Thin-Film Protected Lithium Metal Anode for Lithium Batteries

Abstract

Li batteries based on Li metal anode are expected to provide 2-3 times more energy density than today's Li-ion battery. However, uncontrolled dendritic Li growth (continuous loss of Li) and limited Coulombic efficiency (CE) during Li deposition/stripping of a bulk Li anode, are the main reasons behind low cycle life, which has limited their practical applications [1,2]. Although low CE can be partially compensated by an excess amount of Li, it is achieved at the expense of lower useable Li energy density and elevated risks of fire and other hazards associated with Li. Therefore, efforts should be directed to reduce the use of Li, limited in weight to balance cathode capacity. For example in an effort to balance the sulfur (S)-cathode capacity with Li-anode capacity, we recently demonstrated that 20 µm thick Li can replace 380 µm thick Li and achieve better CE (Figure 1 (a)). Furthermore, the rate of dendrite growth gets amplified when the cell is cycled at high charge-discharge current densities, high charge voltages, and high areal discharge currents. Increasing anode surface area would reduce the effective charge/discharge current densities. We have shown that a 20 mm thick Li deposited on Cu current collector (CC) using thermal evaporation technique showed particulate morphology (high surface area) compared to featureless (low surface area) morphology of commonly used thick Li (Figure 1 (b)) that may have provided improved results as shown in Figure 1 (a). However, with increased surface area, more surface of thin Li is expected to be exposed to electrolytes or catholytes (especially in Li-S battery), and thus react faster than bulk Li causing cell performance degradation during long-term battery cycling. To reduce the high reactivity of thermally evaporated thin film Li, we will apply an artificial SEI (ASEI) consisting of a conducting polymer (poly(3-hexylthiophene) (P3HT)/fullerene (PCBM)/graphite layer developed at UDRI [3]. In thick Li metal based LIB, we have demonstrated that this ASEI as Li protective layer remarkably reduces dendrite growth, enhances cycle life more than 300% and protects Li by physically separating it from the electrolyte (Figure 1 (c)). The protected Li anode may be incorporated into wet cell batteries or solid state batteries having transition metal oxide cathodes (specifically, state-of-the-art Li-ion cathodes), sulfur cathode (Li-S battery), or oxygen cathode (Li-O2 battery). Moreover, similar methodologies can be applied to other rechargeable metal anodes (Na, Al, Mg, etc.) for next generation rechargeable batteries. We will present and discuss detailed research being conducted in our laboratories in order to optimize thin-film Li fabrication parameters, protection of thin-film Li anode and their use in Li batteries including Li-ion and Li-S batteries.