Quantifying Reversibility and Irreversibility of Li-Metal Batteries


Researchers have proposed an analytical approach to quantitatively evaluate the reversibility and irreversibility of practical lithium-metal batteries (LMBs).


The method for decoupling and quantifying "active Li" and "dead Li"


New battery technologies and chemistries feature heavily in this column. This is mainly because better batteries are the holy grail of technology research at the moment. The company or group who manage to overcome outstanding problems and bring new technologies to market that will provide large scale storage for renewable energy, make a fast charging and energy dense battery for electric vehicles, or design a small battery that can power sensor clusters for long periods of time, will have the opportunity to dominate the market, just like Microsoft did in the early days of personal computers. The demand for those types of batteries is incredible. We rely mainly on lithium-ion batteries at the moment, but they have drawbacks. Other types of chemistry have the potential to be cheaper, perform better and operate more safely. However, there always have been some aspect of the design that means that they haven’t reached their full potential yet.


One of those types of batteries is lithium-metal battery (LMB) technology. LMB devices use metal lithium as an anode and have excellent energy density. However, the poor electrochemical reversibility of lithium metal anodes has become a bottleneck to improving the cycle life of LMBs. Accurate analysis of the reversibility of lithium metal anodes is important to the development of longer-life LMBs. Due to the excessive "lithium reservoir" in the anode that continuously compensates for the irreversible loss of lithium during cycling, the true reversibility of the lithium metal anode cannot be accurately determined at the moment.


However, this could change due to a work done by a research group led by Prof. LIU Zhaoping at the Ningbo Institute of Materials Technology and Engineering (NIMTE) of the Chinese Academy of Sciences (CAS). The research group has proposed an analytical approach to quantitatively evaluate the reversibility and irreversibility of practical LMBs and have developed a methodology to quantitatively distinguish active from inactive lithium in the cycled lithium metal anode in a practical lithium battery system. This methodology enables the accurate quantification of the electrochemical reversibility of a lithium metal anode.


The methodology works due to the fact that a solid electrolyte interphase film encapsulates "dead lithium" with an organic solvent barrier. The researchers found a way to use hybrids of biphenyl and tetrahydrofuran as selective solvents to physically separate "active lithium" from "dead lithium." After dissolution of the "active lithium" in biphenyl/tetrahydrofuran, the number of lithium ions was measured by inductively coupled plasma-optical emission spectrometry. Additionally, the amount of hydrogen produced by the reaction of residual "dead lithium" and deionized water can be measured by gas chromatography - achieving precise quantification of "active lithium" and "dead lithium".


Based on the mathematical model that the irreversibility of lithium metal anodes grows exponentially with the number of cycles, the researchers quantified the "active lithium" and "dead lithium" contents at different cycles and obtained the key parameters describing the true reversibility of lithium metal anodes. This quantitative analytical methodology can also be applied to actual pouch cells and reveals intrinsic reversible differences in lithium metal anodes under different operating conditions.


A paper based on the study was published in Nature Energy. This study was supported by the National Natural Science Foundation of China and the China Postdoctoral Science Foundation.