Origins of Irreversibility in Layered NaNixFeyMnzO2 Cathode Materials for Sodium Ion Batteries

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.

Improving Fast-Charging Capability of High-Voltage Spinel LiNi0.5Mn1.5O4 Cathode under Long-Term Cyclability through Co-Doping Strategy

Co-free spinel LiNi0.5Mn1.5O4 (LNMO) is emerging as a promising contender for designing next generation high-energy-density and fast-charging Li-ion batteries, due to its high operating voltage and good Li+ diffusion rate. However, further improvement of the Li+ diffusion ability and simultaneous resolution of Mn dissolution still pose significant challenges for their practical application. To tackle these challenges, a simple co-doping strategy is proposed. Compared to Pure-LNMO, the extended lattice in resulting LNMO-SbF sample provides wider Li+ migration channels, ensuring both enhanced Li+ transport kinetics, and lower energy barrier. Moreover, Sb creating structural pillar and stronger TM─F bond together provides a stabilized spinel structure, which stems from the suppression of detrimental irreversible phase transformation during cycling related to Mn dissolution. Benefiting from the synergistic effect, the LNMO-SbF material exhibits a superior reversible capacity (111.4 mAh g-1 at 5C, and 70.2 mAh g-1 after 450 cycles at 10C) and excellent long-term cycling stability at high current density (69.4% capacity retention at 5C after 1000 cycles). Furthermore, the LNMO-SbF//graphite full cell delivers an exceptional retention rate of 96.9% after 300 cycles, and provides a high energy density at 3C even with a high loading. This work provides valuable insight into the design of fast-charging cathode materials for future high energy density lithium-ion batteries.

Progress and perspectives on iron-based electrode materials for alkali metal-ion batteries: a critical review

Iron-based materials with significant physicochemical properties, including high theoretical capacity, low cost and mechanical and thermal stability, have attracted research attention as electrode materials for alkali metal-ion batteries (AMIBs). However, practical implementation of some iron-based materials is impeded by their poor conductivity, large volume change, and irreversible phase transition during electrochemical reactions. In this review we critically assess advances in the chemical synthesis and structural design, together with modification strategies, of iron-based compounds for AMIBs, to obviate these issues. We assess and categorize structural and compositional regulation and its effects on the working mechanisms and electrochemical performances of AMIBs. We establish insight into their applications and determine practical challenges in their development. We provide perspectives on future directions and likely outcomes. We conclude that for boosted electrochemical performance there is a need for better design of structures and compositions to increase ionic/electronic conductivity and the contact area between active materials and electrolytes and to obviate the large volume change and low conductivity. Findings will be of interest and benefit to researchers and manufacturers for sustainable development of advanced rechargeable ion batteries using iron-based electrode materials.

Defensive and Ion Conductive Surface Layer Enables High Rate and Durable O3-type NaNi1/3 Fe1/3 Mn1/3 O2 Sodium-Ion Battery Cathode

Na-based layered transition metal oxides with an O3-type structure are considered promising cathodes for sodium-ion batteries. However, rapid capacity fading, and poor rate performance caused by serious structural changes and interfacial degradation hamper their use. In this study, a NaPO3 surface modified O3-type layered NaNi1/3 Fe1/3 Mn1/3 O2 cathode is synthesized, with improved high-voltage stability through protecting layer against acid attack, which is achieved by a solid-gas reaction between the cathode particles and gaseous P2 O5 . The NaPO3 nanolayer on the surface effectively stabilizes the crystal structure by inhibiting surface parasitic reactions and increasing the observed average voltage. Superior cyclic stability is exhibited by the surface-modified cathode (80.1% vs 63.6%) after 150 cycles at 1 C in the wide voltage range of 2.0 V-4.2 V (vs Na+ /Na). Moreover, benefiting from the inherent ionic conduction of NaPO3 , the surface-modified cathode presents excellent rate capability (103 mAh g-1  vs 60 mAh g-1 ) at 10 C. The outcome of this study demonstrates a practically relevant approach to develop high rate and durable sodium-ion battery technology.

NASICON-Type NaTi2(PO4)3 Surface Modified O3-Type NaNi0.3Fe0.2Mn0.5O2 for High-Performance Cathode Material for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have shown great potential as energy storage devices due to their low price and abundant sodium content. Among them, O3-type layered oxides are a promising cathode material for sodium-ion batteries; however, most of them suffer from slow kinetics and unfavorable structural stability, which seriously hinder their practical application. O3-NaNi0.3Fe0.2Mn0.5O2 surface modification is performed by a simple wet chemical method of coating NaTi2(PO4)3 on the surface. The NASICON-type NaTi2(PO4)3 coating layer has a special three-dimensional channel, which facilitates the rapid migration of Na+, and the NaTi2(PO4)3 coating layer also prevents direct contact between the electrode and the electrolyte, ensuring the stability of the interface. In addition, the NaTi2(PO4)3 coating layer induces part of the Ti4+ doping into the transition metal layer of NaNi0.3Fe0.2Mn0.5O2, which increases the stability of the transition metal layer and reduces the resistance of Na+ diffusion. More importantly, the NaTi2(PO4)3 coating layer can suppress the O3-P3 phase transition and reduce the volume change of the materials throughout the charge/discharge process. Thus, the NaTi2(PO4)3 coating layer can effectively improve the electrochemical performance of the cathode materials. The NFM@NTP3 has a capacity retention of 86% (2.0-4.0 V vs Na+/Na, 300 cycles) and 85% (2.0-4.2 V vs Na+/Na, 100 cycles) at 1C and a discharge capacity of 107 mAh g-1 (2.0-4.0 V vs Na+/Na) and 125 mAh g-1 (2.0-4.2 V vs Na+/Na) at 10C, respectively. Therefore, this surface modification strategy provides a simple and effective way to design and develop high-performance layered oxide cathode materials for sodium-ion batteries.

Multifunctional Effect of Fe Substitution in Na Layered Cathode Materials for Enhanced Storage Stability

Developing stable cathode materials that are resistant to storage degradation is essential for practical development and industrial processing of Na-ion batteries as many sodium layered oxide materials are susceptible to hygroscopicity and instability upon exposure to ambient air. Among the various layered compounds, Fe-substituted O3-type Na(Ni1/2Mn1/2)1-xFexO2 materials have emerged as a promising option for high-performance and low-cost cathodes. While previous reports have noted the decent air-storage stability of these materials, the role and origin of Fe substitution in improving storage stability remain unclear. In this study, we investigate the air-resistant effect of Fe substitution in O3-Na(Ni1/2Mn1/2)1-xFexO2 cathode materials by performing systematic surface and structural characterizations. We find that the improved storage stability can be attributed to the multifunctional effect of Fe substitution, which forms a surface protective layer containing an Fe-incorporated spinel phase and decreases the thermodynamical driving force for bulk chemical sodium extraction. With these mechanisms, Fe-containing cathodes can suppress the cascades of cathode degradation processes and better retain the electrochemical performance after air storage.

High-Voltage Stabilization of O3-Type Layered Oxide for Sodium-Ion Batteries by Simultaneous Tin Dual Modification

O3-type layered oxide materials are considered to be a highly suitable cathode for sodium-ion batteries (NIBs) due to their appreciable specific capacity and energy density. However, rapid capacity fading caused by serious structural changes and interfacial degradation hampers their use. A novel Sn-modified O3-type layered NaNi_1/3Fe_1/3Mn_1/3O₂ cathode is presented, with improved high-voltage stability through simultaneous bulk Sn doping and surface coating in a scalable one-step process. The bulk substitution of Sn⁴⁺ stabilizes the crystal structure by alleviating the irreversible phase transition and lattice structure degradation and increases the observed average voltage. In the meantime, the nanolayer Sn/Na/O composite on the surface effectively inhibits surface parasitic reactions and improves the interfacial stability during cycling. A series of Sn-modified materials are reported. An 8%-Sn-modified NaNi_1/3Fe_1/3Mn_1/3O₂ cathode exhibits a doubling in capacity retention increase after 150 cycles in the wide voltage range of 2.0-4.1 V vs Na/Na⁺ compared to none, and 81% capacity retention is observed after 200 cycles in a full cell vs hard carbon. This work offers a facile process to simultaneously stabilize the bulk structure and interface for the O3-type layered cathodes for sodium-ion batteries and raises the possibility of similar effective strategies to be employed for other energy storage materials.

A Rational Biphasic Tailoring Strategy Enabling High-Performance Layered Cathodes for Sodium-Ion Batteries

Layered oxide cathodes usually exhibit high compositional diversity, thus providing controllable electrochemical performance for Na-ion batteries. These abundant components lead to complicated structural chemistry, closely affecting the stacking preference, phase transition and Na⁺ kinetics. With this perspective, we explore the thermodynamically stable phase diagram of various P2/O3 composites based on a rational biphasic tailoring strategy. Then a specific P2/O3 composite is investigated and compared with its monophasic counterparts. A highly reversible structural evolution of P2/O3-P2/O3/P3-P2/P3-P2/Z/O3'-Z/O3' based on the Ni²⁺ /Ni^3.5+ , Fe³⁺ /Fe⁴⁺ and Mn^3.8+ /Mn⁴⁺ redox couples upon sequential Na extraction/insertion is revealed. The reduced structural strain at the phase boundary alleviates the phase transition and decreases the lattice mismatch during cycling, endowing the biphasic electrode a large reversible capacity of 144 mAh g^-1 with the energy density approaching 514 Wh kg^-1 .

Spatial and Temporal Analysis of Sodium-Ion Batteries

As a promising alternative to the market-leading lithium-ion batteries, low-cost sodium-ion batteries (SIBs) are attractive for applications such as large-scale electrical energy storage systems. The energy density, cycling life, and rate performance of SIBs are fundamentally dependent on dynamic physiochemical reactions, structural change, and morphological evolution. Therefore, it is essential to holistically understand SIBs reaction processes, degradation mechanisms, and thermal/mechanical behaviors in complex working environments. The recent developments of advanced in situ and operando characterization enable the establishment of the structure-processing-property-performance relationship in SIBs under operating conditions. This Review summarizes significant recent progress in SIBs exploiting in situ and operando techniques based on X-ray and electron analyses at different time and length scales. Through the combination of spectroscopy, imaging, and diffraction, local and global changes in SIBs can be elucidated for improving materials design. The fundamental principles and state-of-the-art capabilities of different techniques are presented, followed by elaborative discussions of major challenges and perspectives.