Fiona C. Strobridge

Capturing metastable structures during high-rate cycling of LiFePO4 nanoparticle electrodes

Introduction The ability to achieve high cycling rates in a lithium-ion battery is limited by the Li transport within the electrolyte; the transport of Li ions and electrons within the electrodes; and, when a phase transformation is induced as a result of the Li compositional changes within an electrode, the nucleation and growth of the second phase. The absence of a phase transformation involving substantial structural rearrangements and large volume changes is generally considered to be key for achieving high rates. This assumption has been challenged by the discovery that some nanoparticulate electrode materials, most notably LiFePO4, can be cycled in a battery at very high rates, even though they cycle between two phases during battery operation. This apparent contradiction has been reconciled by the hypothesis that a nonequilibrium solid solution can be formed during reaction to bypass the nucleation step. Phase transformation from LiFePO4 (blue) to FePO4 (red). The delithiation (indicated by yellow arrows) proceeds at high rates via the formation of a nonequilibrium solid solution phase LixFePO4 (intermediate purple color), avoiding a classical nucleation process (indicated by dashed arrows). When the reaction is interrupted, the particles relax into the equilibrium configuration (shaded region), where only single-phase particles of LiFePO4 and/or FePO4 are present. Rationale To test this proposal, in situ techniques with high temporal resolution must be used to capture the fast phase transformation processes. We performed in situ synchrotron x-ray diffraction (XRD), which readily detects the structural changes and allows for fast data collection, on a LiFePO4-Li battery at high cycling rates, conditions that are able to drive the system away from equilibrium. We used an electrode comprising ~190-nm LiFePO4 particles, carbon, and binder (30:60:10 weight %), along with an electrochemical cell designed to yield reproducible results over multiple cycles, even at high rates. The high carbon content ensures that the reaction at high rates is not limited by either the electronic conductivity or ionic diffusion within the electrode composite. We compared the experimental results with simulated XRD patterns, in which the effects of strain versus compositional variation were explored. We then adapted a whole-pattern fitting method to quantify the compositional variation in the electrode during cycling. Results The XRD patterns, collected during high-rate galvanostatic cycling, show the expected disappearance of LiFePO4 Bragg reflections on charge and the simultaneous formation of FePO4 reflections. In addition, the development of positive intensities between the LiFePO4 and FePO4 reflections indicates that particles with lattice parameters that deviate from the equilibrium values of LiFePO4 and FePO4 are formed. The phenomenon is more pronounced at high currents. Detailed simulations of the XRD patterns reveal that this lattice-parameter variation cannot be explained by a LiFePO4-FePO4 interface within the particles, unless the size of the interface is similar to or greater than the size of the entire particle. Instead, the results indicate compositional variation either within or between particles. Conclusion The results demonstrate the formation of a nonequilibrium solid solution phase, LixFePO4 (0 < x < 1), during high-rate cycling, with compositions that span the entire composition between two thermodynamic phases, LiFePO4 and FePO4. This confirms the hypothesis that phase transformations in nanoparticulate LiFePO4 proceed, at least at high rates, via a continuous change in structure rather than a distinct moving phase boundary between LiFePO4 and FePO4. The ability of LiFePO4 to transform via a nonequilibrium single-phase solid solution, which avoids major structural rearrangement across a moving interface, helps to explain its high-rate performance despite a large Li miscibility gap at room temperature. The creation of a low-energy nonequilibrium path by, for example, particle size reduction or cation doping should enable the high-rate capabilities of other phase-transforming electrode materials. Watching battery materials in action When batteries get rapidly charged and discharged repeatedly, they will often stop working. This is especially true when the cycling changes the crystal structure of the battery components. Liu et al. examined the structural changes in components of a type of lithium battery (see the Perspective by Owen and Hector). Their findings explain why LiFePO4 delivers unexpectedly good electrochemical performances, particularly during rapid cycling. Science, this issue p. 10.1126/science.1252817; see also p. 1451 X-ray diffraction reveals that metastable solid solution reactions undergird the high-rate capability of LiFePO4 electrodes. [Also see Perspective by Owen and Hector] The absence of a phase transformation involving substantial structural rearrangements and large volume changes is generally considered to be a key characteristic underpinning the high-rate capability of any battery electrode material. In apparent contradiction, nanoparticulate LiFePO4, a commercially important cathode material, displays exceptionally high rates, whereas its lithium-composition phase diagram indicates that it should react via a kinetically limited, two-phase nucleation and growth process. Knowledge concerning the equilibrium phases is therefore insufficient, and direct investigation of the dynamic process is required. Using time-resolved in situ x-ray powder diffraction, we reveal the existence of a continuous metastable solid solution phase during rapid lithium extraction and insertion. This nonequilibrium facile phase transformation route provides a mechanism for realizing high-rate capability of electrode materials that operate via two-phase reactions.

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 LiFe(x)Mn(1-x)PO(4) 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 (31)P NMR spectra of the LiFe(x)Mn(1-x)PO(4) (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.

A stepwise mechanism and the role of water in the liquid-assisted grinding synthesis of metal–organic materials

The mechanism of the room-temperature synthesis of coordination polymers from ZnO by liquid-assisted grinding (LAG) was investigated, and using catalytic amounts of water allowed us to extend the scope of this synthetic method to copper compounds. The mechanochemical synthesis of model compounds zinc fumarate and copper(II) acetate proceeds through a stepwise mechanism which involves the intermediate formation of solvates with water (in the case of zinc fumarate) or acetic acid (in the case of copper(II) acetate) as kinetic products. The course of zinc fumarate LAG synthesis was explored using three different types of grinding liquids: water, aqueous organic solvents and pure organic solvents. With liquid water, the formation of the kinetic product switches the reaction mechanism from LAG to a neat grinding process. As a result, the reaction scope is limited to either the tetrahydrate or the pentahydrate as the major products. In contrast, the use of aqueous organic solvents as grinding liquids allows the selective synthesis as well as screening for different hydrated and anhydrous forms of zinc fumarate. Different polymorphs of the zinc fumarate coordination polymer can be obtained by changing the organic liquid. As a first step towards the quantitative understanding of how the liquid phase directs LAG mechanosynthesis, we demonstrate that product formation is regulated by the mole fraction and activity of water in the grinding liquid.

Identifying the Structure of the Intermediate, Li2/3CoPO4, Formed during Electrochemical Cycling of LiCoPO4

In situ synchrotron diffraction measurements and subsequent Rietveld refinements are used to show that the high energy density cathode material LiCoPO4 (space group Pnma) undergoes two distinct two-phase reactions upon charge and discharge, both occurring via an intermediate Li2/3(Co2+)2/3(Co3+)1/3PO4 phase. Two resonances are observed for Li2/3CoPO4 with intensity ratios of 2:1 and 1:1 in the 31P and 7Li NMR spectra, respectively. An ordering of Co2+/Co3+ oxidation states is proposed within a (a × 3b × c) supercell, and Li+/vacancy ordering is investigated using experimental NMR data in combination with first-principles solid-state DFT calculations. In the lowest energy configuration, both the Co3+ ions and Li vacancies are found to order along the b-axis. Two other low energy Li+/vacancy ordering schemes are found only 5 meV per formula unit higher in energy. All three configurations lie below the LiCoPO4–CoPO4 convex hull and they may be readily interconverted by Li+ hops along the b-direction.

Characterising local environments in high energy density Li-ion battery cathodes : a combined NMR and first principles study of LiFexCo1−xPO4

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 LiFexCo1−xPO4 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 M–O–P pathways in the end members LFP and LCP and compared with the fitted values extracted from the LiFexCo1−xPO4 experimental data. Combining both data sets, the FCS for the range of local P environments expected in LiFexCo1−xPO4 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 Co–O–P 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 LiFexMn1−xPO4, where all 31P shifts lay within the LiMnPO4–LFP range. Although there are 24 distinct local P environments in LiFexCo1−xPO4, 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.

A rational approach to screen for hydrated forms of the pharmaceutical derivative magnesium naproxen using liquid-assisted grinding

Variation of water content in liquid-assisted grinding was utilised to mechanochemically screen for different hydrated forms of magnesium naproxen directly from a mixture of magnesium oxide and naproxen. Structure determination from powder and single crystal X-ray diffraction data, supported by solid-state NMR and synchrotron radiation diffraction experiments, revealed a monohydrate coordination polymer, a discrete tetrahydrate complex, and provided a preliminary structural model for a highly hydrated salt.

Localized concentration reversal of lithium during intercalation into nanoparticles

Inhomogeneous Li intercalation and localized concentration reversal in nanoparticles are investigated on a nanometer scale. Nanoparticulate electrodes, such as LixFePO4, 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 LixFePO4 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 11 nm 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.Here the authors show the development of soft X-ray ptychographic tomography to quantify the electrochemical state and resolve phase boundaries throughout the volume of individual nano-particles from a composite battery electrode.

Nanoscale Detection of Intermediate Solid Solutions in Equilibrated LixFePO4 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 LiFePO4 crystals equilibrated after delithiation, SXDM revealed the existence of domains of miscibility between LiFePO4 and Li0.6FePO4. These solid solutions are conventionally thought to be metastable, and were previously undetected by spectromicroscopy. The observation provides experimental verification of predictions that the LiFePO4-FePO4 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.

Research data supporting "Unraveling the Complex Delithiation Mechanisms of Olivine-Type Cathode Materials, LiFexCo1-xPO4"

X-ray diffraction raw data and the Rietveld refinement input files and output files. All of the data was collected at Argonne National Laboratory at the Advanced Photon Source at Sector 11-BM.