Electrochemical modeling of capacity fade in lithium metal batteries: Effects of solid electrolyte interphase formation and dead lithium

With the ever-growing demand for high energy density, anode materials with even higher reactivities, such as Li metal, are being researched and drawn extensive attention owing to its high theoretic capacity (3,860 mAh∙g-1) and low electrode potential (−3.04 V versus standard hydrogen electrode). During the charging process of lithium metal batteries (LMBs), lithium will be electrodeposited on the anode current collector as metallic lithium, and be stripped back to the electrolyte in the discharge process. The reaction is purely lithium deposit/stripping, but several side reactions will deteriorate the reversibility, and the major issues include the incomplete dissolution of metallic lithium and formation of solid electrolyte interphase (SEI). After the discharge process, some dead lithium residue will remain on the anode current collector surface. These inactive substances will gradually accumulate after cycling, which will increase the internal resistance and block ions transportation. The dead lithium was composed of metallic Li within the inner layer, and wrapped by insulating SEI on the outer layer. The formation of SEI will consume lithium, and moreover, the metallic lithium embedded by non-conductive SEI can no longer be stripped back to the electrolyte, and both phenomenon will reduce the Coulombic efficiency. An important factor influencing the reversibility of Li is its electrodeposition morphology on the current collector. If the deposited Li patterns are needle-like and the porosity between each grain is large, the amount of metallic lithium embedded in insulating SEI after discharging will be enormous, and therefore, the consumption rate of usable lithium will be very fast. On the other hand, if lithium can be electrodeposited with large and granular grains in a densely-packed formation, the efficiency of lithium dissolution will be enhanced. The overpotential required for nucleation and growth of lithium during the electrodeposition step will affect the deposition pattern, and evidences from theoretical calculation and experiments have confirmed that lower overpotential can deliver better Li deposition morphology. An important component influencing the internal resistance is the separator. Normally, separators soaked with electrolyte will be placed in the middle of cathode and anode, and act as the agent to transport ions during the charge and discharge process. Most microporous membrane separators are made of polyethylene (PE), polypropylene (PP), or their combinations such as PE/PP and PP/PE/PP. For LMBs, if the electrolyte was conventional LiPF6 salt dissolved in the carbonate-based solvent, the Li deposition pattern with commercially available separator was mostly dendritic. To solve this issue, we proposed a novel type of separator synthesized by electrospinning strategy. The polyamic acid (PAA) precursor solution was used to fabricate polyimide (PI) separator. We compared the in-house synthesized separator with the commercial PP/PE/PP separator, and the results showed that both the volume of electrolyte uptake and ionic conductivity were improved. Also, when using electrospun PI as the separator, the overpotential required for Li nucleation in LMB was clearly lower than commercial one, and PI could ameliorate the lithium deposition morphology, leading to better Li reversibility. In this research, scanning electron microscopy (SEM) was employed to analyze the Li deposition morphology and dead Li after cycling. The battery performance with both types of the separator was evaluated by coin cell configuration, and the cycling ability of LMB was successfully promoted with the PI separator.

Capacity retention in lithium metal batteries needs to be improved if they are to be commercially viable, the low cycling stability and Li corrosion during storage of lithium metal batteries being even more problematic when there is no excess lithium in the cell. Herein, we develop in situ NMR metrology to study “anode-free” lithium metal batteries where lithium is plated directly onto a bare copper current collector from a LiFePO4 cathode. The methodology allows inactive or “dead lithium” formation during plating and stripping of lithium in a full-cell lithium metal battery to be tracked: dead lithium and SEI formation can be quantified by NMR and their relative rates of formation are here compared in carbonate and ether-electrolytes. Little-to-no dead Li was observed when FEC is used as an additive. The bulk magnetic susceptibility effects arising from the paramagnetic lithium metal were used to distinguish between different surface coverages of lithium deposits. The amount of lithium metal was monitored during rest periods, and lithium metal dissolution (corrosion) was observed in all electrolytes, even during the periods when the battery is not in use, i.e., when no current is flowing, demonstrating that dissolution of lithium remains a critical issue for lithium metal batteries. The high rate of corrosion is attributed to SEI formation on both lithium metal and copper (and Cu+, Cu2+ reduction). Strategies to mitigate the corrosion are explored, the work demonstrating that both polymer coatings and the modification of the copper surface chemistry help to stabilize the lithium metal surface.

Engineering a lithium silicate-based artificial solid electrolyte interphase for enhanced rechargeable lithium metal batteries

Anode‐free lithium metal batteries have emerged as strong contenders for next‐generation rechargeable batteries due to their ultra‐high energy density. However, their safety and life span are insufficient because of the easy generation of dendrites and dead lithium during lithium plating and stripping. Understanding the formation mechanism for lithium dendrites and dead lithium is essential to further improve battery performance. By employing in situ solid‐state nuclear magnetic resonance (NMR) spectroscopy, the influence of stacking pressure on dendritic behavior and dead lithium is systematically investigated. At 0.1 MPa, lithium dendrite is rapidly formed, followed by a linear increase of dead lithium. High stacking pressure not only causes lithium metal to fracture but also leads to form dendrites and dead lithium at the fracture site. At 0.5 MPa stacking pressure, the least quantity of dead lithium is attained, and the growth pattern of dead lithium is exponential growth. The exponential growth pattern is distinguished by the high growth of dead lithium early in the battery cycle and essentially no growth later in the cycle. As a result, it is believed that efficient suppression of dead lithium generation early in battery cycling can play a critical role in improving battery performance.

Poly(ethylene oxide) (PEO) solid electrolytes offer great promise to realize all-solid-state lithium metal batteries with both high energy density and safety. However, it remains challenging to fabricate ultrathin PEO-based solid electrolytes that can operate at practical current densities with a long lifespan. Here, we develop a 19 μm-thick PEO-based solid electrolyte with a porous polyethylene support, which provides mechanical strength and blocks lithium dendrites. By repeatedly plating and stripping lithium at a high current density and low areal capacity, we ingeniously transform otherwise detrimental "dead lithium" into functional fillers within the PEO solid electrolytes. Results show that LiOH, Li2CO3, Li2O, and LiF form on the surface of the "dead lithium", blocking electronic transport and thus rendering them as effective fillers. These in situ formed fillers simultaneously enhance lithium-ion transport and act as a barrier to suppress dendrite growth, thus facilitating uniform lithium deposition. As a result, this approach enables Li||Li symmetric cells to achieve a critical current density of as high as 1 mA cm-2 and operate stably for 400 h at 0.5 mA cm-2 and 0.5 mAh cm-2 without short-circuits. Importantly, a precycled Li||LiFePO4 full cell can retain 90.9% capacity after 600 cycles at 1C charging and 3C discharging.

Inactive lithium (more frequently called dead lithium) in the forms of solid–electrolyte interphase and electrically isolated metallic lithium is principally responsible for the performance decay commonly observed in lithium metal batteries. A fundamental solution of recovering dead lithium is urgently needed to stabilize lithium metal batteries. Here we quantify the solid–electrolyte interphase components, and determine their relation with the formation of electrically isolated dead lithium metal. We present a lithium restoration method based on a series of iodine redox reactions mainly involving I3−/I−. Using a biochar capsule host for iodine, we show that the I3−/I− redox takes place spontaneously, effectively rejuvenating dead lithium to compensate the lithium loss. Through this design, a full-cell using a very limited lithium metal anode exhibits an excellent lifespan of 1,000 cycles with a high Coulombic efficiency of 99.9%. We also demonstrate the design with a commercial cathode in pouch cells.

Lithium metal anodes are promising for their high energy density and low working potential. However, high reactivity and dendrite growth of lithium metal lead to serious safety issues. Lithium dendrite may form "dead lithium" or pierce the separator, which will cause low efficiency and short-circuit inside the battery. A nonflammable phosphate-based electrolyte can effectively solve the flammability problem. Also, it shows poor compatibility with lithium metal anodes, resulting in an unstable solid electrolyte interface (SEI), which leads to dendrite growth and poor electrochemical performance. In this study, trimethyl phosphate is used to ensure the safety of lithium metal batteries. By adjusting the concentration of lithium salt and introducing fluoroethylene carbonate, a stable SEI layer is formed on the surface of the lithium metal anode and dendrite growth of the lithium metal anode is inhibited. Lithium metal batteries with a modified electrolyte achieved stable electrochemical plating/stripping, and the full cell has 93.4% capacity left and the coulombic efficiency is nearly 100%. In addition, the modified electrolyte can also enable reversible intercalation and de-intercalation of Li+ in the commercial graphite anode. This work may provide an alternative direction for the development of lithium metal batteries with high safety and high energy density.

Electrolytes are susceptible to reductive decomposition on the surface of negative electrodes leading to the formation and growth of solid-electrolyte interphase (SEI) layer [1]. Stable SEI can be beneficial as a protective layer, given that it can provide insulation to electron transport from the anode to electrolyte, prevent solvent molecules from reaching the anode, and at the same time allow transport of Li+ [2]. Thus, the SEI can contribute critically to the safety and operability of lithium-ion batteries (LIBs), but its functionality heavily depends on the conditions under which it gets synthesized [3]. This can have a significant effect on battery performance. For instance, nucleation and growth of lithium dendrites during cycling is known to result in continuous electrolyte degradation, destruction of the SEI, agglomeration of so-called “dead lithium”, and short-circuiting of the battery [4]. This greatly affects next-generation electric vehicles (EVs), for which all-solid-state lithium-metal batteries (ASSLMBs) have garnered significant attention due to their superior energy storage capacity and safety over LIBs [5]. However, the considerable interfacial impedance originating from poor physical contact and/or parasitic reactions at the Li/SSE interface hinders the development of ASSLMBs. Alternative approaches towards achieving a stable protective interface have been pursued: like use of electrolyte additives to manipulate the constituents and compositions of SEI or designing artificial protective layers to improve performance. For instance, admixture of optimum amounts of fluorine-rich additives like fluoroethylene carbonate (FEC) to traditional carbonate electrolytes (like EC/DMC) has been seen to generate a robust LiF-rich SEI. However, the intrinsic role of LiF remains a topic of uncertainty. Conventional understanding would posit that dominance of LiF leaves the SEI susceptible to poor lithium-ion transport properties. Previously, theoretical works reported significantly lower ionic conductivities in crystalline LiF when compared to other common inorganic SEI materials like Li2O and Li2CO3. Underwhelming transport properties pose serious rate limitations in its effectiveness as a serious candidate for interfacial protection in ASSLMBs. To potentially overcome such bottleneck, this study systematically investigates different phases of LiF: their structures, stabilities and interfacial properties are described using first principles calculations. Careful analysis of the structural models reveals that unlike the widely studied rock salt ordered counterpart, certain phases of LiF exhibit excellent Li+ transport properties with a high predicted diffusivity at room temperature. Mechanically too, improved interfacial qualities are demonstrated with increased flexibility and fracture resistance, opening up important avenues for structural and compositional stability over cell cycling while maintaining its desirable electron-blocking characteristics. However, it is also important to take a step back and note that the rock salt phase is the most energetically stable among LiF phases, which thereby exhibit a propensity for phase transformation under ambient conditions. To overcome this, a strategy of incorporating hetero dopants as impurities to stabilize the host matrix is discussed. By increasing the dopant concentration up to an optimum amount, relative thermodynamic stability of the interface-friendly phases of LiF is achieved. Our examination of the structure reveals unique lithium-dopant interactions which help sustain such LiF phases in the host matrix. The combination of excellent Li-ion transport properties and electron blocking ability makes such LiF-rich composites an excellent candidate for use as an interfacial protective layer that can effectively suppress electrolyte decomposition and Li dendrite propagation, while simultaneously improving the contact and compatibility of the electrode/electrolyte interface. These unique and exceptional traits make them materials of great promise for protecting critical interfaces in ASSLMBs and LIBs.[1] M. Gauthier, T.J. Carney, A. Grimaud, L. Giordano, N. Pour, H.-H. Chang, D.P. Fenning, S.F. Lux, O. Paschos, C. Bauer, F. Maglia, S. Lupart, P. Lamp, Y. Shao-Horn, Electrode−Electrolyte Interface in Li-Ion Batteries: Current Understanding and New Insights, J. Phys. Chem. Lett. 6 (2015) 4653−4672. https://doi.org/10.1021/acs.jpclett.5b01727[2] D. Bedrov, O. Borodin, J.B. Hooper, Li+ Transport and Mechanical Properties of Model Solid Electrolyte Interphases (SEI): Insight from Atomistic Molecular Dynamics Simulations, J. Phys. Chem. C 121 (2017) 16098–16109. https://doi.org/10.1021/acs.jpcc.7b04247[3] D. Aurbach, B. Markovsky, I. Weissman, E. Levi, Y. Ein-Eli, On the Correlation Between Surface Chemistry and Performance of Graphite Negative Electrodes for Li Ion Batteries, Electrochim. Acta 45 (1999) 67−86. https://doi.org/10.1016/S0013-4686(99)00194-2[4] W. Xu, J. Wang, F. Ding, X. Chen, E. Nasybulin, Y. Zhang, J.G. Zhang, Lithium Metal Anodes for Rechargeable Batteries, Energy Environ. Sci. 7 (2014) 513−537. https://doi.org/10.1039/C3EE40795K[5] J. G. Kim, B. Son, S. Mukherjee, N. Schuppert, A. Bates, O. Kwon, M. J. Choi, H. Y. Chung, S. Park, A review of lithium and non-lithium based solid state batteries, J. Power Sources 282 (2015) 299−322. https://doi.org/10.1016/j.jpowsour.2015.02.054

Degradation of lithium metal batteries due to dead lithium accumulation under ultrasound.

The escalating demand for high-performance energy storage, driven by electric vehicles and drones, has intensified research into lithium metal batteries featuring lithium metal anodes. Lithium metal batteries are susceptible to dendrite and dead lithium formation under high current conditions, which impacts their practical applications. This study addresses the challenges of anode stability and dead lithium at high current densities by introducing an inorganic-organic composite artificial solid electrolyte interface (ASEI). We developed a flexible and robust polymer and integrated it with AgNO3 to in situ generate Ag and Li3N to form an ASEI. In the ASEI, these inorganic and organic components are in situ generated during the electrochemical process, exhibiting a gradient transition from the solution to the lithium metal interface. This gradient transition of inorganic and organic layers endows the subsequent deposition and dissolution processes with self-adaptive characteristics, allowing for continuous evolution with lithium insertion and extraction. It provides a stable in situ adaptive ASEI that suppresses the formation of dead lithium and dendrites. The lithiophilic properties of Ag, the ionic conductivity of Li3N and the robustness of the polymers demonstrate superior mechanical strength and electrochemical performance across symmetrical, half, and full cells, showcasing promise for next-generation energy storage solutions.

All-solid-state lithium metal batteries have emerged as a promising solution to overcoming the energy density and safety challenges associated with conventional lithium-ion batteries. Solid polymer electrolytes, particularly those based on poly(vinylidene fluoride) (PVDF) and dimethylformamide (DMF), demonstrate significant potential. However, interfacial side reactions between residual DMF solvents and lithium metal present substantial challenges. In this study, we investigate the in situ formation of solid electrolyte interphase protective layers to mitigate these side reactions. By incorporating F-rich additives, such as fluoroethylene carbonate and lithium difluorophosphate, we successfully establish a dual-layer inorganic SEI structure characterized by an outer LiF layer and an inner Li2O layer. Consequently, our approach extends the cycle life of lithium symmetric batteries to 3000 h. Additionally, the Li||LiFePO4 solid-state battery demonstrates exceptional stability, enduring 400 cycles at a 1C rate with an impressive capacity retention of 84%. This strategic methodology effectively leverages the benefits of residual solvents, ensuring both enhanced battery efficiency and long-term operational stability for PVDF-based all-solid-state lithium metal batteries.

Directing the morphology of lithium metal deposits during electrodeposition is crucial to the development of safe, high energy density batteries with robust cycle life. Towards this end, mechanistic insight is imperative to understand the relationship between electrolytes, additives, cycling conditions, and the resulting lithium morphologies. In this work we used a standard carbonate-based electrolyte while systematically adding water (0-250 ppm, corresponding to ~0-500 ppm HF) in Li||Cu cells to study the links between electrolyte composition, initial solid electrolyte interphase (SEI) formation, and morphology of electroplated lithium metal. Under conditions in which the electrolyte contains several hundred ppm added HF and applied constant currents on the order of 0.5 mA/cm2, this system yields electrodeposited lithium metal with a highly monodispersed columnar morphology, whereas HF-free electrolytes result in lithium metal with a mossy morphology. We used electrochemical characterization to investigate the HF reduction process, X-ray photoelectron spectroscopy to characterize SEI chemistry, wide angle X-ray scattering to probe the nanostructure of the initial SEI and crystallographic texture of electrodeposited lithium metal, scanning electron microscopy to observe the resulting lithium metal morphology, and operando small angle X-ray scattering to understand cyclability of columnar lithium. Our systematic experimental approach enables insights to be drawn concerning the underlying mechanisms of columnar lithium formation. This morphology arises from an SEI layer comprising crystalline LiF deposits, formed through selective reduction of HF, embedded in an amorphous matrix of solvent reduction products. This interphase structure contains fast lithium ion diffusion pathways which lead to a high nucleation density and uniform growth of lithium metal deposits. The mechanism proposed herein will help to inform future electrolyte additive design and rational cycling protocols for lithium metal batteries. Figure 1

The formation of lithium dendrites and dead lithium during deposition/stripping process restricts battery performance especially in wide temperature range. However, due to the lack of real-time detection methods, the intrinsic mechanism of how operational temperature affects the dynamic process on lithium metal anodes is still unclear. Here, an in situ investigation of lithium deposition and dead lithium formation during the first charge-discharge cycle in an ether-based electrolyte system is presented. Both the deposition process and stripping process are found to be temperature dependent. Below 293 K, the lithium deposition is less dense plating and the dead lithium is formed, which contributes to the capacity loss. Above 293 K, the lithium deposition becomes denser, and dead lithium formation is significantly reduced. The capacity loss is primarily driven by the formation of solid electrolyte interphase (SEI) resulting from reactions between lithium and ether-based electrolyte. Further study reveals that the ratio of lithium oligoethoxides on the SEI changes abruptly with temperature above 293 K and thus significantly alters the conductivity and reactivity of SEI, which leads to the abrupt change of the deposition/stripping process. These findings highlight the critical role of temperature in lithium deposition/stripping processes in ether-based anode-free lithium metal batteries.

New insights into “dead lithium” during stripping in lithium metal batteries

Advances in energy storage (ES) devices have the potential to transform nearly every aspect of society, from transportation to communication to electricity delivery, national defense and domestic security. Among the various prevalent energy storage devices, the most prominent to date are related to electrochemical energy storage (EES) technologies. Several EES technologies are either in existence or have evolved over the years. Among the various systems studied, lithium battery technologies (LBs) have emerged in the forefront as a panacea to the high energy and high power problems facing portable electronics, electric powered vehicle, military applications as well as stand-alone stationary power systems integrated into the electric grid. Despite advances in the anode arena, lithium metal anodes due to the inherent dendrite formation limitations have never attained commercial status. Overcoming these barriers would be a major breakthrough in the search for high energy density anode systems due to its extremely high theoretical specific capacity (~3860 mAh/g) and the lowest negative electrochemical potential (-3.040 V vs. the standard hydrogen electrode). Similarly, pursuit of high energy density rechargeable lithium metal battery (LMB) cathodes have led to sulfur (for a Li-S battery), air electrode (for a Li-air battery) or intercalation compounds (e.g. NbSe3, V2O5) to realize high voltage LMBs exhibiting high energy densities. Li-S batteries, owing to their high theoretical energy density (2600 Wh kg-1)1, is considered as one of the most promising Li metal-based batteries. However, the liquid organic electrolytes, S cathodes and Li metal in current LMBs could not be commercialized globally for high energy applications due to low cycle life and safety issues. The low cycle life, low coulombic efficiency (CE) and safety issue of rechargeable LMBs mainly arises due to the uncontrolled cellular and dendritic growth of Li metal during repeated lithium plating/stripping and formation of unstable solid electrolyte interphase (SEI) and the continuous growth of thick SEI during repeated cycling at the solid liquid interface of organic electrolyte based LMBs. In addition, the large volume expansion (~80%)3 of sulfur accompanying the electrochemical cycling, the low utilization of sulfur resulting from poor room-temperature electronic conductivity of sulfur (~10-15 S/cm) 4 , combined with the shuttle effect of highly soluble polysulfide species in the organic ether-based electrolytes has limited the use of Li-S batteries. To address these challenges facing sulfur cathodes, significant efforts have been made to demonstrate advanced composite cathodes using various carbon materials5, polymers6 and metal-organic framework (MOF) materials7. In addition, solid polymer electrolytes8 and electrolytes additives have also been studied to develop a viable Li-S battery system. Thin-film solid-polymer electrolyte batteries offer the potential for improved safety because of the reduced activity of lithium with the solid electrolyte, flexibility in design as the cell can be fabricated in various sizes and shapes, and high energy density. In this work, we demonstrate the use of a composite polymer electrolyte (CPE) with modified physical and chemical properties in Li-S batteries. These CPEs exhibits superior mechanical properties, excellent room-temperature lithium ion conductivity and low electrolyte: sulfur (E:S) ratio. These CPEs, when used as electrolytes for Li-S batteries, helps prevent both polysulfide dissolution and dendrite formation, in addition to providing very high energy density (~750 Wh/kg). Structural, chemical, physical and electrochemical characterization results validating these properties of the CPEs will be presented and discussed. Acknowledgements: The authors acknowledge the financial support of DOE grant DE-EE 0006825 and DE-EE-0008199, Edward R. Weidlein Chair Professorship funds and the Center for Complex Engineered Multifunctional Materials (CCEMM). References V. S. Kolosnitsyn and E. V. Karaseva, Russian Journal of Electrochemistry, 2008, 44, 506-509.Y. Sun, G. Li, Y. Lai, D. Zeng and H. Cheng, Scientific Reports, 2016, 6, 22048.B. Dunn, H. Kamath and J.-M. Tarascon, Science, 2011, 334, 928-935.M. Edeling, R. W. Schmutzler and F. Hensel, Philosophical Magazine Part B, 1979, 39, 547-550.J. Jin, Z. Wen, G. Ma, Y. Lu and K. Rui, Solid State Ionics, 2014, 262, 170-173.P. J. Hanumantha, B. Gattu, O. Velikokhatnyi, M. K. Datta, S. S. Damle and P. N. Kumta, Journal of The Electrochemical Society, 2014, 161, A1173-A1180.P. M. Shanthi, P. J. Hanumantha, B. Gattu, M. Sweeney, M. K. Datta and P. N. Kumta, Electrochimica Acta, 2017, 229, 208-218.B. A. Boukamp, I. D. Raistrick, C. Ho, Y.-W. Hu and R. A. Huggins, in Superionic Conductors, eds. G. D. Mahan and W. L. Roth, Springer US, Boston, MA, 1976, DOI: 10.1007/978-1-4615-8789-7_65, pp. 417-417.E. Peled, C. Menachem, D. Bar‐Tow and A. Melman, Journal of The Electrochemical Society, 1996, 143, L4-L7.