Hao Guo

Layered O3-type oxides are one of the most promising cathode materials for Na-ion batteries owing to their high capacity and straightforward synthesis. However, these materials often experience irreversible structure transitions at elevated cutoff voltages, resulting in compromised cycling stability and rate performance. To address such issues, understanding the interplay of the composition, structure, and properties is crucial. Here, we successfully introduced a P-type characteristic into the O3-type layered structure, achieving a P3-dominated solid-solution phase transition upon cycling. This modification facilitated a reversible transformation of the O3-P3-P3' structure with minimal and gradual volume changes. Consequently, the Na_0.75Ni_0.25Cu_0.10Fe_0.05Mn_0.15Ti_0.45O₂ cathode exhibited a specific capacity of approximately 113 mAh/g, coupled with exceptional cycling performance (maintaining over 70% capacity retention after 900 cycles). These findings shed light on the composition-structure-property relationships of Na-ion layered oxides, offering valuable insights for the advancement of Na-ion batteries.

Ordered layered structures serve as essential components in lithium (Li)-ion cathodes^1,2,3. However, on charging, the inherently delicate Li-deficient frameworks become vulnerable to lattice strain and structural and/or chemo-mechanical degradation, resulting in rapid capacity deterioration and thus short battery life^2,4. Here we report an approach that addresses these issues using the integration of chemical short-range disorder (CSRD) into oxide cathodes, which involves the localized distribution of elements in a crystalline lattice over spatial dimensions, spanning a few nearest-neighbour spacings. This is guided by fundamental principles of structural chemistry and achieved through an improved ceramic synthesis process. To demonstrate its viability, we showcase how the introduction of CSRD substantially affects the crystal structure of layered Li cobalt oxide cathodes. This is manifested in the transition metal environment and its interactions with oxygen, effectively preventing detrimental sliding of crystal slabs and structural deterioration during Li removal. Meanwhile, it affects the electronic structure, leading to improved electronic conductivity. These attributes are highly beneficial for Li-ion storage capabilities, markedly improving cycle life and rate capability. Moreover, we find that CSRD can be introduced in additional layered oxide materials through improved chemical co-doping, further illustrating its potential to enhance structural and electrochemical stability. These findings open up new avenues for the design of oxide cathodes, offering insights into the effects of CSRD on the crystal and electronic structure of advanced functional materials.

To realize concurrently the high-energy density and excellent cycling stability, maximum utilization of redox couple, minimization of detrimental phase transition, and structural degradation of O3-type layered oxide cathodes are critical for developing Na-ion batteries. Ni²⁺/Ni⁴⁺ redox couple showing multielectron reaction and higher redox potential is favorable to increase the energy density. However, the Jahn-Teller distortion of Ni³⁺ generated upon (dis)charging results in a strong anisotropy in the local crystal structure that causes irreversible interlayer bending and chemo-mechanical cracks of the cathode particles, compromising the electrochemical properties eventually. In this work, we show a slight multielement doping strategy that enlarges the amount of active redox components while minimizing the inactive contents. The results show that the uniform distribution of multiple components can help increase the disorder degree of atom arrangement and alleviate the structural changes and detrimental anisotropy cracks. As a proof of concept, a multielement-doped O3-type Na_0.9Ni_0.25Cu_0.05Mg_0.05Zn_0.05Fe_0.05Al_0.05Mn_0.40Ti_0.05Sn_0.05O₂ oxide is rationally prepared that presents better chemo-mechanical stability and delayed O3-P3 phase transition behavior. Compared to the high Ni-content Na_0.9Ni_0.35Fe_0.2Mn_0.45O₂ cathode, this as-prepared multielement material delivers a reversible capacity of about 120 mAh/g in the voltage range of 2-4.0 V, superior cycling stability with 90% of capacity retention after 500 cycles, and excellent rate capability (more than 70% of initial capacity at 5.0 C). This work indicates that the multielement doping method is highly suitable for the development of advanced Na-ion 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 Na_xCu_0.11Ni_0.11Fe_0.3Mn_0.48O₂ 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 Na_0.95Li_0.07Cu_0.11Ni_0.11Fe_0.3Mn_0.41O_1.97F_0.03 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.

P2/O3 composite sodium layered oxide has emerged as a promising cathode for high-performance Na-ion batteries. However, it has been challenging to regulate accurately the phase ratio of P2/O3 composite due to their high compositional diversity, which brings about some difficulty in manipulating the electrochemical performance of P2/O3 composite. Here, we explore the effect of Ti substitution and the synthesis temperature on the crystal structure and Na storage performance of Na_0.8Ni_0.4Mn_0.6O₂. The investigation indicates Ti-substitution and altering synthesis temperature can rationally manipulate the phase ratio of P2/O3 composite, thereby purposefully regulating the cycling and rate performance of P2/O3 composite. Typically, O3-rich Na_0.8Ni_0.4Mn_0.4Ti_0.2O₂-950 shows excellent cycling stability with a capacity retention of 84% (3C, 700 cycles). By elevating the proportion of P2 phase, Na_0.8Ni_0.4Mn_0.4Ti_0.2O₂-850 displays concurrently improved rate capability (65% capacity retention at 5 C) and comparable cycling stability. These findings will help guide the rational design of high-performance P2/O3 composite cathodes for sodium-ion batteries.