Li Counter Electrode Research Articles

Degradation and Safety Analysis of Large Format LiFePO4/Graphite Battery

Large format LiFePO4/Graphite (LFP/Gr) Lithium-ion battery (LIB) have been developed for electric vehicles (EV) and stationary use. In case of large surface electrodes, intrinsic two-phase reaction of LFP has a risk to induce inhomogeneous reaction in planar electrode surface. We applied >200 Ah class LFP/Gr LIBs and cycled them around 80-65 % SOH levels. Among them, we applied non-destructive differential voltage analysis (dV/dQ) to three cells (Cell1:SOH 80.9 %, Cell2:SOH 73.5 %, Cell3:SOH 68.4 %), and determined the degree of inhomogeneity of the electrodes by disassembly of the cell in the Ar-filled glove box.We could obtain dV/dQ peak corresponds to Gr 2nd stage single phase by charge curve analysis between SOC 0 and 100 % at C/20. Based on the assumption that Gr capacity is twice of the Gr 2nd stage peak capacity, Gr capacity was sufficiently larger than cell capacity. Therefore, cell capacity fade was not related to the capacity fade of Gr but caused by the shift of the cathode / anode operation window (SOW) due to loss of lithium inventory (LLI) and/or capacity fade of LFP.After dV/dQ analysis with capacity check, cells were disassembled at 2.5 V, and confirmed LFP and Gr capacity by reassembling coin cells with Li counter electrode. Reassembled LFP/Li cells was discharged to 2 V, and Gr/Li cells were deintercalated to 2.5 V. The subtracted value of these residual capacities, we could estimate SOW capacity. The obtained estimated cell capacity by LFP-SOW showed strong correlation with SOH of cells before disassembly (R=0.98). Therefore, we confirmed that capacity fade of the cell was due to increase in SOW and/or capacity fade of LFP.To determine the degree of inhomogeneity in electrode planar, we applied XRD to LFP. As LFP reaction proceeds typical two-phase reaction, we can indirectly estimate Li content by estimating ratio of LFP and FePO4(FP). The LFP/FP ratio was optimized by Rietveld refinement of the diffraction peaks in various sampling points of the electrode sheet. We confirmed several macroscale inhomogeneous Li distributions in planar LFP electrode. Li poor area was observed at the beginning and the final edge of the roll electrode. The correlation details between cell SOH and degree of inhomogeneity will be discussed in the presentation.In addition, we applied two kinds of abuse test, overcharge and forced internal short circuit test by laser spot heating to three cells with different SOH. In case of overcharge, cell was charged at 0.78 C from SOC 100 %. In case of laser heating, pulse laser with 0.5 sec supply / 0.1 sec rest at 2000 W applied to the cell. The difference of the obtained parameters, and correlation with SOH will be discussed in the presentation.

Understanding the Reaction Processes in Li-Ion Batteries By Voltage Relaxation Analysis

The open circuit voltage (OCV) transient observed after current interruption is a very useful electrochemical signal providing crucial information about a Li-ion battery (LIB), including its state of charge and health, and kinetics of internal processes. Generally, OCV transient is divided into immediate and long-term relaxations, which are probably related to current-dependent kinetic processes and changes in the instantaneous thermodynamic state, respectively. Previous studies have attempted to analyze the pulse and relaxation voltages of LIBs under Li concentration gradients in both the electrolyte and electrode, with much success. Nevertheless, they have been limited in understanding what is really happening inside the cell in detail. This was mainly due to the fact that the experiments were performed on full cells with overlapping anode and cathode signals, which hindered the interpretation of detailed internal reactions, and for the same reason, only unsystematic phenomenological interpretations were possible. So it is necessary to separate relaxation signals from the anode and cathode for more accurate analysis. Even in a half-cell, where the signal contribution of the Li counter electrode (CE) to the overall cell signal is believed to be very small, the degree of potential relaxation attributable to the CE might be non-negligible due to the concentration change of active ions around the electrode. Therefore, in order to fully understand the electrochemical characteristics of the working electrode (WE), a precise analysis of the voltage relaxation of a single electrode is required.In this study, various test cells, including three-electrode half-cells and full cell, and a voltage-controllable symmetrical cell, were fabricated to analyze the pulse and relaxation process in order to further understand the reaction kinetics of the battery. In particular, we aimed to distinguish the detailed processes and to understand the overvoltage evolution during battery operation. For this purpose, Ni-rich layered oxide and graphite were used as the cathode and anode, respectively. Li metal was adopted as the CE and RE in both the half-cell and three-electrode cells. In the two- and three-electrode cathode half-cell experiments, it was confirmed that the overpotential contribution of Li CE in half-cells was one-third to half of total overpotential in specific operating conditions. Considering the relatively fast redox reaction rate of Li, this result strongly implies that a concentration overpotential due to the CE can play a critical role in OCV relaxation. In other words, a typical test cell structure where the opposing areas of WE and CE are similar, total cell overpotential is greatly affected by CE. Supporting this argument, a cell designed with a large CE area and abundant electrolyte showed the very small contribution of CE. In experiments with two- and three-electrode anode half-cells, the contribution of Li CE was similar to that of the cathode half-cell. Next, to investigate the change in the effect of Li CE on the overall overpotential as the cell degrades under repeated cycles, the OCV relaxation with cycles was analyzed for the test cells used above. The analysis revealed an obvious cycle dependence of the long-term OCV relaxation signal, which was confirmed to be largely an effect of Li CE. To investigate the unexpected voltage behavior of Li, Li symmetric cells were configured to obtain voltage curves over time with cycling. From the analysis results, it is proposed that the voltage curve variation is mainly due to different local reaction resistances and highly tortuous surface morphology of the Li surface.In conclusion, the overpotential of Li CE had a large impact on total overpotential in the commonly used two-electrode half-cells, which may cause errors in voltage curve interpretation. Under these circumstances, we further investigated whether the systematic and thorough analysis of the OCV relaxation could reliably separate the cathode and anode overpotential signals in the full cell, as well as the detailed reaction signals at each electrode. To this end, we performed an overpotential analysis using two- and three-electrode cells, as well as a voltage-controllable symmetrical cell, cross-validating the results with those obtained in the half-cell experiments above and electrochemical impedance spectroscopy. In this presentation, we will show how to derive the overpotential contribution of the cathode and anode, and the detailed reaction processes from the short- and long-term OCV relaxation curves obtained from a full-cell. Furthermore, we will discuss the reliability of the separated signals and possible future work to improve their accuracy.

Benchmarking the electrochemical parameters of the LiNi0.8Mn0.1Co0.1O2 positive electrode material for Li-ion batteries

The layered oxide LiNi0.8Mn0.1Co0.1O2 (NMC811, NCM811) is of utmost technological importance as a positive electrode (cathode) material for the forthcoming generation of Li-ion batteries. In this contribution, we have collected 548 research articles comprising >950 records on the electrochemical properties of NMC811 as a cathode material in half-cells with metallic Li counter electrode. The analysis of distribution histograms provided statistically-relevant values of such key characteristics of NMC811 as the first cycle discharge capacity and Coulombic efficiency, discharge capacities at different upper cut-off voltages, capacity fade and capacity retention at the 0.1C–5C current densities. We derived equations describing the relationships between discharge capacity and upper cut-off voltage, Ni content in the LiNixMnyCozO2 compositions in vicinity of NMC811, antisite disorder, and the C-rate. Additionally, the distribution histograms were used for a qualitative comparison between various groups of NMC811 materials, such as benchmarks in various optimizations vs obtained in course of synthesis development, lab-made vs commercial, polycrystalline vs single-crystal. The results of this analysis provide justified values to be used as benchmarks in further works related to optimizing and improving NMC811 and related materials, eliminating random picking up from a huge pool of published data.

How Do Impurities in Electrolytes Affect the Electrochemical Performance of Oxides in Lithium-Ion Batteries?

The recovery and treatment of spent LIBs has already been studied extensively. On one hand, the hydrometallurgical recovery of cathode materials has aroused widespread attention because of its low energy consumption and benefit to the environment. On another hand, the purification of the materials reclaimed from spent batteries is a challenging process with high probability that, later, some remaining impurities such as transition metal carbonates and sulfates could be released into the electrolyte. In this way the impurity content will have a serious impact on the final performance of the battery. To this point different electrochemical and analytical measurements were conducted to elucidate the effects of impurities in the electrolyte formulation, specifically Li, Mn, Co, Ni, and Al carbonates, sulfates, and acetates. The base LP30 electrolyte was intentionally contaminated with a maximum concentration of selected impurity to study the effects of the contaminated electrolyte on the electrode. Different combinations and varieties were explored correspondingly. Test setups were based on both, NMC811 in half cells (vs. Li) and symmetrical Li||Li cells, and the influence of contaminations was thus exploited on the Li counter electrode and on the NMC811 in Li||NMC811 configuration. A number of standard and advanced electrochemical test techniques such as CC-CV, electrochemical impedance, rate capability tests, GITT, and CV was applied for a fully understanding of the electrochemical characteristics of standard cathode active materials such as NMC811 when electrolytes with (artificially downgraded) low purity levels are present. The presentation will focus on discussion of results regarding the presence of transition metals Ni, Co, and Mn as impurities in the electrolyte which showed a significant impact on the electrochemical performance of the tested cells. When the concentration of the impurities increases, both the overpotentials at the cathode and the rate of capacity decay during cycling increase as well. The data shown will be very helpful for the researchers to understand the impact of impurities on the performance of oxide-based LIB cathode materials.

Novel Symmetric Cell Design for Analyzing All-Solid-State Battery Electrode

All-solid-state lithium batteries (ASSBs) with sulfide-based solid electrolytes (SSEs) have drawn much attention recently for the next-generation batteries. But there are some obstacles to overcome for their practical use. The SSE|cathode interface with poor contact and (electro)chemical instability is the subject of special interest, since it critically aggravates the core performance of battery, including cycling stability and rate capability. In addition to the interfacial property, it has been widely known that Li+ transport characteristics inside the cathode is another key factor in terms of fast and efficient operation of all the lithium battery systems, including the ASSBs. Thus, it goes without saying that the accurate characterization of the interfacial and bulk properties of the cathode is essential to develop the ASSBs.There are limited numbers of test cells to analyze the interface and bulk properties of the battery. First, a conventional half-cell includes Li metal as a counter electrode (CE), which is assumed to have a negligible resistance compared to that of the working electrode (i.e., cathode). It has been reported, however, that the effect of Li CE on total electrochemical signal cannot be disregarded. Moreover, the relative contribution of Li CE to total signal is increasing with the use of the state-of-the-art cathode, whose resistance is getting dramatically lower with technological progress. That is, it is getting difficult to obtain a reliable cathode response by using the two-electrode half-cell configuration. Second, a three-electrode test cell provides the signal of each electrode separately and has been extensively used for the electrochemical analysis. When one uses it for the ASSBs, however, the signals can be erroneous due to the structural distortion caused by the reference electrode (RE) insertion and its own chemical instability, which is especially severe in ASSBs with SSEs. Finally, an alternative cell design to avoid the above problems is a symmetric cell including two identical electrodes facing each other. A general symmetric cell is fabricated by charging/discharging two unit-cell, disassembling them, and re-assembling the identical electrodes to face each other. Unfortunately, a series of such procedure, esp. disassembling/re-assembling, is not applicable to the ASSB systems.In this study, a novel non-destructive potential-controllable symmetric cell (PCSC) is proposed for ASSBs. It features a porous temporary CE (TCE) in parallel between two symmetric electrodes. TCE works as an acceptor or provider of Li to charge or discharge, thereby controls the electrode potential. Symmetric electrodes are then evaluated electrochemically after disconnecting the TCE. As a case study, we investigated and comparatively analyze the electrochemical characteristics of an ASSB cathode, LiNbO3-coated LiNi0.8Co0.1Mn0.1O2, obtained from the conventional test cells and PCSC. Figure presents the impedance spectra of test cells. The two-electrode half-cell impedance is noticeably affected by Li CE over a wide frequency range and the three-electrode cell impedance suffers from severe noise probably caused by RE in the low frequency region. This indicates that both test cells are not suitable for interpreting reliably the interfacial and bulk properties of the cathode. In contrast, the impedance spectra from the PCSC have similar trend to that of three-electrode cell, but provides much more stable impedance spectra, allowing more accurate estimation of interface and bulk characters. In this presentation, the results obtained from DC techniques, such as galvanostatic method, are also comparatively analyzed with conventional test cells and our PCSC. Moreover, the reliability of the estimated values from the PCSC will be discussed by the comparison with those of the previous works. Figure 1

Ability of Li4Ti5O12 to suppress Li metal deposition under overpotential conditions confirmed by electrochemical surface plasmon resonance spectroscopy

Undesirable Li metal deposition on the negative electrode increases the risks associated with the use of Li-ion batteries. Generally, spinel-type lithium-titanium oxide Li4Ti5O12 (LTO)-based negative electrodes are well known as safe dendrite-free options, due to their higher working potential. However, the ability of LTO-based materials to prevent Li metal deposition has not been well confirmed under sudden overpotential application. This study examines the behavior of Li metal electrodeposition under various overpotential conditions for unloaded and LTO-loaded Cu electrodes. Electrochemical surface plasmon resonance spectroscopy (EC-SPR) is used as the optical diagnostic technique, owing to its sensitivity in detecting Li metal deposition. Li metal deposition is easily detected for the unloaded Cu electrode at overpotential of − 0.2 V (voltage) versus Li counter electrode (LiCE), whereas no Li metal deposition occurs for the LTO-loaded Cu electrode under the same overpotential. The safety of LTO is investigated by analyzing the reaction kinetics.

Phase Transformation and Microstructural Evolution of CuS Electrodes in Solid‐State Batteries Probed by In Situ 3D X‐Ray Tomography

AbstractCopper sulfide shows some unique physico‐chemical properties that make it appealing as a cathode active material (CAM) for solid‐state batteries (SSBs). The most peculiar feature of the electrode reaction is the reversible formation of µm‐sized Cu crystals during cycling, despite its large theoretical volume change (75%). Here, the dynamic microstructural evolution of CuS cathodes in SSBs is studied using in situ synchrotron X‐ray tomography. The formation of µm‐sized Cu within the CAM particles can be clearly followed. This process is accompanied by crack formation that can be prevented by increasing the stack pressure from 26 to 40 MPa. Both the Cu inclusions and cracks show a preferential orientation perpendicular to the cell stack pressure, which can be a result of a z‐oriented expansion of the CAM particles during lithiation. In addition, cycling leads to a z‐oriented reversible displacement of the cathode pellet, which is linked to the plating/stripping of the Li counter electrode. The pronounced structural changes cause pressure changes of up to 6 MPa within the cell, as determined by operando stack pressure measurements. Reasons for the reversibility of the electrode reaction are discussed and are attributed to the favorable combination of soft materials.

Where Does Sulfur Precipitate in Lithium Sulfur Batteries? an Operando SANS Experiment

In-situ and operando measurements are done to gain a better understanding of the precipitation mechanisms during charge and discharge in lithium sulfur batteries. In this work we used a carbon felt networked with microfibers consisting of pores, 2 nm and smaller, as a freestanding sulfur host. First, three different infiltration methods of sulfur infiltration are explored to determine the best method. Second, in-situ electrochemical impedance spectroscopy measurements were done that showed a solid product formation occurring at the sulfur cathode, both during the high voltage plateau and at the end of discharge. In an additional 3-electrode EIS measurement, a similar solid product formation on the Li counter electrode due to its reaction with polysulfides is also observed. Operando small angle neutron scattering measurements show the solid product formation, in the carbon, both near the beginning and at the end of discharge, confirming the precipitation data via contrast changes as a function of charge and discharge. We will show in this presentation that Li2S precipitates in the pores at the beginning and the end of discharge, whereas S8 precipitates on the surface of the carbon felt.

Chemo-Mechanical Deformations in Lithium Cobalt Oxide Cathode during Li-Ion Intercalation

The overall energy density of the Li-ion batteries depends on the operation voltage and the theoretical capacity of the electrodes. Theoretical capacity of layered lithium cobalt oxide (Li1-xCoO2) cathode is 274 mAh/g, which is higher than the many other commercially available cathode materials, including lithium manganese oxide (148 mAh/g) and lithium iron phosphate (170 mAh/g). However, the irreversible chemo-mechanical deformations at deep charge condition (x>0.5) at high voltages (>4.2V) presents harvesting its full theoretical capacity. Previous XRD and NMR studies demonstrated the structural collapse of the material at deep charge condition [1]. A various material-based strategies such as doping and surface coating have been utilized to improve the stability of the LCO cathodes at deep charge conditions [2]. However, the governing forces behind the chemo-mechanical instabilities in LCO cathodes at high voltages are still poorly understood.In this study, our goal is to elucidate the electrochemically-driven mechanical deformations in the LCO cathode during Li-ion intercalation. To achieve it, we performed in situ curvature measurement and digital image correlation measurements to probe stress and strain evolution in the electrode while cycling the battery. The combination of these techniques previously provided information about the interfacial and structural deformations in LiFePO4 and LiMn2O4 [3,4]. Here, free-standing composite LCO electrodes were fabricated for strain measurements. Thin film LCO electrode was prepared using chemical vapor deposition method on a silicon wafer for stress measurements. The electrodes were cycled in 1 M LiClO4 in 1:1 EC:DMC electrolyte against Li counter electrode. The upper limit of the charge voltage were varied from 4.2 to 4.5V periodically in order to investigate the deformations at deep charge state. Strain and stress derivatives were calculated with respect to applied potential in order to understand the potential-dependent mechanical deformations in the LCO electrode. In this talk, we will present the impact of the structural and interfacial deformations on the chemo-mechanical instabilities of LCO cathodes. Acknowledgment: This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences (Award numberDE-SC0021251). We are also grateful for Dr. Gabriel Veith for fabricating thin film LCO cathodes.

Is Closed-Loop Recycling of Lithium-Ion Batteries Feasible ?

Rechargeable Li-ion batteries have been developed and marketed over the past 30 years and are used today in different fields like electric vehicles, portable electronics, etc. In a few years, the development of this technology has been exponential, and it has quickly imposed on the energy storage market. However, it is important to note that this technology is very dependent on metals (Co, Ni, Cu, Al, Li ...). These metals are rare, expensive and non-renewable. In addition, they are unevenly distributed in the earth's crust and their production is often very polluting. Thus, the massive use of this technology is creating an imbalance between the needs for metals and the existing primary resources, leading irrevocably to environmental and economic problems.Recycling this waste is therefore essential to both limit the overexploitation of primary metal resources and reduce their ecological impact.In terms of batteries recycling, two main families of processes can be distinguished. The first, known as pyrometallurgy, is based on the high-temperature treatment of waste. In contrast, hydrometallurgical processes are processes with significantly lower energy costs, as these processes are carried out at near ambient temperatures. The principle is to dissolve metals in their ionic form. Toxic acids (and their mixtures) are very often used, but other methods are possible (bases, complexing agents, oxidants). Once in solution, the metals can be recovered in the form of salts, hydroxides or in their metallic form by conventional chemical processes (precipitation, crystallization, etc.) or electrochemical processes (electro-deposition). Several separation / purification steps are sometimes necessary. Relatively few studies have looked at closed-loop recycling, i.e. the resynthesis and reuse of recycled materials in batteries.Thus, the objective of this project is to test the feasibility of closed-loop recycling of cathode materials from Li-ion batteries based on the use of innovative and less toxic recycling routes, and to verify the electrochemical performance of recycled materials at the end of the recycling process.To build this green cycle, the present study has been carried out using both model cathode materials (LCO, NMC) and real cathodes from spent drone batteries, that have been discharged, opened and dismantled in a glove box. All materials have, of course, been characterised (X-ray diffraction, SEM) and the present metals content has been quantified (wet digestion followed by AAS analyses) in order to be able to establish reliable material balances at the end of the recycling process.The developed closed-loop recycling process was composed of three subsequent steps:1) The cathode materials recovered have been leached in a deep eutectic solvent medium (DES) composed of ethylene glycol – choline chloride mixture, replacing the concentrated acids traditionally used. DES is a new class of green solvents, that is, non-harmful to human health and the environment compared to conventional organic solvents. They consist of a mixture of products (solids) which, at a given ratio, become liquid (eutectic), and which have the combined properties of the different constituents of the mixture.2) Synthesis of recycling active material from the DES media by precipitation and calcination.3) Assembly of button cells, followed by electrochemical tests and batteries post-mortem analyses (X-ray diffraction, SEM) have been carried-out at the last step.Composite electrodes made of recycled active materials, polymer binder and carbon have been formulated and assembled into button cells with a Li counter-electrode and a conventional separator (e.g. Celgard impregnated with liquid electrolyte). The prepared batteries were then recycled by electrochemical methods coupled with impedance measurements. The aging of the cells was carried out through one hundred charge / discharge cycles and a specified «fast charging» test developed in our laboratory, allowing to calculate the lithium diffusion coefficient in the cathode material.The results have shown that the recycling of metals has been successfully carried out using a deep eutectic solvent (DES), in a closed-loop manner. The leaching protocol have been optimized at the optimum conditions: ChCl: EG (1:2) with 0.8 mol.L-1 of HCl, with a ratio(wt.) S/L of 1/50, heated to 87.5 °C, for 2 h. After being completely dissolved, the metals have been then recovered from the DES by precipitation, calcined and reused as active materials. Using this method, the DES solvent has also been recovered and reused in a closed-loop without suffering any damage. This work has also shown that the recovered battery materials can be reused into the battery manufacturing cycle without changing their electrochemical performance. Indeed, this process allowed to obtain a homogeneous powder with a morphology similar to the initial materials (particle size <1 μm), presenting good capacities (154 mAh.g-1). Figure 1

Cation Mixing and Capacity Loss in Li||Ni0.6Mn0.2Co0.2O2 Cells: Experimental Investigation and Application of the Multi-Site, Multi-Reaction Model

We clarify the degradation phenomena in a pouch cell that contains an insertion electrode (LizNi0.6Mn0.2Co0.2O2 or lithated NMC622) and a Li counter electrode. Greater than 500 cycles have been achieved in these cells employing 4 mAh/cm2 for both the initial Li metal negative and the NMC622 positive, and we find that cation mixing within the NMC622 is prevalent. That is, transition metals (Ni, and to a lesser extent, Mn and Co) in the transition-metal layer of NMC622 irreversibly exchange places with Li in the Li layer of NMC622, corresponding to a loss of Li sites and a concomitant loss of Coulombic capacity. The use of 1) a perturbation procedure of a recent publication employing the multi-site, multi-reaction model for the porous positive electrode and 2) a procedure to average the degradation phenomena over each cycle, which is shown to be consistent with slow degradation, simplifies the analysis of the experimental data and enables straightforward parameter regression. The resulting agreement between the model calculations and the experimental data is quite good, with the differences being similar to experiment-to-experiment variation.

Limitations of Ultrathin Al2O3 Coatings on LNMO Cathodes.

This study demonstrates the application of Al2O3 coatings for the high-voltage cathode material LiNi0.5–xMn1.5+xO4−δ (LNMO) by atomic layer deposition. The ultrathin and uniform coatings (0.6–1.7 nm) were deposited on LNMO particles and characterized by scanning transmission electron microscopy, inductively coupled plasma mass spectrometry, and X-ray photoelectron spectroscopy. Galvanostatic charge discharge cycling in half cells revealed, in contrast to many published studies, that even coatings of a thickness of 1 nm were detrimental to the cycling performance of LNMO. The complete coverage of the LNMO particles by the Al2O3 coating can form a Li-ion diffusion barrier, which leads to high overpotentials and reduced reversible capacity. Several reports on Al2O3-coated LNMO using alternative coating methods, which would lead to a less homogeneous coating, revealed the superior electrochemical properties of the Al2O3-coated LNMO, suggesting that complete coverage of the particles might in fact be a disadvantage. We show that transition metal ion dissolution during prolonged cycling at 50 °C is not hindered by the coating, resulting in Ni and Mn deposits on the Li counter electrode. The Al2O3-coated LNMO particles showed severe signs of pitting dissolution, which may be attributed to HF attack caused by side reactions between the electrolyte and the Al2O3 coating, which can lead to additional HF formation. The pitting dissolution was most severe for the thickest coating (1.7 nm). The uniform coating coverage may lead to non-uniform conduction paths for Li, where the active sites are more susceptible to HF attack. Few benefits of applications of very thin, uniform, and amorphous Al2O3 coatings could thus be verified, and the coating is not offering long-term protection from HF attack.

Electrochemical Ion Exchange in Nasicon Based Solid Electrolytes

Solid electrolytes form the crux of the all-solid-state batteries. Garnets, sulfides, and NASICONs are some of the most promising solid electrolyte material families. An ideal solid electrolyte should have higher ionic conductivity, higher mechanical strength, chemically and electrochemically stable with Li-anode and cathode, and environmentally benign. In the search for a reliable solid electrolyte, researchers are revisiting NASICON material families which have a wide compositional variety [1-2]. Properties of these materials can be easily tuned with substitution and replacement of cations and anions. In this work, we are exploring AM1M2(XO4)3 (where M1=Mn, M2=Zr, X=P) NASICONs for Li-ion based ASSBs. Rhombohedral polymorph of this composition provides the highest ionic conductivity. However, the synthesis of a Li-based NASICON in this composition is not a straightforward through conventional methods.In this work, we have investigated the capability of Na1+xMnx/2Zr2-x/2(PO4)3 (NMZP), which crystallizes in R-3C space group, being able to conduct Li-ions. NMZP was synthesized from x=0.5 to x=2. Neutron diffraction revealed that there are two main sites for Na-ions in this structure denoted by M1 and M2. Rietveld refinement analysis shows a monotonic increase of Na occupancy and Na-O bond length increase in the M2 site with an increase in x suggesting higher concentration of mobile Na in the M2 site (Figure 1a and b). Electrochemical impedance spectroscopy (EIS) analysis revealed that there is an increase in ionic conductivity from x=0.5 to x=1.5. The ionic conductivity of compositions with x=1.5 and x=2 is approximately the same which can be attributed to vacancy-charge-imbalance. Galvanostatic plating and stripping were carried out on symmetric Na | NMZP | Na, and Li | NMZP | Li cells at 70 ⁰C. A stable polarization profile was observed for both cells at various current densities (20, 40, and 80 µA cm-2 for Na | NMZP | Na and 40, 60, and 80 µA cm-2 for Li | NMZP | Li cell) (Figure 1c). Stable polarization observed for Li | NMZP | Li suggests that Na-ions in M2 sites can be readily exchanged with Li-ion from electrodes. Additionally, Na-ions should be pushed out of the structure and platted on the Li-counter electrode. Post-mortem analysis Li-foil from Li|NMZP|Li cell was carried out with SEM after 120 hours of cycling. Li-metal foil showed the presence of Na-metal plating on Li-electrode validating this phenomenon [3]. To further corroborate our findings that NMZP can conduct Li-ions readily, post-mortem XPS analysis of Li-foil and NMZP pellet from Li | NMZP | Li cell was carried out after 120 hours of cycling. Linear scanning map of Li-metal shows the presence of Na metal from 0.15 to 2.07 at% showcasing electroplated Na out of NMZP structure. Furthermore, XPS depth profiling of the cycled NMZP pellet shows the presence of Li within the structure (225 nm depth). These experiments prove the dual-ion conduction capability of NMZP type of NASICON materials. The proof-of-concept work showcased here opens exciting avenues of material combinations which can be leveraged for both Na and Li all-solid-state batteries. References Zhou et al., Small Methods 2020, 4, 2000764.Gao et al., J. Am. Chem. Soc. 2018, 140, 51, 18192–18199Parejiya et al., ACS Energy Lett. 2021, 6, 429−436 Figure 1

A Direct Comparison of Pilot-Scale Li-Ion Cells in the Formats PHEV1, Pouch, and 21700

Li-ion cells of the industrially-relevant formats PHEV1 (prismatic), multi-layer pouch, and 21700 (cylindrical) are directly compared by experiments for the first time. All three cell formats were reproducibly built on pilot-scale with the same anode (graphite), cathode (NMC622), separator, and electrolyte allowing a direct comparison. The main differences between these formats are their capacities (24.6 Ah, 2.2 Ah, 2.3 Ah), volume/surface ratios, as well as tab and the jellyroll/stack configurations (flat-wound, stacked, wound). The comparison involves voltage curves during formation (0.1 C), discharge rate capability (0.5 C−3 C), heating behaviour, cell impedances, geometrical properties such as electrode curvatures and tab configurations, as well as comparison with coin half cells with anode and cathode vs Li counter electrode. The data are put into context with commercial and pilot-line built cells from other studies.

Quantifying the chemical, electrochemical heterogeneity and spatial distribution of (poly) sulfide species using Operando SANS

In-situ and operando measurements are done to gain a better understanding of the precipitation mechanisms during charge and discharge in lithium sulfur batteries. A carbon felt networked with microfibers consisting of pores, 2 nm and smaller, is used as a freestanding sulfur host. Three different methods of sulfur infiltration are explored in order to determine the best one to fill most of the pores. It was identified to be a melt infiltration method in a vacuum oven, where the pores including the ultra-micropores, are successfully filled. In-situ electrochemical impedance spectroscopy measurements show a solid product formation occurring at the sulfur cathode, both during the high voltage plateau and at the end of discharge. In a 3-electrode EIS measurement, a similar solid product formation on the Li counter electrode due to its reaction with polysulfides is also observed. Operando small angle neutron scattering measurements show the solid product formation, in the carbon, both near the beginning and at the end of discharge, confirming the precipitation data via contrast changes as a function of charge and discharge. It is shown that Li2S precipitates in the pores at the beginning and the end of discharge, whereas S8 precipitates on the surface of the carbon felt. This lithium sulfur system shows both the quasi-solid-state and the solid-liquid-solid reactions in a typical ethereal electrolyte solution with a two-plateau discharge profile.

Observation of lithium stripping in super-concentrated electrolyte at potentials lower than regular Li stripping.

Lithium plating/stripping was investigated under constant current mode using a copper powder electrode in a super-concentrated electrolyte of lithium bis(fluorosulfonyl)amide (LiFSA) with methylphenylamino-di(trifluoroethyl) phosphate (PNMePh) and vinylene carbonate (VC) as additives. Typical Li plating/stripping for Cu electrodes in organic electrolytes of conventional lithium batteries proceeds at potentials of several millivolts versus a Li counter electrode. In contrast, a large overpotential of hundreds of millivolts was observed for Li plating/stripping with the super-concentrated electrolyte. When Li stripping started immediately after Li plating and with no rest time between plating and stripping, two potential plateaus, i.e., two-step Li stripping, was observed. The potential plateau for the 1st stripping step appeared at −0.2 V versus a Li metal counter electrode. The electrical capacity for the 1st stripping step was 0.04 mA h cm−2, which indicates irregular Li stripping. Two-step Li stripping was also recorded using cyclic voltammetry. The electrochemical impedance spectroscopy (EIS) studies indicated that the two-step Li stripping behaviour reflected two different solid electrolyte interphases (SEIs) on electrodeposited Li in a Cu electrode. The SEI for the 1st-step stripping was in a transition period of the SEI formation. The open circuit voltage (OCV) relaxation with an order of tens of hours was detected after Li plating and before Li stripping. The in operando EIS study suggested a decrease of the charge transfer resistance in the Cu powder electrode during the OCV relaxation. Since the capacitance for the voltage relaxation was a dozen microfarads, it had a slight contribution to the 1st-step Li stripping behaviour. The voltage relaxation indicated the possibility that it is difficult for Li ions to be electrodeposited or that the Li plating is in a quasi-stable state.

Determination of solid electrolyte interphase formation mechanism on negative electrode surface in Li-O2 battery electrolyte by operando electrochemical atomic force microscopy observation

Solid electrolyte interphase (SEI) on the negative electrode surface affect the electrochemical operation and cycle performance of the Li-O2 batteries. Two-type of SEI formation processes involving the O2 molecule on negative electrode surface have been suggested: (i) direct electrochemical reaction and (ii) indirect multi-step chemical reaction. In this research, the operando electrochemical atomic force microscopy observation was conducted to determine the proper model. Well-polished cupper substrate was used as model working electrode and was observed during the cyclic-voltammetry operation in the tetraglyme (G4) based electrolyte with and without dissolved O2. In the pristine electrolyte, in which O2 is not dissolved, no solid formation was observed. In the O2-dissolved electrolyte, the solid formation was suddenly observed from 1.3 V vs. Li counter electrode (LiCE) in the negative scan, which voltage is remarkably lower than the onset voltage of reduction current at 1.8 V vs. LiCE. This significant delay of solid formation against the electron transfer reaction strongly supported the multi-step solid formation model. The direct observation of electrode surface coupled with electrochemical operation determine the SEI formation mechanism in the Li-O2 battery.

Scalable solvent-free mechanofusion and magnesiothermic reduction processes for obtaining carbon nanospheres-encapsulated crystalline silicon anode for Li-ion batteries

It is unquestionable that Si nanostructures i.e., nanosheets, nanowires, nanoparticles become more and more importance in high-energy lithium ion batteries (>300 Wh kg−1). However, the current commercial Si nanostructures are rather expensive, which is not yet able to complete with the graphite anode in term of USD/Wh kg−1. Herein, the conventional food grade non-porous silicon dioxide (SiO2) which costs 270-times lower than Si micron-sized and 3000-times lower than Si nano-sized (10 nm) is selected as a precursor for synthesizing pure crystalline Si nanoparticles. A novel scalable solvent-free mechanofusion method was firstly introduced to synthesize carbon nanospheres-encapsulated SiO2 (SiO2@C) followed by the reduction process producing silicon-carbon nanoparticles core-shell materials (Si@C) with interparticle void space. The carbon shell can prevent the volume expansion (>400%) of inner Si particles, which is a critical drawback of Si anode. In addition, the pre-lithiated Si@C anode obtained by a direct contact with Li counter electrode can address the poor coulombic efficiency at the first cycle. The pre-lithiated Si@C anode can deliver a reversible discharge specific capacity of 1390 mAh g−1 with a remarkable capacity retention of 90.4% and coulombic efficiency of ∼100% after 1000 cycles at a high rate of 1C. The ex situ TEM and XPS investigated confirm that the inner Si is well-confined within carbon buffer shell without being directly exposed to the electrolyte. Besides, an in operando XRD shows the reversible phase transformation during cycling for which Li15Si4 alloy is the product indicating that Si@C prepared in this work may be an ideal practical anode of high-energy Li-ion batteries.

Porous Electrodes in Lithium-Ion Batteries: Heterogeneity and Performance

The presence of inhomogeneity in the porous electrodes of lithium-ion cells can significantly affect the active material utilization, energy/power density, and ageing dynamics of the cell, among others [1-3]. Inhomogeneity might be defined as a non-zero directional gradient of the physical and/or chemical formulation in a porous electrode. In Li-ion cells, such inhomogeneity is caused by an uneven spatial distribution of the active material, carbon, binder, and electrolyte, or porous electrode micro/nanoscale properties such as the contact resistances, porosity and tortuosity. The existing literature suggests that the heterogeneity could inadvertently be introduced to the electrode with improper combination of design parameters and manufacturing steps, e.g. slurry formulation, thickness, mixing and drying [4].Important insights into the local microstructure of the porous electrodes have been provided by FIB-SEM [5] , X-ray tomography [6] or full simulation of electrode microstructure [7]. Little attention has been paid, however, to develop formalisms to quantify the electrode heterogeneity and its correlation to the design parameters, manufacturing steps, and the battery performance.In this study, a group of 36 NMC porous electrodes with different thickness, porosity, and formulation (NMC loading and carbon/binder ratio) were systematically prepared and were characterized for the microstructure and electrochemical performance. We spotlight the sensitivity of the energy and power density of the electrodes, in front of a Li counter electrode, to the microstructural indexes including tortuosity, porosity, and effective conductivities. The experimental results are analysed with the help of a macroscopic battery model to introduce a series of semi-empirical correlations to quantify the state of heterogeneity in the porous electrodes at a given manufacturing recipe.

Is Li/Graphite Half-Cell Suitable for Evaluating Lithiation Rate Capability of Graphite Electrode?

Li plating on graphite anode is known to be one of the bottlenecks for fast charge of Li-ion batteries and a Li/graphite half-cell has been often used to determine lithiation rate capability of the graphite electrode. In this work, a three-electrode Li/graphite cell with a graphite loading of 3.72 mg cm−2 is used to simultaneously record the cell’s voltage and graphite’s potential during the Li/graphite cell is lithiated at different current rates. It is surprisingly found that the polarization of a Li/graphite cell is dominated by the Li counter electrode, and that the lithiation capacity of graphite is centered in a narrow potential range between 0.068 V and 0.200 V vs Li/Li+. Impedance analysis reveals that the former is because the Li counter electrode has much larger charge-transfer resistance compared with the graphite electrode. As such, over-potential of the Li counter electrode can readily drive the potential of graphite to negative, leading to Li plating, even at moderate lithiation rates. The results of this work indicate that the lithiation rate capability of graphite determined from a Li/graphite half-cell is largely undervalued, and that an alternative technique is needed for accurate determination of the lithiation rate capability of graphite electrode.