Metastable sodium closo-hydridoborates for all-solid-state batteries with thick cathodes

Lithium sulfur (Li-S) batteries are well known for their high theoretical specific capacities, but are plagued with scientific obstacles that make practical implementation of the technology impossible. The success of Li-S batteries will likely necessitate the use of thick sulfur cathodes that enable high specific energy densities. However, little is known about the fundamental reaction mechanisms and chemical processes that take place in thick cathodes, as most research has focused on studying thinner cathodes that enable high performance. In this work, in situ X-ray absorption spectroscopy at the sulfur K-edge is used to examine the back of a 115 μm thick Li-S cathode during discharge. Our results show that in such systems, where electrochemical reactions between sulfur and lithium are likely to proceed preferentially toward the front of the cathode, lithium polysulfide dianions formed in this region diffuse to the back of the cathode during discharge. We show that high conversion of elemental sulfur is achieved by chemical reactions between elemental sulfur and polysulfide dianions of intermediate chain length (Li2Sx, 4 ≤ x ≤ 6). Our work suggests that controlling the formation and diffusion of intermediate chain length polysulfide dianions is crucial for insuring full utilization of thick sulfur cathodes.

Replacing the widely used NMP solvent for processing of lithium-ion battery electrodes with water has both environmental and cost benefits. Compared to aqueous-processing of anodes, the aqueous-processing of cathodes is considerably more challenging. Issues such as damage of the NMC-type active material upon exposure to water, damage of the aluminum substrate due to highly basic aqueous-slurry, and cracking of the water-based cathode slurry coatings upon drying are known and are actively being researched. In this study, we focus on the cracking-issue which is more severe for relatively thick cathodes with loadings of >~11-12 mg/cm2. Our group has previously reported the role of high surface-tension of water compared to NMP on cracking and demonstrated that the addition of 20-30 wt% IPA as a co-solvent lead to crack-free thick (~25 mg/cm2) NMC532 cathodes. This finding was attributed to reduction of surface tension of water, which decreased the capillary forces during drying. Further investigations have resulted in identification of several other factors that are also responsible for the cracking-issue. These factors, which are not necessarily related to the surface tension, will be discussed.

Interest in using thick LiFePO4 cathodes to enhance lithium-ion battery energy density has recently been growing. To obtain thick cathodes with superior electrical conductivity throughout their depth, it is crucial to substitute conventional zero-dimensional conductive agents with one-dimensional carbon nanotubes (CNTs). Nevertheless, the inherent properties of CNT, including their high aspect ratio and strong van der Waals interaction, hinder uniform dispersion, causing poor performance in thick electrodes. In this work, we adopted an electrostatic energy-driven dispersion (EED) method to achieve a homogeneous distribution of multiwalled carbon nanotubes (MWCNTs) with LiFePO4 for thick cathodes. The EED process, guided by the charge residue model and ion evaporation model theories, facilitated the formation of a well-distributed LiFePO4-MWCNT composite. This method yielded e-LiFePO4/MWCNT composites with consistent morphology even at a high MWCNT concentration (5 wt %), as verified by cross-sectional scanning electron microscopy and a microcomputed tomography scan. The e-LiFePO4/MWCNT cathode exhibited reduced overpotential during the Li-ion redox process, along with enhanced areal capacity and capacity retention (7.27 mAh cm-2 at 0.3 C and 80.74% after 90 cycles), outperforming the conventional mixing-only method. These results underline the importance of prioritizing the uniform distribution of active materials and conductive agents in future thick electrode research.

Converting a thick electrode into vertically aligned “Thin electrodes” by 3D-Printing for designing thickness independent Li-S cathode

Lithium-oxygen batteries (LOBs) are considered to be the next generation of high-specific-energy storage devices. To improve the practical specific energy, LOBs typically require thick cathode electrodes to achieve higher areal capacity. However, due to the inefficient O2 diffusion within the electrolyte-flooded thick cathodes, the practical discharge capacity of LOBs is significantly lower than their ultra-high theoretical value. Herein, we propose a strategy to solve the problem of limited O2 diffusion in the thick cathodes of LOBs by applying an O2-enriched localized high-concentration electrolyte (LHCE). With a thick cathode (10mg cm-2), LOBs based on this O2-enriched LHCE deliver impressive discharge capacities of 50.4, 27.1 and 20.3 mAh cm-2 at current densities of 0.1, 0.3 and 0.5mA cm-2, respectively. The discharge product Li2O2 is homogeneously distributed within the thick cathode due to the enhanced O2 diffusion conferred by the O2-enriched LHCE, indicating that the product storage space of the thick cathode is more efficiently utilized. Besides, the O2-enriched LHCE-based LOBs derive a stable solid electrolyte interphase to protect the Li anode from O2 corrosion. Additionally, a pioneering primary Li-O2 pouch cell with the O2-enriched LHCE achieves an exceptional specific energy of 860.6Whkg-1 (based on the total pouch cell weight), providing a promising technical pathway for the practical application of LOBs.

Two-layer cathode architecture for high-energy density and high-power density solid state batteries

Developing high‐performance thick cathodes with optimized inactive‐to‐active material ratios is a promising approach to enhance the energy density of conventional lithium‐ion batteries (LIBs). However, increasing electrode thickness introduces challenges, including elevated resistance and mechanical issues such as cracking and flaking, particularly in slurry‐based wet processes. This highlights the necessity for solvent‐free, low‐resistance thick cathode fabrication methods. In this study, the impact of a solvent‐free mechano–thermal fabrication process on the electrochemical performance of high‐nickel ternary metal oxide‐based thick cathodes is explored. A key challenge identified is the formation of solvophobic crystalline structures on the surface of the active cathode materials. To address this, a fast cooling strategy is implemented at the end of the solvent‐free fabrication process, which successfully reduced the solvophobic crystallinity, ultimately surpassing the electrochemical performance of cathodes produced through wet processes. Furthermore, incorporating a PVDF/succinonitrile (SN) mixture binder via liquid‐phase mixing further minimized crystallinity, resulting in significantly improved electrolyte wettability, ionic conductivity, and mechanical adhesion. As a result, the mixture binder system achieved a high areal capacity of ≈11 mA h cm⁻2 and demonstrated stable cycling performance over 100 cycles. When paired with a lithium metal anode, the thick cathode attained an energy density of ≈418 W h kg⁻¹, translating to ≈335 W h kg⁻¹ with packaging—representing approximately 35% improvement over current LIB technologies.

Coupling of multiscale imaging analysis and computational modeling for understanding thick cathode degradation mechanisms

As one of the methods to improve the energy density of lithium secondary battery, the use of high capacity anode and cathode materials has been focused. Although metals and alloys are mentioned as anode material candidates with high capacity, on the other hand, the candidates for high capacity cathode are insufficient currently. Therefore, a large amount of active material has to be accumulated and compressed to the cathode to achieve high energy density in consideration of the capacity balance with anode (Fig. 1). However, good performance cannot be obtained in the conventional thick coating electrode with a large amount of active material. The thick coating electrode has many problems. The ohmic resistance increases as the distance between a current collector and active material is made longer. In addition, many cracks, dropouts, and exfoliations are caused by volume changes of the coated layer during charge and discharge. In this study, the high capacity thick cathode has been realized by using a porous aluminum as a current collector as a practical method to improve the energy density of lithium secondary battery (Fig. 2). The combination of the thick cathode with a very high capacity per unit area and a high capacity anode can omit the amount of supporting materials such as a separator in batteries, resulting in the high energy density. The various thick cathodes were prepared using lithium iron phosphate, lithium cobalt oxide, and lithium nickel-cobalt-manganese oxide, respectively, and their electrochemical characteristics were evaluated by half-cell test. Though the cathodes had about several times larger capacity than conventional coating cathodes, they exhibited excellent rate performance. For example, the cell comprised of the thick cathode using lithium iron phosphate with a porous aluminum and two pieces of graphite anodes showed approximately 4-5 times capacity of the cell using conventional coating electrode at the same effective electrode area, and no large difference was observed for a in the polarization of charge-discharge at 1.0CA (Fig. 3). In a cycle-life test, the cell exhibited an excellent charge-discharge cycle performance, and retained approximately 80% of initial capacity even at 2,000th cycle (Fig.4). Such excellent performance is expected to be due to the easy electrolyte permeation from one side to the other side of the thick cathode, which is achieved by a porous aluminum current collector in addition to three-dimensional current collection. Of course, the lithium-ion diffusion in the cathode is expected to be insufficient for charge and discharge reactions when the cathode and C rate become thicker and larger, respectively. Thus, further optimization is needed for the cathode as well as for the electrolyte solution and separator to improve the ion supply in the cell. Such battery design will be also mentioned in this study. Figure 1

A combinatorial study is carried out to develop new cathode compositions in alkaline Zn-MnO2 secondary batteries. MnO2 is modified with ternary additions using Bi2O3, Na0.7MnO2, and NiO. A library of thick film cathodes over a wide compositional range is synthesized via sputter deposition. Thick film cathodes are investigated both electrochemically and structurally. Structural studies show that thick film cathodes are largely amorphous in the as-deposited state but upon charge-discharge cycling, are crystallized into δ-MnO2, i.e. layered polymorph of MnO2. The thick film cathodes are evaluated with cyclic voltammetry and charge-discharge cycling. Two compositional regions are noted as significant in terms of useful cathodes for rechargeable batteries. Compositions rich in NiO yield quite a reliable cell performance which might be considered as cathode material for Zn-NiO batteries. More importantly, there was also a region where Mn and Na had similar proportions doped by Bi and Ni, e.g. Bi0.08Na0.39Ni0.09Mn0.44Ox, which might be made the basis for improved cathode for Zn-MnO2 batteries.

High-capacity thick cathode with a porous aluminum current collector for lithium secondary batteries

Electrocatalysis is a powerful approach to accelerate sulfur redox kinetics in lithium-sulfur (Li-S) batteries. However, in practical high sulfur loading and thick cathodes, the severe concentration polarization induced by the rapid depletion of local lithium ions (Li+) greatly restricts catalysis and battery performance, representing an engineering challenge in a closed microelectrochemical reactor. Here, an electrolyte-dispersible Li+-reservoir catalyst is proposed to sustain the local Li+ concentration to ensure the continuous electrochemical reaction in the battery with an energy density of over 400 Wh kg-1. Such a catalyst is realized by anchoring single cobalt atoms onto uniformly dispersed carbon quantum dots (Co-CQD). The negatively charged CQD strongly attracts and enriches Li+ around the Co catalytic sites by robust electrostatic interactions, ensuring a continuous Li+ supply during the catalytic reactions. Moreover, Co-CQDs are homogeneously dispersed in the electrolyte and distributed in thick electrodes, promoting bulk-phase Li+ distribution and effectively eliminating concentration polarization. As a result, this strategy lowers the sulfur conversion activation energy in thick cathodes from 1.27 to 0.72 eV and enables the battery to maintain a high reversible capacity of 13.5 mAh cm-2 under a high sulfur loading of 13 mg cm-2, outperforming conventional catalysts under identical conditions. Moreover, an Ah-level pouch cell delivers a high energy density of 513 Wh kg-1, offering a scalable strategy to overcome ion transport bottlenecks in thick cathodes for practical Li-S batteries.

Reduction of manganese dioxide is not uniform throughout alkaline cells with thick cathodes. Quantification of the degree of reduction of MnO2 as a function of location in the cathode by determining the degree of EMD reduction in discharged alkaline cathodes is described, using a new experimental technique which allows collection and analysis of regionally defined electrolytic manganese dioxide (EMD) samples from commercial primary alkaline batteries of different sizes. This method has been developed for 1.5V D-size and C-size alkaline batteries. The information gained can be used to better explain the behaviour of real cells with thick cathodes.

High-voltage LiNi0.5Mn1.5O4 (LNMO) spinel offers high specific energy and good rate capability with relatively low raw-material cost due to cobalt-free and manganese-rich chemical compositions. Also, increasing mass loading (mg/cm2) by thickening cathodes has been one of the focused areas to greatly improve the energy density of lithium-ion batteries (LIBs) at the cell level. The LNMO cathode made with a polyvinylidene fluoride (PVdF) binder, however, suffers from an oxidative decomposition of liquid electrolytes and cathode delamination from a current collector. This problem is exacerbated with an increase in thickness. In this study, we developed a lithium polyacrylate (LiPAA)-sodium alginate (Na-Alg) composite binder series that offer positive multifunctions such as enhancing cathode adhesion and cohesion, improving cycle life, creating an effective passivating layer at the cathode-electrolyte interface (CEI), and lowering cell impedance. Comprehensive design of systematic experiments revealed a close chemo-mechano-electrochemical relationship in the thick high-voltage cathodes. Among the various binder compositions, the LiPAA (30 wt %)-Na-Alg (70 wt %) binder offered a strong adhesion property and positive multifunctions at the CEI layer, which consequently stabilized the solid-electrolyte interfacial (SEI) layer on the graphite anode and improved LIB performances. This novel composite binder will be applicable to various types of thick cathodes in future studies.

With advances in material synthesis and process technologies, a lot of unique electronic devices could be developed. As a result, a very stable and long life time based energy source is strongly required. Thin film battery (TFB) is a good candidate for satisfying this requirement. Although the TFB has high energy density, it exhibits shortage problem in energy saving amount. To enhance the capacity of TFB, increase thickness of cathode layer has been proposed. However, thick cathode by conventional thick film process cannot give optimal surface roughness for adopting in thin film solid electrolyte. In this presentation, very high capacitive 2-Dimensional all solid state Li ion batteries is shown. Deposition of solid electrolyte could be possible due to well controlled surface morphology of the thick cathode layer. Based on this thick cathode and thin film solid electrolyte, all solid state battery with energy density of 2 mAh/cm2 was prepared.