RESEARCH HIGHLIGHTS From the Resnick Sustainability Institute Graduate Research Fellows at the California Institute of Technology Computationally-guided Design of Polymer Electrolytes
Global Significance While progress of sustainable energyharvesting techniques is promising, tandem advancements in energy storage are required to maintain a stable energy supply. Rechargeable lithium-ion batteries have potential applications for load-leveling in largescale energy generation and as power sources in transportation. However, a critical problem with existing lithium-ion batteries is that the electrolyte solvent is typically a flammable mixture of alkyl carbonates. This hazard virtually prohibits high energy content applications. The design of safer, improved battery technology would be a valuable contribution to the emerging sustainable energy landscape. image per: http://spectrum.ieee.org/ This project applies polymer physics, statistical mechanics, theory and simulation, data mining, materials science, and electrochemistry toward the end goal of improving rechargeable lithium-ion batteries via rational molecular design of enhanced and safer polymer electrolyte systems. A critical problem with existing lithium-ion batteries is that the electrolyte solvent is typically a flammable mixture of alkyl carbonates.
Project Summary Rigid, solid polymeric electrolytes (SPEs) are attractive alternative electrolyte solvents because they are electrochemically stable and non-flammable. The design of better SPEs is complicated by the fact that the molecular mechanisms of ion transport are poorly understood and the design space is large. This project aims to devise new and utilize existing theoretical methods and simulation strategies for the rational molecular design of new polymer electrolyte systems for improved secondary lithium-ion batteries. Potential Impact By answering unresolved questions related to ion transport in polymer systems, the project aims to eliminate ambiguity and guesswork associated with present field-trial design approaches, saving human, monetary, and material resources. If successful, the resulting battery technology should be safe and effective enough for high energy content applications in the kwh to MWh range. With the aid of high-performance computing, our goal is to screen and propose new candidate structures on the basis of insight gleaned from detailed, atomistic simulations and our new transport model. We are leveraging comparative, mechanistic studies in addition to developing a new, general framework for ion transport that is targeted to be applicable to all polymer electrolyte systems. As a part of a collaboration with experimental synthesis and characterization groups, our theoretical efforts are enhanced and refined by experimental feedback and vice versa. SPEs typically suffer from either low ionic conductivity or poor mechanical strength.
The Science Previous theoretical work on polymer electrolytes has demonstrated a number of relevant processes for ion diffusion including bulk polymer chain transport, intra-chain ion migration, inter-chain ionic hopping, ionic cluster-hopping, etc. Most of this work, however, has focused on the study of poly(ethylene oxide), and it is not clear to what extent the mechanisms are transferable to other classes of polymers. Alternatively, we are leveraging comparative studies to identify common trends and essential differences between polymer classes and their performance as lithium ion conductors. Moreover, we are identifying relatively cheap theoretical tools to rapidly quantify key material properties that are essential to ion transport and connecting these to a general transport model. A number of ion transport mechanisms are postulated to exist in polymer systems. A wide array of coordination structures were observed across the different polymers.
Key Results We have simulated dynamics of neat polymer and ion-containing polymer systems in the dilute ion limit on 100 ns timescale. Our dataset comprises nine polymers: six polyesters and three polyethers, including PEO. The polyesters have also been synthesized and characterized by our collaborators so that we can determine strengths and weaknesses of our predictions/models. We have utilized the neat polymer simulations to quantify properties such as the glass transition temperature, the bulk modulus, as well as several relevant features for our transport model (such as the density and distribution of possible solvation sites in each polymer). In the latter set of simulations, we have made predictions regarding the relative diffusion rates of Li +. More importantly, we have uncovered a rich array of solvation structures and connected changes in coordination to available transport pathways. This not only promises to be an interesting design principle, but it also provides the foundation for our theoretical transport model, which is under development. (A) Structures and labels for polymers in data set (B) Mean-squared displacement data for Li + diffusion at 400 K. PEO clearly outcompetes the other polymers in the dataset, consistent with historical precedent and experiment. (C) Correlation between simulated glass transition temperatures and those obtained experimentally using differential scanning calorimetry (Coates group). A comparison of the density of possible solvation sites in PEO and PTMO. We have a number of ways to identify possible solvation sites based on geometric analysis, clustering algorithms, and even short molecular dynamics trajectories.
Future Steps We are actively developing a transport model that considers the ion as moving on a dynamical network of solvation sites based on the mechanistic insights and calculations from molecular dynamics simulations. This model, implemented using a kinetic Monte Carlo scheme, will enable us to access the long-time transport behavior orders of magnitude faster than with molecular dynamics. After additional testing and refinement of our model, we plan to screen a large number of additional polymers to evaluate the effect of certain chemical modifications on ion transport. Instantaneous site-node structures for ion transport in PEO and PTMO.
RESEARCH HIGHLIGHTS From the Resnick Sustainability Institute Graduate Research Fellows at the California Institute of Technology Computationally-guided Design of Polymer Electrolytes Global Significance Secondary lithium-ion batteries have potential applications for load-leveling for large-scale energy generation and as power sources in transportation. However, a critical problem with even state-of-the-art lithium-ion batteries is that the electrolyte solvent is typically a flammable mixture of alkyl carbonates. This hazard virtually prohibits high energy content applications. Project Summary Rigid, solid polymeric electrolytes (SPEs) are particularly attractive alternative electrolyte solvents because they are electrochemically stable and non-flammable. The design of more effective SPEs is complicated by the fact that the molecular mechanisms of ion transport are poorly understood and the design space is large. This project aims to devise new and utilize existing theoretical methods and simulation strategies for the rational molecular design of new polymer electrolyte systems for improved secondary lithium-ion batteries. With the aid of high-performance computing, our goal is to screen and propose new candidate structures on the basis of insight gleaned from atomistic simulations and coarse-grained models. The Science A significant challenge for predicting the net effect of all these contributions is the fact that these mechanisms occur over a range of length and timescales. To address these challenges, we are identifying relative cheap theoretical tools to rapidly quantify key material properties in the design of SPEs so as to reduce the problem to a more workable design space. Additionally, we are leveraging comparative studies with molecular dynamics and a new transport model to identify trends in polymer properties and ion diffusion. Key Results We have used molecular dynamics to compute the diffusion rates of Li + and various other polymer properties in a variety of different systems. An array of different coordination environments were identified; the evident connection between certain changes in coordination and available transport pathways is promising for future design. A transport model that considers the ion as moving on a dynamical network of solvation sites is being developed based on the mechanistic insights and calculations from these molecular dynamics simulations. Future Steps We plan to refine our transport model and extend our methodology to predict the transport properties of more than 20 new polymer electrolyte candidates.