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...

Dynamic Transport in Li‐Conductive Polymer Electrolytes Plasticized with Poly(ethylene glycol)–Borate/Aluminate Ester

“…In addition, a higher transport number with a lowering of the anion mobility and the salt dissociation was expected for these SPEs due to the Lewis acidity of the group-13 elements, which would capture and trap the anions selectively. [5,6] Recently, we fabricated an all-solid-state LPB with Li j SPE j LiFePO 4 constructions: This LPB showed degradation after tens of stable cycles arising from anion decomposition at the electrode surface at elevated temperature (60 8C). [7] Hence, we infer that the reduced anion mobility (or higher transport number) is attractive, because it suppresses the polarization (or overconcentration) of anions at the electrode j electrolyte interface during steady-state charging (concentration gradients of anions at the interface are reduced by the decreasing relative anion diffusivity at the steady state).…”

Nanosized Poly(ethylene glycol) Domains within Reverse Micelles Formed in CO2

“…[6] Recently, roomtemperature ionic liquids (ILs) have been reported to form nanosized domains in reverse micelles, [7] which have found applications in material synthesis [7c, 8] and chemical reactions. [10] Owing to these special properties, liquid PEGs have been widely used in various fields, such as materials science, [11] chemical reactions, [12] electrochemistry, [13] pharmaceutical industry, [14] and others. Liquid PEG is usually regarded as green solvent, because it is economical, environmentally benign, biocompatible, and its properties are tunable by changing the molecular weight.…”

“…Liquid PEG is usually regarded as green solvent, because it is economical, environmentally benign, biocompatible, and its properties are tunable by changing the molecular weight. [10] Owing to these special properties, liquid PEGs have been widely used in various fields, such as materials science, [11] chemical reactions, [12] electrochemistry, [13] pharmaceutical industry, [14] and others.…”

Analyses of structurally modified quasi‐solid‐state electrolytes using electrochemical impedance spectroscopy for dye‐sensitized solar cells

“…It is explained that the BPPO polymer contacting on the TiO 2 layer shifted the flat band in TiO 2 at photo‐anode to a negative range owing to the basicity of the polymer . In addition, coordination of cations in electrolyte to the BPPO functional groups increased V OC . Lower cation concentration vicinity of the TiO 2 layer owing to the coordination suppresses recombination of I 3 − and electron trapping, thus increasing V OC .…”