Ultrathin positively charged electrode skin for durable anion-intercalation battery chemistries

Abstract

The anion-intercalation chemistries of graphite have the potential to construct batteries with promising energy and power breakthroughs. Here, we report the use of an ultrathin, positively charged two-dimensional poly(pyridinium salt) membrane (C2DP) as the graphite electrode skin to overcome the critical durability problem. Large-area C2DP enables the conformal coating on the graphite electrode, remarkably alleviating the electrolyte. Meanwhile, the dense face-on oriented single crystals with ultrathin thickness and cationic backbones allow C2DP with high anion-transport capability and selectivity. Such desirable anion-transport properties of C2DP prevent the cation/solvent co-intercalation into the graphite electrode and suppress the consequent structure collapse. An impressive PF₆^--intercalation durability is demonstrated for the C2DP-covered graphite electrode, with capacity retention of 92.8% after 1000 cycles at 1 C and Coulombic efficiencies of > 99%. The feasibility of constructing artificial ion-regulating electrode skins with precisely customized two-dimensional polymers offers viable means to promote problematic battery chemistries.

Fig. 1. Anion-intercalation chemistries of graphite and morphological characterizations of C2DP.

a Schematic illustration showing challenges associated with the anion-intercalation chemistries of graphite. b Functions of C2DP as the electrode skin in promoting the anion-intercalation chemistries. c Optical microscopy image of C2DP. The inset shows an atomic force microscopy image of a single crystal (size of 60 μm²), and the height profile along the white line indicate that the thickness of the single crystal is 20 nm. d TEM image of C2DP. The inset shows the selected area electron diffraction pattern of a single crystal (the red-cycle zone). e High-resolution TEM image of C2DP with the structural model overlaid.

Fig. 2. Rate capability and PF₆⁻ diffusivity.

a Rate performance of the graphite electrode and the C2DP-G electrode. b Log (σ) as a function of T⁻¹ for PP and C2DP-PP. c EPS plots of C2DP (isosurface value = 0.005 e Å⁻³). The blue and red colours denote deficient and rich electron density, respectively. d Simulated structure and e charge density difference plots (ρ_adsorbed − ∑ρ_isolated, isosurface value = 0.0002 e Å⁻³) of C2DP with PF₆⁻ as counter ions. f Schematic illustration of PF₆⁻-diffusion paths and g the corresponding PF₆⁻-diffusion energy barriers.

Fig. 3. Analysis of electrolyte decomposition.

a Specific capacities and Coulombic efficiencies of the graphite electrode and C2DP-G electrode in the initial 40 charge/discharge cycles at 1 C. b The 1^st, 5^th, 10^th, 40^th GCD curves of the C2DP-G electrode at 1 C. c Nyquist plots of the graphite electrode and the C2DP-G electrode before and after 20 GCD cycles. TEM images of the graphite grain in d the graphite electrode and e the C2DP-G electrode after 3 GCD cycles. f, g F 1 s XPS spectra of (f) the fully charged graphite electrode and (g) the fully charged C2DP-G electrode. h, Cycling performance of the Li//graphite pouch cell and the Li//C2DP-G pouch cell. i Digital photos of the Li//graphite pouch cell (upper) and the Li//C2DP-G pouch cell (lower) after the cycling test.

Fig. 4. Durability assessment of the PF₆⁻-intercalation chemistry.

a Cycling performance of the graphite electrode and C2DP-G electrode at 1 C. b XRD patterns of the graphite electrode and the C2DP-G electrode after 1000 GCD cycles at 1 C. c, d Operando synchrotron XRD maps of (c) the graphite electrode and (d) the C2DP-G electrode during the initial cycle at 0.1 C. e, f TEM images of the graphite grain in (e) the graphite electrode and (f) the C2DP-G electrode after 20 GCD cycles. g, h Raman spectra of (g) the graphite electrode and (h) the C2DP-G electrode at both the fully charged and discharged states.

Fig. 5. Evaluation of other anion-intercalation chemistries.

a, b The 1^st, 5^th, 10^th, 40^th GCD cycles of the C2DP-G electrode in (a) 2 M LiFSI and (b) 2 M LiTFSI. c, d Cycling performance at 1 C of the graphite electrode and the C2DP-G electrode in (c) 2 M LiFSI and (d) 2 M LiTFSI.