Connor P. Aiken

In recent years, the LiFePO ₄ (LFP) positive electrode chemistry has received increasing attention. Comprehensively understanding the high temperature degradation of LFP/graphite cells is still an active research topic. Here, the aging of LFP/graphite cells in cycling and storage experiments was compared, testing cells with three different artificial graphite (AG) negative electrodes, and two different Li salts in the electrolyte, LiPF ₆ , and Li bis(fluorosulfonyl)imide (LiFSI). Capacity retention performance was found to be worse for cells that were stored at 100% SOC compared to cycling, and for both aging conditions the trends were the same over the different artificial graphites and electrolytes tested. Parasitic reactions were probed using ex situ gas evolution measurements and isothermal microcalorimetry (IMC), highlighting the differences between parasitic reactions in full cells and separated charged electrodes for the different cell designs. Cycling and storage data were fit to a two-parameter lifetime model based on SEI growth and Fe deposition on the negative electrode, finding a good fit to the cycling data but a poor fit to the storage data. Finally, it was shown that measurements of parasitic reactions originating solely from the negative electrode correlate well to lifetime for cells under both cycling and storage aging.

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.

Silicon (Si)-based negative electrodes are increasingly being integrated into commercial electric vehicle (EV) batteries due to their significantly higher theoretical capacity compared to graphite, enabling improved energy density. However, Si-based electrode materials show substantial volume expansion during lithiation (150 to 300%), which can lead to mechanical degradation, continuous solid electrolyte interphase reformation, and capacity loss over extended cycling. To mitigate these challenges and enhance cell performance, a novel class of electrolyte additives, alkyl pyrocarbonates, have been investigated for their ability to thermally decompose within the electrolyte, generating CO₂ in situ. In this study, micron-scale silicon (μSi) and a mechanical silicon-carbon composite (Si-C) both blended with natural graphite were employed as negative electrodes in pouch cells with LiN i 0.83 M n 0.06 C o 0.11 O 2 (Ni83) as the positive electrode. The impact of pyrocarbonate additives on cycling stability, gas evolution, and electrochemical performance was systematically evaluated using long-term cycling, in situ volume measurements, EIS, GC-MS, and NMR spectroscopy. Among the tested pyrocarbonates, cells with diallyl pyrocarbonate demonstrated superior electrochemical stability, enhancing cycle life and storage characteristics while minimizing gas evolution and reducing charge transfer resistance (R_ct). Furthermore, the role of electrolyte salts, specifically LiPF₆ and LiFSI, were examined, revealing that LiPF₆ more effectively suppressed gas generation compared to LiFSI.

: Medium-Ni, Co-free NMC640/graphite cells have the potential to compete in volumetric energy density with high-Ni materials like NMC811/graphite cells when charged to high voltages like 4.4 V. Employing NMC640 can significantly reduce cost and alleviate the demand for precious transition metals such as Ni and Co. This study focuses on assessing the impact of electrolyte additives on the operation of high voltage NMC640/graphite pouch cells. Initially, various generations of electrolyte additives were screened in NMC442/graphite pouch cells using a rigorous 24-hour voltage-hold protocol at 40°C, targeting an upper cutoff voltage (UCV) of 4.4 V. This approach accelerates the screening process, allowing for quicker identification of promising additives, as cell failures primarily stem from impedance growth and transition metal loss. Subsequently, promising candidates of electrolyte additives were evaluated in NMC640/graphite cells through long-term cycling (CCCV) to UCVs of 4.3 V, 4.4 V, and 4.5 V at temperatures ranging from 20°C to 55°C, employing a conventional CCCV protocol. Figure 1

The novel Rotational Inertia Measuring (RIM) system provides unique insights into electrolyte motion in cylindrical Li-ion cells¹. As electrode particles expand during charging, a portion of liquid electrolyte is forced out of the electrodes and into the hollow ends of the can, and also into the cavity at the core of the electrode winding. Then, as electrode particles contract while discharging, electrolyte is pulled back into the electrode winding. This movement of electrolyte gives rise to changes in the moment of inertia of a cell that is oscillated about its central diameter, which in turn cause changes in the frequency at which that oscillating cell resonates. RIM is a low-cost and non-destructive torsional oscillator that is used to track small changes in the resonant frequency of a cell as it cycles, as shown in Figure 1. RIM thereby improves our understanding of how electrolyte moves and how electrolyte movement impacts cell performance and lifetime. Animations of high resolution operando synchrotron X-ray CT images verify that conclusions drawn from RIM measurements are accurate. The rate at which electrodes are “re-wetted” as electrolyte exits and re-enters them can be measured with RIM. Insights from RIM could be used to assist in efforts to extend the lifetime of cylindrical cells with electrode materials that exhibit large volume changes, such as silicon, and of cells that undergo fast-charging. RIM can also be used to examine the usefulness of resting cells between cycles in certain scenarios. In addition to the first RIM instrument for horizontal 18650 cells, three new RIM instruments are presented: one for tracking electrolyte movement in horizontal 21700 cells, one for vertical 21700 cells, and one for vertical 46800 cells. The rate at which measurements of resonant frequency are collected by RIM has been doubled to improve the fidelity of measurements taken with high cell cycling rates. New examinations of the correlation between changes in the amplitude of resonant frequency peaks and state of charge have yielded insights into jellyroll expansion in relation to cell age. Using RIM, we will present information on the impact of cell orientation on lifetime, the effects of aging on electrode windings and electrolyte motion, and the effects of increasing the size of high-performance cylindrical cells. Figure 1: RIM instruments have been constructed in four different configurations, including instruments used to examine electrolyte motion in horizontally cycled 18650 cells (left) and vertically cycled 21700 cells (bottom right). Changes in the resonant frequency and in the oscillation amplitude observed at resonance are tracked and associated to a corresponding cell voltage (top right) to obtain insights about electrolyte distributions at all states of charge. References: [1] C. P. Aiken et al 2023 J. Electrochem. Soc. 170 040529 Figure 1

This work involves improving the lifetime of lithium-ion cells during high voltage cycling using electrolyte additives. Three generations of electrolyte additives were investigated and screened in NMC442/graphite pouch cells using a 24 h voltage-hold protocol at 40 °C to accelerate oxidative reactions occurring at 4.4 V. Once promising additives and combinations were identified, they were then tested in cobalt-free NMC640/graphite cells for long-term cycling to upper cutoff voltages of 4.3, 4.4, and 4.5 V at temperatures of 20, 40, and 55 °C. Degradation mechanisms were probed using dV/dQ analysis, micro-X-ray fluorescence spectroscopy, and electrochemical impedance spectroscopy. The primary failure mode of cells held at high voltages is due to increase in cell impedance, which is correlated to the dissolution of transition metals, specifically manganese, originating from the positive electrode. We believe this dissolution is presumably due to the formation of a high impedance rock salt surface layer on the NMC positive electrode particles. Such deleterious outcomes can be limited by selecting an appropriate electrolyte additive package. It is hoped that this paper can provide a starting point for developing NMC Li-ion cells that can operate to voltages as high as 4.4 V and still display long lifetimes.

Physical mixtures of LiMn₂O₄ (LMO) and NMC active cathode materials is a well-known strategy in commercial batteries to achieve better cycling and storage performance than cells with a pure LMO cathode. In this work, we demonstrated a similar synergic effect in LiFePO₄(LFP)/NMC640 cathode material blends. Blending LFP with NMC640 in the weight ratio of 90% to 10% lead to improvements in cycling and storage compared to cells with LFP alone. A clear linear coordination between capacity loss and iron deposition on the graphite anode was observed in these blended cells. This work shows that blending NMC in LFP cathode is a promising strategy to improve the high-temperature stability of LFP/graphite cells for long-term operation.

High energy density cylindrical Li-ion cells are densely packed with active materials, inactive materials and electrolyte. Injected electrolyte generally fills all pore spaces in the electrodes and separators of manufactured cells with possibly some excess. When such cells are charged, the overall volume of the electrode materials increases and therefore some electrolyte is pushed under hydraulic pressure to the spaces outside the electrode winding at the ends of the cylindrical can and also possibly into the hollow core of the cylindrical electrode winding. During discharge this electrolyte reenters the pore spaces of the electrodes as electrode particles contract. Therefore, the moment of inertia of the cell about an axis perpendicular to the axis of the cylindrical can change as the cell is charged and discharged. We have built a torsional oscillator that can measure the resonant frequency, and hence the moment of inertia, of a cylindrical Li-ion cell as it is charged and discharged. Because the moment of inertia of the cell depends on the electrolyte distribution, we can “watch” the electrolyte move within the cell. The design and operation of the instrument is described here as well as experiments that demonstrate the electrolyte motion that occurs in cylindrical cells. Consequences of this electrolyte motion are discussed.

State-of-the-art Li-ion cells can have decades of lifetime (>40 years) and tremendous cycle life greater than 10000 cycles¹. Such incredible cells greatly exceed the goal of 80% capacity retention after 800 cycles, promoted by some as sufficient for electric vehicles (EVs). However, by 2030 it is projected that more than 90% of all Li-ion batteries will be used to power vehicles², with very few remaining for energy storage from intermittent renewables which must displace fossil fuels for power generation. The batteries in electric vehicles will represent a vast amount of energy storage capacity, which can be harnessed using vehicle to grid (V2G) technology. Batteries with that can maintain 80% of their capacity after a mere 800 cycles are unsustainable since most of the charge-discharge cycles in V2G will occur when the EV is stationary. For this reason, ultra-long-lived cells are a critical component of a sustainable future. In this talk, we will discuss the important role of graphite material and upper cut-off voltage (UCV) on the lifetime of NMC811 cells, and how avoiding volume contraction in Ni-rich materials yields cells that should be eminently suitable V2G applications^3,4. Additionally, we will discuss the results of a two-year study on the impact of C-rate, depth of discharge (DoD), UCV, and temperature on the lifetime of NMC811 cells. Finally, we will demonstrate that NMC cells with a low UCV, with an emphasis on Ni-rich materials, yield greater energy throughput compared to their higher energy counterparts due to their prolonged lifetime. The $/kWh metric is surely important when selecting a battery technology, but the levelized cost of energy over the battery lifetime is more important when batteries last for many decades. References Harlow, J. E. et al. A Wide Range of Testing Results on an Excellent Lithium-Ion Cell Chemistry to be used as Benchmarks for New Battery Technologies. Journal of The Electrochemical Society 166, A3031–A3044 (2019). Pillot, C. The Rechargeable Battery Market and Main Trends 2018-2030. https://rechargebatteries.org/wp-content/uploads/2019/02/Keynote_2_AVICENNE_Christophe-Pillot.pdf (2019). Eldesoky, A. et al. Impact of Graphite Materials on the Lifetime of NMC811/Graphite Pouch Cells: Part I. Material Properties, ARC Safety Tests, Gas Generation, and Room Temperature Cycling. Journal of The Electrochemical Society 168, 110543 (2021). Eldesoky, A. et al. Impact of Graphite Materials on the Lifetime of NMC811/Graphite Pouch Cells: Part II. Long-Term Cycling, Stack Pressure Growth, Isothermal Microcalorimetry, and Lifetime Projection. Journal of The Electrochemical Society 169, 010501 (2022).

In search for new classes of additives for high voltage NMC/graphite lithium-ion cells, the precursor additive bis(trimethylsilyl) malonate (bTMSM) is shown to be activated via a spontaneous reaction with LiPF₆ and LiBF₄ salts in carbonate-based electrolyte to form lithium tetrafluoro(malonato)phosphate (LiTFMP), and lithium difluoro(malonato)borate (LiDFMB), respectively. The reaction schemes and rates were studied via NMR spectroscopy and GCMS. The effects of LiTFMP and LiDFMB on high voltage electrochemical performance were then examined up to 4.5 V in Li[Ni_0.4Mn_0.4Co_0.16]O₂ (NMC442)/graphite and Li[Ni_0.6Mn_0.4Co_0.0]O₂ (NMC/640)/graphite pouch cells using aggressive voltage-hold cycling, long-term charge/discharge cycling, storage experiments, electrochemical impedance spectroscopy, and gas evolution measurements. While in situ converted additives suffer from gassing issues due to the presence of trimethylfluorosilane (TMSF) gas, a side product of the in situ reaction of bTMSM with LiPF₆, the cycling and storage capability for the activated additives under study shows competitive performance and controlled impedance when compared to other well-known high voltage additives. Micro X-ray fluorescence spectroscopy (μXRF) confirmed that LiTFMP successfully minimizes the rate of transition metal deposition on the surface of graphite apparently by forming a protective agent at the cathode surface, hence allowing for improved cycling performance at high voltages.