A team at Cornell University led by Dr. Lynden Archer has used simple liquid electrolytes reinforced with halogenated salt blends to stabilize lithium metal anodes in a rechargeable battery. The cells exhibit stable long-term cycling at room temperature over hundreds of cycles of charge and discharge and thousands of operating hours.
In a paper published in the journal Nature Materials, they report that the addition of the salts to the electrolyte spontaneously creates nanostructured surface coatings on the lithium anode thats hinder the development of detrimental dendritic structures that grow within the battery cell. The discovery offers a potential pathway for the use of lithium metal anodes, which are enablers for cost-effective, higher-energy density systems such as Li-sulfur. (Earlier post, earlier post.)
It has long been understood that a rechargeable lithium metal battery (LMB), which eschewed the use of a carbon host at the anode, can lead to as much as a tenfold improvement in anode storage capacity (from 360 mAh g−1 to 3,860 mAh g−1) and would open up opportunities for high-energy un-lithiated cathode materials such as sulphur and oxygen, among others. Together, these advances would lead to rechargeable batteries with step-change improvements in storage capacity relative to today’s state-of-the-art LIBs.
A grand challenge in the field concerns the development of electrolytes, electrode, and battery system configurations that prevent uneven electrodeposition of lithium and other metal anodes, and thereby eliminate dendrites at the nucleation step. It is understood that without significant breakthroughs in this area, the promise of LMBs [lithium metal batteries], as well as of storage platforms based on more earth abundant metals such as Na and Al metal cannot be realized. Specifically, after repeated cycles of charge and discharge, growing metal dendrites can bridge the inter-electrode space, producing internal short circuits in the cell. In a volatile electrolyte, ohmic heat generated by these shorts may lead to thermal runaway and catastrophic cell failure, which places obvious safety and performance limitations on the cells. The ohmic heat generated during a short circuit may also locally melt dendrites to create regions of ‘orphaned’ or electrically disconnected metal that result in a steady decrease in storage capacity as a battery is cycled.—Lu et al.
The dendrite problem has been troubling lithium battery technology for years. Over several charge/discharge cycles, microscopic particles called dendrites form on the electrode surface and spread, causing short circuits and rapid overheating.
We had conflicting insight from two theories under development in the group and by theorists in the Cornell physics department, which suggested that a nanostructured metal halide coating on the anode could help a little—or a lot—in controlling the formation of dendrites. As it turns out, they work spectacularly well in solving what is widely considered a grand-challenge problem in the field.—Lynden Archer
In their paper, the researchers observed that rechargeable batteries based on some metals (such as magnesium) show no evidence of electrode instability and dendrite formation under deposition conditions where dendrites form and proliferate in others such as lithium.
At surface deposition rates common in batteries, the mobility of atoms at the interface determines whether smooth or rough electrodeposits are created.
Density functional theoretical analysis of Mg and Li electrodeposits at a vacuum/metal interface reveals that Mg–Mg bonds are on average 0.18 eV stronger compared with a Li–Li bond. This means that under the same deposition conditions, the probability of a lower-dimension, fibre-like Mg deposit spontaneously transforming to a higher-dimension crystal is more than 1,000 times higher than that for the corresponding transition in lithium. In electrolytes, these differences are only slightly altered by the interfacial tension, which is orders of magnitude lower, perhaps explaining why Li surfaces are more prone to nucleate dendrites irrespective of the electrolyte. A surprising and heretofore unexplored prediction from recent joint density functional theoretical (JDFT) calculations is that the presence of halide anions, particularly fluorides, in an electrolyte produces as much as a 0.13 eV reduction in the activation energy barrier for Li diffusion at an electrolyte/lithium metal electrode interface. If correct, this means that it should be possible to increase the surface diffusivity by more than two orders of magnitude, which may lead to large improvements in the stability of Li electrodeposition and dendrite suppression in simple liquid electrolytes.—Lu et al.
The team studied two configurations of electrolytes: in liquid form; and as liquids infused in nanoporous solids. They found that—consistent with expectations from the JDFT calculations—they essentially eliminated premature cell failure by dendrite growth and proliferation in plate-strip type experiments even at high operating current densities by using common liquid electrolytes reinforced with halogenated lithium salts.
In more aggressive, high-rate polarization experiments, they found levels of dendrite suppression in room-temperature liquid electrolytes that are either comparable to or superior to all previous reports from elevated-temperature studies of polymer and other solid-state electrolytes thought to be necessary for developing reliable lithium metal batteries. Infusing the electrolytes in the pores of a nanoporous ceramic led to much longer cell lifetimes than any previously reported room-temperature LMB.
In their experiments, they confirmed that the interfacial mobility is a strong decreasing function of halogenated lithium salt and is the most likely source of the improved stability of Li electrodeposits in liquids.
Our findings seem significant for at least three reasons. First, they demonstrate that the popular assumption inspired by intuition and supported by continuum modelling, that a high mechanical modulus is a requirement for an electrolyte that can stop growth and proliferation of lithium dendrites, is not generally correct. Second, electrolyte reinforcement by lithium halide salts provides an inexpensive, easy-to-use strategy for stabilizing electrodeposition of lithium metal that promises to enable technological and scientific advances in the field. Third, our findings underscore the benefits of density functional and other atomistic simulation approaches for guiding new materials designs for high-energy batteries.—Lu et al.
Archer and his team have spent two years conducting this research, which was supported by the Energy Materials Center at Cornell, an Energy Frontier Research Center funded by the US Department of Energy.
Yingying Lu, Zhengyuan Tu, and Lynden A. Archer (2014) “Stable lithium electrodeposition in liquid and nanoporous solid electrolytes,” Nature Materials doi: 10.1038/nmat4041