Understanding the diffusion of lithium ions in electrode materials for lithium ion batteries is of great importance for their knowledge-based optimization and development of novel materials and cell designs. The galvanostatic intermittent titration technique (GITT) is widely applied in battery research to study the diffusion of lithium in anode and cathode materials depending on the degree of lithiation. While transport properties of electrode materials at high and ambient temperatures are largely available, low temperature diffusion and rate coefficients are hardly reported in the literature and vary by orders of magnitude for identical active materials. Herein, we demonstrate and discuss several challenges and pitfalls in the application and evaluation of GITT measurements for determining the effective chemical lithium ion diffusion coefficient in lithium insertion electrodes, which become especially important at low temperature. This includes theoretical considerations and an experimental analysis of the promising cathode material LiNi0.5Co0.2Mn0.3O2 (NCM523) in the wide temperature range of −40 °C to 40 °C. We show how the choice of experimental conditions for the GITT measurements and of the subsequent mathematical evaluation significantly influence the derived diffusion coefficient. The results suggest that the large scattering of reported values of the diffusion coefficient could be caused by the use of different evaluation procedures. Simple calculation methods appear to be less suited the lower the temperature is. It is shown that the complementary use of GITT and EIS supplemented by detailed knowledge of the microstructure of the electrode significantly improves the accuracy of determining the diffusion coefficient.

Silicon is a promising negative electrode for secondary lithium-based batteries, but the electrochemical reversibility of particularly nanostructured silicon electrodes drastically depends on their interfacial characteristics, commonly known as the solid electrolyte interface (SEI). The beneficial origin of certain electrolyte additives or different binders is still discussed controversially owing to the challenging peculiarities of interfacial post-mortem investigations of electrodes. In this work, we address the common difficulties of SEI investigations of porous silicon/carbon nanostructures and study the addition of a fluoroethylene carbonate (FEC) as a stabilizing additive as well as the use of two different binders, carboxymethyl cellulose/styrene-butadiene rubber (CMC/SBR) and polyacrylic acid (PAA), for the SEI formation. The electrode is composed of silicon nanocrystallites below 5 nm diameter allowing a detailed investigation of interfacial characteristics of silicon owing to the high surface area. We first performed galvanostatic long-term cycling (400 times) and carried out comprehensive ex situ characterization of the cycled nanocrystalline silicon electrodes with XRD, EDXS, TEM and XPS. We modified the preparation of the electrode for post-mortem characterization to distinguish between electrolyte components and the actual SEI. The impact of the FEC additive and two different binders on the interfacial layer is studied and the occurrence of diverse compounds, in particular LiF, Li2O and phosphates, is discussed. These results help to understand general issues in SEI formation and to pave the way for the development of advanced electrolytes allowing for a long-term performance of nanostructured Si-based electrodes.

Fluoroethylene carbonate (FEC) and vinylene carbonate (VC) are the most frequently used electrolyte additives to enhance the lifetime of anode materials in Li-ion batteries, but for silicon it is still ambiguous when FEC or VC is more beneficial. Herein, a low-cost nanostructured silicon/carbon anode derived from HSiCl3 is tailored by the rational choice of the additive, to obtain an anode material outperforming current complex silicon structures. We demonstrate highly reversible areal capacities of up to 5 mAh/cm2 at 4.4 mg/cm2 mass loading, a specific capacity of 1280 mAh/gAnode, a capacity retention of 81 % after 500 deep-discharge cycles versus lithium metal and successful full-cell tests with high-voltage cathodes meeting the requirements for real application. Electrochemical impedance spectroscopy and post-mortem investigation provide new insights in tailoring the interfacial properties of silicon-based anodes for high performance anode materials based on an alloying mechanism with large volume changes. The role of fluorine in the FEC-derived interfacial layer is discussed in comparison with the VC-derived layer and possible degradation mechanisms are proposed. We believe that this study gives a valuable understanding and provides new strategies on the facile use of additives for highly reversible silicon anodes in Li-ion batteries

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carried out to improve the energy density at the cell level, such as the development of high capacity anode [8,9] and cathode [10,11] materials, improvements of the electrode design [12,13] and composition [14][15][16] as well as optimization of the cell architecture. [17,18] Moreover, researchers analyzed the rate limiting factors of electrodes for LIBs in great detail to understand the complex charge transport and transfer reaction processes and optimize materials and cell components accordingly. [19] It has been found that the rate performance can be improved by increasing solid-state diffusivity, [20] conductive additive type and content [21] or electrode porosity, [22] by the optimization of electrolyte properties, such as concentration [23] and viscosity, [24] and by decreasing the particle size of active materials [13,25] and the electrode thickness. [26][27][28] Although these measures lead to a gradual improvement of decisive performance metrics, a tradeoff between energy and power density became obvious, forbidding arbitrary combinations of high storage capacity and fast charging capability. Understanding this tradeoff and the accompanied practical limits of LIBs is critical for their further improvement and the development of "beyond Li" battery technologies.Herein, we present a simple but powerful electrochemical principle describing the tradeoff between storage capacity and rate capability of electrodes for LIBs. Such electrodes are porous composites consisting of particles of the active storage material, binder and conductive additives coated on a metallic current collector foil. The pores are filled with a liquid electrolyte containing Li-ions as charge carriers. The specific capacity (energy density equivalent) of such an electrode is given by the specific capacity of the active material and its share in the electrode. Figure 1 illustrates the sensitivity of the specific capacity to the most decisive electrode parameters using a typical LiNi 1/3 Mn 1/3 Co 1/3 O 2 (NCM111)-based cathode as the baseline (see Section S3 in the Supporting Information for details on the computations). Please note that here the specific capacity of the electrode also includes the masses of conductive additives, binder, current collector and the pore electrolyte, which is often ignored in the literature but has significant impact on the energy density (see Section S2 in the Supporting Information for details). It turns out that some electrode properties hardly affect the energy density, such as the porosity, the conductive additive and binder content, and the particle size (cf. Figure 1).The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/aenm.201902523.Li-ion batteries (LIBs) were continuously improved over the last decades, aiming at longer operating times of mobile electronic devices and mass implementation in high-energy applications, such as all-electric or hybrid-electric vehicles. [1][2][3][4] However, owing to safety concerns, range limitations, and inad...

Rechargeable batteries with high energy density, long cycle life, and low cost are considered key enablers for sustainable consumer electronics, electric vehicles (EVs), and smart grid energy storage. Lithium-ion batteries (LIBs) have been emerged

Increasing the fast charging capabilities and the driving range of electric vehicles are major goals in Li‐ion battery research to accelerate mass market adoption and reduce greenhouse gas emissions. This requires a fundamental understanding of the performance limiting factors to enable a knowledge‐based optimization of materials, electrode design and cell architectures. Herein, electrochemical fundamentals and recent insights concerning rate performance limitations of Li‐ion batteries at the electrode level are reviewed and discussed from charge and mass transport perspectives. The application of straightforward analytical and semi‐empirical models is highlighted in view of understanding specific performance limiting factors of electrodes for Li‐ion batteries based on experimental investigations. The summarized insights are discussed regarding promising improvement strategies to approach the practical limits of liquid electrolyte‐based Li‐ion batteries.

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