The structural origin of enhanced stability of Na3.32Fe2.11Ca0.23(P2O7)2 cathode for Na-ion batteries

Abstract

The storage of renewable energy depends largely on sustainable technologies such as sodium-ion batteries with high safety, long lifespan, low cost, and non-toxicity. Pyrophosphate Na 3.32 Fe 2.34 (P 2 O 7) 2 cathode could meet this requirement, however, its structural stability needs to be further enhanced for practical purposes. To overcome this problem, Na-deficient Na 3.32 Fe 2.11 Ca 0.23 (P 2 O 7) 2 with exceptional stability is prepared by Ca selective doping in this work. In operando synchrotron-based X-ray diffraction (SXRD) and in situ X-ray absorption near edge spectroscopy (XANES) results reveal that the prepared Na 3.32 Fe 2.11 Ca 0.23 (P 2 O 7) 2 is a single-phase solid-solution reaction with high reversibility. A strong correlation between the voltage curve and lattice parameters is deciphered for the first time. Additionally, the atomic-doping-engineering strategy could significantly enhance the thermal and electrochemical stability of the electrode materials, contributing to their good structural reversibility and enhanced operational safety. Specifically, after 1000 cycles at 1 C, the Ca doped electrode achieves a high capacity retention of 81.7%, which is much better than that of the un-doped electrode (15.5%). Our work may pave a new avenue for designing safe and low-cost cathode materials for battery applications with long cycle life.

Reversible Activation of V4+/V5+ Redox Couples in NASICON Phosphate Cathodes

“…Note that the energy of Fe K‐edge at the pristine state is higher than that of the standard FeO but lower than Fe 2 O 3 , implying the coexistence of Fe 2+ and Fe 3+ ions. [ 42 ] Because of the sensitivity of Fe 2+ to air, this inevitable oxidation phenomenon has also been observed in other similar cathodes, such as Na 4−δ FeV(PO 4 ) 3 , [ 13 ] Na 3.32 Fe 2.34 (P 2 O 7 ) 2 , [ 42 ] etc. Similarly, the corresponding ex situ Fe 2p XPS spectra also reveal the reversible Fe 2+ /Fe 3+ redox during charge/discharge process.…”

Microstructure-Controlled Li-Rich Mn-Based Cathodes by a Gas–Solid Interface Reaction for Tackling the Continuous Activation of Li2MnO3

“…With the development of large-scale energy storage equipment, the demand for high energy density materials is increasing. − Li- and Mn-rich (LMR) layered oxides, x Li 2 MnO 3 ·(1 – x )­LiTMO 2 (0 < x < 1, TM = Ni, Co, Mn, etc.) have attracted great attention owing to their large reversible capacity (≥250 mAh g –1 ). − It is generally agreed that such a high capacity originates from the extra anion redox rather than the sole cation redox process. − Different from Li–O–TM configurations in traditional LiTMO 2 materials, the O 2– ion in Li 2 MnO 3 is surrounded by four Li and two Mn, which represents a different local environment with Li–O–Li configuration due to the honeycomb-like ordering of cations in the TM layer. − Along the Li–O–Li direction, the O 2p orbital generates a nonbonding state located at a high energy band near the Fermi level. , These nonbonding O 2p orbitals are preferentially oxidized above 4.5 V (versus Li/Li + ) during charging and make contribution to the additional capacity in the Li-excess material. , Notably, this O redox is a dynamic redox process related to structure evolution, which is represented by {O 2– + TM} → {O – + TM mig } + e – …”

Strain Engineering by Local Chemistry Manipulation of Triphase Heterostructured Oxide Cathodes to Facilitate Phase Transitions for High‐Performance Sodium‐Ion Batteries

“…Magnesium substitution for nickel is a useful strategy to solve the problem of complex phase transitions and poor electrochemical performance by which the triphase heterostructured oxide cathode LLS-NaNCMM15 was fabricated. [44][45][46][47] The chemical composition of the LLS-NaNCM and LLS-NaNCMM15 samples measured by inductively coupled plasma mass spectrometry (ICP-MS) are shown in Table S1, Supporting Information, which demonstrates that the ratio of metal elements is highly consistent with the designed theoretical value. Meanwhile, as illustrated in Figure 2a and Figure S4, Supporting Information, in situ XRD results and the corresponding intensity contour maps confirmed the structural evolution of the LLS-NaNCMM15 electrode during the cycling process at 0.1 C in a voltage range of 1.5-4.0 V. A thorough systematic understanding of the crystal structural evolution is also discussed in detail.…”