P2-type Cathode Research Articles

High Na-content P2 and P2-O3 layered oxide cathodes for high performance Na-ion batteries

P2-NaxTMO2, (x ≤0.67, TM-transition metal) oxides are promising cathode materials for Na-ion batteries, however they suffer from Na+-vacancy ordering and huge structural transformations (P2→O2→OP4). In contrast, high Na content in P2-type cathodes enables high structural stability and promotes active elements to their lower oxidation state and thereby achieving high capacity and cycling stability. Herein, light-weight elements boron and lithium doped in P2-type Na0.67Ni0.33Mn0.67O2 using cationic potential approach, aiming to improve the Na content. Li-B doped Na0.8Ni0.3Mn0.6Li0.05B0.05O2 delivers a capacity of 85 mAh/g and excellent capacity retention (88 % after 500 cycles). The effects of Li-B doping in P2-type Na0.67Ni0.33Mn0.67O2 are systematically investigated.

Compositionally graded high-voltage P2-type cathode with superior structural stability and redox kinetics for advanced Na-ion batteries

Compositionally graded high-voltage P2-type cathode with superior structural stability and redox kinetics for advanced Na-ion batteries

High-Performance Zr-Doped P3-Type Na0.67Ni0.33Mn0.67O2 Cathode for Na-Ion Battery Applications

Sodium-ion battery (SIB) technology started to bloom along with lithium-ion batteries (LIBs) as a supportive energy source to alleviate the cost of lithium sources for the development of energy storage devices and electric vehicles. Layered cathode materials are considered potential candidates to produce high-energy-density batteries. Among the layered cathode materials, P3-type cathodes are the least investigated in spite of their capacities, which are comparable to those of P2-type cathodes. P3-type cathodes show high polarization, leading to a poor cycle life, which impedes their extensive use in practical applications. In this work, we report on zirconium doping as an effective strategy to improve cycling stability and reduce voltage fading, another serious issue of layered cathode materials. It is found that an optimum composition of the P3-type cathode with Zr doping at the Mn site, leading to a composition of Na0.67Ni0.33Mn0.64Zr0.033O2, shows good electrochemical performance in terms of retention (89% after 100 cycles) when compared to Na0.67Ni0.33Mn0.60Zr0.067O2 (85% after 100 cycles) and an undoped sample (83% after 100 cycles). Also, remarkable performance is delivered by the Na0.67Ni0.33Mn0.64Zr0.033O2 sample, with a retention rate of 72% after 450 cycles. This result is also supported by an analysis of the amount of polarization for undoped and doped samples, which found that doping helps in improving the diffusion of ions, and the least polarization is obtained for the Na0.67Ni0.33Mn0.64Zr0.033O2 sample.

Activating Mn Redox for Boosting Performance Enabled by Highly Oxidized Ru in a P2-Type Cathode Material for Sodium-Ion Batteries

To overcome the inherent limitations of sodium-ion batteries (SIBs) such as relatively low energy density and operating voltage compared to lithium-ion batteries (LIBs), Oxygen redox chemistry is considered to realizing a high-performance battery system. However, oxygen redox commonly accompanied problems such as sluggish kinetics, O2 gas loss, and irreversible structural rearrangement with voltage hysteresis. Herein, we suggest a new model that deviates from the tendency of relying on oxygen redox reaction to realize high capacity. A deep investigation of Na0.6Mg0.2Mn0.8O2 (NMM) and Na0.6Mg0.2Ru0.2Mn0.6O2 (NMRM) is conducted by various analysis tools for the comparison. The highly oxidized Ru5+ lowers the Mn oxidation state in the structure without the Jahn-Teller effect. It drives the increased participation of Mn in the overall electrochemical reaction and the charge compensation by oxygen redox reaction is mostly replaced by the Mn3+/Mn4+ redox pair. A strong covalence of Ru stabilizes the structure and prevents cation migration from TM to Na slap. As a result, even though the specific weight increased, practical capacity was improved by maximizing the participation of Na+ ions, and we confirm that cyclability and rate capability also improved.

Injecting Excess Na into a P2-Type Layered Oxide Cathode to Achieve Presodiation in a Na-Ion Full Cell.

The initial Na loss limits the theoretical specific capacity of cathodes in Na-ion full cell applications, especially for Na-deficient P2-type cathodes. In this study, we propose a presodiation strategy for cathodes to compensate for the initial Na loss in Na-ion full cells, resulting in a higher specific capacity and a higher energy density. By employing an electrochemical presodiation approach, we inject 0.32 excess active Na into P2-type Na0.67Li0.1Fe0.37Mn0.53O2 (NLFMO), aiming to compensate for the initial Na loss in hard carbon (HC) and the inherent Na deficiency of NLFMO. The structure of the NLFMO cathode converts from P2 to P'2 upon active Na injection, without affecting subsequent cycles. As a result, the HC||NLFMOpreNa full cell exhibits a specific capacity of 125 mAh/g, surpassing the value of 61 mAh/g of the HC||NLFMO full cell without presodiation due to the injected active Na. Moreover, the presodiation effect can be achieved through other engineering approaches (e.g., Na-metal contact), suggesting the scalability of this methodology.

A thermodynamically stable O2-type cathode with reversible O2-P2 phase transition for advanced sodium-ion batteries

Low-cost sodium-ion batteries (SIBs) have shown very promise in the applications of renewable energy and low-speed electric vehicles. The development of a new O2-type cathode in SIBs is very challenging in that this compound is only stable as an intermediate product of P2-type oxides during redox reactions. Here, we report a thermodynamically stable O2-type cathode obtained by Na/Li ion exchange from P2-type oxide in a binary molten salt system. It is demonstrated that the as-prepared O2-type cathode exhibits a highly reversible O2–P2 phase transition during Na+ de-intercalation. The unusual O2–P2 transition has a low volume change of ∼11%, much lower than that of 23.2% for P2–O2 transformation in the P2-type cathode. The lowered lattice volume change of this O2-type cathode gives rise to superior structural stability upon cycling. Therefore, the O2-type cathode possesses a reversible capacity of about 100 mAh/g with a good capacity retention of 87.3% even after 300 cycles at 1C, indicating outstanding long-term cycling stability. These achievements will promote the development new class of cathode materials with high capacity and structural stability for advanced SIBs.

Suppressing the formation of OP4 phase in P2-Structured Na0.67Ni0.1Co0.1Mn0.8O2 by in-situ formed NiF2 layer

The P2-structured sodium transition metal oxides suffer from an irreversible phase transition to OP4, which results in the -1.3 Å shrinking of lattice c and locks the following electrochemical cycles between OP4 and P2’, seriously aggravating the cracking of cathode materials. Therefore, strategies to suppress the phase transition from P2 to OP4 are currently pursued to alleviate the destructively volumetric changes. Here, the DFT calculations reveal that the transition to OP4 phase is not driven by thermodynamical mechanism but is induced by structural defects. To verify this prediction, an in-situ formed NiF2 layer via electrolyte additives is coated on the P2-structured Na0.67Ni0.1Co0.1Mn0.8O2 during initial charging, which mitigates the generation of structural defects from the side reaction of electrolyte with active sites of cathode particles. The sample protected with NiF2 layer presents a continuous, moderate and reversible structural evolution, free from phase P2’. Therefore, the coated sample delivers 230.1 mAh/g at 0.1C and retains 83.4% capacity at 0.5C after 150 cycles, significantly higher than the values, 174.5 mAh/g and 63.4%, respectively, of the pristine materials. This work demonstrates the severe phase transition in P2-type cathode can be effectively alleviated by a simple surface modification.

Improved electrochemical performance of P2-type concentration-gradient cathode material Na0.65Ni0.16Co0.14Mn0.7O2 with Mn-rich core for sodium-ion batteries

The full concentration-gradient Na0.65Ni0.16Co0.14Mn0.7O2 (CG-NCM), in which the composition varies gradiently from core Na0.65Ni0.01Co0.01Mn0.98O2 to shell Na0.65Ni0.31Co0.27Mn0.42O2, was prepared by a co-precipitation method plus high-temperature calcination, and was investigated by XRD, SEM, TEM and electrochemical measurements. Benefiting from the special concentration-gradient core-shell structure, CG-NCM possesses higher diffusion coefficients of Na+ and the consequently better rate capability and cyclability than concentration-constant Na0.65Ni0.16Co0.14Mn0.7O2 (CC-NCM). Galvanostatically cycled at 30 mA g−1 between 1.5 and 4.1 V (vs Na+/Na), CC-NCM and CG-NCM have initial discharge capacities of 168.5 and 162.5 mA h g−1, and the capacity retentions of 11.5 % and 51.4 % after 100 cycles, respectively. CG-NCM presents high initial discharge capacities of 144.5, 138.2 and 121.8 mA h g−1 at 75, 300 and 750 mA g−1, respectively, and exhibits capacity retentions of 70.6 %, 73.2 % and 76.9 % after 100 cycles, respectively. However, CC-NCM shows initial discharge capacities of 149.5, 136.5 and 107.3 mA h g−1 at 75, 300 and 750 mA g−1, respectively, and the matching capacity retentions are as low as 49.8 %, 52.3 % and 53.9 %, respectively. The results indicate that the construction of concentration-gradient oxide electrode materials is an effective strategy for improvement of the electrochemical performance of oxides cathodes.

Exploring Optimal Li-ion Substitution for High Na-content P2-Na0.67+a [Li x Ni0.33−y Mn0.67−z ]O2 Cathodes for Sodium-ion Batteries

P2-type Na0.67[Ni0.33Mn0.67]O2 layered oxide cathode is attractive for practical Na-ion battery application due to its high voltage Ni4+/Ni2+ redox and good air-stability. However, it suffers from rapid capacity decay originating from high voltage P2-O2 transformation. To alleviate this issue, herein we explore optimum Li-substitution into the transition metal layer of Na0.67[Ni0.33Mn0.67]O2 to tailor a series of high Na-content P2-type cathodes. Among them, Na0.85[Li0.14Ni0.29Mn0.57]O2 cathode with optimal Li-substitution exhibits reversible capacities of ∼168 mAh g−1 at 0.1C rate and good cycling stability (82% of retention after 100 cycles at 1C rate). In-situ XRD measurement reveals the formation of complete solid-solution and X-ray absorption spectroscopy studies confirm the participation of Ni4+/Ni2+ and Mn4+/Mn3+ redox couples during Na (de)intercalation of the Na0.85[Li0.14Ni0.29Mn0.57]O2. A full Na-ion cell (Na0.85[Li0.14Ni0.29Mn0.57]O2||hard carbon) is demonstrated with an energy density of 420 Wh kg−1.

Suppressing staircase-like electrochemical profile induced by P[sbnd]O transition by solid-solution reaction with continuous structural evolution in layered Na-ion battery cathode

Layered P2-type sodium manganese oxide materials (NaxMnO2) attract substantial interest due to their low cost and high security. However, the staircase-like electrochemical profiles in P2-type NaxMnO2 caused by unfavorable phase transitions and layer gliding upon sodium extraction/re-insertion, particularly from P-type stacking to O-type stacking (e. g. P2 to O2 or OP4), are difficult to be eliminated. In this work, the staircase-like electrochemical profiles induced by PO transition are effectively suppressed in P’2-type Na2/3Mn0.8Ti0.2O2 by a solid-solution reaction with continuous structural evolution during the charge/discharge processes. P’2-Na2/3Mn0.8Ti0.2O2 exhibits smooth voltage profiles and delivers a high specific capacity of 153 mA h g−1 within 2.0–4.0 V. Structural characterizations indicate that P’2-Na2/3Mn0.8Ti0.2O2 undergoes a solid-solution reaction with continuous phase transition from P’2 phase to so-called “Z” phase, considered as an intergrowth of P-type stacking and O-type stacking in the layered lattice. The structural evolution mechanism described in this work provides a solution to suppress the staircase-like electrochemical profiles induced by PO transition in P2-type oxide cathode materials and a versatile strategy for boosting the performance of P2-type cathode materials.

Ti, F Codoped Sodium Manganate of Layered P2-Na 0.7 MnO 2.05 Cathode for High Capacity and Long-Life Sodium-Ion Battery

Ti, F Codoped Sodium Manganate of Layered P2-Na _0.7 MnO _2.05 Cathode for High Capacity and Long-Life Sodium-Ion Battery

Double sites doping local chemistry Adjustment: A Multiple-Layer oriented P2-Type cathode with Long-life and Water/Air stability for sodium ion batteries

P2-type Ni/Mn layered oxides have attracted intensive interest as a kind of promising cathode materials because of their high specific capacities and unique 2D Na+ diffusion path. However, the poor stability and irreversible phase transition at high voltage during charge and discharge process greatly hinders its practical applications. Herein, a novel double site substitution strategy is reported that large-sized K+ is riveted in the prismatic Na+ sites and Cu2+ is occupies in the transition metal sites. The K+ doping lead to the lattice expansion along c-axis, which facilitating sodium ions transport. In addition, the double sites substitution improves the stability of the materials exposed in air and water environments. A combined analysis of first-principles calculation simulation and experimental verification determining that larger interlayer spacing decreases the diffusion barrier of sodium ions. The as-prepared multiple-layer oriented P2-K0.05Na0.67Mn0.6Ni0.3Cu0.1O2 shows excellent rate performance, fast sodium ion transport ability and superior cycling performance with a high-capacity retention of 91.2 % at a high current density of 1 A g−1 after 1800th cycles. Therefore, this work provides a new insight for the design of stable and high-performance cathode materials for sodium ion batteries.

Entropy modulation strategy of P2-type layered transition metal oxide cathodes for sodium-ion batteries with a high performance

Configuration entropy is increased by doping with multiple cations, whereby the material defects and active sites are increased and phase transition is inhibited at high voltage. Meanwhile, the sodium-ion diffusion rate was improved with the co-doping strategy.

Slight compositional variation-induced structural disorder-to-order transition enables fast Na+ storage in layered transition metal oxides

The omnipresent Na+/vacancy orderings change substantially with the composition that inevitably actuate the ionic diffusion in rechargeable batteries. Therefore, it may hold the key to the electrode design with high rate capability. Herein, the influence of Na+/vacancy ordering on Na+ mobility is demonstrated firstly through a comparative investigation in P2-Na2/3Ni1/3Mn2/3O2 and P2-Na2/3Ni0.3Mn0.7O2. The large zigzag Na+/vacancy intralayer ordering is found to accelerate Na+ migration in P2-type Na2/3Ni1/3Mn2/3O2. By theoretical simulations, it is revealed that the Na+ ordering enables the P2-type Na2/3Ni1/3Mn2/3O2 with higher diffusivities and lower activation energies of 200 meV with respect to the P3 one. The quantifying diffusional analysis further prove that the higher probability of the concerted Na+ ionic diffusion occurs in P2-type Na2/3Ni1/3Mn2/3O2 due to the appropriate ratio of high energy ordered Na ions (Naf) occupation. As a result, the interplay between the Na+/vacancy ordering and Na+ kinetic is well understood in P2-type layered cathodes.

Enhanced rate performance and specific capacity in Ti-substituted P2-type layered oxide enabled by crystal structure and particle morphology modifications

In recent years, P2-type layered oxides have received considerable attention as potential cathodes for Na-ion batteries owing to their compositional diversity, good specific capacity, and cyclability. However, poor rate performance and low-capacity retention at high discharge rates have limited their use in commercial battery applications. This study aims to mitigate this issue by synthesizing a series of structurally engineered P2-type cathode materials, through Ti substitution, with high cyclability and improved rate performance. P2-type Na0.70Ni0.20Cu0.15Mn(0.65-x)TixO2 were prepared through a sol–gel route and were characterized for their structural, electrical, and electrochemical properties In the Ti-substituted samples, the Rietveld refinement of XRD data revealed an increased size of the bottleneck area of the Na-O6 prism planes, while the SEM images showed a decrease in the aspect ratio of hexagon-type morphology of particles which facilitates faster Na-ion conduction through the material. These changes in the crystal structure and particle morphology induced by Ti substitution have significantly improved electrical and electrochemical performance compared to the parent material. The sample with x = 7.5% exhibited a specific capacity of 126 mAh/g at a discharge rate of 0.1C in the 2.00–4.25 V window, which was about 25% more than that of the undoped material. At a discharge rate of 1C, the specific capacity of Na0.70Ni0.20Cu0.15Mn0.575Ti0.075O2 was 97 mAh/g compared to 74 mAh/g for Na0.70Ni0.20Cu0.15Mn0.65O2 sample. Na0.70Ni0.20Cu0.15Mn(0.65-x)TixO2 samples also exhibited excellent cyclability, with over 95% of original capacity retained after 300 cycles. Complex impedance measurements corroborated the improved Na-ion conductivity in Ti-doped samples and an associated increase in Na-ion transference number from 0.86 for the sample with x = 0 to 0.97 for the sample with x = 0.075.

Promoting the performances of P2-type sodium layered cathode by inducing Na site rearrangement

The electrochemical performance and structural stability of sodium-ion battery is substantially dependent on the occupancy and distribution of Na+ in cathode materials. However, it is challenging to simultaneously regulate the occupancy and optimize the distribution of Na+ in cathodes for higher capacity and superior cyclability. Here we attempt to adjust the arrangement of Na+ in layered cathode materials by applying a combination approach, including enhancing the Na content, disrupting transition metal ordering and strengthening Na+-TMn+ electrostatic force. Through detailed structural characterizations on cathodes, it is revealed that the rearrangement of Na+ at two distinct Wyckoff positions can be realized in Li/Ti-codoped Na2/3Ni1/3Mn2/3O2 cathodes, contributing to outstanding rate performance and smooth kinetic process. In addition, the inhibited P2-O2 phase transition and intact lattice structure is closely related to the rearranged Na layer and strengthened transition metal slab, jointly resulting in excellent long cycling performance with 90.2% capacity retention after 200 cycles at 1 C (150 mA/g). This work sheds new light on the role of different Na sites and provides a universal and practical approach to adjusting the Na+ distribution in P2-type cathode materials.

Research progress of key materials and engineering exploration for Na-ion batteries

<p indent="0mm">Under the background of “Carbon Peaking and Carbon Neutrality”, Na-ion batteries (NIBs) have attracted much attention due to their advantages such as low cost, high safety, and excellent performance. Low-cost NIBs are beneficial supplements to Li-ion batteries and will show their special advantages in the field of energy storage. Nowadays, NIBs are at a critical point from laboratory exploration to industrialization promotion. Since 2011, the NIBs research group of the Institute of Physics, Chinese Academy of Sciences has been committed to the research and development of low-cost, high-safety and high-performance NIBs technologies. It has obtained more than 40 patents of core materials, and some patents have been authorized from the United States, Japan and the European Union. It is the first time in the world to propose Cu-based oxide cathode materials and low-cost anthracite-based anode materials with independent intellectual properties. In 2017, relying on the core patented technology, the first high-tech enterprise focusing on the development and manufacture of NIBs—HaiNa Battery Technology Co. Ltd. was established. Then, the world’s first example application of <sc>100 kW h</sc> and <sc>1 MW h</sc> NIBs energy storage systems was put forward. This paper firstly focuses on the series of progress in the layered oxide cathode materials made by the research group for NIBs, mainly including: (1) Cost-effective Cu-Fe-Mn layered cathode materials, (2) high-capacity lattice oxygen redox layered oxide cathodes, and (3) stable phase transition layered cathode materials (high entropy and high sodium content P2-type cathodes), and poly-anion compound phosphate cathode materials. In terms of anodes, a series of hard carbon anodes using different biomass materials with excellent sodium storage performance were synthesized, as well as the high-performance soft carbon anode materials based on anthracite and pitch. A long-cycle stable titanium-based oxide anode material was designed. In terms of electrolytes, high-salt-concentration aqueous electrolytes and low-salt-concentration inaqueous electrolytes were designed. In addition, NASICON (Na super ionic conductor) type solid electrolytes and polymer solid electrolytes were modified on purpose. For the mechanism investigation of electrode materials, this paper explores in detail the phase formation mechanism of layered cathode materials, the ordered/disordered arrangement of layered oxides, the sodium storage mechanism of titanium-based oxides, the design methods of new tunnel-type materials, the sodium storage mechanism of amorphous carbon materials, and the sodium storage mechanism of organic anode materials. Finally, in terms of the industrialization exploration of NIBs, the process in the industrialization exploration and example application of NIBs for our group was introduced. The future development directions of NIBs were put forward in order to promote the sustainable development and accelerate the commercial application of NIBs.

Entropy and crystal-facet modulation of P2-type layered cathodes for long-lasting sodium-based batteries

P2-type sodium manganese-rich layered oxides are promising cathode candidates for sodium-based batteries because of their appealing cost-effective and capacity features. However, the structural distortion and cationic rearrangement induced by irreversible phase transition and anionic redox reaction at high cell voltage (i.e., >4.0 V) cause sluggish Na-ion kinetics and severe capacity decay. To circumvent these issues, here, we report a strategy to develop P2-type layered cathodes via configurational entropy and ion-diffusion structural tuning. In situ synchrotron X-ray diffraction combined with electrochemical kinetic tests and microstructural characterizations reveal that the entropy-tuned Na0.62Mn0.67Ni0.23Cu0.05Mg0.07Ti0.01O2 (CuMgTi-571) cathode possesses more {010} active facet, improved structural and thermal stability and faster anionic redox kinetics compared to Na0.62Mn0.67Ni0.37O2. When tested in combination with a Na metal anode and a non-aqueous NaClO4-based electrolyte solution in coin cell configuration, the CuMgTi-571-based positive electrode enables an 87% capacity retention after 500 cycles at 120 mA g−1 and about 75% capacity retention after 2000 cycles at 1.2 A g−1.

Designing a P2-type cathode material with Li in both Na and transition metal layers for Na-ion batteries

P2-type layered oxides have been considered as promising cathode materials for Na-ion batteries, but the capacity decay resulting from the Na+/vacancy ordering and phase transformation limits their future large-scale applications. Herein, the impact of Li-doping in different layers on the structure and electrochemical performance of P2-type Na0.7Ni0.35Mn0.65O2 is investigated. It can be found that Li ions successfully enter both the Na and transition metal layers. The strategy of Li-doping can improve the cycling stability and rate capability of P2-type layered oxides, which promotes the development of high-performance Na-ion batteries.

Regulating Pseudo-Jahn–Teller Effect and Superstructure in Layered Cathode Materials for Reversible Alkali-Ion Intercalation

The Jahn-Teller effect (JTE) is one of the most important determinators of how much stress layered cathode materials undergo during charge and discharge; however, many reports have shown that traces of superstructure exist in pristine layered materials and irreversible phase transitions occur even after eliminating the JTE. A careful consideration of the energy of cationic distortion using a Taylor expansion indicated that second-order JTE (pseudo-JTE) is more widespread than the aforementioned JTE because of the various bonding states that occur between bonding and antibonding molecular orbitals in transition-metal octahedra. As a model case, a P2-type Mn-rich cathode (Na3/4MnO2) was investigated in detail. MnO6 octahedra are well known to undergo either elongation or contraction in a specific direction due to JTE. Here, the substitution of Li for Mn (Na3/4(Li1/4Mn3/4)O2) helped to oxidize Mn3+ to Mn4+ suppressing JTE; however, the MnO6 octahedra remained asymmetric with a clear trace of the superstructure. With various advanced analyses, we disclose the pseudo-JTE as a general reason for the asymmetric distortions of the MnO6 octahedra. These distortions lead to the significant electrochemical degradation of Na3/4Li1/4Mn3/4O2. The suppression of the pseudo-JTE modulates phase transition behaviors during Na intercalation/deintercalation and thereby improves all of the electrochemical properties. The insight obtained by coupling a theoretical background for the pseudo-JTE with verified layered cathode material lattice changes implies that many previous approaches can be rationalized by regulating pseudo-JTE. This suggests that the pseudo-JTE should be thought more important than the well-known JTE for layered cathode materials.