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Polymerization via ultraviolet irradiation of LiTFSI21mH2O + PDA liquid mixtures turned most of these liquids into gels or solids. Here, LiTFSI21mH2O denotes a 21 m aqueous solution of lithium bis(trifluoromethanesulfonyl)imide, and PDA stands for a monomer of poly(ethylene glycol) diacrylate of Mn 575. Systematic thermoconductometric measurement on these electrolytes, both before and after the polymerization, showed the gel electrolytes to be considerably more conductive than their precursor liquid mixtures, especially at lower temperatures. A parallel measurement of glass transition temperature, θ g, revealed in these gel electrolytes a unique double glass transition enveloping two sub-transitions each with its own θ g’s. These and a number of other related experimental observations can be consistently and clearly explained based on the existence of a solution and a polymer substructure in the polymerized electrolytes, and on these substructures becoming codominant in the gels. The exceptional conductivity in these gel electrolytes points to a promising direction to formulating a polymer-supported aqueous electrolyte with a set of desirable physical traits and an uncompromising conductivity.

A thermoconductometric method, with a uniquely designed sample cell and an in situ polymerization procedure, was applied to a polymer-supported aqueous electrolyte of wPDA + (1−w)LiTFSI21mH2O, where w is in mass fraction, PDA stands for a monomer of poly(ethylene glycol) diacrylate of Mn 575, and LiTFSI21mH2O denotes a 21 m aqueous solution of lithium bis(trifluoromethanesulfonyl)imide. The thermoconductometry curves of temperature differential, Δθ, and conductivity, κ, were collected for these samples in the w range of (0, 0.5) and θ range of (−70, 60) °C. The Δθ curves yielded a partial phase diagram revealing a high degree of compatibility between PDA and LiTFSI21mH2O, and the κ curves formed for the unpolymerized and polymerized electrolytes a pair of data sets which were fitted to yield a pair of κ(w, θ) functions for the definition of a conductivity loss function upon polymerization. This function showed that a significant portion of the conductivity was retained upon polymerization in a large part of the (w, θ) space, where a safe and robust electrolyte with a decent conductivity could be formulated. Molecular and thermodynamic considerations were applied to correlate and discuss the results of the Δθ and κ measurements.

Extensive efforts have been made to seek new battery chemistries based on multivalent working ions, with the aim to replace the mature lithium-ion batteries. These efforts were initially driven by the pursuit of higher capacity/energy, better safety and lower cost, and more recently have significantly intensified with the increasing concerns over the climate change, the limited resources of Co and Ni, and the anxieties over geopolitical as well as ethical risks of the corresponding supply chain. But how far are we from a practical multivalent battery? This talk rigorously examines the achievements made in MV batteries as reported in the current literature, while attempting to explore a pathway through the fog-of-war ahead of us.

Highly concentrated solutions of lithium salts in water have made sweeping strides from the time in the early 2010s where aqueous electrolytes could operate a battery within an electrochemical window no more than 1.5V wide. In this presentation, we discuss the construction of a lithium ion battery using graphite as the anode and LiCoO2 as the cathode to make a cell with a 4.2V potential. The primary electrolyte is a water:trimethylphosphate hybrid with a water mole fraction of 0.44 and LiTFSI salt at a concentration of 9 molal. This aqueous hybrid electrolyte can be formed into a gel electrolyte by directly polymerizing acrylate-based monomers and crosslinkers dissolved in the electrolyte. We demonstrate that by protecting the graphite anode using an acrylate gel with a fluoroethylene carbonate-based liquid electrolyte, the battery cell can be cycled repeatedly between 3.0V and 4.2V just like a cell using organic carbonate electrolytes. The advantage of the aqueous hybrid electrolyte is that it is non-flammable, and a cell using aqueous gel electrolytes can withstand damage and even be cut open while operating with no risk of fire or explosion. The manufacturing and performance characteristics of the aqueous 4V battery will be discussed as well as the interfacial issues that come about with the use of aqueous gel electrolytes in a 4V-capable battery system.

The development of aqueous solutions containing comparatively high amounts of dissolved salts has brought water, an unlikely solvent, into the ongoing development of new battery chemistries. Aqueous electrolytes have the advantage of being inherently non-flammable materials, while demonstrating high cation transference number behavior and cation conductivity in the range of 1-10 mS/cm2 in the case of Li+. As a reaction medium, highly concentrated aqueous electrolytes allow for polymerization reactions such as radical polymerization to occur, and it has been demonstrated that the kinetics of these reactions are fast enough to allow rapid processing into films of solid gel electrolytes. While lithium ion batteries were one of the first applications of aqueous gel electrolytes, we have synthesized gel electrolytes for the increasingly popular sodium and zinc systems as well. This talk will focus on the kinetics of polymerization of various radicalized monomers, the formation and properties of the solid gels that result, and the electrochemical behavior and application tree that follow using the new electrolyte materials.

Using a mass spectrum technique and an "SEI-marker", we successfully established a direct link between Li+-solvation sheath structure and the interphasial processes occurring at graphitic anode.

Future advancements in energy storage systems (lithium-ion batteries/LIBs and beyond) rely on the development of novel materials, innovative electrode/device architectures and scalable processing techniques or a combination thereof. In this talk, I will discuss my work on each of these three aspects including in situ characterization of planar devices as well as the development of nanomaterials and additive-free electrode architectures via alternative manufacturing processes such as dry/cold pressing and extrusion printing, respectively. Operando studies can elucidate the reaction behavior of battery electrodes during operation; however, specialized electrochemical cells are required to study these materials in their natural environment (in situ). In this study, a planar microscale battery with an open cell configuration was designed to investigate the structural evolution and concomitant solid electrolyte interphase formation of model electrode-electrolyte systems, such as Na-MoS2.1 By coupling an atomic force microscope with the planar electrochemical cell platform, real-time topographical observations of MoS2 electrodes during sodiation are readily achieved, which leads to crucial information regarding battery operation at the nanoscale. The use of advanced in situ/operando techniques with this newly developed planar battery configuration can elucidate the electrochemical behavior of numerous electrode-electrolyte systems, particularly for alkali-metal-ion batteries, during cycling. The drive towards lighter electric vehicles with longer ranges requires the development of higher energy density systems beyond conventional LIBs, such as lithium-oxygen (Li-O2) batteries. To increase battery performance for practical applications, high mass loading electrodes are advantageous as long as optimal battery reactions are maintained throughout the entirety of the “thick” electrode. Here, I present a facile, alternative electrode processing method known as dry/cold pressing, where a highly porous and compressible carbon nanomaterial (holey graphene or hG) enables the formation of mechanically robust, high mass loading electrodes for Li-O2 batteries.2-3 Dry pressing hG, the compressible matrix, with incompressible materials also enables the preparation of unique mixed and stacked/sandwich electrode architectures under binder- and solvent-free conditions.3 The enhancements in electrochemical performance demonstrate the promise of dry pressed electrode architectures and the reported additive-free processing method for advanced electrochemical energy storage systems. Additive manufacturing (AM) techniques also show promise towards the fabrication of complex 3D designs for advanced electrochemical devices. In particular, the development of inexpensive, sustainable (aqueous), and easily processable material ink systems and the use of a facile and low-cost fabrication process, such as extrusion-based 3D printing, are advantageous for scalable battery manufacturing. In this work, hierarchically porous electrode architectures are readily extruded using additive-free, aqueous graphene oxide-based (GO) ink compositions and demonstrated as the first 3D printed Li-O2 cathodes.4 The development of a nanoporous GO material enabled trimodal porosity (nano-micro-macropores) within the 3D printed mesh architecture, which provides facile mass/ionic transport pathways and enhances active-site utilization during battery operation. The results demonstrate the potential of AM techniques towards the scalable fabrication and improvement of advanced energy storage devices through structurally conscious designs as well as tailored material compositions. References: SD Lacey, J Wan, A Cresce, SM Russell, J Dai, W Bao, K Xu and L Hu, “Atomic Force Microscopy Studies on Molybdenum Disulfide Flakes as Sodium-ion Anodes,” Nano Letters, 15 (2) 1018-1024 (2015). Y Lin, B Moitoso, C Martinez-Martinez, ED Walsh, SD Lacey, JW Kim, L Dai, L Hu and JW Connell, “Ultrahigh-Capacity Lithium-Oxygen Batteries Enabled by Dry-Pressed Holey Graphene Air Cathodes,” Nano Letters, 17 (5), 3252-3260 (2017) SD Lacey, ED Walsh, E Hitz, J Dai, JW Connell, L Hu and Y Lin, “Highly compressible, binderless and ultrathick holey graphene-based electrode architectures,” Nano Energy, 31, 386-392 (2017). SD Lacey, DJ Kirsch, Y Li, JT Morgenstern, BC Zarket, Y Yao, J Dai, LQ Garcia, B Liu, T Gao, S Xu, SR Raghavan, JW Connell, Y Lin and L Hu, “Extrusion-based 3D Printing of Hierarchically Porous Advanced Battery Electrodes,” Advanced Materials, 30 (12), 1705651 (2018).

Sodium ion batteries present a meaningful opportunity to apply knowledge gained from lithium ion batteries. In order to produce functional sodium ion batteries that present real advantages compared to today’s commercial lithium ion batteries, the sodium ion and its interactions in solution must be exploited. From previous work on the subject, our work has shown fundamental differences in the solvation of Na+ as compared to Li+. Specifically, we were able to show that Na+ experiences weaker attractive forces and longer bond lengths between itself and its solvent molecules in organic and aqueous solution, which affects contact ion pairing and aggregation behavior. In this work, we will explore the relationship between ion-solvent interactions as they relate to the transference number and electrochemical stability of sodium organic and aqueous electrolytes. Ion interaction has a significant effect on the reduction potential of both solvent molecules and anions, and so there may be opportunities to take advantage of Na+ to affect both transference number and stability with the goal of producing effective electrolytes for sodium ion batteries.

Water-in-salt electrolytes (WiSE) are one of a growing family of water-based lithium ion electrolytes that use high concentrations of non-hydrolyzable lithium salts such as LiTFSI. WiSE are non-flammable and have a much wider electrochemical stability window than typical dilute aqueous electrolytes through two anion-based mechanisms: the formation of a passive anode layer through the decomposition of the TFSI- anion and exclusion of water by the anion from the cathode surface. This work details the formation of a polymer gel network containing WiSE as its liquid phase using commercially available polymer and crosslinker components. WiSE gel electrolytes are thought to be a natural evolution of the water-in-salt concept, as a WiSE gel should allow for WiSE-containing batteries to decouple themselves from typical cylindrical and prismatic battery form factors. Cycling performance and mechanical properties of the gel electrolytes will be discussed, along with fundamental investigations into the electrode-electrolyte interface and the minimization of charge-transfer resistance. Additionally, the kinetics of gel reactions in WiSE as a solvent will be discussed.

Currently used lithium ion batteries for portable electronics utilize flammable and often toxic non-aqueous electrolytes in order to achieve high energy densities. They also require a low humidity manufacturing environment resulting in an increased cost. Aqueous electrolytes have recently emerged as potential intrinsically nonflammable alternatives after their electrochemical stability window was expanded beyond 3.0 V by employing a new class of “Water-in-Salt” electrolytes. In such super-concentrated electrolyte, the decomposition of salt anion occurs preferentially on the anode before hydrogen evolution takes place, creating a kinetic protection against electrochemical decomposition via a dense solid electrolyte interphase (SEI). In this presentation, results from classical molecular dynamics (MD) simulations using a polarizable APPLE&P force field are analyzed in order to examine in detail the ion transport mechanism in bis(trifluoromethane sulfonyl)imide (LiTFSI-water) “Water-in-Salt” electrolytes (WiSE) for safe, green and low cost aqueous lithium ion batteries. They are complemented by Born Oppenheimer MD simulations of smaller systems that yield similar structural features. Simulations revealed an unusually low activation energy and fast ion transport for highly concentrated solutions even at low temperatures that is quite different from the dramatic increase of the activation energy for conductivity found in traditional battery electrolytes. A high conductivity and lithium transference number in WiSE is attributed to the formation of fast ion transporting pathways that are connected to the unexpected structure of WiSE electrolytes, which was confirmed by small angle neutron scattering experiments (SANS). The ability of MD simulations to describe dynamics of ion and solvent in WiSE electrolytes was further validated via pfg-NMR and conductivity measurements, while IR spectroscopy measurements provide a comprehensive picture of the salt electrolyte aggregation that is coupled with ion transport. The connection between the double layer structure of WiSE electrolytes and its electrochemical stability will be briefly discussed.

Rising interests in sodium ion batteries as low-cost alternatives to lithium ion batteries necessitates a closer look at the interactions between the sodium ion and its anion in electrolyte solution. As demonstrated in lithium ion batteries, solvent and anion species involved in solvation of the lithium ion contribute to the nucleation and growth of the solid electrolyte interphase of the anode. The same is true of sodium ion batteries, which parallel lithium ion batteries in their use of carbonate-based organic solvents and the hexafluorophosphate (PF6 -) anion. Infrared spectroscopy (FTIR) was used to probe the interaction between PF6 - and sodium in various carbonate-based solvent, and these measurements were interpreted using predictions made by density functional theory (DFT) calculations on the model LiPF6 and NaPF6-based solvates. In addition, molecular dynamics (MD) simulations were performed on the EC:DMC/LiPF6 and EC:DMC/NaPF6 electrolytes. A revised many-body polarizable APPLE&P force field was used in MD simulations. Solvate structures extracted from MD simulations were found in good agreement with interpretations of FTIR spectra. Specifically, initial MD results supported our previous finding indicating that the Li+ and Na+ solvation numbers from Raman measurements are found are similar in linear carbonates. Moreover, MD simulations predicted that the PF6 - anion coordination to Li+ and Na+ are quite different. Mono-dentate LiPF6 binding prevailing in LiPF6-DMC, and a combination of bi- and tri-dentate binding dominating in NaPF6-DMC in agreement with the free energies of solvates estimated from DFT cluster – continuum calculations. MD simulations also predicted solvent, anion, Li and Na self-diffusion coefficients in excellent agreement with pfg-NMR measurements.

Nuclear magnetic resonance (NMR) has been productively employed to investigate ion transport and solvation in standard carbonate-based Li ion battery electrolytes and in solid polymer electrolytes based on poly(ethylene oxide) (PEO). Future electrochemical power sources require new electrolytes to adapt to disruptive changes in the basic working chemistry, such as moving from Li ion to Na ion, or to Li metal electrodes. We highlight two recent collaborative activities on electrolytes in our group (i) Na ion; (ii) Li metal polymer. Using a combination of 23Na, 19F and natural abundance 17O NMR, and pulsed field gradient diffusion in a collaboration with the U.S. Army Research Lab, we have examined NaPF6 solutions in binary solvent mixtures ethylene carbonate (EC) and ethylmethyl carbonate (EMC) for possible cation solvation preference as a function of EC/EMC ratio. Spectroscopic evidence demonstrate that the Li+ and Na+ cations share a number of similar ion–solvent and ion-ion interaction trends, such as a preference for a solvation shell rich in cyclic carbonates over linear carbonates and fluorinated carbonates. However some differences are noted, in particular a somewhat weaker ion-solvent interaction for Na+. Ionic Materials, Inc. has invented a novel polymer with extremely high ionic conductivity over a range of temperatures, even surpassing that of a commercial porous separator containing the standard liquid carbonate-based electrolyte at room temperature. This solid polymer can be reliably extruded into very thin films, is non-flammable, has attractive mechanical properties for lithium dendrite suppression, is electrochemically stable against Li, and is compatible with a variety of different anodes and cathodes. The polymer electrolyte is based on an inexpensive semicrystalline commercially available polymer such as polyether ether ketone or polyphenylene sulfide and Li salts familiar to the battery and polymer electrolyte communities. The ionic transport mechanism is unlike that which characterizes the PEO salt complexes – that is ion mobility is completely decoupled from polymer host motion, as verified by differential scanning calorimetry and NMR. In fact NMR pulsed gradient measurements reveal Li self-diffusion coefficients at room temperature that are an order of magnitude higher than in the ceramic ion conductors of the Li10GeP2S12 class and thus the highest in any known solid. Furthermore, the Li+ transference number determined electrochemically and by NMR is > 0.6.

Harnessing the improvements in energy and power offered by high voltage (~5V vs. Li) lithium-ion electrode chemistries relies on development of more robust electrolytes with expanded stability windows. While some success has been demonstrated with incorporation of new solvents/co-solvents with enhanced intrinsic stability, adopting changes of this magnitude to established materials/methods at the industry level is a costly and cumbersome solution. One of the most appealing and affordable alternative approaches utilizes low fraction additives designed to favorably modify interface chemistry in order to stabilize cycling performance and mitigate parasitic reactions. In this work, we synthesize a new additive chemistry and demonstrate its utility in improving cycling performance and limiting parasitic currents, even at high operating voltages (up to 5.5V vs. Li), as compared to baseline electrolytes in Li/LiNi1.5Mn0.5O4 half cells. Quantum chemical modeling and interphase characterization provide some insight with regard to the mechanism for enhanced stability.

As batteries evolve toward higher cell voltage and capacity, developing detailed knowledge of the electrolyte-electrode interface is critically important. The chemical composition and morphology of interphases dictate not only the reversibility (cycle or calendar life), but also the kinetics of the cell chemistry (power density), and it has been well-recognized that the complicated interplay among electrolyte components (solvent, salt anion, additive) and electrode surfaces affect interphasial properties. Our efforts focus on designing new electrolytes to increase the electrochemical stability window. One way to reach that goal is to modify the SEI through electrolyte composition. To advance these efforts and understand the underlying chemistry at electrolyte-electrode interfaces, we perform in operando EC-AFM and in situ confocal Raman experiments, often supported by XPS and DFT studies. With these techniques we investigate the dependence on electrolyte composition on SEI formation, composition, and morphology on graphitic anodes and the elusive interphase on graphitic and thin film LNMO cathodes. The results of such investigations aid in the design of electrolytes that advance lithium ion battery technology and expand beyond traditional lithium ion chemistry to emerging energy storage types such as sodium ion and aqueous systems, which interfaces we have begun to study. This presentation will provide an overview of electrolyte-electrode interactions we have studied and their influence on interphases.

High capacity sodium ion batteries, with the capacity to rival mid-range lithium ion batteries, are developing a solid presence in the field of energy storage research. Sodium ion batteries, at a material level, offer a unique opportunity to apply lessons learned in the study of lithium ion battery behavior. Most important of these is the formation of the solid electrolyte interphase layer at the anode, which shields the electrolyte from continuous decomposition. In lithium ion batteries, the formation of the solid electrolyte interphase is highly dependent (though not exclusively) on the solvation condition of the Li+ ion as it interacts with the surface of the anode. Previous work has shown that the solution behavior of the Na+ ion closely mirrors its alkali metal cousin Li+ in a variety of competitive binary solvent systems, such as ethylene carbonate and dimethyl carbonate. We hypothesize that similar rules of interphase formation will be observed in a Na-ion battery system, and the interphase will serve a similar function. This work will describe the connection between the solvation of Na+ in binary solvent electrolytes, and how this affects the structure and chemistry of the interface at the anode. We will focus on hard carbon, which has sufficient Na+ capacity to support a variety of developing cathode chemistries. Figure 1

Nuclear magnetic resonance (NMR) has been productively employed to investigate ion transport and solvation in standard carbonate-based Li ion battery electrolytes. Future electrochemical power sources require new electrolytes to adapt to disruptive changes in the basic working chemistry, such as moving from Li to Na. This work extends the application of some of the NMR tools developed by our lab and others to study the standard electrolytes to the new electrolytes. Using a combination of 23Na, 19F and natural abundance 17O NMR, and pulsed field gradient diffusion, we have examined NaPF6 solutions in binary solvent mixtures ethylene carbonate (EC) and ethylmethyl carbonate (EMC) for possible cation solvation preference as a function of EC/EMC ratio. Using similar methods, we also investigate the transport properties of newly discovered highly conductive ultra-concentrated aqueous solutions of LiTFSI.

As the essential component in any energy storage device, electrolyte determines a number of key performances of the device, including temperature range for service, rate of electrochemical reaction, safety under both normal and abusive operations, cycle/calendar life as well as the maximum voltage and capacity allowed. Among these, the electrochemical stability window is perhaps the most intriguing property, because in almost all electrochemical devices, the electrodes operate at potentials far beyond what thermodynamics allowed. This group at ARL has been trying to understand this basic phenomenon and exploring various means to affect the surface chemistry at electrode/electrolyte junctions, so that this stability window could be expanded. This talk will summarize these recent efforts.

As lithium-ion batteries evolve toward higher cell voltage and capacity, developing detailed knowledge of the electrolyte-electrode interface is critically important. The chemical composition and morphology of interphases dictate not only the reversibility (cycle or calendar life), but also the kinetics of the cell chemistry (power density), and it has been well-recognized that the complicated interplay among electrolyte components (solvent, salt anion, additive) and electrode surfaces affect interphasial properties. While there is general agreement on solid electrolyte interphase (SEI) formation on graphitic anodes, clear evidence of passivation at the cathode interface remains elusive. Primarily using in-situ electrochemical atomic force microscopy (EC-AFM) and confocal Raman spectroscopy, our studies focus on the in situ and in operando electrolyte-cathode interactions under various chemical and electrochemical environments with the goal of identifying factors that influence phenomena at the interface.

Lithium-ion battery (LIB) is one of the most promising power sources for electronic devices and electric vehicles because of their high energy densities and relatively long lifetime. Graphite is widely used as the anode electrode material in the state-of-the-art LIBs. However, a graphite anode is usually only compatible with an ethylene carbonate (EC)-based electrolyte. In a propylene carbonate (PC)-containing electrolyte, which is advantageous over the EC-based counterpart due to its wider temperature range and higher low-temperature conductivity, the graphite anode experiences significant exfoliation problem during the initial lithium intercalation step. Thus a PC-containing electrolyte cannot be used in LIBs with graphite as the anode unless some solid electrolyte interphase (SEI) film-formation additives are included in the electrolytes. Usually these additives are electrochemically reduced predominantly on the graphite anode surface before PC is intercalated and reduced. Such additives normally contain unsaturated bonds or are unstable cyclic compounds, for instance, vinylene carbonate (VC) and fluoroethylene carbonate (FEC). They play an important role in the protection of the graphitic anode structure from destruction by PC reduction. However, these additives also have certain shortcomings. Usually these additives result in a thick SEI layer, which significantly reduces the rate capability and limits low-temperature performance and cycling stability at elevated temperatures through additional impedance and poor thermal stability. Recently, we developed a novel electrolyte additive that can suppress the PC reductive decomposition on the graphite anode without forming a thick and resistant SEI film. As shown in Figure 1, this additive can not only significantly improve the compatibility between graphite anode and PC-containing electrolyte, but also exhibit superior rate capability and excellent cycling stability at elevated temperatures.This new electrolyte additive can be applied to LIBs and other electrochemical systems using PC-containing electrolytes and graphite anodes. In this work, we systematically investigated the mechanism of the SEI formation by this additive and the protection of graphite anode in PC-containing electrolytes. Several techniques, including computational calculations and simulations, 17O nuclear magnetic resonance (NMR), “soft” electrospray ionization mass spectrometry (ESI-MS), time of flight second ion mass spectrometry (ToF-SIMS), etc. have been used to reveal the reasons behind the compatibility and stability of graphite anode in PC-containing electrolytes with this new additive. The details of the results will be reported at the meeting. Acknowledgements This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technology of the U.S. Department of Energy (DOE). The 17O NMR and ToF-SIMS measurements were performed at the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research and located at PNNL. Figure 1

Ion solvation in electrolytes is a key issue in the understanding of interfacial behavior in lithium ion batteries. It has been shown experimentally that the solvation of Li+ in binary electrolyte solutions directly influences the formation of the solid electrolyte interphase (SEI) layer at the surface of graphitic anodes, and likely other types of anodes as well. Anions in lithium salt systems, especially the important anion PF6 -, exhibit specific solvent preferences as well, though PF6 - and comparable anions (BF4 -, TFSI-, FSI-, ClO4 -) do not form strong or long-lasting interactions with solvent molecules. This talk will explore new evidence of anion-solvent interactions, and will summarize efforts to link anion solvation to cathode SEI formation. Passivation of cathode interfaces with SEI is an important topic, as cathodes are increasingly reaching higher oxidation potentials that cause degradation of electrolytes. New, higher-voltage lithium ion batteries will be limited by development and understanding of oxidation behavior at the cathode-electrolyte interface.

Ever since the birth of Li ion chemistry, carbonate esters have been the main solvents used as the skeleton composition of the electrolytes. This class of polar organic compounds, ethylene carbonate in particular, brings both their merits of supporting reversible intercalation of Li+ at electrodes of potential differences as wide as 4.2 V, and their intrinsic limits of narrow temperature range, chemical reactivity toward conducting salts, and inflammability. Numerous efforts were made during the past decades in an attempt to replace these carbonate, either partially or entirely, but little change has occurred as the result. The emergence of new chemistries such as the high voltage (>4.5 V) cathode intercalation materials and the beyond-Li-ion concepts −− most of which resort to conversion-reaction concepts to maximize the specific capacities −− presents unprecedented challenges to these carbonate esters that have been widely used by Li-ion industry, and activities resumed in seeking new alternatives. This presentation will summarize our recent efforts of designing, synthesizing and characterizing such new electrolyte solvents in both high voltage and Li-ion as well as beyond-Li-ion chemistries.

The chemical composition and morphology of interphases dictate not only the reversibility (cycle or calendar life), but also the kinetics of the cell chemistry (power density), and it has been well-recognized that the complicated interplay among electrolyte components (solvent, salt anion, additive) and electrode surfaces affect interphasial properties. While there is general agreement on the formation of the interphase on graphitic anodes from electrolyte component decomposition, the roles of anions and additives is not well characterized and the existence and nature of an analogous interphase on cathode surfaces remains debatable. This presentation summarizes our efforts in investigating the live-formation of interphases in Li ion batteries on both cathodes and anodes under various chemical and electrochemical environments, and the identified key factors that affect the key interphasial properties.

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In our previously presented work, we were able to show the effectiveness of a new phosphate ester additive tris(hexafluoro-isopropyl)phosphate (HFiP) in increasing the survivability of a conventional carbonate-based electrolyte against two “high-voltage” cathodes, LiNi0.5Mn1.5O4 and LiCoPO4. In this presentation, we would like to explore the effect of the additive structure in an effort to understand the mechanism by which they stabilize the cathode-electrolyte interface region. It was found that the degree of fluorination of the alkyl ester subgroups plays a controlling role in the additive effectiveness of the compound. Mass spectrometry indicates that HFiP and its family, upon ionization, tend to oligomerize, leading to the supposition that they act as network promoters when in the highly redox-active interface regions of a Liion battery. The behavior of these additives on anode is also of interest, as experiments pitting a propylene carbonate electrolyte against a graphitic anode showed no exfoliation in the presence of 1 wt% HFiP in support of a network-promoting hypothesis. This presentation will summarize results from phosphate ester additives of varying degrees of fluorination and will highlight their performance at a variety of cycling temperatures.

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An electrolyte additive based on highly fluorinated phosphate ester structure was identified as being able to stabilize carbonate-based electrolytes on 5 V class cathode surfaces. The synthesis and structural analysis of the additive are described, and preliminary yet encouraging results from electrochemical characterization showed that this additive participates in forming a protective interphasial chemistry not only on transition metal oxide cathode at high voltage (∼5 V vs Li) but also on graphitic graphite at low voltage (∼0 V vs Li), making it possible to formulate an electrolyte supporting reversible Li + -intercalation chemistry in the coveted 5 V region.