Origins of Large Voltage Hysteresis in High-Energy-Density Metal Fluoride Lithium-Ion Battery Conversion Electrodes

Abstract

Metal fluorides and oxides can store multiple lithium ions through conversion chemistry to enable high-energy-density lithium-ion batteries. However, their practical applications have been hindered by an unusually large voltage hysteresis between charge and discharge voltage profiles and the consequent low-energy efficiency (<80%). The physical origins of such hysteresis are rarely studied and poorly understood. Here we employ in situ X-ray absorption spectroscopy, transmission electron microscopy, density functional theory calculations, and galvanostatic intermittent titration technique to first correlate the voltage profile of iron fluoride (FeF3), a representative conversion electrode material, with evolution and spatial distribution of intermediate phases in the electrode. The results reveal that, contrary to conventional belief, the phase evolution in the electrode is symmetrical during discharge and charge. However, the spatial evolution of the electrochemically active phases, which is controlled by reaction kinetics, is different. We further propose that the voltage hysteresis in the FeF3 electrode is kinetic in nature. It is the result of ohmic voltage drop, reaction overpotential, and different spatial distributions of electrochemically active phases (i.e., compositional inhomogeneity). Therefore, the large hysteresis can be expected to be mitigated by rational design and optimization of material microstructure and electrode architecture to improve the energy efficiency of lithium-ion batteries based on conversion chemistry.

Fundamental interplay between anionic/cationic redox governing the kinetics and thermodynamics of lithium-rich cathodes

“…It is now clear, as stated above, that the anionic/cationic redox sequence differs on charge vs. discharge, resulting in path dependence and hysteresis. Such hysteresis mechanism contrasts from LiFePO 4 (non-monotonic equilibrium potential 65 ) or conversion electrodes 66 . Consistent with the anionic/cationic sequence hypothesis, our Ni 2 p 3/2 HAXPES results show a hysteresis loop in Ni oxidation state vs. capacity (Fig.…”

Novel Synthesis of Anhydrous and Hydroxylated CuF2 Nanoparticles and Their Potential for Lithium Ion Batteries

“…This would provide faster kinetics of reaction, better reversibility, and therefore better performances. It is thought that the reaction path length within this kind of material is around 10 nm . Sizing the particles around this diameter should minimize, and hopefully overcome the conductivity problems.…”

An Insoluble Benzoquinone‐Based Organic Cathode for Use in Rechargeable Lithium‐Ion Batteries

“…Thed ischarge curve presents two obvious plateaus and delivers capacities of about 70 and 140 mA hg À1 .A ccording to the capacity distribution, this phenomenon indicates that TPB can reversibly take up six lithium ions with redox couples of Li 6 TPB/ TPB.The charge curve presents one defined plateau and two sloping plateaus.A no verpotential approaching 1.0 Vw as considered responsible for the slow kinetics of lithium-ion extraction from Li 6 TPB during the charging process.A strategy to improve the problem, using electrolyte with ah igher ionic conductivity should be very effective in inhibiting the overpotential of TPB.The two sloping plateaus emerged because the reduction steps (Li 2 TPB/TPB) occur very fast and the voltage gap between the two steps is too small to distinguish at about 3.10 and 3.16 V. Additionally,the voltage hysteresis results in av oltage gap between the redox peaks.Asimilar phenomenon also appears in some conjugated carbonyl-based organic molecules and metal fluorides/oxides with multi-electron reactions. [13] To verify the reversibility of the electrochemical redox process, [14] the TPB electrode was monitored at different discharged and charged states by in situ XRD (see the installation diagram in the Supporting Information, Figure S12). As shown in Figure 2c,the main peaks corresponding to TPB at 14.58 8,1 5.78 8,a nd 32.88 8 decrease little by little during discharging,w hile these peaks gradually appear and enhance during charging.A tt he fully charged state of 3.5 V, the character peaks of TPB shift back to their original positions,indicating that the reaction process is reversible.…”