Designing Electrolytes for High Voltage Dual-Ion Batteries


Student thesis: Doctoral Thesis

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Award date9 Jun 2021


Lithium-ion batteries (LIBs) have become the leading electrochemical energy storage technologies in various fields including portable electronics, battery electric vehicles and energy storage stations. Despite the appreciable advantages of high energy density and long lifetime, the grid/utility applications of LIBs are limited due to low power density, safety, cost and eco-friendliness concerns. Thus, various types of battery systems have been developed to meet the demands for sustainability, low cost, high-power characteristics and fast charging capability, among which dual-ion battery (DIB) is a highly interesting candidate. In contrast to the rocking-chair type cells like LIB which relies on the cations (i.e., Li+) for charge transport upon charging/discharging, DIB exhibits a different working mechanism involving simultaneous intercalation of anions into the cathode and cations into the anode during charge, and simultaneous extraction of both ions during discharge. Compared to LIBs, the higher working voltage of above 4.5 V on the graphite cathode is beneficial for enhancing energy density of DIBs, meanwhile, it also leads to continuous side reactions such as undesirable electrolyte decomposition and structural deterioration of the graphite material. In addition, the electrolyte plays a significant role in the cell performance (e.g., the cell voltage, reversible capacity, energy/power density) for DIBs since it not only serves as charge carrier for ions transport, but also act as active material to supply active ions. Therefore, it is of great significance to enhance the DIBs performance by designing electrolytes and to make clear the underlying mechanism. Herein, the progress of our work based on DIB electrolytes will be introduced systematically; major challenges and future work from both the scientific point of view and with view on the commercial applications will also be discussed. 

This research started with systematically exploring the effects of three linear carbonate solvents, that is, dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC), on the insertion/extraction behavior of PF6- anion into/from graphite cathodes. Our results revealed that the kinetics of PF6- anion intercalation/deintercalation process depend strongly on the type of electrolyte solvent. Specifically, when using EMC-based electrolyte, the graphite||Li cell exhibits the lowest onset potential for anion insertion, the highest reversible capacity, the smallest voltage polarization, further leading to the best cycle stability, lowest self-discharge rate and even the best rate capability over other two electrolytes. Even after counting for self-discharge, EMC-based electrolyte still enables the highest Coulombic efficiency (CE). X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) measurements confirm the features of cathode electrolyte interface (CEI) layer constructed on graphite surface rely heavily on the solvent, and a thinner and more stable CEI layer formed with EMC electrolyte further facilitates the electrochemical performances of anion storage. This finding emphasizes the necessity to understand the interaction between solvents and active material surface, and to enhance DIB performance by designing the interface.

Although EMC exhibits improved cell performance over DMC and DEC, the low CE and short cell lifetime are still far from satisfactory. Herein, via the introduction of a solvent-type additive, fluoroethylene carbonate (FEC, 5% by volume) into the baseline electrolyte (3 M LiPF6-EMC), a robust and stable CEI layer is constructed on the surface of graphite cathode, effectively suppressing parasitic reactions at high voltage, protecting the graphite electrode from structure deterioration and enabling a highly reversible PF6- de-/intercalation. Within the voltage range of 3-5.1 V, the graphite cathode with this protective CEI displays an extremely long cycle life of 5000 cycles with 85.1% capacity retention at average CE of ~99.0%, in sharp contrast with a short calendar life of <200 cycles obtained with baseline electrolyte. Moreover, the FEC-induced CEI exhibits a remarkable rate capability with 93.3% capacity retention at 3 A g-1 (30C), along with largely reduced self-discharge rate. This work reveals that introducing electrolyte additive is an effective method to ameliorate the interfacial stability of graphite cathode and offers insights into the relationship between CEI characteristics and anion intercalation behaviors.

The above work demonstrated that suitable solvent-type additive is effective for enhancing the long-term cycle stability of DIBs. In the past few years, several lithium salt-type additives have been widely investigated in LIBs to promote reversible Li+ cation insertion. However, the role of salt-type additive in manipulating the CEI layer on graphite cathode and its effects on anion de-/intercalation performances has never been reported. Here, the introduction of an optimal amount of ~0.5% lithium difluoro(oxalate) borate salt into the electrolyte forms a robust and durable CEI in situ on the graphite surface, which enables remarkable cycling of the graphite||Li battery with 87.5% capacity retention after 4000 cycles at 5 C. The reduced activation energy and promoted diffusion kinetics for anion intercalation enable an ultrafast charging capability of 88.8% at 40C, achieving high-power of 0.4-18.8 kW kg-1 at energy densities of 422.7-318.8 Wh kg-1. Taking advantage of this robust CEI, a graphite||graphite full battery achieves superior power density and energy density up to 7.8 kW kg-1 and 179.8 Wh kg-1, respectively. The full battery also shows a long cycling life of over 6500 cycles with 92.4% capacity retention and an average CE of ~99.4% at 1 A g-1, which is superior to other dual-graphite (carbon) batteries in the literature. This work sheds light on the strategy of regulating CEI on graphite cathode and provides a promising approach for developing dual-graphite batteries with eco-friendliness, superior lifetime and high-power capability. 

As have been extensively employed in DIBs in the literature and in our above works, utilizing highly concentrated electrolytes has been identified as an effective strategy to improve the electrolyte compatibility with high working voltage (>5.0 V). Though, the substantially increased viscosity with the higher salt concentration above 3M causes poor wettability of the separator and electrode by the electrolyte, leading to poor rate capability and under-utilization of the full battery performance. Besides, the increase in salt concentration raises cost of the battery. These factors ultimately impede the practical applications of DIB. Hence, designing a low-concentration electrolyte (~1 M) that has excellent compatibility with the graphite cathode that facilitates the stable anion de-/intercalation at high voltage is of vital significance for the future developments of DIBs. We demonstrated a dilute fluorinated electrolyte possessing excellent stability towards both high-voltage graphite cathode and the Li-metal anode. It consists of 1 M LiPF6 salt in a mixture of fluoroethylene carbonate/3,3,3-fluoroethylmethyl carbonate (FEC: FEMC, 3:7 by volume). Through the density functional theory (DFT) simulation and electrochemical measurements, we found that 1 M LiPF6-based FEMC and FEC/FEMC electrolytes exhibit superior oxidation stability up to 5.2 V, while 1 M LiPF6-EMC electrolyte shows the highest degree of electrolyte decomposition. Apart from the electrolyte compatibility with the cathode, battery performance is also strongly affected by the parasitic reaction of electrolyte on the Li metal anode. Though FEMC electrolyte exhibits excellent compatibility with graphite cathode at 5.2V, the cell experiences an abnormal fast capacity decay over 250 cycles. This is due to excessive reaction between FEMC electrolyte and the Li metal, causing a continuous growth of surface layer on Li, as evidenced by the rapid increase in overpotential and interfacial resistance of the Li||Li cell. After adding FEC as a co-solvent, FEC/FEMC electrolyte can suppress dendrite growth on the Li metal electrode upon cycling by forming a dense SEI layer, enabling a superb cell lifetime of 5000 cycles with 95.8% capacity retention and a fast-charging rate capability with capacity retention of 91.8% for the graphite||Li cell even at 50C. The electrolyte strategy presented here offers guidelines for further designing electrolyte formulations in battery systems with aggressive chemistries.