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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...

Li-ion batteries with fast charging capabilities will push the adoption of electric vehicles (EVs). The United States Department of Energy has stated goals of achieving “Extreme Fast Charging” (XFC) – charging to 80% capacity in 15 minutes or under – by 2028. The liquid electrolyte plays an extremely important role in achieving this goal. This review considers recent progress made in electrolyte development for high-rate charge applications. Approaches such as using low-viscosity co-solvents, highly concentrated electrolytes, and large polyanions are considered. New methods for measuring electrolyte transport properties are reviewed. The importance of electrolyte additives is also discussed. Finally, the importance of considering cell lifetime in the development of electrolytes for fast charge applications is stressed.

Monitoring the dynamic chemical and thermal state of a cell during operation is crucial to making meaningful advancements in battery technology as safety and reliability cannot be compromised. Here we demonstrate the feasibility of incorporating optical fibre Bragg grating sensors into commercial 18650 cells. By adjusting fibre morphologies, wavelength changes associated with both temperature and pressure are decoupled with high accuracy, which allows tracking of chemical events such as solid electrolyte interphase formation and structural evolution. We also demonstrate how multiple sensors are used to determine the heat generated by the cell without resorting to microcalorimetry. Unlike with conventional isothermal calorimetry, the cell’s heat capacity contribution is readily assessed, allowing for full parametrization of the thermal model. Collectively, these findings offer a scalable solution for screening electrolyte additives, rapidly identifying the best formation processes of commercial cells and designing battery thermal management systems with enhanced safety. Tracking a battery’s chemical and thermal states during operation offers important information on its reliability and lifetime. Here the authors develop optical fibre sensors and decouple temperature and pressure variations in the measurements inside of batteries, allowing chemical and thermal events to be monitored with high accuracy.

In the pursuit of ever more energy-dense and longer lasting Li-ion batteries, the electrolyte is still an area of intense research. A good choice of electrolyte can make or break the long-term performance of a cell. This paper highlights various advances in electrolyte development and evaluation that have been made in the last several years. Blends of electrolyte additives that give superior capacity retention over several thousand cycles, as well as new formulations of electrolyte solvents to achieve higher charging rates and higher voltage operation are presented. The Advanced Electrolyte Model, a theoretical model for the calculation of electrolyte properties, is presented as an effective way to determine the transport properties of a diverse array of electrolyte systems thus speeding electrolyte development.

In an effort to better understand capacity loss mechanisms in LiFePO4 (LFP)/graphite cells, this work considers carbon-coated LFP materials with different surface area and particle size. Cycling tests at room temperature (20 °C) and elevated temperatures show more severe capacity fade in cells with lower surface area LFP material. Measurements of Fe deposition on the negative electrode using micro X-ray fluorescence (μXRF) spectroscopy reveal more Fe on the graphite electrode from cells with low surface area. Measurements of parasitic heat flow using isothermal microcalorimetry show marginally higher parasitic heat flow in cells with low surface area. Cross-sectional SEM images of aged LFP electrodes show micro-fracture generation in large LFP particles, which are more prevalent in the low surface area material. Further, studies on the impact of vacuum drying procedures show that while Fe deposition can be inhibited by removing excess water contamination, the direct impact of Fe deposition on capacity fade is small. Despite the observed particle cracking, differential voltage analysis on aged cells suggested active material loss was not significant, leading to the conclusion that LFP particle fracture instead increases parasitic reaction rates leading to Li inventory loss.

Single crystal Li[Ni0.5Mn0.3Co0.2]O2//graphite (NMC532) pouch cells with only sufficient graphite for operation to 3.80 V (rather than ≥4.2 V) were cycled with charging to either 3.65 V or 3.80 V to facilitate comparison with LiFePO4//graphite (LFP) pouch cells on the grounds of similar maximum charging potential and similar negative electrode utilization. The NMC532 cells, when constructed with only sufficient graphite to be charged to 3.80 V, have an energy density that exceeds that of the LFP cells and a cycle-life that greatly exceeds that of the LFP cells at 40 °C, 55 °C and 70 °C. Excellent lifetime at high temperature is demonstrated with electrolytes that contain lithium bis(fluorosulfonyl)imide (LiFSI) salt, well beyond those provided by conventional LiPF6 electrolytes. Ultra-high precision coulometry and electrochemical impedance spectroscopy are used to complement cycling results and investigate the reasons for the improved performance of the NMC cells. NMC cells, particularly those balanced and charged to 3.8 V, show better coulombic efficiency, less capacity fade and higher energy density compared to LFP cells and are projected to yield lifetimes approaching a century at 25 °C.

The use of LiPF6 in Li-ion battery electrolytes provides sufficient stability, conductivity, and cost in most applications. However, LiPF6 has also been known to cause degradation in Li-ion cells, primarily from its thermal decomposition or hydrolysis to form acidic species. This work considers the use of imide salts lithium bis(fluorosulfonyl)imide (LiFSI) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as an alternative to LiPF6 in LiFePO4/Graphite cells. The use of LiFSI or LiTFSI over LiPF6 improved cycling performance both in control electrolyte (no additives) and electrolyte containing 2% vinylene carbonate (VC). However, while metrics from ultra high precision coulometry, isothermal microcalorimetry, and storage experiments all agreed with long-term cycling results for cells with control electrolyte, the opposite was seen with 2VC electrolyte. Pouch bag experiments elucidated information about the origin of parasitic reactions in LFP/Graphite cells, showing that most parasitic reactions originate at the negative electrode. Additionally, pouch bag experiments reveal a more passivating graphite solid electrolyte interphase (SEI) for LiFSI + 2VC electrolyte, agreeing with long term cycling experiments. It is concluded that in control electrolyte, the use of LiFSI limits redox shuttles, Fe dissolution, and SEI decomposition, while in 2VC electrolyte, LiFSI introduces a minor self-discharge reaction that does not impact long-term cycling.

Part II of this 2-part series examines the impact of competitive 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 due to identical cathodes. We show that an increase in graphite 1st cycle efficiency due to limited redox-active sites—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 A−1 h−1 at 40 oC) compared to cells with natural graphites (NG), and that the cells with the AGs 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 here can have decades-long lifetimes at 20 –30 °C when Li plating is avoided. Such cells are excellent candidates for grid storage applications where energy density is less important compared to long lifetime.

Isothermal microcalorimetry has previously been used to probe parasitic reactions in Li-ion batteries, primarily studying Li[NixMnyCo1-x-y]O2 (NMC) positive electrode materials. Here, isothermal microcalorimetry techniques are adopted to study parasitic reactions in LiFePO4 (LFP)/graphite cells. Features in the heat flow from graphite staging transitions were identified, and the associated heat flow was calculated using simple lattice-gas mean-field theory arguments, finding good agreement with experimentally measured values. Parasitic heat flow was measured in LFP/graphite pouch cells with different electrolyte additives. In an electrolyte without additives, a massive parasitic heat flow was measured suggesting a shuttle reaction unique to the LFP/graphite system. In cells containing electrolyte additives, parasitic heat flow agreed well with long-term cycling results, confirming the value of this technique to rank the lifetime of LFP/graphite cells with different electrolyte additives. Finally, comparing cells with and without unwanted water contamination, it was found that the parasitic heat flow was similar or slightly higher in cells where water was intentionally removed before cycling, seemingly contradicting long-term cycling results. It is concluded that the presence of water (at the 500 ppm level) may slightly reduce parasitic reactions, but at the expense of a more resistive SEI layer.

Adding esters as co-solvents to Li-ion battery electrolytes can improve low-temperature performance and rate capability of cells. This work uses viscosity and electrolytic conductivity measurements to evaluate electrolytes containing various ester co-solvents, and their suitability for use in high-rate applications is probed. Among the esters studied, methyl acetate (MA) outperforms other esters in its impact on the conductivity and viscosity of the electrolyte. Therefore, viscosity and conductivity were measured as a function of temperature and LiPF6 concentration for electrolytes ethylene carbonate (EC): linear carbonate: MA in the ratio 30:(70-x):x, where linear carbonate = {ethyl methyl carbonate (EMC), dimethyl carbonate (DMC)}, and x = {0, 10, 20, 30}. Adding MA leads to an increase in conductivity and decrease in viscosity over all conditions. Calculations of electrolyte properties from a model based on a statistical-mechanical framework, the Advanced Electrolyte Model (AEM), are compared to all measurements and excellent agreement is found. All electrolytes studied roughly agree with a Stokes' Law model of conductivity. A Walden analysis shows that the ionicity of the electrolyte is not significantly impacted by either MA content or LiPF6 concentration. Li[Ni0.5Mn0.3Co0.2]O2/graphite cells containing MA were cycled at charging rates up to 2C and showed improved cycling performance.

A few weight percent of electrolyte additives can dramatically improve Li-ion battery performance and lifetime. A systematic investigation of a series of electrolyte additive formulations was performed on bimodal (BM) and single crystal (SC) LiNi0.8Mn0.1Co0.1O2 (NMC811)/artificial graphite (AG) pouch cells. Long-term cycling tests at different temperatures (20 °C, 40 °C, and 55 °C) and different upper cutoff voltages (4.06 V and 4.20 V) were performed. These tests results were combined with advanced characterization techniques like ultra-high precision charging (UHPC), ex-situ gas measurement, and automatic cell storage tests to rank the additives based on their effectiveness. Radar plots and a figure-of-merit (FOM) approach were further utilized to summarize results for the BM and SC NMC811/AG cells. This work provides a useful benchmark for those developing NMC811/graphite cells and gives important insights for future electrolyte additive studies.

Plating rates and Ni co-deposition were measured for electroless Cu plating baths without and with an added proprietary stabilizer system. The bath has formaldehyde as reducing agent and tartrate as complexing agent. The concentrations of Ni and stabilizer codetermine the plating rate. We investigated this bath over a wide range of compositions, varying the concentrations of stabilizer, Ni, formaldehyde and hydroxyl ions. Stabilized baths without Ni plate very slowly, while with the addition of Ni plating rates are comparable to non-stabilized baths. The amount of Ni in the film is proportional to Ni in the bath for stabilizer-free baths, but becomes dependent on plating rate when the bath is stabilized. Extended absorption fine structure measurements show that the co-deposited Ni is metallic. A quantitative model for time-dependent plating kinetics and Ni incorporation is proposed. The model is based on the inhibition of plating by the adsorption of non-stabilizer and stabilizer species on the plating surface, and it is used to fit plating rates and Ni incorporation for 254 experimental combinations of chemical composition and plating times.

Anode-free Li metal cells are one of the most appealing energy storage technologies beyond Li-ion batteries due to their superior theoretical specific and volumetric energy densities. However, long cycle life in an anode-free cell remains elusive due to difficulties reversibly plating and stripping metallic lithium. Isothermal microcalorimetry was used to study parasitic reactions in anode-free Li metal cells for electrolytes containing different Li salts. A new cycling protocol was used to measure the parasitic heat flow both on freshly plated Li surfaces and anode surfaces after stripping lithium. Existing methods were used to measure parasitic reactions occurring at high voltage. In both low- and high-voltage measurements, electrolytes containing LiDFOB had the highest parasitic heat flow compared to an electrolyte with LiPF6 salt. In contrast to previous studies of parasitic reactions in Li-ion batteries using isothermal microcalorimetry, the LiDFOB-containing electrolytes gave the longest lifetime despite having higher parasitic heat flow. This observation was attributed to decomposition of the LiDFOB salt, and subsequent formation of a favorable SEI layer that greatly improves plating and stripping efficiency. In-situ detection of parasitic heat flow with isothermal microcalorimetry techniques will be valuable for future studies of electrolyte design in anode-free Li metal cells.

Li[Ni0.5Mn0.3Co0.2]O2/graphite pouch cells were cycled using protocols that included 24 h spent at high voltage (≥ 4.3 V) under constant voltage or open circuit conditions to accelerate failure. Compared to traditional cycling, failure was reached up to 3.5 times faster. When this protocol was applied to cells containing low LiPF6 concentrations (≤ 0.4 M) failure was achieved up to 17.5 times faster than traditional cycling with normal LiPF6 concentrations. This represents a time improvement on the order of years and therefore can be used as a high-throughput screening method. Failure mechanisms for cells containing a range of LiPF6 concentrations undergoing these aggressive protocols were investigated using charge-discharge cycling, impedance spectroscopy (including symmetric cell analysis) and isothermal microcalorimetry. Long times at high voltage rapidly increase positive electrode impedance but do not seem to consume lithium inventory. The use of lower LiPF6 concentrations does not seem to introduce new failure mechanisms but makes cells less tolerant to positive electrode impedance growth. The utility of this method is demonstrated by screening cells with a variety of electrolyte additive combinations. Fewer than 3 months were required to distinguish cells containing 1% lithium difluorophospate as superior to cells with other additive combinations.

LiFePO4 (LFP) is an appealing cathode material for Li-ion batteries. Its superior safety and lack of expensive transition metals make LFP attractive even with the commercialization of higher specific capacity materials. In this work the performance of LFP/graphite cells is tested at various temperatures and cycling protocols. The amount of water contamination is controlled to study the impact of water on capacity fade in LFP. Further, several additive systems that have been effective in NMC/graphite chemistries are tested in LFP/graphite cells. The presence of excess water impacts cell performance severely when no electrolyte additives are used, or when the electrodes are poorly passivated. When effective additive systems are used, the existence of up to 500 ppm excess water in the cell is does not strongly affect cycle life and storage performance. Fe dissolution is studied in LFP with micro X-ray fluorescence spectroscopy (μXRF), and most electrolyte additives virtually eliminate Fe dissolution, even at high temperature and with water contamination. Removing excess water contamination suppresses Fe dissolution in cells without electrolyte additives. Finally, the capacity retention of LFP/graphite cells at high temperature is compared with long lifetime NMC/graphite cells, demonstrating challenges for LFP/graphite cells.

Lithium iron phosphate (LiFePO4, or LFP) is a widely used cathode material in Li-ion cells due to its improved safety and low cost relative to other materials such as LiNixMnyCozO2 (x + y + z = 1, NMC). To improve the calendar life of LFP cells, an investigation of their failure mechanisms is necessary. Herein, we use scanning micro X-ray fluorescence (μXRF) to study Fe dissolution from LFP and deposition on the graphite electrode, which is thought to be a contributor to capacity fade. The impacts of the vinylene carbonate (VC) electrolyte additive, cycling conditions, and water content in the positive electrode on Fe dissolution were studied. There was no significant correlation between Fe dissolution and capacity fade found. Furthermore, we proposed that gas generation concomitant with Fe dissolution might be due to the reduction of the organic species coordinating Fe2+ when they reach the negative electrode. Localized regions of increased Fe loading on the anode surface were found, which corresponded to regions with slight non-uniformities in stack pressure or current density. This work demonstrates the effectiveness of μXRF in quantifying transition metal (TM) dissolution in Li-ion cells without any sample treatments that might mask valuable information such as element spatial distribution.

We provide a quantum chemical computational framework to calculate ion association constants relevant to lithium ion battery electrolytes. We compare our method to reported experimental values as the solvent, cation, and anion are varied. For solvent, anion, and cation variations, the standard errors are respectively 0.2 eV, 0.12 eV, and 0.11 eV for the chosen data set, where Pearson correlation values are all above 0.92.

An automated system was developed to measure the viscosity of fluids as a function of temperature using image analysis tracking software. An Ostwald viscometer was placed in a three-wall dewar in which ethylene glycol was circulated using a thermal bath. The system collected continuous measurements during both heating and cooling cycles exhibiting no hysteresis. The use of video tracking analysis software greatly reduced the measurement errors associated with measuring the time required for the meniscus to pass through the markings on the viscometer. The stability of the system was assessed by performing 38 consecutive measurements of water at 42.50 ± 0.05 °C giving an average flow time of 87.7 ± 0.3 s. A device was also implemented to repeatedly deliver a constant volume of liquid of 11.00 ± 0.03 ml leading to an average error in the viscosity of 0.04%. As an application, the system was used to measure the viscosity of two Li-ion battery electrolyte solvents from approximately 10 to 40 °C with results showing excellent agreement with viscosity values calculated using Gering's Advanced Electrolyte Model (AEM).

Ni-rich positive electrode materials for Li-ion batteries have the dual benefit of achieving high energy density while reducing the amount of Co used in cells. However, limitations in cycle life are still an issue for the widespread adoption of these materials. The benefit of using single crystal materials has been demonstrated for LiNi0.5Mn0.3Co0.2O2 (NMC532), LiNi0.6Mn0.2Co0.2O2 (NMC622), and now LiNi0.8Mn0.1Co0.1O2 (NMC811). This work uses long-term cycling, ultra-high precision coulometry (UHPC), and isothermal microcalorimetry to investigate the effect of particle morphology on the lifetime of NMC811/graphite pouch cells. NMC811 with uncoated single crystal (SC) particles, coated polycrystalline (PC) particles, and a composite “bimodal” (BM) material are studied with electrolyte systems that have shown excellent cycle life in other NMC materials. Results from this work show that SC cells have improved cycle life in long-term cycling, as well as higher coulombic efficiency (CE) and lower charge endpoint capacity slippage as seen in the UHPC measurements. This correlates well with the isothermal microcalorimetry results, in which SC cells show the lowest parasitic heat flow over a range of upper-cutoff voltages. This study suggests excellent lifetimes can be achieved in single crystal NMC811/graphite cells with further electrolyte optimization.