Self-Discharge: Another Reason for Performance Deterioration of Lithium-Oxygen Battery?

Abstract

The fast and widespread commercialization of electric vehicles (EVs) demands high performance rechargeable batteries with good safety at low cost. Compared to the prevailing Li-ion battery, lithium-oxygen (Li-O2) battery promises significantly higher energy density in an open-system at notably reduced cost, and has thus become a focus of research on future battery technologies. Despite the encouraging progress in the development of electrocatalytic air-cathode, design of cathode architecture, and understanding of the related electrochemistry and stability of electrolytes, limited success was achieved in the practical aspects of Li-O2 batteries, as represented by low round-trip efficiency and unsatisfactory cycle life. The degradation of electrolyte during the charge/discharge processes is seen as one of the main causes for the battery performance deterioration, originating from the sources of its intrinsic instability, its reactions with discharge products, some synergistic reaction with oxygen, etc. It is worth pointing out that fast self-discharge may also notably contribute to a low round trip efficiency; while no report is available so far on this aspect of Li-O2 batteries. In this paper, we report how the storage condition and duration of Li-O2 cell affect their charge preservation and cell performance. This work also helps further improve the understanding on the stability of electrolyte in Li-O2 cells. Experiments were carried out using CR2032 coin-type cells with the cathode and anode being the stainless steel gauze supported multi-wall carbon nanotubes andlithium foil, respectively. Glass fiber (Whatman®, GF/B) was used as the separator, and the electrolyte was 1 M lithium trifluoromethane-sulfonate (LiOTf) in triethylene glycol dimethyl ether (Triglyme). Both electrolyte preparation and cell assembly were conducted in an Ar-filled glove box. Right after assembly the cells were stored under different conditions (varying storage temperature and time), with the electrochemical impedance spectra (EIS) and open circuit voltage (OCV) being recorded periodically. As the triglyme based electrolyte starts to degrade at a potential of about 4.7 V for cathode of CNTs, in the paper the upper limit of charge potential was set as 4.5 V to keep the electrolyte in a relatively stable state. The degree of self-discharge (DOSD) was quantified by calculating the differences between the discharge capacity of fresh assembled cell and that of the same cell after certain period of storage (Fig. 1a). One can clearly see that the cells lost approximately 44% of discharge capacity after 10 days storage atroom temperature under dry O2 atmosphere, and only ~31% of the discharge capacity can be retained after another 7 days of storage. The EIS spectra of Li-O2 cells showed steadily increased impedance as a result of continuous self-discharge (Fig. 1b). Here, for the real part of the impedance the intercept at high frequencies is generally associated with the separator and electrical contact resistances, as well as the electrolyte resistance. The semicircle in the high- and medium-frequency regions represents the interface resistances of Li-electrolyte, cathode-electrolyte, and charge-transfer resistance. Clearly, both electrolyte and interface resistances have increased. From the data of the first a few days, we can see the self-discharge was initially a slow process. The expedition in the following days indicates that some reactions were triggered by the intermediates of self-discharge, leading to notable electrolyte decomposition. Meanwhile, the accumulation of the self-discharged products elevated the interface resistance. This new understanding should facilitate research for further improvement of Li-O2 battery performance. Further study is in progress. This work was supported by the project IMRE/12-2P0504 under the SERC Advanced Energy Storage Research Programme, and Institute of Material Research and Engineering (IMRE), A*STAR, Singapore. Figure 1