Measuring Li⁺Inventory Losses in LiCoO₂/Graphite Cells Using Raman Microscopy

Abstract

The contribution from loss of Li + inventory to capacity fade is described for slow rates (C/10) and long-term cycling (up to 80 cycles). It was found through electrochemical testing and ex-situ Raman analysis that at these slow rates, the entirety of capacity loss up to 80 cycles can be explained by loss of Li + inventory in the cell. The Raman spectrum of LiCoO 2 is sensitive to the state of lithiation and can therefore be leveraged to quantify the state of lithiation for individual particles. With these Raman derived estimates, the lithiation state of the cathode in the discharged state is compared to electrochemical data as a function of cycle number. High correlation is found between Raman quantifications of cycleable lithium and the capacity fade. Additionally, the linear relationship between discharge capacity and cell overpotential suggests that the loss of capacity stems from an impedance rise of the electrodes, which based on Li inventory losses, is caused by SEI formation and repair. Lithium ion batteries (LIB) provide the highest energy densities compared to other rechargeable battery types.1 For this reason, they are being pursued for usage in advanced applications such as electric vehicles and storage of intermittent renewable energies (solar, wind, etc.). Despite their promise, degradation caused by both chemical and physical mechanisms diminishes LIB performance. Chemically, formation of a solid electrolyte interphase (SEI) on the surface of both the graphite anode and the LiCoO 2 cathode accompanied by other side reactions between the electrolyte and the electrode surface 2 results in the loss of cycleable Li + and thus capacity. These surface films also increase electrode impedance, which can shorten the time for the cell to reach cutoff voltages and affect rate capability of the cell. 3,4 Capacity loss can also originate due to physical mechanisms. For example, stress may accumulate in electrode materials during lithiation and delithiation owing to the large volumetric alterations induced by these processes. 4,5 Volumetric expansion can be up to 10% in graphite and ∼1.6% in LiCoO 2 .6,7 Such expansion and the induced stress can, in turn, lead to fracturing of active particles, cracking and reforming of the SEI, and loss of contact among electrode components (i.e., active material, polymeric binder, and current collectors). 4 Taken together, these chemical and physical degradation mechanisms induce capacity fade with repeated cycling that curtail the lifetime of the battery.Beyond identification, it is necessary to assess the relative influence of individual mechanisms to the totality of capacity fade. 3,8,9 Specifically, comparisons between the relative impact of several mechanisms-primary active material loss (Li + inventory), secondary active material loss (LiCoO 2 and/or graphite), and increased internal cell resistance (caused by surface films)-remain the topic of continued investigation.3 Such effort is complicated by the dependence of these mechanisms to several parameters as it has been...

Ex situ and operando study of LiCoO2 thin films by Raman spectroscopy: Thermal and electrochemical properties

“…This figure shows a sudden downshift of the Raman peak wavenumber that must be due to a quick modification of the whole structure. It means that the change of the structure during Li deinsertion is sudden and not progressive as reported in previous studies . One explanation could be that the Li removing drives to an increase of the stress that is suddenly relaxed by a change of the structure.…”

“…These bands have been attributed to oxygen vibrations involving mainly O─Co─O bending (E g ) and Co─O stretching (A 1g ) vibrations. Raman downshift of the two bands is observed when x is decreased . For the cubic spinel phase (LT) LiCoO 2 , four bands are observed at around 450, 485, 591, and 607 cm −1 .…”

“…Raman downshift of the two bands is observed when x is decreased. [15,16] For the cubic spinel phase (LT) LiCoO 2 , four bands are observed at around 450, 485, 591, and 607 cm −1 . Furthermore, Raman spectroscopy is able to detect the secondary product Co 3 O 4 even as a trace.…”

Ageing and Water-Based Processing of LiFeMnPO4 Secondary Agglomerates and Its Effects on Electrochemical Characteristics

“…This shifts the capacity balancing of anode to cathode towards values >1 with respect to the reversible capacity. A capacity ratio of anode/cathode >1 allows a complete (de-)lithiation of the cathode and avoids safety-critical dendrite formation and lithium plating [18][19][20][21].…”

Performance Improvements of Cobalt Oxide Cathodes for Rechargeable Lithium Batteries

“…Thinner films could protect the electrodes from HF corrosion. But when the film thickness increases, the resistance to the migration of Li-ions increases [19,20]. Studies [21][22][23][24][25] demonstrated that during charging/discharge cycling, the growth of SEI film increases the impedance of the batteries.…”