A new method for quantitative analysis of true reversibility of lithium anode metal for lithium batteries

On September 29, 2022, Liu Zhaoping’s team from the Ningbo Institute of Materials of the Chinese Academy of Sciences published a research paper entitled “Quantifying Reversible and Irreversible Lithium in Practical Lithium-Metal Batteries” in the journal Nature Energy, proposing a new method for quantitatively analyzing the true reversibility of metal lithium anodes in practical lithium batteries. It provides a more accurate and scientific basis for the failure analysis and life prediction of lithium metal batteries.

The corresponding authors are Zhou Xufeng, Liu Zhaoping, Shirley Meng; The first authors are Deng Wei, Yin Xue, and Wurigumula Bao.

Lithium metal secondary batteries with lithium metal as the negative electrode are an important development direction of the next generation of high specific energy battery technology. However, the poor electrochemical reversibility of the metal lithium anode has become the biggest bottleneck restricting the improvement of the cycle life of lithium metal batteries, and the accurate analysis of the reversibility of the metal lithium anode is an important prerequisite for the development of long-life lithium metal batteries. Since the pre-existing metal lithium in the lithium anode continuously compensates for the irreversible lithium loss during the lithium battery cycle, the true reversibility of the lithium anode cannot be known by prior technical means. Recently, Liu Zhaoping’s team at the Power Lithium Battery Engineering Laboratory of the Ningbo Institute of Materials, Chinese Academy of Sciences developed an innovative method to distinguish between the active and inactive lithium of the lithium anode in the actual lithium battery system and quantify it separately, which for the first time realized the accurate quantification of the electrochemical reversibility of the metal lithium anode.

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Figure 1: Flowchart of the quantitative analysis method for the reversibility of the anode of lithium metal.

In this study, the SEI membrane wrapped in dead lithium has the property of blocking organic solvents, and the mixed solvent of biphenyl/tetrahydrofuran is innovatively used to realize the physical separation of active lithium and dead lithium, and the lithium content of active lithium dissolved in biphenyl/tetrahydrofuran after dissolution and gas chromatography (GC) are measured by inductively coupled plasma emission spectroscopy (ICP), respectively, and the amount of hydrogen generated after the reaction of dead lithium and water is measured by inductively coupled plasma emission spectroscopy, so as to achieve accurate determination of the content of active lithium and dead lithium. On this basis, based on the mathematical model of the relationship between the irreversibility of the metal lithium anode and the exponential growth of the number of cycles, the quantitative analysis values of the active lithium and dead lithium content after different cycle turns are fitted, and the key parameters reflecting the true reversibility of the metal lithium negative electrode are further analyzed.

Figure 2: Experimental method for distinguishing and quantitatively analyzing active lithium and inactive lithium.

This quantitative analysis method is applied to the actual soft pack battery system, and the difference in the reversibility of lithium anode intrinsic properties under different operating conditions is compared and analyzed. For example, at the beginning of the cycle, there is almost no difference in the apparent test results such as discharge capacity and coulomb efficiency of lithium metal soft-pack batteries under different external pressures, while the use of this quantitative analysis method clearly reveals significant differences in the true reversibility of lithium anodes at different pressures. For another example, the method is used to quantitatively compare the reversibility of the lithium anode at the discharge magnification of C/5 and C/2, and combined with the morphological characterization of the lithium anode, it breaks through the limitations of the usual prescriptive observation, and gives the structural evolution model of the metal lithium anode with quantitative indicators at different magnifications (Figure 3), which provides a new basis for deducing the dynamic process of battery failure.

Figure 3: Schematic diagram of the evolution of lithium metal anode structure during charge and discharge cycles at different magnifications of C/5(a) and C/2(b) by applying quantitative analysis methods and combined with topography characterization.

The research work provides a quantitative comparison tool for in-depth understanding of the influence of different design parameters and working conditions on the reversibility of the metal lithium anode, and can realize the life prediction of lithium metal batteries, thus providing a more accurate and scientific basis for the rational design and performance optimization of lithium metal batteries, which is of great significance for improving the research level of lithium battery technology in the new system and accelerating the breakthrough of the cycle life bottleneck of high specific energy lithium batteries. (Source: Science Network)

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