Eniko S. Zsoldos

This study presents post-mortem analyses of NMC/graphite pouch cells, which exhibit exceptional capacity retention after up to 8 years or 22,000 charge-discharge cycles. Various NMC/graphite cells with different positive electrode compositions and particle morphologies were examined under a variety of cycling conditions, temperatures, C-rates, and upper cut-off voltages. The findings demonstrate that even after extended periods of continuous cycling, the NMC positive electrodes can maintain the integrity of their crystal structure with minimal transition metal dissolution. Additionally, there is minimal degradation of the electrolyte, and the artificial graphite negative electrode materials only show negligible changes. Ultimately, lithium loss at the negative electrode is the only notable aging mode in these cells. Furthermore, with optimized electrolytes and high-quality artificial graphite (AG), this parasitic reaction occurs at a low rate. As a result, lithium-ion cells can achieve remarkably long lifetimes. For example, a single-crystal NMC532/AG cell studied in this research retained 80% of its capacity, positive electrode crystal structure, and morphology, with minimal changes to the electrolyte after 6.8 years of continuous charge-discharge cycling.

The capacity retention of LiFePO ₄ (LFP)/graphite lithium-ion cells at elevated temperatures is strongly influenced by the initial concentration of vinylene carbonate (VC) additive in the electrolyte. To investigate degradation mechanisms with lithium hexafluorophosphate (LiPF ₆ ) contained electrolytes, a series of nominally identical cells containing either 1 wt% or 4 wt% VC were cycled at 55, 70, 85, and 100°C. Cells were terminated at various cycle numbers to assess gas evolution and charge transfer resistance (Rct), followed by disassembly to analyze VC consumption, iron (Fe) deposition on the graphite electrode, and formation of diethyl-2,5-dioxahexane carboxylate (DEOHC). Results were compared to previous data for cells using lithium bis(fluorosulfonyl)imide (LiFSI) electrolytes under similar conditions. For both electrolyte systems, higher VC content enhances capacity retention and suppresses rapid Fe deposition, though it also leads to increased R _ct during cycling. However, distinct differences were observed between LiPF ₆ and LiFSI-based cells with 4 wt% VC in terms of VC consumption, salt consumption, Fe deposition, and R _ct evolution. This work provides a quantitative perspective on the degradation processes of LFP/graphite cells.

With the growing adoption of LiFePO₄ (LFP)/Graphite (Gr) cell chemistry in electric vehicles and grid energy storage, understanding and enhancing its performance under high-temperature conditions has become increasingly critical. In this study, various concentrations of vinylene carbonate (VC) (1% to 5%) were introduced to LFP/Gr pouch cells cycled at 70 °C to evaluate their impact on cell lifetime. Additionally, two different lithium salts, LiFSI and LiPF₆, were investigated. Upon reaching the end-of-life (80% capacity retention), detailed post-mortem analyses were performed, including qNMR and GC-MS to determine changes in electrolyte composition, micro X-ray fluorescence (μXRF) to quantify Fe deposition on the negative electrode, and electrochemical impedance spectroscopy (EIS) to assess charge-transfer resistance. Various LFP/Gr pouch cells were evaluated, encompassing four distinct graphite types, two LFP surface area variations, and two cell form factors. The results demonstrate that higher VC concentrations significantly improve cell lifetime, reduce Fe dissolution, and suppress electrolyte degradation pathways, including the formation of ethyl methyl carbonate (EMC) and dimethyl 2,5-dioxahexane carboxylate (DMOHC). Furthermore, while LiFSI-based LFP/Gr cells exhibit enhanced performance in certain metrics, they suffer the production of gas at 70 °C, which can be mitigated by incorporating LiPF₆ salt.

Previous findings for LiFePO₄(LFP)/Graphite lithium-ion cells showed that the rate of capacity loss during cycle testing at elevated temperatures is highly dependent on the initial amount of vinylene carbonate (VC) in the electrolyte. To understand the potential mechanism behind this, a series of nominally identical cells with different VC content was prepared and tested for various cycle numbers at 55, 70, 85, and 100 °C. Cells were stopped at different cycle numbers to measure gas evolution and charge transfer impedance (R_ct) and then disassembled to track VC consumption, Fe deposition on the negative electrode, and diethyl-2,5-dioxahexane carboxylate (DEOHC) generation inside the cell. Higher concentrations of VC improves capacity retention, suppresses the generation of DEOHC and the rapid deposition of Fe on the negative electrode but is accompanied by more rapid gas evolution and R_ct increase. Once all of the VC is consumed VC containing cells, Fe deposition accelerates and DEOHC appears in the electrolyte. These findings verify previous suggestions that lithium alkoxide accelerates Fe dissolution and the subsequent Fe deposition on the negative electrode of LFP/Graphite cells.

Lithium iron phosphate (LFP) battery cells are ubiquitous in electric vehicles and stationary energy storage because they are cheap and have a long lifetime. This work compares LFP/graphite pouch cells undergoing charge-discharge cycles over five state of charge (SOC) windows (0%–25%, 0%–60%, 0%–80%, 0%–100%, and 75%–100%). Cycling LFP cells across a lower average SOC results in less capacity fade than cycling across a higher average SOC, regardless of depth of discharge. The primary capacity fade mechanism is lithium inventory loss due to: lithiated graphite reactivity with electrolyte, which increases incrementally with SOC, and lithium alkoxide species causing iron dissolution and deposition on the negative electrode at high SOC which further accelerates lithium inventory loss. Our results show that even low voltage LFP systems (3.65 V) have a tradeoff between average SOC and lifetime. Operating LFP cells at lower average SOC can extend their lifetime substantially in both EV and grid storage applications.

In this work, pouch cells of spinel LiMn₂O₄ (LMO) / artificial graphite (AG) were tested under varying conditions to investigate degradation mechanisms. Mn dissolution from the positive electrode and deposition onto the graphite negative electrodes immediately after formation was found to be significantly suppressed by operating cells at −10 °C during the formation cycle and limiting the upper cutoff voltage. A formation cycle at an elevated temperature of 70 °C greatly increases the Mn deposition and gas generation from electrolyte reduction after just a single cycle and had long term effects at increasing lifetime gassing and Mn deposition. The cold formation advantage disappeared once all cells were cycled at 40 °C, with similarly terrible cycle life (60–200 cycles) regardless of formation conditions. Studying fully lithiated graphite (LiC₆) extracted from LMO cells in an isothermal microcalorimeter (IMC), we found that the parasitic heat flow associated with electrolyte reduction and graphite delithiation increased for anodes with significant Mn deposition. Therefore, Mn deposition on the negative electrode causes cell failure by compromising the anode passivation, increasing lithiated graphite-electrolyte reactivity and thereby accelerating the lithium inventory loss due to electrolyte reduction on the Mn sites on the negative electrode.

This study explores the impact of simple electrolyte additives on the performance of layered oxide/hard carbon sodium-ion pouch cells. The cycle life of these cells between 2.0 and 3.8 V is assessed at various temperatures (20, 40, and 55 °C) with different solvent systems based on ethylene carbonate, diethyl carbonate, and dimethyl carbonate. A particular challenge in these cells is gas generation at high temperature. Pouch bag experiments which separate the charged electrodes to measure their gas generation from reactions with the electrolyte show that hard carbon generates no gas, but the sodium layered oxide produces large amounts of gas. Isothermal microcalorimetry corroborates these results with parasitic heat flow measurements of pouch bags and full pouch cells. A crosstalk mechanism is revealed which lowers gas generation and reduces parasitic heat flows in full cells. The electrolyte additives prop-1-ene-1,3-sultone, sodium difluorophosphate, and 1,3,2-dioxathiolane-2,2-dioxide (DTD) are effective at reducing gas generation and heat flow from the positive electrode. They also reduce self-discharge in elevated temperature storage tests. Overall, 1 M NaFSI in EC:DMC (15:85) with 2% DTD is the best electrolyte for the sodium-ion pouch cells in this work. Eventually, the performance of these cells is compared to optimized LiFePO₄/graphite cells.