https://rdcu.be/dCTb

 The current practice to evaluate and monitor battery performance is by capacity measurement and analysis. Capacity transactions (Q) are measured at the electrode contacts through manipulations of current or voltage in the charge or discharge process. Based on the law of charge neutrality, the amount of capacity is inferred to the same amount of lithium (Li) inventory being transduced in the reaction. However, the Li concentration and its distribution in the electrode is polarized due to slow ionic conduction in the electrode. Therefore, the capacity evaluation cannot reveal the actual electrode potential over the electrode thickness and at the electrode-electrolyte interface during polarization in the charge or discharge process. It is important to note that the most relevant information that should be monitored to understand the electrochemical phenomenon and its reaction is the electrochemical potential at the electrode-electrolyte interface during the polarization. Unfortunately, this critical potential and its variation cannot be measured directly by an electrochemical technique under the polarizing dynamics even with a three-electrode configuration, unless the electrode is a highly conductive electronic conductor without an ionic concentration field distributed over the bulk of the electrode.

Here, we showed a technique (Top Figure) to properly determine such a potential at the electrode-electrolyte interface and use the information to track active Li inventory transaction at the interface to reveal the most relevant state of a Li battery, akin to a fuel gauge for an engine. We used a matrix of 12 Li-Li_xNi_0.8Mn_0.1Co_0.1O₂ (NMC 811) cells that are of different cell formulations, types, and test conditions as case studies to demonstrate the concept, approach, and verification method to rationalize the analytic results and subtle differences in the measurements. The most notable case in the illustration is the reconciliation of the data obtained from a cell formation experiment of a pristine Li-NMC 811 cell using the galvanostatic intermittent titration technique (GITT) as the formation protocol. Fig. 1 shows how this reconciliation is accomplished. In Fig. 1a, the original data is displayed to show the entire GITT formation process. In Fig. 1b, the equilibrium potential (V_eq) of all intermittent states in the charge and discharge segments is plotted against the measured capacity. Discrepancy between the two segments is notable to a degree that is beyond possible experimental uncertainty. To resolve this disparity, the theoretical capacity of NMC 811 (Q_Th = 275.5 mAh/g) and the incremental Q_Th per fraction of the Li content (x in Li_xNMC 811) in the cathode (dQ_Th/dx) were used to rectify the discrepancy. A cathode utilization efficiency U was introduced to reconcile Q (the capacity measured at the electrode contacts during polarization in which a Li inventory distribution existed) and (Q_Th × Δx), which is the anticipated theoretical specific capacity increment based on the Li inventory transaction Δx at the interface. The Li inventory transaction Δx at the interface should be estimated by the stepwise V_eq variation to determine the Li inventory increment in the electrode. In practice, we used the measured dQ/dV in each segment and dQ_Th/dx to transform the V_eq versus Q profiles in Fig. 1b to the corresponding V_eq versus x profiles, as shown in Fig. 1c.

The reduction of the unified V_eq versus x profile provided a universal thermodynamic framework to analyze data obtained from various cells in the matrix that comprises variations in formulations, cell types, and testing protocols and conditions for a consistent cross-platform comparison to rationalize subtle differences among the cells and test results shown in Top Figure, if the polarization potential can be properly removed from the measured cell voltage to give V_eq. We further applied the same technique to analyze cell degradation in cycle aging and rationalize the results from an irregular and irrational dataset to demonstrate the capability and merit of this Li inventory tracking technique.

Since this technique can mitigate and reduce interferences from cell formulations and experimental manipulations, by tracing four key variables (Q, x, U, and Q_Th × Δx) from the formation to end-of-life with a thermodynamic framework, we accurately rationalized subtle differences in Li inventory transaction and utilization in the cells. The approach promises precise battery engineering, evaluation, failure analysis, and risk mitigation. The capability of Li inventory tracking could be applied to applications from cell design optimization, fabrication, to battery management to improve battery performance and reliability.

Authors: 

Meng Li¹, Yulun Zhang¹, Hui Zhou², Fengxia Xin², M. Stanley Whittingham², and Boryann Liaw^1,z

¹ Energy Storage and Electric Transportation, Idaho National Laboratory, Idaho Falls, ID 83415, USA. ² The NorthEast Center for Chemical Energy Storage, Binghamton University (SUNY), Binghamton, NY 13902-6000, USA

^z Corresponding Author E-mail Address [boryann.liaw@gmail.com]