Kinetic Monte Carlo Study of Li Intercalation in LiFePO4

Beyond Constant Current: Origin of Pulse-Induced Activation in Phase-Transforming Battery Electrodes

Mechanistic understanding of phase transformation dynamics during battery charging and discharging is crucial toward rationally improving intercalation electrodes. Most studies focus on constant-current conditions. However, in real battery operation, such as in electric vehicles during discharge, the current is rarely constant. In this work we study current pulsing in LiXFePO4 (LFP), a model and technologically important phase-transforming electrode. A current-pulse activation effect has been observed in LFP, which decreases the overpotential by up to ∼70% after a short, high-rate pulse. This effect persists for hours or even days. Using scanning transmission X-ray microscopy and operando X-ray diffraction, we link this long-lived activation effect to a pulse-induced electrode homogenization on both the intra- and interparticle length scales, i.e., within and between particles. Many-particle phase-field simulations explain how such pulse-induced homogeneity contributes to the decreased electrode overpotential. Specifically, we correlate the extent and duration of this activation to lithium surface diffusivity and the magnitude of the current pulse. This work directly links the transient electrode-level electrochemistry to the underlying phase transformation and explains the critical effect of current pulses on phase separation, with significant implication on both battery round-trip efficiency and cycle life. More broadly, the mechanisms revealed here likely extend to other phase-separating electrodes, such as graphite.

The decisive role of electrostatic interactions in transport mode and phase segregation of lithium ions in LiFePO4

Understanding the mechanism of slow lithium ion (Li+) transport kinetics in LiFePO4 is not only practically important for high power density batteries but also fundamentally significant as a prototypical ion-coupled electron transfer process. Substantial evidence has shown that the slow ion transport kinetics originates from the coupled transfer between electrons and ions and the phase segregation of Li+. Combining a model Hamiltonian analysis and DFT calculations, we reveal that electrostatic interactions play a decisive role in coupled charge transfer and Li+ segregation. The obtained potential energy surfaces prove that ion-electron coupled transfer is the optimal reaction pathway due to electrostatic attractions between Li+ and e- (Fe2+), while prohibitively large energy barriers are required for separate electron tunneling or ion hopping to overcome the electrostatic energy between the Li+-e- (Fe2+) pair. The model reveals that Li+-Li+ repulsive interaction in the [010] transport channels together with Li+-e- (Fe2+)-Li+ attractive interaction along the [100] direction cause the phase segregation of Li+. It explains why the thermodynamically stable phase interface between Li-rich and Li-poor phases in LiFePO4 is perpendicular to [010] channels.

Novel Synthesis of 3D Mesoporous FePO4 from Electroflocculation of Iron Filings as a Precursor of High-Performance LiFePO4/C Cathode for Lithium-Ion Batteries

This study presents an economic and environmentally friendly method for the synthesis of microspherical FePO₄·2H₂O precursors with secondary nanostructures by the electroflocculation of low-cost iron fillers in a hot solution. The morphology and crystalline shape of the precursors were adjusted by gradient co-precipitation of pH conditions. The effect of precursor structure and morphology on the electrochemical performance of the synthesized LiFePO₄/C was investigated. Electrochemical analysis showed that the assembly of FePO₄·2H₂O submicron spherical particles from primary nanoparticles and nanorods resulted in LiFePO₄/C exhibiting excellent multiplicity and cycling performance with first discharge capacities at 0.2C, 1C, 5C, and 10C of 162.8, 134.7, 85.5, and 47.7 mAh·g^-1, respectively, and the capacity of LiFePO₄/C was maintained at 85.5% after 300 cycles at 1C. The significant improvement in the electrochemical performance of LiFePO₄/C was attributed to the enhanced Li⁺ diffusion rate and the crystallinity of LiFePO₄/C. Thus, this work shows a new three-dimensional mesoporous FePO₄ synthesized from the iron flake electroflocculation as a precursor for high-performance LiFePO₄/C cathodes for lithium-ion batteries.

Enhanced electrochemical properties of potassium-doped lithium-rich oxide@carbon as cathode material for lithium-ion batteries

Lithium-rich layered oxides are believed to be the most competitive cathode materials for next-generation lithium-ion batteries (LIBs) due to their high specific capacity, but the poor cycle stability and voltage attenuation severely limit their commercial applications. In this paper, a simple method combining surface treatment via pyrolysis of polyvinyl alcohol (PVA) and potassium ions (K⁺) doping, is designed to improve the above defects of the cobalt-free Lithium-rich material Li_1.2Mn_0.6Ni_0.2O₂ (LMR). The insoluble surface byproduct Li₂CO₃ and amorphous carbon nanolayer derived from the pyrolysis process of PVA alleviate the corrosion of acidic species with a favorable conductivity, while a large radius of K⁺ can enlarge the space of the lithium (Li) layer to facilitate the diffusion of Li⁺, suppress voltage polarization, and synchronously restrain the transformation from a layered structure to a spinel-like structure. After modification, the LMR material exhibits a great initial discharge capacity of 266.0 mAh g^-1 at 0.1C, a remarkable rate capability of 159.1 mAh g^-1 at 5C and an extremely high capacity retention of 98.5% over 200 cycles at 0.5C with a small voltage drop.

Double Donors Tuning Conductivity of LiVPO4F for Advanced Lithium-Ion Batteries

To fulfill the increasing demand of lithium-ion batteries for realizing high energy density and great cycling stability under high rate, the cathode material capable of efficient electron and Li⁺-ion transportation is necessarily demanded. Herein, we propose a double-donor doping strategy by taking the carbon-coated LiVPO₄F as a model system. The Hall effect confirms that either or both Mg²⁺ substitution of Li⁺ and Nb⁵⁺ substitution of V³⁺ cause the carrier-type transformation from p-type to n-type. The great enhancements of electronic conductivity and ionic conductivity are realized in Li_0.995Mg_0.005V_0.98Nb_0.02PO₄F, which also exhibits a markedly improved Li⁺ diffusion coefficient and reduced electrochemical polarization. The carbon-coating layer can effectively prevent the decomposition reaction of electrolyte, allowing for good structural stability of Li_0.995Mg_0.005V_0.98Nb_0.02PO₄F when suffering fast Li⁺ insertion/extraction. As expected, the Li_0.995Mg_0.005V_0.98Nb_0.02PO₄F cathode exhibited superior electrochemical properties with an initial discharge capacity of 124.5 mA h g^-1 and capacity retention of 97.3% after 600 cycles at 1.6C. Even under a high rate of 8C, the discharge energy density was 392 Wh kg^-1 at the beginning and showed a retention rate of 84.4% after 2000 cycles.

Dynamic visualization of the phase transformation path in LiFePO4 during delithiation

Rechargeable lithium-ion batteries have been widely used in portable electronic devices and electric vehicles over the last few decades. The electrochemical performance of lithium-ion batteries is mostly determined using electrode materials, which allow Li to insert/extract in their crystal structure. Conventionally, high-rate electrode materials store Li+via a solid-state reaction (i.e., the single-phase transformation path), and one exception is LiFePO4 (LFP). Although its two-phase transformation path has been widely demonstrated, the abnormal correlation between the lithiation/delithiation mechanism and the high rate performance of LFP is still controversial. Recently, the theory has suggested that the single-phase transformation path at a very low overpotential might be responsible for the abnormal phenomenon. However, direct observation of such a single-phase transformation has been rarely achieved, because once the overpotential is removed, the intermediate solid-solution phase LixFePO4 (0 < x < 1) should separate into thermodynamic LFP and FePO4 (FP). Here, the detailed delithiation path of LFP is directly observed using in situ transmission electron microscopy (TEM) based on a micro-sized solid-state battery (Pt/Li6.4La3Zr1.4Ta6O12/LFP). We first demonstrate a novel two-step solid-solution transformation path during the delithiation of LFP, showing direct evidence for the above assumption. These results provide a new insight into the solid-solution transformation mechanism of electrode materials.

Computational Discovery and Design of MXenes for Energy Applications: Status, Successes, and Opportunities

MXenes (M_n+1X_n, e.g., Ti₃C₂) are the largest 2D material family developed in recent years. They exhibit significant potential in the energy sciences, particularly for energy storage. In this review, we summarize the progress of the computational work regarding the theoretical design of new MXene structures and predictions for energy applications including their fundamental, energy storage, and catalytic properties. We also outline how high-throughput computation, big data, and machine-learning techniques can help broaden the MXene family. Finally, we present some of the major remaining challenges and future research directions needed to mature this novel materials family.

Boosting Rechargeable Batteries R&D by Multiscale Modeling: Myth or Reality?

This review addresses concepts, approaches, tools, and outcomes of multiscale modeling used to design and optimize the current and next generation rechargeable battery cells. Different kinds of multiscale models are discussed and demystified with a particular emphasis on methodological aspects. The outcome is compared both to results of other modeling strategies as well as to the vast pool of experimental data available. Finally, the main challenges remaining and future developments are discussed.

Size-Dependent Memory Effect of the LiFePO4 Electrode in Li-Ion Batteries

In Li-ion batteries, the phase transition usually determines the electrochemical kinetics of some two-phase electrode materials, and it can be adopted to excellently interpret the memory effect of Li-ion batteries, therefore the size dependence of phase transition was expected to affect the memory effect significantly. In this work, we investigated the memory effect and phase transition of olivine LiFePO₄ in Li-ion batteries. Through electrochemical measurements, we found that the memory effect of LiFePO₄ was dependent on the particle size, especially after a long-time relaxation. By using the in situ X-ray diffraction, we found that the phase transition of nano-LiFePO₄ was ahead of the charging and discharging processes, while it took place concurrently or later for micro-LiFePO₄, which might be attributed to the high-specific two-phase boundary of nano-LiFePO₄. Furthermore, the phase-transition diagram was adopted to interpret the size-dependent memory effect schematically. Notably, it is the first time to report the phase transition ahead of (dis)charging for nano-LiFePO₄, which is significant to understand the phase transition of two-phase electrode materials, as well as the relevant phenomena, such as the memory effect.

SQERT-T: alleviating kinetic Monte Carlo (KMC)-stiffness in transient KMC simulations

Lattice based kinetic Monte Carlo (KMC) is often used for simulating the dynamics of systems at a supramolecular scale, based on molecular scale transitions. A common challenge in KMC simulations is rapid 'back-and-forth' reactions, which dominate the events executed during simulations and inhibit the ability for simulations to reach longer time scales. Such processes are fast frivolous processes (FFPs) and are one manifestation of a phenomenon referred to as KMC-stiffness. Here, an algorithm for staggered quasi-equilibrium rank-based throttling geared towards transient kinetics (SQERT-T) is presented. Within the SQERT-T methodology, a pace-restrictor reaction and an FFP floor are utilized along with throttling of the process transition rate constants to accelerate the KMC simulations while still retaining sufficient time resolution for sampling of the data. KMC simulations were performed for CO oxidation over RuO₂(1 1 0) and over RuO₂(1 1 1), and the results were compared to experimental data obtained using RuO₂ powders. The experiments and simulations were for transient conditions: the system was subjected to a temperature program which included temperatures in the range of 363 to 453 K. The timescales that were achieved during the KMC simulations in this study would not have been accessible without KMC acceleration, and were enabled by the use of SQERT-T.