Unraveling the Synergistic Effect of Mg and Ti Codoping to Realize an Ordered Structure and Excellent Performance for Sodium-Ion Batteries

Improving High-Voltage Cycling Stability and High-Rate Capability of Sodium-Ion Layered Cathode Oxides through Trace Amounts of Low-Valence Metals

With the increasing demand for clean energy sources, the need for large-scale energy storage systems to ensure the stable output of renewable energy sources, such as wind and solar, has also increased. Sodium-ion batteries have emerged as a potential solution for these storage systems owing to their high energy density, abundance in the Earth's crust, and low cost. However, the larger atomic radius of sodium ions results in higher energy barriers for ion migration in cathode materials, which can affect the cycle life and rate performance of the battery. Therefore, developing a suitable structure that facilitates rapid sodiation and desodiation and maintains good cycling stability remains a significant challenge. This study aimed to reduce the content of trivalent manganese ions and minimize the impact of the Jahn-Teller effect to enhance the capacity retention of manganese-based layered oxides. Additionally, a series of P2-type Na0.78Li0.1ZnxNi0.15-xMn0.75O2 compounds were successfully synthesized through doping with divalent zinc ions. Structural analyses of the doped material indicated that Zn doping did not alter the crystal structure but increased the interlayer distance of the transition metals. Electrochemical performance tests revealed that appropriate Zn2+ doping promoted sodium-ion diffusion and improved the reversible capacity of the battery. This study provides a promising approach for developing sodium-ion batteries with rapid charging and discharging capabilities.

Recent progress in high-voltage P2-Na x TMO2 materials and their future perspectives

P2-type layered materials (Na x TMO2) have become attractive cathode electrodes owing to their high theoretical energy density and simple preparation. However, they still face severe phase transition and low conductivity. Current research on Na x TMO2 is mostly focused on the modification of bulk materials, and the application performances have been infrequently addressed. This review summarizes the information on current common P2-Na x TMO2 materials and discusses their sodium-storage mechanisms. Furthermore, modification strategies to improve their performance are addressed for practical applications based on a range of key parameters (output voltage, specific capacity, and lifespan). We also discuss the future development trends and application prospects for P2 cathode materials.

Enabling Rapid and Stable Sodium Storage via a P2-Type Layered Cathode with High-Voltage Zero-Phase Transition Behavior

Currently, a major target in the development of Na-ion batteries is the concurrent attainment of high-rate capacity and long cycling stability. Herein, an advanced Na-ion battery with high-rate capability and long cycle stability based on Li/Ti co-doped P2-type Na0.67Mn0.67Ni0.33O2, a host material with high-voltage zero-phase transition behavior and fast Na+ migration/conductivity during dynamic de-embedding process, is constructed. Experimental results and theoretical calculations reveal that the two-element doping strategy promotes a mutually reinforcing effect, which greatly facilitates the transfer capability of Na+. The cation Ti4+ doping is a dominant high voltage, significantly elevating the operation voltage to 4.4 V. Meanwhile, doping Li+ shows the function in charge transfer, improving the rate performance and prolonging cycling lifespan. Consequently, the designed P2-Na0.75Mn0.54Ni0.27Li0.14Ti0.05O2 cathode material exhibits discharge capacities of 129, 104, and 85 mAh g- 1 under high voltage of 4.4 V at 1, 10, and 20 C, respectively. More importantly, the full-cell delivers a high initial capacity of 198 mAh g-1 at 0.1 C (17.3 mA g-1) and a capacity retention of 73% at 5 C (865 mA g-1) after 1000 cycles, which is seldom witnessed in previous reports, emphasizing their potential applications in advanced energy storage.

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.

Enabling an Excellent Ordering-Enhanced Electrochemistry and a Highly Reversible Whole-Voltage-Range Oxygen Anionic Chemistry for Sodium-Ion Batteries

Though considerable Mg-doped layered cathodes have been exploited, some new differences relative to previous reports can be concluded by doping a heavy dose of Mg via two rational strategies. Unlike the common unit cell of the P6₃/mmc group by X-ray diffraction, neutron diffraction reveals a large supercell of the P6₃ group and enhanced ordering for Na_11/18Mg_1/18[Ni_1/4Mg_1/9Mn_11/18]O₂ with Mg occupying both the Na and Mn sites. Compared with only one obvious voltage plateau of Na_0.5[Ni_0.25Mn_0.75]O₂ (NNM), Na_11/18Mg_1/18[Ni_1/4Mg_1/9Mn_11/18]O₂ (NMNMM) shows more severe voltage plateaus but with excellent electrochemical performance. Na_0.5[Mg_0.25Mn_0.75]O₂ (NMM) with Mg only occupying the Ni site displays a highly reversible whole-voltage-range oxygen redox chemistry and smooth voltage curves without any voltage hysteresis. Cationic Ni²⁺/Ni⁴⁺ couples are responsible for the charge compensations of NNM and NMNMM, while only the oxygen anionic reaction accounts for the capacity of NMM between 2.5 and 4.3 V. Interestingly, the Mn³⁺/Mn⁴⁺ pair contributes all capacity for all cathodes between 1.5 and 2.5 V. All cathodes undergo a double-phase mechanism: an irreversible P2-O2 phase transition for NNM, an enhanced reversible P2-O2 phase transition for NMNMM, and a highly reversible P2-OP4 phase transition for NMM. In addition, the designed cathodes display excellent rate capability and long-term cycling stability but with a large difference in the various voltage ranges of 2.5-4.3 and 1.5-2.5 V, respectively. This work provides a good understanding of ion doping and some new insights into exploiting high-performance materials.

Realizing High-Performance Cathodes with Cationic and Anionic Redox Reactions in High-Sodium-Content P2-Type Oxides for Sodium-Ion Batteries

P2-type layered transition-metal oxides with anionic redox reactions are promising cathodes for sodium-ion batteries. In this work, a high-sodium-content P2-type Na7/9Li1/9Mg1/9Cu1/9Mn2/3O2 (NLMC) cathode material is prepared by substituting Li/Mg/Cu for Mn sites in Na2/3MnO2. The Li/Mg ions trigger the anionic redox reaction, while the Cu ions enhance the structure stability during electrochemical cycling. As a result, the oxide has a high reversible capacity of 225 mAh g-1 originating from both cationic and anionic redox activities with a capacity retention of 77% after 100 cycles. The migration energy barrier and Na ion diffusion kinetics are studied using density functional theory (DFT) calculations and the galvanostatic intermittent titration technique. Furthermore, X-ray diffraction, DFT, scanning electron microscopy, and transmission electron microscopy are applied to reveal the structural evolution and charge compensation of NLMC, providing a thorough understanding of the structural and morphology evolution of Na-deficient oxides during cycling. The results are inspiring for the design of a high-Na content P2-type layered oxide cathode for sodium-ion batteries.

In-Plane BO3 Configuration in P2 Layered Oxide Enables Outstanding Long Cycle Performance for Sodium Ion Batteries

P2-phase layered cathode materials with distinguished electrochemical performance for sodium-ion batteries have attracted extensive attention, but they face critical challenges of transition metal layer sliding and unfavorable formation of hydration phase upon cycling, thus showing inferior long cycle life. Herein, a new approach is reported to modulate the local structure of P2 material by constructing a state-of-the-art in-plane BO₃ triangle configuration ((Na_0.67 Ni_0.3 Co_0.1 Mn_0.6 O_1.94 (BO₃ )_0.02 ). Both are unveiled experimentally and theoretically that such a structure can serve as a robust pillar to hold up the entire structure, by inhibiting the H₂ O insertion upon Na (de)intercalation and preventing the structure from deformation, which significantly boost the long cycle capability of P2-materials. Meanwhile, more Na ions in the architecture are enabled to site on the edge sharing octahedrons (Na_e ), thus benefiting the Na⁺ transportation. Consequently, the as produced material demonstrates an ultralow volume variation (1.8%), and an outstanding capacity retention of 80.1% after 1000 cycles at 2 C. This work sheds light on efficient architecture modulation of layered oxides through proper nonmetallic element doping.