Ultrathin, flexible and smooth carbon coating extends the cycle life of dual-ion batteries

The anion storage electrochemistry of carbon cathode is bottlenecked by the severe electrolyte decomposition and significant volume expansion. A surface coating that can impart simultaneous protection against all the drawbacks is challenging to achieve. Herein, the multifaceted benefits of pitch-derived soft carbon are leveraged as a coating layer. An upscalable annealing treatment results in an ultrathin, polycrystalline, uniformly distributed, and porous layer of carbonized pitch on the redox-active core. The coated material demonstrates 25 and 5.5% improvement in capacity retention, and coulombic efficiency, respectively, and a 150 mV reduction in voltage hysteresis. The smooth surface topography of the coated sample alleviates the electrolyte decomposition during high-voltage cycling. The mechanical flexibility of the coated material to withstand the volume change and to preserve the structural integrity are uncovered by the nanoindentation experiments and further supported by the post-cycling characterizations. The coating offers mechanochemical robustness to the interface and minimizes the growth of resistance. The strategy of soft carbon coating on anion-storing cathode can be extended to dual graphite full cells and other anion-storing chemistries irrespective of the anode and electrolyte.