Lithium motion in the anode material LiC6as seen via time-domain7Li NMR

Abstract

Since the commercialization of rechargeable lithium-ion energy storage systems in the early 1990s, graphite intercalation compounds (GICs) have served as the number one negative electrode material in most of today's batteries. During charging the performance of a battery is closely tied with facile Li insertion into the graphite host structure. So far, only occasionally time-domain nuclear magnetic resonance (NMR) measurements have been reported to study Li self-diffusion parameters in GICs. Here, we used several NMR techniques to enlighten Li hopping motions from an atomic-scale point of view. Li self-diffusion in the stage-1 GIC LiC 6 has been studied comparatively by temperature-variable spin-spin relaxation NMR as well as (rotating frame) spin-lattice relaxation NMR. The data collected yield information on both the relevant activation energies and jump rates, which can directly be transformed into Li self-diffusion coefficients. At room temperature the Li self-diffusion coefficient turns out to be 10 −15 m 2 s −1 , thus, slightly lower than that for layer-structured cathode materials such as Li x≈0.7 TiS 2 .

Tailoring Li adsorption on graphene

“…This optimal structure is found in the ( ) 2 3 2 3 30 R ×° supercell (Table 1), which contains 24 C and 4 Li atoms. All the Li atoms prefer to reside over hollow sites of the graphene sheet, consistent with previous results [20][21][22][23][24][25]. However, unlike uniform distribution pattern (which has symmetry group of P6mm); the four Li atoms form a rhombus structure.…”

“…Since the electron localization function (ELF) [44] is widely used to describe the extent of spatial distribution of electrons in molecules [45] 2D slice of ELF of LiC 6 . 15 We now assess the effect of Li clustering on possible applications of the Lifunctionalized graphene, such as Li-ion battery, hydrogen storage material, and superconductor, considered earlier [20][21][22][23][24][25]. We recall that in all these applications Li atoms were assumed to distribute uniformly.…”

“…Our choice of Li decorated graphene sheet [20][21][22][23] as a test case is motivated because of its potential in hydrogen storage [24], Li-ion battery [25,26], and superconducting materials [27][28][29]. To the best of our knowledge, most of the theoretical works thus far assumed, in analogy with multilayer graphite intercalated compounds (GIC), that the Li atoms are uniformly distributed on the graphene surface [24,[27][28][29][30].…”

Diffusion of alkali metals in the first stage graphite intercalation compounds by vdW-DFT calculations

“…The curved diffusion pathway was chosen for Li in LiC 6 and Na in NaC 6 since it leads to a higher diffusion coefficient compared to the straight pathway, which will be discussed further below. Experimental diffusivity data [16][17][18][19][20][21][22][23][24] are included for comparison.…”

“…The microscopic mechanisms of diffusion in AM-GICs is however not fully understood due to the lack of reliable theoretical and experimental methods. Quasielastic Neutron Scattering (QENS) 16,17 and nuclear magnetic resonance (NMR) 18 have been used to derive the diffusion coefficients of Li in LiC 6 and K in KC 8 . Electrochemical methods, including potentiostatic intermittent titration technique (PITT), 19 galvanostatic intermittent titration technique (GITT) 20 and electrochemical impedance spectroscopy (EIS), 21,22 have also been employed to determine the diffusion coefficient of Li in graphite anode materials.…”