Lithium electrode/electrolyte interfaces

Beyond any doubt, the electrode/electrolyte interfaces constitute the foundations for the state-of-the-art lithium ion chemistry and naturally have become the most active research topic during the past decade. However, the characterization of the key attributes of the corresponding surface chemistries proved rather difficult, and significant controversy has been generated. The elusive nature of these interfaces is believed to arise from the sensitivity of the major chemical compounds that originated from the decomposition of electrolyte components. 

This corrosion of the SEI by linear carbonate solvents would undoubtedly produce adverse effects on the performance of lithium ion cells. During longterm cycling, the damaged SEI has to be repaired constantly by the same electrochemical reactions that occurred in the initial formation process, which consumes the limited lithium ion source in the cell and increases the impedance at the electrode/ electrolyte interface. 

In addition to the criticisms from Anderman, a further challenge to the application of SPEs comes from their interfacial contact with the electrode materials, which presents a far more severe problem to the ion transport than the bulk ion conduction does. In liquid electrolytes, the electrodes are well wetted and soaked, so that the electrode/electrolyte interface is well extended into the porosity structure of the electrode hence, the ion path is little affected by the tortuosity of the electrode materials. However, the solid nature of the polymer would make it impossible to fill these voids with SPEs that would have been accessible to the liquid electrolytes, even if the polymer film is cast on the electrode surface from a solution. Hence, the actual area of the interface could be close to the geometric area of the electrode, that is, only a fraction of the actual surface area. The high interfacial impedance frequently encountered in the electrochemical characterization of SPEs should originate at least partially from this reduced surface contact between electrode and electrolyte. Since the porous structure is present in both electrodes in a lithium ion cell, the effect of interfacial impedances associated with SPEs would become more pronounced as compared with the case of lithium cells in which only the cathode material is porous. 

If the electrode material is assumed to be homogeneous, then the concentration gradient of lithium through the electrode is the only factor that drives lithium transport. Hence, lithium will enter/leave the planar electrode only at the electrode/ electrolyte interface, and cannot penetrate into the back of the electrode. Under such an impermeable (impenetrable) constraint, the electric current (I) can be expressed by Equation (5.18) during the initial stage of diffusion, and by Equation (5.19) during the later stage [45] ... 

Serious efforts have been made to explain the atypical lithium transport behavior using modified diffusion control models. In these models the boundary conditions -that is, "real potentiostatic constraint at the electrode/electrolyte interface and impermeable constraint at the back of the electrode - remain valid, while lithium transport is strongly influenced by, for example (i) the geometry of the electrode surface [53-55] (ii) growth of a new phase in the electrode [56-63] and (iii) the electric field through the electrode [48, 56]. 

The above argument, along with the evidences presented in Sections 5.3.2.1-5.3.2.2, indicates that other transport mechanisms than diffusion-controlled lithium transport may dominate during the CT experiments. Furthermore, the Ohmic relationship between Jiiu and A indicates that internal cell resistance plays a critical role in lithium intercalation/deintercalation. If this is the case, it is reasonable to suggest that the interfacial flux of lithium ion is determined by the difference between the applied potential E pp and the actual instantaneous electrode potential (t), divided by the internal cell resistance Keen- Consequently, lithium ions barely undergo any real potentiostatic constraint at the electrode/electrolyte interface. This condition is designated as cell-impedance-controlled lithium transport. 

The contribution of electric field to lithium transport has been considered by a few authors. Pyun et argued on the basis of the Armand s model for the intercalation electrode that lithium deintercalation from the LiCo02 composite electrode was retarded by the electric field due to the formation of an electron-depleted space charge layer beneath the electrode/electrolyte interface. Nichina et al. estimated the chemical diffusivity of lithium in the LiCo02 film electrode from the current-time relation derived from the Nernst-Planck equation for combined lithium migration and diffusion within the electrode. 

The physical model presented in Sections IV. 1 and IV.2 tells us explicitly that the flux of lithium at the electrode/electrolyte interface is mainly limited by the internal cell resistance throughout the entire lithium intercalation/deintercalation. 

So far as lithium intercalation/deintercalation into/from transition metal oxides and graphite proceeds under the cell-impedance controlled constraint Eq. (8), it is unlikely that the disturbance of lithium diffusion inside the electrode due to the presence of the phase boundary and the phase boundary movement causes any significant change in the CTs. It is likely predicted from Eq. (8) that unlike the case of the diffusion controlled phase transformation, the flux of lithium at the electrode/electrolyte interface under the cell-impedance controlled constraint is hardly dependent on the location of the phase boundary within the electrode. 

The calculation procedure of CTs is as follows at r = 0, the values of lithium content (1 - over the electrode (or the electrode potential E) and of internal cell resistance Ran are first initialized to be those values at initial electrode potential And then when the infinitesimal time At has elapsed, i.e., just after the potential step from / , to the final electrode potential Eji = Egpp is applied, the flux at the electrode/electrolyte interface r = R (or current I) is calculated by / = E - EgppyRceii of Eq. (8). After that, the lithium concentration at the electrode surface is evaluated. Next, E, Ran and (1 - < inside the electrode are calculated and re-evaluated after At. The above procedure is repeated until the desired time elapses. Finally, the theoretical CTs of I vs. t plot are obtained. 

The above results of Figures 20 and 21 indicate clearly that both the instantaneous electrode potential, E, and the internal cell resistance Rceii play major roles in the flux of lithium at the electrode/electrolyte interface in the cell-impedance controlled lithium transport model, characterized by the B.C. of Eq. (8). 

Hirayama M., Sonoyama N., Abe T., Minoura M., Ito M., Mori D., Yamada A., Kanno R., Terashima X, Takano M., Tamura K., Mizuki J. Characterization of electrode/electrolyte interface for lithium batteries using in situ synchrotron X-ray reflectometry - A new experimental technique for LiCo02 model electrode, J. Power Sources 2007,168,493-500. 

Dupre N., Cuisinier M., Guyomard D. Electrode/Electrolyte Interface Studies in Lithium... 

Leung, K., Electronic Structure Modeling of Electrocheinical Reactions at Electrode/ Electrolyte Interfaces in Lithium Ion Batteries. J. Phys. Chem. C 2013,117, 1539-1547. 

To be able to understand how computational approaches can and should be used for electrochemical prediction we first of all need to have a correct description of the precise aims. We start from the very basic lithium-ion cell operation that ideally involves two well-defined and reversible reduction and oxidation redox) reactions - one at each electrode/electrolyte interface - coordinated with the outer transport of electrons and internal transport of lithium ions between the positive and negative electrodes. However, in practice many other chemical and physical phenomena take place simultaneously, such as anion diffusion in the electrolyte and additional redox processes at the interfaces due to reduction and/or oxidation of electrolyte components (Fig. 9.1). Control of these additional phenomena is crucial to ensure safe and stable ceU operation and to optimize the overall cell performance. In general, computations can thus be used (1) to predict wanted redox reactions, for example the reduction potential E ) of a film-forming additive intended for a protective solid electrolyte interface (SEI) and (2) to predict unwanted redox reactions, for example the oxidation potential (Eox) limit of electrolyte solvents or anions. As outlined above, the additional redox reactions involve components of the electrolyte, which thus is a prime aim of the modelling. The working agenda of different electrolyte materials in the cell -and often the unwanted reactions - are addressed to be able to mitigate the limitations posed in a rational way. 

R332 N. Dupre, M. Cuisinier and D. Guyomart, Electrode/Electrolyte Interface Studies in Lithium Batteries Using Nuclear Magnetic Resonance , Electro-chem. Soc. Interface, 2011, 20, 61. 

Polymer electrolytes (e.g., poly(ethylene oxide), poly(propylene oxide)) have attracted considerable attention for batteries in recent years. These polymers form complexes with a variety of alkali metal salts to produce ionic conductors that serve as solid electrolytes. Its use in batteries is still limited due to poor electrode/ electrolyte interface and poor room temperature ionic conductivity. Due to its rigid structure it can also serve as the separator. Polymer electrolytes are discussed briefly in the section Separators for Lithium-Ion Batteries. 

The primary molecular modeling methods that have been extensively applied to lithium battery electrolytes and electrode/electrolyte interfaces are molecular orbital calculations and molecular dynamics simulations. The former involves ab initio and density functional methods and will be referred to quanmm chemistry or QC... 

Here, we will briefly summarize lithium-ion conducting solid electrolytes as a key component of all-solid-state batteries. Next, we will discuss about approaches for improving Li-ion conduction across an electrode-electrolyte interface as well as in an active material. 

To boost up the power density of the solid-state batteries, in the following subsections we will focus on the detailed lithium-ion conduction at the electrode-electrolyte interface as well as in the active material. 

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