The Effects of Internal Pressure Evolution on the Aging of Commercial Li-Ion Cells

Abstract

Utilizing a custom developed chamber, the internal pressure evolution of a commercial 2.6 Ah, LiFePO 4 //Graphite lithium-ion battery (LIB) was studied throughout elevated rate lifecycle testing. Verification tests confirmed the custom test chamber and puncture procedure resulted in minimal changes to the LIB performance. Capacity measurements and analysis confirmed the battery's accessible capacity faded 20% after 500 cycles as a result of the increase in DC Equivalent Series Resistance (ESR). Electrochemical Impedance Spectroscopy (EIS) measurements and modeling analysis also established that minimal material change occurred within the LIB during lifecycle testing and the decreased capacity is believed to primarily result from the increase in battery ESR. Analysis of the battery's internal pressure evolution during the baseline discharges yielded information elucidating apparent electrode volume changes during the 1 C discharge. A strong statistically significant correlation was identified between the 1C molar density delta and the battery capacity fade. Differential capacity analysis was used to examine the correlation of the behavior with the de-intercalation process. The pressure data and molar density analysis indicated a reduction in the amount of lithium de-intercalated during discharge as a result of the rise in ESR and thus a measurable shift in the volume as the LIB ages.Over the last two decades, the demand for energy dense electrochemical storage devices has grown exponentially in response to the proliferation of portable electronics. This demand has expanded beyond energy density to include the need for power density as applications such as hybrid electric vehicles, full electric vehicles, aircraft backup power, grid-tie energy storage, and renewable energy emerge. While there are many electrochemical storage devices that work well in either energy or power applications, LIBs have led the way in combining both energy and power density making them the chemistry of choice for these demanding applications. These storage systems are typically driven by performance, cost, and safety. Due to these driving factors, an innovative evaluation of LIB life and failure prediction is needed. The complexity of LIBs creates a challenge to this evaluation and an investigation of the aging process further complicates the pursuit. A comprehensive review of the mechanisms of aging in LIBs was conducted for the FreedomCAR research initiative, and reports that the 'Capacity decrease and power fading do not originate from one single cause, but from a number of various processes and their interactions. Moreover, most of these processes cannot be studied independently and occur at similar timescales, complicating the investigation of aging mechanisms. 1 Numerous studies have examined the causes for capacity fade in LIBs and the result indicates that capacity fade is linked to chemical and structural changes within the battery. Electrolyte decomposition occurs contributing to the growth of passivation layers at the electrode...

A Physically-Based Equivalent Circuit Model for the Impedance of a LiFePO4/Graphite 26650 Cylindrical Cell

“…This is believed to be related to the expansion/contraction of the lithiated/delithiated LFP or C particles. LFP particles are known to shrink by approximately 6.8 vol% with delithiation 58,59 and C particles expand 10.3 vol% with lithiation. 48 The volume reduction of two adjacent LFP particles could highly influence the pores dimension (since the pore dimension is smaller than the dimension of the LFP particles), resulting in the observed significant drop of R ion,L after delithiation.…”

Mechanical Behavior during Electrochemical and Mechanical Deactivation of an Aged Electrode in a Lithium‐Ion Pouch Cell

“…Microstructural changes such as phase transition accompanied by volume expansion result in an abrupt intensive stress increase, and in particular, shear stress at the phase boundary results in structural damage and the fracture of the electrode morphology. The loss of active materials causes the host lattice regression and the degression of the ion transport ability, which results in capacity fading eventually The loss of electrical connectivity: electrons can also be considered as a reactant for the electrode reactions.…”

“…Thus, any interruptions to the pathway for lithium or lithium ions can cause the inefficient utilization of the active materials and the degradation of the capacity. These interruptions of the electron transportation can be summarized as: hindrance of the electron transport inside the electrode because of high electric resistance; precipitation of thick SEI films to lead to the blockage of the contact of Li ions with electrons and considerable shear stress at the electrode surface; particle fracture, which may increase the electrical resistance because of the loss of material connection; gas generation, which imposes a similar effect as a thick surface layer or particle fractures, if the generated gas stays within the electrode or aggregates as a layer on the electrode surface or in the electrolyte; and loss of electrical contact between the electrode materials and current collector caused by binder degradation or impact from mechanical stress . Electrical connectivity between different components of the cells is easily injured by a mechanical impact.…”

“…Thed amage of the active materials can, therefore,e voke the deterioration of the cell as well. Thep otential damage is:e lectrolyte decompositionb ecause of side reactions, for instance, the growth of SEI layers, [7] solventc o-intercalation, gas evolution, and reduction and oxidationo fs olutions pecies at the anode and cathode,r espectively; [8][9][10] the loss of the transition metal at the cathode,w hich includes the dissolution of the transition metal ions into the electrolyte and the subsequent side reaction with electrolyte, or reduction to metallic clusters at the anode; [2] and irreversible structural change because of lithium insertion. Microstructural changes such as phase transition accompanied by volume expansion result in an abrupt intensive stress increase, [2,9] and in particular, shear stress at the phase boundaryr esults in structural damage and the fracture of the electrode morphology.T he loss of active materials causes the host lattice regressiona nd the degression of the ion transport ability,w hich results in capacity fading eventually.…”

The Effects of Defects on Localized Plating in Lithium-Ion Batteries

“…19,3,20,21 The resulting mechanical deformation of the separator is understood to cause transport restrictions through a pore-closure mechanism, 22,14,23 which can occur in a spatially localized manner due to non-uniform mechanical stresses arising from the cell's architecture 24 as well as spatial variations in local electrode structure and mechanical properties. 10,25 * Electrochemical Society Student Member.…”