Jeff R. Dahn

Silicon-containing lithium-ion cells provide higher energy density than cells with only graphite in the negative electrode. This paper investigated the impact of vinylene carbonate (VC) and fluoroethylene carbonate (FEC) on the performance of lithium-ion pouch cells with NMC-type positive electrodes (Ni83-type and Ni65-type) and 20 % chemical silicon-carbon (chemical SiC) in their negative electrode. The additive blends consisted of 2 wt. % VC with either 0, 4, or 10 wt. % FEC. Insitu volume measurements were carried out to study the impact of additives on gas volume produced during the formation step. Long-term cycling tests at 40 °C and 55 °C, and storage experiments at 60 °C, were conducted to investigate the effect of the additives on cell performance. Post-cycle analyses were conducted on cells using impedance spectroscopy (EIS) and gas chromatography-mass spectrometry (GC-MS). The results demonstrate that 2 wt % VC is an effective additive to improve cell performance in both Ni83-type and Ni65-type pouch cells with 20 % chemical SiC. The additive blend, 2 wt % with 4 wt % FEC, shows a positive impact, however, addition of 10 wt % FEC produces minimal additional impact.

Five sets of LFP/artificial graphite 18650 cells were assembled with the aim of limiting degradation to only lithium inventory loss. When this is the case, the well known “square root time model” for cell capacity, Q , as a function of initial capacity, Q 0 , and time, t : Q = Q 0 ( 1 − A t 1 2 ) can be used to model capacity loss fairly well. Data collected at the same charge and discharge rate at various temperatures can be fitted to extract the parameter, A , as a function of temperature, T . In this work, plots of log ⁡ ( A ) versus 1 / T were used to extract the activation energy, E a , and frequency factor, γ , for A = γ exp ^{− Ea/kT} . At extreme temperatures, significant Fe dissolution from LFP and deposition on the graphite electrode occurs, which accelerates capacity loss. In these cases, a modified square root time model is used to extract A . Model parameters for capacity loss were extracted using three different methods, compared, and used to make lifetime predictions as a function of temperature.

Numerous efforts have been made to utilize silicon-based negative electrodes in lithium-ion batteries (LiBs). The main challenge with Si-based anodes is their significant volume expansion and contraction during cycling, which degrades LiB performance . Silicon-carbon composites are known to address many of the challenges of large-volume expansion and can deliver high specific capacity alongside acceptable cycle life. In this study, various chemically synthesized Si–C composites with different porous hard carbon precursors were investigated to examine the evolution of the filling of the carbon precursor pores with Si during impregnation via chemical vapor deposition using SiH ₄ gas. These samples were later etched with a high-pH solution to remove the Si in several steps, returning the structure to that of the carbon precursor to backtrack the changes in the carbon precursor. The as-received carbon precursor and the etched samples were compared using small-angle X-ray scattering and X-ray diffraction. These techniques, combined with high-resolution transmission electron microscopy, compression testing of the samples, and electrochemical performance measurements, provide a broad view of the evolution of the carbon precursor structure during Si deposition and during the charge/discharge processes in the LiB negative electrode, providing insight into the structure-performance-properties correlation of CVD-based Si–C composites.

Well-developed single crystal (SC) LiMn ₂ O ₄ (LMO) particles with distinct exposed facets were synthesized to minimize parasitic reactions between the electrochemically active surface and the electrolyte. Laue diffraction was employed to identify, for the first time, (110)-faceted SC-LMO as distinct from its (111)-faceted counterpart. The (110)-faceted SC-LMO demonstrated superior electrochemical performance relative to (111)-faceted SC-LMO and commercial SC-LMO in half-cells at 55 °C. Scanning Transmission X-ray Microscopy (STXM) revealed a higher surface Mn ²⁺ concentration on (111)-faceted particles after 200 cycles, consistent with more extensive surface reconstruction. Segmented cycling further established that the performance advantage of (110)-faceted SC-LMO is localized to the lower voltage plateau, while the upper plateau remains largely insensitive to both lithium stoichiometry and facet orientation. In single-layer pouch (SLP) full cells at elevated temperature, (110)-faceted SC-LMO retained 80% capacity over 350 cycles, comparable to commercial SC-LMO, while exhibiting the lowest Mn deposition rate on the anode among all materials evaluated.

LiNi _0.7 Mn _0.3 O ₂ (NM73) cathode materials were synthesized with small amounts (0–0.5 mol. %) of tungsten (W) using the all-dry synthesis method. Calcination temperatures between 875 and 975 °C were tested. Pure layered phases were obtained but cycling performance deteriorated when 0.5 mol.% W was added. As more W is added, the particles become less like single crystals and more like polycrystalline agglomerates. By electron-backscattering diffraction, the average grain size of NM73 made with 0.5 mol.% W decreases to 300 nm. Li-ion diffusion studies reveal that even though smaller grain sizes allow for a shorter diffusion path, the addition of W increases ionic resistivity. Adding 0.2 mol.% W creates the desired single crystalline morphology and enables excellent cycling performance. For comparison, LiOH·H ₂ O and a W precursor were mixed with Ni _0.7 Mn _0.3 (OH) ₂ co-precipitated precursors and calcined. At 975 °C, the elemental homogeneity of the all-dry NM73 material matched that of the NM73 material made from the co-precipitated precursor. Characterization by transmission electron microscopy and energy-dispersive spectroscopy reveals that W concentrates in the grain boundaries of the co-precipitated material and on the surface of the single crystals of the all-dry made material.

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.

LiFePO ₄ (LFP) cells show lower gravimetric and volumetric energy density than other types of Li-ion cells. To improve this, a Si-containing anode is employed in LFP cells. However, LFP cells with Si–C composites in the anode show more rapid capacity loss than cells with only graphite in the anode. Lithium alkoxides produced a poorly passivated negative electrode surfaces in carbonate electrolytes travel to the positive electrode and cause Fe dissolution. This Fe moves to the negative and compromises the SEI leading to Li inventory loss and capacity fade. It is more difficult to passivate Si–C effectively in typical electrolytes due to its large volume changes. To solve this issue, we use an electrolyte, based on the solvents sulfolane and toluene, where alkoxide production is impossible and which eliminates Fe dissolution from LFP. LFP/Si–C cells with sulfolane:toluene electrolytes show excellent capacity retention, much better than cells with traditional carbonate electrolytes. Alkoxide-suppressing additives were also explored in this work, and 10% FEC in an EC-free electrolyte also showed promising results. This will open the way to employ a Si–C–containing anode in LFP cells to achieve higher gravimetric and volumetric energy density without sacrificing lifetime.

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.

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.