polyethylene oxide (peo)-based solid polymer electrolytes (Spes) typically reveal a sudden failure in Li metal cells particularly with high energy density/voltage positive electrodes, e.g. Lini 0.6 Mn 0.2 co 0.2 o 2 (NMC622), which is visible in an arbitrary, time-and voltage independent, "voltage noise" during charge. A relation with SPE oxidation was evaluated, for validity reasons on different active materials in potentiodynamic and galvanostatic experiments. the results indicate an exponential current increase and a potential plateau at 4.6 V vs. Li|Li + , respectively, demonstrating that the main oxidation onset of the SPE is above the used working potential of NMC622 being < 4.3 V vs. Li|Li +. obviously, the Spe│NMC622 interface is unlikely to be the primary source of the observed sudden failure indicated by the "voltage noise". Instead, our experiments indicate that the Li | SPE interface, and in particular, Li dendrite formation and penetration through the Spe membrane is the main source. this could be simply proven by increasing the Spe membrane thickness or by exchanging the Li metal negative electrode by graphite, which both revealed "voltage noise"-free operation. The effect of membrane thickness is also valid with Lifepo 4 electrodes. in summary, it is the cell setup (peo thickness, negative electrode), which is crucial for the voltage-noise associated failure, and counterintuitively not a high potential of the positive electrode.

Solid polymer electrolytes (SPEs) are promising candidates for the realization of lithium metal batteries. However, the popular SPE based on poly(ethylene oxide) (PEO) reveals a "voltage noise"-failure during charge, for example, with high energy/high voltage electrodes like LiNi 0.6 Mn 0.2 Co 0.2 O 2 (NMC622), which can be attributed to short-circuits via penetrating Li dendrites. This failure disappears when integrating PEO-based SPE in a semi interpenetrating network, which mainly consists of PEO units, as well. In this work, it is shown that this SPE allows performance improvement via elimination of the crystalline domains without significant sacrifice of mechanical integrity. Hence, a highly amorphous SPE can be obtained by a simple increase of plasticizing Li salts, which overall is beneficial, not only for the ionic conductivity, but also the homogeneity, while remaining mechanically stable and solid in its original shape even after storage at 60 °C for 7 days. These aspects are crucial for the performance of the modified SPE as they can suppress the failure-causing Li dendrite penetration while the electrochemical aspects, that is, anodic stability, are rather unaffected by the modification and remain stable (4.6 V vs Li│Li +). Overall, this optimized SPE enables stable cycling performance in NMC622│SPE│Li cells, even at 40 °C operation temperature.

HIGHLIGHTS PEO-based solid polymer electrolytes (SPEs) are stable up to 4.6 V versus Li|Li + But, linear PEO-based SPE results in a ''voltage noisefailure'' in NMCkLi cells Failure disappears when PEO is incorporated in a semi-interpenetrating network Charge/discharge cycling possible even at 40 C in NMC622kLi cells