The growing demand for advanced portable electronics and electric vehicles calls for the development of Li-ion batteries with enhanced performance and safety. Among the major goals still to achieve is the improvement of cycling stability and safety, where electrolyte and electrode interfacial properties play a central role.

It is generally known that during the first battery charge, a thin film called solid electrolyte interphase (SEI) is formed on the negative electrode due to the decomposition of the electrolyte components. The chemical nature and the morphology of the SEI are important factors for the battery performance. Ideally, the SEI layer is stable and prevents further electrolyte decomposition by blocking the electron transfer through the interface, while concomitantly preserving Li⁺transport.

The most reliable way to control the SEI formation is via electrolyte additives, which have a positive impact on the interface properties without affecting the main electrolyte functions. There is extensive research available on polymerizable additives, where vinylene carbonate (VC) is most widely researched and commercialized. In the last years, however, battery safety is of increasing concern, still limiting the implementation of Li-ion batteries in some industrial fields. In relation to the safety issues one appropriate solution is the design of electrolytes with low flammability.

The application of diphenyloctyl phosphate (DPOF) as an additive with a twofold input, acting as a SEI improving and additionally as flame-retarding component was recently reported [1]. However, the structural aspects of the functional improvement of electrode interfacial properties are not fully understood and require further analysis. The central aim of this paper is to correlate the electrical and structural properties of the SEI layer built on the graphite anode under the influence of DPOF and comparison with its commercial analogue - VC.

Galvanostatic cycling, cyclic voltammetry and electrochemical impedance spectroscopy (EIS) of graphite anodes were performed in 1M LiPF₆in ethylene carbonate (EC) / dimethyl carbonate (DMC) / diethyl carbonate (DEC) (vol. 1:1:1), containing VC or DPOF as additives. The cells with DPOF additive showed the best performance in terms of capacity and rate capability. EIS analysis was performed in symmetric cell configuration, allowing individual interpretation of the impedance parameters for both electrodes [2,3]. After five initial cycles at C/20 used for the formation of the SEI the cells were stopped at 50% SoC and disassembled. The graphite electrodes were re-assembled in symmetric cells, using the same electrolyte type.

The EIS spectra of the graphite symmetric cells consist of at least two overlapping semicircles for higher and a Warburg line at low frequencies. They can be fitted by the equivalent circuit proposed in the literature [3], (Fig.1A). In general, the electrical parameters extracted in the presence of the VC closely resemble these of the control cell (without electrolyte additives). After addition of 2% VC EIS showed a slight decrease of SEI resistance R₁ and at the same time a minimal increase of SEI capacitance C₁. In contrast, the addition of DPOF to the electrolyte resulted in a substantial decrease in R₁ and C₁.

The structural reason for the lower resistance and capacitance of DPOF formed SEI was analysed by means of X-Ray Photoelectron Spectroscopy (XPS). The analysis showed the presence of typically visible SEI features for all samples (Fig. 1B). C1s peaks at around 285eV and 287eV are attributed to a lithium alkyl carbonates. The O1s peaks at 533eV, 532.5eV and 534eV are assigned to σC-O bond in carbonates (Li₂CO₃ and non–lithiated alkyl carbonates) and O₂C=O groups. Beside the discussed C1s components, a low energy peak (dominant for DPOF and less pronounced for VC and control samples) related to the σC-C bonds from the graphite substrate, suggests a formation of much thinner SEI. The F1s core peaks of all samples consist of two main components at 687.0eV and 685.0eV, related to LiPF₆ and LiF, respectively. The P2p spectra are composed of one unresolved doublet (2p_3/2 and 2p_1/2), common for all three samples and attributed to LiPF₆. Additionally, the DPOF samples have a component at 136.8eV, originating from decomposed oxidized phosphorous compounds [1].

The correlation of EIS and XPS analysis indicates that the formation of low-resistive and stable SEI by the assistance of DPOF is related to the growth of much thinner and compact structure, containing oxidized phosphorous compounds.

References:

[1] I. Park, T. Nam, J. Kim, J. Power Sources, 244 (2013) 122-128.

[2] R. Petibon, N. Sinha, J. Burns, C. Aiken, H. Ye, C. VanElzen, G. Jain, S. Trussler, J. Dahn J. Power Sources, 251 (2014) 187-194.

[3] C. Chen, J. Liu, K. Amine, J. Power Sources, 96 (2001) 321-328.