Lithium Metal Deposition Research Articles

Nanobead-reinforced outmost shell of solid-electrolyte interphase layers for suppressing dendritic growth of lithium metal

Plating-stripping reversibility of lithium metal is improved by reinforcing the solid-electrolyte interphase layer by inorganic nanobeads. The outmost solid-electrolyte interphase shell is clearly identified, which is the passive layer formed on current collectors (or lithium metal) before the first lithium metal deposition. The outmost shell is intrinsically brittle and fragile so that it is easily broken by lithium metal dendrites growing along the progress of plating. Lithium metal deposit is not completely stripped back to lithium ions. On the other hand, lithium metal cells containing inorganic nanobeads in electrolyte show high reversibility between plating and stripping. The nanobeads are incorporated into the outmost shell during its formation. The nanobead-reinforced outmost shell having mechanically durable toughness suppresses dendritic growth of lithium metal, not allowing the dendrites to penetrate the shell. In addition to the mechanical effect of nanobeads, the LiF-rich solid-electrolyte interphase layer formation is triggered by HF generated by the reaction of the moisture adsorbed on oxide nanobeads with PF6−. The LiF-rich composition is responsible for facile lithium ion transfer through the passive layers.

Effect of Pore Size in Three Dimensionally Ordered Macroporous Polyimide Separator on Lithium Deposition/Dissolution Behavior

Li dendrite formation has disturbed the usage of Li metal anode in rechargeable battery. Three dimensionally ordered macroporous polyimide (3DOM PI) separator is one of possible technology to suppress the dendrite formation. In order to optimize porous structure, an effect of pore size in 3DOM PI separator was investigated using three different pore size for separator (300 nm, 500 nm, 800 nm). An ionic conductivity of separator depended on size of pore. With decreasing pore size, the ionic conductivity of separator increased. Li | Cu cell was employed to investigate an effect of pore size on morphology of lithium metal. 3DOM separator provided a uniform deposition of lithium metal and a suppression of dendrite formation. This is due to more uniform current distribution in 3DOM PI separator. The highest cycleability of lithium metal anode was realized by using smallest pore size 3DOM PI separator (300 nm). This is also related to uniform current distribution in the cell.

Lithium Plating and Stripping on Carbon Nanotube Sponge.

Lithium metal is an ideal anode material due to its high specific capacity and low redox potential. However, issues such as dendritic growth and low Coulombic efficiency prevent its application in secondary lithium batteries. The use of three-dimensional (3D) porous current collector is an effective strategy to solve these problems. Herein, commercial carbon nanotube (CNT) sponge is used as a 3D current collector for dendrite-free lithium metal deposition to improve the Coulombic efficiency and the cycle stability of the lithium metal batteries. The high specific surface area of the CNT increases the density of the lithium nucleation sites and ensures the uniform lithium deposition while the "pre-lithiation" behavior of the porous CNT enhances its affinity with the deposited lithium. Meanwhile, the lithium plating/stripping on the sponge maintains high Coulombic efficiency and high cycling stability due to the robust structure of graphitic-amorphous carbon composite in the ether-based electrolyte. Our findings exhibit the feasibility of using CNT sponge as a 3D porous current collector for lithium deposition. They shed light on designing and developing advanced current collectors for the lithium metal electrode and will promote the commercialization of the secondary lithium batteries.

Intercalated metal\u2013organic frameworks with high electronic conductivity as negative electrode materials for hybrid capacitors

Hybrid capacitors should ideally exhibit high volumetric energy density, favorable low-temperature performance and safe operation. Here we describe a negative electrode comprising an intercalated metal–organic framework, 4,4′-biphenyl dicarboxylate dilithium [4,4′-Bph(COOLi)2], which forms a repeating organic–inorganic layered structure of π-stacked biphenyl and tetrahedral LiO4 units. The electrode shows a stepwise two-electron transfer and has a capacity of 190 mAh g−1 at 0.7 V vs. Li/Li+, which can suppress the lithium metal deposition reaction occurring an internal short circuit. A hybrid capacitor containing 4,4′-Bph(COOLi)2 negative and activated carbon positive electrodes possesses high volumetric energy density of approximately 60 Wh L−1 and good high-rate performance, particularly at the low temperature of 0 °C, because of low charge-transfer resistance along with low activation energy. Hopping mobility calculations suggest the observed low resistance properties are the result of high electron mobility arising from two electron-hopping pathways between adjacent molecules in the π-stacked biphenyl packing layer by lithium intercalation.

Effect of diethylenetriamine as additive to stabilize the lithium metal anode

Li metal has been regarded as the most promising anode material for next high-energy batteries. However, the dendrite growth during the lithium deposition and low Coulombic efficiencies have restricted their practical applications. In the paper, we investigate the electrodeposition process of lithium metal under different current density in detail and firstly use diethylenetriamine as additive to adjust the deposition of lithium metal in LiPF6-PC and LiPF6-EC/DEC electrolyte. The result shows that trace diethylenetriamine can react with lithium anode producing R-N-Li and LiF, which greatly improves the morphology of lithium deposition by adjusting the components of the SEI film. The content of fast ionic conductors in SEI film (e.g., R-N-Li and LiF) can be identified by the X-ray photoelectron spectroscopy. The electrochemical impedance spectroscopy confirms diethylenetriamine could increase the ionic conductivity of the interface between the anode and electrolyte. Galvanostatic curves also show that the LiPF6-PC electrolyte with 0.2 vol% diethylenetriamine and LiPF6-EC/DEC electrolyte with 0.1 vol% diethylenetriamine has better cycling stability than blank group.

Effects of Polymer Coatings on Electrodeposited Lithium Metal.

The electrodeposition of lithium metal is a key process in next-generation, high energy density storage devices. However, the high reactivity of the lithium metal causes short cycling lifetimes and dendrite growth that can pose a serious safety issue. Recently, a number of approaches have been pursued to stabilize the lithium metal-electrolyte interface, including soft polymeric coatings that have shown the ability to enable high-rate and high-capacity lithium metal cycling, but a clear understanding of how to design and modify these coatings has not yet been established. In this work, we studied the effects of several polymers with systematically varied chemical and mechanical properties as coatings on the lithium metal anode. By examining the early stages of lithium metal deposition, we determine that the morphology of the lithium particles is strongly influenced by the chemistry of the polymer coating. We have identified polymer dielectric constant and surface energy as two key descriptors of the lithium deposit size. Low surface energy polymers were found to promote larger deposits with smaller surface areas. This may be explained by a reduced interaction between the coating and the lithium surface and thus an increase in the interfacial energy. On the other hand, high dielectric constant polymers were found to increase the exchange current and gave larger lithium deposits due to the decreased overpotentials at a fixed current density. We also observed that the thickness of the polymer coating should be optimized for each individual polymer. Furthermore, polymer reactivity was found to strongly influence the Coulombic efficiency. Overall, this work offers new fundamental insights into lithium electrodeposition processes and provides direction for the design of new polymer coatings to better stabilize the lithium metal anode.

Cryo-STEM mapping of solid-liquid interfaces and dendrites in lithium-metal batteries.

Solid-liquid interfaces are important in a range of chemical, physical and biological processes1-4, but are often not fully understood owing to the lack of high-resolution characterization methods that are compatible with both solid and liquid components5. For example, the related processes of dendritic deposition of lithium metal and the formation of solid-electrolyte interphase layers6,7 are known to be key determinants of battery safety and performance in high-energy-density lithium-metal batteries. But exactly what is involved in these two processes, which occur at a solid-liquid interface, has long been debated8-11 because of the challenges of observing such interfaces directly. Here we adapt a technique that has enabled cryo-transmission electron microscopy (cryo-TEM) of hydrated specimens in biology-immobilization of liquids by rapid freezing, that is, vitrification12. By vitrifying the liquid electrolyte we preserve it and the structures at solid-liquid interfaces in lithium-metal batteriesin their native state, and thus enable structural and chemical mapping of these interfaces by cryo-scanning transmission electron microscopy (cryo-STEM). We identify two dendrite types coexisting on the lithium anode, each with distinct structure and composition. One family of dendrites has an extended solid-electrolyte interphase layer, whereas the other unexpectedly consists of lithium hydride instead of lithium metal and may contribute disproportionately to loss of battery capacity. The insights into the formation of lithium dendrites that our work provides demonstrate the potential of cryogenic electron microscopy for probing nanoscale processes at intact solid-liquid interfaces in functional devices such as rechargeable batteries.

Thin Film Surface Coatings for Improved Lithium Metal Cycling Efficiency: A Combinatorial Approach

There is a continual push to improve the energy density of lithium-ion batteries, but this is becoming increasingly difficult as it is believed that many of the best electrode materials have been found and optimized. In order to achieve higher energy densities to meet future storage demands it may be necessary to move “beyond lithium-ion”. Recent research efforts have focused on reviving the lithium metal negative electrode as a potential path to higher energy density.[1] With its high theoretical capacity and lowest reduction potential among metals, using lithium as a negative electrode can certainly increase the energy density of a cell provided the large volume changes of the lithium electrode between the charged and discharged states can be handled. In addition, lithium metal anodes often demonstrate dendrite formation during electrodeposition instead of plating as a smooth film. Since conventional organic electrolytes are thermodynamically unstable to reduction by lithium, dendrite growth leads to continual loss of active lithium, resulting in poor cycling efficiencies (< 90%) and short cell lifetime.[2] These problems are exaggerated as the thickness of the plated lithium increases, making cells with industrially relevant material loadings a significant challenge. Recent studies have shown that the morphology and plating efficiency of lithium can be greatly improved with nanostructured surface coatings. [3],[4] Herein we report on the use of combinatorial materials analysis to investigate the effect of different nanostructured surface coatings on the electrochemical deposition of lithium metal. Custom 64-channel electrochemical cell plates were used along with a combinatorial electrochemical cell to provide an efficient means of screening different electrode coatings. Figure 1 shows a schematic of the 64-channel cell plate as well as the output cyclic voltammograms (CVs) from potentiodynamic lithium plating and stripping on 64 unmodified copper electrodes. All 64 channels show consistent output which was validated against conventional coin type lithium half cells. High throughput sputtering was used to make libraries of thin films with different composition, structure, and thickness. These sputtered libraries were deposited directly on the 64-channel cell plates and used to analyze the morphology and coulombic efficiency of lithium plating as a function of the 64 individual surface coatings. The influence of different electrolyte compositions with these surface coatings was also investigated. These results provide perspective regarding the value of different thin film anode surface coatings as part of a broad strategy for achieving high (>98 %) lithium metal cycling efficiencies. [1] X. Cheng et al., Toward safe lithium metal anode in rechargeable batteries: a review, Chemical reviews, 117 (2015), 10403-10473. [2] Q. Pang, et al. "An In Vivo Formed Solid Electrolyte Surface Layer Enables Stable Plating of Li Metal." Joule 1, (2017): 871-886. [3] Adam P. Cohn et al., Anode-Free Sodium Battery through in Situ plating of Sodium Metal, Nano Letters, 17, (2017) 1296-1301. [4] G. Zheng et al., Interconnected hollow carbon nanospheres for stable lithium metal anodes, Nature Nanotechnology 9, (2014), 618-623. Figure 1

In-Situ Formation of Poly (Vinylene Carbonate) Based Gel Polymer Electrolyte for High Rate Cycling of Lithium Metal

Lithium metal based battery has high theoretical capacity and high energy density, ideal for large-scale energy storages such as in electric vehicles. However, dendritic deposition of lithium metal, which is especially severe at high current charging, causes serious safety problems. Previously, we reported in-situ formation of a single-component coating of crystalline lithium methyl carbonate (LMC) and stable cycling of lithium metals (ACS App. Mater. Interface, 2017, 30635). Here we extend the methodology and report a polymer gel coating on lithium metal surface formed in-situ from a pure vinylene carbonate (VC) solution . The polymerization rate of VC can be controlled by time, temperature, and the initiator, which resulted in a stable polymer gel. The mechanically robust gel can be used directly as an electrolyte layer in battery cells. Remarkably, the Li/Li cell cycled for 1000 hours without increase of resistance at a current density of 5 mA cm−2. A dense layer of lithium metal was observed at the interface of lithium and the gel, which indicated the stabilization of lithium metal by the polymer gel. The chemical structure of the polymer was analyzed by infrared spectroscopy and 1H NMR and evaluated as poly (vinylene carbonate) (polyVC) with a partially branched structure. The polymerization step is proposed to occur in a living radical fashion. The formation of electrolyte layers through in-situ polymerization is a promising approach to stabilize lithium metal electrodes.

Modeling the Voltage Response of Lithium Symmetric Cells

The use of Lithium metal as anode material has been regarded as the "Holy Grail" for Lithium batteries. It has a higher theoretical specific capacity (3860 mAh∙g-1) compared to the graphite (372 mAh∙g-1) used in conventional lithium-ion batteries and one of the lowest electrochemical potentials (−3.04 V vs. standard hydrogen electrode).1 However, its commercialization is greatly hampered by the poor electrodeposition of lithium, characterized by the formation of "dead lithium" and dendrites, which cause low coulombic efficiency, reduced cycle life and possible internal short circuits during operation. Useful insight into lithium electrodeposition dynamics and morphology can be obtained by taking advantage of the simplicity of Li/Li symmetric cells. Electrochemical models can help elucidate these trends in terms of thermodynamics, electrode reactions, and transport phenomena. Models can also help study parametric variations and different operating conditions, giving further information into different electrodeposition regimes and limiting conditions. From a systems perspective, the access to internal state variables (e.g. concentration) can also help capture the complex voltage and current responses that have been observed during Li metal electrodeposition.2,3 These considerations form the motivation for the mathematical model used in this work. The model will include the governing equations for electrolyte transport, ionic current, and electrode reactions. Transport equations are based on the concentrated solution theory.4,5 A modified Ohm’s law for the liquid electrolyte phase is considered and the Butler-Volmer (BV) equation is used for lithium metal deposition and stripping at the electrode. Bulk electrolyte transport is described by Fick’s law. This basic electrochemical model is then extended by modifying the Butler-Volmer equation to incorporate dendrite growth as a parallel electrodeposition pathway on the lines of existing literature.2,6,7 A model for SEI growth will also be incorporated.8 Electrode kinetics equations will be modified to account for external pressure and local stress effects, yielding an integrated model that combines transport and electrode reaction models with SEI growth, external pressure, mechanical effects and heterogeneous plating dynamics.9,10 Efficient simulation techniques will be applied to simulate the model for different parametric combinations and experimentally relevant conditions of galvanostatic cycling. The model will be validated against experimental data in terms of the prediction of electrodeposition trends and cell voltage. References J. Qian, W. A. Henderson, W. Xu, P. Bhattacharya, M. Engelhard, O. Borodin and J.-G. Zhang, Nature communications, 6 (2015).K. N. Wood, E. Kazyak, A.F. Chadwick, K-H. Chen, J-G. Zhang, K. Thornton, and N.P. Dasgupta, ACS Cent. Sci., 2, 790-801 (2016).P. Bai, J. Li, F.R. Brushett, and M.Z. Bazant, Energy Environ. Sci., 9, 3221-3229 (2016).M. Doyle, T. F. Fuller, and J. Newman, J. Electrochem. Soc., 140, 1526–1533 (1993).S-L. Wu, A.E. Javier, D. Devaux, N.P. Balsara, and V. Srinivasan, Electrochem. Soc., 161, A1836–A1843 (2004).R. Akolkar, J. Power Sources, 232, 23-28 (2013).C. Monroe and J. Newman, J. Electrochem. Soc., 150, A1377–A1384 (2003).M. B. Pinson and M.Z. Bazant, J. Electrochem. Soc., 160, A243-A250 (2013).C. Monroe and J. Newman, J. Electrochem. Soc., 151, A880–A886 (2004).X. Zhang, W. Shyy, and A. M. Sastry, J. Electrochem. Soc., 154, A910–A916 (2007).

Development of Novel Ionomer Electrolytes for Alkali Metal Batteries

Single-ion conducting electrolytes are an interesting alternative to liquid binary salt electrolytes, which are currently used in most batteries due to their high ion conductivity. However, the mobility of both cations and anions in these electrolytes results in an ion concentration gradient, and consequent polarisation losses. The ion concentration gradient is especially detrimental for alkali metal batteries. It causes inhomogeneous metal deposition leading to uncontrolled dendrite growth at the alkali metal electrode.1 Thus, it is necessary to eliminate the free movement of anions. One way to achieve that is by covalently tethering the anions to a polymer backbone. So called ionomers have achieved considerable success toward stabilising lithium metal deposition and preventing dendritic structure.2,3 By exchanging the counter ion these ionomers can be used in a wide range of different types of batteries. Here, we report on the synthesis and development of novel ionomers derived from inexpensive and chemically robust polymers. The microporous ionomer membranes infused with liquid achieve cation transference numbers close to unity, high ion conductivity (> 10-5 Scm-1 at room temperature) over wide temperature ranges, and good thermal and mechanical stabilities, see Fig. 1. Stable Na/Li plating stripping performance is achieved using the ionomer membrane. Lin, D. et al., Nanotechnol. 2017, 12, 194–206.Lu, Y. et al., Adv. Energy Mater. 2015, 5, 1402073.Oh, H. et al., Mater., 2016, 28, 188-196. Caption Fig.: Fig 1: (a) Photograph and SEM image of the microporous ionomer membrane. (b) Arrhenius plot of the Na+ ion conductivity of the microporous ionomer membrane infused with EC/DEC (vol. 1:1). Figure 1

Study on Li Metal Deposition, SEI Formation on Anodes and Cathode Potential Change during the Pre-Lithiation Process in a Cell Prepared with Laminated Porous Anodes and Cathodes

The pre-doping of Li+ ions (pre-lithiation) in graphite anodes, which is needed to improve the initial charging/discharging efficiency of lithium ion batteries, was examined from the viewpoints of lithium metal deposition, surface electrolyte interface (SEI) formation and electrode potential changes in the cathode during the pre-lithiation. Using a cell composed of pre-laminated, through-holed anodes and cathodes, the anodes were pre-lithiated with the perpendicular pre-doping method, as explained in this work, which can effectively enhance the pre-lithiation process in laminated cells. In the pre-lithiation system, Li deposition was not observed on the anode surfaces during pre-lithiation, and the thickness of the SEI layers formed on the anodes did not increase. Moreover, the SEI layer has the same composition as that formed by the electrochemical lithiation (charging) process, even when the pre-lithiation of the anodes is accelerated by through-holes formed on the anode and cathode electrodes. In addition, the electrode potential of cathodes inserted between pre-lithiated anodes does not change during the pre-lithiation process, and the capacity of the cathodes does not degrade upon pre-lithiation. The perpendicular pre-doping method examined in this study is found to be applicable to the production process of lithium ion batteries with high charging/discharging efficiency.

Insights into morphological evolution and cycling behaviour of lithium metal anode under mechanical pressure

Dendritic Li formation is one of the critical reasons for the failure of Li batteries. In order to improve the lithium metal anode performance, a better understanding of the growth mechanisms of Li dendrites is necessary. Due to the malleable nature of lithium metal, mechanical pressure should play an important role in determining the morphology and cycling behaviour of Li anode. Here we investigated the effect of an applied external pressure on the electrochemical deposition of lithium metal. Instead of a highly porous, wire-like Li growth in the absence of pressure, a much more compact Li deposition can be achieved when a pressure is applied to the batteries in the charge/discharge processes. The improved Li deposition/stripping behaviour in the pressed cells yields a 5% higher Coulombic efficiency (~90%) and more than 5-fold longer cycling life than the cells without pressure at a current density of 2 mA/cm2. The use of pressure in shaping Li metal is an effective approach to address the Li metal problem and advance Li technologies in the future.

Spatially uniform deposition of lithium metal in 3D Janus hosts

Three-dimensional (3D), high-specific-surface-area, and porous current collectors are strongly considered as the hosts of lithium deposition to avoid dendrite growth of lithium metal in rechargeable batteries. However, a major hurdle in these hosts is the poor affinity of lithium in non-polar framework and favorable lithium deposition toward the conductive separator-facing surface while leaving the interior voids empty. Herein, we demonstrate an effective strategy to address the issue of spatially heterogeneous lithium deposition in 3D Janus current collectors by modifying its separator-away surface with low Li/Li+ over-potential nanoparticles as nucleation sites to guide lithium deposition. The metallic lithium preferentially nucleates around the gold nanoparticles that are sputtered on the separator-away surface of the carbon paper. The lithium metal then grows along the adjacent carbon fiber and renders it spatially homogeneous for deposition/dissolution during the repeated charge/discharge processes. The Janus gold nanoparticle-modified carbon paper (Au/CP) electrode exhibits an excellent Coulombic efficiency of 99.1% over 100 cycles at 1.0 mA cm⁻2 in the ether electrolyte, while the pristine carbon paper (CP) and stainless steel foil (SS) electrodes exhibit Coulombic efficiencies of less than 80.0% after 74 and 59 cycles, respectively. The strategy is universal and similar results are obtained when replacing gold, carbon paper, and ether electrolyte with zinc oxide, nickel foam, and carbonate electrolyte, respectively. Therefore, this strategy presents a general approach to regulate lithium ion distribution, nucleation, and deposition behavior for long-lifespan lithium metal batteries.

The degradation of LiCoO2/graphite batteries at different rates

The LiCoO2/graphite full cells (CP523450A) are cycled at different rates (0.6C, 1.2C, 1.8C and 3.0C), and the degradation mechanisms are evaluated in this paper. The lithium metal is deposited on the anode when the cycling rate is higher than or equal to 1.8C, which is confirmed by the tests of the incremental capacity (IC) curves, XPS, DSC and SEM. The root cause of the lithium deposition is analyzed by the available capacities of the single electrodes as well as the actual capacity ratio of anode to cathode (actual A/C ratio). The rate capability of the anodes recovered from the fresh and the aged full cells is quite poor in comparison with the cathodes. Therefore, the capacity of full cell is limited by the anode when the test rate is relatively high. In such situation, the coulombic efficiency is less than 1, and its value is slightly larger than the actual A/C ratio which is the root cause of the deposition of lithium metal. Compared with the generation of SEI film, the lithium deposition leads to an accelerated loss rate of active lithium which results in the rapid capacity fade of full cell. The research about the actual A/C ratio provides a guidance for the design of the initial reversible capacity ratio of anode to cathode (initial A/C ratio) of lithium ion batteries.

Modeling Lithium Growth in Symmetric Cells

Lithium-metal batteries are receiving great attention in recent years as an anode material. It has a higher theoretical specific capacity (3860 mAh∙g-1) compared to the graphite (372 mAh∙g-1) used in conventional Lithium-ion batteries.1 However, the capacity fade resulted from unexpected dead lithium, and dendrite growth is a major issue that causes shorter cycle life and possible internal short circuits during operation. The purpose of this study is to develop a continuum model that captures the dynamics of lithium metal deposition and stripping during repeated cycling of a lithium-metal cell. The repeated deposition and stripping of Lithium accounts for the presence of a heterogeneous growth of Lithium foil (anode) in a battery electrode. One can take advantage of the simplicity of symmetric cells to understand electrodes and electrolyte in a controlled applied potential.2 The symmetric cell consists of the same materials on both electrodes, so the average potential of the system is zero. This study will compare the continuum model with experimental data for a Li/Li symmetric cell. The mathematical model used in this study is based on the concentrated solution theory models developed by Newman and coworkers.3-5 The mass transport of the Li-ion in the electrolyte, neglecting migration, is governed by Fick’s law. A modified Ohm’s law for the liquid electrolyte phase is considered and the Butler-Volmer equation is accounted for lithium metal deposition and stripping at the electrode. The system will be studied with one-dimensional modeling coupled with a moving boundary representation of lithium stripping and plating.6 Some experimental variations such as high current density cases will also be considered. The one-dimensional model will then be extended to two-dimensional modeling to understand the trend of heterogeneous plating/growth of Lithium metal. Acknowledgements The authors would like to thank for the financial support of this work by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the DOE through the Advanced Battery Material Research (BMR) Program (Battery500 consortium) and the Clean Energy Institute (CEI) at the University of Washington and the Washington Research Foundation (WRF) for their partial monetary support during this work. References J. Qian, W. A. Henderson, W. Xu, P. Bhattacharya, M. Engelhard, O. Borodin and J.-G. Zhang, Nature communications, 6 (2015).J. C. Burns, L. J. Krause, D.-B. Le, L. D. Jensen, A. J. Smith, D. Xiong, and J. R. Dahn. J. Electrochem. Soc., 158, A1417-A1422 (2011).M. Doyle, T. F. Fuller, and J. Newman, J. Electrochem. Soc., 140, 1526–1533 (1993).V. Srinivasan and J. Newman, J. Electrochem. Soc. , 151, A1517–A1529 (2004)S.-L. Wu, A. E. Javier, D. Devaux, N. P. Balsara, and V. Srinivasan, J. Electrochem. Soc. , 161, A1836–A1843Q. Zhang and R. E. White, J. Electrochem. Soc. , 154, A587–A596 (2007)

Use of Asymmetric Average Charge- and Average Discharge- Voltages as an Indicator of the Onset of Unwanted Lithium Deposition in Lithium-Ion Cells

Unwanted lithium-metal deposition on the negative electrode of a lithium-ion cell causes capacity loss due to poor lithium deposition and stripping efficiency and the possibility for internal short circuits. Internal short circuits may cause thermal runaway, which is especially dangerous in applications requiring many individual cells. This article proposes a method capable of identifying the onset of unwanted lithium deposition in-situ using cycles to full depth of discharge in a cell of any form-factor. The two most important factors affecting the average voltage of a Li-ion cell under load are the internal resistance increase and loss of lithium inventory. Increasing internal resistance increases average charge voltage and decreases average discharge voltage. Loss of lithium inventory, which occurs rapidly during unwanted lithium deposition and stripping, increases both average charge and average discharge voltage. Increasing internal resistance and loss of lithium inventory have a linearly additive effect on average voltage; therefore, tracking the average of average charge- and average discharge-voltages versus cycle count allows one to determine where rapid changes to lithium inventory onset, indicative of the onset of unwanted lithium deposition.

Guided Lithium Metal Deposition and Improved Lithium Coulombic Efficiency through Synergistic Effects of LiAsF6 and Cyclic Carbonate Additives

Spatial and morphology control over lithium (Li) metal nucleation and growth, as well as improving Li Coulombic efficiency (CE), are among the most challenging issues for rechargeable Li metal batteries. Here, we report that LiAsF6 and cyclic carbonate additives such as vinylene carbonate (VC) or fluoroethylene carbonate (FEC) can work synergistically to address these challenges. It is revealed that LiAsF6 can be reduced to LixAs alloy and LiF, which act as nanosized seeds for Li growth and form a robust solid electrolyte interface layer. The addition of VC or FEC not only enables the uniform distribution of LixAs seeds but also improves the flexibility of the solid electrolyte interface layer. As a result, highly compact, uniform, and dendrite-free Li film with vertically aligned column structure can be obtained with increased Li CE, and the Li metal batteries using the electrolyte with both LiAsF6 and cyclic carbonate additives can have improved cycle life.

Analysis of Plating in Lithium-Ion Batteries with Anode Control

Lithium-ion batteries (LIBs) are the energy storage system of choice for high energy and power applications, such as electric vehicles. However, LIBs still face multiple challenges, especially during operation under extreme, such as low-temperature and fast charge. Under such conditions, lithium plating may happen, which can deplete the cell capacity, diminish cycle life and may lead to safety incidents. For example, if dendrites are formed, instead of a film-like lithium metal deposition, short circuit and thermal excursion may ensue. Graphite anodes are susceptible to lithium plating due to the proximity of their potential to the reversible potential of lithium. In a typical Li-ion cell, if the graphite anode potential is driven below 0.0 V vs. Li/Li+ during cycling, deposition of lithium may occur. In the present study, an anode control based charging scenario is proposed. Fast charging is conducted by imposing a constant current – constant voltage (CCCV) protocol in the negative electrode with an arbitrary lower cutoff potential (> 0.0 V vs Li/Li+). During the anode controlled charging, both the cathode potential and the full cell voltage are also monitored. Electrochemical analysis based on the anode control strategy and the effect on lithium plating and cell performance will be presented.

Mechanism of Lithium Metal Penetration through Inorganic Solid Electrolytes

Vast energy density improvements may be achieved in Li-ion rechargeable batteries with lithium metal anodes. However, lithium metal batteries containing liquid electrolytes have been unsuccessful thus far due to safety concerns associated with short circuits that occur when Li dendrites grow through the liquid electrolyte during the charging process. In contrast, thin film batteries using Li metal negative electrodes with an inorganic solid electrolyte (e.g., LiPON) appear to withstand dendrite penetration over extended cycling [1]. Recently, however, multiple research groups have reported and investigated cases where ceramic solid electrolytes paired with a Li metal anode experience a short circuit event [2,3]. The mechanism by which short circuit events occur in all-solid-state Li-ion batteries is unclear. Thus, in this project, the growth of lithium metal filled cracks through four types of solid electrolytes, glassy Li2S-P2S5 (LPS), β-Li3PS4­, polycrystalline and single crystal Li6La3ZrTaO12 (LLZTO), was studied using galvanostatic electrodeposition experiments coupled with in-situ and ex-situ microscopy. An electrochemomechanical model for growth of lithium-filled cracks was developed. For current densities up to 5 mA/cm2, only the as-fractured surface of glassy LPS exhibits lithium metal deposition at the surface alone, without penetration of the solid electrolyte. All other surfaces of the LPS and LLZTO samples studied had observable defects, the maximum initial sizes of which were characterized. Electrodeposition of lithium at these surfaces is accompanied by the filling, and then propagation, of lithium metal filled cracks. The experiments and model suggest that above a critical current density, the Li plating overpotentials, and corresponding mechanical stresses, reach values sufficiently large to propagate pre-existing surface defects. The results can be summarized, Figure 1, as a plot of the overpotential and corresponding mechanical stress necessary to extend a pre-existing sharp crack as a function of the crack length, for materials parameters suitable to LPS and LLZO. The prevailing failure mechanism in brittle solid electrolytes is Griffith-like, and differs from the amplification of kinetic perturbations at the metal interface that results in dendrite growth in liquid electrolytes. The results show that the shear-modulus criterion proposed by Monroe and Newman [4,5] for prevention of dendrites is not the determining factor for the high modulus, brittle inorganic electrolytes studied here. We suggest that stabilization of inorganic solid electrolyte interfaces against lithium metal penetration will require scrupulous attention to minimize interfacial defects. Acknowledgments Tushar Swamy and Lukas Porz contributed equally to this work. The authors gratefully acknowledge support from the US Department of Energy, Office of Basic Energy Science, through award number DE-SC0002633 (J. Vetrano, Program Manager). Brian W. Sheldon acknowledges additional support by the US Department of Energy, Office of Basic Energy Sciences under the award number DE-FG02-10ER46771. The authors also acknowledge use of the MIT Nanomechanical Technology Laboratory (A. Schwartzman, Manager).