Electrode Mass Loading Research Articles

Effects of ion mass transport on electrochemical reaction pathways in aluminum-anthraquinone batteries

Aluminum anodes and quinone-based organic cathodes couple as earth abundant, sustainable, and safe battery electrodes. However, different numbers of galvanostatic discharge plateaus have been observed in aluminum-quinone cells depending on the experimental conditions, a phenomenon that has not yet been explained and furthermore affects cell energy density. Here, in aluminum-anthraquinone batteries, we show that ion mass transport affects the electrochemical reaction pathway and controls the occurrence of either one or two reduction plateaus during galvanostatic discharge. The effects of electrode mass loading, porosity, rest periods, and cycling rates were analyzed on the reduction potential and relative specific capacity of the galvanostatic plateaus. When AlCl2+ cations charge compensate the electrochemically reduced carbonyl groups concurrently, a single reduction plateau is observed. When ion mass transport is limiting, sequential reduction and charge compensation of each carbonyl group results in two distinct reduction plateaus. The second plateau occurs at a potential approximately 0.3 V lower than the first, thus decreasing the cell energy density. The potential of the electrochemical oxidation reaction where AlCl2+ cations dissociate, however, is not affected. Ion mass transport is thus shown to be a critical variable that can control the electrochemical reaction pathway, redox potentials, and practical energy densities of aluminum-quinone batteries.

Entropy‐Assisted Anion‐Reinforced Solvation Structure for Fast‐Charging Sodium‐Ion Full Batteries

AbstractAnion‐reinforced solvation structure favors the formation of inorganic‐rich robust electrode‐electrolyte interface, which endows fast ion transport and high strength modulus to enable improved electrochemical performance. However, such a unique solvation structure inevitably injures the ionic conductivity of electrolytes and limits the fast‐charging performance. Herein, a trade‐off in tuning anion‐reinforced solvation structure and high ionic conductivity is realized by the entropy‐assisted hybrid ester‐ether electrolyte. Anion‐reinforced solvation sheath with more anions occupying the inner Na+ shell is constructed by introducing the weakly coordinated ether tetrahydrofuran into the commonly used ester‐based electrolyte, which merits the accelerated desolvation energy and gradient inorganic‐rich electrode‐electrolyte interface. The improved ionic conductivity is attributed to the weakly diverse solvation structures induced by entropy effect. These enable the enhanced rate performance and cycling stability of Prussian blue||hard carbon full cells with high electrode mass loading. More importantly, the practical application of the designed electrolyte was further demonstrated by industry‐level 18650 cylindrical cells.

Entropy-Assisted Anion-Reinforced Solvation Structure for Fast-Charging Sodium-Ion Full Batteries.

Anion-reinforced solvation structure favors the formation of inorganic-rich robust electrode-electrolyte interface, which endows fast ion transport and high strength modulus to enable improved electrochemical performance. However, such a unique solvation structure inevitably injures the ionic conductivity of electrolytes and limits the fast-charging performance. Herein, a trade-off in tuning anion-reinforced solvation structure and high ionic conductivity is realized by the entropy-assisted hybrid ester-ether electrolyte. Anion-reinforced solvation sheath with more anions occupying the inner Na+ shell is constructed by introducing the weakly coordinated ether tetrahydrofuran into the commonly used ester-based electrolyte, which merits the accelerated desolvation energy and gradient inorganic-rich electrode-electrolyte interface. The improved ionic conductivity is attributed to the weakly diverse solvation structures induced by entropy effect. These enable the enhanced rate performance and cycling stability of Prussian blue||hard carbon full cells with high electrode mass loading. More importantly, the practical application of the designed electrolyte was further demonstrated by industry-level 18650 cylindrical cells.

Vertical-channel hierarchically porous 3D printed electrodes with ultrahigh mass loading and areal energy density for Li-ion batteries

Lithium-ion batteries (LIBs) are renowned for their high energy density, long lifespan, and minimal self-discharge, making them a prominent power supply system. However, the persistent demand for improved energy density coupled with mechanical stability remains a significant challenge, as efforts to enhance electrode thickness or achieve geometric complexity through conventional ink-casting methods have yielded limited results. To tackle this challenge, we applied a three-dimensional (3D) printing technique to swiftly construct 3D thick electrodes with ultrahigh mass loading. Combined with unidirectional ice-templating process, high mass loading electrodes with hierarchically porous structures were successfully fabricated. The mass loading of the 3D electrodes can reach up to 73.83 mg cm−2, significantly enhancing the areal capacity. The low tortuosity and good mechanical resilience of these structures enable fast ion and charge transport, leading to significant improvements in areal energy density without sacrificing the power density. The assembled full cell exhibits an impressive areal capacity of 10.34 mAh cm⁻2 at 0.2 C and an excellent energy density of 18.41 mWh cm−2, surpassing other 3D printed lithium-ion batteries. This approach marks a significant advancement in electrode structural design towards higher areal capacity and energy density, showcasing the intriguing potential of 3D printed batteries for practical applications. Additionally, it also highlights the scalability and design versatility provided by 3D printing technique.

Advancements in Dry Coating for Battery Electrodes: Scalable Extrusion Mixing for Control of Electrode Microstructure and Electrochemical Performance

Dry coating of battery electrodes is emerging as a promising alternative technology to the prevailing slurry coating and drying process widely adopted in the lithium-ion battery industry.1,2,3 To date, the most energy-intensive step in cell manufacturing is the drying of slurry-coated electrodes,1 and the adoption of dry coating aims to mitigate both the environmental impact and production costs associated with electrode production. Beyond environmental and economic concerns, producing higher mass-loading electrodes, a crucial aspect for increasing cell energy density, faces inherent challenges with the slurry-coating process: the drying of thick coatings often leads to issues such as electrode cracking and the formation of inhomogeneous electrode microstructures.Bühler and Empa have collaboratively developed and are in the process of scaling up a dry-coating manufacturing method.4 Our process centers around the extrusion mixing of electrode components, followed by calendering of the dry mixture to produce the final electrode and direct lamination onto a current collector. In this presentation, we showcase the adaptability of extrusion mixing in inducing shear fibrillation of polytetrafluoroethylene (PTFE) binder, crucial for achieving the desired electrode microstructures (see Figure 1a and 1b). Furthermore, we demonstrate the capability of our dry-coating technology to produce high-mass loading electrodes with areal capacities of 5 mAh cm-2 or more (see Figure 1c).The presentation encompasses a thorough characterization of dry-coated electrodes, including their microstructure and electrochemical performance in lab-scale batteries. Key aspects, such as rate capability and capacity retention, are evaluated. A comprehensive comparison with slurry-coated electrodes, fabricated using identical active electrode materials, mass fractions, and mass loading, reveals that our dry-coated electrodes exhibit comparable performance to their slurry-coated counterparts.5 The scalability of our dry-coating technology is underscored, highlighting the achievable throughputs necessary for gigafactory-scale production with a single extruder.

A Formula to Customize Cathode Binder for Lithium Ion Battery

AbstractThe binder plays a crucial role in binding the solid electrode components and fixing them to the current collector, ensuring good contact and transformation. The high‐energy‐density cathode poses several challenges for the Polyvinylidene fluoride (PVDF) binder, such as high electrode potential, mass loading, and binding force. Although various new binders are proposed to optimize the electrochemical performance of cathode, the design focus and mechanisms often lack universality. This study introduces a design strategy for the cathode binder from the perspective of molecular structure design. The performance of the binder is optimized by adjusting the soft‐hard and hydrophilic‐hydrophobic balances of the polymer, and the effectiveness of this strategy is verified in the LiCoO2 (LCO) system. The optimized copolymer binder improves the cycling stability of the LCO electrode under high load and low binder content conditions and even achieves stable operation at a high charging voltage of 4.6 V. The mechanism for improving the performance of LCO electrodes is that the optimized binder stabilizes the electrode structure, the active material particle structure, and the active material interface. This design strategy provides a valuable reference for customizing cathode binders.

Continuous Flow‐By Electrochemical Reactor Design for Direct Lithium Extraction from Brines

AbstractThe demand for lithium is expected to increase significantly in the next few years, mainly due to the energy transition in the mobility sector. To electrify the entire global transportation system, it is necessary to have access to a wide variety of lithium deposits to ensure resource sovereignty and sufficient supply to meet demand. For this reason, much effort has recently been invested in developing technologies for direct lithium extraction (DLE) from brines. In this work, a membrane‐based continuous flow‐by reactor for the electrochemical extraction of lithium from brine with a LiMn2O4/λ‐MnO2 system has been studied. A zoned reactor was proposed in which the current density and electrode mass loading gradually decrease along with the depletion of lithium in the brine, obtaining an average electrode capacity of 80 mAh/g throughout three different reactor sections. The limiting Reynolds number was determined for the dilute lithium segment of the reactor to avoid mass transport limitations, with brine concentration equal to Li 3 mM and Na 1.3 M. A homogeneous cycle time was achieved for the three reactor zones and their associated average energy consumption was determined, ranging from 3.9 Wh/mol Li to 9.5 Wh/mol Li.

A Machine-Learning-Based Approach to Analyse the Feature Importance and Predict the Electrode Mass Loading of a Solid-State Battery

Solid-state batteries are currently developing into one of the most promising battery types for both the electrification of transport and for energy storage applications due to their high energy density and safe operating behaviour. The performance of solid-state batteries is largely determined by the manufacturing process, particularly in the production of electrodes. However, efficiently analysing the effects of key manufacturing features and predicting the mass loading of electrodes in the early stages of battery manufacturing remain a major challenge. In this study, a machine-learning-based approach is proposed to effectively analyse the importance of manufacturing features and accurately predict the mass loading of electrodes. Specifically, the importance of four key features during the manufacturing process of solid-state batteries is first quantified and analysed using a machine-learning-based method to analyse the importance of features. Then, four effective machine-learning-based regression methods, including decision tree, boosted decision tree, support vector regression and Gaussian process regression, are used to predict the mass loading of the electrodes in the mixing and coating stages. The comparative results show that the developed machine-learning-based approach is able to provide a satisfactory prediction of the electrode mass loading of a solid-state battery with 0.995 R2 while successfully quantifying the importance of four key features in the early manufacturing stages. Due to the advantages of its data-driven nature, the developed machine-learning-based approach can efficiently assist engineers in monitoring/predicting the electrode mass loading of solid-state batteries and analysing/quantifying the importance of manufacturing features of interest. This could benefit the production of solid-state batteries for further energy storage applications.

3D Printed Freestanding Electrodes for High Energy Density Batteries

The global surge in energy consumption, as well as a rise in greenhouse gases, has created an urgent need for high-capacity battery materials to store intermittent, renewable energy sources such as wind and solar. Directly increasing electrode mass loading using the traditional slurry coating fabrication process introduces unavoidable issues such as slowed kinetics and thermal degradation. Freestanding electrodes are a great alternative as they eliminate the need for inactive components (i.e., binder, additive carbon, and current collector) and shorten the fabrication process. In this work, carbon microlattice templates are designed and fabricated through the use of 3D stereolithography, which allows for design flexibility of any shape or geometry, followed by pyrolysis to create porous hard carbon. These hard-carbon templates are then subjected to a ferrocene/sulfur reaction to grow iron sulfide that serves as active material, which has a theoretical specific capacity higher than that of commercial graphite. The design utilizes controlled microchannels that facilitate ion transport through charging and discharging cycles. The addition of FeS led to a specific capacity of 1204.2 mAh g-1 during cycle two with a current density of 500 mA g-1, which is much higher than that achieved with the control carbon template (≈37.3 mAh g-1 at 300 mA g-1). These results will advance the development of composite electrodes as well as shine light on the immense potential to develop high energy density batteries utilizing 3D printing.

Preparation of MoP-based quasi-two-dimensional belt-like composite fibers and its performance modulation for sodium/potassium ion storage

Beyond round nanofibers, electrospinning method is able to fabricate thin fibers with various cross-sectional shapes. In this work, using phosphomolybdic acid to trigger the polymerization reaction of pyrrole with addition of tin salt for the filler of carbon fibers, we prepared quasi-two-dimensional thick belt-like fibers through single spinneret electrospinning. The side-by-side welding nanofiber bundles with dog-bone-shaped cross-section improved the connection between fibers, endowing the fiber films with some flexibility and enabling it to be used as freestanding electrode. Employed as anode for sodium/potassium ion storage, the carbon fiber encapsulated MoP/SnO2 material exhibited promising electrochemical performance. Controlling the collection process of electrospinning, the electrode mass loading can be increased to 10 mg cm−2. Combined with surface selenizing modification, the performance of as-prepared MoP/SnO2/MoSe2 was further improved, which exhibited areal capacity about 1.5 mAh cm−2 as anode for sodium/potassium ion storage with mass loading of 8–9 mg cm−2.

Fabrication of a 3D structure MnO2 electrode with high MnO2 mass loading as the cathode for high-performance aqueous zinc-ion batteries

Benefiting from the remarkable advantages (eco-friendly, cost-effective, and high safety) of rechargeable aqueous zinc-ion batteries (AZIBs), AZIBs are regarded as viable alternatives to traditional Lithium-ion batteries (LIBs). However, in prevailing research, the excellent electrochemical performance usually achieved by ultra-low active material loadings (e.g., <1 mg/cm²), which is far from the active material mass loading of commercial electrodes in practical (≥10 mg/cm²). Balancing superior properties with high mass loading has posed a challenge. To surmount this, an innovative method combined ultrasonic processing, freeze-drying, and 3D printing technologies is used to engineer a high MnO2 mass loading cathode for AZIBs (3D cathode). Notably, the grid structure of 3D cathode substantially augmenting the surface area for redox reactions during charge/discharge processes, which greatly enhanced the pseudo-capacitance process of cathode. Additionally, the uniform and stable microstructure exhibited the dual capability of expediting ion/electron transfer and sustaining the structural integrity throughout charge/discharge cycles. Consequently, the 3D cathode demonstrated superior comprehensive electrochemical properties at the MnO2 high mass loading of 10 mg/cm2 (138.7 mAh/g at 1 A/g, 87.4 mAh/g after 500 cycles, 82 % energy efficiency) and 15 mg/cm2 (113.3 mAh/g at 1 A/g, 56.3 mAh/g after 400 cycles, 81 % energy efficiency).

Freestanding Carbon Lattice-Based Composites for Lithium-Ion Batteries and Beyond

Advancements in technology in conjunction with rising concerns of global warming have led researchers to search for sustainable and environmentally friendly energy sources. Wind and solar are promising alternatives to fossil fuels; however, storage of these renewable energy sources poses a significant threat to their viability. Lithium-ion batteries (LIBs) are ideal candidates to offset their intermittent nature, functioning as an excellent storage medium. In addition, LIBs have been playing a key role in personal electronics (computers, cellphones, etc.) and are ubiquitous. LIBs in electric vehicles (EVs) are also rising exponentially. However, lithium-ion batteries still face many challenges. Their electrodes are currently made based on the conventional wet film coating fabrication process that utilizes a slurry containing active material coated onto current collectors. This slurry coating technology challenges the production of thicker electrodes with less inactive components. Directly increasing electrode mass loading using the traditional slurry coating fabrication process introduces unavoidable issues such as slowed kinetics and binder degradation. Freestanding electrodes are a great alternative as they eliminate the need for inactive components (i.e., binder, additive carbon, and current collector) and shorten the fabrication process. In addition, it preserves the microstructures of the electrodes and alleviates agglomeration. In this work, carbon microlattice templates are designed and fabricated through the use of 3D stereolithography, which allows for design flexibility of any shape or geometry, followed by pyrolysis to create hard carbon. These hard-carbon templates are then subjected to grow iron sulfide that serves as active material, which has a theoretical specific capacity higher than that of commercial graphite. The design utilizes controlled microchannels that facilitate ion transport through charging and discharging cycles and carbon networks for fast electron transfer. The composites show much improved electrochemical performance. These results demonstrate the potential for advancing the development of composite electrodes and shed light on the enormous potential for developing high-energy density batteries using 3D printing.

Machine-learning-assisted material discovery of oxygen-rich highly porous carbon active materials for aqueous supercapacitors

Porous carbons are the active materials of choice for supercapacitor applications because of their power capability, long-term cycle stability, and wide operating temperatures. However, the development of carbon active materials with improved physicochemical and electrochemical properties is generally carried out via time-consuming and cost-ineffective experimental processes. In this regard, machine-learning technology provides a data-driven approach to examine previously reported research works to find the critical features for developing ideal carbon materials for supercapacitors. Here, we report the design of a machine-learning-derived activation strategy that uses sodium amide and cross-linked polymer precursors to synthesize highly porous carbons (i.e., with specific surface areas > 4000 m2/g). Tuning the pore size and oxygen content of the carbonaceous materials, we report a highly porous carbon-base electrode with 0.7 mg/cm2 of electrode mass loading that exhibits a high specific capacitance of 610 F/g in 1 M H2SO4. This result approaches the specific capacitance of a porous carbon electrode predicted by the machine learning approach. We also investigate the charge storage mechanism and electrolyte transport properties via step potential electrochemical spectroscopy and quasielastic neutron scattering measurements.

Regulating the interconnected microporous structure of porous carbon by alkali metal ions for improved surface and space utilization for organic supercapacitors

Developing porous carbon with suitable microporous structure and large effective surface area is crucial for pursuing high volumetric performance of organic supercapacitors. Herein, we proposed a homogeneous activation method to regulate the interconnected microporous structure of porous carbon through choosing the alkali metal ion activators (Li+, K+, Cs+). We have revealed the pore evolution mechanism from the points of ion activation strength, cluster transport, and metal boiling points. Compared to others, the optimized sample activated by Cs+ shows large micropore volume, high effective surface area (2463 m2 g−1), and high compaction density (0.75 g cm−3), performing an improved volumetric capacitance (126 F cm−3 at 1 A g−1) and good rate performance (79 F cm−3 at 20 A g−1) in organic electrolyte. The symmetric supercapacitor at the commercial electrode mass loading shows a competitive energy density (23.9 Wh L−1 at 211.4 W L−1, 4 Wh L−1 at 1638 W L−1). Moreover, this pore-regulating strategy has a general availability when using other non-foaming carbon precursors, which will open up new paths for improving the surface and space utilization of porous carbon for compact capacitive energy storage.

Full Parameterization Study of a High-Energy and High-Power Li-Ion Cell for Physicochemical Models

For physicochemical modelling of lithium ion batteries, an extensive parametrization is necessary. These parameters need to be derived cell specifically as they vary with cell design. In this study, two cells from the same manufacturer are investigated which are optimized for high power and high energy applications. After opening the cells under argon atmosphere, the battery materials are extracted to conduct various chemical and physical measurements to define the active material type, microstructure, conductivity and mass loading of the electrodes. Furthermore, laboratory cells were built from the extracted materials to evaluate tortuosity and exchange current density by electrochemical impedance spectroscopy, open circuit voltages and solid diffusion coefficient by galvanostatic intermittent titration technique (GITT). The differences and similarities of these parameters for both cell types are discussed and compared to literature. Main differences are the electrode area, thickness, porosity, and thus, mass loading and areal capacity of the electrodes. Both cells have a NCA cathode, but only the high energy cell has a blend anode consisting of graphite and Si/SiOx whereas the anode active material of the high power cell is only made of graphite. The derived parameters are finally used for the parameterization of a P2D model.

Multifunctional MXene–Fe3O4–Carbon Nanotube Composite Electrodes for High Active Mass Asymmetric Supercapacitors

Ti3C2Tx–Fe3O4–carbon nanotube composites were prepared for electrochemical energy storage in the negative electrodes of supercapacitors. The electrodes show a remarkably high areal capacitance of 6.59 F cm−2 in a neutral Na2SO4 electrolyte, which was obtained by the development of advanced nanofabrication strategies and due to the synergistic effect of the individual components. Enhanced capacitance was achieved using the in-situ synthesis method for the Fe3O4 nanoparticles. The superparamagnetic behavior of the Fe3O4 nanoparticles facilitated the fabrication of electrodes with a reduced binder content. Good mixing of the components was achieved using a celestine blue co-dispersant, which adsorbed on the inorganic components and carbon nanotubes and facilitated their co-dispersion and mixing. The capacitive behavior was optimized by the variation of the electrode composition and mass loading in a range of 30–45 mg cm−2. An asymmetric device was proposed and fabricated, which contained a Ti3C2Tx–Fe3O4–carbon nanotube negative electrode and a polypyrrole–carbon nanotube positive electrode for operation in an Na2SO4 electrolyte. The asymmetric supercapacitor device demonstrated high areal capacitance and excellent power-density characteristics in an enlarged voltage window of 1.6 V. This investigation opens a new avenue for the synthesis and design of MXene-based asymmetric supercapacitors for future energy storage devices.

Design and electrochemical properties of carbonized pure bacteria electrode for supercapacitor

Carbonized bacteria are prepared on a nickel plate without any additives (binder, conductor, solvent) to examine the bacteria's own capacitance. The absence of any additives makes it suitable to investigate the capacitance of carbonized bacteria. The Gram-negative bacteria are used as a function of different electrode mass loading to analyze morphological properties and electrochemical properties. At a current density of 1 A∙g−1, the sample prepared showed specific capacitance of 37F∙g−1 after 5000 cycles. These results could prove the manufacture and electrochemical properties of the bacteria for energy storage devices and fundamental research in terms of bacteria-doped composites.

How to Measure Solid State Lithium-ion Diffusion using the Atlung Method for Intercalant Diffusion

A systematic investigation of the factors that affect lithium diffusion coefficient measurements in the Atlung Method for Intercalant Diffusion (AMID) is carried out. Single crystal LiNi0.6Mn0.2Co0.2O2 is used for method development. The factors include electrode mass loading, separator thickness, electrolyte solvent choice and salt molarity, voltage interval sizes, open circuit relaxation time, C-rate choice, and charge vs discharge direction. In an ideal diffusivity measurement method, none of these factors listed should impact the true material diffusivity since it should be a purely material property. However, as we demonstrate here, all these experimental parameters should be selected carefully to minimize cell and electrode resistance contributions so that one can measure true lithium diffusivity in a material that is independent of current direction. We propose a low electrode loading cell design that is applicable for all diffusion measurement methods consisting of an ultrathin electrode coating (0.2 mAh cm−2), thin separator and 1.5 M LiPF6 ethylene carbonate: dimethyl carbonate 1:1 electrolyte. Additionally, we show diffusivity is not dependent on charge or discharge direction in NMC622, and we compare AMID results to Galvanostatic Intermittent Titration. Specific to AMID 15 min OCV time with 0.1 V intervals are suitable for this measurement.

Reconciling Mass Loading and Gravimetric Performance of MnO2 Cathodes by 3D‐Printed Carbon Structures for Zinc‐Ion Batteries

AbstractTo promote the real application of zinc‐ion batteries (ZIBs), reconciling the high mass loading and gravimetric performance of MnO2 electrodes is of paramount importance. Herein, the rational regulation of 3D‐printed carbon microlattices (3DP CMs) enabling an ultrathick MnO2 electrode with well‐maintained gravimetric capacities is demonstrated. The 3DP CMs made of graphene and carbon nanotubes (CNTs) are fabricated by direct ink 3D printing and subsequent high‐temperature annealing. 3D printing enables a periodic structure of 3DP CMs, while the thermal annealing contributes to high conductivity and defective surfaces. Due to these structural merits, uniform electrical field distribution and facilitated MnO2 deposition over the 3DP CMs are permitted. The optimal electrode with MnO2 loaded on the 3DP CMs can achieve a record‐high specific capacity of 282.8 mAh g−1 even at a high mass loading of 28.4 mg cm−2 and high ion transfer dynamics, which reconciles the loading mass and gravimetric performance. As a result, the aqueous ZIBs based on the 3DP CMs loaded MnO2 afford an outstanding performance superior to most of the previous reports. This study reveals the essential role of interaction between active materials and current collectors, providing an alternative strategy for designing high‐performance energy storage devices.

Surficial Sulfur Loss of Jet-Milled Li6PS5Cl Powder under Mild-Temperature Heat Treatment

All-solid-state batteries employing inorganic solid electrolytes are considered a promising next-generation energy storage system. Especially, argyrodite-type Li6PS5Cl (LPSCl) was highlighted for its good ionic conductivity and malleability. In the electrode level, achieving an efficient Li+ pathway even with the maximized active material proportion and electrode mass loading is vital for a high energy density. Therefore, a smaller solid electrolyte particle is highly sought for. Herein, we demonstrated a spiral jet mill process for refining LPSCl particle size. The mean particle size (D50) of LPSCl was reduced from 39.9 to 1.9 μm where the ionic conductivity was decreased from 2.0 to 0.23 mS cm–1. The post-annealing in a mild-temperature range (T < 250 °C) led to the even more decreased ionic conductivity. Although the ionic conductivity was restored when T > 300 °C, this high-temperature annealing resulted in severe particle agglomeration. Spectroscopic observations revealed that particle surfaces were damaged during pulverization and irreversible sulfur losses occurred during the mild-temperature annealing.