A Highly Reversible 5V-Class Li2CoPO4f/Graphite Battery
Authors: Ko Seongjae , Yamada Yuki , Yamada Atsuo 
Published: February 2020

Introduction

To increase the energy density of the Li-ion rechargeable batteries, a use of high-voltage cathode materials as well as their stable operating systems are of importance. However, the issues represented by degradation of the cathode materials, electrolytes, and other cell components have obstructed the development of a high-voltage system over past decade. Here, we report that concentrated LiBF4 in a mixed solvents, propylene carbonate (PC) and fluoroethylene carbonate (FEC), allows a stable cycling 5 V-class Li2CoPO4F/graphite full-cell for more than 600 cycles. In the presentation, the optimized cell components with special attention to the compatibility between the carbon conductive agents and electrolytes, will be discussed.

Method

The LiBF4-based electrolytes were prepared by mixing a certain amount of salt with solvents in an Ar-filled glove box. The Raman measurement was conducted to study the solution structure of the electrolytes. The linear sweep voltammetric (LSV) was performed to evaluate the anodic limits of the electrolytes with three-electrode cells consisting a Pt plate as a working electrode and Li metals as reference and counter electrodes. The carbon compatibility with the electrolytes was estimated using cyclic voltammetric (CV) with two-electrode cells of carbon electrode and Li metal. The transition metals deposited on the cycled graphite electrodes in the LCPF/graphite full-cells after charge and discharge test were analyzed by an Energy-dispersive X-ray spectroscopy (EDS).

Results

As a result of LSV measurement, the anodic current started to flow from 5.8 V (vs. Li/Li+) in the concentrated 1:1.8 LiBF4:PC/FEC (5/5,n/n), which was far higher than that in the commercial electrolyte of 1M LiPF6 EC/DMC (1/1,v/v). The Raman spectroscopy analysis presented that the concentrated LiBF4 PC/FEC electrolyte had an aggregate (AGG)-predominant solution structure. An upshift of bands corresponding to the anion and free solvent molecules was observed with an increase of salt concentration, denoting strengthened coordination of anion and solvents with Li+. Therefore, it was considered that the highest occupied molecular orbital (HOMO) level of the anion and solvents was decreased since the Li+ ion functioned as a strong Lewis acid that resulted in partial electron donation from the anion and solvents to the Li+ ion.
Moreover, it was revealed that the main causes of cell deterioration at a high potential were (i) decomposition of electrolyte, (ii) anion intercalation into the carbon additives, and (iii) transition metal dissolution from cathode material. Especially, the carbon conductive agents with a low graphitization degree was selected to increase carbon compatibility with the electrolyte. It was because the anion intercalation occurred easily to the carbon additives with a high graphitization degree, leading to the damage of the graphene layers by continuously exposing the sites where were highly reactive with the electrolyte. Also, it was obvious that an intensified coordination between the anion and Li+ in the highly concentrated electrolyte helped to impede the anion intercalation reaction.
The concentrated LiBF4 PC/FEC electrolyte and the optimized cell design including carbon conductive agents with a low graphitization degree were applied to a 5 V-class Li2CoPO4F/graphite system. As a result, a stable charge and discharge for more than 600 cycles was obtained. Besides, it was revealed that the concentrated electrolyte contributed to the stable operation of the high voltage battery by suppressing the transition metal dissolution from the cathodes. In conclusion, on the basis of the various design factors as noted above; (i) improvement of oxidation stability, (ii) impeding the anion intercalation, and (iii) suppression of transition metal dissolution, we succeed to obtain a stable reversible operation of the 5 V-class Li2CoPO4F/graphite full-cell for the first time.

Figure 1