Document Type

Article

Date of Original Version

3-7-2015

DOI

10.1149/2.0601506jes

Abstract

A novel carbonate free electrolyte, 1 M lithium difluoro(oxalato) borate (LiDFOB) in 1:1 gamma-butyrolactone (GBL)/methyl butyrate (MB), has been compared to a standard electrolyte, 1 M LiPF6 in 1:1:1 EC/DMC/DEC, and a 1 M LiDFOB in 1:1:1 EC/DMC/DEC electrolyte. The conductivity of 1 M LiDFOB in GBL/MB is higher at low temperature, but slightly lower at higher temperature compared to the standard electrolyte. The 1 M LiDFOB in GBL/MB electrolyte has comparable cycling performance to the standard electrolyte, and better cycling performance than the 1 M LiDFOB in EC/DMC/DEC electrolyte. The reversible cycling performance suggests that the LiDFOB in GBL/MB electrolyte forms a stable anode solid electrolyte interface (SEI) in the presence of GBL. Ex-situ surface analysis of the extracted electrodes has been conducted via a combination of XPS, FTIR-ATR and SEM which suggests that the stable anode SEI results is primarily composed of reduction products of LiDFOB.

The widespread implementation of electric vehicles (EVs) requires further improvements in lithium ion batteries.13 Some of the biggest challenges for lithium ion batteries in EVs are cost, low temperature performance and battery lifetime.2,3 Improvements in the electrolyte can assist in the resolution of each of these problems.1,4,5 Most commercial electrolytes are composed of LiPF6 in a mixture of carbonate solvents.5 However, the high cost and poor thermal and hydrolytic stability of LiPF6 is problematic for the electrolyte.68 In addition, ethylene carbonate (EC) is typically a required component of the electrolyte due to the role of EC in the formation of the solid electrolyte interphase (SEI) on the anode.5,914 Since EC is a solid at room temperature, electrolytes containing EC frequently have poor performance at low temperature.15 Despite the shortcomings of LiPF6 / EC based electrolytes, these formulations have proven very difficult to replace. While there have been significant efforts to develop novel electrolytes with superior performance to LiPF6 in carbonates, there has been limited success. The development of novel solvent systems has been more limited and frequently targeted toward specific problems such as high voltage cathodes, salt solubility, or reactivity issues.1621 The development of novel salts has encountered problems related to salt solubility and corrosion of the aluminum current collector on the cathode.21,22

One of the more interesting and promising alternative salts is lithium difluoro(oxalato) borate (LiDFOB).1,4,23,24 LiDFOB is promising due to good solubility, thermal stability, passivation of the aluminum current collector, stable SEI formation, and potentially lower cost. While there have been a limited number of investigations of LiDFOB as the conducting salt in the electrolyte,4,16,25 there have been several reports of the use of LiDFOB and the related salts lithium bis(oxalato) borate (LiBOB) and lithium tetrafluoro(oxalato) phosphate (LiTFOP) as additives to LiPF6 based electrolytes to form a more stable SEI.1,14,23,2630 There have also been reports of the use of oxalate salts enabling the cycling of PC based electrolytes due to better SEI formation.30 The presence of the oxalate group in the anode thus may enable the use of EC free electrolytes and electrolytes with non-carbonate solvents.

The investigation of LiDFOB has been expanded to include carbonate free electrolyte formulations. Esters and lactones are an interesting alternative to carbonate solvents. Linear esters have been studied as co-solvents due to the high dielectric constants and low freezing points which have been reported to improve the low temperature performance of lithium ion batteries,15 while lactones such as γ-butyrolactone (GBL) have high dielectric constants5 and a very wide liquid temperature range (−43.5 to 204°C). However, the use of GBL as a primary solvent in lithium ion battery electrolytes has been plagued by problems with the stability of the anode SEI.5 Despite the issues with GBL as a solvent in carbonate based electrolytes, GBL has been studied with LiBOB based electrolytes due to the limited solubility of LiBOB in carbonates.18,31 In order to investigate the use of novel electrolyte formulations for lithium ion batteries, a comparative study of three electrolytes has been conducted; a standard LiPF6 electrolyte in 1:1:1 EC/ dimethyl carbonate (DMC)/ diethyl carbonate (DEC) was tested against 1 M LiDFOB in 1:1:1 EC/DMC/DEC and 1 M LiDFOB in 1:1 GBL/MB (MB is methyl butyrate).

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