Hydroxyl terminated Poly(dimethylsiloxane) as an electrolyte additive to enhance the cycle performance of lithium-ion batteries
Graphical abstract
Introduction
Lithium ion batteries (LIBs) have become a main stream battery choice for consumer and automotive applications as well as large-scale energy storage due to their advantages over other existing battery chemistries [[1], [2], [3], [4]]. However, there are still problems with lithium ion battery technology that are yet to be solved, like the formation of better passivating films on the graphitic anodes for better cycle life and on the cathodes for high voltage performance [[1], [2], [3], [4]]. The electrolyte is one component of LIBs that is currently being researched and electrolyte additives have attracted much attention for enhancing various electrochemical characteristics such as ionic conductivity and improved SEI formation [1,2,4,5].
When Dahn et al. found that the use of ethylene carbonate (EC) reduced the first cycle capacity loss (due to a much thinner SEI layer formation which resulted in reduction of graphite exfoliation), solvent mixtures with high EC concentration began to be used mainly [6]. Because of the need for a stable and durable SEI, more studies are being done to improve the formation of such by using additives that result in (i) better SEI layer films, and (ii) modification of the anode reaction kinetics and also inventing other alternative electrolyte salts that result in less lithium loss during SEI formation in the first charge– discharge cycle [[4], [5], [6]].
Of late, polydimethylsiloxane (PDMS)-based co-polymers have been shown to have good chemical properties such as a highly flexible backbone, small dielectric constant, oxidation stability, high permeability for various gases, hydrophobicity and anti-adhesiveness, and chemical inertness [1,2,7]. For instance, a PDMS co-polymer (PDMS-A) together with a lithium–modified silica nanosalt was reported to improve the electrochemical and interfacial stabilities of LIBs at lower temperatures due to their surface functional groups [2]. PDMS has also been used to suppress to a larger extent, the formation of dendrites in lithium metal surfaces caused by stress arising within the electrodes during charging and discharging [8,9].
This research utilizes Hydroxyl terminated Poly(dimethylsiloxane) (PDMS-HT), which consists of an –OH group at the end of each polymer chain as shown in Fig. 1, as an electrolyte additive for lithium ion batteries. The hydroxyl group has a low hindrance to bond rotation, can donate lone electron pairs for coordinating with cations, and has the ability to form hydrogen bonds in a liquid media to form networks that can facilitate Li+ ion movement through the electrolyte medium leading to an improved ionic conductivity [[10], [11], [12], [13]]. The –OH groups also have the ability to interact with electrolyte components such as EC leading to their decomposition that generates an SEI layer [[10], [11], [12]]. The presents of –OH groups on the graphite edges also cause a high chemical potential for lithium thus creating an environment highly concentrated with Li+ ions which also leads to SEI formation [14] and also higher lithium ion transfer velocities [15]. Considering all these, the cycle performance of PDMS-HT, which bears only one –OH polymer end group, is investigated in LiPF6 electrolytes with varying ratios of EC/dimethyl carbonate (DMC)-in LCO/graphite full cells.
Section snippets
Experimental method
To examine the electrochemical influence of PDMS-HT, standard commercial liquid electrolytes (PanaX StarLyte) consisting of 1 M LiPF6 dissolved in various EC/DMC ratios (0:1, 1:9, 3:7, 4:6, and 1:1) were used. The following three electrolytes samples were prepared for each electrolyte in an Argon-filled glove box: (i) Electrolyte, E (ii) electrolyte with 0.2 wt% PDMS-HT (Sigma-Aldrich, Mw ≈ 550, viscosity ~25 cSt), E + 0.2% and (iii) electrolyte with 0.5 wt% PDMS-HT, E + 0.5%. Coin-type half
Ionic conductivity and linear sweep voltammetry
To ensure that PDMS-HT additive did not negatively affect the ionic conductivity of the electrolytes, the ionic conductivities, of electrolyte samples with and without PDMS-HT additive were examined using the first resistance value obtained on the Z’ axis of the Nyquist impedance plot and substituting it in into Equation (1) below,where is the separation between the electrodes, and the total surface area of the electrodes. Ionic conductivities of 10.813; 11.913 and 11.375 mS cm−1
Conclusion
The effects of adding PDMS-HT to the conventional electrolyte solutions are examined in this study with the aim of improving the electrochemical performance of LIB cells, in particular, the cycle life stability. It is apparent that the cycle life performance is enhanced as a result of the formation of a stable SEI layer due to addition of PDMS-HT to the electrolyte. Based on the results of this study, PDMS-HT has proven to be a promising electrolyte additive to commercially available
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This research was supported by the research fund of Hanbat National University in 2019.
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