Understanding the Chemical Stability of Polymers for Lithium–Air Batteries

Abstract

Recent studies have shown that many aprotic electrolytes used in lithium-air batteries are not stable against superoxide and peroxide species formed upon discharge and charge. However, the stability of polymers often used as binders and as electrolytes is poorly understood. In this work, we select a number of polymers heavily used in the Li-air/Li-ion battery literature, and examine their stability, and the changes in molecular structure in the presence of commercial Li 2 O 2 . Of the polymers studied, poly (acrylonitrile) (PAN), poly(vinyl chloride) (PVC), poly(vinylidene fluoride) (PVDF), poly(vinylidene fluoride-cohexafluoropropylene) (PVDF-HFP), and poly(vinyl pyrrolidone) (PVP) are reactive and unstable in the presence of Li 2 O 2 . The presence of the electrophilic nitrile group in PAN allows for nucleophilic attack by Li 2 O 2 at the nitrile carbon, before further degradation of the polymer backbone. For the halogenated polymers, the presence of the electron-withdrawing halogens, and adjacent alpha and beta hydrogen atoms that become electron-deficient due to hyperconjugation makes PVC, PVDF, and PVDF-HFP undergo dehydrohalogenation reactions with Li 2 O 2 . PVP is also reactive, but with much slower kinetics. On the other hand, the polymers poly(tetrafluoroethylene) (PTFE), Nafion ® , and poly(methyl methacrylate) (PMMA) appear stable against nucleophilic Li 2 O 2 attack. The lack of labile hydrogen atoms and the poor leaving nature of the fluoride group allows for the stability of PTFE and Nafion ® , while the methyl and methoxy functionalities in PMMA reduce the number of potential reaction pathways for Li 2 O 2 attack in PMMA. Polyethylene oxide (PEO) appears relatively stable, but may undergo some crosslinking in the presence of Li 2 O 2 . Knowledge gained from this work will be essential in selecting and developing new polymers as stable binders and solid or gel electrolytes for lithiumair batteries.

Examining the Role of Electrolyte and Binders in Determining Discharge Product Morphology and Cycling Performance of Carbon Cathodes in Li-O2Batteries

“…The total integrated area for the tests in DMSO decreased in the order of PTFE>PEO>PVP>PVDF, which compares well with the discharge capacities seen in the galvanostatic tests (Figure 2). The higher discharge capacities for PTFE and PEO based cathodes in DMSO (where discharge proceeds primarily through a single discharge mechanism) correlate well with the relative stabilities of the binders as described by Amanchukwu et al 58 SEM analysis was used to identify the morphology of discharge products on the different carbon cathodes after linear voltage sweeps to 2 V in DMSO ( Figure 5). This was used as a means of examining the differences in peak location and the magnitudes of currents between the two electrolyte systems and between different cathodes.…”

“…Amanchukwa et al recently compared the stability of different polymers in the Li-O 2 system and classed them as either stable or unstable in contact with Li 2 O 2 . 58 According to their results, PVP and PVDF were deemed unstable while PTFE and PEO were stable (with the latter possibly prone to some crosslinking in the presence of Li 2 O 2 ). The unusual discharge profiles for the PVP cathode seen here after the second discharge likely indicated poorer stability of PVP compared to the other binders.…”

“…The anomalously high coulombic efficiency values for the PVP test in DMSO suggest extensive side reactions caused by the fundamental instability of PVP in the Li-O 2 system as recently reported. 58 To further investigate the electrochemical response for the various cathodes in the two different electrolytes outlined above, LSV tests were conducted (Figure 4). LSV has recently been used as a useful means of fingerprinting the nature of Li 2 O 2 growth on Li-O 2 battery cathodes.…”

XPS Study of the Interface Evolution of Carbonaceous Electrodes for Li-O2Batteries during the 1st Cycle

“…27 PTFE based binders are fully fluorinated and already widely reported to be chemically stable toward Li 2 O 2 . 38 In addition, several previous studies demonstrated the presence of LiF solely when fluorinated salts were available, such as LiTFSI, 39 thus supporting our assumption that LiF originates from LiTFSI salt and not the PTFE binder. Furthermore, the degradation of the LiTFSI salt is also observed on the S 2p spectrum with the formation of double split peaks at ∼167.3 eV and ∼168.5 eV, most probably related to salt decomposition products formed during the last stages of discharge.…”

Surface engineering and self-cleaning properties of the novel TiO2/PAA/PTFE ultrafiltration membranes

“…In particular, porous PTFE membranes have found special importance in water treatment [1], separators of lithium-ion batteries [1,2], pervaporation [3], and blood purification [4]. However, the presence of strong C-F bonding and water repellence makes the application of PTFE membranes in water treatment field less competitive because of the rather low water flux.…”