A High Electrode‐Reaction Rate for High‐Power‐Density Lithium‐Ion Secondary Batteries by the Addition of a Lewis Acid

Abstract

Lithium-ion secondary batteries using polymer electrolytes based on lithium-salt complexes of polyethers have attracted much attention because of their potential for practical applications, such as electric-, hybrid-, or fuel-cell vehicles. [1] Although enhancement of the charge-transfer reaction rate is important to fabricate high-power-density batteries, only a few investigations have been focused on the charge-transfer reaction at the electrode/polymer electrolyte interfaces.[2]Herein we describe a significant enhancement of the charge-transfer reaction rate by addition of Lewis acid to polymer electrolytes. This is, to our knowledge, the first report about the achievement of a high rate by the addition of additives to electrolytes.Recently, we reported that poly(ethylene glycol) (PEG)-borate ester increases the ionic conductivity and transport number of lithium ions or magnesium ions of polymer electrolytes.[3] Since the borate ester groups, which act as Lewis acids, prefer to interact with anions, the increase in the conductivity and transport number is induced by enhancing the dissociation of the salts in polymer electrolytes.[3] These results indicate that the activity of lithium ions in the polymer electrolytes increases on addition of the PEG-borate ester. Therefore, the charge-transfer reaction rate should be enhanced by the addition of the Lewis acid, because the rate is proportional to the activity of lithium ions.[4]Herein, we investigate the influence of the PEG-borate ester on the electrokinetics of the Li + /Li couple in poly-(ethylene glycol) dimethyl ether (PEGDME) based electrolytes. PEGDME solutions of LiCF 3 SO 3 are used as a model of polymer electrolytes, and is similar to the amorphous conducting phase in high-molecular-weight poly(ethylene oxide). To evaluate the charge-transfer reaction rate at the electrode/electrolyte interfaces, the exchange current densities were studied by chronoamperometry with a microelectrode technique. Microelectrodes have several properties that facilitate the electrochemical analyses, such as minimization of the ohmic (IR) drop and charge current, and enhancement of the transport of electroactive ions to the electrode surface by spherical diffusion. [2,5] Chronoamperometry was performed to obtain the exchange current densities. The measured coulombic efficiency of lithium deposition and dissolution was over 90 % in all cases, which means that no significant secondary reaction occurred at the interfaces. The exchange current densities obtained in the electrolytic solutions with various amounts of the PEG-borate ester are summarized in Figure 1. Significant increases in the exchange current densities of the electrolytes were found when the PEG-borate ester was added. Furthermore, the values of the exchange current densities show a maximum at each temperature with a PEG-borate ester content of 25 wt % to the standard solvent, PEGDME, which corresponds to a molar ratio of PEG-borate ester:the anion of almost 1:1. The maximum value at 333 K was 2.88 mA cm À2 whi...

Poly(ethylene oxide)-based electrolytes for lithium-ion batteries

“…The PEG-borate ester showed good thermal stability and high ionic conductivity of $10 À4 S cm À1 at 30 C. The authors also investigated the effect of EO chain length of PEGBE on the ionic conductivity and thermal stability. [485][486][487][488][489][490][491][492][493][494][495] Based on Wakihara's studies, many researchers reported PEGBE-based polymer electrolytes with PVAc, 496 PEO, 305,497 PAN, 498 silica, 499 and crosslinked systems. 500,501 Comb-like borate ether polymer electrolytes (PECBE) bearing EO units both in the main chain and side chain were also prepared, and were expected to be good candidates for use in LIBs.…”

Gel Polymer Electrolytes Containing Anion-Trapping Boron Moieties for Lithium-Ion Battery Applications

“…The t Li+ values of LIB could be increased by introducing single-ion conductor, , ceramic fillers such as SiO 2 , Al 2 O 3 , and TiO 2 , , and Lewis acids to the electrolytes . Especially boron moieties in LIB systems can improve the electrochemical properties because they effectively trap anions in the lithium salts through Lewis acid–base interaction between the vacant p orbital of the boron and basic anions of the lithium salt. − ,− …”

Nanocomposites based on borate esters as improved lithium-ion electrolytes

“…In particular, borate ester based electrolytes have been investigated for lithiumbased battery electrolytes. [9][10][11][12][13][14][15] The anion coordinating effect of the boron atom imparts a high lithium transference number in these electrolytes. Furthermore a low vapor pressure of these electrolytes makes them attractive from a safety view point as well.…”