Current-induced transition from particle-by-particle to concurrent intercalation in phase-separating battery electrodes

Abstract

Many battery electrodes contain ensembles of nanoparticles that phase-separate upon (de)intercalation. In such electrodes, the fraction of actively-intercalating particles directly impacts cycle life: a vanishing population concentrates the current in a small number of particles, leading to current hotspots. Reports on the active particle population in the phase-separating electrode lithium iron phosphate (LFP) vary widely, ranging from around 0% (particle-byparticle) to 100% (concurrent intercalation). Using synchrotron-based X-ray microscopy, we probed the individual state-of-charge for over 3,000 LFP particles. We observed that the active population depends strongly on the cycling current, exhibiting particle-by-particle-like behaviour at low rates and increasingly concurrent behaviour at high rates, consistent with our phase-field porous electrode simulations. Contrary to intuition, the current density, or current per active internal surface area, is nearly invariant with the global electrode cycling rate. Rather, the electrode accommodates higher current by increasing the active particle population. This behaviour results from thermodynamic transformation barriers in LFP, and such a phenomenon likely extends to other phase-separating battery materials. We propose that modifying the transformation barrier and exchange current density can increase the active population and thus the current homogeneity. This could introduce new paradigms to enhance the cycle life of phaseseparating battery electrodes. 3Electrochemical systems can provide clean and efficient routes for energy conversion and storage. Many electrochemical devices such as batteries, fuel cells, and supercapacitors consist of porous electrodes containing ensembles of nanoparticles 1 . For typical microstructures, the particle density can reach as high as 10 15 cm -3 . To further increase complexity, many intercalation battery electrodes, such as graphite 2 , lithium iron phosphate 3,4 , lithium titanate 5 , and spinel lithium nickel manganese oxide 6 , phase-separate upon (de)intercalation. Such electrodes are physically and chemically heterogeneous on the nanoscale, and likely exhibit inhomogeneous current distributions.In phase-separating electrodes, the active particle population is a crucial factor in determining the overall electrode current and the degree of current homogeneity. The electrode current is given by:where is the reaction area of the th actively-intercalating particle, and is the current density of that particle. Under the approximation of similar particle size, we obtain the final expression in equation 1, where ̅ is the average current density of all actively-intercalating particles, is the total internal surface area of all particles (rather than the projected electrode area), and is the so-called active population. When approaches 0%, the electrode intercalates particle-by-particle with a heterogeneous current distribution; when approaches 100%, the electrode intercalates concurrently with a more homogeneous current ...

Kinetics of Nanoparticle Interactions in Battery Electrodes

“…At low current densities, particles transform sequentially, whereas the sequential transformation is suppressed at high current densities (in agreement with Ref. 38, in which a similar transition from sequential to simultaneous transformation is also observed for both the case of two-phase lithiation and the case of suppressed intraparticle phase separation). However, there are some differences between the two cases.…”

Three-Dimensional Visualization of Gas Evolution and Channel Formation inside a Lithium-Ion Battery

“…57,60 Li et al conducted an in-depth characterization and observed that only 5% to 8% of particles are actively intercalating during lithiation, while during delithiation the active population ranges from 8 to 32%. 24 They further confirm that the current is heterogeneously distributed in the electrode, that is to say, only a small number of active particles carries most of the current regardless of the total electrode current. Taking into account that lithiation of Si will not occur until the native silicon oxide is at least partially reduced by Li ions 49 and the gas evolution behavior presented here, we assume that diffusion of Li ions takes place as follows: during lithiation, Li ions that pass though the separator and/or the native Li ions in the native lithium salt electrolyte will first diffuse to the potentially active electrode particles, reduce the electrolyte and/or lithium salts with the obtain of electrons from the electrode and successively reduce the native silicon oxide, thus forming the SEI layer with inorganic inner species and organics outer species, releasing gas, and finally lithiate the active particles of the electrode, leaving non-active local regions intact.…”

“…[24][25]56 For example, a discrepancy is observed between electrochemical measurements that represent the overall state of the cell and spectroscopic data that represent a local state. [57][58] A recent experiment by Delmas et al showed the coexistence of fully lithiated and fully delithiated individual LiFePO 4 cathode particles by Xray diffraction after full lithiation.…”

Charge Heterogeneity and Surface Chemistry in Polycrystalline Cathode Materials

“…[10][11][12] For example, a heterogeneous SOC distribution has recently been reported in spinel LiMn1.5Ni0.5O4 (LMNO) micro-crystals. 13 The evolution of SOC distribution within a LiCoO2 particle upon long term 2 cycling was also reported recently.…”