P2-Na 0.67 Ni 0.33 Mn 0.67 O 2 (NNMO) is promising cathode material for sodium-ion batteries (SIBs) due to its high specific capacity and fast Na + diffusion rate. Nonetheless, the irreversible P2-O2 phase transformation, Na + /vacancy ordering, and transition metal (TM) dissolution seriously damage its cycling stability and restrict its commercialization process. Herein, Na occupation manipulation and interface stabilization are proposed to strengthen the phase structure of NNMO by synergistic Zn/Ti co-doping and introducing lithium difluorophosp (LiPO 2 F 2 ) film-forming electrolyte additive . The Zn/Ti co-doping regulates the occupancy ratio of Na e /Na f at Na sites and disorganizes the Na + /vacancy ordering, resulting in a faster Na + diffusion kinetics and reversible P2-Z phase transition for P2-Na 0.67 Ni 0.28 Zn 0.05 Mn 0.62 Ti 0.05 O 2 (NNZMTO). Meanwhile, the LiPO 2 F 2 additive can form homogeneous and ultrathin cathode-electrolyte interphase (CEI) on NNZMTO surface, which can stabilize the NNZMTO-electrolyte interface to prevent TM dissolution, surface structure transformation, and micro-crack generation. Combination studies of in situ and ex situ characterizations and theoretical calculations were used to elucidate the storage mechanism of NNZMTO with LiPO 2 F 2 additive. As a result, the NNZMTO displays outstanding capacity retention of 94.44% after 500 cycles at 1C with 0.3 wt% LiPO 2 F 2 , excellent rate performance of 92.5 mA h g −1 at 8C with 0.1 wt% LiPO 2 F 2 , and remarkable full cell capability. This work highlights the important role of manipulating Na occupation and constructing protective film in the design of layered materials, which provides a promising direction for developing high-performance cathodes for SIBs.
Activating anionic redox reactions (ARR) in P2-type layered oxide cathodes has drawn great attention as a promising strategy to break through the bottleneck of their energy density limit. However, irreversible lattice oxygen loss and detrimental structure distortion associated with ARR are still challenges, resulting in severe capacity fading and sluggish reaction kinetics. Herein, we designed a novel P2-Na 0.67 Li 0.15 Mg 0.1 Mn 0.75 O 2 (NLMOMg0.1) sphere as a cathode for sodium-ion batteries (SIBs), leveraging the synergistic effect of Li/Mg co-doping and a LiPO 2 F 2 film-forming additive to enhance the reversibility and reaction kinetics of ARR. The Li/Mg co-doping effectively activates ARR and reduces the Mn 3+ ratio, weakening the Jahn-Teller effect and and minimizing structure distortion. The LiPO 2 F 2 film-forming additive in the electrolyte can form a stable cathode-electrolyte interphase film on the surface of NLMOMg0.1, significantly suppressing side reactions between the electrode and electrolyte and thereby preventing irreversible lattice oxygen loss. Impressively, NLMOMg0.1 with 0.2 wt% LiPO 2 F 2 additive attains a high capacity of 159.1mAh g −1 and an outstanding retention rate of 97.1 % at 0.5C after 300 cycles. In-situ X-ray diffraction (XRD) experiments confirm the excellent structural stability, and in-situ differential electrochemical mass spectrometry (DEMS) and in-situ optical microscope tests demonstrate the inhibition of lattice oxygen loss during the cycling process. This study provides practical guidelines for designing layered oxide cathode materials for SIBs with high stability and dynamics.
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