Binder Chemistry to Realize High Capacity Li-/Na-Ion Batteries

Li-ion batteries are most attractive among commercial rechargeable batteries to achieve higher energy storage.  By properly selecting lithium insertion materials and electrolyte, we can further design lithium-ion batteries having higher energy / power, longer cycle life, safety, or cost friendliness.  As is widely known, cobalt is a minor-metal as well as lithium, therefore, rechargeable batteries which are free from minor-metals and operable at high voltage of > 3 volts under ambient conditions are attractive to achieve much low-priced batteries.  From this motivation, we first succeeded in the demonstration of 3 volt class Na-ion batteries operable at room temperature [1].

For the higher energy Li- and Na-ion batteries, we need higher capacity electrode materials and high voltage.  However, there is a dilemma because the higher capacity uptake of Li and Na insertion inevitably leads to larger volume change, which accelerates capacity fade.  To overcome this dilemma, we have studied advanced polymeric binders for the powdery composite electrodes.  In this study, we will introduce our recent achievements on the polymeric binders for high capacity electrodes in Li- and Na-ion batteries.

Li-ion batteries: Binders for Si-based electrode

Although silicon-based negative electrodes have high theoretical capacity, they suffer from capacity fade due to the huge volume change by repeated lithiation between Si and Li₁₅Si₄.  Electrode reversibility of powder Si-based electrodes is significantly improved by using poly(acrylic acid) (PAA) and its salt, poly(sodium acrylate) (PANa) as a binder [2].  Binder plays a key role to improve their mechanical, morphological, and SEI properties of the negative electrodes.  Since polyacrylate is calboxylate polymer, the polymeric conformation is highly influenced by neutralization degree of calboxylate.  This leads to the difference in the electrode performance associated with the neutralization degree [3].  Furthermore, we design crosslinkage of PAA [4] and utilize natural polymers; branched polysaccharides as binders for Si-graphite composite electrodes [5].

Na-ion batteries: Binders for Sn and P electrodes

For Na-ion electrodes, acceptable cycle life of hard carbon negative electrode is succeeded by careful selection of the electrolyte solution [1].  Furthermore, electrolyte additive of fluoroethylene carbonate further improves the cycle life of the Na-ion batteries.  By exploiting electrochemical reversible formation of intermetallic Na-Sn phases, we succeeded stable charge and discharge cycle for Sn composite electrode with the polyacrylate binder in electrolyte with the additive, which shows 400 – 700 mAh g^-1 due to the formation of Na₁₅Sn₄ alloy [6].  Furthermore, instead of Sn, elemental phosphorus electrodes are prepared with the polyacrylate binder, and they shows reversible three-electron redox of P/P^3- by electrochemical sodium uptake.  As a result, the phosphorus-PANa composite electrode can deliver reversible capacity over 2000 mAh (g of phosphorus)^-1 [7].

To realize the piratical use of new binders, we will further discuss the importance not only of negative electrodes but also of positive electrodes and electrolyte to improve the cycle-life and cost-friendriness for large-format Li-ion and Na-ion batteries.

 

References

[1] S. Komaba et al., Adv. Funct. Mater. 2011, 21, 3859.

[2] N. Yabuuchi et al., Adv. Energy Mater. 2011, 1, 759, and S. Komaba et al., J. Phys. Chem. C 2012, 116, 1380.

[3] Z.-J. Han et al., Energy Environ. Sci. 2012, 5, 9014.

[4] Z.-J. Han et al., ECS Electrochem. Lett. 2013, 2, A17.

[5] M. Murase et al., ChemSusChem 2012, 5, 2307.

[6] S. Komaba et al., Electrochem. Commun. 2012, 21, 65.

[7] N. Yabuuchi et al., ChemElectroChem in press, DOI: 10.1002/celc.201