Exploring the Reversibility of Phase Transformations in Aluminum Anodes through Operando Light Microscopy and Stress Analysis

Abstract

Aluminum is well-known to possess attractive properties for possible use as an anode material in Li-ion batteries (LIBs), but effort is still needed to understand how and why it degrades. Herein, investigations of the delithiation and the re-lithiation processes in Al thin films using an established operando light microscopic platform are pursued. Operando videos highlight that the extraction of Li from the β phase (LiAl) is accompanied by fracture and crack formation leading to the detachment of the α phase (Al) from the rest of the electrode. The evolution of mechanical stress in Al thin film electrodes is tracked and shows severe stress asymmetry as phase transformations progress. Combining with the observations from light and electron microscopy, the mechanical stress during dealloying can be explained by Li solubility with the β phase, formation of cracks and of a highly porous Al nanostructure. Although the results pave a difficult path for utilization of the Al/LiAl/Al (α/β/α) phase transformations in future LIBs, they also suggest excellent opportunities when structural changes can be prevented, which otherwise impact the stability of Al-based electrodes.

Aluminum Foil Anodes for Li-ion Rechargeable Batteries: The Role of Li Solubility within β-LiAl

“…10 This solubility feature was also acknowledged by a previous study, but for a fully lithiated Al film instead of a partly lithiated one. 9 Interestingly, the stress data of the previous study is consistent with the β-LiAl of a partly lithiated solid Al film being (de-)saturated without propagating the phase boundary in this work. Subsequently, a nanoporous α-Al matrix will be created by further removing the Li atoms, like other dealloying processes, 28 thereby giving a stable stress signal from 11 hours.…”

“…Cantilevers made of aluminum oxide with the size of 15×5×0.25 mm 3 are double-side polished prior to the similar PVD processes described in a previous study. 9 The thickness of TiN (current collector) and Al films (electrode material) for stress measurement are characterized to be ~160 nm and ~420 nm, respectively. The GCD tests were run for the in situ stress cell at a rate and C/10, determined by the total charge of the Al film.…”

“…[4][5] To date, only limited success has been attained for Al-based anodes due to the issues that are not yet resolved, including significant mechanical strain, 6 brittleness of the β-LiAl, [7][8] and electrode pulverization during delithiation. [9][10] Our previous study suggests that the Al/LiAl/Al (α/β/α) phase transformations might be intrinsically challenging to utilize due to the formation of nanopores, which cause a loss of electrolyte due to secondary SEI formation on the large surface area. Therefore, a strategic pathway is required to mitigate degradation.…”

“…Therefore, a strategic pathway is required to mitigate degradation. 9 From a more holistic perspective, conventional composite anodes may be ill-suited to wide-spread energy storage efforts if cradle-to-grave costs and sustainability are considered. In today's state-of-the-art devices, both the cathode and the anode consist of composites of active materials, polymer binders and conductive additives.…”

Interface engineering towardhigh‐efficiencyalloy anode for next‐generation energy storage device

“…Many strategies have been proposed to stabilize the alloy anodes, such as nanostructure design, electrolyte formulation, and composite materials 19,20 . In recent years, interface engineering has been proposed as one of effective strategies to alleviate the volume expansion and reduce parasitic reactions 21,22 . The ideal SEI for alloy anodes should be homogeneous, robust, and minimizes active ions loss and volumetric expansion.…”