Active materials and electrolytes are widely explored to improve the energy and power density of the batteries. However, polymer binder is the key component for the control of the electrode (composite) interfaces, which include the conductive agent/active materials interface, the conductive agent/current collector interface, and the initial active materials/electrolyte interface, facilitating electron and ion conductivity, retaining the physical structure of electrode and stable cycling improving the performance of the LIB. According to Sugita et al. [1] and the approach based on the thermodynamic work of adhesion, polar binders improve the composite electrode wettability, increasing the ionic conductivity in conventional composites; these showed highest capacity, suggesting that Li⁺ conduction inside the polymeric binder contributes to the enhancement of electrochemical performance.

In this work, we report a serie of binders based on sp³ boron atoms and poly(ethylene glycol) PEG400 bridges and other alcohols. These binders were obtained by polymerization in-situ with lithium tetrametoxyborate (LTMB), on the LiFePO₄ and SP-carbon surfaces (120°C, 12h), after homogenized in methanol by magnetic stirring. The electrodes based on polyborates as binders were processed and compared with electrodes with PVDF as binder, according to the methodology used by G. Guzman et al. [2] In addition, the effect of charge delocalization present in BO₄^- groups, associated to strong electron-withdrawing substituents and their effect on the ionic conductivity in polyborates binders, was studied by electrochemical impedance spectroscopy EIS.

The results suggest that increased in ionic conductivity of the SLICB relative to the PVDF, decreases the charge transfer resistance in LiFePO₄ cathodes. During the process of stabilization of the cell at C-rate of C/10, the cathodes based on PVDF as binder have better capacity, however, it is easily degraded with the increase of the C-rate. In general, the use of SLICB allows improving the load capacity of LiFePO₄ cathodes, mainly at high C-rate values. Besides, they allow for a lower degree of degradation, which is possible to observe in the reversibility tests at low C-rate values, see figure 1.

Figure 1. Rate capabilities collected during 40 cycles for a Li-ion cell at subsequent C-rates of 0.1, 1, 10 and 0.1C. Cell contains a cathode composite comprised: LiFePO₄: SP-Carbon and different polymer binders 86-07-07 %w) and electrolyte of 1 M LiPF₆ in a mixture of PC:EC:DMC (1:1:1 V%).

Acknowledgements This work was supported, GR thanks (project seciti/080/2017) IG thanks CONACYT (proyect 237343) for financial support. G. Guzman is grateful to CONACYT for the scholarship granted to pursue his doctoral studies.

References:

[1] M. Sugita, H. Nakamura, M. Yoshio, IMLB-2006, Abstract No. 486.

[2] G. Guzmán, J. Vazquez, G. Ramos, M. Bautista, I. González, Electrochimica Acta 247 (2017) 451–459