Tailoring Electronic Structure to Achieve Maximum Utilization of Transition Metal Redox for High-Entropy Na Layered Oxide Cathodes.

Charge compensation from cationic and anionic redox couples accompanying Na+ (de)intercalation in layered oxide cathodes contributes to high specific capacity. However, the engagement level of different redox couples remains unclear and their relationship with Na+ content is less studied. Here we discover that it is possible to take full advantage of the high-voltage transition metal (TM) redox reaction through low-valence cation substitution to tailor the electronic structure, which involves an increased ratio of Na+ content to available charge transfer number of TMs. Taking NaxCu0.11Ni0.11Fe0.3Mn0.48O2 as the example, the Li+ substitution increases the ratio to facilitate the high-voltage TM redox activity, and further F-ion substitution decreases the covalency of the TM-O bond to relieve structural changes. As a consequence, the final high-entropy Na0.95Li0.07Cu0.11Ni0.11Fe0.3Mn0.41O1.97F0.03 cathode demonstrates ∼29% capacity increase contributed by the high-voltage TMs and exhibits excellent long-term cycling stability due to the improved structural reversibility. This work provides a paradigm for the design of high-energy-density electrodes by simultaneous electronic and crystal structure modulation.

Conversion of Surface Residual Alkali to Solid Electrolyte to Enable Na-Ion Full Cells with Robust Interfaces.

The deposition of volatilized Na+ on the surface of the cathode during sintering results in the formation of surface residual alkali (NaOH/Na2 CO3 NaHCO3 ) in layered cathode materials, leading to serious interfacial reactions and performance degradation. This phenomenon is particularly evident in O3-NaNi0.4 Cu0.1 Mn0.4 Ti0.1 O2 (NCMT). In this study, a strategy is proposed to transform waste into treasure by converting residual alkali into a solid electrolyte. Mg(CH3 COO)2 and H3 PO4 are reacted with surface residual alkali to generate the solid electrolyte NaMgPO4 on the surface of NCMT, which can be labeled as NaMgPO4@NaNi0.4 Cu0.1 Mn0.4 Ti0.1 O2 -X (NMP@NCMT-X, where X indicates the different amounts of Mg2+ and PO4 3- ). NaMgPO4 acts as a special ionic conductivity channel on the surface to improve the kinetics of the electrode reactions, remarkably improving the rate capability of the modified cathode at a high current density in the half-cell. Additionally, NMP@NCMT-2 enables a reversible phase transition from the P3 to OP2 phase in the charge-discharge process above 4.2 V and achieves a high specific capacity of 157.3 mAh g-1 and outstanding capacity retention in the full cell. The strategy can effectively and reliably stabilize the interface and improve the performance of layered cathodes for Na-ion batteries (NIBs).

In situ synthesis of a nickel concentration gradient structure of Ni-rich LiNi0.8Co0.15Al0.05O2 with promising superior electrochemical properties at high cut-off voltage.

Nickel-rich layered cathode materials have aroused widespread interest due to their high discharge capacity, which is a basic requirement for next-generation high energy density lithium batteries. However, with the increase of nickel content, cathode materials face the serious challenge of capacity degradation, which is attributed to the formation of rock salt-type oxides such as NiO on the surface of cathode particles. To overcome this shortcoming, a novel Ni concentration gradient LiNi0.8Co0.15Al0.05O2 (NCG-NCA) cathode material was successfully synthesized using the characteristic reaction of Ni2+ and dimethylglyoxime. The final synthesized nickel concentration gradient material combines the advantages of high discharge capacity and excellent stability, which are attributed to the high nickel content in the core and high cobalt content on the surface of the material particles. The cycling stability of the NCG material is remarkably improved, exhibiting an excellent capacity retention of 75% after 200 cycles at a current density of 10C (1C = 160 mA g-1) under a high cut-off voltage of 4.5 V, much higher than that of a pristine NCA (P-NCA) cathode without NCG (50%). The excellent cycling stability of NCG-NCA is due to formation of a stable surface, which is not prone to serious atomic rearrangement on the surface. More importantly, with the structural analysis of NCA materials by neutron diffraction, we find that the proportion of Li/Ni mixing of NCA is reduced by utilizing the NCG structure; in turn, the rate performance of NCG-NCA cathode materials is improved greatly.

Na+-Conductive Na2Ti3O7-Modified P2-type Na2/3Ni1/3Mn2/3O2 via a Smart in Situ Coating Approach: Suppressing Na+/Vacancy Ordering and P2-O2 Phase Transition.

Sodium-ion batteries (SIBs) have shown great superiority for grid-scale storage applications because of their low cost and the abundance of sodium. P2-type Na2/3Ni1/3Mn2/3O2 cathode materials have attracted much attention for their high capacities and operating voltages as well as their simple synthesis processes. However, Na+/vacancy ordering and the P2-O2 phase transition are unavoidable during Na+ insertion/extraction, leading to undesired voltage plateaus and deficient battery performances. We show that this defect can be effectually eliminated by coating a moderate Na+ conductor Na2Ti3O7 with a smart in situ coating approach and a concomitant doping of Ti4+ into the bulk structure. Based on the combined analysis of ex situ X-ray diffraction, scanning electron microscopy, electrochemical performance tests, and electrochemical kinetic analyses, Na2Ti3O7 coating and Ti4+ doping effectively refrain Na+/vacancy ordering and P2-O2 phase transition during cycling. Additionally, the Na2Ti3O7 coating layer suppresses particle exfoliation and accelerates Na+ diffusion at the cathode and electrolyte interface. Hence, Na2Ti3O7-coated Na2/3Ni1/3Mn2/3O2 exhibits excellent cycling stability (almost no capacity decay after 200 cycles at 5 C) and outstanding rate capability (31.1% of the initial capacity at a high rate of 5 C compared to only 10.4% for the pristine electrode). This coating strategy can provide a new guide for the design of prominent cathode materials for SIBs that are suitable for practical applications.

CuO-Coated and Cu2+-doped Co-modified P2-type Na2/3[Ni1/3Mn2/3]O2 for sodium-ion batteries.

Layered P2-type CuO-coated Na2/3[Ni1/3Mn2/3]O2 (NNMO@CuO) with excellent rate capability and cycling performance was investigated as a sodium-ion battery cathode material for the first time. The NNMO@CuO cathode material combines the advantages of CuO coating and Cu2+ doping. Transmission electron microscopy (TEM) images, TEM elemental line scan analysis and ex situ scanning electron microscopy (SEM) images show that CuO has been successfully coated on the particle surface uniformly, and that this CuO layer effectively suppresses the exfoliation of the metal oxide layers and unfavorable side reactions. Furthermore, Cu2+ is also partially incorporated into the host structure, according to the X-ray diffraction (XRD) patterns and refinement results. Although incorporated Cu2+ does not take part in the redox reactions of the battery cell, the refinement results indicate that the d-spacing of the Na+-ion diffusion layer is enlarged due to Cu2+ doping in the crystal structure, which results in better Na+ kinetics. Thus, the CuO-coated cathode material shows prominent cycling performance and rate capability. We believe that this CuO-coating and Cu2+-doping co-modification strategy provides a promising approach to designing advanced cathode materials for sodium-ion batteries.

Synthesis Method for Long Cycle Life Lithium-Ion Cathode Material: Nickel-Rich Core-Shell LiNi0.8Co0.1Mn0.1O2.

High-nickel materials with core-shell structures, whose bulk is rich in nickel content and the outer shell is rich in manganese content, have been demonstrated to improve cycle stability. The high-nickel cathode material LiNi0.8Co0.1Mn0.1O2 is a very promising material for lithium-ion batteries; however, its low rate performance and especially cycle performance currently hamper further commercialization. This study presents a new synthesis method to prepare this core-shell material (LiNi0.8Co0.1Mn0.1O2@ x[Li-Mn-O], x = 0.01, 0.03, 0.06). Electrochemical data show that LiNi0.8Co0.1Mn0.1O2@ x[Li-Mn-O] ( x = 0.03, CS-0.03) exhibits the best high-rate performance, cycle stability, and thermal stability. The initial discharge capacity of the core-shell sample CS-0.03 is 118 mAh g-1, which is almost the same as the discharge capacity of pristine LiNi0.8Mn0.1Co0.1O2 (117 mAh g-1) at the rate of 10 C in the voltage range of 3.0-4.3 V. Notably the capacity decay of CS-0.03 is 18.4% after 200 cycles compared to 27% decay in capacity of the pristine sample. Furthermore, CS-0.03 exhibits better thermal cycling stability. The capacity retention of the CS-0.03 sample reached 65.1% which is over 1.3 times than that of the pristine one, whose capacity retention is 49.2% after 105 cycles (55 °C). Evidently, the core-shell structured CS-0.03 sample has excellent cycle stability and this synthesis method can be applied to other cathode materials.