Researchers at the University of Waterloo (Canada) have developed a low-cost and scalable approach that tackles the stabilization of Li metal electrodes by forming a single-ion-conducting and stable protective surface layer in vivo.
They use a rationally designed electrolyte additive complex that reacts with the Li surface to form the membrane. In a paper in the journal Joule, they reported demonstrating stable Li plating/stripping for 2,500 hr at 1 mA cm-2 in symmetric cells, and efficient Li cycling at high current densities up to 8 mA cm-2. More than 400 cycles were achieved at 5-C rate in cells with a Li4Ti5O12 counter electrode at close to 100% coulombic efficiency. The increased energy density enabled by safely using a Li metal anode could significantly increase the range of electric vehicles.
Li metal batteries offer promise as next-generation electrochemical storage devices for electric vehicle applications due to lithium’s highest specific capacity (3,840 mA hr g-1) and its lowest reduction potential (-3.04 V versus standard hydrogen electrode) among all metals. … [However] Li metal anodes are plagued with dendrite-like electro-deposition behavior instead of plating Li smoothly as a film. Since liquid organic electrolytes (LE) are thermodynamically unstable to reduction by Li, dendritic growth leads to the dynamic loss of active Li, buildup of a high-impedance solid electrolyte interphase (SEI), and electrolyte dry-out causing cell failure. Upon prolonged cycling, penetration of dendrites through the separator can cause hazardous cell short-circuits. Dendrite growth is also exacerbated at the higher currents necessary for fast charging.
… Herein, we demonstrate a facile and scalable approach to build a single-ion-conducting SEI layer with controlled compositions in vivo (i.e., inside the assembled cell) that maintains complete and intimate contact with the locally uneven Li metal surface. This comprises a thin amorphous “Li3PS4” layer formed by using a low-concentration electrolyte additive. It reduces the reactions with the electrolyte and eliminates the heterogeneity of the SEI, thus allowing a non-impeding and uniform Li+ flux.
The nature of in vivo formation distinguishes it from ex situ deposition of solid electrolytes (SE), such as atomic layer deposition. More importantly, the Li3PS4 layer is a Li+ single-ion conductor with a theoretical Li+ transference number of unity, which ideally eliminates the ion depletion and strong electric field buildup at the Li surface that inspire dendrite growth. This also contrasts with other types of artificial or additive-driven ion-passivating SEIs. We show experimental evidence of these two important aspects and demonstrate that their interplay allows long-life dendrite-free Li plating.—Pang et al.
The researchers designed an electrolyte additive complex—Li2S6-P2S5 (denoted as LSPS)—to enable the direct formation of the Li3PS4 layer on the Li metal surface in dimethoxyethane (DME).
Because the reactivity of Li with Li2S6 is higher than that with DME, Li3PS4 is the predominant component in the SEI upon conditioning. Even when microstructured Li forms upon extreme plating conditions, the plated Li can react with the accessible LSPS additive in the electrolyte to repair the locally damaged Li3PS4 layer.
Quan Pang, Xiao Liang, Abhinandan Shyamsunder, Linda F. Nazar (2017) “An In Vivo Formed Solid Electrolyte Surface Layer Enables Stable Plating of Li Metal” Joule doi: 10.1016/j.joule.2017.11.009u