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

Abstract

Nickel-rich layered cathodes (for example, NCM811) hold great promise for practical applications in high-energy density lithium-ion battery technology. However, a nickel-rich cathode has poor cyclic stability because of the structure degradation induced by repeated cycles and excessive formation of thick cathode–electrolyte interface (CEI) layers. Poly(vinylidene difluoride) (PVDF) is a typical binder used to form a positive electrode, which nevertheless lacks the functionality to suppress structure degradation and control CEI layer formation. A new concept is proposed in this work to use a multi-functional block copolymer of poly(ether imide)-block-poly(dimethyl siloxane) (PEI-b-PDMS; abbreviated as PEMS) as the binder for the nickel-rich cathode. Unlike the conventional PVDF binder, the PEMS binder is capable of inhibiting crack formation within the NCM811 particles to retard structure degradation over cycles because of its elastomeric feature. The PEMS binder also enables tuning the composition of the CEI layer and inhibits the formation of overly thick CEI layers because the carbonate solvent molecules are excluded from the solvation sheath by the PEMS binder. Enhanced electrolyte affinity is exhibited by the electrode with the PEMS binder as evidenced by the contact angle measurement. Improved carbon black dispersion within the electrode is achieved by the PEMS binder because the PEI aromatic chain has strong affinity toward the carbon black particles. Both the cycling and rate performance of the NCM811 cathode is 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 cycles at 0.5 C (1 C = 205 mAh g–1) when the PVDF binder is replaced by the PEMS binder. The rate performance at current densities of 2 and 5 C is increased significantly due to enhanced electrochemical kinetics.