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For next generation lithium batteries, solid polymer electrolytes (SPEs) are essential to meet the challenges of higher safety standards, higher specific energy, and easy processing. Linear polyethylene glycol (PEG) based SPEs are by far the most investigated systems for these requirements. However, the weak mechanical properties, high crystallinity, and consequently moderate ionic conductivity prevent these systems from being used in electrochemical storage devices. We address the question of the influence of the polymer architecture on the above properties by synthesizing bottlebrush copolymers carrying PEG side chains and comparing their electrochemical properties and ionic conductivity with those of the respective linear PEG polymers. For obtaining bottlebrush polymers, first methacrylate (PEGMEMA) and norbornene (Nb-PEGME) macromonomers carrying PEG side chains were synthesized and polymerized using either free radical polymerization or ring-opening metathesis polymerization (ROMP), respectively. We ...

Solid polymer electrolytes (SPEs) with good thermal, mechanical and electrochemical cycling stability are required for application in all-solid-state lithium metal batteries (LMBs) using non-intercalating Li metal anodes at room temperature. In this context, the polymer architecture plays a significant role in influencing the above parameters. Therefore, we studied systematically Poly(MA)m-graft-PEGME2k in comparison to the linear poly(ethylene oxide) (PEO) homopolymer as SPEs in all-solid-state LMBs using LiFePO4 as a cathode. Additionally, nanocomposite electrolytes using bottlebrush (SPNE1) and PEO (SPNE2) with improved mechanical and electrochemical properties were prepared by adding different amounts of TiO2 nanoparticles. Among them, the SPNE1-10 (with 10 wt% TiO2) showed a homogenous distribution of nanoparticles throughout the polymer matrix, exhibited a good ionic conductivity of 3·10–5 at 25 ᴼC and 5.2·10–4 at 70 ᴼC, as well as a high electrochemical stability of up to 5.2 V vs. Li/Li+. Moreover, the symmetric Li/SPNE1-10/Li cells displayed a constant current up to 40 cycles without any fluctuations indicating good interfacial compatibility between the electrode and electrolyte. Furthermore, extended distribution of relaxation times (eDRT) studies provide evidence of a stable solid-electrolyte interface (SEI) layer formation, which is further supported by ex-situ X-ray photoelectron spectroscopy (XPS) analysis of the cycled lithium surface. The LMBs with the SPNE1-10 electrolyte delivered a high discharge capacity of 132 mAh g−1 at 70 ᴼC at a 0.2C. Even, when the current rate was increased to 2C, the cell maintained a good discharge capacity after 400 cycles. The SPNE1-10 nanocomposite based on the bottlebrush polymer outperforms considerably the SPNE2-10 consisting of linear PEO for the whole temperature range from 25 to 80 ᴼC enabling efficient all solid-state LMBs using SPEs below 70 ᴼC.

Solid polymer electrolytes in solvent-free lithium batteries may overcome some of the disadvantages of liquid electrolytes such as flammability and instability.1 Thereby, the polymer electrolyte acts as both ion transport medium and electrical separator between the electrodes. Compared to rigid separators (e.g. fiber glass) a higher shape flexibility is a further advantage, as these SPEs can compensate volume changes of the electrodes by elastic and plastic deformation.2 Poly(ethylene glycol) (PEG) possesses one of the highest ionic conductivities among solvent-free SPEs but suffers from a conductivity drop below its melting temperature about 50-60°C due to high crystallinity depending on the molecular weight.1 In addition to linear PEG polymers, ion-conducting bottlebrush graft copolymers can be obtained by attaching PEG side chains to a polymer backbone in order to reduce the crystallinity maintaining very high molecular weight. In this work we synthesized five new bottlebrush polymers using free radical polymerization as well as ring-opening metathesis polymerization (ROMP). These brush polymers contain different lengths of PEG side chains (1 kg mol-1 and 2 kg mol-1) and two different backbones, poly(methacrylate) and poly(norbornene). We present the influence of the polymer architecture on mechanical stability, ionic conductivity, Li-ion transport number and electrochemical stability of a series of SPEs obtained thereof by mixing with different amounts of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). We also compare the results to the respective linear PEG counterparts. The differences in ionic conductivity were analyzed and correlated with respect to thermal properties such as T g and ΔH melt in order to understand the fundamental factors which influence the properties of solvent-free PEG-containing bottlebrush SPEs. We also examined the implications of changing a linear polymer system to a brush architecture for potential applications in batteries and correlate the occurring processes in the cell with the distribution of relaxation times (DRT). Detailed comparative measurements under similar cell configurations for diverse O/Li ratios in a temperature range of 25 to 80 °C for different SPEs were carried out to elucidate structure-property relationships. The interesting findings are that by applying a brush architecture, we suppress the crystallinity of PEG and improve the mechanical strength without losing ionic conductivity. We obtained conductivities in the range of 10-3 to 10-4 S cm-1 for solvent-free SPEs. Furthermore, the best ionic conductivities for any system correlate strongly with their respective T g.3 Nevertheless, there are still several unresolved questions regarding these bottlebrushes compared to linear PEG in terms of interfacial as well as bulk processes in both blocking steel (ionic conductivity) and lithium (lithium plating/stripping, interfacial resistance) electrode setups. In addition to the polymer component, the lithium salt has a major effect on the properties of the electrolyte. LiTFSI is probably the most common Li-ion source in SPEs.4 Besides that, lithium borate salts have gained high interest due to their high thermal stability, cost-effectiveness, favorable solid electrolyte interface (SEI) formation and ionic conductivities in the same range as LiTFSI.5 For example, Lithium bis(oxalate)borate (LiBOB) and its asymmetric counterpart Lithium difluoro(oxalate) borate (LiDFOB) are stable in organic solvents and the electrochemical stability is higher than 4.5 V vs. Li/Li+.6 Different salts in an electrolyte can influence the Li-ion transport as well as the processes at the interfaces or the formation of the SEI. This again requires a detailed comparative analysis. For this, we prepared promising bottlebrush polymer electrolytes (1 kg mol-1 PEG sidechain) containing LiBOB and LiDFOB and subsequently analyzed them electrochemically by impedance spectroscopy. The measurement data was finally interpreted by the extended Distribution of Relaxation Times (eDRT). Both SPEs showed similarities in the lithium-ion conducting process in possessing one major, resistive-capacitive bulk conductivity mechanism. The resulting, temperature-dependent conductivities were evaluated and are in the range of 10-4 to 10-5 S cm-1. During cycling, the SPEs showed increased interface resistances over time, which are higher than respective bulk resistances. By applying eDRT, the time-dependent formation of an interphase layer in the SPEs is identified, separated from the slower charge transfer process and quantified. Thus, the electrolytes cannot be considered electrochemically stable against metallic Li, which is similar for liquid electrolytes.7 References (1) Scrosati, B.; Energy Environ. Sci. 2011, 4, 3287. (2) Janek, J.; Nat. Energy 2016, 1, 16141. (3) Rosenbach, D.; ACS Appl. Energy Mater. 2019, 2, 3373–3388. (4) Etacheri, V.; Energy and Environmental Science 2011, pp 3243–3262. (5) Xu, K. Chem. Rev. 2014, 114, 11503–11618. (6) Liu, Z.; Coord. Chem. Rev. 2015, 292, 56–73. (7) Hahn, M.; Electrochim. Acta 2020, 344, 136060. Figure 1

Solid state electrolytes based on polymer or ceramic materials are a safe alternative to liquid electrolytes based on organic solvents. Yet their ionic conductivity does not meet the required specification for state-of-the-art lithium-ion batteries. Furthermore, their conductivity mechanisms and interfacial behavior are not fully understood, making in-depth electrical characterization necessary. The calculation of the Distribution of Relaxation Times from the impedance spectrum of an electrochemical component is a powerful approach to gain insight into the processes and mechanisms responsible for the electrical and electrochemical behavior. Here we introduce an extended Distribution of Relaxation Times to avoid error-prone preprocessing. This method includes non-resistive-capacitive elements in its impedance function to overcome the constraints usually limiting the Distribution of Relaxation Times. In this study we investigated solid polymer and ceramic electrolytes regarding their conductivity mechanisms, charge transfer mechanisms and interphase formation. While the materials all possess one major conductivity mechanism, significant differences in charge transfer and interphase behavior were observed. In the case of solid polymer electrolytes, poly (ethylene glycol) bottlebrush polymers were prepared using two different boron-based lithium salts. Additionally, solid polymer electrolytes based on triethylene glycol grafted onto a poly (glycidyl propargyl ether) backbone blended with lithium bis(trifluoromethanesulfonyl)imide were compared to a similar single-ion conducting solid polymer electrolyte with the bis(trifluoromethanesulfonyl)imide anion spatially fixed to the backbone by sequential co-click synthesis. For the ceramic electrolyte, Li5·6Al0·3La3Zr1·5Ta0·5O12 powder was synthesized via a mixed oxide route and the dense ceramic electrolytes layers were fabricated by the Powder Aerosol Deposition Method. The produced layers were post-treated at 400 °C and the influence of the thermal annealing atmosphere on the conductivity of the solid electrolytes was investigated. In this study, we present the feasibility of the extended Distribution of Relaxation Times for the characterization of the investigated materials. The time-dependent formation of an interphase layer in the polymer electrolytes is identified, separated from the charge transfer process and quantified. For the ceramic electrolyte, the influence of the annealing is depicted and the charge transfer reaction is detected.