Zhengyao Li

Designing an advanced P2-Na0.67Mn0.65Ni0.2Co0.15O2 layered cathode material for Na-ion batteries

A high performance layered P2-Na0.67Mn0.65Ni0.2Co0.15O2 cathode material for sodium ion batteries with high rate capability and excellent long-life cyclic performance has been successfully designed and synthesized by a simple sol–gel method. In comparison with the reported Na0.7MnO2, the designed P2-Na0.67Mn0.65Ni0.2Co0.15O2 cathode material can be charged and discharged in an extended voltage range of 1.5–4.2 V and shows reversible capacities of 155, 144, 137, 132 and 126 mA h g−1 at different current densities of 12, 24, 48, 120 and 240 mA g−1, respectively. Even at high current densities of 480 (2C), 1200 (5C) and 1920 mA g−1 (8C) it can still deliver capacities of 117, 93 and 70 mA h g−1, respectively, which are much higher than those of the recently reported Na0.5[Ni0.23Fe0.13Mn0.63]O2. In addition, the Na0.67Mn0.65Ni0.2Co0.15O2 cathode material also displays an excellent capacity retention ca. 85% and 78% after 100 cycles at 0.05C and 0.5C, respectively. It is also proposed that Mn4+ may be “activated” in a low voltage range, especially below 2.0 V, which contributes to the additional capacity. The Na-ion diffusion coefficient, DNa+, is ca. 10−14 cm2 s−1 as calculated by the PITT and the discharge diffusion coefficient is a little larger than the charge one. The designed Na0.67Mn0.65Ni0.2Co0.15O2 shows great potential as a cathode material for sodium ion batteries.

New insights into designing high-rate performance cathode materials for sodium ion batteries by enlarging the slab-spacing of the Na-ion diffusion layer

Recently, the design and synthesis of high performance cathode materials for sodium ion batteries have attracted great interest. In this study, we propose a novel strategy to design high-rate performance cathode materials for sodium ion batteries through enlarging the d-spacing of the Na-ion diffusion layer. More importantly, some new insights into the expansion mechanism of the interplanar spacing for Na0.67Mn0.8Ni0.1Mg0.1O2 induced by Ni and Mg co-doping and the resulting high-rate capability have been presented for the first time. We find that Mg and Ni co-doping leads to the shortening of the TM–O (TM = transition metal) bond lengths and the shrinkage of the TMO6 octahedrons, which might be largely responsible for the expansion of the interplanar spacing of the Na-ion diffusion layer. In comparison with Na0.67Mn0.8Ni0.2O2 and Na0.67Mn0.8Mg0.2O2, Mg and Ni co-doped Na0.67Mn0.8Ni0.1Mg0.1O2 has a higher Na-ion diffusion coefficient and can deliver around 160, 145, 133 and 124 mA h g−1 at 24, 48, 120 and 240 mA g−1, respectively. In particular, at the high current densities of 480 (2C), 1200 (5C) and 1920 mA g−1 (8C), MMN can still offer reversible capacities of 110, 66 and 37 mA h g−1, respectively. In addition, the cycling stability has also been enhanced via Mg and Ni co-doping at the same time, which means that Mg and Ni co-doping also has a positive effect on the stability of the layered structure.

Unveiling the Role of Co in Improving the High-Rate Capability and Cycling Performance of Layered Na0.7Mn0.7Ni0.3-xCoxO2 Cathode Materials for Sodium-Ion Batteries.

Co substitution has been extensively used to improve the electrochemical performances of cathode materials for sodium-ion batteries (SIBs), but the role of Co has not been well understood. Herein, we have comprehensively investigated the effects of Co substitution for Ni on the structure and electrochemical performances of Na0.7Mn0.7Ni0.3-xCoxO2 (x = 0, 0.1, 0.3) as cathode materials for SIBs. In comparison with the Co-free sample, the high-rate capability and cycle performance have been greatly improved by the substitution of Co, and some new insights into the role of Co have been proposed for the first time. With the substitution of Co(3+) for Ni(2+) the lattice parameter a decreases; however, c increases, and the d-spacing of the sodium-ion diffusion layer has been enlarged, which enhances the diffusion coefficient of the sodium ion and the high-rate capability of cathode materials. In addition, Co substitution shortens the bond lengths of TM-O (TM = transition metal) and O-O due to the smaller size of Co(3+) than Ni(2+), which accounts for the decreased thickness and volume of the TMO6 octahedron. The contraction of TM-O and O-O bond lengths and the shrinkage of the TMO6 octahedron improve the structure stability and the cycle performance. Last but not least, the aliovalent substitution of Co(3+) for Ni(2+) can improve the electronic conductivity during the electrochemical reaction, which is also favorable to enhance the high-rate performance. This study not only unveils the role of Co in improving the high-rate capability and the cycle stability of layered Na0.7Mn0.7Ni0.3-xCoxO2 cathode materials but also offers some new insights into designing high performance cathode materials for SIBs.

Understanding the effect of an in situ generated and integrated spinel phase on a layered Li-rich cathode material using a non-stoichiometric strategy

Recently, spinel-layered integrated Li-rich cathode materials have attracted great interest due to the large enhancement of their electrochemical performances. However, the modification mechanism and the effect of the integrated spinel phase on Li-rich layered cathode materials are still not very clear. Herein, we have successfully synthesized the spinel-layered integrated Li-rich cathode material using a facile non-stoichiometric strategy (NS-LNCMO). The rate capability (84 mA h g-1vs. 28 mA h g-1, 10 C), cycling stability (92.4% vs. 80.5%, 0.2 C), low temperature electrochemical capability (96.5 mA h g-1vs. 59 mA h g-1, -20 °C), initial coulomb efficiency (92% vs. 79%) and voltage fading (2.77 V vs. 3.02 V, 200 cycles@1 C) of spinel-layered integrated Li-rich cathode materials have been significantly improved compared with a pure Li-rich phase cathode. Some new insights into the effect of the integrated spinel phase on a layered Li-rich cathode have been proposed through a comparison of the structure evolution of the integrated and Li-rich only materials before and after cycling. The Li-ion diffusion coefficient of NS-LNCMO has been enlarged by about 3 times and almost does not change even after 100 cycles indicating an enhanced structure stability. The integration of the spinel phase not only enhances the structure stability of the layered Li-rich phase during charging-discharging but also expands the interslab spacing of the Li-ion diffusion layer, and elongates TM-O covalent bond lengths, which lowers the activation barrier of Li+-transportation, and alleviates the structure strain during the cycling procedure.

Different Effects of Al Substitution for Mn or Fe on the Structure and Electrochemical Properties of Na0.67Mn0.5Fe0.5O2 as a Sodium Ion Battery Cathode Material

P2-type layered oxides based on the elements Fe and Mn have attracted great interest as sodium ion battery (SIB) cathode materials owing to their inexpensive metal constituents and high specific capacity. However, they suffer from rapid capacity fading and complicated phase transformations. In this study, we modulate the crystal structure and optimize the electrochemical performances of Na0.67Mn0.5Fe0.5O2 by Al doping for Mn or Fe, respectively, and the roles of Al in the enhancement of the rate capability and cycling performance are unraveled. (1) The substitution of Al for Mn or Fe decreases the lattice parameters a and c but enlarges d spacing and lengthens Na-O bonds, which enhances Na+ diffusion and rate capability especially for Na0.67Mn0.5Fe0.47Al0.03O2. (2) Al doping reduces the thickness of TMO2 and strengthens TM-O/O-O bonding. This enhances the layered structure stability and the capacity retention. (3) Al doping mitigates Mn3+ and Jahn-Teller distortion, mitigating the irreversible phase transition. (4) Al doping also alleviates the lattice volume variation and the structure strain. This further improves the stability of the layered structure and the cycling performances particularly in the case of Al doping for Fe. The in-depth insights into the roles of Al substitution might be also useful for designing high-performance cathode materials for SIBs through appropriate lattice doping.

Improving the Performance of Layered Oxide Cathode Materials with Football-Like Hierarchical Structure for Na-Ion Batteries by Incorporating Mg2+ into Vacancies in Na-Ion Layers

The development of advanced cathode materials is still a great interest for sodium-ion batteries. The feasible commercialization of sodium-ion batteries relies on the design and exploitation of suitable electrode materials. This study offers a new insight into material design to exploit high-performance P2-type cathode materials for sodium-ion batteries. The incorporation of Mg2+ into intrinsic Na+ vacancies in Na-ion layers can lead to a high-performance P2-type cathode material for sodium-ion batteries. The materials prepared by the coprecipitation approach show a well-defined morphology of secondary football-like hierarchical structures. Neutron power diffraction and refinement results demonstrate that the incorporation of Mg2+ into intrinsic vacancies can enlarge the space for Na-ion diffusion, which can increase the d-spacing of the (0 0 2) peak and the size of slabs but reduce the chemical bond length to result in an enhanced rate capability and cycling stability. The incorporation of Mg2+ into available vacancies and a unique morphology make Na0.7 Mg0.05 Mn0.8 Ni0.1 Co0.1 O2 a promising cathode, which can be charged and discharged at an ultra-high current density of 2000 mA g-1 with an excellent specific capacity of 60 mAh g-1 . This work provides a new insight into the design of electrode materials for sodium-ion batteries.

Modulating the Electrochemical Performances of Layered Cathode Materials for Sodium Ion Batteries through Tuning Coulombic Repulsion between Negatively Charged TMO2 Slabs

Exploiting advanced layered transition metal oxide cathode materials is of great importance to rechargeable sodium batteries. Layered oxides are composed of negatively charged TMO2 slabs (TM = transition metal) separated by Na+ diffusion layers. Herein, we propose a novel insight, for the first time, to control the electrochemical properties by tuning Coulombic repulsion between negatively charged TMO2 slabs. Coulombic repulsion can finely tailor the d-spacing of Na ion layers and material structural stability, which can be achieved by employing Na+ cations to serve as effective shielding layers between TMO2 layers. A series of O3-type NaxMn1/3Fe1/3Cu1/6Mg1/6O2 (x = 1.0, 0.9, 0.8, and 0.7) have been prepared, and Na0.7Mn1/3Fe1/3Cu1/6Mg1/6O2 shows the largest Coulombic repulsion between TMO2 layers, the largest space for Na ion diffusion, the best structural stability, and also the longest Na-O chemical bond with weaker Coulombic attraction, thus leading to the best electrochemical performance. Meanwhile, the thermal stability depends on the Na concentration in pristine materials. Ex situ X-ray absorption (XAS) analysis indicates that Mn, Fe, and Cu ions are all electrochemically active components during insertion and extraction of sodium ion. This study enables some new insights to promote the development of advanced layered NaxTMO2 materials for rechargeable sodium batteries in the future.