Engineering d-band center of MnO2 to promote semi-ionic C-F bonding for high-rate Li-CFx batteries

Abstract

Lithium/carbon fluoride (Li/CF_x) primary batteries suffer from severe voltage hysteresis and rapid capacity degradation at high current densities, presenting significant challenges for achieving superior rate performance. This work proposes a modification strategy for fabricating an amorphous MnO₂-anchored CF composite cathode via rapid in-situ reduction. The amorphous MnO₂ provides abundant active sites and diffusion channels on the CF substrate surface, thereby improving the utilization efficiency of the conversion reaction. Compared with CF and c-MnO₂@CF, a-MnO₂@CF-2 exhibits enhanced electrical conductivity, effectively mitigating voltage hysteresis and delivering superior electrochemical performance under high-rate discharge conditions. Experimental results demonstrate that a-MnO₂@CF-2 achieves a maximum energy density of 2.05 × 10³ Wh kg⁻¹ at 0.1 C. Compared with pristine CF, the discharge rate performance improves from 2 C to 15 C, while the power density increases from 3.11 kW kg⁻¹ to 27.9 kW kg⁻¹. Ex situ XPS and XRD analyses reveal an “in situ electrochemical activation” mechanism: a-MnO₂ preferentially undergoes lithiation at 2.5 V to form highly conductive Li_XMnO₂ networks, which reduce interfacial resistance and activate CF discharge at elevated potentials (>2.5 V). DRT analysis reveals that the abundant surface defects of amorphous MnO₂ facilitate the Li⁺ transport pathways. Additionally, theoretical calculations reveal that, compared with c-MnO₂@CF, the d orbital of Mn in a-MnO₂@CF is closer to the Fermi level. This shift leads to greater electron transfer from MnO₂, to CF, and consequently reduces the overlap between C and F p-orbitals in the CF component of the composite. This reduction in orbital overlap weakens C-F p-p orbital hybridization, thereby enhancing the semi-ionic character of the C-F bonds. This work demonstrates a highly feasible chemical modification strategy for constructing composite cathodes, enabling significant performance improvements in Li/CF_X primary batteries.