A new type of polyamide imidazole (PAID) polymer with two functional groups for tight binding with silicon (Si) particles and carbon black is investigated as a binder for the Si anode in lithium-ion batteries (LIBs). PAID is synthesized via three reaction phases. The first phase is polyamide polymerization (p-PAID), the second phase is formation of imide and imidazole rings (i-PAID), and the last phase is ring cyclization for the PAID structure. Among these stages, i-PAID shows ambidextrous binding characteristics for LIBs. The planar π-conjugated backbone in the i-PAID provides a strong π−π stacking interaction with carbon black, thus sustaining the electrical conduction pathway in the Si electrode during cycling. The amine and carboxylic acid functional group in the i-PAID have a strong interaction with Si particles, which efficiently suppresses the volume expansion of the Si electrode, confirmed by in situ electrochemical dilatometry and ex situ SEM observation. The Si anode with the bifunctional i-PAID binder shows not only a higher reversible capacity but also a greatly enhanced cycle performance over 200 cycles in comparison to the Si anode with a simple polyimide binder. This ambidextrous polymer binder offers a new opportunity to positively impact the development of a mechanically robust Si anode for lithium-ion batteries.

Huge volume changes of silicon particles upon alloying and dealloying reactions with lithium are a major reason for the poor cycle performance of silicon-based anodes for lithium-ion batteries. To suppress dimensional changes of silicon is a key strategy in attempts to improve the electrochemical performance of silicon-based anodes. Here, we demonstrate that a conductive agent can be exploited to offset the mechanical strain imposed on silicon electrodes caused by volume expansion of silicon associated with lithiation. Hollow graphene particles as a conductive agent inhibit volume expansion by absorbing the swelling of silicon upon lithiation through flattening the free voids surrounded by the graphene shell. As a result, silicon electrodes with hollow graphene showed a height expansion of 20.4% after full lithiation with a capacity retention of 69% after 200 cycles, while the silicon electrode with conventional carbon black showed an expansion of 76.8% under the same conditions with a capacity retention of 38%. Some of the deflated hollow graphene returns to its initial shape on delithiation due to the mechanical flexibility of the graphene shell layer. Such a robust microstructure of a silicon electrode incorporating hollow graphene that serves as both an expansion inhibitor and a conductive agent greatly improves capacity retention compared with silicon electrodes with the conventionally used carbon black.

In-situ electrochemical dilatometry is applied as an analytical technique to investigate the effect of both solid electrolyte interface (SEI) formation and the microstructure of the electrode on the dilation behavior of the graphite electrodes during lithium ion intercalation at the same time. It was found that the dilation and contraction behavior of the graphite electrode was dependent on the microstructure of the graphite electrode. The relationship between the microstructure, such as preferred orientation of graphite particles and the pore structure, and the volume changes of the graphite electrode were investigated by half-cell type in-situ electrochemical dilatometer during cycling. Dilation behavior of graphite electrodes monitored by in-situ dilatometer clearly showed the volume changes of the graphite electrodes induced by Li ion intercalation/de-intercalation as well as the SEI layer formation, and how the microstructure of the electrodes affects their volume expansion upon lithium ion intercalation. It was also found that differential dilation plots of the graphite electrodes upon lithiation and delithiation gave us more detailed information of dilation and contraction behaviors with respect to the amount of Li ion intercalated into graphite.

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Polymer binders are essential for Silicon (Si) anode‐based lithium‐ion batteries (LIBs). However, the synthetic guidance for aromatic polymer binder is relatively less explored compared to aliphatic polymer binders. In this study, polyimide‐based aromatic polymer binders are developed that have strong binding affinity with Si particles, a conductive agent and copper (Cu) current collector, and they show an improved initial discharge capacity of 2663 mAh g−1, which is 29% higher than that of Kapton‐based one (2071 mAh g−1). The copolymerization between “hard” and “soft” segments is crucial to achieve reversible volume expansion/contraction during the repeated charging/discharging process, resulting in the best cycle performance. The new binder ensures both excellent volume retention after full‐delithiation and allowed volume expansion at least to some extent upon full‐lithiation. This Study finds a power‐law relationship between the capacity of Si anode and the mechanical properties of the binder, i.e., the tensile stress (σ) and strain (ɛ). The initial discharge capacity is proportional to σn · ɛ (n = 2.3–2.7). Such an understanding of the relationships between polymer structure, mechanical properties of the polymer and binder performance clearly revealed the importance of the soft‐hard polymer structure for aromatic binders used in Si‐based high‐capacity lithium storage materials.