18 research outputs found
Localized concentration reversal of lithium during intercalation into nanoparticles.
Nanoparticulate electrodes, such as Li x FePO4, have unique advantages over their microparticulate counterparts for the applications in Li-ion batteries because of the shortened diffusion path and access to nonequilibrium routes for fast Li incorporation, thus radically boosting power density of the electrodes. However, how Li intercalation occurs locally in a single nanoparticle of such materials remains unresolved because real-time observation at such a fine scale is still lacking. We report visualization of local Li intercalation via solid-solution transformation in individual Li x FePO4 nanoparticles, enabled by probing sub-angstrom changes in the lattice spacing in situ. The real-time observation reveals inhomogeneous intercalation, accompanied with an unexpected reversal of Li concentration at the nanometer scale. The origin of the reversal phenomenon is elucidated through phase-field simulations, and it is attributed to the presence of structurally different regions that have distinct chemical potential functions. The findings from this study provide a new perspective on the local intercalation dynamics in battery electrodes
Three-dimensional localization of nanoscale battery reactions using soft X-ray tomography.
Battery function is determined by the efficiency and reversibility of the electrochemical phase transformations at solid electrodes. The microscopic tools available to study the chemical states of matter with the required spatial resolution and chemical specificity are intrinsically limited when studying complex architectures by their reliance on two-dimensional projections of thick material. Here, we report the development of soft X-ray ptychographic tomography, which resolves chemical states in three dimensions at 11ânm spatial resolution. We study an ensemble of nano-plates of lithium iron phosphate extracted from a battery electrode at 50% state of charge. Using a set of nanoscale tomograms, we quantify the electrochemical state and resolve phase boundaries throughout the volume of individual nanoparticles. These observations reveal multiple reaction points, intra-particle heterogeneity, and size effects that highlight the importance of multi-dimensional analytical tools in providing novel insight to the design of the next generation of high-performance devices
Density Functional Theory-Based Bond Pathway Decompositions of Hyperfine Shifts: Equipping Solid-State NMR to Characterize Atomic Environments in Paramagnetic Materials
Solid-state
nuclear magnetic resonance (NMR) of paramagnetic samples has the potential
to provide a detailed insight into the environments and processes
occurring in a wide range of technologically-relevant phases, but
the acquisition and interpretation of spectra is typically not straightforward.
Structural complexity and/or the occurrence of charge or orbital ordering
further compound such difficulties. In response to such challenges,
the present article outlines how the total Fermi contact (FC) shifts
of NMR observed centers (OCs) may be decomposed into sets of pairwise
metalâOC bond pathway contributions via solid-state hybrid
density functional theory calculations. A generally applicable âspin
flippingâ approach is outlined wherein bond pathway contributions
are obtained by the reversal of spin moments at selected metal sites.
The applications of such pathway contributions in interpreting the
NMR spectra of structurally and electronically complex phases are
demonstrated in a range of paramagnetic Li-ion battery positive electrodes
comprising layered LiNiO<sub>2</sub>, LiNi<sub>0.125</sub>Co<sub>0.875</sub>O<sub>2</sub>, and LiCr<sub>0.125</sub>Co<sub>0.875</sub>O<sub>2</sub> oxides; and olivine-type LiMPO<sub>4</sub> and MPO<sub>4</sub> (M
= Mn, Fe, and Co) phosphates. The FC NMR shifts of all <sup>6/7</sup>Li and <sup>31</sup>P sites are decomposed, providing unambiguous
NMR-based proof of the existence of local Ni<sup>3+</sup>-centered
JahnâTeller distortions in LiNiO<sub>2</sub> and LiNi<sub>0.125</sub>Co<sub>0.875</sub>O<sub>2</sub>, and showing that the presence of
M<sup>2+</sup>/M<sup>3+</sup> solid solutions and/or M/MâČ isovalent
transition metal (TM) mixtures in the olivine-type electrodes should
lead to broad and potentially interpretable NMR spectra. Clear evidence
for the presence of a dynamic JahnâTeller distortion is obtained
for LiNi<sub><i>x</i></sub>Co<sub>1â<i>x</i></sub>O<sub>2</sub>. The results emphasize the utility of solid-state
NMR in application to TM-containing battery materials and to paramagnetic
samples in general
Spin-Transfer Pathways in Paramagnetic Lithium Transition-Metal Phosphates from Combined Broadband Isotropic Solid-State MAS NMR Spectroscopy and DFT Calculations
Substituted lithium transition-metal (TM) phosphate LiFexMn1-xPO4, materials with olivine-type structures are among the most promising next generation lithium ion battery cathodes. However, a complete atomic-level description of the structure of such phases is not yet available. Here, a combined experimental and theoretical approach to the detailed assignment of the P-31 NMR spectra of the LiFexMn1-xPO4 (x = 0, 0.25, 0.5, 0.75, 1) pure and mixed TM phosphates is developed and applied. Key to the present work is the development of a new NMR experiment enabling the characterization of complex paramagnetic materials via the complete separation of the individual isotropic chemical shifts, along with solid-state hybrid DFT calculations providing the separate hyperfine contributions of all distinct Mn-O-P and Fe-O-P bond pathways. The NMR experiment, referred to as aMAT, makes use of short high-powered adiabatic pulses (SHAPs), which can achieve 100% inversion over a range of isotropic shifts on the order of 1 MHz and with anisotropies greater than 100 kHz. In addition to complete spectral assignments of the mixed phases, the present study provides a detailed insight into the differences in electronic structure driving the variations in hyperfine parameters across the range of materials. A simple model delimiting the effects of distortions due to Mn/Fe substitution is also proposed and applied. The combined approach has clear future applications to TM-bearing battery cathode phases in particular and for the understanding of complex paramagnetic phases in general
Spin-Transfer Pathways in Paramagnetic Lithium Transition-Metal Phosphates from Combined Broadband Isotropic Solid-State MAS NMR Spectroscopy and DFT Calculations
International audienceSubstituted lithium transition-metal (TM) phosphate LiFexMn1-xPO4, materials with olivine-type structures are among the most promising next generation lithium ion battery cathodes. However, a complete atomic-level description of the structure of such phases is not yet available. Here, a combined experimental and theoretical approach to the detailed assignment of the P-31 NMR spectra of the LiFexMn1-xPO4 (x = 0, 0.25, 0.5, 0.75, 1) pure and mixed TM phosphates is developed and applied. Key to the present work is the development of a new NMR experiment enabling the characterization of complex paramagnetic materials via the complete separation of the individual isotropic chemical shifts, along with solid-state hybrid DFT calculations providing the separate hyperfine contributions of all distinct Mn-O-P and Fe-O-P bond pathways. The NMR experiment, referred to as aMAT, makes use of short high-powered adiabatic pulses (SHAPs), which can achieve 100% inversion over a range of isotropic shifts on the order of 1 MHz and with anisotropies greater than 100 kHz. In addition to complete spectral assignments of the mixed phases, the present study provides a detailed insight into the differences in electronic structure driving the variations in hyperfine parameters across the range of materials. A simple model delimiting the effects of distortions due to Mn/Fe substitution is also proposed and applied. The combined approach has clear future applications to TM-bearing battery cathode phases in particular and for the understanding of complex paramagnetic phases in general
Nanoscale Detection of Intermediate Solid Solutions in Equilibrated Li<sub><i>x</i></sub>FePO<sub>4</sub> Microcrystals
Redox-driven
phase transformations in solids determine the performance
of lithium-ion batteries, crucial in the technological transition
from fossil fuels. Couplings between chemistry and strain define reversibility
and fatigue of an electrode. The accurate definition of all phases
in the transformation, their energetics, and nanoscale location within
a particle produces fundamental understanding of these couplings needed
to design materials with ultimate performance. Here we demonstrate
that scanning X-ray diffraction microscopy (SXDM) extends our ability
to image battery processes in single particles. In LiFePO<sub>4</sub> crystals equilibrated after delithiation, SXDM revealed the existence
of domains of miscibility between LiFePO<sub>4</sub> and Li<sub>0.6</sub>FePO<sub>4</sub>. These solid solutions are conventionally thought
to be metastable, and were previously undetected by spectromicroscopy.
The observation provides experimental verification of predictions
that the LiFePO<sub>4</sub>âFePO<sub>4</sub> phase diagram
can be altered by coherency strain under certain interfacial orientations.
It enriches our understanding of the interaction between diffusion,
chemistry, and mechanics in solid state transformations
Characterising local environments in high energy density Li-ion battery cathodes : a combined NMR and first principles study of LiFexCo1âxPO4
Olivine-type LiCoPO4 (LCP) is a high energy density lithium ion battery cathode material due to the high voltage of the Co2+/Co3+ redox reaction. However, it displays a significantly poorer electrochemical performance than its more widely investigated isostructural analogue LiFePO4 (LFP). The co-substituted LiFexCo1âxPO4 olivines combine many of the positive attributes of each end member compound and are promising next-generation cathode materials. Here, the fully lithiated x = 0, 0.25, 0.5, 0.75 and 1 samples are extensively studied using 31P solid-state nuclear magnetic resonance (NMR). Practical approaches to broadband excitation and for the resolution of the isotropic resonances are described. First principles hybrid density functional calculations are performed on the Fermi contact shift (FCS) contributions of individual MâOâP pathways in the end members LFP and LCP and compared with the fitted values extracted from the LiFexCo1âxPO4 experimental data. Combining both data sets, the FCS for the range of local P environments expected in LiFexCo1âxPO4 have been calculated and used to assign the NMR spectra. Due to the additional unpaired electron in d6 Fe2+ as compared with d7 Co2+ (both high spin), LFP is expected to have larger Fermi contact shifts than LCP. However, two of the CoâOâP pathways in LCP give rise to noticeably larger shifts and the unexpected appearance of peaks outside the range delimited by the pure LFP and LCP 31P shifts. This behaviour contrasts with that observed previously in LiFexMn1âxPO4, where all 31P shifts lay within the LiMnPO4âLFP range. Although there are 24 distinct local P environments in LiFexCo1âxPO4, these group into seven resonances in the NMR spectra, due to significant overlap of the isotropic shifts. The local environments that give rise to the largest contributions to the spectral intensity are identified and used to simplify the assignment. This provides a tool for future studies of the electrochemically-cycled samples, which would otherwise be challenging to interpret
Unraveling the Complex Delithiation Mechanisms of Olivine-Type Cathode Materials, LiFe<sub><i>x</i></sub>Co<sub>1â<i>x</i></sub>PO<sub>4</sub>
The
delithiation mechanisms occurring within the olivine-type class
of cathode materials for Li-ion batteries have received considerable
attention because of the good capacity retention at high rates for
LiFePO<sub>4</sub>. A comprehensive mechanistic study of the (de)Âlithiation
reactions that occur when the substituted olivine-type cathode materials
LiFe<sub><i>x</i></sub>Co<sub>1â<i>x</i></sub>PO<sub>4</sub> (<i>x</i> = 0, 0.05, 0.125, 0.25,
0.5, 0.75, 0.875, 0.95, 1) are electrochemically cycled is reported
here using in situ X-ray diffraction (XRD) data and supporting ex
situ <sup>31</sup>P NMR spectra. On the first charge, two intermediate
phases are observed and identified: Li<sub>1â<i>x</i></sub>(Fe<sup>3+</sup>)<sub><i>x</i></sub>(Co<sup>2+</sup>)<sub>1â<i>x</i></sub>PO<sub>4</sub> for 0 < <i>x</i> < 1 (i.e., after oxidation of Fe<sup>2+</sup> to Fe<sup>3+</sup>) and Li<sub>2/3</sub>Fe<sub><i>x</i></sub>Co<sub>1â<i>x</i></sub>PO<sub>4</sub> for 0 †<i>x</i> †0.5 (i.e., the Co-majority materials). For the
Fe-rich materials, we study how nonequilibrium, single-phase mechanisms
that occur discretely in single particles, as observed for LiFePO<sub>4</sub> at high rates, are affected by Co substitution. In the Co-majority
materials, a two-phase mechanism with a coherent interface is observed,
as was seen in LiCoPO<sub>4</sub>, and we discuss how it is manifested
in the XRD patterns. We then compare the nonequilibrium, single-phase
mechanism with the bulk single-phase and coherent interface two-phase
mechanisms. Despite the apparent differences between these mechanisms,
we discuss how they are related and interconverted as a function of
Fe/Co substitution and the potential implications for the electrochemistry
of this system