P2-Type Moisture-Stable and High-Voltage-Tolerable Cathodes for High-Energy and Long-Life Sodium-Ion Batteries

Medium-entropy P2-type layered metal oxide cathode demonstrating complete solid-solution behavior for improved Na-ion battery performance

P2-type layered metal oxides are promising cathode materials for Na-ion batteries (NIBs), but they suffer from Na+/vacancy ordering and P2-O2 phase transitions during charge-discharge processes, resulting in rapid capacity decay and poor rate performance. In this study, a non-equimolar five-component medium-entropy strategy was employed to design a Na0.67Ni0.19Cu0.07Zn0.07Mn0.60Ti0.07O2 (NNCZMT) material. Unlike equimolar high-entropy designs, Ni and Mn remain as the main components, with Ni providing high capacity through the Ni2+/Ni4+ redox couple and Mn stabilizing the P2 structure. The introduction of Cu2+, Zn2+, and Ti4+, which have slightly larger ionic radii and the same valence states as Ni2+ and Mn4+, respectively, allows for mutual substitution, synergistically promoting a solid-solution reaction and excellent Na+ diffusion kinetics. The NNCZMT electrode delivers a high reversible capacity of 110 mAh g-1 at 10 mA g-1, an initial Coulombic efficiency of 96.8 %, remarkable cycling stability (94.7 % capacity retention after 100 cycles at 200 mA g-1), and excellent rate performance (57.2mAh g-1 at 2000 mA g-1).

Negative Enthalpy Doping Stabilizes P2-Type Oxides Cathode for High-Performance Sodium-Ion Batteries

P2-type Na0.67Ni0.33Mn0.67O2 (NNMO) as cathode material for sodium-ion batteries (SIBs) largely suffers from continuous accumulation of local stress caused by destructive structural evolution and irreversible oxygen loss upon cycling, leading to rapid capacity degradation. Herein, a strategy of negative enthalpy doping (NED), wherein transition metal (TM) sites are substituted with 0.01 mol each Sn, Sb, Cu, Ti, Mg, and Zn to increase the stability of the TM layers, is proposed. The robust structure of NED-NNMO significantly suppresses the P2 to O2 phase transition and improves the Na+ kinetics upon long-term cycling. Consequently, the NED-NNMO exhibits much smoothened voltage platforms and improved oxygen redox reversibility, thus considerably extended lifetime as compared with the pristine NNMO sample. The NED-NNMO delivers a high capacity of 138.9 mAh g-1 with an operation voltage of 3.51 V under 0.1 C and prominent capacity retention of 94.6% after 100 cycles under 1 C, and 90.0% over 3000 cycles under ultra-high rate of 30 C, which is among the best over previous reports. Moreover, an ampere-hour scale pouch cell based on the NED-NNMO demonstrates an energy density of 139 Wh kg-1. This work sheds light on a route of negative enthalpy doping to design high-performance sodium-ion batteries.

Moisture-Resistant, Expansive, and Disordered Interlayer Microenvironment-Enabled Robust Sodium Oxide Cathodes

Layered transition metal oxides are some of the most attractive cathode candidates for sodium-ion batteries (SIBs). The main challenge of achieving superior storage performance is to simultaneously boost the ion diffusion kinetics and restrain the undesirable OP4 phase transition upon long-term cycling. In this report, a step-by-step molecule-ion exchange approach is presented to design the high air-stability disordered Ca0.065Na0.55MnO2.05 (CNMO-1) cathode functionalized with an expansive and disordered interlayer microenvironment. Theoretical and experimental investigations revealed that water mediation and ion exchange enhance ion diffusion, while Ca ions stabilize the alkali metal layer, preventing phase transition and manganese (Mn) migration during high-voltage cycling. It exhibits a high specific capacity of 135.4 mA h g-1 at 0.2 A g-1. Beyond that, it can also deliver 81.3 mA h g-1 at the harsh condition of 5 A g-1 with a high 93.3% retention even after 2000 cycles, surpassing most previous achievements. This proposed strategy can be extended to other K+, Zn2+, and La3+ cases, showing an innovative method for designing robust cathodes that enhance the performance of SIBs.

Coordination Chemistry Regulation Suppressing Voltage Hysteresis for Na3MnTi(PO4)3 in High-Rate Sodium-Ion Batteries

As a natrium superionic conductor, NASICON-type Na3MnTi(PO4)3 (NMTP) has garnered increasing attention for large-scale sodium-ion batteries due to its high stability and power densities. Nevertheless, it still suffers from an inferior rate capability and poor cycling longevity, arising from sluggish intrinsic kinetics and severe structural degradation. Herein, vanadium (V) is used as a dopant for equal substitution of manganese (Mn) and titanium (Ti) in NMTP to alleviate voltage hysteresis and enhance the cycling performance. V-doping regulates the local coordination chemistry of transition metals and reduces derivative antisite defect concentration upon cycling. Through density functional theory analysis, Na3Mn0.9V0.2Ti0.9(PO4)3 (NMTP-V0.2) demonstrates a lower bandgap and higher electronic conductivity. Additionally, V-doping significantly lowers the diffusion barrier of Na2, leading to Na+ diffusivity that is approximately two orders of magnitude higher than that of NMTP during the Mn2+/Mn3+ redox process. The as-prepared NMTP-V0.2 delivers an excellent rate capability of 85.3 mAh g-1 under 50 C and satisfactory cycling retention of 81% with a high capacity over 1400 cycles. Thus, the assembled NMTP-V0.2/hard carbon sodium-ion full cell achieves a high energy density of 292.3 Wh kg-1 as well as outstanding capacity retention of 92% after 500 cycles under 10 C. This result not only provides an approach for suppressing voltage hysteresis in polyanion cathodes but also offers guidance for designing high-power SIBs.

Sodium-Rich, Co-Ni-Free P2-Layered Manganese Oxide Cathodes for Sodium-Ion Batteries

The rapid capacity loss attributed to irreversible phase reactions and structural instability has consistently affected the development of P2-layered cathode materials. Moreover, the introduction of costly elements such as single or multiple dopants has failed to resolve the sustainability challenges in designing an optimal Mn-based layered oxide cathode. This study proposes a Co-Ni-free, poly-elemental doping strategy (Li, Mg, and Cu) combined with high sodium content for an Mn-based P2-layered cathode designed for Na+ ion storage. In situ X-ray diffraction analysis confirms the absence of P2 - O2 phase transitions during cycling for both NLMMC87 (Na0.87Li0.1Mg0.1Mn0.7Cu0.1O2) and NLMMC77 (Na0.77Li0.1Mg0.1Mn0.7Cu0.1O2). This can be attributed to the co-substitution of the electronegative Cu element and the stabilizing dopants Li and Mg, which suppress oxygen evolution. Simultaneously, the high sodium content within the host structure promotes high reversible capacity, enhances structural stability, and minimizes internal stress due to volume changes. Moreover, NLMMC87 demonstrates a reversible capacity of 167.9 mA h g-1 at 0.1 C (97% Coulombic efficiency) and maintains excellent stability across various current rates. Further investigations into the practical application of NLMMC87 in a sodium full cell will be a critical step toward realizing an ideal cathode for sodium-ion batteries.

A W/Al Co-doped Na0.44MnO2 Cathode Material for Enhanced Sodium-Ion Storage

The Na0.44MnO2 cathode has attracted enormous interest owing to its low cost, low toxicity, and stable structure, but its practical application is still hindered by the limited sodium storage sites. Element doping is widely used to improve its capacity. However, cation and anion substitution could barely reach a satisfactory compromise between the structural stability and reversible capacity. Herein, we show that the above issue could be overcome via the synergetic effect of W and Al substitution. Combining the electrochemical and in situ X-ray powder diffraction (XRD) measurements, we reveal that the substitution of W effectively facilitates the tunnel-to-layered phase transformation, while the further substitution of Al eliminates the Na+/vacancy ordering during the extraction/insertion of Na+ ions, resulting in a layered Na0.44Mn0.94W0.01Al0.05O2 (NMO-1W5Al) with negligible Na+/vacancy ordering upon cycling. In addition, NMO-1W5Al represents a wider Na+ interlayer spacing to accelerate the diffusion of Na+, which improves the rate performance. The high specific capacity, remarkable rate performance, and high cycling stability of NMO-1W5Al are promising for large-scale energy storage systems.

Ti-Substituted O3-NaNi0.5Mn0.3Ti0.2O2 Material with a Disordered Transition Metal Layer and a Stable Structure

O3-type NaNi0.5Mn0.5O2 (NNM) is very competitive for sodium-ion batteries (SIBs) due to its high capacity and easy production. Nevertheless, the intricate phase transitions during the charging-discharging significantly impede its practical application. This paper proposes a strategy for successfully synthesizing NaNi0.5Mn0.3Ti0.2O2 (NNMT) by combining coprecipitation and a high-temperature solid-state method. This method introduces Ti elements while retaining the electrochemically active Ni2+ content, thus, the NNMT has a high initial specific capacity of 151.4 mAh g-1 at 1 C. It is demonstrated that introducing Ti4+ leads to the transition metal layers becoming disordered by ex situ XRD, thus mitigating the irreversible phase transition of the material. In addition, Ti4+ does not have an outer electron, which can reduce electron delocalization in the transition metal layer and improve the material's cyclic stability. The NNMT possesses a capacity retention rate of 60.66% after 150 cycles, much higher than the initial NNM's 18.96%. It also exhibits an excellent discharge capacity of 86.8 mAh g-1 at 5 C. In conclusion, the cycling and rate performance of the Ti-substituted NNMT are greatly improved without capacity loss, which offers innovative concepts for the modification means of the SIBs layered oxide cathode materials.

Realizing long-term cycling stability of O3-type layered oxide cathodes for sodium-ion batteries

O3-type layered oxide cathodes are promising for practical sodium-ion batteries (SIBs) owing to their high theoretical capacity, facile synthesis, and sufficient Na+ storage. However, they face challenges such as rapid capacity loss and poor cycling stability, mainly attributed to irreversible phase transitions. To address these challenges, a novel cathode material, Li/Sn co-substituted O3-Na0.95Li0.07Sn0.01Ni0.22Fe0.2Mn0.5O2 (LSNFM), has been designed by regulating the electronic structure, in which Li+ activates more redox reactions of Ni2+/3+ and Fe3+/4+ above 2.5 V and suppresses the redox reactivity of Mn3+/4+ below 2.5 V, while Sn4+ can prevent the charge delocalization in the transition metal layer, contributing to structural stability. Due to this synergistic effect, the as-prepared LSNFM electrode with high structural reversibility displays a 27.2% capacity increase contributed by the high-voltage transition metal ion redox activity and exhibits excellent long-term cycling stability, an 84.0% capacity retention after 500 cycles at 1 C and an 84.7% capacity retention after 2000 cycles at 5 C. The fundamental mechanism is fully investigated using systematic in situ/ex situ characterization techniques and density functional theory computations. This work provides a paradigm for designing long-term cycle life cathode materials by synergistically regulating the electronic structure in practical SIBs.

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 a Deeply Desodiated Stabilized Cathode Material by the High Entropy Strategy for Sodium-ion Batteries

Manganese-based layered oxides are currently of significant interest as cathode materials for sodium-ion batteries due to their low toxicity and high specific capacity. However, the practical applications are impeded by sluggish intrinsic Na+ migration and poor structure stability as a result of Jahn-Teller distortion and complicated phase transition. In this study, a high-entropy strategy is proposed to enhance the high-voltage capacity and cycling stability. The designed P2-Na0.67Mn0.6Cu0.08Ni0.09Fe0.18Ti0.05O2 achieves a deeply desodiation and delivers charging capacity of 158.1 mAh g-1 corresponding to 0.61 Na with a high initial Coulombic efficiency of 98.2 %. The charge compensation is attributed to the cationic and anionic redox reactions conjunctively. Moreover, the crystal structure is effectively stabilized, leading to a slight variation of lattice parameters. This research carries implications for the expedited development of low-cost, high-energy-density cathode materials for sodium-ion batteries.

Polypyrrole-coated sodium manganate microspheres cathode for superior performance Sodium-ion batteries

P2-Na0.67Mn0.67Ni0.33O2 is a promising cathode material for sodium ion batteries (SIBs) due to its low cost, high theoretical capacity, and non-toxicity. However, it still suffers from unsatisfactory cycling stability mainly incurred by the Jahn-Teller effect of Mn3+ and electrolyte decomposition on the electrode/electrolyte interface. Herein, the P2-Na0.67Ni0.33Mn0.67O2@PPy (NNMO@PPy) composite applied as cathode materials for SIBs is obtained by introducing conductive polypyrrole (PPy) as coating layer on the P2-Na0.67Ni0.33Mn0.67O2 (NNMO) microspheres. Numerous physical characterization methods indicate that the PPy layer was uniformly coated on the surface of NNMO microspheres without change in phase structure and morphology. The PPy coating layer can alleviate Mn dissolution and effectively suppress the side reactions between the electrolyte and electrode during cycling. The optimal NNMO@PPy-9 with 9 wt% PPy delivers a high capacity of 127.4 mAh/g at the current density at 150 mA g-1, an excellent cyclic stability with high capacity retention of 80.5 % after 300 cycles, and enhanced rate performance (169.3 mAh/g at 15 mA g-1 while 89.8 mAh/g at 600 mA g-1). Furthermore, hard carbon (-)//NNMO@PPy-9 (+) full cell delivers a high energy density of 305.1 Wh kg-1 and superior cycling stability with 88.2 % capacity retention after 150 cycles. In-situ X-ray diffraction experiment and electrochemical characterization verify the highly reversible structure evolution and robust P2-type phase structure of NNMO@PPy-9 for fast and stable Na+ diffusion. This effective strategy of using conductive PPy as a coating layer may provide a new insight to modify NNMO surface, improving the cycling stability and rate capability.

Developing an abnormal high-Na-content P2-type layered oxide cathode with near-zero-strain for high-performance sodium-ion batteries

Layered transition metal oxides (NaxTMO2) possess attractive features such as large specific capacity, high ionic conductivity, and a scalable synthesis process, making them a promising cathode candidate for sodium-ion batteries (SIBs). However, NaxTMO2 suffer from multiple phase transitions and Na+/vacancy ordering upon Na+ insertion/extraction, which is detrimental to their electrochemical performance. Herein, we developed a novel cathode material that exhibits an abnormal P2-type structure at a stoichiometric content of Na up to 1. The cathode material delivers a reversible capacity of 108 mA h g-1 at 0.2C and 97 mA h g-1 at 2C, retaining a capacity retention of 76.15% after 200 cycles within 2.0-4.3 V. In situ diffraction studies demonstrated that this material exhibits an absolute solid-solution reaction with a low volume change of 0.8% during cycling. This near-zero-strain characteristic enables a highly stabilized crystal structure for Na+ storage, contributing to a significant improvement in battery performance. Overall, this work presents a simple yet effective approach to realizing high Na content in P2-type layered oxides, offering new opportunities for high-performance SIB cathode materials.