A novel templating method to create 3D bicontinuous structured hybrid electrolytes with improved mechanical properties for all-solid-state lithium batteries.

Dopants used to stabilise the cubic phase of LLZO also play a crucial role in the cell's critical current density.

In this work, we reveal the impact of moisture-induced chemical degradation and proton-lithium exchange on the Li-ion dynamics in the bulk, the grain boundaries and at the interface with lithium metal in highly Li-conducting garnet electrolytes. A direct correlation between chemical changes as measured by depth-resolved secondary ion mass spectrometry and the change in transport properties of the electrolyte is provided. In order to probe the intrinsic effect of the exchange on the lithium kinetics within the garnet structure, isolated from secondary corrosion product contributions, controlled-atmosphere processing was first used to produce proton-free Li6.55Ga0.15La3Zr2O12 (Ga0.15-LLZO), followed by degradation steps in a H2O bath at 100 C, leading to the removal of LiOH secondary phases at the surface. The proton-exchanged region was analysed by focussed ion beam-secondary ion mass spectrometry (FIB-SIMS) and 2 found to extend as far as 1.35 m into the Ga0.15-LLZO garnet pellet after 30 minutes in H2O. Impedance analysis in symmetrical cells with Li metal electrodes evidenced a greater reactivity in grain boundaries than in grains and a significantly detrimental effect on the Li transfer kinetics in the Li metal/garnet interface correlated to a threefold decrease in the Li mobility in the protonated garnet. This result evidences that the deterioration of Li charge transfer and diffusion kinetics in proton-containing garnet electrolytes have fundamental implications for the optimisation and integration of these systems in commercial battery devices.

Garnet-type structured lithium ion conducting ceramics represent a promising alternative to liquidbased electrolytes for all-solid-state batteries. However, their performance is limited by their polycrystalline nature and the inherent inhomogeneous current distribution due to the different ion dynamics at grains, grain boundaries and interfaces. In this study we use a combination of electrochemical impedance spectroscopy, distribution of relaxation times analysis and solid state nuclear magnetic resonance (NMR), in order to understand the role that bulk, grain boundary and interfacial processes play in the ionic transport and electrochemical performance of garnet based cells.Variable temperature impedance analysis reveals the lowest activation energy for Li transport in the bulk of the garnet electrolyte (0.15 eV), consistent with pulsed field gradient NMR spectroscopy measurements (0.14 eV). We also show a decrease in grain boundary activation energy at temperatures below 0 °C , that is followed by the total conductivity, suggesting that the bottleneck to ionic transport resides in the grain boundaries. We reveal that the grain boundary activation energy is heavily affected by its composition that, in turn, is mainly affected by the segregation of dopants and Li. We suggest that by controlling the grain boundary composition, it would be possible to pave the

Lithium (Li) metal is being intensively pursued as a negative electrode due to its high specific capacity. Yet, irreversible Li loss, dendrite formation of plated Li metal, and short circuiting, when being cycled as an electrode in battery cells presently impede its use. Furthermore, the flammability of liquid electrolytes poses a severe safety hazard. Replacement of organic liquids by solid electrolytes could pave the way for using Li metal, simultaneously improving specific energy and battery safety. Significant attention has been paid to ceramic electrolytes due to their high Li-ion conductivity at room temperature and robustness. Nevertheless, processing and manufacturing of the ceramics into sufficiently thin, highly dense, pinhole-free sheets, required for high specific energy and power devices, remains an active challenge. Additionally, the electrolyte must maintain contact with the solid electrodes while simultaneously inhibiting Li dendrites as well as be durable towards electrode volume changes and to external shock. Polymer electrolytes have been developed for a number of years. The advantages include good adhesion and contact to the electrodes. Still, their low conductivity at room temperature, especially in solid polymer electrolytes, along with their limited capability of suppressing Li dendrite formation, are persistent drawbacks.[i] To date, ceramic and polymer electrolytes remain a challenge. Hybrids with particulate ceramics embedded in polymer electrolytes have been investigated to improve the conductivity and mechanical properties. [ii] Our approach is to create continuously ordered hybrid electrolytes, which allows modification of the mechanical properties of the hybrid while retaining a high ionic conductivity through the ceramic phase. The ionic conductivity of the hybrid electrolyte is 1.5 x 10-4S/cm at 25 °C. The results of different ceramic-polymer compositions indicate that the mechanical properties of the hybrids can be adjusted by the presented approach. Electrochemical, mechanical, and microstructural characterizations of the hybrid will be reported. [i] G. M. Stone, S. A. Mullin, A. A. Teran, D. T. Hallinan Jr., A. M. Minor, A. Hexemer, N. P. Balsara, J. Electrochem. Soc., 2012, 159, A222–A227. [ii] Y.-C. Jung, S.-M. Lee, J.-H. Choi, S. S. Jang and D.-W. Kim, J. Electrochem. Soc., 2015, 162, A704–A710.

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Previous research in the area of polymer electrolytes has been mainly focused on complexes prepared with alkali metal salts and polydispersed poly(ethylene oxides) (abbreviated to PEO).1 Previous studies have shown that polymer chain length may play a significant part in the ion transport in the crystalline phase but high distribution of molecular weight of the commercially available polymers complicates the understanding of the conductivity mechanism.2 By employing monodispersed polymers (with a discrete chain length) we attempt to reduce a number of variables that are likely to influence the ion transport process. However, monodispersed poly(ethylene oxides) are not readily available and therefore, a synthetic method has been developed to obtain a series of monodispersed homologues in a variety of chain lengths. These monodispersed polymers were used to prepare a series of crystalline complexes with a lithium salt (PEO6:LiPF6).  Correlation between chain length of the polymer and unit cell parameters of the crystalline complexes in combination with molecular dynamics simulations3 revealed different structural arrangements at polymer chain-end regions. These arrangements and their role in coordination of lithium ions, ion mobility and transport, and the resulting conductivities will be presented. [1] P. G. Bruce, B. Scrosati and J. M. Tarascon, Angewandte Chemie-International Edition 2008, 47, 2930-2946. [2] E. Staunton, Y. G. Andreev and P. G. Bruce, Faraday Discussions 2007, 134, 143-156. [3]A. Liivat, D. Brandell, A. Aabloo and J. O. Thomas, Polymer 2007, 48, 6448-6456.

Lithium metal is being intensively pursued as an anode due to its high specific capacity. However, dendrite formation, Li loss and short circuiting upon cell cycling presently impede its use. Furthermore, the flammability of liquid electrolytes poses a severe safety hazard. Replacement of organic liquids by solid electrolytes could pave the way for using lithium metal, simultaneously improving specific energy and battery safety. Efforts continue on ceramic electrolytes. Challenges include the formation and manufacture at scale of sufficiently thin, highly dense, pinhole-free sheets of ceramic electrolytes, required for high power devices. Such electrolytes must also maintain contact with the solid electrodes while inhibiting Li dendrites and be durable towards volume changes of the electrodes as well as to external shock. Polymer electrolytes have been developed for a number of years. They can exhibit good adhesion and contact to the electrodes, but typically have lower conductivity and limited capability of suppressing dendrite formation.[i] To date, ceramic and polymer electrolytes remain a challenge. Composites of particulate ceramics embedded within polymer electrolytes have been investigated to improve the conductivity and mechanical properties. [ii] Our approach is to create ordered composite electrolytes, which allows modification of the mechanical properties of the composite while retaining good overall ionic conductivity. The conductivity of the polymer-ceramic composite is 1.5E-4 S/cm at 25 °C. The results for different ceramic-polymer compositions indicate that the mechanical properties of the composite can be modified by the presented approach. Mechanical testing and the results of galvanostatic cycling will be reported. [i] G. M. Stone, S. A. Mullin, A. A. Teran, D. T. Hallinan Jr., A. M. Minor, A. Hexemer, N. P. Balsara, J. Electrochem. Soc., 2012, 159, A222–A227. [ii] Y.-C. Jung, S.-M. Lee, J.-H. Choi, S. S. Jang and D.-W. Kim, J. Electrochem. Soc., 2015, 162, A704–A710.