The lithium-ion battery provides high-performance energy storage, enabling energy to be efficiently stored and delivered on demand. It is this type of battery that is widely used for the rechargeable batteries of portable electronic devices, such as mobile phones1. The reliable efficacy demonstrated by lithium-ion batteries as effective energy storage devices has resulted in this becoming the battery type of choice to power electric cars2. With the marked increase in electric car production to achieve global emission targets and protect the environment, demand for lithium batteries has soared.
Lithium-ion batteries comprise a negative graphite electrode and a positive intercalated lithium electrode separated by a suitable electrolyte. Lithium ions move from the negative electrode through the electrolyte to the positive electrode to provide energy, and the reverse occurs during recharging. With the increased production of lithium batteries of considerably greater size to support the mass introduction of electric vehicles, there has been a sudden surge in demand for the component chemicals. Since the increase in battery production is designed to reduce the transportation sector's carbon footprint, it is important that raw materials used in the manufacture of lithium-ion batteries can be sourced in a sustainable manner2.
The latest research investigates the potential of obtaining electrolytes suitable for use in lithium-ion batteries from biomass and agricultural waste to reduce the depletion of natural resources.
Commercial Lithium Batteries
The electrolytes used in commercial lithium-ion batteries typically comprise lithium hexafluorophosphate (LiPF6) dissolved in organic-carbonate-based solvents. These solvents are volatile and flammable and thus represent a serious chemical hazard under harsh conditions and can cause fires3.
Furthermore, LiPF6 is thermally unstable and decomposes at about 343K in an organic-solvent based electrolyte, producing hydrogen fluoride that is toxic and corrosive. There is, therefore, the risk that hydrogen fluoride reacts with cell components releasing transition metals from the positive electrode and corrodes the current collectors. The heat generation and thermal runaway that, in addition to detrimentally affecting battery performance, can contribute to water and soil pollution with the potential for harming and human health during recycling4.
Consequently, there is a need to replace the large fluorine and flammable organic solvent content of lithium-ion batteries to improve the safety and performance of next-generation batteries, especially now that numerous large batteries of this type are entering routine circulation. With this in mind, many new salts have been tested as battery components, but the majority of them were too unstable for thermal and electrochemical applications5.
However, aromatically stable lithium salts, having high thermal stability and being readily soluble in organic solvents or ionic liquids have a great potential in battery applications6. Ionic liquids are thus emerging as potential safe alternatives for use as electrolytes in lithium-ion batteries.
Ionic Liquid Electrolytes
Ionic liquids are molten salts at room temperature that are not flammable and have high thermal stability and good ionic conductivity. They are thus promising safer substitutes for the volatile organic-solvent-based electrolytes currently used in lithium-ion batteries7. The cations identified as being the most effective cations for ionic liquids destined for use in lithium-ion batteries are tetraalkylammonium, cyclic aliphatic quaternary ammonium and imidazolium7
Most recently, research is being conducted to produce these fluorine-free electrolytes from renewable sources8. A recent study prepared fluorine-free electrolytes using anions obtained from biomass and agricultural waste produced on a large scale; 2-furoic acid was produced from lignocellulosic biomass. It is hoped that such a process will contribute to the development of renewable electrolytes for batteries.
The structure of the lithium salt and electrolytes produced were characterized by nuclear magnetic resonance (NMR) spectroscopic analysis using a Bruker Ascend Aeon WB 400 spectrometer. NMR diffusion and relaxation measurements were obtained through pulsed gradient spin echo-NMR using a Bruker Avance III spectrometer. The attenuated total reflection Fourier transform infrared (ATR-FTIR) spectra of samples were recorded using a Bruker IFS 80v spectrometer equipped with a deuterated triglycine sulfate (DTGS) detector and diamond ATR accessory.
The electrolytes were found to have a Tonset higher than 568 K and acceptable ionic conductivities in a wide temperature range. The pulsed field gradient NMR analysis confirmed that the lithium ion interacted strongly with the carboxylate functionality in the electrolyte and diffused slower than other ions over the whole studied temperature range. The interaction of the lithium ion with the carboxylate group was also confirmed by NMR and Fourier transform infrared spectroscopy.
The transference number of the lithium ions increased with increasing lithium salt concentration. Linear sweep voltammetry suggested lithium underpotential deposition and bulk reduction at temperatures above 313 K.
These data highlight the feasibility of developing thermally and electrochemically stable fluorine-free electrolytes in a cost-effective, environmentally friendly and sustainable process. It is hoped that this research marks the start of meeting the challenges associated with the safety, recyclability, accessibility, affordability and service life of lithium-ion batteries.
Bruker’s unparalled technology portfolio is used at various positions within the Li-Ion Battery supply and value chains. This includes NMR and FTIR spectrometers for novel electrolyte fomulations as described here. But it also spans from investigating the phenomenon of metallic Li deposition at anode materials which is called Li-plating. The key technology used there is Electron Paramagnetic Resonance or EPR2. Solid-State Magic Angle Spinning (MAS) NMR spectroscopy is used to understand ion-mobility during the charging and dis-charging processes of batteries. Finally, sensitivity enhanced cryogenically cooled CP-MAS probes can be used to identify and quantify valuable trace elements in the black mass produced during battery recycling processes. Novel recycling processes assisted by means of magnetic resonance analysis will be vital in applying circular economy concepts to the battery industry.