**4. Conclusions**

A polymer-carbon composite current collector foil (PCCF) for bipolar lithium-ion battery applications is developed and evaluated in comparison to state-of-the-art Al-foil collector. The PCCF shows su fficient mechanical properties, which allow the processing of the PCCF collector in a roll-to-roll industrial electrode coater. The PCCF proved to be hermetical dense, which is important to avoid liquid electrolyte penetration through the collector. The applicability for lithium-ion batteries was studied based on water-processed LiNi0.5Mn1.5 O4 (LMNO) cathode and Li4Ti5O12 (LTO) anode coatings with the integration of a thin carbon primer at the interface to the collector. Despite the fact that the laboratory-manufactured PCCF shows a much higher film thickness of 70 μm compared to Al-foil of 19 μm, the electrode resistance was measured to be by a factor of five lower compared to Al collector, which was attributed to the low contact resistance between PCCF, carbon primer and electrode microstructure. The PCCF-C-primer collector shows a su fficient voltage stability up to 5 V vs. Li/Li+ and low Li-intercalation losses into the carbon primer of the PCCF (~0.1 mAh/cm2), which makes him compatible to a wide range of anode and cathode active materials. Electrochemical cell tests demonstrate the applicability of the developed PCCF for LMNO and LTO electrodes, with no obvious disadvantage compared to Al collector. The advantage of a nearly 50% lower raw material density of the PCCF polymer collector compared to metal Al-foil along with expected improvements in collector thickness reduction and cost savings, due to a scaled industry manufacturing approach, will o ffer the possibility to significantly reduce the mass loading of the collector in the battery cell. Overall, the developed PCCF collector appears to be advantageous, especially for bipolar battery architectures, where a combination of the abovementioned properties is needed which cannot be fulfilled by today´s metal-, bimetal- or carbon-based collectors.

*Batteries* **2020**, *6*, 60

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2313-0105/6/4/60/s1, Figure S1: Voltage profiles of measured coin cells from Figures 9 and 10 (first three cycles at 0.1 C); Top left: LTO on PCCF, Top right LTO on Al-collector, Bottom left: LMNO on PCCF, Bottom right: LMNO on Al, Figure S2: Post-mortem picture of PCCF-foil of a LTO cell after cycling test according to Figure 9; Left: PCCF C-primer side in contact to LTO electrode after cycling, Right: Backside of the PCCF after cycling of LTO in coin cell, Figure S3: Post-mortem picture of PCCF-foil of a LMNO cell after cycling test according to Figure 10; Left: PCCF C-primer side in contact with LMNO electrode coating (some separator residue white) after cycling on coin cell; Right: Backside of the PCCF after cycling of LMNO in coin cell.

**Author Contributions:** Conceptualization, M.F. and M.C.; methodology, M.F., M.C., B.K., P.P.; validation, M.F. and M.C.; investigation, M.F., M.C., P.M., B.K.; writing—original draft preparation, M.F., M.C., K.K., B.K. and P.P.; writing—review and editing, M.F.; visualization, M.F., M.C., B.K. and P.P.; supervision, M.W. and A.M.; project administration, M.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by German Federal Ministry of Education and Research (BMBF) project EMBATT2.0 No. 03XP0068G and No. 03XP0068E.

**Acknowledgments:** The authors thank the processing laboratory of IPF, headed by I. Kühnert, and especially F. Pursche for carrying out the melt compounding and film extrusion experiments. We thank O. Kobsch (IPF) for performing roughness measurements and S. Höhn (IKTS, Ceramography and Phase Analysis) for sample preparation and SEM analysis.

**Conflicts of Interest:** The authors declare no conflict of interest.
