Electrochemical Li-ion storage in defect spinel iron oxides: the critical role of cation vacancies

Abstract

Rechargeable lithium-ion batteries are the preferred power source for consumer electronic devices, but the cost and toxicity of many cathode materials limit their scale-up. Worldwide research efforts are addressing this concern by transitioning from conventional Co-and Ni-based intercalation hosts towards Fe-and Mn-based alternatives. The unfavorable energetics of the Fe 2+/3+ redox couple and limited Li-insertion capacities render the use of iron oxides impractical. We address this limitation with the defect spinel ferrite g-Fe 2 O 3 as a model structure for Li-ion insertion by replacing a fraction of the Fe 3+ sites with highly oxidized Mo 6+ to generate cation vacancies that shift the onset of Li-ion insertion to more positive potentials as well as increase capacity. In the present study, native and Mo-substituted iron oxides are synthesized via base-catalyzed precipitation in aqueous media, yielding nanocrystalline spinel materials that also exhibit short-range disorder characteristic of a proton-stabilized structure. The Mo-substituted ferrite reported herein is estimated to have $3Â as many cation vacancies as g-Fe 2 O 3 with a corresponding increase in the Li-ion capacity to >100 mA h g À1 between 4.1 and 2.0 V vs. Li/Li + . This dual enhancement in capacity and insertion potential will enable these and related defect spinel ferrites to be explored as positive electrode materials for lithium batteries, while retaining the cost advantages of a material whose metal composition is still predominately iron based.

The Role of Cation Vacancies in Electrode Materials for Enhanced Electrochemical Energy Storage: Synthesis, Advanced Characterization, and Fundamentals

“…The replacement of constituent cations with aliovalent dopants not only creates cation vacancies, but may also increases the cell volume of the host lattice, thus provides new intercalation sites, facilitates ion diffusion, and improves the electronic conductivity. [ 75,115,126 ] Hahn et al [ 97 ] have prepared defective γ‐Fe 2 O 3 by replacing a fraction of Fe 3+ with highly oxidized Mo 6+ to generate more cation vacancies. Detailed chemical formula for native and Mo‐substituted ferrite can be written as Fe 3 + 2.22 Fe 2 + 0.67 □ 0.11 O 4 · n H 2 O and Mo 6 + 0.59 Fe 3 + 1.50 □ 0.91 O 4 · n H 2 O, respectively, which clearly indicate that Mo‐doping leads to the formation of more Fe vacancies in the as‐prepared ferrite.…”

“…It is also more favorable for the electrochemical intercalation of electrolyte cations near the vacancies, which leads to enhanced rate capability in the defective MnO 2 for electrochemical intercalation/deintercalation. Following this pioneering study, more research works have been conducted to intentionally introduce cation vacancies into different transition metal oxides/carbides, such as MnO 2 , [ 89–93 ] TiO 2 , [ 94–96 ] Fe 2 O 3 , [ 77,78,97,98 ] ZnCo 2 O 4 , [ 99 ] ZnMn 2 O 4 , [ 76 ] and MXenes, [ 79,100–102 ] providing better and more fundamental understanding of the role of cation vacancies in boosting electrochemical performance ( Figure 2 ). Several density functional theory (DFT) calculations have also indicated that the presence of cation vacancies results in a decrease of energy barrier for ion diffusion, [ 95,103 ] and an increase of the materials' electronic conductivity, [ 80 ] thus benefits the charge storage process.…”

“…Atomic resolution transmission electron microscope (TEM), with the ability to see single atom, is typically used to give a direct visualization of the cation vacancies. [ 90,95,107 ] Other powerful techniques for detecting cation vacancies in nanomaterials, especially the materials without long‐range order, are extended X‐ray absorption fine structure (EXAFS) [ 77,78,97,105,111 ] and X‐ray pair distribution function (PDF) analysis, [ 89,94,96,106 ] both of which are sensitive to the local structure variations that enable accurate determination of cation vacancies. Additionally, multiple analytical techniques, such as X‐ray photoelectron spectroscopy (XPS), [ 91,105,113 ] nuclear magnetic resonance (NMR), [ 94–96 ] Raman/Fourier transform infrared (FTIR) spectroscopy, [ 104 ] electron paramagnetic resonance (EPR), [ 110 ] photoluminescence (PL), [ 110 ] and positron annihilation spectrometry (PAS), [ 114 ] have also been reported to serve as complementary tools for cation vacancies characterization.…”

Defective by design: vanadium-substituted iron oxide nanoarchitectures as cation-insertion hosts for electrochemical charge storage

“…For example, maghemite (g-Fe 2 O 3 , more accurately denoted as Fe 3+ 2.67 , 0.33 O 4 ) is a cation-decient spinel phase that inserts Li + into structural cation vacancies at more positive potentials than observed in the defect-free analogue, magnetite (Fe 3 O 4 ). 8,9 The population of electrochemically active defects in metal oxides can also be amplied when these materials are expressed in nanoscale forms, 10 ranging from hollow nanospheres 11,12 to three-dimensionally (3D) ultraporous nanoarchitectures such as aerogels. 6,7 We recently reported that a nanocrystalline Mo-substituted ferrite (Mo 0.59 Fe 1.50 , 0.91 O 4 $nH 2 O) provides >4Â improvement in specic capacity for Li + -insertion relative to an analogous g-Fe 2 O 3 material.…”

Electrochemical energy storage to power the 21st century

“…38 for the unsubstituted ferrite control) and increased the electrochemical potential of the oxide to move the capacity into a cathode-relevant range of 4.1 and 2.0 V.…”