Exploring the Limits of the Rapid-Charging Performance of Graphite as the Anode in Lithium-Ion Batteries

Graphite is, in principle, applicable as a high-power anode in lithium-ion batteries (LIBs) given its high intralayer lithium diffusivity at room temperature. However, such cells are known to exhibit poor capacity retention and/or undergo irreversible side reactions including lithium plating when charged at current rates above ~2C (~740 mA g-1). To explore the inherent materials properties that limit graphite anodes in rapid-charge applications, a series of full-cells consisting of graphite as the anode and a standard Li[Ni0.8Mn0.1Co0.1]O2 (NMC811) cathode was investigated. Instead of a conventional cathode-limited cell design, an anode-limited approach was used in this work to ensure that the overall cell capacity is only determined by the graphite electrode of interest. The optimized N:P capacity ratio was determined as N/P = 0.67, enabling stable cycling across a wide range of charging rates (4-20C) without inhibition by the NMC811 cathode. The results show that unmodified, highly crystalline graphite can be an excellent anode for rapid-charge applications at up to 8C, even with a standard electrolyte and NMC811 cathode and in cells with 1.0 mAh cm-2 loadings. As a rule, capacity and specific energy are inversely proportional to crystallite size at high rates; performance can likely be improved by electrolyte/cathode tuning.

Fine Spin-Dependent Splitting of Electronic Excitations and Their Dispersion in Single-Layer Graphene and Graphite

The dispersion dependences of electronic excitations in single-layer graphene and crystalline graphite have been studied taking the electron spin into consideration. Compatibility conditions for two-valued irreducible projective representations characterizing the symmetry of spinor excitations in the above structures and the distributions of spinor quantum states over projective classes and irreducible projective representations at all high-symmetry points in the corresponding Brillouin zones are determined for the first time. The principal existence of the spin-dependent splitting (or merging) of the electronic energy states, in particular, the electronic п-bands at the Dirac points, is established. The magnitude of spin-dependent splitting can be significant, e.g., for the transition-metal chalcogenides belonging to the same spatial symmetry group as crystalline graphite. However, because of the weak spin-orbit interaction for carbon atoms, it turns out small for all carbon structures including single-layer graphene and crystalline graphite.

Energy Spectra of Electron Excitations in Graphite and Graphene and Their Dispersion Making Allowance for the Electron Spin and the Time-Reversal Symmetry

The dispersion dependences of electron excitations in crystalline graphite and single-layer graphene have been studied taking the electron spin into consideration. The correlations of the energy spectra of electron excitations and, for the first time, the compatibility conditions for two-valued irreducible projective representations characterizing the symmetry of spinor excitations in the indicated structures are determined, as well as the distributions of spinor quantum states over the projective classes and irreducible projective representations for all high-symmetry points in the corresponding Brillouin zones. With the help of theoretical symmetry-group methods for the spatial symmetry groups of crystalline graphite and single-layer graphene (in particular, the splitting of п-bands at the Dirac points), the spin-dependent splittings in their electron energy spectra are found. The splitting magnitude can be considerable, e.g., for dichalcogenides of transition metals belonging to the same spatial symmetry group. But it is found to be small for crystalline graphite and single-layer graphene because of a low spin-orbit interaction energy for carbon atoms and, as a consequence, carbon structures.