Unwanted redox shuttles can lead to self-discharge and inefficiency in lithium-ion cells. This study investigates the generation of a redox shuttle in LFP/graphite and NMC811/graphite pouch cells with common alkyl carbonate electrolyte. Visual inspection of the electrolyte extracted after formation at temperatures between 25 and 70°C reveals strong discoloration. Such extracted electrolytes with intense red and brown color show relatively large shuttling currents in Al/Li coin cells. Two weight percent of vinylene carbonate is effective at preventing the redox shuttle generation as indicated by the absence of electrolyte discoloration and shuttling current. Ultra-high precision coulometry demonstrates that the presence of the shuttle molecule during cycling of LFP/graphite and NMC811/graphite pouch cells leads to significant charge endpoint capacity slippage and coulombic inefficiency. A brief constant voltage hold at 4.2 V can eliminate the shuttle molecule.
Part II of this 2-part series examines the impact of various graphite materials on NMC811 pouch cell performance using Ultra-High Precision Coulometry (UHPC), isothermal microcalorimetry, and in-situ stack growth. A simple lifetime projection of the best NMC811/graphite cells as a function of operating temperature is made. We show that graphite choice greatly impacts fractional fade, while fractional charge endpoint capacity slippage was largely unchanged. We show that an increase in 1st cycle efficiency due to limited redox-active sites, which is favourable for minimizing Li inventory loss, is concomitant with an increase in negative electrode charge transfer resistance. Further, we demonstrate that cells with competitive artificial graphites (AG) have a lower parasitic heat flow (~0.060 mW/Ahr at 40oC) compared to cells with natural graphites (NG), and that the cells with the AG materials had minimal stack thickness change with cycling. Finally, we model SEI growth for NMC811 cells limited to 4.06 V with the square-root time model, and project that the best NMC811/graphite cells can have a century of lifetime at 15 oC when Li plating during charge is avoided. Such cells are an excellent candidate for grid storage applications where energy density is less important compared to long lifetime.
The impact of graphite materials on capacity retention in Li-ion cells is important to understand since Li inventory loss due to SEI formation, and cross-talk reactions between the positive and negative electrodes, are important cell failure mechanisms in Li-ion cells. Here, we investigate the impact of five graphite materials from reputable suppliers on the performance of NMC811/graphite cells. We show that natural graphites (NG) here have a mixture of 3R and 2H phases, while artificial graphites (AG) were 2H only. We find that there are differences between the N2 BET surface area and the electrochemically-accessible area where redox reactions can take place and it is the latter that is most important when optimizing graphite-containing cells. Part I of this 2-part series investigates physical and electrochemical differences between the graphite materials of interest here, as well as room temperature cycling to probe improvements in capacity retention. We demonstrate that advanced AG materials with small accessible surface areas can improve safety, 1st cycle efficiency (FCE) and long-term cycling compared to NG materials with higher accessible surface areas. Part II of this work examines elevated temperature cycling, cell swelling, and makes lifetime predictions for the best NMC811/graphite cells.
Unwanted self-discharge of LFP/AG and NMC811/AG cells can be caused by in-situ generation of a redox shuttle molecule after formation at elevated temperature with common alkyl carbonate electrolyte. This study investigates the redox shuttle generation for several electrolyte additives, e.g., vinylene carbonate and lithium difluorophosphate, by measuring the additive reduction onset potential, first cycle inefficiency and gas evolution during formation at temperatures between 25 and 70°C. After formation, electrolyte is extracted from pouch cells for visual inspection and quantification of redox shuttle activity in coin cells by cyclic voltammetry. The redox shuttle molecule is identified by GC-MS and NMR as dimethyl terephthalate. It is generated in the absence of an effective SEI-forming additive, according to a proposed formation mechanism that requires residual water in the electrolyte, catalytic quantities of lithium methoxide generated at the negative electrode and, surprisingly, polyethylene terephthalate tape within the cell.
Unwanted parasitic reactions in lithium-ion cells lead to self-discharge and inefficiency, especially at high temperatures. To understand the nature of those reactions, this study investigates the open circuit storage losses of LFP/graphite and NMC811/graphite pouch cells with common alkyl carbonate electrolytes. The cells perform a storage test at 40°C with a 500 h open circuit period after formation at temperatures between 40 and 70°C. Cells formed at elevated temperature showed a high reversible storage loss that could be assigned to a redox shuttle generated in the electrolyte during formation. A voltage hold after formation can reduce the shuttle-induced self-discharge as indicated by significantly lower reversible storage losses, the absence of shuttling currents in cyclic voltammetry and improved metrics in ultra-high precision cycling. The addition of two weight percent vinylene carbonate can prevent redox shuttle generation and leads to almost zero reversible self-discharge.
Lithium-ion cells can be inadvertently subjected to overcharge or other off-nominal conditions during their use in the field, compromising user safety. Off-nominal tests are typically carried out on fresh cells. The goal of this work is to characterize the interplay between cycle life aging and the off-nominal events. Cylindrical cells aged to 10, 15 and 20% capacity fade (CF) and battery modules aged to 20% CF, both cycled under two operating voltage windows, were subjected to overcharge and external short tests. Additionally, single cells were aged to 20% CF using a drive cycle profile at three temperatures of 10 °C, 25 °C and 40 °C. Under overcharge conditions, the single fresh cells experience slower activation of the current interrupt device (CID) compared to the aged cells and the cathode displayed severe degradation in spite of the CID activation and the anode exhibited lithium plating on the edges of the electrode. At the module level, the fresh module experiences fire while the aged module shows sequential CID activation with no thermal runaway. No major trends were observed with the external short tests of the aged cells compared to the fresh ones due to protection provided by the positive temperature coefficient (PTC).
Lithium-ion cells and batteries pose safety risks along with their favorable characteristics such as high energy and power densities. The numerous differences in chemistries and form-factors along with poor manufacturing quality in some cases, can lead to unpredictable field failures with this battery chemistry. The safety of lithium-ion cells and batteries at various states of charge (SOC) has not been studied comprehensively in the past and the goal of this study was to determine if the result of off-nominal conditions would vary with SOC. The study includes cells and batteries of different form factors, cathode chemistries, and capacities. The off-nominal conditions that the cells were exposed to were high-temperature and low impedance external short. In addition to this, voltage stability for the cells and batteries at various SOC was studied for a period of 9 months. The results demonstrate the differences in the level of safety for the cells and batteries at different SOC.
Spent coffee grounds, which represents the vast solid residual matter generated from consumed coffee beans, requires proper reutilization. This work represents the production of an alternate material from spent coffee grounds to replace expensive metal based catalysts currently used as electrodes in fuel cells. A novel microwave assisted technique which is easy, rapid, and economical is utilized for the synthesis of Phosphorous, Nitrogen co-doped carbon (PNDC) from spent coffee grounds and ammonium polyphosphate. SEM analysis revealed that PNDC is composed of distinct, spherical shaped particles. PNDC has a BET surface area of 507 m 2 g 21 and is predominantly mesoporous. XPS reveals that PNDC contains about 1.90% N and 3.02% P besides C and O.PNDC exhibits good O 2 reduction response in 0.1M KOH, which was found to be comparable to that of 20% Pt/C.
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