MSDE MINI REVIEW Cite this: Mol. Syst. Des. Eng., 2019, 4, 850 Received 9th April 2019, Accepted 15th April 2019 DOI: 10.1039/c9me00050j rsc.li/molecular-engineering Managing transport properties in composite electrodes/electrolytes for all-solid-state lithium- based batteries† Marisa Falco, a Stefania Ferrari, b Giovanni Battista Appetecchi c and Claudio Gerbaldi * a In the global competition for ultimate electrochemical energy storage systems, the increasing tendency of original equipment manufacturers (OEM) worldwide is to consider solid-state technology as a solution to replace the current Li-ion batteries operating with liquid electrolytes. The reason for this is the need of enhanced energy density batteries which are also durable and inherently safe. Proper understanding of the electrode/electrolyte interface is of paramount importance for this purpose. Indeed, all-solid-state lithium-based secondary batteries require efficient ion conductive pathways through the whole thickness of the electrode to properly access all the active material particles, thus providing full electrode capacity. In this respect, here, we propose an overview of the strategies adopted to achieve this goal, including polymeric and inorganic ion conductors and composites thereof as well as their preparation procedures and characterisation techniques, which currently represent highly important topics in the academic/in- dustrial community to provide solutions for the shortcomings of poor safety, low ion mobility and short cycle life. 1 Introduction The pressing demand for long-lasting, high-power portable electronics and the emerging large-scale diffusion of electric vehicles (EVs) and energy storage from renewable sources re- quire batteries with lower cost and improved energy density along with enhanced cycle life and safety. 1 Lithium ion batte- ries (LIBs) currently on the market contain liquid electrolytes (LEs); these are inexpensive, easy to prepare and ensure opti- mum wetting of the electrodes, thus enabling ionic pathways throughout the whole thickness of the cell and minimising internal resistance. 2 To achieve higher energy density, using high potential cathode materials (>4.5 V vs. Li + /Li) in cells with LEs may result in unwanted reactions despite the use of additives, thus compromising the stability and operational life of the cells. 3 Moreover, attempting to decrease the thick- ness of the separator below 10 μm can result in safety issues, as in the case of the recent problems with the Samsung Note 7. 1,4 Additional limitations of LEs include difficulty of design- ing flexible cells, risk of leakage and flammability, restricted thermal stability and low Li + transference number (t Li +, viz. the fraction of Li + ion conductivity with respect to the overall ionic conductivity σ i ), leading to cell polarisation. 1 Within this context, a variety of solid-state electrolytes (SEs) have been investigated to date; the development of ion- 850 | Mol. Syst. Des. Eng., 2019, 4, 850–871 This journal is © The Royal Society of Chemistry 2019 a Group for Applied Materials and Electrochemistry (GAME Lab), Department of Applied Science and Technology (DISAT), Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy. E-mail: claudio.gerbaldi@polito.it b Department of Pharmacy, University of Chieti–Pescara “G. d'Annunzio”, Via dei Vestini 31, 66100 Chieti, Italy c ENEA, Agency for New Technologies, Energy and Sustainable Economic Development, SSPT-PROMAS-MATPRO, Via Anguillarese 301, Rome, 00123, Italy † Electronic supplementary information (ESI) available: PDF file including a ta- ble summarising relevant cell performances of the most relevant solid-state cells reported in the literature. See DOI: 10.1039/c9me00050j Design, System, Application Moving beyond the current state-of-the-art, where solid polymer electrolytes operate well in rechargeable batteries only at high temperature, which is unrealistic for practical applications, this review article presents a thorough overview of the strategies adopted to achieve practical operation under ambient conditions; we discuss polymeric and inorganic ion conductors and the composites thereof as well as their preparation procedures and characterisation techniques, which currently represent highly important topics in the academic/industrial community to provide solutions to the shortcomings of poor safety, low ion mobility and short cycle life of Li-based batteries. A very high quality state-of-the-art account of the subject matter and a balanced assessment of the current primary literature is given. The implications of recent developments for the wider scientific community are emphasised with the aim of stimulating progress in the field.