Interfacial Engineering of P2-Type Ni/Mn-Based Layered Oxides by a Facile Water-Washing Method for Superior Sodium-Ion Batteries.

Owing to the strong basicity and reactivity, residual sodium compounds (RSCs) on the surface of Na-based layered oxides for sodium-ion batteries (SIBs) cause the deterioration of the electrochemical performance and processability of the oxide cathode materials. Herein, considering P2-type Na0.66Ni0.26Zn0.07Mn0.67O2 as the model material, the water-washing treatment is proven to be a facile, economic, and highly efficient method to improve the electrochemical performance of P2-type Ni/Mn-based layered oxides. Experimental results show that RSCs on material surfaces can be effectively removed by water washing without causing severe damage to the bulk structure. Notably, water washing triggers the formation of an ultrathin (2-3 nm thick) Na-poor disordered interfacial layer on the surface of Na0.66Ni0.26Zn0.07Mn0.67O2. This layer plays a passivating role in further enhancing the material's resistance to water and reduces the reactivity of the material surface with the electrolyte. These compositional and structural optimizations for P2-type Na0.66Ni0.26Zn0.07Mn0.67O2 effectively suppress the release of gaseous CO2, formation of thick cathode-electrolyte interphase films, and consumption of active Na+, enabling good Na+ transport kinetics during cycling. The water-washed Na0.66Ni0.26Zn0.07Mn0.67O2 exhibits significantly improved cycling stability with a capacity retention of 89.1% at 100 mA g-1 after 100 cycles and rate capability with a discharge capacity of 76.3 mA g-1 at 2000 mA g-1; these values are higher than those of the unwashed Na0.66Ni0.26Zn0.07Mn0.67O2 (83.3%, 71.4 mA h g-1). This work provides fundamental insights into the detrimental effect of RSCs on the electrochemical performance of layered oxides and highlights the importance of regulating interfacial compositions for developing high-performance layered-oxide cathode materials for SIBs.

Single-Crystal Growth of P2-Type Layered Oxides with Increased Exposure of {010} Planes for High-Performance Sodium-Ion Batteries.

An increase in the size of single-crystal particles can effectively reduce the interfacial side reactions of layered oxides for sodium-ion batteries at high voltages but may result in sluggish Na+ transport. Herein, single-crystal Na0.66Ni0.26Zn0.07Mn0.67O2 with increased proportions of {010} planes is synthesized by adding low-cost NaCl as the molten salt. With the assistance of a NaCl molten salt, the median diameter (D50) of single-crystal Na0.66Ni0.26Zn0.07Mn0.67O2 increases to 10.46 µm relative to that of the comparison sample without NaCl (6.57 µm). Electrolyte decomposition on the surface of single-crystal Na0.66Ni0.26Zn0.07Mn0.67O2 is considerably suppressed, owing to a decrease in the specific surface area. Moreover, the increased exposure of {010} planes is favorable for improving the Na+ transport kinetics of single-crystal particles. Therefore, at 100 mA g-1, single-crystal Na0.66Ni0.26Zn0.07Mn0.67O2 exhibits a high-capacity retention of 96.6% after 100 cycles, which is considerably greater than that of the comparison sample (86.8%). Moreover, the rate performance of single-crystal Na0.66Ni0.26Zn0.07Mn0.67O2 (average discharge capacity of 81.2 mAh g-1) is superior to that of the comparison sample (average discharge capacity of 61.2 mAh g-1) at 2000 mA g-1. This work provides a new approach for promoting the single-crystal growth of layered oxides for highly stable interfaces at high voltages without compromising Na+ transport kinetics.

Stabilizing P2-Type Ni-Mn Oxides as High-Voltage Cathodes by a Doping-Integrated Coating Strategy Based on Zinc for Sodium-Ion Batteries.

The key to development of high-voltage P2-type Na0.66Ni0.33Mn0.67O2 is the modification methods that can effectively improve its electrochemical reversibility. Herein, a doping-integrated coating strategy based on zinc element is proposed to modify P2-type Na0.66Ni0.33Mn0.67O2, which can be achieved by a facile one-step solid-state reaction. The formation mechanism of Na0.66Ni0.26Zn0.07Mn0.67O2@0.06ZnO (NNZM@0.06ZnO) is investigated, revealing that the spinel and P3 intermediate phases appear prior to the formation of the P2 phase. Ni2+ can be preferentially incorporated into the P2 structure in competition with Zn2+ at high temperature, resulting in a uniform enrichment of ZnO on the surface. A small amount of Zn2+ doping significantly suppresses the Na+/vacancy ordering effect and improves the structural reversibility. Furthermore, the electrolyte decomposition is effectively reduced because of the presence of the ZnO coating layer, leading to the formation of a thin cathode electrolyte interphase film that is favorable to fast Na+ diffusion. In virtue of the Zn2+ doping and in situ formed ZnO coating, NNZM@0.06ZnO exhibits excellent cycling stability with a capacity retention of 83.7% after 100 cycles at 100 mA g-1 and rate performance with a discharge capacity of 56.4 mAh g-1 at 2000 mA g-1, which significantly outperforms the uncoated Na0.66Ni0.26Zn0.07Mn0.67O2 and the Na0.66Ni0.26Zn0.07Mn0.67O2/0.06ZnO with the coating layer introduced by mechanical milling. This work provides a new strategy to design high-performance cathode materials for sodium-ion batteries.

Toward a High-Energy-Density Cathode with Enhanced Temperature Adaptability for Sodium-Ion Batteries: A Case Study of Na3MnZr(PO4)3 Microspheres with Embedded Dual-Carbon Networks.

Polyanionic cathode materials that have high energy density and good temperature adaptability are in high demand for practical applications in sodium-ion batteries (SIBs). In this study, a scalable spray-drying strategy has been proposed to construct interconnected conductive networks composed of amorphous carbon and reduced graphene oxide in Na3MnZr(PO4)3 microspheres (NMZP@C-rGO). The dual-carbon conductive networks provide fast electron migration pathways in the microspheres. Moreover, they significantly increase the porosity and specific surface area of the microspheres, which are conducive to accommodating the volume change and improving the electrode/electrolyte contact interface and the contribution of the pseudocapacitance effect to achieve fast sodium storage. As a result, NMZP@C-rGO exhibits excellent rate performance (50.9 mAh g-1 at 50C and 30 °C, 35.4 mAh g-1 at 50C and -15 °C) and long-term cycling stability (capacity retentions of 97.4 and 79.6% after 1500 cycles at 30 and -15 °C, respectively) in a wide temperature range.

Insights into the Effects of Zinc Doping on Structural Phase Transition of P2-Type Sodium Nickel Manganese Oxide Cathodes for High-Energy Sodium Ion Batteries.

P2-type sodium nickel manganese oxide-based cathode materials with higher energy densities are prime candidates for applications in rechargeable sodium ion batteries. A systematic study combining in situ high energy X-ray diffraction (HEXRD), ex situ X-ray absorption fine spectroscopy (XAFS), transmission electron microscopy (TEM), and solid-state nuclear magnetic resonance (SS-NMR) techniques was carried out to gain a deep insight into the structural evolution of P2-Na0.66Ni0.33-xZnxMn0.67O2 (x = 0, 0.07) during cycling. In situ HEXRD and ex situ TEM measurements indicate that an irreversible phase transition occurs upon sodium insertion-extraction of Na0.66Ni0.33Mn0.67O2. Zinc doping of this system results in a high structural reversibility. XAFS measurements indicate that both materials are almost completely dependent on the Ni(4+)/Ni(3+)/Ni(2+) redox couple to provide charge/discharge capacity. SS-NMR measurements indicate that both reversible and irreversible migration of transition metal ions into the sodium layer occurs in the material at the fully charged state. The irreversible migration of transition metal ions triggers a structural distortion, leading to the observed capacity and voltage fading. Our results allow a new understanding of the importance of improving the stability of transition metal layers.

Nanostructured Black Phosphorus/Ketjenblack-Multiwalled Carbon Nanotubes Composite as High Performance Anode Material for Sodium-Ion Batteries.

Sodium-ion batteries are promising alternatives to lithium-ion batteries for large-scale applications. However, the low capacity and poor rate capability of existing anodes for sodium-ion batteries are bottlenecks for future developments. Here, we report a high performance nanostructured anode material for sodium-ion batteries that is fabricated by high energy ball milling to form black phosphorus/Ketjenblack-multiwalled carbon nanotubes (BPC) composite. With this strategy, the BPC composite with a high phosphorus content (70 wt %) could deliver a very high initial Coulombic efficiency (>90%) and high specific capacity with excellent cyclability at high rate of charge/discharge (∼1700 mAh g(-1) after 100 cycles at 1.3 A g(-1) based on the mass of P). In situ electrochemical impedance spectroscopy, synchrotron high energy X-ray diffraction, ex situ small/wide-angle X-ray scattering, high resolution transmission electronic microscopy, and nuclear magnetic resonance were further used to unravel its superior sodium storage performance. The scientific findings gained in this work are expected to serve as a guide for future design on high performance anode material for sodium-ion batteries.