Lithium Intercalation in Anatase Titanium Vacancies and the Role of Local Anionic Environment

Abstract

The structure of bulk and nondefective compounds is generally described with crystal models built from well mastered techniques such the analysis of an X-ray diffractogram. The presence of defects, such as cationic vacancies, locally disrupt the long-range order, with the appearance of local structures with order extending only a few nanometers. To probe and describe the electrochemical properties of cation-deficient anatase, we investigated a series of materials having different concentrations of vacancies, i.e., Ti1–x–y □ x+y O2–4(x+y)F4x (OH)4y , and compared their properties with respect to defect-free stoichiometric anatase TiO2. At first, we characterized the series of materials Ti1–x–y □ x+y O2–4(x+y)F4x (OH)4y by means of pair distribution function (PDF), 19F nuclear magnetic resonance (NMR), Raman and X-ray photoelectron spectroscopies, to probe the compositional and structural features. Second, we characterized the insertion electrochemical properties vs metallic lithium where we emphasized the beneficial role of the vacancies on the cyclability of the electrode under high C-rate, with performances scaling with the concentration of vacancies. The improved properties were explained by the change of the lithium insertion mechanism due to the presence of the vacancies, which act as host sites and suppress the phase transition typically observed in pure TiO2, and further favor diffusive transport of lithium within the structure. NMR spectroscopy performed on lithiated samples provides evidence for the insertion of lithium in vacancies. By combining electrochemistry and DFT-calculations, we characterized the electrochemical signatures of the lithium insertion in the vacancies. Importantly, we found that the insertion voltage largely depends on the local anionic environment of the vacancy with a fluoride and hydroxide-rich environments, yielding high and low insertion voltages, respectively. This work further supports the beneficial use of defects engineering in electrodes for batteries and provides new fundamental knowledge in the insertion chemistry of cationic vacancies as host sites.

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

“…Ma et al [ 96 ] prepared a series of cation‐deficient TiO 2 using solvothermal approach, with the cation vacancy content controlled by the reaction temperatures. To determine the titanium vacancy concentration, they used two approaches: i) the real‐space refinement of PDF data and ii) individual peaks fitting.…”

“…The presence of cation vacancies will affect the frequencies of vibrational modes of the host atomic bonds around defect sites, which are highly sensitive to the local structures and therefore can be used to identify the presence of cation vacancies. Ma et al [ 96 ] characterized the titanium vacancies in defective Ti 1 − x − y □ x + y O 2 − 4( x + y ) F 4 x (OH) 4 y using Raman spectroscopy, with the Ti vacancies induced by different reaction temperatures. Through varying the reaction temperature, the vacancy concentration of the as‐prepared Ti 1 − x − y □ x + y O 2 − 4( x + y ) F 4x (OH) 4 y can be manipulated from 0 to 0.3.…”

“…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.…”

“…There are several ways for incorporating cation vacancies and tailoring its content in transition metal oxides/carbides through synthetic approaches, such as replacing the constituent cations or anions with aliovalent substitutes, [ 94–96,104,105 ] equilibration at solutions with different pH values, [ 89,90,99,106,107 ] selective removal of constituent cations, [ 101,102,108 ] thermal annealing in defects inducing atmospheres, [ 109,110 ] and plasma etching. [ 111 ] The characterization of cation vacancies, however, is usually challenging.…”

“…[ 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. Some of these techniques are commonly used together to elucidate the presence and content of cation vacancies, which will give a better understanding of the physical nature of these cation‐deficient nanomaterials.…”