A Critical Review of the Li Insertion Mechanisms in LiFePO₄Electrodes

Abstract

With the ever-growing public and now commercial sentiment supporting the widespread adoption of low and zero-emission vehicles, it is unsurprising that Li-ion batteries which currently assume the bulk of the cost of electrified vehicles (a significant proportion coming from cost of battery materials) have become prolific not only in the primary research literature but have also entered the general public consciousness. Since the initial work in 1997, over 2000 research publications have been authored on lithium iron phosphate (LiFePO 4 ), one of only a handful of commercially viable Li-ion battery cathode materials currently used in electrified vehicles. Despite the sheer quantity of research devoted to the subject and the well-demonstrated excellent battery performance of LiFePO 4 , what is surprising are the number of apparent discrepancies and ensuing contention in the literature regarding its (de)lithiation kinetics. This Review forgoes re-summarizing the well-accepted materials properties of LiFePO 4 and instead seeks to address headlong the specific areas of dispute in the field, specifically by considering how the thermodynamics and kinetics of Li intercalation are modified from the bulk to single-particle to multi-particle scales, shedding light on what makes LiFePO 4 such a unique material and ideally shaping future discourse of LiFePO 4 research.

Nanoscale Imaging of Lithium Ion Distribution During In Situ Operation of Battery Electrode and Electrolyte

“…42 These methods typically rely on bulk particle analysis which convolutes many particle effects. 3 We observed the evolution of many individual particles under high rate conditions in aqueous solution. The evolution of one cluster of particles is shown in Figure 4, corresponding to the voltage profile in Figure 4a.…”

“…Energy storage materials, such as battery electrodes, often display inhomogeneous behavior on the nanoscale. 3 Thus, the most illuminating and useful characterization methods are those capable of providing detailed mechanistic information of charge/discharge dynamics of individual grains and particles. TEM investigations specialize in revealing structural and compositional information with nanoscale spatial resolution and sub-second temporal resolution.…”

“…Here, as a demonstration of tracking lithiation and degradation in an in situ battery in the TEM, we studied the cathode material LiFePO 4 , which has surged in interest due to its attractive capacity, ability to sustain high charging and discharging rates, abundance, low toxicity, relative operational safety, and low cost. 17,19 There is much discussion in the literature on the mechanism of lithiation and delithiation, 3,17,[20][21][22][23][24][25][26][27] with evidence of a two-phase reaction or a metastable solid solution. Within the two-phase reaction pathway, there are different theories for the propagation of lithiation.…”

“…Some of the disagreement may be attributed to many-particle effects, where bulk measurements (both in situ and ex situ) convolve signals from the entire area of the electrode probed. 3 Ex situ studies are inherently compromised by removal of the particles from their native -and often reactive -environment, which leads to questions of relaxation or reactions caused by the foreign surroundings. Here we use a liquid cell in situ TEM, which can probe, in real time, the evolution of individual grains and nanoparticles in the native environment of a battery.…”

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

“…On the other hand, these different models for the single-particle delithiation mechanism are mainly based on an entire electrode, assuming that multi-particle behaviour also applies to single-particle systems. In fact, the free energy and phase transition voltage varies with each individual particle, which makes them have different behaviour from the entire electrode 21 .…”