Abstract
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.
Highlights
The global rechargeable battery market is forecasted to reach250 billion euros per year by 2025, around 60% of which is committed for the advanced materials market and the rest is for investments in manufacturing capacity, R&D, and other supporting activities
We focused on the development of novel poly[(ionic liquid)-b-(carbonate)] block copolymers (Scheme 1) with single Li-ion conducting features, showing greatly enhanced performance toward the state of the art of solid-state electrolyte systems, in terms of electrochemical stability and compatibility with both high voltage cathodes (NMC) and lithium metal anode, which resulted in reversible cycling near theoretical capacity in lab-scale Li-metal cells
Recent advances in the ring opening polymerization (ROP) of cyclic carbonates allowed elaborating a new metal-free green method, which uses various alcohols as initiators and highly basic amines (1,8diazabicyclo(5.4.0)undec-7-ene (DBU), 1,5,7triazabicyclo[4.4.0]dec-5-ene (TBD), 4-dimethylaminopyridine (DMAP), etc.) as catalysts.[45,47,48]. This method was further expanded when reversible addition-fragmentation chain transfer (RAFT) chain transfer agents containing a hydroxyl functionality like 4-cyano-4(dodecylsulfanylthiocarbonyl)sulfanylpentanol (CDP) or (S)2-cyano-5-hydroxypentan-2-yl benzodithioate were exploited as dual initiators, allowing the subsequent realization of ROP and RAFT polymerizations.[45,46]
Summary
250 billion euros per year by 2025, around 60% of which is committed for the advanced materials market and the rest is for investments in manufacturing capacity, R&D, and other supporting activities. The absolute values of G′ in the case of copoly[8] were several orders of magnitude higher than those of poly(LiMm-r-PEGMk) under the same measurement conditions These outcomes are definitely relevant considering that the increase of the solid polymer electrolyte modulus is reported to effectively suppress/limit the formation and growth of lithium dendrites.[5,61] As the compared copolymers are both linear and of similar molecular weight (Table 2), the observed change in viscoelastic properties can only be attributed to the presence of the poly(TMC) block in copoly[8]. The CE exceeded 99% during initial and prolonged cycling at low as well as at high rates, confirming the reversibility of the lithium ion intercalation process and the electrochemical and interfacial stabilities of the single-ion conducting block copolymer electrolyte
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