Molecular Dynamics Simulations and Experimental Study of Lithium Ion Transport in Dilithium Ethylene Dicarbonate

301

190

Understanding the factors limiting Li + charge transfer kinetics in Li-ion batteries is essential in improving the rate performance, especially at lower temperatures. The Li + charge transfer process involved in the lithium intercalation of graphite anode includes the step of de-solvation of the solvated Li + in the liquid electrolyte and the step of transport of Li + in the preformed solid electrolyte interphase (SEI) on electrodes until the Li + accepts an electron at the electrode and becomes a Li in the electrode. Whether the de-solvation process or the Li + transport through the SEI is a limiting step depends on the nature of the interphases at the electrode and electrolyte interfaces. Several examples involving the electrode materials such as graphite, lithium titanate (LTO), lithium iron phosphate (LFP), lithium nickel cobalt aluminum oxide (NCA) and solid Li + conductor such as lithium lanthanum titanate or Li-Al-Ti-phosphate are reviewed and discussed to clarify the conditions at which either the de-solvation or the transport of Li + in SEI is dominating and how the electrolyte components affect the activation energy of Li + charge transfer kinetics. How the electrolyte additives impact the Li + charge transfer kinetics at both the anode and the cathode has been examined at the same time in 3-electrode full cells. The resulting impact on Li + charge transfer resistance, R ct , and activation energy, E a , at both electrodes are reported and discussed. To improve the power performance of Li-ion batteries, it is important to understand the factors that limit the Li + charge transfer kinetics. Li-ion batteries comprised of a graphite anode and a lithium cobalt oxide cathode in an electrolyte consisting of 1 M LiPF 6 in ethylene carbonate (EC)-dimethyl carbonate (DMC)-diethyl carbonate (DEC) carbonate solvent mixture could not deliver their room temperature capacity at a rate of C/2 at −30 and −40• C. 1 When DEC was replaced by a linear ester solvent, such as ethyl acetate (EA) or methyl butyrate (MB), the Li-ion batteries at −30 and −40• C could deliver over 80% of their room temperature capacity at the same rate.2 When the LiPF 6 salt is replaced by lithium bis(oxalato)borate (LiBOB) in EC-ethyl methyl carbonate (EMC) (1:1 wt ratio) carbonate solvent mixture, the impedance of the graphite-electrolyte interface measured using graphite/Li half cells in the electrolyte with LiBOB is three times that in the electrolyte with LiPF 6 .3 These examples illustrate that the electrolyte components play crucial roles in affecting Li + charge transfer kinetics in Li-ion batteries.The Li + charge transfer process starts from the solvated Li + in the electrolyte to the reception of an electron (e − ) from the electrode and becomes Li (i.e., Li x C in graphitic anodes). This involves the desolvation step of Li + before entering into a layer of solid electrolyte interphases, or SEI, that is often referred to that on the anode such as carbonaceous materials, and the diffusion step of Li + through the SEI, which is pre-form...

Section: Discussionmentioning

confidence: 99%

Factors Limiting Li⁺Charge Transfer Kinetics in Li-Ion Batteries

Jow

Delp

Allen

et al. 2018

301

190

“…The relatively rapid diffusion of Li + through the SEI is apparently a consequence of the arrangement of the EC-derived polymer chains that are "zipped" by Li + ions that have a tetrahedral coordination and are shared by two carbonate groups; 60 Li + conduction can occur through an ion-exchange mechanism, with the incoming Li + ion releasing bound Li + ions. 61 In the inorganic layer diffusion of Li + ions likely occurs by a repetitive knock-off mechanism than by direct hopping through the empty spaces between the lattice sites. 62 Both knock-off (in the inorganics) and zipping (in the organics) mechanisms are facilitated by the abundance (and increasing content) of Li + ions in the SEI layer.…”

Section: Summary and Concluding Remarksmentioning

confidence: 99%

Cycling Behavior of NCM523/Graphite Lithium-Ion Cells in the 3–4.4 V Range: Diagnostic Studies of Full Cells and Harvested Electrodes

Gilbert

Bareño

Spila

et al. 2016

160

180

Energy density of full cells containing layered-oxide positive electrodes can be increased by raising the upper cutoff voltage above the present 4.2 V limit. In this article we examine aging behavior of cells, containing LiNi 0.5 Co 0.2 Mn 0.3 O 2 (NCM523)-based positive and graphite-based negative electrodes, which underwent up to ∼400 cycles in the 3-4.4 V range. Electrochemistry results from electrodes harvested from the cycled cells were obtained to identify causes of cell performance loss; these results were complemented with data from X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS) measurements. Our experiments indicate that the full cell capacity fade increases linearly with cycle number and results from irreversible lithium loss in the negative electrode solid electrolyte interphase (SEI) layer. The accompanying electrode potential shift reduces utilization of active material in both electrodes and causes the positive electrode to cycle at higher states-of-charge. Full cell impedance rise on aging arises primarily at the positive electrode and results mainly from changes at the electrode-electrolyte interface; the small growth in negative electrode impedance reflects changes in the SEI layer. Our results indicate that cell performance loss could be mitigated by modifying the electrode-electrolyte interfaces through use of appropriate electrode coatings and/or electrolyte additives. The pursuit of high energy density lithium-ion batteries for transportation applications continues in order to increase the driving range of vehicles on a single charge. Because graphite (Gr) remains the active material of choice in the negative electrode, the search mainly revolves around layered oxides for the positive electrode. In recent years there has been immense interest in the lithium and manganese-rich layered oxides which are capable of delivering high energy density cells. [1][2][3][4][5][6][7][8][9][10] This interest has ebbed somewhat because the voltage fade exhibited by these oxides lowers the usable energy, and complicates state-ofcharge determination, of the battery cells. In this article we focus our attention on the NCM523 oxide which has been the subject of several recent articles. For example, Bak et al. conducted in situ X-ray diffraction studies on various (delithiated) Li x Ni a Co b Mn c O 2 materials and concluded that NCM523 is an optimized composition offering the thermal stability of lower-Ni oxides (such as NCM333) while displaying capacities closer to the higher-Ni oxides (such as NCM811).27 From AC impedance and DC polarization studies Amin and Chiang reported that the electronic conductivity of NCM523 increases with decreasing Li-content, from 10 −7 S/cm for a fully-lithiated oxide (Li 1.0 ) to 10 −2 S/cm for a significantly delithiated (Li 0.25 ) oxide.28 Dixit et al. confirmed from first * Electrochemical Society Member. z E-mail: abraham@anl.gov principle-based simulation studies that during oxide delithiation Ni oxidizes first, followed by Co, while Mn rem...

“…Most striking is the fact that the mechanism for lithium-ion transport through the SEI is still debated. Shi et al propose a "knock-of" diffusion mechanism for lithium-ion interstitials in Li 2 CO 3 .18 Diffusion of lithium-ions through Li 2 EDC is modeled by Borodin et al 19 At the same time …”

mentioning

confidence: 99%

Revealing SEI Morphology: In-Depth Analysis of a Modeling Approach

Single

Horstmann

Latz

2017

104

134

In this article, we present a novel theory for the long term evolution of the solid electrolyte interphase (SEI) in lithium-ion batteries and propose novel validation measurements. Both SEI thickness and morphology are predicted by our model as we take into account two transport mechanisms, i.e., solvent diffusion in the SEI pores and charge transport in the solid SEI phase. We show that a porous SEI is created due to the interplay of these transport mechanisms. Different dual layer SEIs emerge from different electrolyte decomposition reactions. We reveal the behavior of such dual layer structures and discuss its dependence on system parameters. Model analysis enables us to interpret SEI thickness fluctuations and link them to the rate-limiting transport mechanism. Our results are general and independent of specific modeling choices, e.g., for charge transport and reduction reactions. In the near future, automotive and mobile applications demand power storage with large energy and power density. Currently, lithiumion batteries (LIBs) are the technology of choice for devices with these demands. They operate at high cell potentials and offer high specific capacities while providing long lifetimes. The latter is a consequence of the stable chemistry of modern LIB systems. A significant part of this stability can be attributed to the passivation ability of the solid electrolyte interphase (SEI). This thin layer forms between the negative electrode and the electrolyte. Hence contact between these phases is prevented and the continuous reduction of electrolyte molecules is suppressed. These reduction processes occur because the operating potential of the negative electrode lies well below the stability window of the electrolyte.1 They are suppressed because reduction products quickly form the SEI during the first charge of a pristine electrode. The self passivating ability is one of the most important distinctions between a well and a badly performing lithium-ion battery chemistry. It is of such importance because the reduction reactions consume lithium-ions, directly reducing battery capacity. However, a real SEI is not perfectly passivating and electrolyte reduction is never completely suppressed. Consequently, the lifetime of a battery is directly related to the long-term passivating ability of the SEI.Numerous studies on SEI have been conducted since Peled reported on this correlation in 1979.2 Most of these studies are experimental, investigating cycling stability as well as SEI impedance and composition. Theoretical studies are scarce in comparison, despite established methods such as DFT and DFT/MD derivatives. This can be partially explained with the chemical diversity of SEI, which has been investigated by Aurbach et al. for decades. Results are summarized in Refs. 3, 4 and include the study of SEI formation on graphite electrodes in organic solvent mixtures. The most significant finding of this time is that ethylene carbonate (EC) forms a stable SEI on graphite as opposed to propylene carbonate (PC). Another...