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Chemistry in a Li-air battery: Live monitoring with operando NMR

Chemistry in a Li-air battery: Live monitoring with operando NMR

Lithium-oxygen (Li-O2) or lithium-air batteries are possible alternatives to lithium-ion batteries for energy storage. They offer a high specific energy of ~ 3500 Wh kg-1 that is more than ten times higher than current Li-ion batteries. However, Li-O2 batteries are far from mature and have not yet reached their full potential, mainly due to their limited number of charge-discharge cycles. More knowledge about the operation of this type of batteries is needed to improve their performance.

Operando studies, where analytical data are acquired on an operating battery, are essential to obtain a clear view of the reaction mechanisms involved. However, it is a challenge to adapt these techniques to a metal-oxygen battery, since it requires a continuous supply of oxygen. A team from IRAMIS designed a cell with an oxygen gas tank, allowing the operation of a Li-O2 cell inside a magnet of a nuclear magnetic resonance (NMR) spectrometer during the acquisition of spectra. The results allow to detail the electrochemical and degradation mechanisms within an operational Li-O2 battery.

Metal-oxygen (M-O2) batteries have attracted attention in recent years because they offer the highest energy density. Among them, the lithium-oxygen battery theoretically offers a specific energy of ~ 3500 Wh kg-1 in the discharged state, with the combination as active materials of lithium, a light metal, and oxygen, naturally abundant in the atmosphere.

During the discharge of a Li-O2 battery, an electrochemical reaction between Li+ ions and oxygen O2 leads to the formation of lithium peroxide Li2O2. However, the performance of Li-O2 batteries remains rather poor, due to parasitic chemical reactions during charging, involving the electrolyte solutions, singlet oxygen, Li2O2 and carbon electrodes and leading to the formation of the compounds: CO2, Li2CO3, lithium-formate (CHLiO2) and -acetate (CH3COOLi). In situ studies have provided valuable initial information on the real-time chemistry of Li-O2 batteries [1].

Nuclear magnetic resonance (NMR) can provide critical information on the mechanisms involved, particularly on the formation and behavior of amorphous products. Until now it has only been applied ex situ, after operating and the dismantling of the cell. These measurements do not give a realistic view of the operating cell and can lead to biased data and erroneous interpretations, in particular because of the high reactivity of the products formed in Li-O2 batteries, such as singlet oxygen and peroxide. The first developments of operando NMR were mainly applied to metal or metal-ion batteries based on the intercalation or insertion of the charge carrier ion. This had not yet been realized for Li-O2 batteries, due to the difficulty of incorporating an O2 gas electrode in the NMR apparatus.

The NIMBE/IRAMIS team has succeeded in designing a closed system (see figure below) with a cylindrical cell including two compartments, the first being an oxygen reservoir and the second an electrode stack. The first advantage of this closed system is that it can be easily manipulated during NMR experiments, without connection to a gas supply. Another important advantage of this setup, for the type of battery to be studied, is the absence of evaporation of the electrolyte by the gas flow. In the design of the cell, other requirements were also taken into account to allow NMR measurements:

  1. cell geometry adapted to the space available inside the NMR probe head;
  2. absence of metallic materials to avoid any interaction with the strong magnetic field;
  3. robust sealing and compression of the cell for cyclability and durability of the battery;
  4. sufficient mass of electrode material for enhanced measurement sensitivity.
 
Left: picture of the cell for static operando NMR measurements placed in the NMR probe; Center: schematic diagram of the operating principle of a Li-O2 battery with formation of the Li2O2 product from the electrochemical reaction between the Li+ ion, an e– and the O2 gas. Curves: evolution of the 7Li NMR spectra during successive charge-discharge cycles.

After verifying the quality of the chemical process in the newly designed Li-O2 cell, the team observed its operation with a carbon cathode by 7Li NMR (see figure below). The operando measurements provide information both on the lithium anode (see figure below, following the left peak with a chemical shift ~ 250 ppm) and on the evolution of the electrolyte and cathode (peak with a low chemical shift ~ 4 ppm). During the discharge, a decrease of the lithium metal concentration is observed, as well as a contribution of slow diamagnetic Li+ ions. This is mainly attributed to the formation of the expected Li2O2 discharge product.

Upon charging, the lithium metal redeposits on the anode, mainly in the form of “mossy / foamy” lithium and dendrites, but without returning to the initial level. Thus the lithium exchanged for Li2O2 formation during discharge does not reversibly return to the anode during charging. The oxidation of Li2O2 to Li and O2 is therefore not complete and some of the Li is consumed in parasitic processes. This is in agreement with the evolution of the solid phase NMR resonance during charging (right peak, chemical shift ~ 4 ppm, for t=40h), suggesting that irreversible and parasitic products were also formed such as Li2CO3 and Li acetate, which explains the low yield of the cell.

7Li operando NMR spectra for a Li-O2 cell at t = 2 h (beginning of discharge), 18 h (end of the first discharge) and 40 h (end of first charge). NMR peaks on the left show a chemical shift around 250 ppm characteristic of the metallic lithium electrode. Peaks on the right with a small chemical shift (~ 4 ppm) correspond to electrolyte and Li-based products formed on the cathode.

These operando NMR experiments highlight the challenges and obstacles of Li-air batteries, such as the high reactivity of the discharge products and electrolyte, preventing, for now, the commercialization of this technology.

In addition to the Li-O2 battery, this study opens the way for operando NMR analysis of other metal-oxygen batteries such as Na-O2, Al-O2 or Zn-O2, or any electrochemical device requiring a gas supply. As such, the NMR cell design could be adapted to study fuel cells, if a continuous flow of fuels and oxidants is provided within the cell itself.

References:

[1] “Recent progress in applying in situ/operando characterization techniques to probe the solid/liquid/gas interfaces of Li–O2 batteries3
Z. Liang, Q. Zou, Y. Wang, Y.-C. Lu, Small Methods 1 (2017) 1700150.

[2] “Operando NMR characterization of a metal-air battery using a double-compartment cell design”
M. Gauthier, M. H. Nguyen, L. Blondeau, E. Foy, A. Wong, Solid State Nuclear Magnetic Resonance 113 (2021) 101731.
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Contact CEA-IRAMIS : Alan Wong (NIMBE/LSDRM) ; Magali Gauthier (NIMBE/LEEL).

Luc BARBIER