Analysis of the new ternary phase with C_(6)Cr_(23) structure in Mg-Co-B system by Rietveld method and physical properties of its Ni-substituting effect

A new ternary Mg_(1.4)Co_(21.6)B_(6) compound in the Mg-Co-B system was synthesized via a conventional solid-state reaction method and the effect of Ni-substitution on its crystal structure,thermal stability,solid solubility and physical properties were systematically investigated.The crystal structure of the Mg_(1.4)Co_(21.6)B_(6) compound was fully determined by the X-ray diffraction technique with Rietveld refinement method.It is found that Mg_(1.4)Co_(21.6)B_(6) crystallizes in the form of C_(6)Cr_(23) structure type(space group:Fm-3m(No.225),a=10.5617(2)A,Z=4).The results showed that the 4a sites have been occupied completely by Co atoms in present compound which with M_(2-x)Ni_(21+x)B_(6) form belonging to the W_(2)Cr_(21)C_(6)-type.When Mg_(1.4)Co_(21.6)B_(6) is repeatedly sintered at elevated temperatures,it becomes unstable and decomposes into Co3B and Mg.The lattice parameters of the Mg_(1.4)Co_(21.6)B_(6) solid solution alters dramtically with increasing Ni substitution,with no regular trend being observed.The electrical and magnetic performances of the 3.6Mg:3Co:17Ni:6B and 3.6Mg:3Co:18Ni:6B(nominal compositions)samples suggest that both samples are typical ferromagnetic materials.The temperature in the maximum drop of theρ(T)curve decreases as a function of the Ni content.Base on the correlation between the critical temperature and Ni content,a linear fitting equation is obtained and the critical temperature of Mg_(1.4)Co_(21.6)B_(6) calculated utilizing the linear fitting equation.The findings in this work may provide certain reference values for material science on electrical magnetic properties and other references for researching the material further.

Interfacial Chemistry Enables Highly Reversible Na Extraction/Intercalation in Layered-Oxide Cathode Materials

Comprehensive Summary Layered transition-metal oxides are promising cathode candidates for sodium-ion batteries.However,the inferior interphase formation and particulate fracture during sodiation/desodiation result in structure degradation and poor stability.Herein,the interface chemistry of P2-Na_(0.640)Ni_(0.343)Mn_(0.657)O_(2)in an electrolyte of 1.0 mol/L NaPF6 in diglyme is unveiled to enable highly reversible Na extraction and intercalation.The uniform and robust cathode-electrolyte interphase layer is in situ formed with decomposition of diglyme molecules and anions in initial cycles.The NaF-and CO-rich CEI film exhibits high mechanical strength and ionic conductivity,which suppresses the reconstruction of its electrode interphase from P2 phase to spinel-like structure and reinforces its structure integrity without cracks.This favours facile Na+transport and stable bulk redox reactions.It is demonstrated to show long cycling stability with capacity retention of 94.4%for 180 cycles and superior rate capability.This investigation highlights the cathode interphase chemistry in sodium-ion batteries.