Origin and hysteresis of lithium compositional spatiodynamics within battery primary particles

Abstract

Abstract:The kinetics and uniformity of ion insertion reactions at the solid/liquid interface govern the rate capability and lifetime, respectively, of electrochemical devices such as Li-ion batteries.We develop an operando X-ray microscopy platform that maps the dynamics of the Li composition and insertion rate in LiXFePO4, and show that nanoscale spatial variations in rate and in composition control the lithiation pathway at the sub-particle length scale. Specifically, spatial variations in the insertion rate constant lead to the formation of nonuniform domains, and the composition dependence of the rate constant amplifies nonuniformities during delithiation but suppresses them during lithiation, and moreover stabilizes the solid solution during lithiation. This coupling of lithium composition and surface reaction rates controls the kinetics and uniformity during electrochemical ion insertion.One Sentence Summary: X-ray microscopy reveals the nanoscale evolution of composition and reaction rate inside a Li-ion battery during cycling Main Text: The insertion of a guest ion into the host crystal is the fundamental reaction underpinning insertion electrochemistry and has been applied to store energy (1), tune catalysts (2), and switch optoelectronic properties (3). In Li-ion batteries, for example, Li ions from the 2 liquid electrolyte insert into solid host particles in the electrode. Nanoscale intraparticle electrochemical inhomogeneities in phase and in composition are responsible for mechanical strain and fracture which decrease the reversibility of the reaction (4). Moreover, these nonuniformities make it difficult to correlate current-voltage measurements to microscopic ion insertion mechanisms. Simultaneously quantifying nonuniform nanoscale reaction kinetics and the underlying material composition at the solid-liquid interface holds the key to improving device performance.A gold standard material for investigating ion insertion reactions is LiXFePO4 (0<X<1), which separates into two phases at equilibrium (5). Recent studies using in situ X-ray diffraction observed a continuous distribution of lattice constants at high rates of (de)lithiation (6-9). This finding supports the hypothesis that phase separation is suppressed during (de)lithiation and replaced with a solid solution crystallographic insertion pathway (10), consistent with theoretical predictions (11, 12). Precise quantification of the Li composition (X) is difficult because the lattice constant change convolves information from both Li composition and mechanical strain (8). Whereas heterogeneous current distributions between particles have been studied (7, 13, 14), there exists little understanding of how compositional nonuniformities evolve within individual particles.Diffuse interfaces have been proposed from diffraction patterns (7), but it is unclear where they occur or how they develop over time. Even less understood is the effect of interfacial reactivity on the single-particle lithiation pathway, which has been explored using models (11...

Mesoscopic Modeling of a LiFePO4Electrode: Experimental Validation under Continuous and Intermittent Operating Conditions

“…5,32,53 Very recently, the physical picture of the mesoscopic elementary units in our model has been confirmed by high-resolution scanning transmission X-ray microscopy (STXM) observation of a very dilute LFP electrode. 54 This showed that stationary boundaries (as opposed to moving phase boundaries in conventional views of phase transformation) divide micron-sized particles into many single-phase solidsolution domains which are lithiating/delithiating to settle into either Li-rich or Li-poor phases at different rates and independently of one another (see movie S2 in Ref. 54).…”

“…54 This showed that stationary boundaries (as opposed to moving phase boundaries in conventional views of phase transformation) divide micron-sized particles into many single-phase solidsolution domains which are lithiating/delithiating to settle into either Li-rich or Li-poor phases at different rates and independently of one another (see movie S2 in Ref. 54). It has also been observed that the domains that lithiate first also delithiate first and those that lithiate last also delithiate last, confirming that such a meso-scale phase distribution within each particle is not related to nucleation and growth or spinodal decomposition mechanisms as proposed in other models.…”

Better linkage of smart materials to energy scale

“…Applying advanced manufacturing principles such as ink-jets could enable tailor-made thin structures reducing charge transfer losses and raise the efficiency of electrochemical devices. Advanced microscopy and characterization such as 3D tomography and nanoscale visualization [5] accompanied with interdisciplinary research (e.g., molecular dynamics simulations and machine learning for material science) could in turn broaden our understanding of the underlying phenomena for inefficiencies, heterogeneities, and aging. …”

Probing the local structure of Prussian blue electrodes by 113Cd NMR spectroscopy

“…Gaining information on the structural and electronic changes occurring in the host material at the atomic scale is now crucial to improve the batteries performance. [17][18][19][20][21] For cathodes materials containing iron ions, 57 Fe Mössbauer spectroscopy allows to probe the redox state of the active iron(II/III) ions. 22 In the field of Li-ion batteries, 6 Li/ 7 Li NMR is also often used to investigate the Li + (de)intercalation processes during the charge and discharge.…”