Tingfei Yang,Na Chen,Anqi Li,Anrui Feng,Yihan Li,Yaru Qin,Xue Qin,Chenglong Shi
P2-Na 0.67 Ni 0.33 Mn 0.67 O 2 cathode material is considered as an ideal cathode material for sodium-ion batteries due to its high capacity. However, it has P2-O2 phase transition, which leads to fast capacity decay. This paper investigates the effect of Zn/Al co-substitution on their chemical-electrical properties. The experimental results show that this strategy successfully transforms the unfavorable P2-O2 phase transition that occurs in the material during cycling into a reversible P2-OP4 phase transition, and the reversible capacity and capacity retention of the P2-type cathode material are significantly improved. It maintains a high energy density of up to 444 Wh·Kg −1 in the first five charge/discharge cycles, facilitating the future practical implementation and commercialization of the material.
Chi Shuxiu,Wang Chenhui,Liao Jie,Sun Peng,Fei Yaohan,Han You,Zhang Jinli,Wu Jiangjiexing,Li Wei
Aiming at inhibiting the irreversible P2–O2 phase transition of conventional P2-type cathode materials at high voltage and enhancing the cycling stability of sodium-ion batteries, in this article, based on a strategy of adjusting the Na + ion occupancy within the crystal structure, Na 0.67 Ni 0.33 Mn 0.67– x Fe x O 2 (NM– x Fe, x = 0.10, 0.15, 0.20) cathode materials were synthesized by high shear mixer (HSM)-assisted co-precipitation method and evaluated the electrochemical performance at high voltage (4.35 V). The optimal sample NM–0.15Fe exhibits an initial discharge capacity of 130.8 mAh/g (0.1 C), with exceptional retention of 95.9% after 100 cycles (1 C). XRD analysis reveals that Fe intercalation promotes the more amount of Nae-similar occupation; the Nae/Naf ratio equals 1.93 for NM–0.15Fe versus 1.62 for NM, which enhances Na + diffusion kinetics, as confirmed by GITT tests. Through characterizations of in situ XRD, XPS, HRTEM, CV, etc., it is illustrated that the Fe 3+ intercalation can effectively disrupt the Na + /vacancy ordering and inhibit the harmful P2–O2 phase transition, and then improve the cycling stability of the cathode. DFT calculations disclose that intercalated Fe can reduce the electron densities of adjacent transition metallic elements, generating more repulsive forces impacted on sodium and consequently appearance of more Nae sites, leading to a lower Na + diffusion energy barrier. Such strategy of modulating Na occupation sites in crystal structure is conducive to the development of low-cost and high-performance layered cathode materials for sodium-ion batteries.
P2-type manganese-based cathode materials are notable for their high theoretical capacity and open prismatic channels. However, rapid capacity decay caused by harmful irreversible phase transformations (P2-O2) and oxygen release in the high-voltage region (>4.2 V) limits their commercial application. Here, a co-doping strategy with lithium and niobium is proposed to enhance the structural stability at both room-temperature (RT) and low-temperature (LT) over extended cycles. The X-ray absorption near-edge spectra and first-principles calculations reveal that Li doping elevates the valence states of transition metals, thereby enhancing the reversibility of anionic redox reactions. Additionally, Nb doping leads to the formation of a rock-salt-like phase on the surface, which improves the stability of the electrode/electrolyte interface. In situ X-ray diffraction and differential electrochemical mass spectrometry show that Li and Nb co-doping prevents the P2-O2 phase transition, suppresses gas release, and enhances long-cycle stability. The P2-Na 0.67 Ni 0.24 Mn 0.64 Li 0.1 Nb 0.02 O 2 (NNMLNb) exhibits a reversible specific capacity of 127.4 mAh g −1 and 101.5 mAh g −1 at -20 °C and -30 °C, respectively. Remarkably, the sample cycles stably for 1200 cycles with a discharge capacity of 85 mAh g −1 , corresponding 90 % capacity retention. This work provides a novel strategy to improve the cycling life of sodium-ion batteries at both RT and LT.
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