Interphases Set to Stun!

Economic markets have supply, demand, and some sort of storage infrastructure in place that enables supply when demand is higher than the production. However, the electrical energy market lacks such a storage infrastructure.1,2 The storage of tangible items is trivial, but due to the abstract nature of energy, the current electrical energy market lacks a robust storage platform. For this reason, power plants are manufactured to meet peak production.1 It is not economically desirable to design a power plan to meet peak production needs, which only constitutes a small fraction of an annual production, resulting is excessive over design and excessive initial investment costs that affect ROI.

Electrochemical energy storage is uniquely suited to store electrical energy in the form of chemical potential by physically separating chemical reactants.1 When the energy needs to be accessed, the reactants are allowed to be in contact and chemical energy is converted to electrical energy in the form of electron flow. In primary batteries, the reactants cannot be re-separated, however in a secondary battery, the passage of current in the opposite direction induces an electrolytic process, re-separating the reactants and allowing for another discharge cycle. In secondary batteries this process can be repeated multiple times, however inefficiencies limit the cycle life.3

In Li-ion batteries, electrode degradation processes are just one of the inefficiencies that limit battery lifetime. Due to potential differences in initial cycles of the battery, and thermodynamic instabilities of solid lithium or carbon electrodes in the presence of organic electrolyte components, interphases can form on the surface of the electrode materials during the first few battery cycles (Figure 1).4 These phases remain for the duration of the battery life and interfere with electrochemical performance, most notably limiting the lifecycle of the entire battery through capacity fading.5 The composition of these solid-electrolyte interphases (SEIs) is highly dependent on the composition of the electrolyte, electrodes, and initial electrochemical environment.6

Figure 1: A secondary battery during a) charge and b) discharge and the formation of a solid-electrolyte interphase (SEI) on the anode.

Although the SEI introduces inefficiencies in the battery, the formation of the SEI is necessary to the function of the battery, as it promotes electrode stability. Carbon electrode materials in lithium-ion batteries are known to be unstable in the presence of the organic components of the electrolyte, and the SEI introduces a stabilizing layer on their surfaces.7 However, the presence of an SEI can also be detrimental to the battery function, and can introduce continuous capacity loss.8 The components of the electrolyte can be chosen in order to design desirable properties of the SEI. Ions must still be able to transport through the SEI for the battery to remain operable. The SEI must have some mechanical flexibility to facilitate electrode expansion and recoil during charge and discharge of secondary batteries.4 Electrolyte components with high dielectric constants are implemented in battery design to integrate these desirable properties into the formation of the SEI, but they must not also interfere with the overall battery performance.9

The storage of tangible items is trivial, but due to the abstract nature of energy, the current electrical energy market lacks a robust storage platform.

Batteries are dynamic and complicated systems. There are a multitude of design components that must be addressed to develop a robust chemical energy storage system. A well working battery requires interdisciplinary understanding and technologies. The physics of solid-liquid interfaces, solid-state transport, liquid transport, non-equilibrium conditions, electrochemical reactions and interphase development. SEIs are just one of the many components in the battery that must be optimized. SEIs are necessary for carbon electrode stability, but can introduce unwanted capacity fading and therefore shorten battery lifetimes. A well-designed SEI will stabilize the electrode but will not affect the long-term electrochemical performance of the battery.

1. Yang, Z.; Zhang, J.; Kintner-Meyer, M. C. W.; Lu, X.; Choi, D.; Lemmon, J. P.; Liu, J. Chem. Rev. 2011, 111, 3577–3613; 2. Chen, H.; Cong, T. N.; Yang, W.; Tan, C.; Li, Y.; Ding, Y. Prog. Nat. Sci. 2009, 19, 291–312; 3. Huggins, R. A. In Understanding Batteries; Dell, R. M.; Rand, D. A. J., Eds.; 2001.; 4. Balbuena, P. B.; Wang, Y. Lithium-ion batteries: solid-electrolyte interphase; Imperial College Press: London, 2004; 5. Phul, S.; Deshpande, A.; Krishnamurthy, B. Electrochim. Acta 2015, 164, 281–287; 6. Bhatt, M. D.; O’Dwyer, C. Curr. Appl. Phys. 2014, 14, 349–354; 7. Li, S.; Xu, X.; Shi, X.; Li, B.; Zhao, Y.; Zhang, H.; Li, Y.; Zhao, W.; Cui, X.; Mao, L. J. Power Sources 2012, 217, 503–508; 8. Sacci, R. L.; Black, J. M.; Balke, N.; Dudney, N. J.; More, K. L.; Unocic, R. R. Nano Lett. 2015, 15, 2011–2018; 9. Ong, M. T.; Verners, O.; Draeger, E. W.; van Duin, A. C. T.; Lordi, V.; Pask, J. E. J. Phys. Chem. B 2015, 119, 1535–1545.

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