Michael Metzger

This work evaluates O ₃ -type layered oxide/hard carbon sodium-ion pouch cells (SIBs) against conventional LiFePO ₄ (LFP)/graphite pouch cells under a realistic, constant-power solar-duty protocol across a temperature envelope of 20 °C to 55 °C. At elevated temperatures (40 °C and 55 °C), the SIBs exhibit superior capacity retention compared to the LFP baseline. Specifically, restricting the SIB operational window to a 3.8 V upper cut-off voltage mitigates irreversible structural phase transitions, yielding a remarkable 97% capacity retention after 500 d at 40 °C. However, despite this superior high-temperature capacity retention, a crossover in absolute deliverable energy does not occurring this time. Due to the underlying energy density disparity, the LFP cells continue to deliver substantially higher absolute energy throughout this time. Additionally, the LFP configurations exhibit virtually no polarization growth across all testing temperatures.

Electrolyte motion–induced salt inhomogeneity (EMSI) is a critical degradation mechanism that can limit lifetime of large-format lithium-ion cells. Here, ex-situ scanning micro–X-ray fluorescence (µ-XRF) spectroscopy is employed to map electrolyte salt concentration across entire electrodes harvested from 18 650 cells. Lithium hexafluoroarsenate (LiAsF ₆ ) is used as a spectroscopically favorable salt, enabling arsenic fluorescence as an indicator of local salt concentration. Transport properties of LiAsF ₆ are shown, via Advanced Electrolyte Model calculations, to closely match those of LiPF ₆ in relevant carbonate solvents, validating its use as a surrogate salt for this technique. We confirm that the measured As distributions reflect true electrolyte salt concentration by tracking their growth and relaxation over cycling and rest, and by showing consistent patterns across the separator–anode–cathode stack, indicating a cell-level redistribution of salt. The maps reveal that the axial salt gradient along the entire length of the anode is strongly non-uniform. We observe pronounced tab-correlated banding and localized salt-depleted regions beneath the tab, which we interpret as signatures of pressure-dependent electrolyte pumping and tab-driven modulation of EMSI. This work establishes µ-XRF mapping of LiAsF ₆ as a powerful new technique to quantify electrolyte salt distributions with high spatial resolution.

Thermal management of Li-ion cells within battery packs for stationary energy storage and electric vehicles is critical to ensure longevity. Cells within packs are often heated or cooled using a thermal plate underneath the pack, which could lead to non-uniform temperature in the cells. To test the effects of temperature gradients on LFP/graphite cylindrical cells, a specialized cell holder was designed to apply a constant temperature to the top of a cell and a different constant temperature to the bottom. Temperatures of 30 °C–40 °C, 25 °C–45 °C, and 15 °C–55 °C were applied to the bottom and tops of the cells, respectively, and verified using an infrared camera. Cells were cycled at a variety of rates for approximately six months, and the capacity loss was compared between the different conditions. For most conditions there was no difference in the capacity fade based on temperature gradient and performance closely matched that of cells tested at a constant temperature of 35 °C. Thermal modelling of the jelly roll suggests that the temperature gradient does not penetrate deeply into the jelly roll due to the excellent axial thermal conductivity and this can explain the independence of capacity loss on the magnitude of the applied temperature gradient.

A novel multi-channel Online Electrochemical Mass Spectrometer was built to quantify gases in up to six battery cells for the study of interfacial gas generating reactions in alkali-ion cells. It measures an integral signal of the total evolved/consumed gases by sampling each cell once per hour using an automated switching sequence of computer-controlled valves optimized to remove residual gases from the central gas line connecting the six cells without affecting the vacuum chamber containing the quadrupole mass analyzer. It is demonstrated that the new multi-OEMS system enables reproducible gassing studies on machine-made double-sided graphite electrodes since the evolved gas volume easily exceeds the electrode pore volume, so that sluggish gas diffusion around the solid current collector can be avoided. Multi-OEMS can be used for rapid electrolyte additive screening as demonstrated for common CO ₂ forming additives like VC and FEC, C ₂ H ₄ forming additives like DTD, and novel additives like EC-bisDTD, a polycyclic compound combining EC and DTD, that can suppress gas generation. The multi-sampling approach accelerates systematic gassing experiments and still resolves interfacial gas generating reactions at relevant timescales for alkali-ion cells.

Sodium-ion batteries are emerging as a viable alternative to lithium-ion batteries, particularly for cost-sensitive applications. A key component for such batteries is a durable cathode active material based on abundant and cost-effective resources. NaNi _1/3 Fe _1/3 Mn _1/3 O ₂ has shown promise due to its excellent cycling stability and relatively high energy density in cells with hard carbon anodes. The addition of Ca to NaNi _1/3 Fe _1/3 Mn _1/3 O ₂ has been shown to improve processability by reducing its sensitivity to moisture. Here, we investigate the stabilizing role of calcium in NaNi _1/3 Fe _1/3 Mn _1/3 O ₂ . Specifically, we identify the location of calcium within cathode active material particles using X-ray photoelectron spectroscopy, examine gas evolution during operation with online electrochemical mass spectrometry, and evaluate the influence of calcium on cycling stability, material degradation, and internal resistance evolution in full-cells with a Na reference electrode at two different upper cutoff voltages, 3.9 V and 4.1 V. The stability enhancements are most pronounced at 3.9 V, leading to reduced capacity fading over 1000 cycles and suppressed cathode impedance build-up. Our findings demonstrate that reactive sodium metal—even when used only as reference electrode with a small surface area—can skew full-cell testing due to detrimental side reactions involving sodium alkoxides formed on the sodium metal surface.

Incorporating silicon–carbon (Si–C) composite negative electrodes into Li[Ni _{(1− x − y )} Mn _{( x )} Co _{( y )} ]O ₂ (NMC) lithium-ion cells offers a viable route toward higher energy density. However, the impact of increasing Si–C content in the negative electrode on cell abuse tolerance remains insufficiently understood. Here, we systematically evaluate the thermal stability of single-crystal Li[Ni _0.65 Mn _0.30 Co _0.05 ]O ₂ (Ni65) pouch cells containing 0%, 20% and 50% Si–C in the negative electrode across upper cut-off voltages of 3.9–4.4 V using accelerating rate calorimetry. By maintaining constant cathode chemistry while independently varying Si–C fraction, operating voltage, and Si–C architecture, we isolate the relative contributions of cathode delithiation levels and anode composition to self-heating and thermal runaway. Self-heating was found to scale primarily with the degree of cathode delithiation rather than Si–C content, with increasing operating voltage consistently accelerating thermal reactivity. Accordingly, at a given operating voltage, graphite-only and Si–C-containing cells exhibited nearly identical self-heating profiles. In contrast, thermal runaway severity, as reflected by gas generation and quantified by the mass ejected from the pouch cells, scaled strongly with energy density, reaching a maximum in 4.4 V cells containing 50% Si–C. These findings demonstrate the pivotal role of Si–C content, along with operating voltage, in determining the severity of thermal runaway.

This work presents an evaluation of 1,3,2-dioxathiolane 2,2-dioxide (DTD) as an electrolyte additive for sodium layered oxide/hard carbon pouch cells. Several DTD concentrations (1–4 wt.%) were examined at 55 °C with cycling, open-circuit storage, gas generation, and symmetric cell impedance experiments. Increasing the DTD concentration suppresses gas evolution but also shortens cycle life and accelerates impedance growth. X-ray Photoelectron Spectroscopy analysis of the cycled electrodes reveals that higher concentrations of DTD directly correlate with an increased deposition of sulfate species on both the positive and negative electrode surfaces. Post-mortem electrolyte characterization by gas chromatography–mass spectrometry and nuclear magnetic resonance shows that DTD suppresses the formation of alkyl dicarbonates during long-term cycling, consistent with its effective passivation of the hard carbon electrode. In addition, ethylene carbonate (EC) formation in the initially EC-free solvent scales with the DTD content, suggesting a H ₂ O-induced decomposition pathway of DTD that generates EC. The proposed reaction pathway was validated using the hydrolysis-resistant derivative 4,4′-bi-1,3,2-dioxathiolane 2,2,2′,2′-tetraoxide (bis-DTD). Electrolyte analysis of post-cycled electrolytes confirmed the absence of EC formation. However, suppressing this reaction did not translate to improved cell longevity as DTD-containing cells exhibited enhanced capacity retention and lower impedance growth compared to their bis-DTD counterparts.

An impurity phase, Na ₃ MnO ₄ , is often overlooked during the synthesis of Mn-based sodium layered oxides due to its absence in X-ray diffraction (XRD) patterns, leading to misjudged phase purity and overestimated Na/TM ratios in the targeted materials. In this work, we confirmed its presence through titration and elemental analysis and showed its profound influence on the electrochemical responses. We proposed that Na ₃ MnO ₄ , formed at elevated temperatures, rapidly degrades in ambient air because of its highly hygroscopic nature and reactivity with water, rendering it “invisible” to XRD. Remarkably, incorporating 2 at.% Ca not only improves the air stability of the host materials but also stabilizes the Na ₃ MnO ₄ impurity phase, making it detectable by XRD. This simple yet effective strategy is applicable to various Mn-based sodium layered oxides and offers new insights into their phase purity and accurate Na/TM ratio.

LiMn _0.8 Fe _0.2 PO ₄ (LMFP)/graphite Li-ion cells are an emerging low-cost, Ni and Co free alternative to the existing NMC battery technology. Although cells with this chemistry are promising, various issues impact the commercialization of this material. A major failure mechanism identified in these cells is the loss of lithium inventory due to solid-electrolyte interphase breakdown when Mn dissolves from the positive electrode at high temperature and deposits on the negative electrode. Herein, we propose the use of various blends of conducting salts to minimize this failure mode in LMFP/graphite cells and extend cycle life. Using LiFSI or LiTFSI as the primary salt in combination with 20 wt% of different co-salts such as LiDFOB, LiBOB, and LiPF ₆ resulted in reduced Mn dissolution and deposition on the negative electrode and improved cycle life, especially for blends containing LiDFOB. However, Mn dissolution could not be completely prevented, and salt consumption occurred during cycling, requiring further optimization before these electrolytes can be used in applications.

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.