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DOE OSTI2024

Structural and chemical evolutions of a magnesium vanadium oxide cathode under electrochemical cycling in magnesium batteries

The design of cathode materials that remain chemically and structurally stable during repetitive ion insertion and extraction poses a significant challenge in developing multivalent batteries. The cycling stability of traditional metal oxide-based cathode is challenged by sluggish diffusion of multivalent cations and parasitic reactivity at interfacial regimes, including the cathode electrolyte interphase layer (CEI). Understanding the reactions at the cathode-electrolyte interface, particularly those induced by non-stoichiometric surface layers, is a crucial design parameter for both cathode materials and electrolytes. Here, in this study, we employed multimodal analysis, including in situ and ex situ X-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (TEM) and electrochemical impedance spectroscopy (EIS) to examine the surface reactions and subsequent structural and chemical evolutions of the CEI on high voltage magnesium vanadium oxide (MgV 2 O 4 ) spinel cathode during the Mg 2+ insertion/extraction processes. The results revealed that the presence of non-stoichiometric surface layers in the magnesium vanadium oxide cathode drive the decomposition of bis(trifluoromethanesulfonyl)imide (TFSI - ) anion, leading to the formation of the CEI layer. The CEI layer could inhibit the Mg 2+ ion transfer processes. Accompanying this reactivity-driven degradation, the magnesium vanadium oxide cathode undergoes pulverization, forming clusters of nanosized particles. This process likely improves cycling ability by creating new intercalation sites and shortening the diffusion pathway for the Mg 2+ cations. This study demonstrates that controlling surface stoichiometry and engineering morphological properties are critical design parameters for high performance cathodes for multivalent batteries.

25 ENERGY STORAGE↗
DOE OSTI2022

Enhancing Cycling Stability and Capacity Retention of NMC811 Cathodes by Reengineering Interfaces via Electrochemical Fluorination

High-capacity cathodes (LiNi 0.8 Mn 0.1 Co 0.1 O 2 ) that can boost the energy density of lithium-ion batteries are promising candidates for vehicle electrification. However, several factors specific to high energy density materials entailing electrode reactions inhibit their application. Fluorination has shown a promising ability to combat the detrimental electrochemical performances of cathode materials, however, it remains difficult to achieve the desired functionality. In this work, a novel electrochemical fluorination (ECF) that demonstrates a promising electrochemical performance enhancement via stabilization of the cathode–electrolyte-interphase (CEI) by forming conformal LiF is proposed. Besides LiF surface layer formation, ECF reduces the degree of fluorination-induced Ni/Li disordering and enhances the layered structural stability as probed by X-ray diffraction. Because of the robust CEI, ECF-NMC811 cathodes deliver 203.0 mAh g -1 first discharge capacity at the current rate of C/10, with ≈98% capacity retention up to 100 cycles. Similarly, it delivers ≈180 mAh g -1 capacity at a 1 C rate with 86.4% capacity retention up to 200 cycles with average coulombic efficiency of > 99.5%. Comprehensive characterization with a multitude of probes reveals that ECF enhances the cycling stability of the electrode without altering bulk structure and morphology.

25 ENERGY STORAGE↗
DOE OSTI2022

In-situ synthesis of porous metal fluoride@carbon composite via simultaneous etching/fluorination enabled superior Li storage performance

Transition metal fluorides as Li-free conversion-type cathode materials have high theoretical specific capacities, however, their preparation strategy, sluggish electrochemical kinetic and poor cyclability have impeded their wide adoption in lithium-ion batteries. Herein, a facile in-situ synthesis of porous metal-fluoride-carbon composites is accomplished via simultaneous polytetrafluorethylene-based hard template etching and metal fluorination. This not only facilitates fast electron transfer and lithium-ion diffusion kinetics, but also buffers severe volume fluctuation during lithiation/delithation and enables the formation of a uniform and thin Li 2 CO 3 /LiF-rich cathode-electrolyte interphase. Here, as a proof of concept, the as-prepared porous FeF 3 @C (p-FeF 3 @C) indeed exhibits a high specific capacity of 230 mAh g -1 at 0.1 C together with an excellent capacity retention of 92.5% at 1 C for 200-cycles. Moreover, the practicality of the strategy is demonstrated by the superb electrochemical performance of the full-cells coupled with pre-lithiated graphite anodes. Therefore, the proposed novel synthetic strategy will enlighten the future design of high-performance metal-fluoride-carbon composites with porous structure for energy storage applications.

25 ENERGY STORAGE↗
DOE OSTI2025

Exploring the Electrochemical Stability Window of an All-Solid-State Composite Cathode via a Novel Operando Tender XPS Setup

All-solid-state batteries (ASSBs) have the potential to provide greater energy density than conventional batteries based on liquid electrolytes. Here, an operando ASSB cell setup for tender X-ray photoelectron spectroscopy (XPS) was developed, and the interface of a Ni-rich layered transition metal oxide cathode active material (CAM) and an Li 6 PS 5 Cl (LPSCl) solid electrolyte (SE) was evaluated during initial charge/discharge cycles. After validating the cell performance against a conventional pouch cell operated at high compression, intermittent galvanostatic cycling was performed, and XPS data were recorded as a function of state of charge (SOC). Upon the initial charge of the cell to ≈3.3 V Li , the LPSCl appears to decompose into LiCl, Li 3 PS 4 , and polysulfides, whose amount gradually increases with potential. Upon further charge, at a potential higher than ≈3.8 V Li , initially, present sulfate and sulfite impurities decompose, and at ≈74% SOC (corresponding to a cathode potential of ≈4.10 V Li ), surface reconstruction of the CAM particles due to lattice oxygen release is detected. In addition, at potentials beyond ≈4.6 V Li , a decrease of the S 1s counts of the sum of the LPSCl, the thiophosphate, and polysulfide species suggests the formation of elemental sulfur that is lost via sublimation into the vacuum chamber.

25 ENERGY STORAGE↗
DOE OSTI2024

Alumina – Stabilized SEI and CEI in Potassium Metal Batteries

Aluminum oxide (Al 2 O 3 ) nanopowder is spin-coated onto both sides of commercial polypropene separator to create artificial solid-electrolyte interphase (SEI) and artificial cathode electrolyte interface (CEI) in potassium metal batteries (KMBs). Here, this significantly enhances the stability, including of KMBs with Prussian Blue (PB) cathodes. For example, symmetric cells are stable after 1,000 cycles at 0.5 mA/cm 2 –0.5 mAh/cm 2 and 3.0 mA/cm 2 –0.5 mAh/cm 2 . Alumina modified separators promote electrolyte wetting and increase ionic conductivity (0.59 vs. 0.2 mS/cm) and transference number (0.81 vs. 0.23). Cryo-stage focused ion beam (cryo-FIB) analysis of cycled modified anode demonstrates dense and planar electrodeposits, versus unmodified baseline consisting of metal filaments (dendrites) interspersed with pores and SEI. Alumina-modified CEI also suppresses elemental Fe crossover and reduces cathode cracking. Mesoscale modeling of metal – SEI interactions captures crucial role of intrinsic heterogeneities, illustrating how artificial SEI affects reaction current distribution, conductivity and morphological stability.

25 ENERGY STORAGE↗
DOE OSTI2026

Enhanced cycling stability of Ni-rich Li-metal cells enabled by dual vinylene carbonate and tris(trimethylsilyl)borate electrolyte additives

NMC811 (LiNi 0.8 Co 0.1 Mn 0.1 O 2 ) and other high-Ni chemistries are promising cathode candidates for high-performance electric vehicles, owing to their high energy density and reduced cobalt content. However, their long-term cycling stability is hindered by surface degradation, particularly when paired with conventional electrolytes and a lithium metal anode. Electrolyte additives represent a practical approach to enhance interfacial stability and improve overall battery performance by promoting the formation of a robust electrolyte–electrode interphase (EEI). In this study, we revisit the effects of vinylene carbonate (VC) and tris(trimethylsilyl)borate (TMSB) additives on single-crystal SC-NMC811||Li cells. While TMSB only increases the open-circuit voltage and initial overpotential, it delivers superior capacity retention at C/3 compared to cells containing only VC or a dual additive system (VC and TMSB). Notably, under fast-charging conditions (1C, 2C, and 5C), the dual-additive system significantly outperforms other formulations, achieving markedly enhanced long-term capacity retention. Comprehensive electrochemical and spectroscopic analyses reveal that the VC/TMSB dual-additive system suppresses surface transition in NMC811, mitigates structural degradation by forming a thin, LiF-deficient cathode-electrolyte interface (CEI) layer. Moreover, they promote smooth and dense Li deposition and generate a LiF-deficient solid-electrolyte interphase (SEI). Consequently, the synergistic stabilization of both the CEI and SEI effectively limits the overall cell impedance growth during extended cycling. These findings provide key insights into co-additive strategies for engineering stable interfaces in high-energy Ni-rich Li-metal batteries.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Mitigating Calendar Aging in Si-NMC Batteries with Advanced Dual-Salt Glyme Electrolytes

In addressing the critical challenge of calendar aging in silicon (Si)-based lithium-ion batteries, this study introduces a groundbreaking strategy utilizing glyme-type dual-salt electrolytes (lithium bis(trifluoromethanesulfonyl)imide [LiTFSI] and lithium difluoro(oxalato)borate [LiDFOB]). These electrolytes are demonstrated to significantly mitigate parasitic reactions and capacity loss in Si-NMC (lithium nickel manganese cobalt oxide) full cells, especially when compared with traditional carbonate-based electrolytes. Further, our exhaustive mechanistic analysis reveals that such electrolytes not only preserve the integrity of the Si anode but also improve the cathode/electrolyte interphases (CEI) through the formation of a conformal coating on the high-voltage cathode surface. This dual-salt approach, enhanced by the addition of a phosphate additive, effectively decelerates calendar aging, marking a substantial advance in the quest for durable and reliable Si-based energy storage technologies. The findings underscore the vital role of electrolyte composition in extending the calendar life of Si batteries, offering an alternative avenue toward maximizing the performance and longevity of next-generation Li-Si batteries.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Interphasial engineering for Ni-rich NMC cathode materials

Ni-rich LiNi 1-x-y Mn x CoyO 2 , (NMC: 1-x-y ≥ 0.6) are promising cathode materials for lithium-ion batteries (LIBs) due to high reversible capacity and low cost. However, the fast capacity decay and voltage fading caused by interphasial instability requires improvement. The unstable cathodeelectrolyte interphase (CEI) and transition metal (TM) dissolution at higher voltages can lead to the drastic deterioration of electrochemical performance. In this review, recent approaches to novel electrolyte and additive design, cathode engineering including artificial CEI, and doping/coating on the surface of particles or the whole electrode are comprehensively summarized. In conclusion, the importance of multi-model and multi-scale characterization methods in understanding the effects of interphasial chemistry on battery performance is highlighted.

25 ENERGY STORAGE↗
DOE OSTI2023

Probing the Formation of Cathode-Electrolyte Interphase on Lithium Iron Phosphate Cathodes via Operando Mechanical Measurements

Interfacial instabilities in electrodes control the performance and lifetime of Li-ion batteries. While the formation of solid-electrolyte interface (SEI) on anodes has received much attention, there is still lack of understanding about the formation of cathode-electrolyte interface (CEI) on the cathodes. To fill this gap, we report on dynamic deformations on lithium iron phosphate, LiFePO 4 cathodes during charge / discharge by utilizing in-operando digital image correlation, impedance spectroscopy and Cryo X-ray photoelectron spectroscopy. LiFePO4 cathodes were cycled in either LiPF 6 , LiClO 4 or LiTFSI- containing organic liquid electrolytes. Beyond the first cycle, Li-ion intercalation results in a nearly linear correlation between electrochemical strains and the state of (dis)-charge, regardless of the electrolyte chemistry. However, during the first charge in LiPF 6 - containing electrolyte, there is a distinct irreversible positive strain evolution at the onset of anodic current rise as well as current decay at around 4.0V. Impedance studies show the increase in surface resistance in the same potential window, suggesting the formation of CEI layers on the cathode. The chemistry of the CEI layer was characterized by X-ray photoelectron spectroscopy. LiF is detected in CEI layer starting as early as 3.4 V and Li $x$ PO $y$ F $z$ appeared at voltages higher than 4.0 V during the first charge. In conclusion, our approach offers new insights into the formation mechanism of CEI layers on the cathode electrodes, which is crucial for the development of robust cathode and electrolyte chemistries for higher performance batteries.

25 ENERGY STORAGE↗
DOE OSTI2022

Review of Multifunctional Separators: Stabilizing the Cathode and the Anode for Alkali (Li, Na, and K) Metal–Sulfur and Selenium Batteries

Alkali metal batteries based on lithium, sodium, and potassium anodes and sulfur-based cathodes are regarded as key for next-generation energy storage due to their high theoretical energy and potential cost effectiveness. However, metal–sulfur batteries remain challenged by several factors, including polysulfides’ (PSs) dissolution, sluggish sulfur redox kinetics at the cathode, and metallic dendrite growth at the anode. Functional separators and interlayers are an innovative approach to remedying these drawbacks. Here we critically review the state-of-the-art in separators/interlayers for cathode and anode protection, covering the Li–S and the emerging Na–S and K–S systems. The approaches for improving electrochemical performance may be categorized as one or a combination of the following: Immobilization of polysulfides (cathode); catalyzing sulfur redox kinetics (cathode); introduction of protective layers to serve as an artificial solid electrolyte interphase (SEI) (anode); and combined improvement in electrolyte wetting and homogenization of ion flux (anode and cathode). It is demonstrated that while the advances in Li–S are relatively mature, less progress has been made with Na–S and K–S due to the more challenging redox chemistry at the cathode and increased electrochemical instability at the anode. Throughout these sections there is a complementary discussion of functional separators for emerging alkali metal systems based on metal–selenium and the metal–selenium sulfide. The focus then shifts to interlayers and artificial SEI/cathode electrolyte interphase (CEI) layers employed to stabilize solid-state electrolytes (SSEs) in metal–sulfur solid-state batteries (SSBs). The discussion of SSEs focuses on inorganic electrolytes based on Li- and Na-based oxides and sulfides but also touches on some hybrid systems with an inorganic matrix and a minority polymer phase. The review then moves to practical considerations for functional separators, including scaleup issues and Li–S technoeconomics. The review concludes with an outlook section, where we discuss emerging mechanics, spectroscopy, and advanced electron microscopy (e.g. cryo-transmission electron microscopy (cryo-TEM) and cryo-focused ion beam (cryo-FIB))-based approaches for analysis of functional separator structure–battery electrochemical performance interrelations. Finally, throughout the review we identify the outstanding open scientific and technological questions while providing recommendations for future research topics.

25 ENERGY STORAGE↗
DOE OSTI2021

F and N Rich Solid Electrolyte for Stable All-Solid-State Battery

We report the instability of sulfide solid electrolytes to Li anode and high-voltage LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC811) cathodes limits the cyclic performance of all-solid-state lithium battery (ASSLB). Herein, the stability of Li 6 PS 5 Cl against Li anode is enhanced by mixing a small amount (0.32 wt%) of CuF 2 -LiNO 3 (CL) into Li 6 PS 5 Cl electrolyte layer to in-situ form a mixed-conductive-lithiophobic and self-healing LiF-Li 3 N-Cu solid electrolyte interphase (SEI) at Li 6 PS 5 Cl-CL/Li interface. The critical current density (CCD) of Li 6 PS 5 Cl-CuF 2 -LiNO 3 increases to 1.4 mA cm –2 /1.4 mAh cm –2 at room temperature, which is much higher than that of pristine Li 6 PS 5 Cl (0.4 mA cm –2 /0.4 mAh cm –2 ) even though mixing 0.32 wt% CL into Li 6 PS 5 Cl slightly reduces the ionic conductivity from 2.9 × 10 –3 to 1.5 × 10 –3 S cm –1 . The compatibility of Li 6 PS 5 Cl-CL electrolyte to single-crystalline NMC811 (S-NMC811) is further enhanced by adding a small amount (0.02 wt%) of AlF 3 into Li 6 PS 5 Cl-CL forming Li 6 PS 5 Cl-CuF 2- LiNO 3 -AlF 3 (Li 6 >PS 5 Cl-CLA) as a cathode electrolyte and by doing Cl – on S-NMC811 (Cl@S-NMC811) surface. The Cl@S-NMC811-Li 6 PS 5 Cl-CLA|Li 6 PS 5 Cl-CL|Li cells with areal capacity of 2.55 mAh cm -2 achieve a capacity retention of 69.4% after 100 cycles at 1C (1C = 200 mAh g -1 ). Adding a small amount of SEI and cathode/electrolyte interphase (CEI) former into the sulfide electrolytes with minimal reduction (48.3%) of ionic conductivity is an effective method to enhance the performance of ASSLB.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Multiscale Cryo Electron Microscopy Reveals Interfacial Degradation and Stabilization in Battery Electrodes

Electrochemical interfaces are dynamic systems, evolving based on their local environment and reactant surface structures. The electrode-electrolyte interface in Li-ion batteries can be protective, limiting parasitic reactions with the electrolyte to passivate the surface [1]. Additionally, this interphase has an impact on the Li-ion transport through that layer based on its composition, bonding environment, and thickness. These parameters are challenging to collect and may vary depending on the electrode surface site investigated relative to its spatial position in a coin cell. This study will detail a multiscale cryogenic electron microscopy approach where millimeter-scale cross-sections through the coin cell batteries were made using a cryogenic stage within a fs-laser plasma focused ion beam (laser PFIB) with complementary energy dispersive X-ray spectroscopy able to detect variations in the composition at electrode interfaces [2]. Microscale cross-sectioning and lamella sample preparation of battery electrodes was conducted at the Center for Integrated Nanotechnologies using a Ga-ion focused ion beam (FIB) with air-free and cryo-transfer [3], followed by nanoscale mapping of composition and bonding within the CEI through cryo-scanning transmission electron microscopy (cryo-STEM) electron energy loss spectroscopy [4]. This multiscale approach enabled identification of millimeter-scale features of a battery stack with visualization of degradation in electrodes such as cracks in cathode particles, gas evolution, and SEI evolution; microscale interfacial characteristics, such as heterogeneity in the SEI or barrier layer and identification of electrolyte networks to the electrode surfaces; and nanoscale measurement of the CEI thickness, mapping of transition metal bonding within the cathode particles to identify loss of active materials, and identification of beneficial electrolyte additives incorporated into the CEI structure. This multiscale approach allows for a statistical understanding of the primary mechanisms and parasitic degradation pathways that impact performance by limiting the ion transport pathways within Li+ batteries.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Designing a hybrid electrode toward high energy density with a staged Li + and PF 6 - deintercalation/intercalation mechanism

Existing lithium-ion battery technology is struggling to meet our increasing requirements for high energy density, long lifetime, and low-cost energy storage. Here, a hybrid electrode design is developed by a straightforward reengineering of commercial electrode materials, which has revolutionized the “rocking chair” mechanism by unlocking the role of anions in the electrolyte. Our proof-of-concept hybrid LiFePO 4 (LFP)/graphite electrode works with a staged deintercalation/intercalation mechanism of Li + cations and PF 6 - anions in a broadened voltage range, which was thoroughly studied by ex situ X-ray diffraction, ex situ Raman spectroscopy, and operando neutron powder diffraction. Introducing graphite into the hybrid electrode accelerates its conductivity, facilitating the rapid extraction/insertion of Li + from/into the LFP phase in 2.5 to 4.0 V. This charge/discharge process, in turn, triggers the in situ formation of the cathode/electrolyte interphase (CEI) layer, reinforcing the structural integrity of the whole electrode at high voltage. Consequently, this hybrid LFP/graphite-20% electrode displays a high capacity and long-term cycling stability over 3,500 cycles at 10 C, superior to LFP and graphite cathodes. Importantly, the broadened voltage range and high capacity of the hybrid electrode enhance its energy density, which is leveraged further in a full-cell configuration.

25 ENERGY STORAGE↗
DOE OSTI2020

Influence of Binder Coverage on Interfacial Chemistry of Thin Film LiNi 0.6 Mn 0.2 Co 0.2 O 2 Cathodes

In this work, we explore the influence of binder coverage and chemistry on the interfacial properties of the textured Ni-rich cathode LiNi 0.6 Mn 0.2 Co 0.2 O 2 . We find that the formation of the cathode/electrolyte interphase (CEI) composition varies significantly for cathodes coated with either poly(vinylene fluoride) (PVDF), carboxymethyl cellulose (CMC), or lithium polyacrylate (LiPAA) after cycling to high upper cutoff voltages (4.5 V vs Li/Li). The PVDF-coated samples had a thinner CEI and twice the relative concentration of LiF and Li 2 CO 3 to Li x PO y F z species in the CEI compared to the uncoated sample. This correlated with significantly lower interfacial impedance (285 vs ~1700 Ohm-cm 2 ) and improved capacity retention between cycles of the PVDF-coated samples compared to the other binder compositions and the uncoated sample. CMC-coated samples performed worst, with a CEI comprised of greater amounts of Li x PO y F z . In addition, we find the choice of binder results in the selective protection or promotion of electrolyte reactions at the (104) surface of the 622 cathode. This suggests that the choice of binder can impact the surface chemistry and performance of high voltage cathodes and supports an avenue for interest in multifunctional binders for stabilizing the CEI.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗