Simon F. Lux

Future generations of cathode materials: an automotive industry perspective

Future generations of electrified vehicles require driving ranges of at least 300 miles to successfully penetrate the mass consumer market. A significant improvement in the energy density of lithium batteries is mandatory, maintaining at the same time similar, or improved, rate capability, lifetime, cost, and safety. Several new cathode materials have been claimed over the last decade to allow for this energy improvement. The possibility that some of them will find application in the future automotive batteries is critically evaluated here by first considering their theoretical and experimentally demonstrated energy densities at the material level. For selected candidates, the energy density at the automotive battery cell level for electric vehicle applications is calculated using an in-house developed software. For the selected cathodes, literature results concerning their power capability and lifetime are also discussed with reference to the automotive targets.

Electrode–Electrolyte Interface in Li-Ion Batteries: Current Understanding and New Insights

Understanding reactions at the electrode/electrolyte interface (EEI) is essential to developing strategies to enhance cycle life and safety of lithium batteries. Despite research in the past four decades, there is still limited understanding by what means different components are formed at the EEI and how they influence EEI layer properties. We review findings used to establish the well-known mosaic structure model for the EEI (often referred to as solid electrolyte interphase or SEI) on negative electrodes including lithium, graphite, tin, and silicon. Much less understanding exists for EEI layers for positive electrodes. High-capacity Li-rich layered oxides yLi2-xMnO3·(1-y)Li1-xMO2, which can generate highly reactive species toward the electrolyte via oxygen anion redox, highlight the critical need to understand reactions with the electrolyte and EEI layers for advanced positive electrodes. Recent advances in in situ characterization of well-defined electrode surfaces can provide mechanistic insights and strategies to tailor EEI layer composition and properties.

Low Cost, Environmentally Benign Binders for Lithium-Ion Batteries

The stringent environmental requirements regarding the mobility energy usage are forcing most automakers to develop hybrid electric vehicles, which allows for a more efficient and thus less polluting use of fossil combustibles. A vast deployment of such vehicles involves producing and recycling of batteries on the thousand tons per year scale. Present Li-ion technologies involve the use of fluorinated binders, which are costly, and the use of environmentally unfriendly volatile organic compounds for the processing, which are difficult to recycle. In this paper, it is shown that the fluorinated binders can be replaced with greener and cost-effective polymers derived from cellulose. .

Dual-ion Cells Based on Anion Intercalation into Graphite from Ionic Liquid-Based Electrolytes

Abstract Electrochemical energy storage systems using graphite as both the negative and the positive electrode have been proposed as “dual-graphite cells”. In this kind of electrochemical system, the electrolyte cations intercalate into the negative electrode and the electrolyte anions intercalate into the positive electrode, both based on graphite, during the charging process. On discharge, cations and anions are released back into the electrolyte. So far, the systems proposed in literature are primarily based on Li+ and PF6- intercalation/de-intercalation into/from graphite from non-aqueous organic solvent based electrolytes. As the positive electrode potential during charging always exceeds 4.2 V vs. Li/Li+, the organic electrolyte starts to decompose at these highly oxidizing conditions resulting in insufficient discharge/charge efficiencies. The replacement of organic solvent by ionic liquids (ILs) leads an increased stability of the electrolyte towards oxidation and thus to remarkably higher efficiencies as well as an increased cycling stability. In fact, ionic liquids provide extended anodic electrochemical stability and in addition, no solvent co-intercalation occurs in parallel to anion intercalation at high potentials. Here, we present highly promising results for “dual-ion cells” based on a graphite cathode and an ionic liquid based electrolyte, namely N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr14TFSI). As the compatibility of this IL with graphite anodes is poor, alternative anodes such as metallic lithium or lithium titanate (Li4Ti5O12, LTO) are used. Consequently, the “dual-graphite” cell is renamed to “dual-ion” cell. In addition, the calculation of the specific energy of these systems will be in the focus of the discussion.

Mechanism of interactions between CMC binder and Si single crystal facets.

Interactions of the active material particles with the binder are crucial in tailoring the properties of composite electrodes used in lithium-ion batteries. The dependency of the protonation degree of the carboxyl group in the carboxymethyl cellulose (CMC) structure on the pH value of the preparation solution was investigated by Fourier transform infrared spectroscopy (FTIR). Three different distinctive chemical states of CMC binder were chosen (protonated, deprotonated, and half-half), and their interactions with different silicon single crystal facets were investigated. The different Si surface orientations display distinct differences of strength of interactions with the CMC binder. The CMC/Si adhesion forces in solution and Si wettability of the silicon are also strongly dependent on the protonation degree of the CMC. This work provides an insight into the nature of these interactions, which determine the electrochemical performance of silicon composite electrodes.

Kinetic Study of Parasitic Reactions in Lithium-Ion Batteries: A Case Study on LiNi0.6Mn0.2Co0.2O2

The side reactions between the electrode materials and the nonaqueous electrolytes have been the major contributor to the degradation of electrochemical performance of lithium-ion batteries. A home-built high-precision leakage current measuring system was deployed to investigate the reaction kinetics between the delithiated LiNi(0.6)Mn(0.2)Co(0.2)O2 and a conventional nonaqueous electrolyte. It was found that the rate of parasitic reaction had strong dependence on the upper cutoff potential of the cathode material. The kinetic data also indicated a change of reaction mode at about 4.5 V vs Li(+)/Li.

Mixtures of Ionic Liquid in Combination with Graphite Electrodes: The Role of Electrolyte Additives and Li-salt

Presently, commercially available lithium-ion batteries use graphite based anodes in combination with organic carbonate (e.g. Propylene Carbonate, PC, Ethylene Carbonate, EC) electrolytes. In this kind of Li-ion batteries the solid electrolyte intephase (SEI) formation process on the surface of graphite is crucial since it strongly influence the performances of the batteries systems [1]. It is known that the use of electrolyte additives (e.g. containing vinylene groups) improves the design of the resulting SEI on the graphite and leads to more efficient cycling of the material [2]. For that, several types of additives have been already studied and tested and intense research is now focused on the optimization of their design for a more effective film-forming efficiency. Ionic Liquids (ILs), room temperature molten salts typically showing a very low vapor pressure, high thermal stability, wide electrochemical windows and good conductivity at room and sub-room temperatures [3-4]. These properties make them very attractive candidates for the use as electrolytes in electrochemical devices such as batteries, particularly to increase the safety and the operative temperature range. So far, different types of ILs have been already used in combination with graphite electrodes with promising results [5-7]. However, only few reports studied the SEI formation process on graphite electrodes when ILs are used in the electrolytes as well as the contribution of additives to the SEI formation in such electrlytes [8]. Recently, we investigated the role of the additive Vinylene Carbonate (VC) in ILs-based electrolyte. The results of our studies indicated that when ultrapure ILs are used as electrolytes in combination with graphite electrodes, the need of additives in the electrolyte solution is strictly related to the film-forming ability of the ILs [9] themselves. For instance, in electrolyte solution based on the ultrapure ionic liquid N-butyl-N-methylpyrrolidinium bis(trifluoromethansulfonyl)imide (PYR14TFSI) the use of VC appears to be indispensable because such IL does not display film-forming ability. To the contrary, in electrolyte solution based on the ultrapure N-methyl-Npropylpyrrolidinium bis(fluorosulfonyl)imide (PYR13FSI) the presence of VC was not strictly required because this IL displays film-forming ability. In order to investigate the film-forming ability of PYR13FSI and the possibility of using this IL as additive or co-solvent in pure IL-based solutions, we prepare different mixture of PYR13FSI -PYR14TFSI with and without VC. These solutions have been used in combination with the graphite electrode and their influence on the specific capacity, the cycling efficiency and the cycling stability of the electrodes have been investigated. As example, Fig. 1 shows the cyclic voltammetry at 50 μV sec of graphite electrode in 0.3 M LiTFSI + PYR14TFSI + 5%wt. VC (A); 0.3 M LiTFSI + PYR14TFSI [50%] PYR13FSI [50%] + 5%wt. VC (B) and 0.3 M LiTFSI + PYR13FSI + 5%wt. VC (C). Also the influence of two different Lithium salt (Lithium bis(trifluoromethansulfonyl)imide, LiTFSI and Lithium exafluophosphate, LiPF6) on the performance of graphite electrode in the mixtures of PYR13FSI PYR14TFSI has been investigated. These studies clearly evidence that the Li-salt strongly influence the performance of graphite electrode in combination with ILs-based solutions.

Chemical Reactivity Descriptor for the Oxide-Electrolyte Interface in Li-Ion Batteries

Understanding electrochemical and chemical reactions at the electrode-electrolyte interface is of fundamental importance for the safety and cycle life of Li-ion batteries. Positive electrode materials such as layered transition metal oxides exhibit different degrees of chemical reactivity with commonly used carbonate-based electrolytes. Here we employed density functional theory methods to compare the energetics of four different chemical reactions between ethylene carbonate (EC) and layered (LixMO2) and rocksalt (MO) oxide surfaces. EC dissociation on layered oxides was found energetically more favorable than nucleophilic attack, electrophilic attack, and EC dissociation with oxygen extraction from the oxide surface. In addition, EC dissociation became energetically more favorable on the oxide surfaces with transition metal ions from left to right on the periodic table or by increasing transition metal valence in the oxides, where higher degree of EC dissociation was found as the Fermi level was lowered into the oxide O 2p band.

Enhanced lithium ion transport in garnet-type solid state electrolytes

Al-substituted Li7La3Zr2O12 samples processed under argon show enhanced Li-ion transport and interfacial properties in symmetrical cells with lithium electrodes, compared to those prepared in air. In particular, the samples prepared under argon have higher ionic conductivities and lower interfacial impedances in symmetrical lithium cells, and show better DC cycling characteristics. The electronic conductivities are also somewhat higher. Pellets subjected to thermal treatment under the two types of atmospheres have different colors but exhibit similar microstructures. X-ray diffraction experiments suggest that there are slight structural differences between the two types of samples, but few dissimilarities were observed in elemental composition, distribution of ions, oxidation states, or bond lengths using laser-induced breakdown spectroscopy (LIBS), x-ray photoelectron spectroscopy (XPS), and extended x-ray absorption fine structure spectroscopy (EXAFS) to analyze the materials. Additionally, there was no evidence that La or Zr were reduced during the processing under Ar. Possible explanations for the improved electrochemical properties of the sample prepared under Ar compared to the one prepared in air include differences in grain boundary chemistries and conductivities and/or a small concentration of oxygen vacancies in the former.

Molecular Spring Enabled High-Performance Anode for Lithium Ion Batteries

Flexible butyl interconnection segments are synthetically incorporated into an electronically conductive poly(pyrene methacrylate) homopolymer and its copolymer. The insertion of butyl segment makes the pyrene polymer more flexible, and can better accommodate deformation. This new class of flexible and conductive polymers can be used as a polymer binder and adhesive to facilitate the electrochemical performance of a silicon/graphene composite anode material for lithium ion battery application. They act like a “spring” to maintain the electrode mechanical and electrical integrity. High mass loading and high areal capacity, which are critical design requirements of high energy batteries, have been achieved in the electrodes composed of the novel binders and silicon/graphene composite material. A remarkable area capacity of over 5 mAh/cm2 and volumetric capacity of over 1700 Ah/L have been reached at a high current rate of 333 mA/g.