Interlayer Structure and Chemistry Engineering of MXene-Based Anode for Effective Capture of Chloride Anions in Asymmetric Capacitive Deionization.

Negatively charged surfaces and readily oxidizabile characteristics fundamentally restrict the use of MXene building blocks as anodes for anion intercalation. Herein, by embedding bacterial cellulose nanofibers with conformal polypyrrole coating (BC@PPy) and populating them between MXene (Ti3C2Tx) interlayers, we enable the fabricated MXene/BC@PPy (MBP) composite films to be highly efficient anodes for Cl--capturing in asymmetric capacitive deionization (CDI) systems. Performance gains are realized due to the surface electronegativity of MXene nanosheets becoming compensated by positively charged BC@PPy nanofibers, alleviating electrostatic repulsion, thus realizing reversible Cl- intercalation. More crucially, the anodization voltage of MBP is effectively enhanced as a result of the increase of the Ti valence state in MXene nanosheets with the addition of the BC@PPy spacer. Furthermore, BC@PPy nanopillars effectively enlarge the interlayer space for facile Cl- de-/intercalation, improve the vertical electron transfer between loosely deposited MXene nanosheets, and perform as additional active materials for Cl--capturing. Consequently, the MBP anode exhibits a promising desalination capacity of up to 17.56 mg g-1 at 1.2 V with a high capacity retention of 94.6% after 30 cycles in an asymmetric CDI system. This work offers a simple and effective strategy to unlock the application potential of MXene building blocks as anodes for Cl--capturing in electrochemical desalination.

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

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 ß 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 ß 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/ß/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.

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

Aluminum is an attractive anode material for lithium-ion batteries (LIBs) owing to its low cost, light weight, and high specific capacity. However, utilization of Al-based anodes is significantly limited by drastic capacity fading during cycling. Herein, a systematic study is performed to investigate the kinetics of electrochemical lithiation of Al thin films to understand the mechanisms governing the phase transformation, by using an operando light microscopy platform. Operando videos reveal that nuclei appear at random positions and expand to form quasi-circular patches that grow and merge until the phase transformation is complete. Based on this direct evidence, models of the lithiation processes in Al anodes are discussed and reaction-controlled kinetics are suggested. The growth rate of LiAl depends on the potential and increases considerably as higher overpotentials are approached. Lastly, improved cycling performance of Al-based anodes can be realized by two approaches: 1) by controlling the lithiation extent, the cycling life of Al thin film is extended from 5 cycles to 25 cycles; 2) the performance can be optimized by adjusting the kinetics. Together, this work offers a renewed promise for the commercialization of Al-based anodes in LIBs where the performance requirements are compatible with the proposed cycle life-extending strategies.