Theoretical study on the synergistic mechanism of Fe–Mn in sodium-ion batteries

This article conducts first-principles calculations to initially explore the construction of two configurations,NaFeO_(2)(NFO)and NaMnO_(2)(NMO),and studies the mixing enthalpies under different Fe–Mn ratios.The results indicate that NaFe_(3/8)Mn_(5/8)O_(2)(NFMO)exhibits the most thermodynamically stable structure.Subsequent calculations on the mixing enthalpies and volume changes during the sodium extraction process for NFO,NMO,and NFMO configurations are presented,along with the partial density of states(PDOS)and Bader charges of transition metals(TM)and oxygen.These calculations reveal the synergistic mechanism of Fe and Mn.Fe and Mn can engage in more complex electron exchanges during sodium extraction,optimizing the internal electron density distribution and overall charge balance,thereby stabilizing the crystal structure and reducing the migration of Fe^(3+)to the sodium layers during deep sodium extraction.The interaction between Fe’s 3d electrons and Mn’s 3d electrons through the shared oxygen atoms’2p orbitals occurs in the Fe–Mn–O network.This interaction can lead to a rebalancing of the electron density around Mn³⁺atoms,mitigating the asymmetric electron density distribution caused by the d4 configuration of the lone Mn³⁺and suppressing the Jahn-Teller effect of Mn^(3+).Moreover,the synergistic effects between Fe and Mn can provide a more balanced charge distribution,reducing extreme changes to the charge state of oxygen atoms and decreasing the irreversible oxygen release caused by anionic redox reactions during deep sodium extraction,thereby enhancing the material’s stability.This in-depth study of the interaction mechanism at the microscopic level when co-doping Fe and Mn offers valuable insights for the rational design and development of high-performance cathode materials.

Origin of enhanced electrochemical performance in lithium-rich cathode materials via the fast ion conductor Li_(2)O-B_(2)O_(3)-LiBr

Lithium-rich cathode materials have garnered significant attention in the energy sector due to their high specific capacity.However,severe capacity degradation impedes their large-scale application.The employment of fast ion conductors for coating has shown potential in improving their electrochemical performance,yet the structural and chemical mechanisms underlying this improvement remain unclear.In this study,we systematically analyze,through first-principles calculations,the mechanism by which Li_(2)O-B_(2)O_(3)-LiBr(Hereafter referred to as LBB)coating enhances the electrochemical performance of the lithium-rich layered cathode material 0.5Li2MnO3·0.5LiNi1/3Co1/3Mn1/3O2(Hereafter referred to as OLO).Our calculations reveal that the LBB coating introduces a more negative valence charge(average−0.14 e)around the oxygen atoms surrounding transition metals,thereby strengthening metal-oxygen interactions.This interaction mitigates irreversible oxygen oxidation caused by anionic redox reactions under high voltages,reducing irreversible structural changes during battery operation.Furthermore,while the migration barrier for Li+in OLO is 0.61 eV,the LBB coating acts as a rapid conduit during the Li+deintercalation process,reducing the migration barrier to 0.32 eV and slightly lowering the internal migration barrier within OLO to 0.43 eV.Calculations of binding energies to electrolyte byproducts HF before and after coating(at−7.421 and−3.253 eV,respectively)demonstrate that the LBB coating effectively resists HF corrosion.Subsequent electrochemical performance studies corroborated these findings.The OLO cathode with a 2%LBB coating exhibited a discharge capacity of 157.12 mAh g^(−1) after 100 cycles,with a capacity retention rate of 80.38%,whereas the uncoated OLO displayed only 141.67 mAh g^(−1) and a 72.45%capacity retention.At a 2 C rate,with the 2 wt%LBB-coated sample maintaining a discharge capacity of 140.22 mAh g^(−1) compared to only 107.02 mAh g^(−1) for the uncoated OLO.

Effect of Mo^(6+)doping on the structure and electrochemical properties of spinel-type LiMn_(2)O_(4)cathode materials

The Jahn-Teller effect and the dissolution of Mn are significant factors contributing to the capacity degradation of spinel LiMn_(2)O_(4) cathode materials during charging and discharging.In this study,Mo^(6+)-doped polycrystalline octahedral Li_(1.05)Mn_(2-x)Mo_(x)O_(4)(x=0,0.005,0.01,0.015)cathode materials were prepared by simple solid-phase sintering,and their crystal structures,microscopic morphologies,and elemental compositions were characterized and analyzed.The results showed that the doping of Mo^(6+)promoted the growth of(111)crystalline facets and increased the ratio of Mn^(3+)/Mn^(4+).The electrochemical performance of the materials was also tested,revealing that the doping of Mo^(6+)significantly improved the initial charge/discharge specific capacity and cycling stability.The modified sample(LMO-0.01Mo)retained a reversible capacity of 114.83 mA h/g with a capacity retention of 97.29%after 300 cycles.Additionally,the doping of Mo^(6+)formed a thinner,smoother SEI film and effectively inhibited the dissolution of Mn.Using density-functional theory(DFT)calculations to analyze the doping mechanism,it was found that doping shortens the Mn-O bond length inside the lattice and increases the Li-O bond length.This implies that the Li^(+)diffusion channel is widened,thereby increasing the Li^(+)diffusion rate.Additionally,the modification reduces the energy band gap,resulting in higher electronic conductivity.