전기수력학적 방법을 이용한 PVA 나노구조가 적용된 리튬이온전지용 Si/C음극의 제조

Abstract

Silicon (Si) is considered to be a promising anode material for next-generation lithium ion batteries (LIBs) due to its extremely large capacity of 4,200 mAh/g (Li4.4Si phase). However its large volume change during charging and discharging leads to a severe crack in the electrode and rapid capacity decrease. Fortunately, a size dependence of fracture was discovered and it turned out that there is a critical size of ~150 nm for avoiding fracture in Si nanoparticles. Meanwhile, a binder is homogeneously mixed with active materials to withstand the volume change but large volume expansion drops its cohesiveness. Polyvinyl alcohol (PVA) is known to form a hydrogen bonding with partially hydrolyzed silicon oxide layer on Si nanoparticles. For this reason, PVA replaces existing binders like PVDF, SBR/CMC in the Si based LIBs but the cracks in the overall structure of the electrode caused by large volume change still happens. To overcome this binder problem, we have introduced electrohydrodynamic methods (electrospray/electrospinning). By using electrospraying method, PVA nanoparticles were added to make adhesive pores through the drying process to withstand the expansion of Si nanoparticles. By using electrospinning method, the resulting nanofibrous PVA structures were expected to stably weave the active materials. Stresses from the large volume expansion of Si nanoparticles could be spread out and the loss of the electrical contact from the contraction of Si nanoparticles could be eliminated by the elasticity of nanofibrous PVA structures. Accordingly, we have confirmed that the capacity retention of Si-based LIBs using electrospun PVA matrix is much higher compared to the electrospraying method and the conservative method (only dissolving in the slurry); the 25th cycle capacity retention ratio based on the 2nd cycle was 37% for the electrode with electrospun PVA matrix, compared to 22, 27, and 8% for the electrodes with the electrosprayed PVA, PVdF and PVA binders, respectively.