Simultaneously promoting the surface/bulk structural stability of Fe/Mn-based layered cathode for sodium ion batteries.

Layered sodium iron manganese oxide cathodes have attracted great interest owing to their high specific capacity and cost-effective metal resources, while the detrimental phase transitions and surface structural degradation severely limit their commercial applications. In this work, the bulk and surface structure stability of a P2-Na0.67Fe0.5Mn0.5O2 cathode can be synergically enhanced by a one-step Li/Nb co-doping strategy. Structural characterizations reveal that Li doping promotes the formation of P2/O3 biphasic structure and makes the unfavorable P2-OP4 phase transition convert into a smooth solid-solution reaction. Nb doping enhances the mobility of sodium ions and forms strong Nb-O bonds, thereby enhancing the stability of the TMO2 layer structure. In particular, the Nb element induces the surface reorganization of an atomic-scale NaNbO3 coating layer, which could effectively prevent the dissolution of metals and surface side reactions. The synergistic mechanism of enhanced electrochemical performance is proved by multiple characterizations during cycling. As a result, the as-prepared Na0.67Li0.1Fe0.5Mn0.38Nb0.02O2 exhibits improved capacity retention of 85.4 % than raw material (45.7 %) after 100 cycles at 0.5C (1C = 174 mA g-1) within 2.0-4.0 V. This co-regulating strategy provides a promising approach to designing highly stable sodium-ion battery cathodes. Furthermore, a full cell of Na0.67Li0.1Fe0.5Mn0.38Nb0.02O2 with hard carbon displays excellent cycling stability (85.1 % capacity retention after 100 cycles), making its commercial operation possible. This synergistic strategy of biphasic structure and surface reorganization is a critical route to accelerate the application of layer oxide cathodes.

Relieving Stress Concentration through Anion-Cation Codoping toward Highly Stable Nickel-Rich Cathode.

Nickel-rich LiNi0.8Co0.15Al0.015O2 (NCA) with excellent energy density is considered one of the most promising cathodes for lithium-ion batteries. Nevertheless, the stress concentration caused by Li+/Ni2+ mixing and oxygen vacancies leads to the structural collapse and obvious capacity degradation of NCA. Herein, a facile codoping of anion (F-)-cation (Mg2+) strategy is proposed to address these problems. Benefiting from the synergistic effect of F- and Mg2+, the codoped material exhibits alleviated Li+/Ni2+ mixing and demonstrates enhanced electrochemical performance at high voltage (≥4.5 V), outperformed the pristine and F-/Mg2+ single-doped counterparts. Combined experimental and theoretical studies reveal that Mg2+ and F- codoping decreases the Li+ diffusion energy barrier and enhances the Li+ transport kinetics. In particular, the codoping synergistically suppresses the Li+/Ni2+ mixing and lattice oxygen escape, and alleviates the stress-strain accumulation, thereby inhibiting crack propagation and improving the electrochemical performance of the NCA. As a consequence, the designed Li0.99Mg0.01Ni0.8Co0.15Al0.05O0.98F0.02 (Mg1+F2) demonstrates a much higher capacity retention of 82.65% than NCA (55.69%) even after 200 cycles at 2.8-4.5 V under 1 C. Furthermore, the capacity retention rate of the Mg1+F2||graphite pouch cell after 500 cycles is 89.6% compared to that of the NCA (only 79.4%).

NiSb/nitrogen-doped carbon derived from Ni-based framework as advanced anode for lithium-ion batteries.

Antimony anode has attracted much attention owing to its low lithium-embedded platform and high specific capacity. However, the dramatic volume expansion during the insertion and detachment of Li+ seriously affects its application in lithium-ion batteries. In this work, NiSb alloy embedded in nitrogen-doped carbon (NiSb/C) derived from a Ni-based framework was synthesized by a simple hydrothermal reaction followed by annealing treatment. NiSb alloy nanoparticles could alleviate significant volume expansion during lithium/delithiation owing to the good buffering action of Ni. Nitrogen-doped carbon provides abundant active sites for Li+ and serves as a conductive network to accelerate electron transport. Moreover, the uniformly dispersed NiSb alloy particles and the nitrogen-doped carbon can effectively cooperate to retain the structural completeness of antimony, which promotes the cycling stability and high-rate performance of the NiSb/C anode. At a high density of 2 A g-1, the prepared NiSb/C anode exhibits a reversible specific capacity of 426 mAh g-1 after 450 cycles. It can also exhibit a superior rate capability of 387 mAh g-1 at 5.0 A g-1, which can provide a possibility for designing new anode materials for rechargeable batteries.

Synthesis and Mechanism of High Structural Stability of Nickel-Rich Cathode Materials by Adjusting Li-Excess.

It has been a long-term challenge to improve the phase stability of Ni-rich LiNixMnyCo1-x-yO2 (x ≥ 0.6) transition metal (TM) oxides for large-scale applications. Herein, a new structure engineering strategy is utilized to optimize the structural arrangement of Li1+x(Ni0.88Mn0.06Co0.06)1-xO2 (NMC88) with a different Li-excess content. It was found that structure stability and particle sizes can be tuned with suitable Li-excess contents. NMC88 with an actual Li-excess of 2.7% (x = 0.027, Li/TM = 1.055) exhibits a high discharge capacity (209.1 mAh g-1 at 3.0-4.3 V, 0.1 C) and maintains 91.7% after the 100th cycle at 1 C compared with the NMC88 sample free of Li-excess. It also performs a delayed voltage decay and a good rate capacity, delivering 145.8 mAh g-1 at a high rate of 10 C. Multiscale characterization technologies including ex/in situ X-ray diffraction (XRD), focused ion beam (FIB) cutting-scanning electronic microscopy (SEM), and transmission electron microscopy (TEM) results show that a proper Li-excess (2.7%) content contributes to the formation of a broader Li slab, optimized cation mixing ratio, and even particle sizes. Therefore, NMC88 with a proper Li-excess is a good choice for next-generation cathode materials.

Influence of Nb Doping on Electrochemical Performance of Nanostructured Cation Disordered Li1+x/100Ni1/2-x/100Ti1/2-x/100Nbx/100O2 Composites Cathode for Li-Ion Batteries.

Li-excess cation-disordered rock-salts Li1+x/100Ni1/2-x/100Ti1/2-x/100Nbx/100O2 (x = 0, 5, 10, 15, 20) were synthesized in this study, and effects of Nb doping on their electrochemical performance were also investigated. X-ray diffraction (XRD) indicated that the rock-salt structure was maintained, but with lower crystallization. The scanning electron microscopy (SEM) displayed Nb-doped samples' morphology and particles were uniformly distributed within 100 nm. The transmission electron microscopy (TEM) analysis also confirmed that these Nb-doped samples still maintained the cationdisordered rock-salt structure. Charge-discharge test showed that the electrochemical performance was greatly improved after Nb-doping. The Li1.2Ni0.3Ti0.3Nb0.2O2 sample delivered highest capacity of up to 226.5 mAh·g-1, at 20 mA·g-1 density. Moreover, the sample still delivered 130 mAh·g-1 capacity of even 400 mA·g-1 density. This result indicates that the Nb-doping improved the cycling performance and rate capacity. In addition, electrochemical impedance spectroscopy (EIS) test showed that the Nb-doping further enhanced mobility of Li-ions when forming the 0-TM channel.

Synthesis and Redox Mechanism of Cation-Disordered, Rock-Salt Cathode-Material Li-Ni-Ti-Nb-O Compounds for a Li-Ion Battery.

Cation-disordered oxide materials working as cathodes for Li-ion batteries have been at a standstill because of their structurally limited specific capacities (below 175 mAh g-1 in most cases). In this work, we have introduced 4d0 Nb5+ into host material LiNi0.5Ti0.5O2 to synthesize Ni-based cation-disordered Fm3̅m Li-Ni-Ti-Nb-O compounds of Li1+x/100Ni1/2-x/100Ti1/2-x/100Nbx/100O2 (x = 0, 5, 10, 15, 20) through a sol-gel method, showing particle sizes of less than 200 nm. Taking Li1.2Ni0.3Ti0.3Nb0.2O2 with the best performance (an average voltage of ∼2.7 V and high discharge capacity of 221.5 mAh g-1) among oxides as a model, we study the relationship between the structure, morphology, redox mechanism, and electrochemical performance of cation-disordered oxides through a combination of X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and X-ray absorption near-edge spectroscopy tests and in situ XRD with electrochemistry. The obtained results indicate that the improved capacity is mainly ascribed to Nb5+, which optimizes the Ni2+/Ni4+ practical capacity and effectively stabilizes the O2-/O- redox reaction. The results emphasize that Li-Ni-Ti-Nb-O compounds are promising members in the family of cation-disordered transition-metal oxide materials.