Multifunctional Additives for High-Energy-Density Lithium-Ion Batteries: Improved Conductive Additive/Binder Networks and Enhanced Electrochemical Properties

Abstract

Cylindrical-type cells have been widely adopted by major battery and electric vehicle manufacturers owing to their price competitiveness, safety, and easy expandability. However, placement of electrodes at the core of cylindrical cells is currently restricted because of high electrode curvature and the lack of specialized electrodes and electrode materials. Here, we report the realization of highly flexible high-energy-density electrodes (active material loading of >98.4%) that can be used at the cores of cylindrical cells. The improved properties result from the introduction of a multifunctional poly(melamine-co-formaldehyde) (MF copolymer) additive, which yields a relatively more fluidic and well-dispersed slurry using only 0.08 wt %. MF copolymer-mediated formation of completely wrapped CNT/PVDF networks on LiCoO 2 (LCO) and accompanying contact enhancement between LCO and carbon nanotubes (CNTs) resulted in an increase of electrical and mechanical properties of the two types (composites with or without collectors) of electrodes compared with those of additive-free electrodes. Flexibility tests were carried out by rolling electrodes onto cylinder substrates (diameters of ca. 1 and 10 mm); this process resulted in relatively lower resistance changes of the MF copolymer-containing electrodes than for the reference electrodes. In addition, capacity retention after 100 cycles for cells with and without MF copolymers was approximately 10−20% better for the samples with the MF copolymer than for those without. CNT/PVDF networks with MF copolymers were proven to induce a relatively thin and stable cathode electrolyte interface layer accompanying the chemical bond formation during cycling.

Ultrahigh loading dry-process for solvent-free lithium-ion battery electrode fabrication

“…Presumably, this is due to the sheet-to-point contact mode present in DPCEs as opposed to the line-to-point contact mode prevalent in SCEs 35 . The PVDF binder facilitates the interweaving of the dispersed MWNT subunits and creates a web structure that tightly secures the active material within its matrix 36,37 . This contact mode is more effective, especially in high-loading electrodes because an increase in the number of composite sheet layers enhances the structural rigidity of the electrode body and allows the mechanical stability to be maintained as the electrode thickness increases 38 .…”

Charge Transport Limitations to the Power Performance of LiNi0.5Mn0.3Co0.2O2 Composite Electrodes with Carbon Nanotubes

“…significantly improved cyclability of LiCoO 2 -based electrode with a high active mass loading of 16 mg.cm −2 and prepared with CNT, if an additive, poly(melamine-co-formaldehyde) copolymer was added in tiny amount, less than 0.1 wt%. 22 This additive favored the disentanglement of the CNTs in the NMP-based electrode slurry and the formation of a very uniform CNT/PVdF network wrapped on the LiCoO 2 particles, which resulted in improved mechanical strength of the electrode as well as the formation of a thinner cathode−electrolyte interface (CEI). Here, the CNTs appear much more entangled and forming clusters than a net on the surface of the NMC clusters.…”

“…20 The validation of the interest of CNTs as a conductive additive requires their evaluation in highly loaded and dense electrodes, based on conventional active materials to be relevant for EV application. 21,22 For example, Kim et al, studied LiNi 0.6 Mn 0.2 Co 0.2 O 2 (NMC 622)/PVdF/CB-CNT based electrodes with 23 mg.cm −2 active mass loading, 3.6 g.cm −3 density, and 96:2:2 wt% composition. Interestingly, they found the best performance for the 1 wt% CNT-incorporated electrode.…”

Crystallization behavior of polyvinylidene fluoride (PVDF) in NMP/DMF solvents: a molecular dynamics study

“…7,8 In this case, the aggregation issue of CNTs is controlled by adding hydrogenated NBR (HNBR) which can be used as a dispersant. 9 While NMP is the commercially preferred solvent up to now, it is a toxic compound that can damage the skin, eyes, and respiratory system, which has led to legislation limiting the use of NMP to solutions below 0.3% by Registration, Evaluation, Authorization and Restriction of Chemicals (REACH). [10][11][12] As organic solvents are expensive and consume lots of energy in the recovery process, research on replacing organic solvents with aqueous solutions has been recently conducted.…”

“…7,8 In this case, the aggregation issue of CNTs is controlled by adding hydrogenated NBR (HNBR) which can be used as a dispersant. 9…”