Unique Carbonate-Based Single Ion Conducting Block Copolymers Enabling High-Voltage, All-Solid-State Lithium Metal Batteries

Safety and high-voltage operation are key metrics for advanced, solid-state energy storage devices to power low- or zero-emission HEV or EV vehicles. In this study, we propose the modification of single-ion conducting polyelectrolytes by designing novel block copolymers, which combine one block responsible for high ionic conductivity and the second block for improved mechanical properties and outstanding electrochemical stability. To synthesize such block copolymers, the ring opening polymerization (ROP) of trimethylene carbonate (TMC) monomer by the RAFT-agent having a terminal hydroxyl group is used. It allows for the preparation of a poly(carbonate) macro-RAFT precursor that is subsequently applied in RAFT copolymerization of lithium 1-[3-(methacryloyloxy)propylsulfonyl]-1-(trifluoromethylsulfonyl)imide and poly(ethylene glycol) methyl ether methacrylate. The resulting single-ion conducting block copolymers show improved viscoelastic properties, good thermal stability (Tonset up to 155 °C), sufficient ionic conductivity (up to 3.7 × 10–6 S cm–1 at 70 °C), and high lithium-ion transference number (0.91) to enable high power. Excellent plating/stripping ability with resistance to dendrite growth and outstanding electrochemical stability window (exceeding 4.8 V vs Li+/Li at 70 °C) are also achieved, along with enhanced compatibility with composite cathodes, both LiNiMnCoO2 – NMC and LiFePO4 – LFP, as well as the lithium metal anode. Lab-scale truly solid-state Li/LFP and Li/NMC lithium-metal cells assembled with the single-ion copolymer electrolyte demonstrate reversible and very stable cycling at 70 °C delivering high specific capacity (up to 145 and 118 mAh g–1, respectively, at a C/20 rate) and proper operation even at a higher current regime. Remarkably, the addition of a little amount of propylene carbonate (∼8 wt %) allows for stable, highly reversible cycling at a higher C-rate. These results represent an excellent achievement for a truly single-ion conducting solid-state polymer electrolyte, placing the obtained ionic block copolymers on top of polyelectrolytes with highest electrochemical stability and potentially enabling safe, practical Li-metal cells operating at high-voltage.


I. SYNTHESIS AND CHARACTERIZATION OF SICS S4
A. Materials S4

A. Materials
Poly ( Table S1). Typical polymerization procedure is given bellow by an example of ROP 9 synthesis:

RAFT synthesis of poly[TMCn-b-LiMm] block copolymers
RAFT polymerization technique was used to grow the poly(LiM) block from poly(TMC) macro-RAFT agent and, as a result, to prepare a set of poly[TMCn-b-LiMm] copolymers (Table 1, copoly1 -copoly3). Typical polymerization procedure is given bellow by an example of copoly3

Determination of number average molar mass (Mn) and monomers ratio by NMR
where NLiM -number of protons under 7 (Figure S10b), when number of protons under 10 ( Figure   S10b) is set to 3.

Gel permeation chromatography (GPC)
For Degree of polymerization (DP) was determined using the following equation (Eq. 3): where Mn GPC is the molar mass of the polymer determined by GPC, while Mmonomer -molar mass of the monomer.

Thermal gravimetric analysis (TGA) and Differential Scanning Calorimetry (DSC)
TGA was carried out in air on a TGA2 STAR e System (Mettler Toledo) applying a heating rate of samples were loaded directly onto the aluminum plate of the rheometer and special care was taken to exclude bubbles. Measurements were recorded in the oscillation mode at an imposed 1% strain amplitude (), ensuring that both moduli G′ and G″ were obtained in the linear viscoelastic regime.
All measurements were carried out at 25 and 70°C. Tests were repeated at least twice to insure good repeatability of the results.

Atomic Force Microscopy (AFM)
AFM-images were recorded with MFP-3D infinity microscope (Asylum Instruments/Oxford Instruments) in the tapping mode (-20°C, in air). AC160TS-R3 (Olympus) cantilevers were applied with a stiffness of 26 N m -1 and resonance frequency of 300 KHz. The domains periodicity was evaluated from 3 different 1x1m 2 images. On each image two profiles were taken and for each the distance over 10 consecutives periods was recorded. The images were recorded in the socalled 'soft tapping mode', to avoid deformation and indentation of the polymer surface by the tip.
All the images were collected with the maximum available number of pixels (512) in each direction. General procedure for the preparation of the samples for AFM was as follow: films were cast from 10 wt% solution of copoly8 in DMF on a microscope glass slide, and allowed to slowly evaporate at 80°C. The obtained thin films were dried at 80°C/1 mm Hg for 24 h.

Electrochemical impedance spectroscopy (EIS)
Ionic conductivity () was determined by electrochemical impedance spectroscopy (EIS) with a VSP potentiostat/galvanostat (Bio-Logic Science Instruments). To avoid any influence of moisture/humidity on the conductivity of polymer electrolytes, the latter were preliminary dried at 60 °C/1 mm Hg for 12 h in the B-585 oven (Buchi Glass Drying Oven, Switzerland) filled with P2O5 and were transferred under vacuum inside an argon-filled glovebox (MBRAUN MB-Labstar, H2O and O2 content <0.5 ppm). Polymers were sandwiched between two stainless steel (SS-316) blocking electrodes. The distance between the electrodes (d) was kept equal to 250 m using a Teflon spacer ring with the inner area (A) of 0.502 cm 2 . Symmetrical stainless steel/copolymer/stainless steel assembly was clamped into the 2032 coin cell and afterwards was taken out from glovebox. Cell impedance was measured at the open circuit potential (OCV) by applying a 50 mV perturbation in the frequency range from 10 -2 to 2×10 5 Hz and in a temperature S14 range from 20 to 100 °C. Temperature was controlled using the programmed M-53 oven (Binder, Germany), where cells were allowed to reach thermal equilibrium for at least 1h before each test.
The ohmic resistance (R, ) of the polymer electrolyte sample, obtained from the Nyquist plot at the low frequency end of the semicircle, was used to calculate the ionic conductivity using the following equation (Eq. 4):

Cyclic voltammetry (CV)
CV was used to determine the electrochemical stability window ( The lithium-ion transference number (tLi + ) was determined at 70 °C in a symmetric lithium metal/copolymer/lithium metal cell, which was subjected to a 120 mV polarization bias (V) with the aim to determine the initial (Io) and the steady state (Iss) currents. EIS was performed on VMP3 S15 multipotentiostat (20 V, ±400 mA, Bio-Logic Science Instruments) by applying a 50 mV perturbation between 300 kHz and 0.5 mHz at OCV conditions to obtain the resistance of the passivation layer before (RSEI+CT,o) and after (RSEI+CT,ss) polarization. The tLi + was calculated using an Abrahams equation 3

Li cells assembly
Lab-scale LiFePO4/copoly8/Li and LiNiMnCoO2/copoly8/Li battery prototypes assembly was performed inside the Ar-filled glovebox using the ECC-Std test cells (EL-Cell GmbH). A 100 m thick polyethylene terephthalate (Mylar) round spacer with a 10 mm internal diameter was layered on top of the composite cathode tape. Afterwards a layer of copoly8 electrolyte was applied manually directly on the composite cathode's surface within the internal diameter of the spacer.
The assembly was completed with a lithium metal disk anode.

Li cells testing
Lab-scale LiFePO4/copoly8/Li cells were galvanostatically cycled on a VMP3 multipotentiostat (20 V, ±400 mA, BioLogic Science Instruments) at 70 °C between 2.5 and 3.8 V vs. Li + /Li at fixed charge/discharge current regime of C/20, corresponding to a full discharge or full charge of the theoretical cathode capacity (170 mAh g -1 ) in 20 hours.

S17
Lab-scale LiNiMnCoO2/copoly8/Li cells were cycled at 70 °C between 3 and 4.3 V vs. Li + /Li at fixed charge/discharge current regime of C/20, corresponding to a full discharge or full charge of the theoretical cathode capacity (185 mAh g -1 ) in 20 hours.
Cycling tests were also conducted at higher current rates, where the rate is denoted as C/n, corresponding here to a full discharge or full charge of the theoretical cathode capacity (C) in n hours. S18   (copoly8) block copolymers after precipitation. Figure S4. 1