Enhanced high-rate performance of ball-milled MmNi_3.55Co_0.75Mn_0.4Al_0.3hydrogen storage alloys with graphene nanoplatelets

The poor high-rate dischargeability (HRD) of negative electrode materials (hydrogen storage alloys) has impeded applications of nickel metal hydride batteries in high-power fields, electric vehicles, hydride electric vehicles, power tools, modern military devices, etc. Here we report a facile and effective strategy to enhance the HRD performance of commercial MmNi_3.55Co_0.75Mn_0.4Al_0.3hydrogen storage alloy (here Mm denotes mischmetals) with high-energy ball milling and addition of graphene nanoplatelets (GNPs). At a discharge current density of 3000 mA g⁻¹, the capacity retention rate of the alloy electrode could reach 53.0% after ball-milling, and 68.3% after further addition of GNPs, which is 3.2 times that of original alloy electrode (21.5%). Such a superior HRD performance is contributed by (1) smaller particle size of alloys to reduce the diffusion distance of hydrogen atoms; (2) high conductivity of GNPs to accelerate the charge transfer; and (3) interconnected GNPs among alloys to decrease the internal resistance.