: Tuning Properties To Enhance the Electrochemical Performance of Manganese-Based Layered Oxide Cathode

Ionic liquids (ILs) are considered as appealing alternative electrolytes for application in rechargeable batteries, including next-generation sodium-ion batteries, because of their safe and eco-friendly nature, resulting from their extremely low volatility. In this work, two groups of advanced pyrrolidinium-based IL electrolytes are concerned, made by mixing sodium bis(fluorosulfonyl)imide (NaFSI) or sodium bis(trifluoromethanesulfonyl)imide (NaTFSI) salts salts with N-methyl- N-propylpyrrolidinium bis(fluorosulfonyl)imide (Pyr₁₃FSI), N-butyl- N-methylpyrrolidinium bis(fluorosulfonyl)imide (Pyr₁₄FSI), and N-butyl- N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr₁₃FSI). The characterization of eight different electrolytes, including single anion electrolytes and binary anion mixtures, in terms of thermal properties, density, viscosity, and conductivity, as well as electrochemical stability window and cycling performance in room-temperature sodium cells, is reported here. Among all of the blends, those containing Pyr₁₄FSI outperform the others in terms of cell performance enabling the layered P2-Na_0.6Ni_0.22Al_0.11Mn_0.66O₂ cathode to deliver about 140 mAh g^-1 for more than 200 cycles.

Improving the Electrochemical Properties of the Manganese-Based P3 Phase by Multiphasic Intergrowth

Layered transition-metal oxides are one kind of the most promising cathode materials for sodium-ion batteries. In this study, we propose a strategy to enhance the electrochemical properties of P3-type manganese-based layered oxide cathode by introducing a small amount of layered P2 and Li-O'3 phases. Powder X-ray diffraction (PXRD) structural refinement and aberration-corrected scanning transmission electron microscopy (STEM) are performed to confirm the microstructures of different samples. PXRD refinement results show that the elevated annealing temperature leads to a partial conversion of the P3 phase to the P2 phase and the addition of lithium results in the formation of a new O'3 phase in the P3/P2-layered matrix. STEM results identify the intergrowth of P3/P2 and P3/P2/O'3 in biphasic and triphasic materials, respectively. Electron energy loss spectroscopy verifies that the alkali metal layer in the O'3 phase is occupied by the lithium ion. The intergrowth of biphasic and triphasic materials in these layered P3/P2 and P3/P2/O'3 hybrid structures brings forth a positive effect on the electrochemical properties. In particular, the formation of P3/P2/O'3-intergrown hybrid structures greatly improves the cycling stability of the P3 phase that the capacity retention of P3/P2/O'3 hybrid structures remains 78%, while capacity retention of the pure P3 phase is only 54.1% after 50 cycles at a rate of 0.2 C, and the rate performance of the P3 phase has also been enhanced.