Rational design of N-doped C-encapsulated flower-like nickel-based heterostructured microsphere anodes for high-capacity and stable lithium storage

Designing a unique morphology and nanoarchitecture with a heterostructure is regarded as an efficient strategy to achieve lithium-ion batteries (LIBs) with high capacity and cycle life. Herein, N-doped C-encapsulated flower-like NiS/Ni3(BO3)2 heterostructures (NiS/Ni3(BO3)2/NC) with a core-shell morphology are successfully synthesized by a facile general method to improve the rate performance and prolong the cycle life of LIBs. The coated NC layer and core-shell structure with elasticity can relieve the volume expansion during the lithiation/delithiation process to strengthen the stability of the structure. Moreover, the NC layer and NiS/Ni3(BO3)2/NC heterostructure can enhance the electronic conductivity of the electrode and guarantee fast and unimpeded electron transfer channels, thereby improving the electrochemical reaction kinetics. Owing to the synergy of heterostructures and core-shell layer, the as-synthesized NiS/Ni3(BO3)2/NC anode acquires a specific charge capacity of 549 mA h g-1 at 0.2 A g-1 after 100 cycles; meanwhile, a reversible capacity of 322 mA h g-1 can be maintained even at 1 A g-1 after 500 cycles. This study develops a universal interface manipulation strategy for the synthesis of M3B2O6-based or/and other advanced transition metal compound anode materials for the practical applications of LIBs.

Increasing the Magnetic Order Temperature in Co3O2BO3:In Ludwigite

Below 42 K, the homometallic Co3O2BO3 ludwigite forms magnetic planes separated by nonmagnetic low-spin Co3+ ions. The substitution of Co3+ by other nonmagnetic ions enhances the magnetic interactions, raising the magnetic ordering temperature. However, depending on the nonmagnetic dopant ion, the remaining Co3+ ions could adopt a high-spin state, creating magnetic frustration and lowering the magnetic transition temperature. Doping Co3O2BO3 with nonmagnetic In3+ ions favors the appearance of both high-spin Co2+ and Co3+. The In3+ ions preferentially occupy sites 4 and are randomly distributed in each site. The two-dimensional magnetic character of the parent compound, Co3O2BO3, is preserved, and the magnetic transition temperature increases to 47.8 K. Measurements of magnetization, which show metamagnetic transitions at low temperatures, and specific heat are consistent with ferrimagnetic ordering in this system. Thus, using these results and those reported in the literature, the effects caused by doping of Co3O2BO3 with different nonmagnetic +3 ions are discussed in terms of the presence of high-spin Co2+ and Co3+ in the compounds.

Combustion Activation Induced Solid-State Synthesis for N, B Co-Doped Carbon/Zinc Borate Anode with a Boosting of Sodium Storage Performance

Zinc borates have merits of low voltage polarization and suitable redox potential, but usually suffer from low rate capability and poor cycling life, as an emerging anode candidate for Na⁺ storage. Here, a new intercalator-guided synthesis strategy is reported to simultaneously improve rate capability and stabilize cycling life of N, B co-doped carbon/zinc borates (CBZG). The strategy relies on a uniform dispersion of precursors and simultaneously stimulated combustion activation and solid-state reactions capable of scalable preparation. The Na⁺ storage mechanism of CBZG is studied: 1) ex situ XRD and XPS demonstrate two-step reaction sequence of Na⁺ storage: Zn₆ O(OH)(BO₃ )₃ +Na⁺ +e^- ↔3ZnO+Zn₃ B₂ O₆ +NaBO₂ +0.5H₂ ①, Zn₃ B₂ O₆ +6Na⁺ +6e^- ↔3Zn+3Na₂ O+B₂ O₃ ②; reaction ① is irreversible in ether-based electrolyte while reversible in ester-based electrolyte. 2) Electrochemical kinetics reveal that ether-based electrolyte possesses faster Na⁺ storage than ester-based electrolyte. The composite demonstrates an excellent capacity of 437.4 mAh g^-1 in a half-cell, together with application potential in full cells (discharge capacity of 440.1 mAh g^-1 and stable cycle performance of 2000 cycles at 5 A g^-1 ). These studies will undoubtedly provide an avenue for developing novel synthetic methods of carbon-based borates and give new insights into the mechanism of Na⁺ storage in ether-based electrolyte for the desirable sodium storage.

Electronic and magnetic states of Fe ions in Co2FeBO5

The ludwigite Co2FeBO5 has been studied experimentally using 57Fe Mössbauer spectroscopy and theoretically using DFT + GGA calculations. The room-temperature Mössbauer spectra are composed of four quadrupole doublets corresponding to the high-spin Fe3+ ions in octahedral oxygen coordination. All components undergo splitting below 117 K due to the magnetic hyperfine fields. The DFT + GGA calculations performed for three models of Fe ion distributions have revealed that the ground state corresponds to the "Fe4(HS)" model with the high-spin Fe3+ ions located at the M4 site and the high-spin Co2+ ions located at the M1, M2, and M3 sites. A ferrimagnetic ground state, with the Co and Fe magnetic moments being nearly parallel to the b-axis and a total magnetic moment of circa 1.1μB f.u.-1, was found. The other Fe distributions cause an increase in the local octahedral distortions and transformation of the spin state. The calculated quadrupole splitting values are in good agreement with the experimental values obtained by Mössbauer spectroscopy.