Energy Density Li-ion Batteries Research Articles

Wet-Chemical Synthesis of a Protective Coating on NCM111 Cathode: The Quantified Effects of Washing, Sintering and Coating

LiNixCoyMn1-x-yO2 (NCM) cathodes, especially with high Ni content, are widely expected to keep advancing the energy density of Li-ion batteries. However, ensuring a good cycle life remains a key challenge. Applying inert protective coatings on the surface of NCMs is a common route for mitigating surface-based degradation. In this study a sustainable ethanol-based wet-chemical coating method for covering the material with Al2O3 is developed and demonstrated on NCM111. The effect of the synthesis procedure is carefully evaluated to distinguish the benefits of the protective coating from the contributions of re-sintering and removal of surface contaminants, all taking place during the synthesis of the coated material. We show that while the cycling stability is significantly improved by the material regeneration alone (65% vs 79% state-of-health after 500 charge-discharge cycles at voltage range 2.7–4.3 V vs Li/Li+), the Al2O3-coated material displays further cycle life gains, maintaining 88% of initial capacity after 500 charge-discharge cycles. This work thus demonstrates both a sustainable wet-chemical coating method and the importance of establishing a proper baseline for characterization of inert protective coatings in general. The importance of both gains further prominence with the transition to inherently less stable higher Ni content NCMs.

Rational design of the temperature-responsive nonflammable electrolyte for safe lithium-ion batteries

The development of nonflammable electrolytes is critical for breaking the trade-off between the safety and energy density in Li-ion batteries (LIBs). Here, a rational design strategy of temperature-responsive nonflammable electrolytes (TRNEs) is proposed which are capable to prevent the heat accumulation and extinguish the fire efficiently during thermal runaway. Compared to the conventional phosphate- or halogen-based flame retardants, the TRNE based on low-cost and multifunctional methylurea (MU) was demonstrated with the lowest volatility (11.6 % weight loss) below 250 °C, and the highest efficacy to extinguish the fire at >210.4 °C through heat absorption, inert gases generation and char layer formation. In addition, the developed MU-based TRNEs enable higher stability and rate capability of LIBs compared to various nonflammable electrolytes. A Li||LiFePO4 (LFP) cell employing MU-based TRNE achieved higher stability (94.9 % capacity retention for 1500 cycles) than commercial electrolyte. A Ni-rich Li||LiNi0.8Mn0.1Co0.1O2 (NMC811) system was demonstrated with superior rate capability and high stability for 700 cycles (2.72 months) with the capacity retention of 89.9 %. Combining low cost and volatility, as well as high stability, rate capability and fire extinguishing efficacy, we demonstrate a promising design strategy to improve the battery safety for high-energy-density LIBs.

Vertical-channel hierarchically porous 3D printed electrodes with ultrahigh mass loading and areal energy density for Li-ion batteries

Lithium-ion batteries (LIBs) are renowned for their high energy density, long lifespan, and minimal self-discharge, making them a prominent power supply system. However, the persistent demand for improved energy density coupled with mechanical stability remains a significant challenge, as efforts to enhance electrode thickness or achieve geometric complexity through conventional ink-casting methods have yielded limited results. To tackle this challenge, we applied a three-dimensional (3D) printing technique to swiftly construct 3D thick electrodes with ultrahigh mass loading. Combined with unidirectional ice-templating process, high mass loading electrodes with hierarchically porous structures were successfully fabricated. The mass loading of the 3D electrodes can reach up to 73.83 mg cm−2, significantly enhancing the areal capacity. The low tortuosity and good mechanical resilience of these structures enable fast ion and charge transport, leading to significant improvements in areal energy density without sacrificing the power density. The assembled full cell exhibits an impressive areal capacity of 10.34 mAh cm⁻2 at 0.2 C and an excellent energy density of 18.41 mWh cm−2, surpassing other 3D printed lithium-ion batteries. This approach marks a significant advancement in electrode structural design towards higher areal capacity and energy density, showcasing the intriguing potential of 3D printed batteries for practical applications. Additionally, it also highlights the scalability and design versatility provided by 3D printing technique.

Accelerated Exploration of Li-Rich Cathode Materials with High Energy Density for Li-Ion Batteries

Li-rich cathode materials have been widely researched as promising cathode materials for Li-ion batteries due to the abundant numbers of Li+ in their formula units. There are, however, still some problems, such as the fading discharge voltage during cycling, to be overcome before its commercial application can be realized. In recent years, there has been a global expansion of efforts to innovate processes in material development by utilizing data science and materials informatics (hereafter referred to as MI), moving away from the traditional trial-and-error approach based on experiments. These initiatives have been surpassing the limitations of human-designed patterns and the number of feasible experiments, and examples of such endeavors are starting to be reported even in the field of battery materials[1]. In this study, we challenged a MI technology combined with first-principles calculation and database technology to widely search for suitable compositions of Li-rich cathode materials at high speed.Li1.4(Mn0.667Ni0.333)1-x Mx O2.35F0.05 (hereafter referred to as LMNOF, M: a dopant element, 0 ≤ x ≤ 0.01) was fabricated by a conventional method. The cathode was fabricated by mixing powder of the active material, acetylene black as a conducting powder and PTFE as a binder at the ratio of 7:2:1 by weight. The electrochemical performance was investigated using 2032 coin type half cells. The models for predicting battery performance were constructed by machine learning techniques. Composition formula information, synthesis conditions, and cell weight were used as candidate explanatory variables. XenonPy[2] was used for elemental-derived information. Principal component analysis was performed on the candidate explanatory variables, and only the variables determined to be important were used to construct the predictive model. The number of candidate M was narrowed down from approximately 100 to 30 based on the values of the enthalpy change between LMNOF with and without M. The enthalpy was calculated to determine the dopant elements for LMNOF by first-principles calculation. Specific M elements will be reported at the time of the presentation. The oxygen desorption energies of LMNOF with the 30 extracted elements were calculated to evaluate the stability of the LMNOF structure against the reaction in the fully charged state. Ten elements with negative values of the energy were selected as the dopant ones (Ms) for experimental demonstration of this study and the rest were used to predict the performance by using the database of the Materials Project[3]. The battery performance of M-doped LMNOF were predicted by the constructed model based on machine learning techniques with experimental data. This model indicated that the performances of LMNOF with a certain element M1 or M2 tended to be higher than ones with another M dopant elements at the stage where there had been no experimental data for M1 and M2. When the experiment was conducted for M1 and M2, the obtained data was turned out to be close to the predicted values and higher than ones with another M. These experimental demonstrations revealed that the constructed model at the initial stage in this study was suitable to predict the reasonable values for the composition without adequate experimental data. The loop of accumulating the experimental data and constructing the prediction model have been continuously carried out to improve the accuracy of predictions. As the number of experimental trials increased, the performance was gradually improved and the composition including M1 alone, M2 alone, M1 and other multiple Ms together, or M2 and other multiple Ms together, showed relatively higher performance, which, we considered, surpassed the maximum values obtained by the conventional trial-and-error experimental method. As a result, our new MI technology was demonstrated as an effective method to search for an unknown and unexplored composition with high performance due to the prediction accuracy of machine learning. In future, we are going to further improve prediction accuracy to find multiple Ms for even higher performance.

Glassy Oxides as Positive Electrode Materials for Sustainable and High Energy Density Li-Ion and Na-Ion Batteries: First Attempts to Establish Relationships between Glass Compositions, Physicochemical Characterizations and Electrochemical Properties with Machine Learning and Design of Experiments Approaches

Today, rechargeable lithium-ion batteries (LIBs) represent one of the best alternatives to reduce our dependency on fossil fuels. LIBs are using cathode materials based on polycrystalline oxides or polyanionic compounds [1]. However, their performances can be limited by their crystalline structure when other compounds suffer from metallic dissolution and irreversible phase changes during cycling [2]. To overcome these key shortcomings, implementing glasses or glass-ceramics as cathode materials is an interesting approach [3]. Glasses have a structure composed of more free volume, and, in the literature that is mainly concerning glasses containing Vanadium transition metal, the glasses can so accept a large amount of lithium [4] and easily accommodate structural changes upon lithium ions extraction/insertion [5]. Furthermore, glass production is a scalable process and can be commercially easier to implement than most of synthesis processes of conventional cathode materials.The main objective targeted is to develop high capacity positive electrodes (up to 300mAh/g) for Li-ion batteries based on glassy oxides without critical transition metals such as Cobalt and Vanadium. The ambitious target is to exceed 1000Wh/kg at the active material level and so enable energy densities of 300-350 Wh/kg at the Li-Ion cell level. Additionally, the electrochemical behavior of some alkali-free glass compositions will be evaluated in sodium-ion configuration. The major scientific objective is to correlate the most relevant physicochemical properties of glass with the corresponding electrochemical performances using machine learning approaches or design of experiment methodology. For a better understanding of involved processes, three oxides glass families are studied as promising cathode materials for sustainable and high energy density lithium batteries. These oxides are composed of: i) a glass network-forming element (Si, B, P) leading to the 3 different families, ii) a transition metal (Mn, Fe, ...) and optionally iii) an alkali (Li, Na).The influence of the nature of the transition metal and the polyanionic group corresponding to the network-forming element (PO4, BO3, SiO4) on the electrical properties of the as-prepared glasses measured at different temperatures will be presented mainly for the lithiated compounds. For microstructural characterization, several techniques were coupled such as X-Ray Diffraction (XRD), SEM-EDX, DTA/TGA, Raman, IR and UV-Vis spectroscopies, Electrochemical Impedance Spectroscopy (EIS) and chronoamperometry.Finally, the electrochemical properties in terms of specific capacity, redox potentials, first cycle capacity loss, coulombic and energetic efficiencies of these materials were investigated in coin-cells vs lithium (or sodium) metal electrode. To understand the electrochemical processes involved in these materials at room temperature, SEM-EDX, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Mossbauër Spectroscopy for iron based compounds were coupled and performed on the as-prepared glasses and on ex-situ (after cycling) materials at different electrochemical states of charge.All the generated data and measurements were analyzed by machine learning approach and tools and in some cases compared with a more classical Design of Experiments (DOE) approach. For instance, figure 1 illustrates the contour diagram of first discharge capacities in the mixture diagram of phosphate glasses based on 3 different transition metal oxides. The final goal is to establish more precisely the relationships between the different glassy oxides structures and their electrochemical performances. First results of this important work of elaboration, characterization and data analysis will be presented here. This new study will bring significant elements to optimize both the glass composition and its elaboration conditions to obtain high performance cathode materials without critical materials.[1] Nitta, N. et al., Li-ion battery materials: present and future. Materials Today 18, 252–264 (2015).[2] Tesfamhret, Y. et al., On the Manganese Dissolution Process from LiMn2O4 Cathode Materials. ChemElectroChem 8, 1516–1523 (2021).[3] K. Kercher et al., « Mixed polyanion glass cathodes: Glass-state conversion reactions », J. Electrochem. Soc., vol. 163, no 2, p. A131-A137, 2016, doi: 10.1149/2.0381602jes [4] Afyon, S. et al., New High Capacity Cathode Materials for Rechargeable Li-ion Batteries. Scientific Reports 4, 7113 (2015).[5] Kindle, M. et al., Alternatives to Cobalt. ACS Sustainable Chemistry Engineering 9, 629–638 (2021). Figure 1

Development of a 3-Electrode Setup for the Operando Detection of Parasitic Side Reactions: Exemplified at the Quantification of Released Oxygen

The most considerable push to higher energy density Li-ion batteries (LiBs) has been achieved by incremental improvements of the positive electrode cathode active materials (CAM).1 One approach has been to increase the nickel content of layered transition metal oxide CAMs like lithium nickel cobalt aluminum oxide (NCA; LiNixCoyAlzO2, with x+y+z=1) to above 80%, which has already been achieved on a commercial level. While this increase is accommodated by a higher achievable discharge capacity at a given upper cut-off potential, it also reduces the structural stability of the material at high degrees of delithiation (i.e. at high state-of-charge (SOC)).2,3 The structural instability of the CAM is associated with a release of lattice oxygen at high SOC, starting at approximately 80% SOC.2,3 The release of reactive oxygen species is reported to cause a cascade of degradation mechanisms resulting in capacity fade. During this process, the near-surface region of layered transition metal oxide particles is converted into an electrochemically inactive rock-salt phase, which results in the formation of a resistive surface layer.4 Furthermore, the released oxygen can stimulate the chemical oxidation of the typically employed electrolyte solvents, such as ethylene carbonate (EC), resulting in the formation of CO, CO2, and H2O, which in turn hydrolyzes the electrolyte salt.2,5 Consequently, the detection and quantification of released oxygen for different CAMs and cycling conditions is of high interest. Gas quantification methods such as differential electrochemical mass spectrometry (DEMS) and on-line electrochemical mass spectrometry (OEMS) are powerful tools for the qualitative and quantitative measurement of gaseous species such as O2 and CO2.2,6 However, such methods require an expensive mass spectrometer setup and a sophisticated interface between the LiB cell hardware and the mass spectrometer.Thus, in this study, we will present a simplified method that can detect and quantify released oxygen. The method is based on a setup consisting of a dual working electrode (WE) configuration, namely a primary WE with an NCA CAM (LiNi0.80Co0.15Al0.05O2, from BASF TODA Battery Materials LLC, Japan) and an auxiliary WE with only carbon (Vulcan-type carbon, XC-72, Tanaka, Japan), both being coated with a ; a lithium iron phosphate (LFP) electrode coated on aluminum serves as counter electrode (CE). The latter is delithiated to a degree of 90% and capacitively oversized, allowing the NCA working electrode to be cycled against a constant potential (with the LFP potential being at ~3.43 V vs. Li+/Li).8 While charging the NCA working electrode within the desired potential window, the carbon electrode is polarized to a constant potential of 1.21 V vs. the LFP counter electrode corresponding to ~2.20 V vs. Li+/Li. Since oxygen is reduced at that potential, a reductive current is detected once lattice oxygen is being evolved.9 By the precise knowledge of the number of electrons involved in the reduction of oxygen at the carbon electrode, we are able to convert the reductive current into an amount of released oxygen (e.g., in units of moles of oxygen per gram of cathode active material).We apply the developed cell setup to study the oxygen release characteristics of the NCA when being charged to 5.0 V vs. Li+/Li at 0.1 C and 25 °C, and compare these data to the gassing profile analyzed via OEMS. As will be shown, there is a good agreement between integrated and converted reductive current and the amount of detected O2 in the OEMS.

Spray-Flame Synthesis of NASICON-Type Rhombohedral (α) Li1+xYxZr2-x(PO4)3 [x = 0-0.2] Solid Electrolytes.

Since solid electrolytes have a broad electrochemical stability window, are exceptionally electrochemically stable against Li metal, and function as a physical separator to prevent dendrite growth, they are at the forefront of alternate possibilities, further increasing the stability and energy density of Li-ion batteries. NASICON-type electrolytes are a promising candidate due to their negligible moisture sensitivity, which results in outstanding stability and a lower probability of Li2CO3 passivity under the ambient atmosphere. However, one of the most promising representatives, Li1+xYxZr2-x(PO4)3 (LYZP), has multiple stable phases with significant variation in their corresponding Li-ion conductivity. In this paper, we have successfully synthesized the highly ionically conductive rhombohedral phase of LYZP via spray-flame synthesis. Two different solvent mixtures (e.g., 2-ethyl hexanoic acid/ethanol, propanol/propanoic acid) were chosen to explore the effect of precursor composition and combustion enthalpy on the phase composition of the nanoparticle. The as-synthesized nanoparticles from spray-flame synthesis consisted of the crystalline tetragonal zirconia (t-ZrO2) phase, while lithium, yttrium, and phosphate were present on the nanoparticles' surface as amorphous phases. However, a short annealing step (1 h) was sufficient to obtain the NASICON phase. Moreover, we have shown the gradual phase conversion from orthorhombic β phase to rhombohedral α phase as the annealing temperature increased from 700 °C to 1300 °C (complete removal of β phase). In this context, Y3+ doping was also crucial, along with the appropriate solvent mixture and annealing temperature, for obtaining the much-desired rhombohedral α phase. Further, 0.2 at% Y3+ doping was added to the solvent mixture of 2-ethyl hexanoic acid/ethanol, and annealing at 1300 °C for 1 h resulted in a high ionic conductivity of 1.14∙10-5 S cm-1.

Evaluating Polyacrylic Acid as a Universal Aqueous Binder for Ni-Rich Cathodes NMC811 and Si Anodes in Full Cell Lithium-ion Batteries.

Silicon (Si) and silicon/graphite (Si/Gr) composite anodes are promising candidates due to their high theoretical capacity, low operating potential and natural abundance for high energy density Li-ion batteries. Green electrode production, eliminating organic volatile solvents require advancement of aqueous electrodes. Engineering the binder plays a critical role for improving waterborne electrodes. Lithium substituted polyacrylic acid LiPAA has been demonstrated as a promising binder for Si/Gr anodes and for Ni-rich cathodes in different cell configurations. LiPAA is utilized to minimize the volume expansion during cycling for Si/Gr anodes. LiPAA is formed in situ during cathode slurry preparation to regulate the pH and dimmish the Li loss. Using advanced characterization techniques, we investigated the slurries, electrodes, and active material reaction with LiPAA and its effect to the cycling performance. Our results indicate that the performance of high Si containing anode is limited by the amount of Si in the electrode. The failure mechanism with respect to high Si content was studied thoroughly. Aqueous processed cathodes with LiPAA binder in combination with Si anodes outperformed NMP based cathodes. Hence, LiPAA was successfully utilized as an active binder for both a high Si containing anode and for a Ni rich cathode.

From Oxygen Redox to Sulfur Redox: A Paradigm for Li-Rich Layered Cathodes.

The utilization of anionic redox chemistry provides an opportunity to further improve the energy density of Li-ion batteries, particularly for Li-rich layered oxides. However, oxygen-based hosts still suffer from unfavorable structural rearrangement, including the oxygen release and transition metal (TM)-ion migration, in association with the tenuous framework rooted in the ionicity of the TM-O bonding. An intrinsic solution, by using a sulfur-based host with strong TM-S covalency, is proposed here to buffer the lattice distortion upon the highly activating sulfur redox process, and it achieves howling success in stabilizing the host frameworks. Experimental results demonstrate the prolonged preservation of the layered sulfur lattice, especially the honeycomb superlattice, during the Li+ extraction/insertion process in contrast to the large structural degeneration in Li-rich oxides. Moreover, the Li-rich sulfide cathodes exhibited a negligible overpotential of 0.08 V and a voltage drop of 0.13 mV/cycle, while maintaining a substantial reversible capacity upon cycling. These superior electrochemical performances can be unambiguously ascribed to the much shorter trajectories of sulfur in comparison to those of oxygen revealed by molecular dynamics simulations at a large scale (∼30 nm) and a long time scale (∼300 ps) via high-dimensional neural network potentials during the delithiation process. Our findings highlight the importance of stabilizing host frameworks and establish general guidance for designing Li-rich cathodes with durable anionic redox chemistry.

Design of interfacial Li-ion transfer channels for practical improvement of a multicomponent high-voltage olivine cathode

The development of a high-voltage olivine cathode is of great interest to increase the energy density of Li-ion batteries. Focusing on the multicomponent LiFe0·4Mn0·3Co0·3PO4 (LFMCP) olivine material, in which the involvement of Co is necessary to extend the operating voltage range of the olivine materials up to 5.0 V despite having many critical issues, our research shows that the construction of Li-ion transfer channels on the particle surface is a key factor for the practical realization of the high-voltage cathode. A systematic electrochemical investigation reveals that the weakening and disappearance of the Mn activity due to the redox shift and overlap during cycling are responsible for the poor capacity delivery and capacity retention of the multicomponent LFMCP material. Surface coating of the LFMCP material with either carbon (LFMCP@C) or with a combination of carbon and LATP (LFMCP@C_LATP) helps to identify and stabilize the redox regions of the Fe, Mn, and Co components. The presence of LATP as a Li-ion conductor and an electrochemically active component on the LFMCP particle surface can induce the formation of the Li-ion concentration gradient between the core and the shell to facilitate the Li-ion diffusion during cycling. As a result, the LFMCP@C_LATP has faster Li-ion diffusion in all redox regions, more stable operating plateaus, and can therefore provide better capacity and retention than the conventional LFMCP@C. The LFMCP@C_LATP can provide a discharge capacity of 139.2 mA h g−1 with 80.5 % retention after 50 cycles, while the LFMCP@C provides a discharge capacity of 135.1 mA h g−1 with 68.4 % retention under the same cycling conditions, using a common electrolyte without the addition of any high-voltage additive.

Ion Transport in (Localized) High Concentration Electrolytes for Li-Based Batteries.

High concentration electrolytes (HCEs) and localized high concentration electrolytes (LHCEs) have emerged as promising candidates to enable higher energy density Li-ion batteries due to their advantageous interfacial properties that result from their unique solvent structures. Using electrophoretic NMR and electrochemical techniques, we characterize and report full transport properties, including the lithium transference numbers (t+) for electrolytes ranging from the conventional ∼1 M to HCE regimes as well as for LHCE systems. We find that compared to conventional electrolytes, t+ increases for HCEs; however the addition of diluents to LHCEs significantly decreases t+. Viscosity effects alone cannot explain this behavior. Using Onsager transport coefficients calculated from our experiments, we demonstrate that there is more positively correlated cation-cation motion in HCEs as well as fast cation-anion ligand exchange consistent with a concerted ion-hopping mechanism. The addition of diluents to LHCEs results in more anticorrelated motion indicating a disruption of concerted cation-hopping leading to low t+ in LHCEs.

Advantageous Electrochemical Behaviour of New Core-Shell Structured Cathodes over Nickel-Rich Ones for Lithium-Ion Batteries

Currently, layered Ni-rich oxides cathodes of LiNi1-xMnyCozO2 (x ≥ 0.8) have gained a major attention for the high energy density Li-ion batteries (LIBs), due to their high specific capacity of...

Enhanced cycling, safety and high-temperature performance of hybrid Li ion/ Li metal batteries via fluoroethylene carbonate additive

Hybrid graphite/Li metal anode has been proved to be a feasible approach to enlarge the energy density of Li-ion batteries. However, there is still a gap between the electrochemical performance of the hybrid Li-ion/Li metal batteries (HLI-LMBs) and the practical requirements. In this work, the cycling, safety and high-temperature performance of the HLI-LMBs have been successfully enhanced via adding fluoroethylene carbonate (FEC) in the electrolyte. It is found that FEC can facilitate the formation of a LiF-rich SEI layer on the surface of Li metal, which effectively suppresses the formation of Li dendrites and enables uniform deposition of Li metal on the surface of graphite. The LiF-rich SEI layer can restrain the exothermal reaction between Li metal and electrolyte under short-circuit. Moreover, the LiF-rich SEI slows down the reaction of anode with electrolyte and largely suppresses the Li dendrite formation at high temperature, improving the high-temperature performance of the HLI-LMBs. This work sheds the light on the critical roles of electrolyte additive and can serve as a guideline for the development of high-performance HLI-LMBs.

Capacity Fading Mechanism of Ni-Rich Cathode Materials Focusing on Particle Interior

With the prevalence of electric vehicles, Ni-rich layered cathodes have been extensively developed to increase the energy density of Li-ion batteries (LIBs). However, cathodes with a high Ni content suffer from inherent structural and chemical instabilities, which lead to rapid capacity fading and thermal instability.1 The degradation of Ni-rich cathode materials is largely caused by the high proportion of reactive Ni4+ combined with the mechanical instability originated from microcracks. This unstable Ni4+ can be readily reduced to Ni2+ by parasitic reactions with the electrolyte concurrently with oxygen release, forming a NiO-like rocksalt impurity phase.2 This phase increases the charge-transfer resistance and thus deteriorates the electrochemical performance of Ni-rich cathodes. As the degradation mainly occurs at the surface of cathode by deleterious electrolyte attack, the rate of degradation is significantly determined by the area exposed to the electrolyte. For cathodes with low Ni contents of ≤60%, the cathode−electrolyte interphase is only limited to the exterior surface of cathode particles in which microcracks are not severe; thus, the decrease in the cycling stability of cathodes is directly proportional only to their Ni4+ fraction. However, as the Ni content in the cathode increases above 60%, the extent of microcracks rapidly increases owing to the buildup of anisotropic strain induced by abrupt lattice volume changes during the H2−H3 phase transition.2 The resultant microcracks in Ni-rich cathode particles can create channels through which the electrolyte infiltrates into the particle interior, thereby increasing the surface area exposed to the electrolyte attack. Janek et al. reported that the specific surface area of 0.2 m2 g−1 for pristine LixNi0.8Co0.1Mn0.1O2 cathode increased to ∼1.4 m2 g−1 when charged to 4.2 V (vs Li/Li+) owing to crack formation.3 This increased surface area further accelerated the rate of capacity fading for Ni-rich cathodes (especially Ni fraction ≥90%), as not only the particle exteriors but also the interiors were exposed to deleterious electrolyte attack and were thus also passivated by NiO-like impurity phases caused by surface degradation.In this study, we performed comparative post-mortem analyses of Li[Ni0.8Co0.1Mn0.1]O2 (NCM811) and Li[Ni0.90Co0.05Mn0.05]O2 (NCM90) cathodes after long-term cycling to elucidate the degradation mechanism of Ni-rich cathodes, focusing on the particle interior. Furthermore, we demonstrated correlations between the degradation of cathodes and the resultant changes in electrochemical properties (e.g., ionic and electrical conductivities). This systematic approach enables us to identify the mechanism by which cathode degradation deteriorates the electrochemical performance, which we expect to lead to breakthroughs in developing Ni-rich cathodes appropriate for EV applications. Reference s : [1] H.-J. Noh, S. Youn, C. S. Yoon, Y.-K. Sun, J. Power Sources, 2013, 233, 121.[2] H.-H. Ryu, K.-J. Park, C. S. Yoon, Y.-K. Sun, Chem. Mater. 2018, 30, 1155.[3] E. Trevisanello, R. Ruess, G. Conforto, F. H. Richter, J. Janek, Adv. Energy Mater. 2021, 11, 2003400.

Aqueous Processing of Ni-Rich NMC Electrodes with High Areal Capacity and Excellent Long-Term Cycling Stability

Research efforts in the last three decades have resulted in a steady increase in the energy density of Li-ion batteries (LIBs), leading to their integration into electric vehicles (EVs). Therefore, car manufacturers have increased the number of EVs in their fleets and successfully introduced them to the mass market, with 200M of EVs expected to be sold by 2025 [1] . In such a fast-growing market, the cost and environmental impact of LIB production play major roles. A crucial step in battery manufacturing is the processing of active materials to produce electrode coatings. Commercial cathode electrodes (i.e. LiNixMnxCoxO2 (NMC), LiNi0.5Mn1.5O4 (LNMO) etc.) are still largely processed using organic solvents, specifically N-methyl-2-pyrrolidone (NMP), with poly(vinylidene difluoride) (PVDF) as a binder. However, the NMP-PVDF combination has many disadvantages [2]. Firstly, the binder is considerably more expensive than water-soluble binders and not easy to dispose of at the end of the battery life. Secondly, NMP is costly and toxic, and being a volatile organic compound (VOC), needs solvent recovery at commercial scale. On the other hand, water-soluble binders such as Na-carboxymethylcellulose (Na-CMC) and styrene butadiene rubber (SBR) are not only cheaper but also easier-to-recycle [3]. Additionally, the use of water as a solvent would remove the need for solvent recovery. It is then clear that switching to water-based processing for cathode materials would decrease cost and improve the environmental sustainability of LIB production. Nevertheless, aqueous cathode processing has many challenges [2], the major one being the instability of the active material in water. The leaching of Li+ ions gives rise to very high pH (~ 12) values which, in turn, leads to the corrosion of the current collector (aluminium) and gas evolution during coating. Consequently, electrodes usually show cracks and pinhole defects and exhibit poor flexibility. These issues are even more enhanced for thick coatings, which are needed to achieve high energy density cells. Therefore, the electrochemical performance of water-based cathode electrodes is generally poorer than that for NMP-based electrodes.In this work, the effect of additions of phosphoric acid (H3PO4) on slurry rheology, adhesion strength, electrode morphology and the electrochemical performance of Ni-rich NMC cathodes prepared via water-based processing was investigated. To effectively bridge the gap between lab-scale investigations and industrially applicable procedures, pH control of the slurries and roll-to-roll electrode coating were performed at the pilot scale, and only electrodes exhibiting commercially relevant areal capacities (~ 4-4.5 mAh cm-2), high active material contents (> 90%) and low binder amounts (3 wt.%) were investigated. Phosphoric acid was added in amounts between 1.5 and 3 wt.%, and a fast-mixing procedure was developed to minimize the contact time between the NMC particles and water. The pH evolution over time showed that at least 2 wt.% of H3PO4 is required to keep the pH below the corrosion threshold of aluminium (pH < 9) for 6-8 hours, which is a usual time scale for industrial electrode processing. All slurries showed shear thinning behaviour with the viscosity of the water-based slurries being slightly lower than that for NMP-based slurries at the shear rate used for coating. Optical micrographs of coated electrodes showed no evidence of defects or agglomerates and good homogeneity. The adhesion to the current collector decreased with increasing amount of acid, e.g., electrodes prepared using 3 wt.% H3PO4 showed cracks at the edges of the coating and delamination.The long-term performance and rate capability of the electrodes were investigated in full-cell configuration vs graphite (coin-cell setup). NMC cathodes prepared with 1.5 wt.% acid showed a specific capacity of around 180 mAh g-1 at C/10 and 165 mAh g-1 at C/3. The specific capacity of the cells decreased with increasing amount of acid. The water-based and NMP electrodes retained around 50% and 60% of the initial capacity at 1C, respectively. Furthermore, electrodes prepared with 1.5 wt.% and 2.25 wt.% acid achieved a capacity retention above 90% after 500 cycles, which is comparable to NMP-based electrodes. In a further step, the production of the electrodes was upscaled in order to manufacture NMC/graphite pouch cells of 10 Ah capacity. These cells showed excellent cycling stability, reaching 80% SoH after 1500 cycles.

Investigating the Mechanisms of Li Dendrite Formation in Sulfide Solid Electrolytes for All-Solid-State Batteries

As the primary use of Li-ion batteries shifts from small electronic devices to electric vehicles, there is a need to increase the stability and energy density of Li-ion batteries. In order to achieve high energy density, use of lithium metal as an anode has being considered due to its high theoretical capacity (3860 mAh g-1) and low reduction potential (-3.04 V vs. SHE). However, the liquid electrolyte used in Li-ion batteries is not only dangerous due to the risk of fire in case of leakage, but also has the disadvantage of shortening the lifetime of the battery due to the generation of lithium dendrites during cycling when lithium metal is used as anode. To overcome these disadvantages, all-solid-state batteries using solid electrolytes have emerged, which are expected to improve safety and cycling stability even when using lithium metal anodes. Among the various types of solid electrolytes, sulfide solid electrolytes are widely used due to their high ionic conductivity and ductility, which allows easy formation of good interfaces, resulting in high performance all-solid-state batteries.To date, many attempts have been made to use lithium metal anodes with solid electrolytes, but dendrite growth has not been completely suppressed.1 Dendrite growth is affected by many interface-related factors, such as void ratio, grain size, elastic modulus, reductive decomposition products, and electrical conductivity of the electrolyte, which must be clarified before the use of lithium metal anodes can be realized.2 It is essential to observe and analyse the changes during electrochemical tests in order to reveal the formation of Li dendrites under the conditions in which actual real cells operate. We have attempted to elucidate the dynamic structural changes at the interface using multimodal/multiscale operando X-ray CT under an applied pressure, as well as the reactions taking place at the interface by observing the interface products using X-ray absorption spectroscopy. In this study, X-ray diffraction, X-ray absorption spectroscopy, time-resolved impedance measurements and multi scale operando X-ray CT are used to determine the electrochemical and chemical mechanisms of Li dendrite formation in different sulfide solid electrolytes. By investigating and comparing the particle size, porosity, doping effect of halides and reduction resistance of solid electrolytes used in lithium metal solid-state batteries, the mechanism of Li dendrite growth and how each factor affects dendrite growth are investigated.Here, we discuss Li3PS4, halogen-doped Li3PS4, and argyrodite-based Li6PS5Cl as typical sulfide-based solid electrolytes; Li3PS4 is known to have a low elastic modulus, and a simple cold press could be used to reduce the inter-particle voids. However, Li3PS4 has a very narrow potential difference, which resulted in unwanted oxidation and reduction byproducts during battery cycling. The high electronic conductivity of the reduction products at the interface also led to the formation of electronic pathways, resulting in the concentration of current in several locations and ultimately triggering the formation of Li dendrites. On the other hand, in argyrodite Li6PS5Cl, the formation of an interfacial phase with low electronic conductivity, which is mainly composed of lithium chloride at the interface, suppresses the reduction reaction and thus the formation of Li dendrites. In the presentation, the relationship between the lithium dendrite formation and the operating conditions such as temperature and pressure will also be presented.

Electrochemical Scanning Probe Microscopy Evaluation of Li-Ion Battery Cathode Degradation through in Situ Oxygen Evolution Measurement and Interfacial Property Characterization

Degradation processes involving oxygen evolution significantly limit the performance, reversibility, and capacity of high energy density Li-ion battery (LiB) cathodes. In addition, oxygen evolution processes create hazardous conditions for LiB operation. Despite the importance of these processes, characterization of evolved O2 with sufficient spatial and temporal versatility is hampered by the complex setups required to perform electrochemical mass spectrometry, the gold standard for evaluating this process. Recently, we introduced a scanning electrochemical microscopy (SECM) approach that enables in-situ, real-time (i.e., few ms temporal resolution), chemically selective detection of O2.[1] Measurements were accomplished by positioning an SECM probe (i.e., an Au ultramicroelectrode) near operating cathode surfaces for detecting O2 via the oxygen reduction reaction (ORR). The superior signal-to-noise of this method enabled the observation of two peaks corresponding to a steady-state evolution at high voltage and a transient O2 evolution at low voltage on materials such as lithium cobalt oxide (LCO) and NMC electrodes.Here, we expand the capabilities of our technique by incorporating correlated measurements aimed at understanding how oxygen evolution profiles are related to surface electrochemical and mechanical properties. Specifically, we will focus on the use of feedback-mode SECM and the use of reactive mediators to evaluate the changes in the rates of electron transfer measured on materials undergoing O2 and after structural degradation. Furthermore, the use of electrochemical AFM allows us to measure the mechanical properties of these interfaces, in an attempt to tie the structural changes and the evolution of oxidizing species with the formation of cathode-electrolyte interfaces. Altogether, these multimodal SECM and AFM methods elucidate the manifold chemical, structural, and performance changes underpinning LiB cathode operation.[1] Mishra, A.; Sarbapalli, D.; Hossain, M.S.; Gossage, Z.T.; Li, Z.; Urban, A.; Rodríguez-López, J. Highly Sensitive Detection and Mapping of Incipient and Steady-State Oxygen Evolution from Operating Li-Ion Battery Cathodes via Scanning Electrochemical Microscopy. J. Electrochem. Soc., 2022, 169, 086501. Figure 1

NMC811 Electrodes with High Mass Loadings Enabled By Non-Solvent Induced Phase Inversion

Higher energy density Li-ion batteries that can enable longer driving ranges are currently subject to intensive research interest. Increasing the thickness of electrodes and reducing the proportion of inactive components per cell is one way to achieve this. In thick electrodes, a higher electronic and ionic overpotential and mechanical failure (cracking, delamination etc.) induced by binder migration during the drying process leads to sluggish (dis)charge performance or even cell failure respectively. Here we report non-solvent induced phase inversion as a scalable, effective method to arrive at thick NMC811 electrodes.By tuning the non-solvent properties and other processing conditions to be compatible with Ni-rich electrodes, it was possible to obtain NMC811 electrodes with mass loadings up to 11 mAh/cm2 (single sided) which outperform their conventional processed counterparts in Li/NMC half cells. This improvement could be attributed to the altered carbon-binder structure, which improves the pore connectivity and lowers the electrode tortuosity factor. The rapid solvent removal also reduces the long-range binder migration during drying. With imaging and porosimetry techniques we show the structural change which leads to the observed improved electrochemical performance. The method also shows good compatibility with double sided slot-die electrode coating procedure, making this a technique with potentially high industrial feasibility towards making thicker NMC811 and other electrodes.

Temperature-Dependent Interplay of Chemical Versus Electrochemical Electrolyte Oxidation at Ni-Rich Cathodes

Increasing demands for high energy density Li-ion batteries as a source for power supply are demanding an optimization of cathode active materials (CAMs) and of the overall cell chemistry of batteries. Thus, the main interest of researchers has focused on nickel-rich (>80%) layered transition metal oxides such as NCMs (LiNixCoyMnzO2, with x+y+z=1) or NCAs (LiNixCoyAlzO2, with x+y+z=1) as CAMs, owing to their high capacity. However, as a result of the increase in Ni-content, the structural stability of these CAMs is significantly decreased at high upper cut-off potentials compared to those with lower Ni-content, resulting in capacity fading, which restricts their practically achievable capacity.[1] One possible culprit behind this capacity fading at high state-of-charge (SOC) is the release of reactive lattice oxygen, which is known for layered TM-oxides to occur at high SOC (i.e., starting at approx. 80% SOC).[2] The released oxygen is proposed to lead to the chemical oxidation of ethylene carbonate (EC), resulting in HF formation, which in turn is proposed to trigger the dissolution of TMs.[3] Furthermore, the electrochemical oxidation of the electrolyte is known to occur at high potentials, which as a result of its reactive reaction products, can induce further transition metal (TM) dissolution from the CAM.[4,5] The proposed chemical oxidation of the electrolyte by lattice oxygen suggests that this process should be strongly correlated to the dissolution of TMs from the CAM. However, as shown by operando X-ray absorption spectroscopy (XAS) and on-line electrochemical mass spectroscopy (OEMS) studies, the onset for the TM-dissolution, depending on the CAM under investigation, occurs at ~300 mV higher potentials compared to the onset for oxygen evolution.[2,3] This suggests that the dissolution of metals is rather induced by the electrochemical oxidation of the carbonate-based electrolyte, which was reported to initiate at potentials above 4.7 VLi at room temperature.[5] Both the electrochemical oxidation of battery electrolytes, as well as the dissolution of TMs from the CAM were reported to be strongly affected by temperature, while the onset potential for lattice oxygen loss is essentially unaffected, which further hints towards the significance of former.[5-7] Within this work we therefore conduct a detailed study on the temperature dependency of both the gas formation due to (electro)chemical electrolyte oxidation and the dissolution of TMs by means of OEMS and operando hard XAS, respectively. By applying a measurement protocol consisting of constant voltage-holds in steps of 50 mV at elevated potentials (see Figure 1), an excellent time as well as potential resolution enables the deconvolution of the onset for lattice oxygen release and the (electro)chemical oxidation of LP47 (1 M LiPF6 in EC:DEC 3:7 by weight) for a LiNi0.80Co0.15Al0.05O2 (NCA) CAM via OEMS. It will be shown that the onset for oxygen evolution coincides with a significant evolution of CO2 (see Figure 1b), indicating chemical oxidation of the electrolyte. However, the onset for POF3/PF5 evolution (marked in Figure 1b as “POF3”, since both gasses appear on the same mass channel)[8], which is a reaction product from the decomposition of LiPF6 caused by electrochemical electrolyte oxidation, is shifted to 300 – 450 mV higher potentials (depending on the temperature). It suggests a chemical oxidation mechanism of the electrolyte, which proceeds without significant H2O/H+ production. A change in temperature strongly affects this onset of “POF3” evolution, while O2 is released at the same degree of de-lithiation, regardless of temperature. To correlate this effect observed in the temperature-dependent gassing behavior to the dissolution of transition metals for this CAM, an operando hard XAS study at different temperatures will be conducted.

Building oxygen-rich vacancy coating for fast Li-ion diffusion kinetics and stable cycling of lithium-rich layered oxide cathode at wide-temperature

As a cathode with a high energy density for Li-ion batteries, Li-rich layered oxide (LLO) cathodes suffer from short lifespans and poor rate performance, mainly due to their irreversible lattice oxygen evolution leading to severe structural degradation and oxygen release. Herein, we propose the use of Zn-doped SnO2 coated with LLO cathode to tackle these issues. Compared to the traditional method of coating oxides on the surface of lithium-rich cathodes, Zn doping of SnO2 coating can artificially create abundant oxygen vacancies for LLO cathode, which can provide a buffer boundary for active oxygen generated by lithium-rich cathode during circulation, effectively promoting the reversible reaction of oxygen redox couple and stabilizing lattice oxygen. The experimental results show that the surface of the LLO cathode's Zn-doped SnO2 is rich in oxygen vacancies could significantly boost the kinetics of lithium ion transfer, and effectively inhibit layer-to-spinel phase transition, and lattice oxygen release. By taking advantage of the Zn-doped SnO2 coating, the capacity stability of LLO cathode is raised from 58.1 % to 86.7 % at 1C after 500 cycles and also obtains outstanding rate performance and cycling durability at wide-temperature (−10 to 55 °C).