Evaluating Li Ion Storage Mechanism in Carbon Nanotubes (CNT) Anodes for High Energy Density Li-Ion Batteries: An Atomistic Perspective

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Current Lithium-ion batteries (LIBs) are limited to energy densities of ~150 Wh/kg and pose safety issues associated with dendrite formation and the use of flammable liquid electrolytes that can result in the common battery failure known as thermal runaway and explosion. Lithium metal is an ideal anode for high energy density LIBs due to its high theoretical capacity (3860 mAh/g) but is dangerous due to its propensity to form dendrites. Carbon nanotubes (CNTs) with superior electronic, mechanical, and structural properties provide an exciting alternative as anode for high energy density LIBs. LIBs with vertically aligned CNTs forest help achieve the capacity retention > 80% and superior efficiency over 1000 charging-discharging cycles. However, a fundamental understanding of Li+ ion storage within CNT anodes is largely unexplored and lacking. Experimental characterization of Li+ ion storage in CNT anodes is expensive and challenging. Previous theoretical studies have evaluated storage mechanism for isolated Li+ ions within small diameter CNTs (< 1.5 nm) forest. In common organic liquid electrolytes, Li+ ions prefer to be solvated and therefore isolated Li+ ions cannot accurately describe the Li+ storage mechanism especially in large diameter CNTs. In the current study, we carefully investigated the de-solvation mechanisms for Li+ ions in EC: EMC (3/7 w/w) 1.2 M LiPF6 liquid electrolytes both inside and outside of pristine CNT. The calculated de-solvation energies for Li+ ion adsorption onto the outside surface (~0.4-0.5 eV) of CNTs show relatively lower energies as compared to inside surface (~ 0.9-1.0 eV) of CNTs. We therefore claim that the Li+ ion storage strongly occurs on the outside surface of pristine defect-free CNTs. We further employ classical molecular dynamics simulations to investigate the Li+ ion storage on the outside of CNTs. The classical molecular dynamics simulations reveal a capacitive-type storage mechanism for pristine CNTs forest. Recent experimental reports suggest a large fraction of storage mechanism in CNT anode to be capacitive type (> 55 %) which supports the findings from atomistic modeling. Introducing structural defects (n-membered C-C rings), functional groups (-OH, =O, -COOH) etc... can lead to intercalation or plating-type Li ion storage in CNT forest which would result in higher capacity as well as energy density. However, for pristine CNTs we report the Li+ ion storage mechanism to be purely capacitive and therefore would serve as potential supercapacitor rather than battery. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.