A new "hot formation" protocol is proposed to improve lower temperature cycling of lithium metal batteries. The cycling stability of anode-free pouch cells under low pressure (75 kPa) is shown to decline significantly as the cycling temperature is decreased from 40°C to 20°C. At low pressure and 40°C the initial morphology of the lithium anode is dense and columnar, far superior to that plated at 20°C. For "hot formation" two initial 40°C cycles (C/10 charge C/2 discharge) are conducted prior to extended low temperature (20°C) cycling. These two initial cycles have a surprisingly large impact; capacity retention to 80% is increased from only 18 cycles without hot formation to 60 cycles with hot formation at low pressure. When the applied pressure is increased to 1200 kPa, the hot formation (20°C cycling) cells show 85% capacity retention at 100 cycles. The benefits established during these two initial formation cycles are apparently carried forward to improve the longer term performance of lithium metal cells tested at room temperature.

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.

Electrolyte systems based on binary mixtures of organic carbonate ester cosolvents have limitations in ionic transport and thus limit extreme fast charge (XFC) and high-rate cycling of energy dense lithium-ion cells with thick electrodes (>80 μm per side) at ambient temperature and below. Here, we present LiPF6 in methyl acetate (MA) as an ester-based liquid electrolyte that offers substantial improvements in ionic transport, doubling the conductivity of conventional electrolyte systems. Density functional theory-based molecular dynamics (DFT-MD) simulations give insights into the experimentally observed low solvation number for lithium ions in MA solutions and show a solution system with highly mobile, loosely bound ionic species. We show that MA-based electrolytes with suitable additive formulas enable high cycling rates and excellent low-temperature cycling performance in lithium-ion cell designs with thick electrodes but come with a trade-off in lifetime at elevated temperature. While there are inherent practical issues with MA as an electrolyte solvent, including a low flash point (−10 °C) and lifetime penalties compared to state-of-the-art electrolytes, this work demonstrates that excellent ionic transport in the electrolyte can enable fast charging without the energy density sacrifice inherently associated with specifically tailored electrodes. Further work in electrolyte design, particularly in increasing ionic conductivity without sacrificing stability, has the potential to enable XFC in practical lithium-ion cell chemistries and cell designs.

Many studies of Li-ion cells examine compositional changes to electrolyte and electrodes to determine desirable or undesirable reactions that affect cell performance. Cells involved in these studies typically have a limited test lifetime due to the resource intensive and time-consuming nature of these experiments. Here, electrolyte and electrode analyses were performed on a large matrix of cells tested at various conditions and with various cycle lifetimes. The matrix included LiNi0.5Mn0.3Co0.2O2 (NMC532)/graphite and LiNi0.6Mn0.2Co0.2O2 (NMC622)/graphite pouch cells with excellent performing electrolyte mixtures, both cycling and storage protocols at 40 °C and 55 °C with both 4.3 V and 4.4 V upper cutoff potentials. This study presents post-test analysis (electrochemical impedance spectroscopy, differential voltage analysis, differential thermal analysis), electrolyte analysis (gas chromatography, quantitative nuclear magnetic resonance), and electrode analysis (micro X-ray fluorescence) for these cells after 3, 6, 9, and 12 months of testing. Many products and reactants, such as fraction of transesterification, gas production, transition metal dissolution appeared to have a constant rate of increase in this 12-month observation period. In most cases, results from cells after 3 to 6 months of testing could be used to reasonably estimate the status of the cells (electrolyte composition, gas production, transition metal dissolution) at 12 months.

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.