Modeling solid-electrolyte interfacial phenomena in silicon anodes
Introduction
Energy storage devices such as batteries are necessary for the advancement of Plugless electric vehicles (EVs), low-power battery-on-a-chip, electricity storage in distribution networks, portable electronics and so forth. Due to the market penetration of rechargeable batteries in the automotive industry, the United States Advanced Battery Council (USABC) has established long-term goals that demand batteries to provide EVs with driving ranges greater than 200 miles and a battery life greater than 100,000 miles (2016, www.uscar.org). Lithium-ion batteries (LIBs) are suitable candidates to drive the technology to harvest a higher energy density to power portable devices and EVs. A typical electrochemical cell for this system includes graphite-lithium cobalt oxide (LiCoO2) with lithium hexafluorophosphate (LiPF6) and organic solvents as the electrolyte mixture [1, 2]. Graphite has replaced lithium metal as the anode electrode material due to the uncontrollable dendritic Li growth and limited Coulombic efficiency (C.E.) during Li deposition/stripping in Li metal anode-based batteries [3, 4, 5, 6, 7, 8, 9]. In the graphite anode-based batteries, graphite has a Li charge storage capacity of 372 mAh/g (with a theoretical capacity of up to one Li+ for every six carbon atoms, LiC6) [10]. Hence, the combination of these standard materials results in a relatively low voltage (i.e. 3.4 eV) which is deemed insufficient for EV's battery packs. It is widely known that modern cathode materials such as LiCoO2, lithium manganese oxide (LiMn2O4), and lithium iron phosphate (LiFePO4) are limited to intercalation/insertion compounds [11]. Thus, any considerable improvement in the electrode must be made of the anode material. Finding a suitable anode material is an effective way to solve the problem related to Li dendrite growth and to improve Li resource utilization [5, 12, 13, 14, 15]. Specifically, LIBs consisting of a transition metal oxide cathode and a silicon anode are a promising solution due to their potentially high energy and power density [16, 17].
Silicon (Si) as an anode material offers a tenfold increase in capacity per unit weight (4200 mAh/g, lithiated to Li4.4Si) when compared to the widely-used graphite anode [18, 19, 20, 21, 22]. Furthermore, the lithiation voltage plateau (0.2–0.3 V versus Li/Li+) for Si could prevent lithium plating, dendrite formation, and growth [19, 23]. Despite these improvements over graphite, it is still a significant challenge to implement Si as an anode in LIBs due to the volume change that takes place during cycling (up to four times its volume when charged with lithium ions) which leads to low C.E. and poor capacity retention [24, 25, 26, 27]. On the other hand, the reaction mechanisms that result in the decomposition of the electrolyte and simultaneous solid-electrolyte interphase (SEI) formation remain elusive. This article briefly summarizes the computational research activities in the field of LIBs with Si as an anode material and reports recent works and improvements on this subject. We emphasize the study of LIBs with Si anodes on the formation (i.e. understanding of the processes that lead to electrolyte decomposition at the Si surface) and growth of the SEI layer.
Section snippets
Solvent decomposition at the lithiated Si surface
Martinez de la Hoz, Balbuena, and co-workers [28•, 29•, 30•] studied the interaction of electrolyte molecules with lithiated Si anodes, using ab-initio molecular dynamics (AIMD) simulations. First, reduction mechanisms of the ethylene carbonate (EC) solvent molecule and those of the additives fluoroethylene carbonate (FEC) and vinylene carbonate (VC) were determined as a function of the lithiation degree of the Si anode. It is important to note that as the Li/LixSiy cell is cycled
Summary and concluding remarks
Overall, the results we have discussed in this article have important implications for the initial and growth stages of SEI layer formation on Si anodes. Since EC reduces to different adsorbed species and charged fragments at different lithiation stages, the composition of the SEI layer is expected to vary during the first cycling processes. Furthermore, the composition of the SEI layer varies depending on the anode material and decomposed solvent further away from the surface can lead to
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgements
This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231, Subcontract No. 7060634 under the Advanced Batteries Materials Research (BMR) Program, and by the Qatar National Research Fund (QNRF) through the National Priorities Research Program (NPRP 7-162-2-077). PPM and ZL acknowledge financial support from NSF Grant No. 1438431. Computational resources from
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Performance degradation modeling in silicon anodes
2021, Silicon Anode Systems for Lithium-Ion BatteriesSEI layer and impact on Si-anodes for Li-ion batteries
2021, Silicon Anode Systems for Lithium-Ion BatteriesKey functional groups defining the formation of Si anode solid-electrolyte interphase towards high energy density Li-ion batteries
2020, Energy Storage MaterialsCitation Excerpt :The inorganic species is composed of Li2O, Li2CO3, and LiF [65,66]. Here, LiF is more electron resistive than Li2O (14.2 eV and 7.99 eV band-gap, respectively) [67]. Among the inorganic species, LiF is the rate limiting inorganic compound since cation diffusivity is the lowest [68].
The influence of FEC on the solvation structure and reduction reaction of LiPF<inf>6</inf>/EC electrolytes and its implication for solid electrolyte interphase formation
2019, Nano EnergyCitation Excerpt :Through this technique, FEC was found to decompose into a range of products including HCO2Li, Li2C2O4, Li2CO3, and polymerized vinylene carbonate (VC), which supports a decomposition mechanism where FEC reduces to form VC and LiF, followed by subsequent VC reduction. Other complementary approaches include theoretical modeling the electrolyte [44], Si anode [45], and their interphase [46,47]. Quantum chemical calculations confirm that defluorination reactions significantly increase the reduction potential of FEC [48].
Chemical and mechanical degradation and mitigation strategies for Si anodes
2019, Journal of Power SourcesCitation Excerpt :Also, since the cutoff value used in the MD simulation is 12 Å, as the expansion increases, the interaction between the atoms from the SEI with the inner TDEP decreases. The calculated volume expansion is comparable to the one from previous molecular dynamics work done with silicon anodes using the same force field as well as to experimental data [49]. Mechanical properties such as cracking mechanism and deformation behavior of the LiF SEI are studied at different charging currents emulated with the TDEP model.
Review on multi-scale models of solid-electrolyte interphase formation
2019, Current Opinion in ElectrochemistryCitation Excerpt :The collective dynamics of molecules and atoms can then be calculated with molecular dynamics simulations (MD). In this section, we give a brief outline of results from atomistic simulations, but refer to recent reviews for further details [46–49]. Borodin et al. highlight general challenges for calculations of electrolyte stability [50].
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