Electrochemical Impedance Analysis for Li-ion Batteries (& economy a bit).

Electrochemical Impedance Analysis for Li-ion Batteries | LinkedIn

I. Main Issue

(1) Equivalent circuit elements

(a) Anode (e.g., graphite)

1. A solvation/desolvation resistance at high frequencies (*4), and a diffusion resistance in the liquid electrolyte that fills the void space in the electrode at low frequencies (Warburg impedance) in series.

2. A capacitance of the Solid-Electrolyte Interphase (SEI) film in parallel with the above-mentioned resistance components (*5).

3. The rate determining step is the lithium-ion diffusion in the liquid electrolyte that fills the void space in the electrode. Note that the Warburg impedance is represented as the distributed constant circuit (*2).

• Lithium ion diffusion in the intercalation materials: With the enough amount of lithium ions near the graphite active-material particles, a graphite particle can respond in a faradaic manner at, e.g., the C-rate of 600C (*1), thus the lithium ion diffusion in the intercalation materials is fast enough.

(b) Cathode (e.g., lithium transition-metal complex oxides)

1. A solvation/desolvation resistance at high frequencies (*4) and the Warburg impedance at low frequencies in series.

2. A capacitance of an Cathode-Electrolyte Interphase (CEI) film in parallel with the solvation/desolvation resistance (*5).

3. The rate determining step is the lithium-ion diffusion in the liquid electrolyte that fills the void space in the electrode. Note that the Warburg impedance is represented as the distributed constant circuit (*2).

• Lithium ion diffusion in the intercalation materials: With the enough amount of lithium ions near the cathode active-material particles, and with the enough electron paths, e.g., a LiCoO2 particle can respond in a faradaic manner, e.g., at 360C, a LiFePO4 particle or a LiMn2O4, particle e.g., at 36,000C etc. (*1), thus the lithium ion diffusion in the intercalation materials is fast enough.

• The impedance of the cathode component is highly dependent on its state of charge (SOC); this can help the analysis. The high-frequency cathode component usually lies at lower frequencies than the anode high-frequency component. The high-frequency cathode impedance often becomes a bit larger than the anode counterpart, needless to say, depending on the conductive material weight ratio. The CEI component overlaps with the anode SEI component.

(c) Electrolyte

1. An electrolyte resistance, which fills the void space in the separator, in the highest frequencies (*3).

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Supplementary

*1: Lithium ion diffusion in the intercalation materials:

- With the enough amount of lithium ions near the graphite active-material particles, a graphite particle can respond in a faradaic manner at, e.g., the C-rate of 600C, thus the lithium ion diffusion in the intercalation materials is fast enough.

- With the enough amount of lithium ions near the cathode active-material particles, and with the enough electron paths, e.g., a LiCoO2 particle can respond in a faradaic manner, e.g., at 360C, a LiFePO4 particle or a LiMn2O4, particle e.g., at 36,000C etc., thus the lithium ion diffusion in the intercalation materials is fast enough.

1. Microvoltammetric Studies on Single Particle Voltammetry of LiNiO2 and LiCoO2. In situ Observation of Particle Splitting during Li-ion Extraction/Insertion.

2. High Resolution Cyclic Voltammograms of LiMn2-xNixO4 with a Microelectrode Technique.

3. High-Speed voltammetry of Mn-doped LiCoO2 using a microelectrode technique.

4. Effect of Hetero-contacts at Active Material Conductive Additives on Lithium Intercalation/Deintercalation of LiCoO2.

5. Architectures of Positive Electrodes for Rapid Charging/Discharging Performances of Lithium Ion Secondary Batteries.

6. Electrochemical performance of single Li4Ti5O12 particle for lithium ion battery anode.

7. Express Charging/Discharging Lithium Ion Secondary Batteries.

*2: But only with the enough amount of lithium ions near the active-material particles.

1. Express Charging/Discharging Lithium Ion Secondary Batteries.

2. The electrode porosity in which an electrolyte solution is reserved is, e.g., 30%, then the cathode density becomes 3.5 g/cm^3. The weight ratio of the positive active material (theoretically 5 g/cm^3) is, e.g., 94%, then after 50% deintercalation (e.g., 136 mAh/g for the reaction, LiCoO2 -> 0.5 Li+ + Li0.5CoO2), 16.8 mmol/cm^3 of lithium ions are required in order to re-intercalate to become LiCoO2. The reserved Li+ in the 1 mol/L of the electrolyte solution, only 0.3 mmol/cm^3 (1.8% of 16.8 mmol/cm^3) is available; thus, in order to refill the Li+ ion into Li0.5CoO2, the offshore Li+ which is reserved in the separator that has only 0.3-0.4 mmol/cm^3 of Li+ (40% of porosity is assumed). When the electrode thickness is 100 um and the separator thickness is 25 um, the offshore Li+ can only supply 0.6% of 16.8 mmol/cm^3. Only 2.4% of Li+ is available in the electrolyte solution. Under the rocking-chair scheme, this is OK but at a relatively low C rate. However, when the response speed of the two active materials are different, the enough amount of the Li+ is not supplied to the relatively high-response active material accompanying Li+ deficiency in the electrolyte solution (concentration polarization).

3. The higher porosity (the more Li+ in the electrode), the reaction occurs in a more homogeneous manner.