Direct Evidence of Concurrent Solid-Solution and Two-Phase Reactions and the Nonequilibrium Structural Evolution of LiFePO₄

Abstract

Lithium-ion batteries power many portable devices and in the future are likely to play a significant role in sustainable-energy systems for transportation and the electrical grid. LiFePO(4) is a candidate cathode material for second-generation lithium-ion batteries, bringing a high rate capability to this technology. LiFePO(4) functions as a cathode where delithiation occurs via either a solid-solution or a two-phase mechanism, the pathway taken being influenced by sample preparation and electrochemical conditions. The details of the delithiation pathway and the relationship between the two-phase and solid-solution reactions remain controversial. Here we report, using real-time in situ neutron powder diffraction, the simultaneous occurrence of solid-solution and two-phase reactions after deep discharge in nonequilibrium conditions. This work is an example of the experimental investigation of nonequilibrium states in a commercially available LiFePO(4) cathode and reveals the concurrent occurrence of and transition between the solid-solution and two-phase reactions.

In operando tracking phase transformation evolution of lithium iron phosphate with hard X-ray microscopy

“…In addition, most conventional tools only specialize in revealing the structural and compositional information without chemical mapping or imaging capability, lacking effective spatial resolution information 12,13 . As a result, the homogeneous and concurrent phase transformation on the multiple-particle scale has become increasingly inadequate.…”

“…To explore the widely debated mechanism stated above, some advanced techniques (for example, in situ) have been applied in recent years to understand and examine the dynamic phase transformation in a real battery electrode as it cycles [22][23][24][25] . With a synchrotron X-ray diffraction (XRD) technique, Ogumi and colleagues 22 directly detected a metastable crystal phase of Li x FePO 4 during electrochemical phase transformation under non-equilibrium conditions (at high rate of 10C).…”

In Situ Powder Diffraction Studies of Electrode Materials in Rechargeable Batteries

“…Using ar oll-over cylindrical cell, researchers were able to show how the reaction mechanismse volved in aL iFePO 4 cathode.T hey showed the first experimental evidence for the presence of simultaneous reaction mechanismso ccurring in this electrode during battery function. [28] Structural characteristics of both the solid-solution-type and two-phase-type reaction mechanism were observed duringc ertain states of charge, as shown in Figure 16. The changing lattice parameter is indicative of as olid-solution reaction, whereas the changing phase fraction of LiFePO 4 and FePO 4 is indicative of the twophase reaction.…”

“…Shaded vertical regionsa re locations where simultaneouss olid-solution and two-phase reactionsa re observed. [28] Reprinted with permission from [28].Copyright 2012 American Chemical Society. ChemSusChem 2015ChemSusChem , 8,2826ChemSusChem -2853 www.chemsuschem.org sion in these devices.…”

“…This includes 1) reaction mechanism evolution and how one type of reaction evolvesi nto another,f or example, in LiFePO 4 the interface between solid-solutiona nd two-phase reactions; [28] 2) the crystallographicdistribution of the chargecarrierduring electrochemical charge/discharge processes; [38,41,42] 3) the influence of electrochemical conditions, such as voltage limits and appliedc urrent, on the electrode structurea nd evolution; [5c, 13, 38] 4) differences between operando ande xs itum easurements; [79a, 80] 5) kinetics of structural evolution; [5c, 38] 6) functionality of composite electrodes, in which each component varies during charging/discharging; [90] and 7) the existence of intermediate phases. [5a] To determine these and other electrochemical-structural relationships, optimized electrochemical cells need to be constructed that are both representative of the electrochemical performance observed in the laboratory or commercial use and also provideadiffraction signal sufficient for structural and phase-evolution questions to be addressed.…”