Binder Selection for Copper Oxide Anodes: Why SBR+CMC and LA133 Outperform PVDF in Lithium‑Ion Batteries

Background

Lithium‑ion batteries are ubiquitous in portable electronics, aerospace, and electric transportation, owing to their high specific energy, long cycle life, and compact form factor [1‑5]. Conventional graphite anodes, while inexpensive and stable, are limited by a theoretical capacity of 372 mAh g⁻¹, prompting the exploration of high‑capacity alternatives such as 3d transition‑metal oxides (CuO, Fe₂O₃, Co₃O₄, NiO) [6‑10]. However, these materials suffer from rapid capacity fade driven by mechanical pulverization and SEI instability [8‑10]. Binder chemistry has emerged as a key lever for mitigating these issues. In 2014, our group demonstrated that CuO electrodes with PVDF 301F delivered robust cycling; yet two years later, the same formulation collapsed to <100 mAh g⁻¹ after 50 cycles, highlighting binder sensitivity [11]. Recent studies confirm the importance of binders: SBR+CMC has shown superior cycling and rate performance in TiO₂, ZnFe₂O₄, and Si anodes [12‑16].

PVDF remains the industry standard due to its electrochemical and thermal stability, yet it requires toxic, volatile solvents such as NMP, DMAc, and DMF, which pose environmental and safety challenges [17‑21]. Consequently, the field is shifting toward aqueous binders—CMC, SBR, LA133, PAA, PVA, PEG, PAI—whose solubility in water eliminates hazardous solvents while providing good adhesion and flexibility [22‑31]. Among these, the SBR+CMC combination offers high flexibility, strong adhesion, and excellent cycle stability, making it a prime candidate for metal‑oxide anodes.

Synopsis of chemical structures of polymers introduced in this work

In this study, we systematically examined the performance of CuO anodes prepared with five binders—PVDF HSV900, PVDF 301F, PVDF Solvay5130, SBR+CMC, and LA133—using identical electrode formulations (except for binder). The electrochemical metrics (galvanostatic cycling, cyclic voltammetry, rate capability, EIS) collectively demonstrate the superiority of SBR+CMC.

Experimental

Preparation of Anode Electrode

CuO nanoparticles were synthesized via a chemical reduction route previously reported by our group [11]. For PVDF binders, a slurry of 60 wt % CuO, 10 wt % acetylene black, and 30 wt % PVDF (dissolved in NMP) was cast on copper foil and dried at 80 °C for 5 h. For SBR+CMC, a slurry of 80 wt % CuO, 10 wt % acetylene black, 5 wt % SBR, and 5 wt % CMC (1 % aqueous solution, viscosity >1900 mPa s) was cast and dried at 50 °C for 4 h. For LA133, the slurry composition was 80 wt % CuO, 10 wt % acetylene black, 10 wt % LA133 (water‑soluble) and dried similarly. The active‑material ratio was tuned for each binder to achieve optimal performance.

Cell Assembly and Electrochemical Studies

CR2025 coin cells were assembled in an argon glove box (<1 ppm H₂O/O₂). Metallic lithium served as both reference and counter electrode, with Celgard 2320 as separator. The electrolyte was 1 M LiPF₆ in EC:DMC (1:1 v/v). Galvanostatic charge–discharge tests were conducted on a LAND CT2001A at 0.2 C (0.01–3.00 V vs. Li⁺/Li). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed on a CHI604D workstation (frequency 0.01–100 kHz, 5 mV AC). All measurements followed the protocols outlined in the Supporting Information.

Results and Discussion

Galvanostatic Cycling Performance

PVDF Binder

Figure 2 presents the charge–discharge curves for CuO electrodes prepared with PVDF HSV900, 301F, and Solvay5130 at 0.2 C. While PVDF Solvay5130 offered the highest initial capacity (~870 mAh g⁻¹), capacity retention dropped below 30 % after 50 cycles, with the 301F binder yielding <100 mAh g⁻¹. The poor performance is attributed to the low adhesion strength of PVDF in NMP, leading to electrode delamination and excessive SEI growth [33‑35]. Even varying the PVDF molecular weight could not overcome these limitations.

Charge‑discharge curves of CuO with different PVDF binders (a–c) and cycling performance (d). (a) PVDF HSV900, (b) PVDF 301F, (c) PVDF Solvay5130 at 0.2 C.

SBR+CMC Binder

Using the SBR+CMC blend dramatically improved both capacity and stability. At the optimal 80:10:10 (CuO:acetylene black:SBR+CMC) ratio, the electrode delivered 461.3 mAh g⁻¹ after 50 cycles with 86.9 % retention (Figure 3e). The enhanced adhesion is attributed to the elastomeric SBR providing flexible, spot‑type bonding, while CMC supplies carboxylate groups that form strong interactions with CuO and the copper foil [14, 32]. This binder configuration has also been successful in ZnFe₂O₄ and Si anodes, achieving capacities >1000 mAh g⁻¹ and exceptional cycle life [36].

Charge‑discharge curves with SBR+CMC binder at different CuO ratios (a–d) and cycling performance (e). (a) 70:10:20, (b) 75:10:15, (c) 80:10:10, (d) 90:5:5.

LA133 Binder

LA133, a polyacrylonitrile derivative, also yielded superior results. The 80:10:10 ratio achieved a 99 % retention and 450.2 mAh g⁻¹ after 50 cycles (Figure 4g). LA133 can be applied to both anodes and cathodes, whereas SBR+CMC is best suited for anodes due to its oxidation sensitivity at high potentials. Nevertheless, both binders outperform PVDF in terms of adhesion and electrochemical stability.

Charge‑discharge curves with LA133 binder at different CuO ratios (a–f) and cycling performance (g). (a) 70:10:20, (b) 75:10:15, (c) 77.5:10:12.5, (d) 80:10:10, (e) 85:10:5, (f) 87.5:10:2.5.

Conclusions of Binders

Figure 5 summarizes the comparative performance: both SBR+CMC and LA133 deliver markedly higher capacities and retention than any PVDF variant. PVDF’s weak adhesion leads to rapid delamination and high SEI resistance, while the aqueous binders maintain robust contact and lower charge‑transfer resistance. These findings are consistent with recent reports on MnCo₂O₄ and other oxide anodes, where CMC+SBR achieved >1600 mAh g⁻¹ over 700 cycles [37].

Charge‑discharge curves of CuO with different binders (a–c) and cycling performance (d). (a) PVDF, (b) SBR+CMC, (c) LA133.

Morphological and Structural Characterization

Optical imaging (Figure 6) shows that PVDF‑based electrodes suffered extensive delamination after 50 cycles, whereas SBR+CMC and LA133 maintained intact films. SEM cross‑sections (Figure 7) confirm that the gap between electrode and copper foil is smaller for SBR+CMC (1.4 µm) compared to LA133 (1.8 µm), reflecting superior adhesion and reduced mechanical strain.

Optical image of CuO electrodes before (left) and after (right) charge‑discharge cycles using different binders.

SEM and cross‑section SEM of CuO electrodes using different binders. (a, c) SBR+CMC before cycle; (b, d) after cycle; (e, g) LA133 before; (f, h) after.

Rate Performance

Rate tests (Figure 8) revealed that SBR+CMC achieved the highest specific capacities across all rates (0.2→5.0 C) and fully recovered to 87 % of the 0.2 C capacity when the current was reduced again, outperforming LA133 (71.7 %) and PVDF (61.3 %). This underscores the lower polarization and superior ionic/electronic pathways afforded by the SBR+CMC network.

Rate performance (a) and charge‑discharge curves (b–d) for PVDF, SBR+CMC, and LA133 binders.

Cyclic Voltammetry

CV curves (Figure 9a–c) show that SBR+CMC yields the sharpest redox peaks at 0.85 V (CuO→Cu₂O) and 1.28 V (Cu₂O→Cu), indicating low kinetic barriers. The linear dependence of peak current on the square root of scan rate (Figure 9d–f) confirms diffusion‑controlled processes, with the diffusion coefficient for SBR+CMC‑based electrodes being the highest (Table 1). Lower polarization and higher Li⁺ diffusivity explain the superior cycling performance.

Cyclic voltammograms (left) and peak current vs. √scan rate (right) for PVDF (a, d), SBR+CMC (b, e), and LA133 (c, f).

Electrochemical Impedance Spectroscopy

EIS (Figure 10) shows that SBR+CMC and LA133 yield semicircles of ~50 Ω cm², far lower than PVDF’s >200 Ω cm², indicating faster charge‑transfer and reduced SEI resistance. Table 2 confirms that SBR+CMC electrodes have the lowest bulk resistance (≈200 Ω) compared to PVDF (≈500 kΩ). These impedance metrics correlate with the observed high-rate performance.

Electrochemical impedance spectra of CuO electrodes using different binders after 50 cycles.