More than Just a Binder: Versatile Block Copolymer Enhances the Electrochemical Performance of a Nickel-Rich Cathode

Nickel-rich layered cathodes (for example, NCM811) holdgreat promise for practical applications in high-energy density lithium-ionbattery technology. However, a nickel-rich cathode has poor cyclicstability because of the structure degradation induced by repeatedcycles and excessive formation of thick cathode-electrolyteinterface (CEI) layers. Poly(vinylidene difluoride) (PVDF) is a typicalbinder used to form a positive electrode, which nevertheless lacksthe functionality to suppress structure degradation and control CEIlayer formation. A new concept is proposed in this work to use a multi-functionalblock copolymer of poly(ether imide)-block-poly(dimethylsiloxane) (PEI-b-PDMS; abbreviated as PEMS) as thebinder for the nickel-rich cathode. Unlike the conventional PVDF binder,the PEMS binder is capable of inhibiting crack formation within theNCM(811) particles to retard structure degradation over cyclesbecause of its elastomeric feature. The PEMS binder also enables tuningthe composition of the CEI layer and inhibits the formation of overlythick CEI layers because the carbonate solvent molecules are excludedfrom the solvation sheath by the PEMS binder. Enhanced electrolyteaffinity is exhibited by the electrode with the PEMS binder as evidencedby the contact angle measurement. Improved carbon black dispersionwithin the electrode is achieved by the PEMS binder because the PEIaromatic chain has strong affinity toward the carbon black particles.Both the cycling and rate performance of the NCM811 cathodeis improved with the PEMS binder compared to the PVDF binder. In detail,the capacity retention is increased from 63.9 to 80.6% after 200 cyclesat 0.5 C (1 C = 205 mAh g(-1)) when the PVDF binderis replaced by the PEMS binder. The rate performance at current densitiesof 2 and 5 C is increased significantly due to enhanced electrochemicalkinetics.