Anode-free Lithium Metal Batteries Research Articles

Dynamically lithium-compensated polymer artificial SEI to assist highly stable lithium-rich manganese-based anode-free lithium metal batteries

Dynamically lithium-compensated polymer artificial SEI to assist highly stable lithium-rich manganese-based anode-free lithium metal batteries

Improved plating/stripping in anode-free lithium metal batteries through electrodeposition of lithiophilic zinc thin films

The new concept of anode-free Li batteries (AFLBs) with no excess lithium, while retaining numerous strengths, does come with certain challenges, such as high capacity loss and low coulombic efficiency. These challenges can be addressed through the utilization of modified lithiophilic current collectors to enhance the electrochemical performances. Herein, we investigated the surface modification of the Cu current collector by zinc electrodeposition to provide a lithiophilic thin layer. This process aims to facilitate a smoother plating/stripping process, leading to a uniform and dendrite-free lithium deposition on the LixZny phase formed at the first stages of plating. The zinc-modified current collectors improved cell's performances, including smooth and dense lithium deposition, reduced overpotential at various applied current densities (44.5 mV to 16.8 mV @0.25 mA cm−2), prolonged cycling stability for about 300 cycles and provided high coulombic efficiency of 95 % when compared to bare Cu electrodes. These improvements are attributed to a favorable lithium alloying reaction on the Zn-coated surface. Galvanostatic Intermittent Titration Technique (GITT) and Electrochemical Impedance Spectroscopy (EIS) revealed high diffusion coefficients for lithium (ranging from 10−9 to 10−6 cm² s-1), despite of the solid-state Li diffusion in the Zn film coating. We anticipate that implementing this modified current collector in a half-cell configuration could lead to the realization of a high-performance, anode-free lithium battery. This holistic approach proposes a scalable and cost-effective technique for boosting the performance of AFLBs.

Stoichiometric Ti3C2Tx Coating for Inhibiting Dendrite Growth in Anode‐Free Lithium Metal Batteries

Lithium metal batteries (LMBs) and anode‐free LMBs (AFLMBs) present a solution to the need for batteries with a significantly superior theoretical energy density. However, their adoption is hindered by low Coulombic efficiency (CE) and rapid capacity fading, primarily due to the formation of unstable solid electrolyte interphase (SEI) layer and Li dendrite growth as a result of uneven Li plating. Here, we report on the use of a stoichiometric Ti3C2Tx (S‐Ti3C2Tx) MXene coating on the copper current collector to enhance the cyclic stability of an anode‐free lithium metal battery. The S‐Ti3C2Tx coating provides abundant nucleation sites, thereby lowering the overpotential for Li nucleation, and promoting uniform Li plating. Additionally, the fluorine (−F) termination of S‐Ti3C2Tx participates in the SEI formation, producing a LiF‐rich SEI layer, vital for stabilizing the SEI and improving cycle life. Batteries equipped with S‐Ti3C2Tx@Cu current collectors displayed reduced Li consumption during stable SEI formation, resulting in a significant decrease in capacity loss. AFLMBs with S‐Ti3C2Tx@Cu current collectors achieved a high initial capacity density of 4.2 mAh cm−2, 70.9% capacity retention after 50 cycles, and an average CE of 98.19% in 100 cycles. This innovative application of MXenes in the energy field offers a promising strategy to enhance the performance of AFLMBs and could potentially accelerate their commercial adoption.

Multiple protective layers for suppressing Li dendrite growth and improving the cycle life of anode-free lithium metal batteries

Anode-free lithium metal batteries (AFLMBs) have sparked considerable attention in recent years because of their potential for high energy density; however, they suffer from severe Li dendrite growth and unstable solid electrolyte interphase (SEI), which typically result in rapid capacity decay. Herein, we demonstrate a long-life anode-free pouch cell by designing a dual-coating protective layer (Cu-Sn@SFPH) electrode with Sn-coated Cu (denoted as Cu-Sn) as the bottom layer and SrF2 nanoparticles strengthened by poly (vinylidene fluoride)-co-hexafluoropropylene (PVDF-HFP) as the top layer. The in-situ formed LiF-rich SEI enables fast Li+ transfer, while the lithiophilic Li-Sn and Li-Sr alloy layers serve as nucleation seeds for uniform Li deposition. The dual-coated Cu electrode in the Cu-Sn@SFPH||Li cell exhibits remarkable cycling stability for more than 3,200 h at a capacity of 2 mAh cm−2. The NCM111||Cu-Sn@SFPH pouch cell demonstrates outstanding performance with a capacity retention of 72.1 % and an average Coulombic efficiency (CE) of 99.9 % for 120 cycles. Under practical conditions, with NCM cathodes and a lean electrolyte volume, this design strategy opens a new approach to AFLMBs.

A high-current initiated formation strategy for improved cycling stability of anode-free lithium metal batteries

Anode-free lithium metal batteries (AFLMBs) are considered as one of the most promising candidates for next-generation high-energy-density rechargeable lithium batteries. An efficient high-current initiated formation strategy is developed for AFLMBs.

Mechanism of lithium dendrite growth on iron surfaces toward high-performance and safe anode-free lithium metal batteries

Selecting the Fe(111) surface, reducing surface cracks, and controlling the surface nanogroove structures can effectively prevent the formation of irreversible Li dendrites on the Fe collector, resulting in better performance of anode-free LMBs.

Constructing Stable Solid Electrolyte Interphase Using Gel Polymer Electrolytes for Anode-Free Lithium Metal Batteries with Enhanced Cycle Life

Anode-free lithium metal batteries (AFLMBs) have recently emerged as next-generation batteries due to high energy density, reduced cost, and simplified manufacturing process due to the absence of reactive metallic lithium. However, developing high-performance AFLMBs faces several challenges for practical applications, such as low coulombic efficiency and poor cyclability, due to the dendrite growth of lithium and side reactions of lithium with liquid electrolytes. To develop the AFLMBs with long cycle life, it is necessary to attain high coulombic efficiency since there is no excessive lithium to compensate for irreversible Li inventory losses in the AFLMB. Three-dimensional cross-linked gel polymer electrolyte (GPE) can be a good candidate for stabilizing the lithium metal by encapsulating organic solvents in the polymer matrix, leading to reduction of the parasitic reactions between lithium metal and liquid electrolyte, which results in high coulombic efficiency of AFLMBs. In this work, we prepared a cross-linked GPE using a multi-functional cross-linking agent to realize high coulombic efficiency and long cycle life of AFLMBs with LiNixCoyAl1-x-yO2(NCA) cathode (areal capacity of 4.76 mAh cm-2). Moreover, the GPE capable of coordinating with lithium ions could construct stable solid electrolyte interphase by regulating lithium ion solvation structures. Cycling performance was investigated in the voltage range of 3.6 – 4.3 V at 0.5 C rate. The Cu/GPE/NCA cell exhibited a capacity retention of 80% after 150 cycles, which was higher than that of AFLMB with liquid electrolyte. Electrochemical characterization and spectrometry were performed to investigate how the GPE could improve the cycling performance of AFLMBs.

Development of Anode-Free Lithium Metal Batteries

The continuous demand for high energy density storage devices, researchers has focused on a new energy storage system for their practical life. Absolutely, Lithium metal is a best anode candidate for high energy density Lithium Metal Battery (LMB) because it has high theoretical capacity, light weight and much lower negative reduction potential.However, the growth of Li dendrite and low Coulombic efficiency upon Li plating/stripping causes the safety issues and poor cycle life of LMBs because of challenges embedded from the interaction of the Li metal with electrolyte and unstable SEI layer formation. For that reason, people have focused on developing new chemistry/system to improve the energy density, cycle life and safe energy storage system for practical applications. Anode free lithium metal battery (AFLMB) is one of the alternate choices and new system for high energy density storage system because it has no active anode material initially. In addition, the fabrication of AFLMB is simple, low-cost, safe since Li metal is not directly used as an anode. However, since the Li amount is limited and electrolyte decomposition, consuming Li during the SEI formation, the capacity can fade quickly relative to LMB. The dead Li can affect the Coulombic efficiency, polarization, and cycle life of the cell during continues cycles of AFLMB. However, AFLMB is the best system to understand the detail chemistry like quantification of dead Li, polarization effect, electrolyte consumption easily compared to the LMB. In this presentation, various approaches including functional electrolytes and functional coating on Cu surface will be reported to improve the coulombic efficiency, energy density and cycle life for the AFLMB.

Nanotwinned Structure in Copper Affects Lithium Deposition Behavior at Electrolyte-Electrode Interface in Anode-Free Lithium Metal Battery

Anode-free lithium metal battery (AFLMB) is one of new promising energy storage systems. In contrast to lithium-ion battery, the negative electrode of AFLMB does not contain any active materials, resulting in a two-fold increase in energy density. As a result, the charging mechanism on the negative electrode in AFLMB is based on electrodeposition of lithium, rather than intercalation of lithium ions. Unfortunately, that causes a series of problems, such as lithium dendrite formation, volume expansion, broken SEI and dead lithium accumulation, leading to the low cycling stability of AFLMB. Consequently, interfacial characteristics of the negative electrode (usually copper) play a critical role in determining the morphology of lithium deposits which largely affects the issues mentioned above. Understanding the mechanism of lithium electrodeposition across the electrode-electrolyte interface is crucial to design a better copper electrode for long-cyclability AFLMB.The interfacial characteristics of copper are mainly influenced by crystal phase, surface roughness and defects. While the effects of crystal phase and surface roughness on lithium electrodeposition have been extensively studied, the role of micro-scale defects has been seldom discussed, despite their potential to strongly influence the surface properties of copper. Therefore, this work focuses on elucidating the elementary process about how the defects on copper affect the lithium nucleation and subsequent growth. Specifically, we use (111)-nanotwinned and (111)-preferred orientation copper foils with the same surface roughness to conduct a series of electrochemical measurements. The presence of nanotwin structure can reduce the grain boundary and atomic step density, a type of the defects with high surface energy, on the copper. Before lithium electrodeposition, solvents and anions in an electrolyte adsorb on the electrode-electrolyte interface to form the electric double layer. During this process, the atomic steps with high surface energy on copper can establish strong adsorption with the substance in the electrolyte. The solvents adsorbed on the defects may hinder the reduction of anion. Accordingly, the (111)-nanotwinned Cu foil with fewer defects induces the formation of LiF-rich solid electrolyte interphase (SEI), which is beneficial to the transportation of lithium ions. In cyclic voltammetry test, the (111)-nanotwinned Cu foil exhibits higher reduction current density of the precipitation of LiF (Fig. 1).For lithium electrodeposition, during the initial stage, lithium adatoms tend to adsorb on the defects with high surface energy on the copper surface, where other adatoms accumulate to form stable nucleus. Subsequently, the nucleus on each nucleation sites might have different growth direction and crystal phase, resulting in a lot of independent lithium grains. In other words, compared to the (111)-preferred orientation substrate, the (111)-nanotwinned Cu foil with less nucleation sites provides more space for the nucleus to grow in 2-D direction, which leads to the lower nucleation overpotential (Fig. 2 a). That result also facilitates the formation of larger lithium grains with smaller surface area exposed to the electrolyte (Fig. 3). As shown in Figure 2 (b), quasi-steady state over-potential of Li plating on the (111)-nanotwinned Cu is generally larger than that on the (111)-preferred orientation Cu foil, which has been ascribed to the high local current density on larger lithium grains. The quasi-steady state overpotential of Li plating on the (111)-preferred orientation Cu foil gradually increased with the cycle number, which might be attributable to the relatively unstable SEI and the accumulation of dead lithium with the GCD cycling. Furthermore, the (111)-nanotwinned Cu foil always exhibits a lower quasi-steady state overpotential of lithium stripping in comparison with the (111)-preferred orientation substrate at each cycle, probably resulting from the uniform Li deposit and the special LiF-rich SEI formation due to the nanotwinned structure. In the cycling stability test (Figure 2 c), for the Cu//Li cell using the (111)-nanotwinned Cu foil, the coulombic efficiency was kept being around 90% in this 160-cycle test. However, for the Cu//Li cell using the (111)-preferred orientation Cu foil, the coulombic efficiency is generally lower than that of the cell using the (111)-nanotwinned Cu foil at the cycle number > 15. In addition, the coulombic efficiency starts to obviously decay at the 135th cycle and becomes lower than 5% at the 160th cycle.To sum up, we proposed a mechanism about how the defects affect the reaction at electrode-electrolyte interface. The nanotwinned columnar grains reduces the atomic density on the copper surface, which effectively reduces the formation of lithium dendrite, the accumulation of dead lithium and broken SEI. The (111)-nanotwinned Cu foil is proved to be a superior substrate for lithium deposition and stripping, significantly improving the Li+/Li redox reversibility and cycling stability. Figure 1

Cu Foil Modification By Ag Inkjet Printing for Improved Li Plating in Anode-Free Li Batteries

The rush in the development of new generation of lithium-ion batteries (LIB) is focused on enhancing the specific energy of batteries by employing innovative materials and manufacturing techniques. Anode-free lithium metal batteries (AFLBs) are an emerging technology that can almost double the specific energy of LIBs by eliminating the conventional graphite negative electrode or excess lithium metal 1. However, AFLBs are well-known to suffer of fast capacity decay due to the poor lithium plating and stripping efficiency process associated to the typical porous growth of Li during charge 2. Tuning the interfacial electrochemistry was widely demonstrated to be one the most performing key to enable efficient and smooth Li deposition. In this regards, functional coatings of metals as Sn, Al, Zn and Ag have been widely demonstrated to dramatically improve the quality of Li plating process respect to the neat Cu foil substrate, by forming lithium-rich alloys that provide minimum Li nucleation overpotentials 3. Inkjet printing (IJP) could be a viable technique for electrode fabrication in LIBs manufacturing, being a quick, easy and scalable technology able to deposit both thin film and patterned structures 4. However, very poor attention has been paid so far to the potentiality of IJP for Cu modification for improved Li plating in AFLBs.In this work, we employed a Fujifilm Dimatix inkjet printer to achieve “lithiophilic” coatings of Ag nanoparticles deposited on a Cu foil. In particular, potenziodynamic and galvanostatic electrochemical test were employed to investigate the effect of the Ag printed structures on the Li nucleation overpotential. Ex-situ morphological analysis by electron scanning microscopy (SEM) were usefully employed to investigate the surface evolution of the printed electrodes during lithium alloying and plating. In particular, the effect pf patterned Ag structured rather than fully coated Cu was investigated, revealing that patterning can provide more mechanically robust electrode and efficient catalytic sites for Li electrodeposition respect to fully coated inkjet printed electrodes. Bibliography L. Lin et al., Advanced Materials, 34, 2110323 (2022).L. Su and A. Manthiram, Small Structures, 3, 2200114 (2022).X. Gu, J. Dong, and C. Lai, Engineering Reports, 3, e12339 (2021).P. Viviani, E. Gibertini, F. Iervolino, M. Levi, and L. Magagnin, Journal of Manufacturing Processes, 72, 411–418 (2021).

Role of Copper as Current Collectors in the Reductive Reactivity of Polymers for Anode-Free Lithium Metal Batteries - Insights from DFT and AIMD Studies

Understanding the role of current collectors (CCs) in the reductive reactivity of polymers on Li metal and the resultant solid electrolyte interphase (SEI) formation is essential for improving the performance of anode-free lithium metal batteries (AFLMBs). In this study, we have examined the reactivity of three polymeric hosts: poly(ethylene oxide) (PEO), poly(ε-caprolactone) (PCL), and poly(trimethylene carbonate) (PTMC) at Li metal supported on Cu surfaces (Li/Cu) using density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations. In particular, the effect of copper (Cu) CCs on polymer stability, electronic structure, and their reduction reactions is investigated and compared to that of pure Li (100) surface. Through time-dependent Bader charge transfer analysis, electron transfer is identified as the triggering factor for polymer reduction. Based on the simulations, we find that the Cu CCs have a significant influence on the charge distribution of the Li metals, which increases electron transfer to the polymers and thereby accelerates polymers reduction. This thereby leads to different reaction mechanisms as compared to on Li-metal. The findings suggest that utilization of Cu CCs avoid production of CO molecules and improves the quality of the formed SEI layer. Figure 1

Lithium Dendrite Propagation in Anode-Free Lithium Metal Batteries: An in-Operando Scanning Electron Microscopic Study

In the current technological landscape, lithium metal batteries have significantly surpassed the energy density of conventional lithium-ion batteries. Therefore, The advent of anode-free lithium metal batteries (AFLMBs) offers higher energy densities.. They also provide advantages such as reduced process complexity and lower costs due to their design that eliminates the need for anode materials. Unlike the lithium-ion insertion and extraction reactions in traditional lithium-ion batteries. This makes them easier to lithium dendrite formation, which can puncture the separator or solid electrolytes, among other issues. In this work, we developed anin-operando electron microscopic technique with a unique vacuum square-cell design to ensure air-tight transfer of in-operandocells with vacuum protection into the SEM chamber. We significantly improved the imaging resolution with a windowless design, enabling the direct electron beam to focus on the electrode interfaces. Thus, real-time imaging of Li growth could be achieved via a stable current supply and controlled temperature. we systematically analyzed the Li growth and dendrite propagation behavior in AFLMBs using solid polymer electrolyte (SPE). In particular, we used polyethylene oxide (PEO)-based SPEs containing different lithium salts, namely 25wt% Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), Lithium difluoro (oxalateo)borate (LiDFOB), and Lithium bis(oxalate)borate (LiBOB), within a LiFePO4||Cu anode-free cell configuration. Under a fixed current density, Li deposition and stripping were conducted, in which the PEO-LiTFSI SPE exhibited uniform deposition with a mossy lithium dendrite morphology. In the subsequent deposition process, some branching lithium dendrites were observed. Meanwhile, PEO-LiBOB showed uneven Li deposition with needle-like dendritic formation, potentially disrupting the SPE/electrode interface and leading to safety hazards. Lastly, PEO-LiDFOB initially showed mossy lithium deposition. In contrast, a few dendritic lithium structures appeared as the deposition process continued into the intermediate and later stages. The study successfully established a novel in-situ electron microscopy observation platform isolating water and oxygen. This platform recreated the actual battery operating environment, enhancing the credibility of experimental results. Additionally, we successfully elucidated the growth mechanisms of lithium dendrites in SPE with different lithium salts in the LFP||Cu configuration. This in-situ microscope technology aims to observe the relevant mechanisms behind dynamic deposition. Understanding how this deposition affects battery performance and safety and combining it with advanced operational and ex-situ characterization techniques can further promote the development of all-solid -state batteries in the future.

Enhancing Performance of Anode-Free Li-Metal Batteries

Because of their higher energy density, compared to lithium-ion batteries, rechargeable lithium-metalbatteries (LMB) have been considered one of the most attractive next-generation energy-storagesystems (ESS). A promising approach to improving LMB performance, that has gained interest inrecent years, is the use of anode-free lithium-metal batteries (AFLMB). Such battery configurationenables elimination of the problem of using excessive amounts of lithium in LMBs, hence increasingthe specific energy of the battery. This work explores the beneficial effects of integrating metal-oxidenanoparticles (MONPs) into the liquid electrolyte of AFLMB. It was found that the addition of lowconcentrations of MONPs significantly improves coulombic efficiency (CE), capacity retention (CR)and the Solid-Electrolyte-Interphase (SEI) properties. SEM images revealed a smoother lithiummorphology for MONPs cells and XPS analysis showed great differences in the SEI composition. Theresistance of the cell components, extracted via electrochemical impedance spectroscopy (EIS), hasbeen significantly lowered due to the addition of MONPs. The performance of the AFLMB, withoutMONPs, was 97.1% CE and CR of 70% within 12 cycles, while cells with 0.5% SnO2 , 1% Al2O3 and1% TiO2 was 99.1%, 98.8% and 99.4% CE, and CR of 70% within 39, 38 and 36 cycles, respectively.Combinations of two MONPs resulted in 99.9% CE, which is among the highest found in the literaturefor AFLMB.

Electrodeposited Lithiophilic Zn Thin Film on the Current Collector for Improved Cyclability of Anode-Free Lithium Metal Battery

Anode-free lithium metal batteries (AFMBs) are recently presented as the new generation of Li metal batteries benefiting high gravimetric and volumetric energy densities due to the zero excess lithium. However, poor electrochemical stability and maximum cyclability of around 40-50 cycles due to the zero excess Lithium prevent the AFLMBs to be functional1. Here, lithiophilic electrodeposited Zn thin film on the current collector is reported in order to decrease the lithium nucleation overpotential and improve the plating/stripping process during alloying and dealloying. Moreover, columbic efficiency is increased by applying more zinc coated on the cupper current collector and by providing a more hospitable site for Li nuclei to plate2. Due to the alloy formation of Li-Zn, more conformal Li growth on the current collector is provided and homogeneous Li distribution and smooth morphology are observed by SEM images. The cells in different zinc thin layer coating are cycled with different current densities as well as long-term cycling by galvanostatic cycling charge-discharge cycling. As a result, increasing the thin film thickness up to 1 micron significantly enhances the electrochemical stability of the cells such as higher columbic efficiency and cyclability, lower Li nucleation overpotential. Finally, the Li-ion chemical diffusion coefficients DLi, are evaluated by galvanostatic intermittent titration technique (GITT) and electrochemical impedance spectroscopy (EIS) and the results are compared to each other3.References Maddukuri, S. et al. On the challenge of large energy storage by electrochemical devices. Electrochim Acta 354, 136771 (2020).Merso, S. K. et al. An in-situ formed bifunctional layer for suppressing Li dendrite growth and stabilizing the solid electrolyte interphase layer of anode free lithium metal batteries. J Energy Storage 56, 105955 (2022).Xie, J. et al. Determination of Li-ion diffusion coefficient in amorphous Zn and ZnO thin films prepared by radio frequency magnetron sputtering. Thin Solid Films 519, 3373–3377 (2011).

Improving Cycling Performance of Anode-Free Lithium Metal Batteries By Electrolyte Design

The anode-free lithium metal battery (AFLMB) is gaining attention due to its straightforward yet innovative cell configuration. However, the direct plating and stripping of lithium on the copper current collector presents challenges in material design and cell fabrication. Uncontrolled lithium growth and dendrite formation are common in these challenging conditions and can result in low coulombic efficiency. In this presentation, we summarize recent efforts to develop new electrolytes for high-voltage applications based on AFLMB protocols, including concentrated dual-salt and locally concentrated electrolytes, as well as additives. The solvation structure and corresponding interfacial analysis are significant factors in this development. Additionally, we aim to establish a titration-based method to estimate dead and inactive lithium in various electrolytes, which could facilitate the quick screening of various electrolyte candidates and offer further insight into the fundamentals of lithium growth and solid-electrolyte interface formation.

(Invited) Lithium Battery Interfacial Engineering with Thin Conformal Polymer Coatings

Coupled chemical and mechanical phenomena at electrode interfaces largely govern the reversibility and long term stability of lithium ion and lithium metal batteries. A well-known example in lithium ion batteries is the formation of the electrically insulating, ionically conductive, continuous solid electrolyte interface (SEI) on graphitic anodes, which passivates the electrode against further electrochemical reduction of the electrolyte. Unfortunately, this stable passivation does not occur in Si and Li metal anodes. High voltage cathodes are also prone to detrimental interfacial processes, resulting in electrolyte oxidation, gas formation, and transition metal dissolution. In response, a myriad of coating technologies has been developed to mitigate and control these interfacial phenomena and enable next-generation electrodes. These technologies must provide thin, conformal coatings over the entire electrochemically active surface area of the electrode; the coatings must also possess the requisite mechanical properties to accommodate volume dilation due to lithium insertion/extraction.This presentation will describe the development of conformal, compliant polymer thin film coatings for lithium battery electrodes. The polymer thin films are prepared by initiated chemical vapor deposition (iCVD), which provides exquisite control over film composition and morphology by facilitating heterogeneous free-radical polymerization of vinyl monomers with thickness precision on the order of 1 nm. Poly(1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane) (pV4D4) was deposited onto silicon thin film electrodes using iCVD. 25 nm-thick pV4D4 films on Si electrodes improved initial coulombic efficiency by 12.9% and capacity retention over 100 cycles by 64.9% relative to untreated electrodes. PV4D4 coatings also improved rate capabilities, enabling higher lithiation capacity at all current densities. Post-cycling FTIR and XPS showed that pV4D4 inhibited electrolyte reduction and altered the SEI composition, with enrichment in LiF. In another example, thin conformal coatings of perfluorinated polymer were deposited onto battery-grade Cu current collectors to function as an artificial solid electrolyte interface. The effect of the film thickness on Li cycling reversibility was characterized. By galvanostatically cycling Li in an asymmetric Li||Cu cell using 1M LiFSI in a 1:1 (v/v) mixture of dioxolane:dimethoxyethane at 30°C, it was shown that the average coulombic efficiency averaged over 50 cycles increased from 98.36% for bare, untreated Cu to 99.08% for Cu coated with 25 nm of the fluoropolymer. Moreover, the 25 nm coating suppressed interfacial impedance growth on the current collector; the resistance of the coated Cu was 40% lower than the untreated Cu after 100 cycles. The mechanism by which the polymer coating enhances coulombic efficiency and suppresses side reactions will be discussed, along with discussion of the performance in practical anode-free lithium metal batteries.

Exploring the Efficiency of Lithium Stripping/Plating in Solid Polymer Anode-Free Lithium Cells

Traditional Li-ion batteries (LIBs) are widely used in various electronic devices due to their advantages in energy storage applications. However, finding ways to push the energy densities even higher would be highly desirable for the further rapid development of electric vehicles and electromobility. Li metal batteries and anode-free batteries (AFBs) – where the Li metal is plated in situ from Li+ ions taken from the cathode – have received extensive attention and research due to their very high specific capacity.[1] However, Li ions are unevenly deposited on the surface of anodes to form Li dendrite during the charging and discharging process, which severely deteriorates their electrochemical performance and battery safety.[2] To solve this problem, many researchers choose solid polymer electrolytes (SPEs) instead of liquid electrolytes in the batteries, because of the stability of SPEs towards Li metal.Long-term cycling of AFBs requires high-efficiency Li metal stripping and plating, which in a solid polymer cell is highly dependent on the polymer electrolyte system used, so finding appropriate host materials and salts to for the SPEs is very necessary. Poly(ethylene oxide) (PEO)-based SPEs are one of the most popular host materials for SPEs, due to their excellent chemical stability. Poly(ε-caprolactone) (PCL)-based SPEs have higher ionic conductivity and can be cycled at room temperature.[3] Poly(trimethylene carbonate) (PTMC)-based SPEs have high lithium transference number and better mechanical properties.[4] Lithium bis(trifluoromethanesulphonyl)imide (LiTFSI) is one of the most popular salts used for SPEs. Recently, lithium difluoro(oxalate)borate (LiDFOB) and lithium bis(oxalato)borate (LiBOB) have been considered a promising alternative as electrolyte because they can passivate electrode surfaces and protect electrode materials.[5] Combinations of these materials are all potentially relvant for application in AFBs.In this work, three different polymer host materials, PEO, PTMC, and PCL, were explored as polymer electrolytes together with LiTFSI salt for their performance in anode-less cell setups, testing the plating/striping coulombic efficiency (CE) in Cu || Li cells. The results reveal clear issues for the CE of cycling in the PEO:LiTFSI system, while low-concentration PTMC:LiTFSI instead shows much more promising results. Changing the LiTFSI salt for LiBOB or LiDFOB also improves the CE of the cells.[1] Beyene, T. T., Jote, B. A., Wondimkun, Z. T., Olbassa, B. W., Huang, C. J., Thirumalraj, B., Wang C. H., Su W. N., Dai H., Hwang B. J., Effects of concentrated salt and resting protocol on solid electrolyte interface formation for improved cycle stability of anode-free lithium metal batteries, ACS applied materials & interfaces, 2019, 11(35), 31962.[2] Huang W. Z., Zhao C. Z., Wu P., Yuan H., Feng W. E., Liu Z. Y., Lu Y., Sun S., Fu Z. H., Hu J. K., Yang S. J., Anode‐Free Solid‐State Lithium Batteries: A Review, Advanced Energy Materials, 2022, 12(26), 2201044.[3] Chiu, C. Y., Chen, H. W., Kuo, S. W., Huang, C. F., Chang, F. C., Investigating the effect of miscibility on the ionic conductivity of LiClO4/PEO/PCL ternary blends. Macromolecules, 2004, 37(22), 8424.[4] Li Z., Mogensen R., Mindemark J., Bowden T., Brandell D., Tominaga Y., Ion‐conductive and thermal properties of a synergistic poly (ethylene carbonate)/poly (trimethylene carbonate) blend electrolyte. Macromolecular rapid communications, 2018, 39(14):1800146.[5] Shkrob, I. A., Zhu, Y., Marin, T. W., Abraham, D. P., Mechanistic Insight into the Protective Action of Bis (oxalato) borate and Difluoro (oxalate) borate Anions in Li-Ion Batteries, The Journal of Physical Chemistry C, 2013, 117(45), 23750.

Functionalizing Separator by Coating a Lithiophilic Metal for Dendrite-Free Anode-free Lithium Metal Batteries.

A stable anode-free lithium metal battery (AFLMB) is accomplished by the adoption of a facile fabricated amorphous antimony (Sb)-coated separator (SbSC). The large specific surface area of the separator elevates lithium (Li)-Sb alloy kinetic, improving Li wetting ability on pristine copper current collector (Cu). When tested with LiNi0.8 Mn0.1 Co0.1 O2 (NMC811) as cathode, the full cell with SbSC demonstrates low nucleation overpotential with compact, dendrite-free and homogeneous Li plating, and exhibits a notable lithium inventory retention rate (LIRR) of 99.8 % with capacity retention of 93.6 % after 60 cycles at 0.5 C-rate. Conversely, full cells containing pristine separator/Cu (i. e., SC) and pristine separator/Sb-coated current collector (i. e., SSbC) display poor cycling performances with low LIRRs. Density functional theory corroborates the nucleation behaviours observed during in-situ half-cell Li deposition. Functionalizing polymeric separator by metallic coating in AFLMB is a novel approach in improving the cycle life of an AFLMB by promoting homogeneous Li plating behavior. This innovative approach exemplifies a promising applicability for uniform Li-plating behavior to achieve a longer cycle life in AFLMB.

Unveiling the effect of alloy-type coating for lithium metal plating and stripping on anode-free lithium metal batteries

Li dendrite formation is the root cause of capacity fading in anode-free lithium metal batteries (AFLMBs). Previous studies have shown that Au coating on Cu can seed Li nucleation by forming Li3Au for smooth Li plating and improves the cycle life of Li||Cu cells. However, unlike Li||Cu cells with excessive Li supply, in practical AFLMBs (i.e., full-cell), the active lithium is limited from the lithium intercalation cathode. The introduction of alloy-type coatings may adversely affect the cycling stability due to the pulverization effect during alloying with Li (e.g., the volume expansion rate from Au to Li3Au is 337%). Here, we evaluate the full-cell performance of AFLMBs focusing on different thicknesses of Au coating (20, 50, and 100nm) and four coating layers (alloy-type: Au, Ag; non-alloy-type: Ni, Ti). Our results show that although the alloy-type coatings (i.e., Au and Ag) lower the lithiation overpotential, the full-cells equipped with these coatings experience faster capacity fading than those with pristine and non-alloy-type coatings. When the Au-coated Cu is paired with high-mass loading NMC622 cathode, the full-cells AFLMBs exhibit inferior performance with a low initial Coulombic efficiency (89.97% and 79.75% for 20 and 100nm thick Au coating) and accelerated capacity fading, producing plated Li with porous structure and carbonyl-rich solid electrolyte interphase. This study underscores the necessity of employing a full-cell configuration for evaluating Li plating-stripping efficacy in AFLMBs, emphasizing a transition from laboratory research to practical applications.

Interrelation Between External Pressure, SEI Structure, and Electrodeposit Morphology in an Anode‐Free Lithium Metal Battery

AbstractThe interrelation is explored between external pressure (0.1, 1, and 10 MPa), solid electrolyte interphase (SEI) structure/morphology, and lithium metal plating/stripping behavior. To simulate anode‐free lithium metal batteries (AF‐LMBs) analysis is performed on “empty” Cu current collectors in standard carbonate electrolyte. Lower pressure promotes organic‐rich SEI and macroscopically heterogeneous, filament‐like Li electrodeposits interspersed with pores. Higher pressure promotes inorganic F‐rich SEI with more uniform and denser Li film. A “seeding layer” of lithiated pristine graphene (pG@Cu) favors an anion‐derived F‐rich SEI and promotes uniform metal electrodeposition, enabling extended electrochemical stability at a lower pressure. State‐of‐the‐art electrochemical performance is achieved at 1MPa: pG‐enabled half‐cell is stable after 300 h (50 cycles) at 1 mA cm−2 rate −3 mAh cm−2 capacity (17.5 µm plated/stripped), with cycling Coulombic efficiency (CE) of 99.8%. AF‐LMB cells with high mass loading NMC622 cathode (21 mg cm−2) undergo 200 cycles with a CE of 99.4% at C/5‐charge and C/2‐discharge (1C = 178 mAh g−1). Density functional theory (DFT) highlights the differences in the adsorption energy of solvated‐Li+ onto various crystal planes of Cu (100), (110), and (111), versus lithiated/delithiated (0001) graphene, giving insight regarding the role of support surface energetics in promoting SEI heterogeneity.