Ultra High-Precision Studies of Degradation Mechanisms in Aged LiCoO₂/Graphite Li-Ion Cells

Abstract

Twenty LiCoO 2 /Graphite cells with ages between 0 and 12 years having different cycling histories but identical chemistry and construction were obtained from Medtronic Inc. These cells presented a unique opportunity to learn about aging mechanisms in Liion cells. The cells were studied using the ultra-high precision chargers (UHPC) at Dalhousie University, electrochemical impedance spectroscopy (EIS) and differential voltage versus capacity (dV/dQ vs Q) measurements. Even after 12 years of operation at 37 • C, the cells still retained 80% of their initial capacity and their coulombic efficiency was 0.99985 when measured at C/20 and 40 • C. The capacity loss of the aged cells could be explained by loss of lithium inventory through growth of the solid electrolyte interphase (SEI) at the negative electrode which led to impedance increase. There is no evidence of any active material loss due to electrical disconnect in these cells suggesting the cells have excellent electrodes. A low upper cutoff voltage (4.075 V) is crucial to the long lifetime of these cells due to electrolyte oxidation reactions at the positive electrode, revealed by the UHPC experiments. Comparing similar cells that were cycled to either 4.075 or 4.175 V showed that those cycled to higher voltages under the same conditions failed significantly sooner. These results suggest that electrolyte additives that reduce electrolyte oxidation at the positive electrode, will prolong the cycle and calendar life of these Li-ion cells.

Studies of the Capacity Fade Mechanisms of LiCoO2/Si-Alloy: Graphite Cells

“…The time dependence was set to be proportional to the square root of time as reported by several researchers. [86][87][88][89][90][91] The cycle number dependence can come from the SEI repair at the surface of the Si-alloy particles due to the volume changes upon lithiation and delithiation. In these fits, the possibility of lithium plating was ignored since no plating was observed in cells cycled at C/2.…”

A Review of Existing and Emerging Methods for Lithium Detection and Characterization in Li‐Ion and Li‐Metal Batteries

“…Finally, some of the Li is also lost irreversibly to side reactions at either electrode with reduced and oxidized electrolyte species on the anode (solid electrolyte interphase, SEI) and cathode (cathode electrolyte interphase, CEI), respectively. [18] Here, we refer to the Li lost to such reactions as inactive Li. Thus, we define Li detection techniques as having two broad goals:…”

“…Finally, some of the Li is also lost irreversibly to side reactions at either electrode with reduced and oxidized electrolyte species on the anode (solid electrolyte interphase, SEI) and cathode (cathode electrolyte interphase, CEI), respectively. [ 18 ] Here, we refer to the Li lost to such reactions as inactive Li. Thus, we define Li detection techniques as having two broad goals: Detection of Li in the battery, separation of irreversible Li from reversible Li, and correlating it to the performance of the battery. Characterization of the irreversible Li (spatial heterogeneity or Li microstructure, morphology and chemistry once Li loss has occurred) to understand the underlying degradation mechanisms. …”

Differential thermal voltammetry for tracking of degradation in lithium-ion batteries

“…Therefore, a more practical approach is to implement diagnostic techniques that do not necessarily require the use of a reference electrode. These include: Electrochemical Impedance Spectroscopy (EIS) [5,8,[17][18][19][20][21][22], Slow Rate Cyclic Voltammetry (SRCV) [5,8], differential voltage (dV/dQ) [15,16,[23][24][25][26] analysis, differential capacity (dQ/dV) [19,[27][28][29][30][31][32] analysis and statistical methods [33,34]. Whilst no single technique provides all the required information to decouple physical mechanisms of operation in a whole cell, their use in combination can often provide additional insight.…”

“…Here conclusions about the degradation processes occurring were alluded to by fitting the dV/dQ data to half-cell measurements of fresh electrodes. Fathi et al [23] also demonstrated the use of dV/dQ with other techniques including EIS and coulombic efficiency measurements to probe the degradation processes in Lithium Cobalt Oxide (LCO)-graphite cells.…”