Bifunctional Sulfonated Graphene-Modified LiNi_0.5Mn_1.5O₄ for Long-Life and High-Energy-Density Lithium-Ion Batteries

In this paper, sulfonated graphene (SA-GE) with the functions of both high electronic conductivity and fast Li+ transport is utilized to modify high-voltage LiNi0.5Mn1.5O4. We show that LiNi0.5Mn1.5O4 modified with 2 wt % SA-GE exhibits extremely long cycling life, achieving a great capacity retention of 98.0% after 1000 cycles at the loading of 2.6 mg cm–2 and 99.0% after 350 cycles at the loading of 6.0 mg cm–2 under 1C. Besides, the rate capability is also greatly improved (discharge capacity remains 78.5 mAh g–1 at 10C). It is revealed that the SA-GE-involved layer could significantly inhibit the rapid growth of the cathode electrolyte interphase (CEI) layer at 5.0 V, which mainly contributes to an enhanced Li+ diffusion kinetics without sacrificing the electronic conductivity of graphene coating. Thus, the polarization degree and charge transfer resistance are significantly mitigated. Furthermore, structure degradation of the cathode is avoided by reducing the side reactions and the dissolution of transition-metal ions. It is thus well demonstrated that chemical decorations of graphene modifiers with an optimized amount of functionized sulfonic acid groups can be a promising strategy to construct lithium-ion batteries with long life and high energy density.