Li-ion Transport Research Articles

Impact of Morphological Dimensions in Carbon‐Based Interlayers on Lithium Metal Anode Stabilization

AbstractLithium metal batteries (LMBs) offer high energy density and promise as a future technology. Yet, their adoption is hindered by safety concerns and cycle life stability, arising from Li dendrite formation, solid electrolyte interphase instability, and volume changes during cycling. In response to these challenges, carbon‐based materials have been utilized as an artificial interface layer for modifying the surface of copper current collector in LMBs. Among the diverse carbon‐based materials, 0D carbon, with its high specific surface area, is advantageous for enhancing Li ion transport rates and ensuring uniform current distribution. 1D carbon structures foster a network that facilitates Li ion diffusion, while 2D carbon establishes a protective layer, mitigating side reactions. 3D carbon structures promote Li deposition within their internal cavities, effectively controlling volume fluctuations. With this understanding, this review delves into the latest advancements in carbon‐based materials for modifying copper current collectors. It offers a detailed exploration of how each dimension of carbon‐based materials contributes to regulating Li deposition. Furthermore, the ongoing challenges and potential avenues in the development of carbon‐modified copper current collectors for LMBs are spotlighted, aiming to provide insightful guidance for the design of anode‐free Li metal batteries.

Borate-Based Artificial Solid-Electrolyte Interphase Enabling Stable Lithium Metal Anodes.

Lithium (Li) metal is considered as the "holy grail" of anode materials for next-generation high energy batteries. However, notorious dendrite growth and interfacial instability could induce irreversible capacity loss and safety issues, limiting the practical application of Li metal anodes. Herein, we develop a novel approach to construct a borate-based artificial solid-electrolyte interphase (designated as B-SEI) through the reaction of metallic Li with triethylamine borane (TEAB). According to our cryogenic electron microscopy (Cryo-EM) characterization results, the artificial SEI adopts a glass-crystal bilayer structure, which facilitates uniform Li-ion transport and inhibits dendrite growth during Li plating. Benefiting from such an artificial SEI, the Li anode delivers an improved rate performance and prolonged cycle life. The symmetric Li/B-SEI||Li/B-SEI cell can maintain stable cycling for 700 h at a high current density of 3 mA cm-2. The full-cell pairing Li/B-SEI with LiFePO4 only exhibits minimal capacity decay after 500 cycles in a conventional carbonate-based electrolyte. This work demonstrates the feasibility of building a boride-based artificial SEI to stabilize the Li metal anode based on microscopic characterization results and comprehensive electrochemical data, which represents a promising avenue to develop practical Li metal batteries.

LiPF6 Induces Phosphorization of Garnet-Type Solid-State Electrolyte for Stable Lithium Metal Batteries.

Garnet solid electrolyte Li6.4 La3 Zr1.4 Ta0.6 O12 (LLZTO) is an excellent inorganic ceramic-type solid electrolyte; however, the presence of Li2 CO3 impurities on its surface hinders Li-ion transport and increases the interface impedance. In contrast to traditional methods of mechanical polishing, acid corrosion, and high-temperature reduction for removing Li2 CO3 , herein, a straightforward "waste-to-treasure" strategy is proposed to transform Li2 CO3 into Li3 PO4 and LiF in LiPF6 solution under 60°C. It is found that the formation of Li3 PO4 during LLZTO pretreatment facilitates rapid Li-ion transport and enhances ionic conductivity, and the LLZTO/PAN composite polymer electrolyte shows the highest Li-ion transference number of 0.63. Additionally, the dense LiF layer serves to safeguard the internal garnet solid electrolyte against solvent decomposition-induced chemical adsorption. Symmetric Li/Li cells assembled with treated LLZTO/PAN composite electrolyte exhibit a critical current density of 1.1mAcm-2 and a long lifespan of up to 700h at a current density of 0.2mAcm-2 . The Li/LiFePO4 solid-state cells demonstrate stable cycling performances for 141mAhg-1 at 0.5 C, with capacity retention of 93.6% after 190 cycles. This work presents a novel approach to converting waste into valuable resources, offering the advantages of simple processes, and minimal side reactions.

The oxygen-doped self-supporting functional carbon nanotube sponge realized the synergistic enhancement of lithium ion transport and lithium polysulfide reaction

Lithium-sulfur (Li-S) batteries are one of the most promising energy storage systems, in which the severe shuttle effect causes the loss of the sulfur active species and unacceptable capacity and cycling stability. Here, a self-supporting functional carbon nanotube sponge (FCNTS), electrostatically repelling polysulfide anions and allowing Li+ to pass through, is synthesized by etching channels and introducing enriched anions on CNTs based on the “Donnan effect”. The experimental results confirmed that FCNTS accelerated the Li-ion transport, that is, the Li+ transfer rates of the FCNTS materials were 8.226 (Li2S4 reduction) and 3.627 times (Li2S oxidation) that of the CNTs blank samples, respectively. Meanwhile, defects and O-group modified FCNTS regulated the local charge density of carbon-based materials, which enriched the reduction current response of the soluble Li2S4 and the oxidation current of Li2S2/Li2S around the cathode more than 1.9 and 1.6 times that of the CNTs blank samples, respectively. This significantly improved the kinetics and suppressed the overpotential. Thus, the cells assembled by S@FCNTS achieved a high-rate performance (866.9 mAh∙g−1 at 2 C), excellent stability (fading rate of only 0.035% per cycle after 500 cycles at 2 C), and a high area capacity of 13.2 mAh∙cm−2 at a high sulfur load of 10.9 mA∙cm−2. This work demonstrates a dynamic polysulfide-interception strategy to facilitate the utilization of polysulfides and provides a promising idea for the design of multi-functional sulfur storage electrode materials for high-load LSBs.

In Situ construction of PVDF/LLTO electrolyte with stable electrolyte/electrode interphase for integrated solid-state lithium batteries

Solid-state lithium (Li) metal batteries (LMBs) are promising owing to enhanced safety and high energy density. However, the issues of Li dendrite growth and large electrolyte-electrode interfacial resistance still hinder their practical applications in LMBs. Here, a Li anode-electrolyte structure integration has been developed using in-situ chemical reactions between the polyvinylidene fluoride/Li3xLa2/3−xTiO3/N-methyl pyrrolidone (PVDF/LLTO/NMP) solution and the Li metal interphase. After gradually coating the PVDF/LLTO/NMP solution onto Li metal, NMP as catalyst, inducing PVDF reacted with Li spontaneously to form lithium fluorine (LiF), and the accompanying Ti4+ ions (in LLTO) are reduced to Ti3+ ions to form a LLTO ionic/electronic conductor (LixLLTO). After NMP evaporation, an ionic/electronic conductor protective layer of LiF/LixLLTO is created between the PVDF/LLTO electrolytes and Li metal. When applying the integrated solid-state Li batteries (ISSLBs), the protective layer can reduce interfacial resistance and the formation of Li dendrites by regulating the even Li deposition redistributing the Li-ion transport. As a result, the as-assembled LiNi0.8Co0.1Mn0.1O2/ISSLBs achieves an initial capacity of 181 mA h g−1 and retains 83% capacity after 200 cycles at 0.5 C. This design can be an effective strategy to tailor the properties of Li-metal anodes for achieving dendrite-free LMBs with high performance and high safety.

In-situ interfacial passivation and self-adaptability synergistically stabilizing all-solid-state lithium metal batteries

The function of solid electrolytes and the composition of solid electrolyte interphase (SEI) are highly significant for inhibiting the growth of Li dendrites. Herein, we report an in-situ interfacial passivation combined with self-adaptability strategy to reinforce Li0.33La0.557TiO3 (LLTO)-based solid-state batteries. Specifically, a functional SEI enriched with LiF/Li3PO4 is formed by in-situ electrochemical conversion, which is greatly beneficial to improving interface compatibility and enhancing ion transport. While the polarized dielectric BaTiO3-polyamic acid (BTO-PAA, BP) film greatly improves the Li-ion transport kinetics and homogenizes the Li deposition. As expected, the resulting electrolyte offers considerable ionic conductivity at room temperature (4.3 × 10−4 S cm−1) and appreciable electrochemical decomposition voltage (5.23 V) after electrochemical passivation. For Li-LiFePO4 batteries, it shows a high specific capacity of 153 mA h g−1 at 0.2 C after 100 cycles and a long-term durability of 115 mA h g−1 at 1.0 C after 800 cycles. Additionally, a stable Li plating/stripping can be achieved for more than 900 h at 0.5 mA cm−2. The stabilization mechanisms are elucidated by ex-situ XRD, ex-situ XPS, and ex-situ FTIR techniques, and the corresponding results reveal that the interfacial passivation combined with polarization effect is an effective strategy for improving the electrochemical performance. The present study provides a deeper insight into the dynamic adjustment of electrode-electrolyte interfacial for solid-state lithium batteries.

High-Areal Capacity, High-Rate Lithium Metal Anodes Enabled by Nitrogen-Doped Graphene Mesh.

Hosts hold great prospects for addressing the dendrite growth and volume expansion of the Li metal anode, but Li dendrites are still observable under the conditions of high deposition capacity and/or high current density. Herein, a nitrogen-doped graphene mesh (NGM) is developed, which possesses a conductive and lithiophilic scaffold for efficient Li deposition. The abundant nanopores in NGM can not only provide sufficient room for Li deposition, but also speed up Li ion transport to achieve a high-rate capability. Moreover, the evenly distributed N dopants on the NGM can guide the uniform nucleation of Li so that to inhibit dendrite growth. As a result, the composite NGM@Li anode shows satisfactory electrochemical performances for Li-S batteries, including a high capacity of 600 mAh g-1 after 300 cycles at 1 C and a rate capacity of 438 mAh g-1 at 3 C. This work provides a new avenue for the fabrication of graphene-based hosts with large areal capacity and high-rate capability for Li metal batteries.

Grain boundaries boost the prelithiation capability of the Li2CO3 cathode additives for high-energy-density lithium-ion batteries

Due to high theoretical capacity, Si anodes have attracted tremendous attention. Nevertheless, huge volume change during cycling together with large lithium loss substantially hinders their large-scale application. Prelithiation has emerged as a highly attractive strategy to compensate for the loss of active lithium. Li2CO3, which is ambient stable and delivers more than 5 times the theoretical capacity of the existing cathode materials, serves as an excellent cathode prelithiation additive. However, the practical application of Li2CO3 is limited by its sluggish charge transport and high critical decomposition potential. Herein, we report a facile route to fabircating nanosized Li2CO3 (N-Li2CO3) materials enriched with grain boundaries (GBs). First-principles investigations reveal that the Li-ion transport behavior is dependent on the concentration of GBs. The Σ(001) GB model exhibits higher Li-ion and electronic conductivities compared to those of the bulk counterpart. As a result, the ∼ 100 % capacity utilization of N-Li2CO3 with a delithiation capacity of 724 mAh/g is obtained, which is much higher than that of commercial Li2CO3 (115 mAh/g), greatly enhancing the reversible capacity, energy density and lifetime of the LiNi0.5Co0.2Mn0.3O2//Si-graphite full cell. These findings provide valuable understanding of the role of GBs and the correlation between microstructure engineering and performance optimization.

Enabling long-term oxide based solid-state lithium metal battery through a near room-temperature sintering process

The huge Li ion transport resistance through the grain boundaries (GBs) among rigid oxide particles forces the adoption of high-temperature sintering (HTS) process over 1000 °C. Nevertheless, the severe side reactions and uncontrollable lithium loss are always companied during the high-cost HTS process, which slows down the pace of oxide solid electrolyte (OSE) for practical application and accelerates the exploration of a new OSE sintering process. Herein, a near-room-temperature (60 °C) cold-sintering process is proposed by filling the GBs with a low-melting-point plastic crystal electrolyte (PCE). Due to the soft property and high-ionic conductivity of PCE, the Li ion transport rate through the GBs is 10 times faster than the bulk phase, endowing the OSE (Li1.5Al0.5Ge1.5(PO4)3 chosen as a representative) with a room temperature ionic conductivity of 0.25 mS cm−1. As proof of the concept, the assembled Li symmetrical cells perform a low over-potential of 50 mV with a capacity of 1 mA h cm−2 and full cells delivers a capacity retention of roughly 70% after 820 cycles (1.5 years) at 0.1C.

Superlattice and defect engineering enabled NC@VS2–x as trifunctional promoter for polysulfides catalytic conversion

The shuttling effect and sluggish reaction kinetics of the intermediate lithium polysulfides (LiPSs) are the main barriers limiting the application of Li–S batteries. Herein, superlattice and sulfur-vacancy engineering are concurrently constructed in the N-doped carbon embedded VS2 (NC@VS2–x) for LiPSs catalytic conversion. The superlattice and defect engineering with NC imbedded produce high activity trapping sites, enhance the electronic conductivity, and provide fast Li diffusion channels, achieving the massive precipitation of Li2S, promoting the dissolution of solid sulfur species, and accelerating the Li ions transport. Therefore, the trifunctional LiPSs promoter in Li–S cell yields high rate performance (4.0 C) and a low capacity decay rate of 0.036% at 2.0 C. Impressively, the NC@VS2–x cell delivers a capacity of 5.15 mA h cm−2 with S loading of 6.2 mg cm−2 and lean E/S ratio of 4.0 µL mg−1. When assembled in the pouch cell, the NC@VS2–x electrode can work well over 45 cycles, indicating a potential application in practical Li–S batteries. The proposed dual-engineering strategy to form the trifunctional promoter is anticipated to be applied in other multielectron energy related systems.

Thick Architected Silicon Composite Battery Electrodes Using Honeycomb Patterned Carbon Nanotube Forests

To meet the growing performance demands for personal electronics and electric vehicles the energy density of lithium-ion batteries can be increased by incorporating thicker electrodes. We present thick “honeycomb” electrodes based on patterned, vertically aligned carbon nanotubes (CNTs) on Cu foils. Thick electrodes are created by Si deposition on >100 μm tall honeycomb patterned CNTs. Si-CNT electrodes are cycled in half-cells, demonstrating electronic connection between the Si and Cu foil via the aligned CNTs. For ~4.7 mAh cm−2 capacity the honeycomb patterning improves capacity retention (78%) over 30 cycles compared to non-patterned electrodes (58%). We attribute this improvement to the honeycomb’s ability to accommodate Si expansion, thereby reducing cracking that causes active material loss and solid electrolyte interphase instability, and to provide pathways for Li-ion transport into the electrode. The Si-CNT electrode capacity is further increased to 20 mAh cm−2 by increasing the Si loading. Finally, a fluoroethylene carbonate containing electrolyte is used to increase cell lifetime. Here, the honeycomb electrodes have a higher areal (~10.2 mAh cm−2) and retained (65%) capacity over 180 cycles, and exhibit superior rate performance to their non-patterned counterparts. Our work demonstrates the role of patterning in enabling aligned CNTs as a robust template for thick battery electrodes.

Multiscale Modeling of Heterogeneous Interfaces in All Solid-State Batteries

Ion transport and degradation at the solid-electorlyte/cathode interfaces dictate the performance of all solid-state batteries. For example, it has been routinely observed that in many superionic oxide conductors, Li-ion transport kinetics are orders of magnitude slower at the interfaces, such as grain boundaries or interfaces with common electrode materials, compared to the bulk phases. Upon electrochemical cycling, interdiffusion and/or electrolyte/cathode decomposition at these interfaces can further deteriorate Li-ion transport kinetics. Therefore, the ability to understand, predict and precisely control ion transport kinetics and the degree of interfacial degradation are critical and would have a transformative impact on the development of solid-state battery technologies for practical applications. Here I will address the need of a multiscale and multiphysics modeling framework to understand Li-ion transport mechanisms at complex interfaces and predict the kinetics of Li-ion transport in solid-electrolyte/cathode composite architectures that are relevant for all solid-state Li batteries. I will also emphasize the importance of using a closely coupled theory-experiment approach to reveal the atomic details of the interfaces and unravel the degradation mechanisms of interfaces and its impact on Li-ion transport. I will use the garnet-type solid-state electrotype and lithium cobalt oxide cathode as an example to demonstrate the predictive power of such a multiscale, multiphysics modeling framework towards establishing the correlations between ion transport and local atomic structure, chemical environment, and mesoscopic microstructure features in the cathode/electrolyte composites.This work was sponsored by the Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Office and was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. An award of computer time was provided by the Innovative and Novel Computational Impact on Theory and Experiment (INCITE) program. This research used resources of the Argonne Leadership Computing Facility, which is a DOE Office of Science User Facility supported under Contract DE-AC02-06CH11357.

Modeling the Anisotropic Transport Behavior for Axisymmetric 2D Electrode Particles Using a Modified Pseudo-2D Model Framework

This presentation extends widely used pseudo-2D (P2D) Li-ion battery models to include electrode particles that have strong crystal-scale anisotropies and possibly non-spherical shapes. Typical P2D models predict Li transport with representative spherical electrode particles by solving a one-dimensional diffusion equation in spherical coordinates. However, battery electrodes (graphite, NMC, etc.) may have strong crystalline anisotropies, leading to transport and mechanical properties that vary by orders of magnitude depending on crystallographic orientations. The anisotropic transport can cause large concentration gradients, possibly leading to particle fracture associated with concentration-induced stress. The remainder of the P2D model, such as Li-ion transport within the electrolyte solvent, remains essentially unchanged. The charge-transfer boundary conditions at the particle surfaces do need to accommodate the crystallographic anisotropy.The present model discretizes representative electrode particles in a two-dimensional finite-volume axisymmetric geometry, accounting for the anisotropic behaviors. The computational burden is greater than it is for the one-dimensional spherical solution. However, the P2D model with two-dimensional axisymmetric particle discretization still runs in only a few minutes on a typical personal computer.The model shows how electrode-particle anisotropy affects the battery performance, such as charge-discharge characteristics and electrochemical impedance spectroscopy. The model also predicts the concentration gradients and correlated stress for various axisymmetric, non-spherical particles.

Enabling Stable Li Metal Anodes for Solid-State Batteries

Solid-state batteries have the potential to transform energy storage by providing significantly higher energy densities and improved safety compared to conventional Li-ion batteries. However, robust high-capacity Li metal anodes are required to realize this technology. Current Li metal anode designs suffer from significant degradation over time due to non-uniform Li plating/stripping and loss of contact with the solid electrolyte during cycling. Carbon scaffold hosts have been shown to mitigate this problem in liquid electrolyte systems by creating a uniform electric field and providing abundant Li nucleation sites, but they have not been well explored in solid-state systems, which require the fabrication of more complicated mixed ion/electron conducting scaffolds to provide transport of both Li ions and electrons throughout the anode. We investigated the effect of scaffold architecture and chemistry on Li morphology and electrochemical performance using a combined experiment/theory approach. Scaffolds were characterized by SEM, Raman, and XPS before cycling in cells to evaluate their performance as hosts for Li plating/stripping. Atomistic and mesoscale modeling were used to help predict Li nucleation and current density hotspots and assist in guiding scaffold design. This work provides further insight into the relationship between anode design and Li metal cycling stability and will facilitate the development of safer, high-energy-density solid-state batteries.Lawrence Livermore National Laboratory is operated by Lawrence Livermore National Security, LLC, for the U.S. Department of Energy, National Nuclear Security Administration under Contract DE-AC52-07NA27344.

(Invited) Multi-Site, Multi-Reaction Model for the Simulating Intercalation and Alloy Electrodes

We overview enabling research for the characterization and design of cells using intercalation and alloy electrodes. Specifically, we have recently developed and implemented a multi-site, multi-reaction (MSMR) model. We shall demonstrate the approach with various chemistries of interest today, and the creation of reduced order models to simply calculations.We first discuss some of the similarities and differences between the MSMR model and the original Doyle-Fuller-Newman model [1, 2], which we shall refer to as the DFN model, first published in 1993. This model represents an intercalation electrode using porous electrode theory, but it also includes a detailed description of transport of Li ions in the electrolyte, transport of lithium in the solid phase, and the kinetics of the intercalation reaction as a Butler-Volmer equation. The MSMR model is based on the same porous electrode theory used in the original DFN model, and it uses the same treatment of transport in the electrolyte. However, the MSMR model introduces significant changes in the representation of both transport in the solid phase and the kinetics of the intercalation reaction. The main reason for the changes in the MSMR model is because it hypothesizes that intercalated lithium consists of not just one species, but multiple species. There is significant evidence to support this point of view, both from cyclic voltammetry experiments as well as X-ray diffraction analyses [4].The DFN model, as originally described and often used today, represents solid phase diffusion as a linear equation with a constant diffusion coefficient. This representation is known to be highly inaccurate, for example, in the case of graphite, where measurements have shown that the diffusion coefficient can vary by several orders of magnitude depending on the mole fraction of intercalated lithium [3,4]. More sophisticated treatments of solid phase diffusion use the chemical potential as the driving force for diffusion and because the chemical potential can be written in terms of the OCV, this explains how the Open Circuit Voltage now plays a role in the diffusion equation. Since the MSMR model uses multiple species, the solid phase diffusion problem should also involve multiple species, but the entire system of multi-species diffusion equations simplifies to a single equation, if one assumes that the various species of lithium are locally equilibrated with each other within the active material particles. The details of this analysis are given in [6].The DFN model uses a Butler-Volmer equation to represent the kinetics of the intercalation reaction, whereas the MSMR model uses a Butler-Volmer equation for each species; the intercalation reaction rate thus becomes the sum of these multiple Butler-Volmer equations, which are formulated using activities instead of concentrations, as is done in the original DFN model.Finally, the MSMR model proposes a simple empirical relationship, based on a generalized Nernst equation, to describe the relationship between the mole fraction of each intercalated species and the OCV. This formulation provides a systematic way to derive analytical formulas to represent the OCV relationship x(U), where x is the total mole fraction of intercalated species and U is the corresponding OCV. Several examples [5,6] show that this formalism suffices to provide accurate fits of x(U) to experimental data for many different electrode materials in common use, and it also provides analytical expressions for the derivative dx/dU, which shows up in the expressions for the solid phase diffusion equation.In summary, the MSMR model attempts to give a more accurate representation of solid phase diffusion and a more complete representation of intercalation interfacial kinetics, and, in this context, our hope is that the MSMR model provides a useful complement to the original DFN model. M. Doyle, T. F. Fuller, and J. Newman, J. Electrochem. Soc., 140(1993)1526.T. F. Fuller, M. Doyle, and J. Newman, J. Electrochem. Soc., 141(1994)1.D. Levi, E. A. Levi, and D. Aurbach, Journal of Electroanalytical Chemistry, 421, 89 (1997).D. R. Baker and M.W. Verbrugge, J. Electrochem. Soc., 159 (8) A1341-A1350 (2012).M. W. Verbrugge, D. R. Baker, and X. Xiao, J. Electrochem. Soc., 163(2016)A262-A271.M. W. Verbrugge, D. R. Baker, B. J. Koch, X. Xiao, and W. Gu, J. Electrochem. Soc., 164(2017)E3243-E3253.D. R. Baker, M. W. Verbrugge and W. Gu, J. Electrochem. Soc., 166 (4) A521-A531 (2019).

(Invited) Lithium Tetrafluoroborate Based Ester Electrolyte System for Wide Operating Temperatures Ingraphite/ Lini 0.6 Co 0.2 Mn 0.2 O 2 Cells

As Lithium ion batteries (LIB) are utilized in wider scale of applications, operation at a wide range of operating temperatures without a penalty to the energy density is desired. Current LIB only produces a fraction of the room temperature capacity when operated at subzero temperatures. Optimization of the electrolyte composition has long been considered a way of improving low temperature battery performance, as the electrolyte dictates the ionic mobility and the stability of the solid electrolyte interface (SEI), which in turn determine the Li ion intercalation kinetics and cycle life of the cell. Different operating temperatures has different demands from the electrolyte. For room temperature performance, formation of a stable solid electrolyte interface is thought to dictate the capacity and the cycle life of a cell. For low temperature performance, Li ion transport properties and charge-transfer polarization may have a dominant effect on cycling performance. Therefore, in this work, a novel electrolyte system was developed that included lithium tetrafluoroborate and methyl acetate for good low temperature performance and different additives were incorporated to this system to improve the room temperature performance by formation of a stable SEI.

Stable Hexaazatrinaphthalene-Based Planar Polymer Cathode Material for Organic Lithium-Ion Batteries.

Organic materials have garnered intensive focus as a new group of electrodes for lithium-ion batteries (LIBs). However, many reported organic electrodes so far still exhibit unsatisfying cycling stability because of the dissolution in the electrolytes. Herein, a novel azo-linked hexaazatrianphthalene (HATN)-based polymer (AZO-HATN-AQ) is designed and fabricated by the polymerization of trinitrodiquinoxalino[2,3-a:2',3'-c]phenazine (HATNTN) and 2,6-diaminoanthraquinone (DAAQ). The abundant redox-active sites, extended π-conjugated planar conformation, and low energy gap endow the AZO-HATN-AQ electrode with high theoretical capacity, excellent solubility resistance, and fast Li-ion transport. In particular, the fully lithiated AZO-HATN-AQ still keeps the planar structure, contributing to the excellent cycling stability. As a result, AZO-HATN-AQ cathodes show high specific capacity (240 mAh g-1 at 0.05 A g-1), prominent rate capability (98 mAh g-1 at 8 A g-1), and outstanding cycling stability (120 mAh g-1 after 2000 cycles at 4 A g-1 with 85.7% capacity retention) simultaneously. This study demonstrates that rational structure design of the polymer electrodes is an effective approach to achieving excellent comprehensive electrochemical performance.

Effects of Cathode Loadings on the Performance of the LiNi0.8Mn0.1Co0.1O2 Cathode in Lithium Batteries

LiNixMnyCozO2 (NMCs, x + y + z = 1) represent one promising cathode material for next-generation lithium-ion batteries (LIBs) and beyond, due to their high specific capacity and low cost.1-3 The current commercialized batteries tend to utilize high loadings of cathode materials and thin Li anodes for producing higher energy densities. However, a systematic study on the effects of cathode loadings is missing. To this end, we recently conducted a comparative study on the effects of different cathode loadings on the performance of LiNi0.8Mn0.1Co0.1O2 (NMC811). The NMC811 loading ranges from 2 – 14 mg/cm2. The resultant Li||NMC811 cells were tested for their capacity retention and sustainable capacity at different C-rates. Our study revealed that the cathode loadings have remarkably affected the cell performance. With increasing NMC811 loadings, the cells are more liable to fade in their capacity, due to a faster growth of solid electrolyte interface (SEI) on NMC811 particles and thereby a more sluggish Li-ion transportation. We also noticed that the Li anode side had remarkable effects on the Li||NMC811 cells. Particularly, we demonstrated that a polymeric surface coating via molecular layer deposition could protect the Li anode from degradation and thereby sustain Li||NMC811 cells for a better performance. Therefore, we concluded that the performance of the Li||NMC811 cells suffers from the issues of both the NMC811cathode and Li anode. It is significant to address their issues simultaneously for high performance.

Solid Electrolytes As Li-Ion Conducting Additive and Interfacial Stabilizer for High-Voltage Cathodes

Li-ion batteries (LIBs) have been a great facilitator to enable portable devices and electric vehicles (EVs) because of their high energy and power densities compared with other energy-storage devices. The high energy of LIBs is attributed to the high operating voltages of oxide-based cathodes (e.g., ~ 4 Vvs.Li) and wide electrochemical stability windows of organic liquid electrolytes (e.g., 1 – 4 Vvs.Li). In pursuit of extending energy density and lowering cost of LIBs further, R&D efforts have been devoted to finding the chemistries that can increase the cell operating voltages.However, increasing the voltages beyond the highest occupied molecular orbital (HOMO) of the electrolytes (c.a., ~ 4 Vvs.Li) leads to unwanted parasitic reactions at cathode-electrolyte interphase (CEI). In addition to the electrolyte oxidations at CEI, the reaction byproducts such as transition metal ions, HF, and CO2 can migrate to anodes and degrade their solid-electrolyte interphase (SEI) layers. As a result, high-voltage LIBs suffer from rapid capacity fading, growth of cell impedance, and gas generation.To mitigate the issue, various approaches have been proposed such as artificial CEI layer on cathode, tuning chemical compositions of cathodes, coating graphite anodes, using electrolyte additives, and adopting functional binders for cathodes. However, there is no simple solution to resolve all the complex issues occurring in the high-voltage cells. Considering that such parasitic reactions originate from the CEI and propagate to the SEI of anodes, there is a dire need of multimodal strategies that can create synergistic effect on stabilizing the electrode-electrolyte interphases in cell-level.Our research team has been developing multi-strategy that can coherently improve the high-voltage stability of LIB cells. Among them, we will present our recent approach of using solid-electrolyte (SE) powders as Li-ion conducting additive and interfacial stabilizer for high-voltage cathodes. As the proof-of-concept, Li6.7La3Zr1.7Ta0.3O12 (LLZT) SE was blended with Ni-rich Li(Ni1-xMnx/2Cox/2)O2 (NMC) or LiNi0.5Mn1.5O4 (LNMO) cathodes by relying on conventional electrode fabrication processes in LIB cells. This simple process offers advantages over the conventional coating methods in terms of manufacturing friendliness, energy saving, and cost effectiveness.Among various SE materials, garnet-type LLZT shows great promises based on its wide electrochemical stability window, good mechanical properties, and reasonable Li-ion conductivity (10-3 – 10-4 S/cm). First, the LLZT blended cathode significantly improved specific capacity and its retention during cycling at high voltages: e.g., 4.5 Vvs.Li for Ni-rich NMC and 5.0 Vvs.Li for LNMO spinel. Compared with the SE-free cathodes (baseline samples), the SE blended cathodes consistently delivered improved specific capacities, capacity retentions, and Coulombic efficiencies in full-cells made with graphite anodes.The improvement mechanism of the LLZT blended cathodes can be explained by two folds. First, LLZT contacts with cathodes (e.g., NMC or LNMO) and improves Li-ion transport properties of CEI layer, as evidenced by electrochemical impedance spectroscopy (EIS) and distribution of relaxation time (DRT) analyses. Second, LLZT can scavenge moisture/proton in the liquid electrolytes and subsequently suppress the degradation of CEI and SEI layers during extended cycles, as evidenced by X-ray photoelectron spectroscopy (XPS). As a result, 5 wt% LLZO blended cathodes delivered a stable electrochemical performance even at the presence of 5000 ppm moisture (and thus HF) in an electrolyte. In contrast to the traditional surface coating methods, solid-electrolyte blending approach is cost-effective and manufacturing/environmentally friendly, and thereby can serve as a practical pathway for improving performances and stability of current battery cells for EV and small electronics.

Ion Transport in Polymer/Inorganic Composite Electrolytes – a Comparison between Broadband Dielectric Spectroscopy and Impedance Spectroscopy

Significant efforts have been made to develop composite electrolytes combining polymer matrix with Li-ion conducting inorganic solids for feasible construction of solid-state batteries. However, the Li-ion transfer kinetics at the interface between polymer/inorganic electrolyte is unfavorable for Li-ion conduction in the composite electrolyte because of the interfacial resistance and the activation energy barrier for interfacial Li-ion transfer. The activation energy barrier for the Li-ion transfer reaction at the interface is correlated with the interaction between Li-ion and polymer matrix in the polymer electrolyte, interaction between Li-ion and anions in inorganic electrolytes and the interaction between polymer and inorganic electrolytes, therefore, the electrolyte composition significantly affects the interfacial charge transfer kinetics in composites.In this work, Li-ion transport properties in the composite electrolytes are investigated by using broadband dielectric spectroscopy (BDS) and impedance spectroscopy (IS). We particularly focus on Li-ion charge transfer at the polymer/inorganic interface from the dual ion conducting polymer to the single ion conducting polymer electrolytes with different Li-ion conducting salts such as LiN(SO2CF3)2 (LiTFSI) and (LiMTFSI) in vinyl ethylene carbonate (VEC) or polymerized vinyl ethylene carbonate (PVEC). The interfacial resistances of inorganic electrolytes Li0.34La0.56TiO3 (LLTO) in polymer electrolytes are elucidated. To further evaluate the polymer/inorganic electrolyte interfaces, various types of cell are assembled in a glove box filled with argon gas and characterized by a series of techniques such as BDS and IS. The Li-ion transfer kinetics at the interface significantly affects the Li-ion flux through the composite electrolytes and will be presented. In addition, we will present a systematic comparison between BDS and IS results in order to build connections between the two communities. Acknowledgements This work was supported as part of the Fast and Cooperative Ion Transport in Polymer-Based Materials (FaCT), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences at Oak Ridge National Laboratory under contract DE-AC05-00OR22725.