Coordinatively Cross-Linked Binders for Silicon-Based Electrodes for Li-Ion Batteries: Beneficial Impact on Mechanical Properties and Electrochemical Performance

Abstract

A simple and versatile preparation of Zn(II)−poly(carboxylates) reticulated binders by the addition of Zn(II) precursors (ZnSO 4 , ZnO, or Zn(NO 3 ) 2 ) into a preoptimized poly(carboxylic acids) binder solution is proposed. These binders lead systematically to a significantly improved electrochemical performance when used for the formulation of silicon-based negative electrodes. The formation of carboxylate-Zn(II) coordination bonds formation is investigated by rheology and FTIR and NMR spectroscopies. Mechanical characterizations reveal that the coordinated binder offers a better electrode coating cohesion and adhesion to the current collector, as well as higher hardness and elastic modulus, which are even preserved in the presence of a carbonate solvent (i.e., in battery operation conditions). Ultimately, as shown from operando dilatometry experiments, the electrode expansion during lithiation is reduced, mitigating electrode mechanical failure. Such coordinatively reticulated electrodes outperform their uncoordinated counterparts with an improved capacity retention of over 30% after 60 cycles.

Impact of the Cu Current Collector Grade and Ink Solid Fraction on the Electrochemical Performance of Silicon‐Based Electrodes for Li‐Ion Batteries

“…Then, Zn(II) ion are introduced in the form of a salt (ZnSO 4 , Zn(NO 3 ) 2 ) or an oxide (ZnO) for coordinated formulations. The effects of the nature of the metallic precursor [ 17 ] and the coordination ratio, [ 16 ] which is the molar ratio of Zn over COOH moieties, have been previously evaluated. Interestingly, regardless of the Zn(II) ion source, the electrochemical performance is significantly improved by the formation of a cross‐linked network of Zn‐carboxylate coordination bonds as compared to the reference (Zn‐free) binder electrodes.…”

“…Interestingly, regardless of the Zn(II) ion source, the electrochemical performance is significantly improved by the formation of a cross‐linked network of Zn‐carboxylate coordination bonds as compared to the reference (Zn‐free) binder electrodes. The Zn cross‐linked binder particularly improves the electrode mechanical properties [ 17 ] and solid electrolyte interphase (SEI) stability, [ 15 ] thus improving the overall electrochemical performance. Table 1 and S1, Supporting Information, summarizes the ZnSO 4 and Zn‐free formulations which are used in this study.…”

“…The latter formulation has better mechanical properties than the former, i.e., superior cohesion, hardness, and elastic modulus, which ultimately lead to a reduced expansion of the electrode during electrochemical cycling. [ 17 ]…”

“…On the contrary, such a dependence is not noted for the Zn cross‐linked electrodes, which have a much higher cohesion. [ 17 ] Therefore, for the reference formulation of lower cohesion, it appears that the increase in adhesion at the CC's interface via the roughening of the surface of the current collector and/or the decrease of its yield strength contribute to mitigate the electrode film's mechanical damage, therefore preserving its cyclability.…”

“…[14] As such, in the present study, the influence of the Cu current collector (Cu CC) grade and the ink SF on the performance of Si-based electrodes is investigated. Two preoptimized binders were considered for this study: a purely organic binder of carboxymethylcellulose and citric acid, which has been used as a reference in previous works, [15][16][17] and the other which consists of the same organic components together with a divalent cation (here Zn(II)) which has been shown to further cross-link the carboxylate moieties through coordination bonds, enhancing the mechanical properties and acting to some extent as an artificial SEI in the resulting electrodes. [15][16][17] Here, we show that the Cu CC grade and the SF significantly influence the cyclability of silicon-based electrodes while highlighting the robustness of the formulation including cross-linked polymers with Zn(II) cations whose electrochemical behavior appears much less sensitive to the Cu CC grade and ink SF.…”