Compatibility between lithium difluoro (oxalate) borate-based electrolytes and Li1.2Mn0.54Ni0.13Co0.13O2 cathode for lithium-ion batteries

https://doi.org/10.1016/j.jelechem.2018.07.019Get rights and content

Highlights

  • The LiODFB forms a uniform and stable CEI film on the surface of the cathode.

  • The CEI can inhibit decomposition of electrolyte and the side reaction of material in high voltage.

  • The CEI film hinders phase transformation from layered to spinel-like phases.

  • The cells present excellent electrochemical performance with the LiODFB-based electrolyte.

Abstract

Lithium-rich layered oxide is a promising cathode material for high-energy density lithium ion batteries. Generally, it is essential to develop high-voltage electrolyte because electrolyte is one of the key factors that determines the capacity of cathode materials. In this work, lithium difluoro(oxalato) borate (LiODFB) is introduced as a novel lithium-salt for lithium-rich cathodes. The investigation reveals that the LiODFB modifies the surface film and forms a uniform and electrochemical stable cathode electrolyte interface (CEI) on the lithium-rich cathode. The LiODFB-derived CEI layer effectively suppresses severe electrolyte decomposition at high voltages and hinders undesirable phase transformation from layered to spinel-like phases during cycling. Furthermore, the Li1.2Mn0.54Ni0.13Co0.13O2/Li cell with the LiODFB-based electrolyte exhibits high capacity retention of 91.73% after 50 cycles and better rate capability of 195 mAh g−1 at 2 C. The unique function of the LiODFB on the surface chemistry of lithium-rich cathodes is confirmed through X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy analyses.

Introduction

Lithium-ion battery (LIBs) technology has grown promptly due to the shortages of fossil energy sources and the increased environmental pollution [[1], [2], [3]]. However, cycle life, cost and energy density are the major bottleneck to the widespread use of LIBs in vehicle application [[4], [5], [6], [7]]. In an effort to accomplish higher energy density, significant efforts have been directed to expand the operating potential of the cathode material.

However, the commercialized cathode materials are restricted by their low energy density etc. Based on the previous researches, the energy density of current LIBs with LiCoO2 [8], LiMn2O4 [9], LiFePO4 [10], and LiMO2 [11] (M = Ni,Co,Mn) cathode materials are all below 300 W h kg−1. In addition, these materials are typically charged to ~4.5 V or below vs. Li/Li+, and cannot meet the increased requirements in EVs and energy storage. Recently, lithium-rich cathode materials, represented by xLi2MnO3·(1-x)LiMO2 (M = Mn,Ni,Co,Fe,Cr, etc.), as one of the most hopeful cathode materials for future generation of LIBs have been studied intensively due to their high reversible capacities (>250 mAh g−1) and high operating voltages (≥4.8 V vs. Li/Li+) [[12], [13], [14]]. In addition, the cost of this materials is lower than that of LiCoO2, the most widely used cathode for lithium ion battery up to date, due to the abundance of manganese.

Despite their excellent capacities and high energy density, several elementary challenges for lithium-rich layered oxides cathode materials still need to be solved before large-scale commercial application. Generally, the Li2MnO3 can be activated and release O2 in the time of initial charge process. The released O2 can be translated into oxygen free radicals with high reactive and intensify the undesired electrolyte decomposition on the surface of the electrode materials. Accordingly, the anodic stability of the electrolyte at high voltages may be decreased further. Furthermore, the structure of Li-rich materials would be changed from layer to spinel. As a result, the irreversible phase transformation leads to a poor retention rate of capacity [[15], [16], [17], [18], [19], [20]].

To resolve these issues, the detrimental reactions of the electrolyte on the surface of the high voltage cathode materials were suppressed by various methods that have been proposed. One method is to prepare materials with inert surface coatings, such as metal oxides [21,22], metal phosphates [23], spinel-type materials [24,25] and olivine-type materials [26]. Although this kind of approach has been reported to restrain the oxidation of the electrolyte, it still has problems related to imperfect surface coatings, difficulty in scale up for commercial applications and additional processing cost.

Opportune lithium salts have been considered as another efficient tactics to maintain the interfacial stability of Li-rich cathodes. At present, the mixture of LiPF6 salt and organic carbonate (EC/DEC) is the most essential and widely used ingredient of commercial electrolytes. However, as we all know, the LiPF6 can experience an autocatalytic decomposition into unacceptable products as LiF and PF5. The irreversible reaction with organic solvents such as ethylene carbonate (EC) and the hydrolysis reaction with any trace amounts of water present in the electrolyte occurred because of a strong Lewis acid PF5 [27]. And then, HF can degrade the electrochemical performance because of the lithium-rich material/electrolyte interface reactions during charging/discharging cycling.

Recently, lithium difluoro(oxalato)borate (LiODFB) has been proposed as a potential alternatives for LiPF6 [[28], [29], [30], [31], [32]]. As an interesting and promising alternative salts, LiODFB helps to the formation of stable cathode electrolyte interphase (CEI) film on the surface of positive electrode, which can effectively protect positive material from dissolution and corrosion. Besides, LiODFB exhibits high conductivity and good solubility in carbonate solvents, which can improve Li+ ions transfer rate.

Previous works on Li1.2Mn0.54Ni0.13Co0.13O2 (LR-NCM) focus on preparation and modification of the cathode material, while compatibility with electrolyte is rarely investigated. Herein, we testified the distinct effect of LiODFB as a novel salt-type that can elevate the outstanding cycle and rate performance of lithium-rich cathode. The effect of LiODFB on the surface chemistry of the Li-rich cathodes was revealed by means of X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM).

Section snippets

Synthesis of LR-NCM

LR-NCM was prepared using a citric acid-assisted sol-gel method as follows. Stoichiometric amount of CH3COOLi, Mn(CH3COO)2∙4H2O, Ni(CH3COO)2∙4H2O, and Co(NO3)2∙6H2O were dissolved in 100 mL of deionized water to achieve a solution with Li/metal (Ni, Co, and Mn) molar ratio of 1.05. Excess lithium was added to compensate for loss of Li occurred at high temperature. The mixed solution was added dropwise to 100 mL of a suitable amount of citric acid solution. The molar ration of citric acid to

Results and discussion

The crystal structures of LR-NCM have been characterized by XRD as shown in Fig. 1. Obviously, all the diffraction peaks are consistent well with α- NaFeO2-type unit cells, which shows a symmetrical R3¯m space group. In addition, a weak diffraction peak can be observed around 2θ = 20–25°. This phenomenon can be ascribed to the monoclinic structure of Li2MnO3 with a space group of C/2 m. The results reflect the arrangement of LiMn6 cation in the transition metal layers [34]. In the XRD patterns,

Conclusions

In this work, LR-NCM has been synthesized successfully via a facile chemistry method using a cheap chelating agent of citric acid. Compared with LiPF6, LiODFB is a promising salt-type that match with high-voltage LR-NCM cathode correctly. The results indicate that cells with LiODFB-based electrolyte present superior rate capability and cycling performance at room temperature. Besides, LiODFB played an important role in forming a thin and stable CEI layer on the cathode surface through the

Acknowledgements

This work is supported by the Natural Science Foundation of China (No. 21566021 and 21766017), the Transformation of Scientific and Technological Achievements of Gansu Institutions of Higher Education (No. 2017 D-04), and the Gansu Province Science and Technology Major Project (17ZD2GC011).

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