Study of Defect Chemistry in the System La_2–<i>x</i>Sr_<i>x</i>NiO_4+δ by ¹⁷O Solid-State NMR Spectroscopy and Ni K‑Edge XANES

The properties of mixed ionic–electronic conductors (MIECs) are most conveniently controlled through site-specific aliovalent substitution, yet few techniques can report directly on the local structure and defect chemistry underpinning changes in ionic and electronic conductivity. In this work, we perform high-resolution ¹⁷O (<i>I</i> = 5/2) solid-state NMR spectroscopy of La_2–<i>x</i>Sr_<i>x</i>NiO_4+δ, an MIEC and prospective solid oxide fuel cell (SOFC) cathode material, showing the sensitivity of ¹⁷O hyperfine (Fermi contact) shifts and quadrupolar coupling constants due to local structural changes arising from Sr substitution (<i>x</i>). Previously, we resolved resonances from three distinct oxygen sites (interstitial, axial, and equatorial) in the unsubstituted <i>x</i> = 0 material (Halat et al., <i>J. Am. Chem. Soc.</i> <b>2016</b>, 138, 11958). Here, substitution-induced changes in these three spectral features indirectly report on the ionic conductivity, local octahedral tilting, and electronic conductivity, respectively, of the (substituted) materials. In particular, the intensity of the ¹⁷O resonance arising from mobile interstitial defects decreases, and then disappears, at <i>x</i> = 0.5, consistent with reports of lower bulk ionic conductivity in Sr-substituted phases. Second, local distortions among the split axial oxygen sites diminish, even on modest incorporation of Sr (<i>x</i> < 0.1), which is also accompanied by faster spin–lattice (<i>T</i>₁) relaxation of the interstitial ¹⁷O resonances, indicating increased mobility of the associated sites. Finally, the hyperfine shift of the equatorial oxygen resonance decreases due to conversion of Ni²⁺ (d⁸) to Ni³⁺ (d⁷) by charge compensation, a mechanism associated with improved electronic conductivity in the Sr-substituted phases. Valence and coordination changes of the Ni cations are further supported by Ni K-edge X-ray absorption near-edge structure (XANES) measurements, which show a decrease in the Jahn–Teller distortion of the Ni³⁺ sites and a Ni coordination change consistent with the formation of oxygen vacancies. Ultimately, these insights into local atomic and electronic structure that rely on ¹⁷O solid-state NMR spectroscopy should prove relevant for a broad range of aliovalently substituted functional paramagnetic oxides

Ab Initio Structure Search and in Situ ⁷Li NMR Studies of Discharge Products in the Li–S Battery System

The high theoretical gravimetric capacity of the Li–S battery system makes it an attractive candidate for numerous energy storage applications. In practice, cell performance is plagued by low practical capacity and poor cycling. In an effort to explore the mechanism of the discharge with the goal of better understanding performance, we examine the Li–S phase diagram using computational techniques and complement this with an in situ ⁷Li NMR study of the cell during discharge. Both the computational and experimental studies are consistent with the suggestion that the only solid product formed in the cell is Li₂S, formed soon after cell discharge is initiated. In situ NMR spectroscopy also allows the direct observation of soluble Li⁺-species during cell discharge; species that are known to be highly detrimental to capacity retention. We suggest that during the first discharge plateau, S is reduced to soluble polysulfide species concurrently with the formation of a solid component (Li₂S) which forms near the beginning of the first plateau, in the cell configuration studied here. The NMR data suggest that the second plateau is defined by the reduction of the residual soluble species to solid product (Li₂S). A ternary diagram is presented to rationalize the phases observed with NMR during the discharge pathway and provide thermodynamic underpinnings for the shape of the discharge profile as a function of cell composition

Local Structure Evolution and Modes of Charge Storage in Secondary Li–FeS₂ Cells

In the pursuit of high-capacity electrochemical energy storage, a promising domain of research involves conversion reaction schemes, wherein electrode materials are fully transformed during charge and discharge. There are, however, numerous difficulties in realizing theoretical capacity and high rate capability in many conversion schemes. Here we employ <i>operando</i> studies to understand the conversion material FeS₂, focusing on the local structure evolution of this relatively reversible material. X-ray absorption spectroscopy, pair distribution function analysis, and first-principles calculations of intermediate structures shed light on the mechanism of charge storage in the Li–FeS₂ system, with some general principles emerging for charge storage in chalcogenide materials. Focusing on second and later charge/discharge cycles, we find small, disordered domains that locally resemble Fe and Li₂S at the end of the first discharge. Upon charge, this is converted to a Li–Fe–S composition whose local structure reveals tetrahedrally coordinated Fe. With continued charge, this ternary composition displays insertion–extraction behavior at higher potentials and lower Li content. The finding of hybrid modes of charge storage, rather than simple conversion, points to the important role of intermediates that appear to store charge by mechanisms that more closely resemble intercalation

Electrochemical Performance of Nanosized Disordered LiVOPO₄

ε-LiVOPO₄ is a promising multielectron cathode material for Li-ion batteries that can accommodate two electrons per vanadium, leading to higher energy densities. However, poor electronic conductivity and low lithium ion diffusivity currently result in low rate capability and poor cycle life. To enhance the electrochemical performance of ε-LiVOPO₄, in this work, we optimized its solid-state synthesis route using in situ synchrotron X-ray diffraction and applied a combination of high-energy ball-milling with electronically and ionically conductive coatings aiming to improve bulk and surface Li diffusion. We show that high-energy ball-milling, while reducing the particle size also introduces structural disorder, as evidenced by ⁷Li and ³¹P NMR and X-ray absorption spectroscopy. We also show that a combination of electronically and ionically conductive coatings helps to utilize close to theoretical capacity for ε-LiVOPO₄ at C/50 (1 C = 153 mA h g^–1) and to enhance rate performance and capacity retention. The optimized ε-LiVOPO₄/Li₃VO₄/acetylene black composite yields the high cycling capacity of 250 mA h g^–1 at C/5 for over 70 cycles