Polymer layers enhance the compatibility of LATP and electrodes, leading to the superb cycling stability of all-solid-state lithium batteries.

Dry solvent-free and gel polymer electrolytes were analyzed based on poly[bis(2-(2-methoxyethoxy) ethoxy) phosphazene] (= MEEP) with the salts LiBOB, LiPF6, and LiTFSI with regard to stability versus lithium deposition at lithium metal electrodes under dc current flow. Symmetrical cells were used with two lithium metal electrodes. The interfaces were monitored using direct optical microscopy and accompanying intermediate impedance measurements. The results were compared with measurements on a dry polymer electrolyte of dissolved LiTFSI in PEO (Li : EO = 1 : 10). The PEO-based electrolyte and the saltin-MEEP electrolytes have shown a comparable ability of dendrite inhibition. The MEEP based gel polymer electrolytes containing~50 wt % of a 1 : 1 mixture of EC/DMC, however, showed a much-enhanced ability of inhibition towards dendrite formation made evident by increased dendrite onset time (t0) and short-circuit time (ts) when observed in special visualization cells. This could be explained by an increased lithium ion conductivity, an increased lithium transference number and a lower interface resistance at the interface Li/MEEP gel polymer electrolyte. Among the three different salts investigated in the MEEP based polymer electrolytes, LiBOB and LiTFSI show much better stability at the lithium metal interface as compared to LiPF6 which hints to a more stable and conductive SEI at the Li/ MEEP interface with dissolved LiBOB and LiTFSI. For MEEP/LiBOB polymer electrolytes, the dendrites grow directly towards the positive electrode with a fast velocity at the early stage which then decays with time in a later phase. This can be explained by the stress in electrolyte and the 'competitive growth' of dendrite tips.

The combination of mechanically modified lithium electrodes and solid polymer electrolytes (SPEs) is investigated. So far, modified lithium electrodes, e.g., via structuring, are successfully used in combination with liquid electrolytes like ionic liquid‐based and organic solvent‐based ones. Specific SPEs have proven to be well compatible with lithium metal, even better than with liquid electrolytes. Nevertheless, the interface between lithium and SPEs is a complex issue. In this work, the challenge of the adequate wetting of the lithium electrode surface, especially a structured lithium surface, with a solid polymer is faced to successfully improve the interfacial properties and thus the performance, by a new method for the coating of such electrodes. The wetting of the micropatterned lithium surface by the SPE and the directed lithium electrodeposition in the patterns are verified by scanning electron microscopy analysis. The electrochemical properties of Li|SPE assemblies are investigated on the basis of constant current cycling and impedance experiments.

A modified polyphosphazene was synthesized using a mixed substitution at phosphorus consisting of 2-(2-methoxyethoxy)ethoxy side groups and anionic trifluoroborate groups. The primary goal was to increase the low lithium ion conductivities of the conventional lithium salt containing poly[2-(2-methoxyethoxy)ethoxy-phosphazene] (MEEP) by the immobilized anionic groups. As in previous studies, the mechanical stability was stabilized by UV induced radiation cross linking. By variation of the molar ratio between different side groups, mechanical and electrochemical properties are controllable. The polymer demonstrated large electrochemical stability windows ranging between 0 and 4.5 V versus the Li/Li+ reference. Total and lithium conductivities of 3.6 × 10−4 S·cm−1 and 1.8 × 10−5 S·cm−1 at 60 °C were revealed for the modified MEEP. When observed in special visualization cells, dendrite formation onset time and short-circuit time were determined as 21 h and 90 h, respectively, under constant current polarization (16 h and 65 h for MEEP, both with 15 wt % LiBOB), which hints to a more stable Li/polymer interface compared to normal MEEP. The enhanced dendrite suppression ability can be explained by the formation of a more conductive solid electrolyte interphase (SEI) and the existence of F-contained SEI components (such as LiF). With the addition of ethylene carbonate–dimethyl carbonate (EC/DMC) to form MEE-co-OBF3P gel polymer, both total and lithium conductivity were enhanced remarkably, and the lithium transference numbers reached reasonable values (σtotal = 1.05 mS·cm−1, σLi+ = 0.22 mS·cm−1, tLi+ = 0.18 at 60 °C).

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