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All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Electrolyte and Cutoff Potential Effects on Cycle Life of Li4Ti5O12/LiNi0.9Mn0.1O2 Batteries for Behind-the-Meter Storage Applications

Behind-the-Meter Storage (BTMS) is a stationary battery energy storage system that is connected to the electrical distribution system on the customer's side of the utility's service meter. BTMS systems are used to store electrical energy from the grid as well as inconstant, renewable energy, such as local solar and wind generation. A successful BTMS system will allow the customer to pair their energy generation and storage to optimize electrical consumption from the grid, improving reliability and minimizing cost. For BTMS applications, batteries must be designed and optimized with different set of criteria from other leading segments of the Li-ion battery market, like transportation, due the system being stationary and proximal to the residential or commercial building it's benefitting. BTMS applications prioritize safety, cost (low/no-critical materials), reliability (20-year calendar life), and durability (10,000 cycle life), while having the ability to (minimally) compromise energy density and rate capability. Lithium titanate (Li4Ti5O12-, LTO) is a promising anode candidate for BTMS applications due to its high safety and capacity retention, while maintaining a reasonable 160 mAhg-1 reversable capacity and composition of relatively abundant materials. (1) Specifically, LTO has a high working voltage which helps to prevent Li dendrite formation, improving safety. Furthermore, LTO also has negligible lithiation-based volume change, leading to less mechanical pulverization, or loss of active material, upon cycling. For the cathode, materials with little or no Co are of high interest due to the high cost and low abundance of Co. LiMn2O4 (LMO) has been paired with LTO for BTMS applications in the past due to its safety, low cost (abundancy), and reasonably high operating voltage. (2-4) However, the low capacity of LMO limits energy density and specific energy. While not the highest priority for BTMS applications, increasing energy density will enable deployment in space constrained BTMS applications and decrease total cost. LiNi0.9Mn0.1O2 (LN-MO) is a recently developed material with promise due to its high operating voltage and relatively low price. (5) However, Ni-rich layered oxides, including LNMO, tend to struggle with capacity retention during high-voltage cycling due to mechanical pulverization, irreversible phase transitions, and unstable solid-electrolyte interphase. The study presented here focuses on building an understanding of how electrolyte solvent and varied cutoff potentials will impact the cycle life of LTO/LN-MO cells. Specifically, a comparison is provided between ethylene carbonate (EC), ethyl methyl carbonate (EMC), fluoroethylene carbonate (FEC), and Gen2 electrolyte solvents with 1M Lithium hexafluorophosphate (LiPF6) salt, cycling to two upper termination potentials, 2.6V and 2.7V. Electrochemical testing and diagnostics (e.g., differential capacity analysis, area specific impedance, constant voltage hold, and rate capability) and post-mortem characterization will be used to understand the aging behavior and failure mechanisms of the 8 cell combinations (four electrolytes and two voltage cutoffs). Cells with FEC electrolyte showed a lower initial capacity compared to cells with Gen2, EMC, and EC cycling at both voltages; however, the cells with FEC showed consistent trends in capacity retention with 2.6V and 2.7V termination potentials, while the cells with the other electrolytes showed much higher rates of capacity loss when cycling to the higher voltage. These results indicate that FEC may play a role in improving durability of high-voltage, Ni-rich electrode systems for use in high-cycle applications, such as BTMS.

electrolyte↗
DOE OSTI2021

Investigating Ternary Li–Mg–Si Zintl Phase Formation and Evolution for Si Anodes in Li-Ion Batteries with Mg(TFSI) 2 Electrolyte Additive

Improved electrochemical performance of Si was recently reported by adding multivalent cation salts (such as Mg 2+ , Al 3+ , Ca 2+, etc.) in the electrolyte. This is achieved via the formation in an in situ manner of relatively more stable Li-M-Si ternary phases with less chemical reactivity. These phases stabilize Si anions and thus reduce side reactions with electrolytes at the surface and eventually benefit the overall electrochemistry. To understand the mechanism of ternary Zintl phase formation and its dynamics upon lithiation/delithiation, high-resolution solid-state 7 Li and 29 Si nuclear magnetic resonance (NMR) are utilized to directly probe the local Li and Si environments on Si electrodes harvested from coin and pouch cells at various states of (de)lithiation. The NMR spectra along with the electrochemical characterization reveal that lithiation of Si starts from the surface Si-O layer further confirmed by 7 Li– 29 Si cross-polarization NMR. Lithiation progresses with heterogeneous silicon clustering with Si -4 anions at high states of lithiation. At a fully lithiated state, the formation of overlithiated Si species is detected. At a low-voltage region (below 100 mV), direct evidence for Mg-ion insertion is found, postulated by two possible mechanisms: ion exchange with fully or overlithiated binary domains (Li 3.75+x Si) and/or a coinsertion with slightly underlithiated domains (similar to Li 3.55 Si). Upon delithiation, Li extraction starts from overlithiated Si domains. No evidence is found for electrochemical Mg removal. Evidence for a lithium-deficient Li y Mg 0.1 Si phase is found as a result of Li removal during charging. This investigation sheds light on the possible mechanisms of a new Si anode chemistry, which could enable the development of stable Si-based anodes for lithium-ion batteries.

25 ENERGY STORAGE↗
DOE OSTI2024

Inactive Overhang in Silicon Anodes

Li-ion batteries contain excess anode area to improve manufacturability and prevent Li plating. These overhang areas in graphite electrodes are active but experience decreased Li + flux during cycling. Over time, the overhang and the anode portions directly opposite to the cathode can exchange Li + , driven by differences in local electrical potential across the electrode, which artificially inflates or decreases the measured cell capacity. Here, we show that lithiation of the overhang is less likely to happen in silicon anodes paired with layered oxide cathodes. The large voltage hysteresis of silicon creates a lower driving force for Li + exchange as lithium ions transit into the overhang, rendering this exchange highly inefficient. For crystalline Si particles, Li + storage at the overhang is prohibitive, because the low potential required for the initial lithiation can act as thermodynamic barrier for this exchange. We use micro-Raman spectroscopy to demonstrate that crystalline Si particles at the overhang are never lithiated even after cell storage at 45 °C for four months. Because the anode overhang can affect the forecasting of cell life, cells using silicon anodes may require different methodologies for life estimation compared to those used for traditional graphite-based Li-ion batteries.

25 ENERGY STORAGE↗
DOE OSTI2022

Paving Pathways Toward Long–Life Graphite/LiNi 0.5 Mn 1.5 O 4 Full Cells: Electrochemical and Interphasial Points of View

The high-voltage spinel cathode LiNi 0.5 Mn 1.5 O 4 (LNMO) with no cobalt and low nickel content is promising for lithium-ion batteries due to its high energy and power densities, good thermal stability, and low cost. However, its high operating voltage (≈4.7 V) results in a decomposition of the electrolyte, severe chemical crossover, and deterioration of electrode-electrolyte interphases (EEIs), hindering its practical viability. It is demonstrated here that by electrochemically pre-cycling the graphite in an electrolyte containing 30 wt.% fluoroethylene carbonate, a robust LiF-rich artificial solid electrolyte interphase can be constructed for surface protection; on the other hand, an electrochemical pre-lithiation of Fe-doped LNMO serves as a lithium source for graphite at a full-cell voltage of 2.6 V. As a result, the full cells with the as-modified electrodes deliver a high capacity of 129 mA h g –1 with an excellent capacity retention of 93% after 200 cycles, vastly outperforming the full cells with fresh electrodes (120 mA h g –1 initial capacity with 78% retention). Lastly, pathways toward long-life graphite||LNMO full cells are pictured based on the inspirations from the electrochemical modifications and in-depth analyses of the EEIs in this study.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Dynamic In‐Plane Heterogeneous and Inverted Response of Graphite to Fast Charging and Discharging Conditions in Lithium‐Ion Pouch Cells

Solutions for improving fast charging of lithium‐ion batteries have largely focused on alleviating through‐plane lithiation gradients while little is understood about in‐plane heterogeneities and how to resolve them. Herein, high‐speed synchrotron X‐ray diffraction (XRD) resolves graphite lithiation spatially and temporally during 6 C charging and 2 C discharging. At every point during operation, considerable differences in the state of lithiation across the pouch cell are present. Some regions are more responsive to operation than others, reaching full lithiation early during charge and full delithiation during discharge. Other regions within the cell never fully delithiate during discharge, despite a prolonged voltage hold at 2.8 V. Using time‐resolved XRD data, the calculated local current density (mA cm −2 ) at the graphite surface shows an unexpected occurrence of local inverted current densities where regions of graphite are observed to delithiate during charging and lithiate during discharging. A pseudo‐3D model is developed for the graphite electrode with spatially varying microstructural tortuosity to show how microstructural heterogeneity could influence spatial charge dynamics. The model could not predict the complex in‐plane charge behavior observed within the cell. Consequently physics‐based charging protocols based on homogeneous electrode assumptions may underestimate the local variations in charge dynamics and occurrence of lithium plating.

25 ENERGY STORAGE↗
DOE OSTI2020

The Study of the Binder Poly(acrylic acid) and Its Role in Concomitant Solid–Electrolyte Interphase Formation on Si Anodes

In this work we use neutron reflectometry to study how the polymeric binder, poly(acrylic acid) (PAA), affects the in situ formation and chemical composition of the solid–electrolyte interphase (SEI) formation on a silicon anode at various states of charge. The reflectivity is correlated with electrochemical quartz crystal microbalance to better understand the viscoelastic effects of the polymer during cycling. The use of model thin films allows for a well-controlled interface between the amorphous Si surface and the PAA layer. If the PAA perfectly coats the Si surface and standard processing conditions are used, the binder will prevent the lithiation of the anode. The PAA suppresses the growth of a new layer formed at early states of discharge (open circuit voltage to 0.8 V vs Li/Li+), protecting the surface of the anode. At 0.15 V, the SEI layer underneath the PAA changes in chemical composition as indicated by an increase in the scattering length density and thickness as the layer incorporates components from the electrolyte, most likely the salt. At lithiated and delithiated states, the SEI layer changes in chemical composition and grows in thickness with delithiation and shrinks during lithiation.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Impact of lithium wall conditioning and wave-frequency on high density lower hybrid current drive experiment on EAST

A series of dedicated lower hybrid current drive (LHCD) experiments on EAST shows that lithium wall conditioning extends LH current drive and heating up to the line-averaged density of n ̅_e≈ 4x1019 m-3 for both 2.45 and 4.6 GHz. Current drive at such a high density is crucial for the development of long-pulse non-inductive scenarios on EAST. With lithiation, the LH power injection of 1.5 MW at 2.45 GHz resulted in a drop of loop voltage of ~ 0.3 V, which is a comparable loop voltage drop observed with 1.1 MW at 4.6 GHz. The observed decrease in loop voltage is attributed mostly to the RF heating effect. Another LHCD experiment suggests that lithium wall coating has a more significant impact on the scrape-off-layer (SOL) properties than changes in the Greenwald fraction. LHCD at 2.45 GHz still suffers from a loss of efficiency. Enhanced power ionization in front of the launcher may cause the onset of density-dependent wave instabilities. The rise in the midplane SOL density may also accelerate a transition in the divertor regime, leading to additional ionization and collisional losses in the X-point divertor plasma. Ray-tracing modeling supports that a lower wave frequency is more prone to collisional power loss. The experiments confirm that lithiation is a useful tool to control the SOL plasma, and suggest that density control in front of the launcher may be critical to mitigating power loss mechanisms in the plasma boundary.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Assessing cathode property prediction via exchange-correlation functionals with and without long-range dispersion corrections

In this work, we benchmark calculated interlayer spacings, average topotactic voltages, thermodynamic stabilities, and band gaps in layered lithium transition-metal oxides (TMOs) and their de-lithiated counterparts, which are used in lithium-ion batteries as positive electrode materials, against available experimental data. Specifically, we examine the accuracy of properties calculated within density functional theory (DFT) using eight different treatments of electron exchange-correlation: the strongly constrained and appropriately normed (SCAN) and Perdew–Burke–Ernzerhof (PBE) density functionals, Hubbard-U-corrected SCAN and PBE (i.e., SCAN+U and PBE+U), and SCAN(+U) and PBE(+U) with added long-range dispersion (D) interactions (i.e., DFT(+U)+D). van der Waals interactions are included respectively via the revised Vydrov-Van Voorhis (rVV10) for SCAN(+U) and the DFT-D3 for PBE(+U). We find that SCAN-based functionals predict larger voltages due to an underestimation of stability of the MO2 systems, while also predicting smaller interlayer spacings compared to their PBE-based counterparts. Furthermore, adding dispersion corrections to PBE has a greater effect on voltage predictions and interlayer spacings than with SCAN, indicating that DFT-SCAN – despite being a ground-state theory – fortuitously captures some short and medium-range dispersion interactions better than PBE. While SCAN-based and PBE-based functionals yield qualitatively similar band gap predictions, there is no significant quantitative improvement of SCAN-based functionals over the corresponding PBE-based versions. Finally, we expect SCAN-based functionals to yield more accurate property predictions than the respective PBE-based functionals for most TMOs, given SCAN's stronger theoretical underpinning and better predictions of systematic trends in interlayer spacings, intercalation voltages, and band gaps obtained in this work.

25 ENERGY STORAGE↗
DOE OSTI2021

A solid electrolyte interphase to protect the sulfurized polyacrylonitrile (SPAN) composite for Li–S batteries: computational approach addressing the electrolyte/SPAN interfacial reactivity

This study addresses the reactivity of multiple solvents and lithium bis(fluorosulfonyl)imide (LiFSI) at the interface with sulfurized polyacrylonitrile (SPAN) in multiple stages of lithiation via ab initio molecular dynamics simulations. Here, both ether 1,3-dioxolane (DOL) and dimethyl carbonate (DMC) proved stable on the lithiated SPAN surface regardless of the lithium content, meaning that neither of these species likely contributes to growing a solid electrolyte interphase (SEI) coating to protect the SPAN composite. Conversely, cyclic carbonates, ethylene carbonate (EC) and fluoroethylene carbonate (FEC) are shown to be very active. The EC reduction occurs only on a highly lithiated surface with a 3.0 Li/S molar ratio, the equivalent of the SPAN composite in an over-discharge regime with voltages close to 0.0 V vs. Li/Li + . The FEC reduction starts with a 2.0 Li/S molar ratio and above, suggesting that FEC could act as a useful additive in electrolyte formulations with EC. Both EC and FEC follow multiple reduction mechanisms to produce complex reduction products and LiF in the FEC case. We provide a mechanistic description for each detected decomposition path. The LiFSI salt also proves reactive against the lithiated SPAN surface. The FSI – defluorination is the dominant reduction path. However, the SO 2 NSO 2 F – and SO 2 NSO 2 2– species proved stable against S–N cleavage. This behavior makes the LiFSI salt a potential candidate for SPAN-based Li–S batteries because it produces LiF without releasing SO 2 .

25 ENERGY STORAGE↗
DOE OSTI2022

Regulation of Surface Defect Chemistry toward Stable Ni-Rich Cathodes

Surface reconstruction of Ni-rich layered oxides (NLO) degrades the cycling stability and safety of high-energy-density lithium-ion batteries (LIBs), which challenges typical surface-modification approaches to build a robust interface with electrochemical activity. Here, a strategy of leveraging the low-strain analogues of Li- and Mn-rich layered oxides (LMR) to reconstruct a stable surface on the Ni-rich layered cathodes is proposed. The new surface structure not only consists of a gradient chemical composition but also contains a defect-rich structure regarding the formation of oxygen vacancies and cationic ordering, which can simultaneously facilitate lithium diffusion and stabilize the crystal structure during the (de)lithiation. These features in the NLO lead to a dramatic improvement in electrochemical properties, especially the cyclability under high voltage cycling, exhibiting the 30% increase in capacity retention after 200 cycles at the current density of 1 C (3.0-4.6 V). We report the findings offer a facile and effective way to regulate defect chemistry and surface structure in parallel on Ni-rich layered structure cathodes to achieve high-energy density LIBs.

25 ENERGY STORAGE↗
DOE OSTI2025

Precise Synthesis of 4.75 V-Tolerant LiCoO 2 with Homogeneous Delithiation and Reduced Internal Strain

The rapid advancements in 3C electronic devices necessitate an increase in the charge cutoff voltage of LiCoO 2 to unlock a higher energy density that surpasses the currently available levels. However, the structural devastation and electrochemical decay of LiCoO 2 are significantly exacerbated, particularly at ≥4.5 V, due to the stress concentration caused by more severe lattice expansion and shrinkage, coupled with heterogeneous Li + intercalation/deintercalation reactions. Herein, employing the molten-salt synthesis technique, we propose a universal morphology-shaping strategy to attain bulk reaction homogeneity and reduce internal strains, even at extremely high charge voltages. The newly designed flattened polygon prismlike LiCoO 2 (P-LCO) particle, featuring a regular symmetrical arrangement along the c-axis, demonstrates a more homogeneous Li + extraction/insertion reaction, which results in a restrained transformation to detrimental O1 phase and reduced variation in lattice volume throughout the (de)lithiation processes. This benefits the mitigation of the local stress accumulation misfit dislocations and particle cracking, ultimately maintaining the mechanical stability of the cathode. Consequently, P-LCO is capable of breaking the voltage ceiling and exhibits exceptional long-term cycling capability at an ultrahigh voltage of 4.75 V. In conclusion, this work offers a brand-new perspective for the rational design of cathode morphology to address capacity deterioration caused by inhomogeneous delithiation and internal strain, thus inspiring the development of high-energy-density cathodes with improved durability.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Anionic Oxygen Redox in the High-Lithium Material Li 8 SnO 6

Lithiated transition metal oxide cathode materials that combine oxygen-anion redox [O 2 – /(O 2 ) n– ] with cation redox offer substantial capacities and higher voltage than cathodes without oxygen redox, but oxygen-anion redox is usually accompanied by the formation of peroxo or superoxo species that may cause oxygen release. To clarify the relationship between the oxygen release and peroxide and superoxide dimers without the complication of transition-metal redox, we performed density functional theory calculations to study the layered Li 8 SnO 6 cathode because it has only oxygen-anion redox during delithiation. Features with O–O distances of ~1.5 Å (corresponding to the formation of a peroxide) were observed at 75% lithium concentration, and further delithiation to 62.5% lithium concentration shortened the O–O bond distance to ~1.3 Å (corresponding to the formation of superoxide). The solely anionic oxygen redox stabilizes the delithiated structure of Li 8–x SnO 6 , and it does not cause structural disorder or release oxygen gas. Ab initio molecular dynamics calculations in the supercell at 300 K indicate that oxygen dimerization may occur at 75% lithium concentration. On the (0001) surface with a high lithium concentration (98%), oxygen dimerization is unfavorable. The calculations by HSE06 show that oxygen redox in Li 8 SnO 6 offers an initial voltage (vs Li/Li + ) greater than 4.2 V. The GGA+U calculations indicate that the Li + -polaron in Li 95 Sn 12 O 72 migrates with a migration barrier of only 0.43 eV. Here, this work also quantifies a solely anionic oxygen redox mechanism in Li 8 SnO 6 , and it provides a deeper understanding of anionic oxygen redox in Li-excess oxide cathode materials.

25 ENERGY STORAGE↗
DOE OSTI2020

Sulfurized Polyacrylonitrile for High-Performance Lithium–Sulfur Batteries: In-Depth Computational Approach Revealing Multiple Sulfur’s Reduction Pathways and Hidden Li + Storage Mechanisms for Extra Discharge Capacity

Like no other sulfur host material, polyacrylonitrile-derived sulfurized carbon (SPAN) promises improved electrochemical performance for lithium–sulfur batteries, based on its compatibility with carbonate solvents and ability to prevent self-discharge and shuttle effect. However, a complete understanding of the SPAN’s lithiation mechanism is still missing because its structural features vary widely with synthesis conditions, and its electrochemical performance deviates from elemental sulfur. This study continues our research on the elucidation of the SPAN’s structural characteristics and lithiation mechanisms via computational approaches. Here, our models reproduce most experimental data regarding carbon’s graphitization level and conjugated ordering, sulfur–carbon covalent bonding, sulfur loading, and elemental composition. Our simulations emulate the discharge voltage observed in experiments for the first discharge, which reveals that sulfur follows multiple reduction pathways based on its interaction with the carbon backbone. Sulfur reduction takes place above 1.0 V vs Li/Li + mostly in the SPAN-like material, with no long-chain lithium polysulfide formation. Below 1.0 V vs Li/Li + , the backbone’s electrochemical activity occurs via multiple C–Li and N–Li interactions, mostly with edge carbon atoms and pyridinic nitrogen. Moreover, we identify Li + binding sites throughout the graphitized backbone that might lead to prohibited energy costs for Li + deintercalation, which may explain the irreversible capacity loss between the first and second discharges. This work improves understanding of lithiation mechanisms in sulfurized carbon, which is useful for rationally designing SPAN synthesis pathways tailored to increase sulfur loading and enhanced electrochemical performance.

25 ENERGY STORAGE↗
DOE OSTI2021

Potentiometric entropy and operando calorimetric measurements reveal fast charging mechanisms in PNb 9 O 25

The Wadsley–Roth compound PNb 9 O 25 is a promising fast charging lithium ion battery anode material with high operating voltage to prevent solid electrolyte interface formation. Here, in this work, we present potentiometric entropy measurements featuring signatures of semiconductor-to-metal transition and intralayer ordering upon lithiation in the anode material PNb 9 O 25 that could not be observed with in situ X-ray diffraction. In addition, the instantaneous heat generation rates at the PNb 9 O 25 working electrode and at the lithium metal counter electrode during galvanostatic cycling were measured individually for the first time by operando isothermal calorimetry. The heat generation rate decreased at the PNb 9 O 25 electrode upon lithiation due to the decrease in electrical resistivity caused by the semiconductor-to-metal transition observed in potentiometric entropy measurements. Furthermore, the heat generation rate at the lithium metal electrode was positive during delithiation due to the exothermic plating of Li + ions on the lithium metal counter electrode associated with dendrite formation. Furthermore, calorimetric measurements established that the entropy change dominated the reversible heat generation rate at each electrode. Finally, the contribution of enthalpy of mixing was relatively small even at high C-rates thanks to the high Li + ion mobility in PNb 9 O 25 confirming its promises as a fast charging anode material.

25 ENERGY STORAGE↗
DOE OSTI2023

Structure design enables stable anionic and cationic redox chemistry in a T2-type Li-excess layered oxide cathode

Coupled with anionic and cationic redox chemistry, Li-rich/excess cathode materials are prospective high-energy-density candidates for the next-generation Li-ion batteries. However, irreversible lattice oxygen loss would exacerbate irreversible transition metal migration, resulting in a drastic voltage decay and capacity degeneration. Herein, a metastable layered Li-excess cathode material, T2-type Li 0.72 [Li 0.12 Ni 0.36 Mn 0.52 ]O 2 , was developed, in which both oxygen stacking arrangement and Li coordination environment fundamentally differ from that in conventional O3-type layered structures. By means of the reversible Li migration processes and structural evolutions, not only can voltage decay be effectively restrained, but also excellent capacity retention can be achieved upon long-term cycling. Moreover, irreversible/reversible anionic/cationic redox activities have been well assigned and quantified by various in/ex-situ spectroscopic techniques, further clarifying the charge compensation mechanism associated with (de)lithiation. These findings of the novel T2 structure with the enhanced anionic redox stability will provide a new scope for the development of high-energy-density Li-rich cathode materials.

25 ENERGY STORAGE↗
DOE OSTI2023

Trimer Quinoxalines as Organic Cathode Materials for Lithium-Ion Batteries

Due to synthetic ease, high redox potentials, low solubility in polar electrolytes, and good electric conductivity of their semiconducting crystals, discotic quinoxaline trimers (3Q) have been considered as possible candidates for 4 V organic cathodes in lithium-ion batteries. To assess their feasibility as such materials, several 3Q derivatives have been synthesized and tested in half-cells. In voltage limited cycling tests at 1.2–3.9 V vs Li/Li + , the specific discharge capacities of 40–180 mAh g -1 were obtained at a rate of 1 C, and multiple lithiation of 3Q and its derivatives was observed during discharge. However, the obtained discharge capacity was only a fraction of the theoretical capacity expected for reversible six-electron reduction; there was also rapid capacity fade. Our spectroscopic studies indicate the reversible three-electron lithiation at 2 V vs Li/Li + , but suggest instability of more highly discharged states. Finally, our conclusion is that while the 3Q derivatives combine several traits that are desirable in an organic cathode material (including negligible solubility, capacity for multiple charging, and near-100% coulombic efficiency), these materials are still impractical to use.

25 ENERGY STORAGE↗
DOE OSTI2021

Benefits of Fast Battery Formation in a Model System

Lithium-ion battery formation affects battery cost, energy density, and lifetime. An improved understanding of the first cycle of solid-electrolyte interphase (SEI) growth on carbonaceous negative electrodes could aid in the design of optimized formation protocols. In this work, we systematically study SEI growth during the formation of carbon black negative electrodes in a standard carbonate electrolyte. We show that the initial ethylene carbonate (EC) reduction reaction occurs at ~0.5-1.2 V during the first lithiation, except under fast lithiation rates (≥10C). The products of this EC reduction reaction do not passivate the electrode; only the SEI formed at lower potentials affects the second-cycle Coulombic efficiency. Thus, cycling quickly through the voltage regime of this reaction can decrease both formation time and first-cycle capacity loss, without an increase in subsequent-cycle capacity loss. We also show that the capacity consumed by this reaction is minimized at low temperatures and low salt concentrations. Finally, we discuss the mechanism behind our experimental results. This work reveals the fundamental processes underlying initial SEI growth on carbonaceous negative electrodes and provides insights for both optimizing the battery formation process and enabling novel electrolytes.

25 ENERGY STORAGE↗
DOE OSTI2021

In Situ ATR-FTIR Study of the Cathode–Electrolyte Interphase: Electrolyte Solution Structure, Transition Metal Redox, and Surface Layer Evolution

We present a study of the lithium nickel manganese cobalt oxide (LiNi 0.6 Mn 0.2 Co 0.2 O 2 , NMC622) cathode-electrolyte interphase (CEI) during galvanostatic charging and discharging using in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) methods to investigate the voltage dependent electrolyte solution structure changes at the interface, transition metal (TM) redox chemistry, and cathode/electrolyte interfacial layer evolution. Furthermore, our in situ cell design provides both reliable electrochemical device testing and strong FTIR vibrational absorption signals near the cathode surface. Specifically, advanced spectral analysis elucidates changes of near-surface Li+ ion (de)solvation by solvent molecules during galvanostatic cycling. Moreover, cathode metal-oxygen vibrational absorptions, sensitive to TM redox behaviors and subsequent local structural variations, were correlated to cathode de-lithiation (and lithiation) and electrolyte solution structure changes. In addition, we have detected the formation and evolution of a CEI surface layer on the NMC622 cathode that contributes to the cell's capacity fade.

25 ENERGY STORAGE↗