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We have studied the cycle life of two polyhedral oligomeric silsesquioxane-b-poly(ethylene oxide)-b-polyhedral oligomeric silsesquioxane (POSS-PEO-POSS) block copolymer electrolytes differing primarily in molecular weights and composition using lithium/polymer/lithium symmetric cells. The higher molecular weight electrolyte, labeled H, has a higher storage modulus, G el . However, the volume fraction of the conducting phase in the low molecular weight electrolyte, labeled L, is higher and this leads to a four-fold increase in limiting current density, i L. Measurement of ionic conductivity provides insight into the reason for the observed differences in limiting current density. The average lifetime of symmetric cells with electrolyte L was slightly higher than that of cells with electrolyte H. The combined effect of mechanical and electrochemical properties of electrolytes on the stability of lithium electrodeposition was quantified by examining two dimensionless parameters, i/i L and G el /G Li , introduced in the theory developed by Barai and Srinivasan [Phys. Chem. Chem. Phys., 19, 20493–20505 (2017)]. This theory predicts the regime of stable lithium electrodeposition as a function of these two parameters. Despite large differences in G el and i L between the two electrolytes, we show that similar cell lifetimes are consistent with the theoretical predictions of unstable lithium electrodeposition without resorting to any adjustable parameters.

Author(s): Wang, DR; Shah, DB; Maslyn, JA; Loo, WS; Wujcik, KH; Nelson, EJ; Latimer, MJ; Feng, J; Prendergast, D; Pascal, TA; Balsara, NP | Abstract: The reduction of sulfur during discharge in a lithium-sulfur (Li-S) cell is known to occur in a series of reaction steps that involve lithium polysulfide intermediates. We present an operando study of the discharge of a solid-state Li-S cell using X-ray absorption spectroscopy (XAS). In theory, the average chain length of the polysulfides, xavg,cell, at a given depth of discharge is determined by the number of electrons delivered to the sulfur cathode. The dependence of xavg,cell measured by XAS on the depth of discharge is in excellent agreement with theoretical predictions. XAS is also used to track the formation of Li2S, the final discharge product, as a function of depth of discharge. The XAS measurements were used to estimate rate constants of a series of simple reactions commonly accepted in literature.

Author(s): Maslyn, JA; Mcentush, KD; Harry, KJ; Frenck, L; Loo, WS; Parkinson, DY; Balsara, NP | Abstract: Lithium metal is a high-energy-density battery electrode material, but the largely irreversible growth of lithium protrusions on an initially planar electrode during cycling makes it unsuitable for incorporation into a commercial battery. In this study, a lithium electrode with globular protrusions was stripped electrochemically, and the local morphology of the electrode as a function of time was determined by hard X-ray tomography. We demonstrate that globules are preferentially stripped compared to a planar electrode in our system, which incorporates a nanostructured block copolymer electrolyte. We report current density at the electrode as a function of micron-scale position and time. The local current density during the electrode healing process calculated from a reference frame at the electrode/electrolyte interface provides insight into the driving forces responsible for selective stripping of the globule. These results imply the possibility of discharging protocols that may return a lithium electrode to its initial planar state.

Successful prevention of lithium dendrite growth would enable the use of lithium metal as an anode material in next-generation secondary batteries. Mechanically stiff solid polymer electrolytes have been shown to prolong the life of lithium metal cells by partially suppressing lithium dendrite growth. However, we lack fundamental knowledge about the nature of lithium electrodeposition and stripping as a function of time, as direct observation of this phenomenon is non-trivial. Synchrotron X-ray microtomography was used observe lithium metal plating and stripping through these solid polystyrene-block-poly(ethylene oxide)-LiTFSI electrolyte membranes in the vicinity of lithium protrusions. Local current density was calculated from these experimental measurements and mapped on the top and bottom electrode under conditions of lithium metal plating and stripping. Local current density at one electrode was correlated to local current density at the other electrode and to electrode thickness. We show quantitatively how lithium protrusions are stripped at a faster rate than planar lithium and gain insight into the lithium ion transport governing protruding lithium growth in a cycled lithium metal electrode.

It is now commonplace to report the data obtained during cycling of symmetric lithium-electrolyte-lithium cells with solid polymer electrolytes. The parameter that is usually the focus is the number of cycles prior to cell failure. Quantitative prediction of cell failure is a daunting task. As a step toward that aim, we seek to predict the potential needed to cycle such cells at a function of the applied current density. We show that such predictions cannot be made on the basis of conductivity alone. The importance of complete characterization of polymer electrolytes will be discussed. This includes determination of the thermodynamic factor, salt diffusion coefficient and transference number using Newman’s concentrated solution theory. The connection between these parameters and dendrite growth is being explored. If new results are obtained, I will present them at the meeting.

Successful prevention of lithium dendrite growth would enable the use of lithium metal as an anode material in next-generation secondary batteries. Mechanically stiff solid polymer electrolytes have been shown to prolong the life of lithium metal cells by partially suppressing lithium dendrite growth. A series of diblock copolymers of high modulus and molecular weight were synthesized, and their nanostructures and electrochemical properties were characterized as a function of polymer composition and lithium salt concentration, including measurements of the limiting current. X-ray tomography was used to observe lithium metal plating and cycling through these solid polymer electrolyte membranes under a range of electrochemical conditions. Insight into the interplay between electrochemical and mechanical properties, polymer microstructure, and lithium electrodeposition morphology will aid in the design of next-generation electrolytes.

In a society increasingly concerned by its impact on the environment and the urgent issue of climate change, there is great demand for an energy storage device with higher energy density and power capability (especially for electric vehicles and power grid). The use of the high energy density lithium metal anode jointly with a solid polymer electrolyte could enable the development of large-scale battery applications. Yet, dendritic lithium formation leading to short circuit currently limits its practical applications. Therefore, understanding the fundamentals of dendrite nucleation and growth is necessary to design robust battery systems. In this work, the nucleation and growth of lithium dendrite is studied in function of salt concentration in lithium-lithium symmetric cells in a rigorous manner. Complete electrochemical characterization of a solid block copolymer polystyrene-polyethylene oxide mixed with lithium salt (LiTFSI) will be presented, as well as the cell lifetime under galvanostatic cycling. We have discovered an unexpected relationship between cell lifetime and salt concentration. X-ray micro-tomography was used to underpin the relationship between salt concentration and dendrite nucleation and morphology.