(Invited) Functional Conductive Polymer Binders for High-Performance Silicon-Based Anodes in Lithium-Ion Batteries

Electrode design has been a key aspect to achieve the high energy and power density, and superb cycling performance. Current lithium-ion-batteries (LIBs) use intercalation materials such as LiCoO₂ for the cathode and graphite for the anode, with a theoretical specific capacity of around 370 mAh/g. Si, with rich natural abundance, exhibits theoretically 10 times higher capacity (4200 mAh/g) than the graphite (370 mAh/g). However, the problem associated with Si-based materials is that they involve a large volume change during cycling, and poor electronic conductivity. Fully alloying Si on charge to form Li₄Si results in an almost 300% volume increase. Because of this high volume change, the electronic integrity of the composite electrode is disrupted, and a high and continuous surface side reaction is induced, both leading to a drastic capacity fade.

Our state-of-the-art electrode materials design has made a big step forward to use a polymer binder to ensure the integrity of the composite electrode for a dimensionally stable laminate. The polymer binder plays a critical function in maintaining mechanical stability and electrical conduction during the lithium insertion and removal process. We proposed a specific designed conductive polymer structure to provide molecular-level electronic connections between the active material and the conductive polymer matrix. Not only cycling stability of the silicon is significantly enhanced, being conductive itself, the use of conductive polymer binder eliminates the necessity of conductive additive, which considerably increases the loading of active material.

This approach achieves unparalleled capacity retention during cycling. An underlying principle to use conductive polymer as a negative electrode binder was developed and implemented in the synthesis of the new conductive and adhesive binders Poly(9,9-dioctylfluorene-co-fluorenone-co-methylbenzoic ester) (PFFOMB). These binders are cathodically (reduction) doped in the Si electrode environment to achieve electrical conductivity. Adhesion groups have been incorporated into the polymer binder to improve the electrode’s integrity. Aldrich commercial nano-Si powder has been used with the conductive polymer binder to form a Si electrode, without the need for acetylene black conductive additive. The electrode demonstrated over 650 cycles between 1volt (V) and 0.01 V at the 2500 milliamp hours per gram (mAh/g)-Si with only 20% capacity fade (Fig. 1). The conductive polymer binders have opened a new paradigm of electrode design for Si materials: they provide molecular-level electrical interaction between the electrode matrix and active materials and accommodate volume expansion of high-capacity Si materials.