2, MEET Battery Research Center, University of Münster, Münster, NRW, Germany
Lithium metal constitutes an attractive anode material mainly due to its high theoretical specific capacity of 3860 mAh g−1, ten times higher than graphite (372 mAh g−1). The use of lithium metal in rechargeable batteries with typical liquid organic solvent based electrolytes suffers so far from severe safety problems associated with the formation of high surface area metallic lithium (HSAL) upon repeated charge/discharge. Solid polymer electrolytes (SPEs) designed to be compatible with lithium metal are able to mechanically suppress HSAL formation and are considered as viable alternative. Solvent-free SPEs exhibit advantages in terms of mechanical stability, operational safety and simplicity of cell design. However, application of polymer electrolytes to all-solid-state lithium ion batteries (ASS-LIBs) and all-solid-state lithium ion batteries (ASS-LMBs), requires improvements in respect to lithium ion conductivity, especially at ambient temperature.
Although high ionic conductivities can be achieved by high chain mobility linked to low molecular weight polymers, they are mostly too soft and therefore cause deterioration in mechanical stability of the SPE. In order to use low molecular weight polymers for fast lithium ion transport with sufficient mechanical strength at the same time, one strategy is related to utilization of a hyperbranched co-polymer where one segment represents a stable, hard backbone while the second segment is derived from a soft polymer with high ionic conductivities. With this in line, a new generation of Li+-conducting SPEs obtained from supramolecular self-assembly of PEO, cyclodextrin (CD) and lithium salt was designed and thoroughly investigated for application in lithium metal batteries (LMBs) and LIBs. When mixing an aqueous solution of PEO together with an aqueous solution of CD, a precipitate forms where the CD is threaded onto a PEO chain. The channel-type structure formed by self-assembly of PEO and CD can be used as the backbone structure whereas the hydroxyl groups of CD rings can be modified. Here, we use the ability of CD being the initiator for ring-opening polymerization of cyclic carbonates. This strategy enables synthesis of grafted polycarbonate side chains with low molecular weight. The obtained inclusion complexes show impressive ionic conductivity up to 1 mS cm-1 at 60 °C, together with high oxidative stability and allow for application in LFP/Li cells at 40 °C for more than 200 charge/discharge cycles. Post mortem XPS and SEM studies confirm that the polymer/LiTFSI penetrates the cathode upon cycling, facilitating improved contacts. This new system provides a platform for further modifications of the polymer side-chains.