Performance of Full Cells Containing Carbonate-Based LiFSI Electrolytes and Silicon-Graphite Negative Electrodes

Abstract

The energy density of lithium-ion cells can be significantly increased by the use of silicon-containing negative electrodes. However, the long-term performance of these cells is limited by the stability of the silicon electrode-electrolyte interface, which is continually disrupted during electrochemical cycling. Therefore, the development of electrolyte systems that enhance the stability of this interface is a critical need. In this article, we examine the cycling of ∼20 mAh pouch cells with lithium bis(fluorosulfonyl)imide (LiFSI)-containing carbonate-based electrolytes, silicon-graphite negative electrodes, and Li 1.03 (Ni 0.5 Co 0.2 Mn 0.3 ) 0.97 O 2 based positive electrodes. The effect of fluoroethylene carbonate (FEC) and vinylene carbonate (VC) addition on cell performance is also examined and compared to the performance of our baseline LiPF 6 -containing cells. Our data show that cells containing only LiFSI show rapid loss of capacity, whereas additions of FEC and VC significantly improve cell capacity retention. Furthermore, the performance of LiFSI-FEC and LiPF 6 -FEC cells are very similar indicating that the electrolyte salts play a much smaller role in performance degradation than the electrolyte solvent. Future efforts to enhance longevity of cells with silicon-graphite negative electrodes will thereby focus on developing alternative solvent systems. The ever-increasing demand for high energy density lithium-ion cells has led to a resurgence of interest in silicon-based negative electrodes.1-4 Unlike conventional graphite-based negative electrodes which have a theoretical capacity of 372 mAh/g-graphite, siliconbased negative electrodes can deliver capacities up to 3579 mAh/gsilicon, thus making possible the development of thinner, highercapacity electrodes. However, the commercialization of silicon anodes has been limited by factors that include the following: (a) the relatively large amounts of conduction additives required to ensure good interparticle electric conductivity because of silicon's semi-conducting nature; (b) the large volume expansion/contraction resulting from silicon lithiation/delithiation processes leading to excessive cracking and delamination from the current collector; (c) the development of binder-systems that can maintain cohesion between the coating components and adhesion to the current collector during electrochemical cycling; (d) the excessive solid electrolyte interphase (SEI) formation on the electrodes that irreversibly trap lithium leading to rapid capacity fade. 1In order to improve the performance of silicon-based negative electrodes, researchers have developed various strategies that include the following: (a) nanosizing the silicon particles, whereby the spaces between the particles can accommodate the volume expansion of individual particles; the use of nanoparticles, nanowires, and nanotubes have been shown to significantly improve cyclability; 5-7 (b) using non-traditional binders including water soluble polymers, such as polyacrylic acid (PAA), carboxymethyl ...

Layered Oxide, Graphite and Silicon-Graphite Electrodes for Lithium-Ion Cells: Effect of Electrolyte Composition and Cycling Windows

“…The contribution of the Gr or Si-Gr negative electrode to the impedance rise is small. We also noted previously 18 that it is difficult to obtain high-current pulse DC impedance data on the 100 cycle Gen2 electrolyte cell, because of its low capacity.…”

“…We present the effect of electrolyte composition, by adding various amounts of VC and FEC to our baseline electrolyte; such reports on electrolyte composition effects on silicon-containing full cells are still relatively few. 18,[21][22][23][24][25][26] We also describe the effect of voltage windows on cell performance and durability. We remind the reader that the effect of voltage windows on cell cycle life is intrinsically related to the negative and positive electrode's capacity matching (n:p ratio), and to the initial coulombic efficiency, which together dictate the electrode potential limits within a cell.…”

“…Fluoroethylene carbonate (FEC) specifically, and to some extent vinylene carbonate (VC), added to Si-containing cells has been shown to improve cycle life compared to cells containing the standard carbonate mixtures of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC). [14][15][16][17][18][19] Additionally, alternative strategies of usage could improve longevity, 20 for example by cycling the cells in a narrower voltage window thereby limiting material expansion, albeit at a cost of lower capacity per cycle. * Electrochemical Society Member.…”

“…A similar result was observed during the cycling of NCM523/Si-Gr cells with an LiFSI-electrolyte containing either 10 wt% FEC or VC. 18 This result may simply be a consequence of the difference in molecular weights; VC has lower molecular weight than FEC and hence will have a higher molarity in the electrolyte (see Table II). Alternatively, the better performance of VC could be related to the large amount of graphite in the Si-Gr electrode.…”

Effect of Fluoroethylene Carbonate in the Electrolyte for LiNi0.5Mn1.5O4 Cathode in Lithium-ion Batteries

“…The electrolytes for conventional graphite anodes in LIBs usually contain a small quantity of additives, less than 10 vol%. However, the electrolytes for alloyingtype anodes with high capacity require a large amount of additives such as FEC owing to the everincreasing surface area of the active materials resulting from pulverization [12,13].…”