A Calculation Model to Assess Two Irreversible Capacities Evolved in Silicon Negative Electrodes

Abstract

A novel calculation model is devised to quantitatively assess two irreversible capacities evolved in Si negative electrodes: electrolyte decomposition and Li trapping. In this model, the capacity of the electrode reaction (Li-Si alloy formation, Q n alloy ), which is the only implicit value on the galvanostatic charge/discharge voltage profiles, is calculated with the data obtained from GITT (galvanostatic intermittent titration technique) experiment. When the calculation model is applied to two Si electrodes of different particle sizes, the particle size is found to significantly affect the nature of the irreversible reactions. In the bulk-sized Si electrode, Li trapping is dominant over electrolyte decomposition. This feature must be due to an electrical contact loss that is caused by crack formation, which is more vulnerable to bulk-sized Si particles. The hump behavior in the Coulombic efficiency profiles is also explained by the Li trapping. In the nano-sized Si electrode, electrolyte decomposition is the major irreversible reaction because of its larger surface area. Because of a stronger endurance against mechanical stress, crack formation and subsequently Li trapping are less severe than that of the bulk-sized one.Silicon has emerged as an alternative material to conventional carbonaceous materials for negative electrodes because of its high theoretical capacity (Li 15 Si 4 : 3579 mA h g −1 at room temperature) and working potential (<0.5 V vs. Li/Li + ) which is close to that of Li metal. 1-4 However, the use of Si in practical lithium-ion batteries (LIBs) is still not practical because of low Coulombic efficiency and poor cycle performance. The low Coulombic efficiency is because of irreversible consumption of Li + ions and the equivalent amount of electrons during charge/discharge. Here, the Li + ions/electrons are supplied from the positive electrode. Hence, if irreversible consumption continues to proceed, Li + ions/electrons are gradually exhausted in the positive electrode, which appears as a steady decrease in cell capacity (that is, poor cycle performance). 5 Two major routes for the irreversible consumption of Li + ions/electrons are assumed: reductive electrolyte decomposition and Li trapping, both of which are in turn deeply associated with severe volume change upon charge/discharge of Si electrodes. 6-9 Namely, the volume expansion of Si upon charging and contraction during discharge causes crack formation within the Si particles. The new Si surface is then exposed to the electrolyte solution, and subsequently, the electrolyte is reductively decomposed by taking Li + ions/electrons supplied from the positive electrode. [10][11][12] This electrolyte decomposition reaction is irreversible, such that the Li + ions/electrons cannot be recovered. Li trapping is also caused by a volume change in the Si particles. That is, crack formation as a result of volume change leads to electrically isolated Si particles, or poor contact between the Si particles themselves or with other conductive carbon parti...

Capacity Fading Mechanisms of Silicon Nanoparticle Negative Electrodes for Lithium Ion Batteries

“…In addition, particle pulverization has also been frequently reported to contribute to capacity fade in silicon electrodes. 4,35 However, in this study there are only small differences in the capacity fade as a function of particle size when comparing 0.7, 0.2, and 0.05 μm silicon, as depicted in Fig. 4.…”

“…In addition, the systematic electrochemical analysis of the crucial factors dictating capacity retention of nano-structured silicon electrodes has been limited. 19,[31][32][33][34][35][36] The prime focus of this work is to elucidate the dominant mechanisms of capacity fade of nano-structured silicon electrodes. To this end, different sizes of nano-structured silicon particles were investigated and the electrochemical properties, such as voltage profile and differential capacity, have been carefully analyzed.…”

Mechanical Damage of Surface Films and Failure of Nano-Sized Silicon Electrodes in Lithium Ion Batteries

“…From the second cycle, lithiation was performed to a voltage of 0.1 V (vs. Li/Li + ) to avoid the formation of the crystalline Li 15 Si 4 phase, which makes it difficult to estimate the state of charge at quasi-open circuit voltage (QOCV) values. 6 The de-lithiation cutoff voltage was fixed at 1.5 V and current density was fixed at 300 mA g Si −1 for each cycle. After one lithiation/de-lithiation cycle, the cells were rested for 30 min to achieve the QOCV.…”

“…[1][2][3] However, in practice, the use of Si electrodes continues to be limited owing to poor capacity retention and low Coulombic efficiency, which are mainly caused by two irreversible reactions, namely electrolyte decomposition/film deposition and Li trapping. [4][5][6] While these two parasitic reactions also occur in graphite electrodes, Li trapping is not severe in graphite because the commonly used graphite materials are highly crystalline. Further, electrolyte decomposition/film deposition in the case of graphite is appreciable only during the initial few cycles.…”

“…From the charge/discharge cycling data obtained in the two electrolyte solutions, the irreversible capacity observed in each cycle is considered in two parts (electrolyte decomposition and Li trapping) by using the calculation model proposed in our previous work. 6 Evolution of the two irreversible capacities is then tracked as a function of cycle number, and the causes and effects of the above-mentioned events are analyzed during cycling. Additionally, the advantages of using FEC are discussed.…”