Effect of Substituted Styrene‐Butadiene Rubber Binders on the Stability of 4.5 V‐Charged LiCoO₂ Electrode

Abstract

The combination of synthetically modified styrene‐butadiene rubber (SBR) and sodium carboxymethyl cellulose (CMC) was studied as an aqueous binder for LiCoO2 composite electrodes under high‐voltage operation. Methyl methacrylate (MMA) and 2‐vinylpyridine (VP) substituted SBR copolymers were synthesized and examined as functional polymer binders. The electrolyte‐solvent uptake of the binder‐polymer film was increased by the introduction of the MMA units, but the solubility of the polymer in the solvent remained negligibly low. Hard‐X‐ray photoelectron spectroscopy revealed the formation of a relatively thick passivation layer on the LiCoO2 surface with MMA‐substituted SBR and CMC binder, which suppressed the self‐discharge of 4.5 V‐charged LiCoO2. The capacity retention during battery cycling experiments was similar for the LiCoO2 electrodes with MMA‐modified and unmodified SBR binders. While the effect of VP‐substitution was not significant, incorporation of the MMA unit enhanced the stability of the charged LiCoO2 electrode and provided improved self‐discharge performance.

Sulfated Alginate as an Effective Polymer Binder for High-Voltage LiNi0.5Mn1.5O4 Electrodes in Lithium-Ion Batteries

“…As phosphorus is contained only in the electrolyte, the surface of the LNMO particles is concluded to be covered by both the binder and electrolyte decomposition products. This conclusion is consistent with previous reports on the use of high-solvent-uptake binders, wherein the binders covering the cathode surface were shown to decompose and form a passive layer by anodic decomposition of the electrolyte. ,, …”

“…This behavior is attributed to the fact that SO 3 -ALG exhibits the proper coating properties by forming the low-resistance passivating layer that suppresses electrolyte decomposition even at high voltages. 55 The foregoing XPS results indicate that the electrolyte decomposition products are accumulated with the number of cycles, because of the insufficient coverage of LNMO by the PVdF and ALG binders. On the other hand, the employment of SO 3 -ALG as binder is effective in forming a stable protective layer on the initial cycle, and decomposition of the electrolyte is suppressed during subsequent cycles.…”

“…This conclusion is consistent with previous reports on the use of high-solventuptake binders, wherein the binders covering the cathode surface were shown to decompose and form a passive layer by anodic decomposition of the electrolyte. 24,45,55 The surface layer on LNMO was further examined by the SOXPES and HAXPES of electrodes after zero (pristine electrode), 5, and 50 cycles. These techniques provide information at depths of several to several tens of nanometers from the surface, respectively.…”

“…To elucidate the origin of the improved cycling and rate performances observed for sulfated binders, we performed EIS measurements and determined the interfacial resistances on the cathode side. LNMO electrode with PVDF binder is known to show increasing interfacial resistance. , Figure shows the Nyquist plots obtained after 10 and 50 cycles at 50 mAh g –1 charging. As the assignment of Nyquist plots for composite electrodes is complicated even in the case of three-electrode measurements, ,− we only discuss the size of the entire capacitive semicircle as an indicator of interfacial resistance.…”

“…As seen in Figure S10 and Table S1, the SO 3 -ALG electrode shows a greater Coulombic efficiency, indicating a suppression of self-discharge compared to with the ALG one. This behavior is attributed to the fact that SO 3 -ALG exhibits the proper coating properties by forming the low-resistance passivating layer that suppresses electrolyte decomposition even at high voltages …”