Insights into Phase Transformations and Degradation Mechanisms in Aluminum Anodes for Lithium-Ion Batteries

Abstract

Attempts to use aluminum-based anodes in lithium-ion batteries often fail due to fast capacity fading. Generally, this has been attributed to pulverization of the electrode and the large volume changes associated with the phase transformation between the crystalline α and β phases of Li-Al alloys. In this study, these transformations were investigated in aluminum films that were lithiated either electrochemically or via direct reaction with lithium metal. Scanning electron microscopy was used to image the samples at different stages of (de)lithiation. By imaging the same locations before and after each step, it can be seen that alloying between Li and Al proceeds from distinct nucleation sites. In situ and ex situ observations reveal that the α-to-β phase transformation is highly anisotropic and causes strong distortions of the film morphology, but only a relatively small amount of mechanical damage such as cracks and delamination. Comparisons between films that were lithiated to 70% and 100% of the theoretical capacity of LiAl indicate that the critical, irreversible damage is more dependent on depth of discharge than on the volume contraction caused by delithiation. Our observations challenge the pessimistic view that pulverization is unavoidable during the phase transformations of the Li-Al system.

Revisiting the Electrochemical Lithiation Mechanism of Aluminum and the Role of Li‐rich Phases (Li1+xAl) on Capacity Fading

“…As reported by Liu et al., Li + ions can diffuse rapidly through the external Li x AlO y layer. However, the nucleation of the crystalline β‐LiAl phase is known to be kinetically hindered, as indicated by the high overpotential, which is responsible for the relative minimum in the potential profile [see the white circle in the (de‐)lithiation profile in Figure ] . In such a scenario, it is reasonable to expect an accumulation of Li + ions at the Li x AlO y /Al interphase.…”

“…However, this value is substantially lower than that of a micro‐sized Si electrode (with the same active material ratio), which shows a 135 % thickness increase even in the first cycle . The lower electrode displacement is in line with the substantially lower compressive stress developed in Al compared with Si, as reported by Boles and co‐workers A more detailed look at the displacement and potential profiles reveals some interesting features (see Figure b). First, a sudden and large electrode thickness change (≈3.8 μm, ≈20 % relative to pristine electrode thickness) occurred after 1.5 h, corresponding to the onset of the lithiation plateau at approximately 0.16 V (vs. Li/Li + ).…”

Improvement of the Cycling Performance of Aluminum Anodes through Operando Light Microscopy and Kinetic Analysis

“…Moreover, the quasi‐one‐dimensional (1 D) thickening behavior described by the Deal–Grove model is not expected to be applicable for the lithiation of Al thin films. The nucleation and formation of the β phase are not uniform, and the growth of quasi‐circular two‐dimensional (2 D) patches occurs at apparently random positions . The degradation of the Al anodes will depend on the details of the phase transition between the α (Al‐based, face‐centered cubic, fcc) and the β (LiAl) phase.…”

“…2 D growth in Al thin films was also observed ex situ by SEM in a study that pointed out the randomness of the position of nuclei and the non‐uniformity of the patch sizes . Apart from the 2 D circular growth of the β phase, progressive nucleation and patch coalescence are also observed during stage 3.…”

“…Although there are noticeable advantages of Al as a negative electrode in LIBs, significant capacity fading during cycling and poor coulombic efficiency (CE) are the main obstacles that limit the commercial utilization of Al‐based LIBs . Some studies claim that the capacity fading is caused by the volume expansion of nearly 100 %, associated with pulverization and delamination . Other groups suggested another explanation for the capacity fading: lithium trapping might be responsible, even if no delamination takes place .…”