Mg/Fe site-specific dual-doping to boost the performance of cobalt-free nickle-rich layered oxide cathode for high-energy lithium-ion batteries

Layer-type LiNi0.9Mn0.1O2is promising to be the primary cathode material for lithium-ion batteries(LIBs)due to its excellent electrochemical performance.Unfortunately,the cathode with high nickel content suffers from severely detrimental structural transformation that causes rapid capacity attenuation.Herein,site-specific dual-doping with Fe and Mg ions is proposed to enhance the structural stability of LiNi0.9Mn0.1O2.The Fe3+dopants are inserted into transition metal sites(3b)and can favorably provide additional redox potential to compensate for charge and enhance the reversibility of anionic redox.The Mg ions are doped into the Li sites(3a)and serve as O_(2)^(-)-Mg^(2+)-O_(2)^(-)pillar to reinforce the electrostatic cohesion between the two adjacent transition-metal layers,which further suppress the cracking and the generation of harmful phase transitions,ultimately improving the cyclability.The theoretical calculations,including Bader charge and crystal orbital Hamilton populations(COHP)analyses,confirm that the doped Fe and Mg can form stable bonds with oxygen and the electrostatic repulsion of O_(2)^(-)-O_(2)^(-)can be effectively suppressed,which effectively mitigates oxygen anion loss at the high delithiation state.This dual-site doping strategy offers new avenues for understanding and regulating the crystalline oxygen redox and demonstrates significant potential for designing high-performance cobalt-free nickel-rich cathodes.

Unraveling the degradation mechanism of LiNi_(0.8)Co_(0.1)Mn_(0.1)O_(2) at the high cut-off voltage for lithium ion batteries

LiNi_(0.8)Co_(0.1)Mn_(0.1)O_(2)(NCM811)layered oxides have been regarded as promising alternative cathodes for the next generation of high-energy lithium ion batteries(LIBs)due to high discharge capacities and energy densities at high operation voltage.However,the capacity fading under high operation voltage still restricts the practical application.Herein,the capacity degradation mechanism of NCM811 at atomic-scale is studied in detail under various cut-off voltages using aberration-corrected scanning transmission electron microscopy(STEM).It is observed that the crystal structure of NCM811 evolution from a layered structure to a rock-salt phase is directly accompanied by serious intergranular cracks under 4.9 V,which is distinguished from the generally accepted structure evolution of layered,disordered layered,defect rock salt and rock salt phases,also observed under 4.3 and 4.7 V.The electron energy loss spectroscopy analysis also confirms the reduction of Ni and Co from the surface to the bulk,not the previously reported only Li/Ni interlayer mixing.The degradation mechanism of NCM811 at a high cut-off voltage of4.9 V is attributed to the formation of intergranular cracks induced by defects,the direct formation of the rock salt phase,and the accompanied reduction of Ni^(2+)and Co^(2+)phases from the surface to the bulk.

Sulfur-deficient CoNi_(2)S_(4)nanoparticles-anchored porous carbon nanofibers as bifunctional electrocatalyst for overall water splitting

Water electrolysis technology is considered to be one of the most promising means to produce hydrogen.Herein,aiming at the problems of high overpotential and slow kinetics in water splitting,N-doped porous carbon nanofibers-coupled CoNi_(2)S_(4)nanoparticles are prepared as bifunctional electrocatalyst.In the strategy,NaCl is used as the template to prepare porous carbon nanofibers with a large surface area,and sulfur vacancies are created to modulate the electronic structure of CoNi_(2)S_(4).Electron spin resonance confirms the formation of abundant sulfur vacancies,which largely reduce the bandgap of CoNi_(2)S_(4)from 1.68 to 0.52 eV.The narrowed bandgap is conducive to the migration of valence electrons and decreases the charge transfer resistance for electrocatalytic reaction.Moreover,the uniform distribution of CoNi_(2)S_(4)nanoparticles on carbon nanofibers can prevent the aggregation and facilitate the exposure of electrochemical active sites.Therefore,the composite catalyst exhibits low overpotentials of 340 mV@100 mA·cm^(-2)for oxygen evolution reaction and 380 mV@100 mA·cm^(-2)for hydrogen evolution reaction.The assembled electrolyzer requires 1.64 V to achieve 10 mA·cm^(-2)for overall water-splitting with good long-term stability.The excellent performance results from the synergistic effect of porous structures,sulfur deficiency,nitrogen doping,and the well-dispersed active component.