Mathematical Modeling and Experimental Studies of the Role of Electrolytes on the Durability of the Electrodes for Lithium-Ion Batteries

The efficiency of a hybrid electric vehicle (HEV) is greater than that of pure petroleum based vehicles because of the use of regenerative braking and suspension in HEVs, which helps to minimize energy loss and recover the energy used to slow down or stop a vehicle. In addition, HEVs have lighter engine because HEV engines can be sized to accommodate average load, not peak load, and lighter vehicle weight because special lightweight materials can be used in their manufacture.  Much of research on the electrochemical storage for HEVs has focused on circumventing a so-called “range anxiety”, which alludes to two most fundamental parameters of electrochemical batteries: energy density and power density. In other words, in a HEV, the low energy density of a fix volume battery package results in a limited mileage range, and (2) long recharge time is often needed, compared with the fast process of refueling a tank. Equally important factor to be considered in developing commercial HEVs is the capacity loss in lithium-ion batteries largely due to the chemical changes of the electrodes during cycling processes. Numerous factors contribute to the capacity loss of a lithium-ion battery, including electrolyte solvent decomposition, active material dissolution, phase transition, abrupt lithium dendrite formation, passive layer deposition, and so on. During the past few years, extensive research on the capacity fade of Si-based electrodes has revealed the origin of a continuous growth of solid electrolyte interfaces (SEI).^1,2 Fluoroethylene carbonate (FEC) was found to be capable of achieving improved capacity retention as an additive or a co-solvent.^1,2

In this presentation, we report our recent studies on the roles of FEC as a co-solvent for binary and ternary solvent electrolytes used for sub-micron sized germanium electrodes. The binary solvent electrolyte consisting of 1 M LiPF₆ in DMC/EC (1:1 by volume) displayed rapid capacity fading with the charge capacity retention of ~33% at the end of 120^th cycle (361 mAh/g) while the one containing 1 M LiPF₆ in DMC/FEC (1:1) by volume, retained ~96% of the charge capacity (~1210 mA/g) at the charge and discharge current density of ~0.31 C (500 mA/g). The ternary solvent electrolyte displayed a similar behavior, as shown in Figure 1. When cycled at 0.5 C (800 mA/g), the ternary solvent electrolyte consisting of 1 M LiPF₆ in EC/DMC/DEC (1:1:1 by volume), the capacity retention at the end of 50th cycle was observed to be ~58% (645 mAh/g) whereas, using 1 M LiPF₆in DMC/DEC/FEC (1:1:1 by volume), about 94% of the charge capacity was retained.

These results demonstrated that FEC as co-solvent significantly improves the performance of the Ge electrodes in a binary or ternary solvent electrolytes. Post analysis is carried out by using scanning electron microscopy and x-ray photoelectron spectroscopy to relate the SEI structure – electrochemical property relationship in our battery system. A continuum mechanics model is developed by considering the reduction and of diffusion of solvent.  The model will be used to extract solvent diffusivity and predict the thickness of SEI layer in both Ge- and Si-based electrodes in binary and ternary compounds by using experimental data of Ge electrodes from this work and Si-based electrode in literature. 

References:

(1)   Chen, X.; Li, X.; Mei, D.; Feng, J.; Hu, M. Y.; Hu, J.; Engelhard, M.; Zheng, J.; Xu, W.; Xiao, J.; Liu, J.; Zhang, J.-G. ChemSusChem 2014, 7, 549.

(2)   Etacheri, V.; Haik, O.; Goffer, Y.; Roberts, G. A.; Stefan, I. C.; Fasching, R.; Aurbach, D. Langmuir 2011, 28, 965.

Figure 1. Cyclic capacity performance of Germanium electrodes in (a) binary solvent electrolyte containing 1 M LiPF₆ in DMC/EC (1:1 by vol) at current density of  500 mA/g, (b) 1 M LiPF6 in DMC/FEC (1:1 by vol) at current density of 500 mA/g.