Lignin-Functionalized Supramolecular Binder Enables Aggressive Cathode Chemistries in Advanced Li-Ion Batteries

Cathode chemistries directly dictate the energy densities of advanced batteries; however, these chemistries of high voltage and high capacity present a severe challenge to the reversibility of the batteries, incurring irreversible reactions between electrolytes and cathodes, resulting in electrolyte and lithium inventory consumption as well as interfacial degradation of cathode lattice structures. Here, departing from the conventional approach of electrolyte engineering, we report that the polymeric binder, an often-overlooked ingredient in the electrode composite, can mitigate all these parasitic reactions in a more economical and effective manner. The lignin-functionalized polymonofluoroacrylic acid (PFA) was shown to stabilize a wide spectrum of aggressive cathode chemistries, including LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811), LiCoO₂ (LCO), and 5-V class LiNi_0.5Mn_1.5O₄ (LNMO), with superior performances. Besides providing robust adhesive force to keep the active and conductive ingredients within the cathode composite in intimate contact, especially when the cathode experiences extreme mechanical and electrochemical stress under high voltages, PFA also serves as an F source to form LiF-rich interphases on cathode materials that insulate parasitic reactions with electrolytes. Lignin, on the other hand, makes PFA well-dispersed in the cathode composite, rendering it with excellent radical-scavenging ability. Altogether, the lignin-PFA binder combination proves versatile in stabilizing these cathodes in both lithium-ion and lithium-metal configurations, enabling the Li||NCM811 cell to maintain 77% capacity after 500 cycles at 4.6 V, or a 2.2 Ah graphite||NCM811 pouch cell with 91.2% capacity retention after 500 cycles at the high cutoff voltage of 4.6 V. This molecular design approach for an advanced binder offers a completely different and unique route to increase the energy densities of advanced batteries.