Solid-state batteries are seen as a possible revolutionary technology, with increased safety and energy density compared to their liquid-electrolyte-based counterparts. Composite polymer/ceramic electrolytes are candidates of interest to develop a reliable solid-state battery due to the potential synergy between the organic (softness ensuring good interfaces) and inorganic (high ionic transport) material properties. Multilayers made of a polymer/ceramic/polymer assembly are model composite electrolytes to investigate ionic charge transport and transfer. Here, multilayer systems are thoroughly studied by electrochemical impedance spectroscopy (EIS) using poly(ethylene oxide) (PEO)-based polymer electrolytes and a NaSICON-based ceramic electrolyte. The EIS methodology allows the decomposition of the total polarization resistance (R_p) of the multilayer cell as being the sum of bulk electrolyte (migration, R_el), interfacial charge transfer (R_ct), and diffusion resistance (R_dif), i.e., R_p = R_el + R_ct + R_dif. The phenomena associated with R_el, R_ct, and R_dif are well decoupled in frequencies, and none of the contributions is blocking for ionic transport. In addition, straightforward models to deduce R_el, R_dif, and t⁺ (cationic transference number) of the multilayer based on the transport properties of the polymer and ceramic electrolytes are proposed. A kinetic model based on the Butler–Volmer framework is also presented to model R_ct and its dependency with the polymer electrolyte salt concentration (C_Li⁺). Interestingly, the polymer/ceramic interfacial capacitance is found to be independent of C_Li⁺.

The energy conversion and storage are great challenges for our society. Despite the progress accomplished by the Lithium(Li)-ion technology based on flammable liquid electrolyte, their intrinsic instability is the strong safety issue for large scale applications. The use of solid polymer electrolytes (SPEs) is an adequate solution in terms of safety and energy density. To increase the energy density (resp. specific energy) of the batteries, the positive electrode thickness must be augmented. However, as for Li-ion liquid electrolyte, the cationic transference number of SPEs is low, typically below 0.2, which limits their power performance because of the formation of strong gradient of concentration throughout the battery. Thus, for a given battery system a compromise between the energy density and the power has to be found in a rapid manner. The goal of this study is to propose a simple efficient methodology to optimize the thickness of the SPE and the positive electrode based on charge transport parameters, which allows to determine the effective limiting Li⁺ diffusion coefficient. First, we rapidly establish the battery power performance thanks to a specific discharge protocol. Then, by using an approach based on the Sand equation a limiting current density is determined. A unique mother curve of the capacity as a function of the limiting current density is obtained whatever the electrode and electrolyte thicknesses. Finally, the effective limiting diffusion coefficient is estimated which in turn allows to design the best electrode depending on electrolyte thickness.

X-ray and neutron imaging are widely employed for battery materials, thanks to the possibility to perform noninvasive in situ and in operando analyses. X-ray tomography can be performed either in synchrotron or in laboratory facilities and is particularly well-suited to analyze bulk materials and electrode/electrolyte interfaces. Several post-lithium-ion (Li-ion) devices, such as Li–sulfur, Li–O₂, or all-solid-state Li batteries, have an anode made of metallic Li in common. The main failure mode of Li batteries is the inhomogeneity of the Li electrodeposits onto the Li anode during charge steps, leading to dendrite growth and low Coulombic efficiency. X-ray tomography is a powerful tool for studying dendrites as it provides useful information about their locations, dynamics, and microstructures. So far, the use of neutron tomography is scarcely reported for Li deposit analysis due to the difficulty in reaching sufficient image resolution to capture the deposit microstructure, that is, typically below 10–20 µm. The very different interactions of X-rays and neutrons with Li, which has significantly different opacity in the two cases, make the two techniques highly complementary. Notably, the capacity of neutrons to discern different Li isotopes is pivotal to getting an insight into the composition of Li deposits by distinguishing between Li originating from an electrode (⁶Li in this study) and Li originating from the Li salt electrolyte (mainly in ⁷Li here). Indeed, the theoretical linear neutron attenuation coefficient of ⁶Li is about 15 and 2,000 times larger than that of natural Li and ⁷Li, respectively. Therefore, a high imaging contrast difference is obtained between ⁶Li (high attenuation) and natural Li and ⁷Li (lower attenuations), which could allow a better understanding of the origin of the Li comprising the electrodeposits. In this work, we report, as a proof of concept, an in situ neutron tomography imaging of Li electrodeposits in a cycled Li symmetric cell. The electrochemical cell comprises a natural Li electrode, a ⁶Li electrode, and a deuterated liquid electrolyte. The neutron tomographies are compared with X-ray tomography images of the same electrochemical cell acquired both at an X-ray synchrotron beamline and at a laboratory X-ray tomograph. Neutron tomography is shown to be compatible with in situ analysis and capable of capturing the overall morphology of the Li deposits in good accordance with X-ray tomography analyses.