Electrolyte Wettability Research Articles

Insight into the Improvement Strategies of Aqueous Electrolyte for Aqueous Rechargeable Batteries

AbstractAqueous rechargeable batteries (ARBs) have attracted wide attention due to their rich resources, environmental friendliness, and safety. The high ionic conductivity, safety and interfacial wettability of the aqueous electrolyte are essential for energy storage devices. However, the narrow electrochemical stability window (ESW) of the aqueous electrolyte and the side reactions of the electrode/electrolyte lead to the low energy density of the ARBs. In the past studies, the above problems were generally solved by improving the ion transport characteristics of the electrolyte and the electrode/electrolyte interface. Herein, we discuss the background, limitations, and key concepts for performance improvement of aqueous electrolytes, aiming to provide a comprehensive perspective and inspiration for further research on aqueous electrolytes and promote their development and application in the field of energy storage.

The impact of 3-dimensional anode geometry on the electrochemical response of high temperature gas evolution reactions in molten salts

Bulk electrodes are used in molten salt electrolysis as consumable graphite anodes for existing aluminium production (producing CO2) as well as for oxygen evolution anodes in future zero emission manufacturing processes, such as direct iron electrolysis. Here, graphite electrodes with different geometry, roughness, density, and size were assessed for their kinetic behaviour and gas evolution stability in a molten alkali-metal (Li-Na-K) carbonate salt at 600 °C. It is suggested through comparative assessment of controlled electrode sizes and geometry that radial diffusion at a small (5 mm diameter disc) electrode is likely to influence performance. Roughened electrodes and electrodes of varying porosity also impacted polarisation behaviour which was ascribed to changes in electrolyte wetting observed in separate studies, conducted under similar conditions. It was also seen that 3-dimensional cone and slant type electrodes on angles of 75° increased the electrode performance. Enhanced kinetics were observed by a decreasing Tafel slope from 254 mV.decade−1 to 188 mV.decade−1 for 45° and 75° slant electrodes, respectively. The apparent improved performance for identical electrode material is discussed to involve influence from radial diffusion of reactants, modified via electrode geometry. Additional work is required to confirm this. It was concluded that to fairly compare oxidation of graphite materials in molten carbonate salts, electrodes of 10 mm diameter equivalent should have the same geometry and consistent inclination angle (of below 45°), as well as control of bulk density and resulting porosity. This observation may well also apply to other systems and should be considered in experimental design.

Effect of thermal processing conditions on the structure and properties of porous PENK/PVDF composite films and its application as lithium-ions separators

By developing high-performance lithium-ion battery technology, the demands on the design of separator modification have increased. However, battery performance and safety are limited by conventional polyolefin separators' poor thermal resistance and electrolyte wettability. Biphenyl-type poly (arylene ether nitrile ketone) (BP-PENK) is synthesized. PENK polymers are a class of poly (aryl ether) with excellent properties of both Poly (aryl ether nitrile) (PEN) and Poly (ether ketone) (PEEK). The BP-PENK/PVDF composite film is further processed by non-solvent induced phase separation (NIPS), and the impact of annealing temperatures on the thermal resistance and crystalline phase structure of composite separators has been studied. The composite separator owns enhanced ionic conductivity, excellent electrolyte wettability, and superior thermal durability. The cell equipped with the composite separator has a discharge-specific capacity of 146.5 mAh·g−1 and a capacity retention rate of 86.64 %. In this way, A novel approach to producing lithium-ion battery separators that possess outstanding electrochemical properties and high thermal durability is provided.

Constructing a rapid ion-transport anode interface protective layer for zinc ion batteries to suppress solvation and improve surface electronic structure

Rechargeable Aqueous Zinc-Ion Batteries (AZIBs) have attracted significant attention in recent years due to their high energy density, safety, and cost-effectiveness. However, despite these advantages, AZIBs face challenges such as anode side reactions, passivation, corrosion, hydrogen evolution, and the growth of zinc dendrites, which hinder their widespread use. To address these issues, this study involves the creation of a zinc-affinitive coating using a WSe2/ZIF composite on zinc foils. The amorphous WSe2 protective layer, formed through electrochemical induction as ZnSe, enhances the anode's zinc affinity, accelerates Zn2+ transfer at the interface, and effectively inhibits zinc dendrite growth. Additionally, the coating improves electrolyte wettability, reducing interface resistance. Compared to pure zinc, anodes with a WSe2/ZIF protective layer exhibit excellent performance in both symmetric cells and full-cell systems using MXene/MnO2 as the cathode. Symmetric cells can cycle for over 1000 h at a current density of 5 mA/cm2, and the full cell maintains a capacity retention of 91.4 % after 2000 charge-discharge cycles. This interface engineering strategy, supported by density functional theory calculations, provides a strong foundation for practical applications.

Mechanically enhanced battery separator prepared by hot‐pressing electrospun polyvinylidene fluoride@polymethyl methacrylate membranes with different fiber orientations in adjacent layers

AbstractCompared with commercial polyolefin membranes, electrospun polymeric membranes have the advantages of higher porosity and better electrolyte wettability, but their poor mechanical performance limits their industrial use as battery separators. In this work, a layer‐to‐layer electrospinning is used for fabrication of PVdF@PMMA membranes, in which fibers in adjacent layers are arranged at a certain angle (0°, 30°, 60°, 90°). The as‐electrospun membranes are further hot‐pressed to boost their mechanical strength. Various techniques are employed to evaluate the membranes, such as scanning electron microscopy, differential scanning calorimetry, contact angle measurement, electrochemical impedance spectroscopy, and linear sweep voltammetry. The results show that polymethyl methacrylate partially melts during the hot‐pressing treatment, causing adjacent fibers in the membranes to bridge. The obtained membranes show different tensile strength values with the various angles between fibers. When the angle is 30°, the tensile strength can reach 22.09 MPa and higher porosity (74.68%), greater liquid electrolyte absorption (612.36%) than that of commercial polyolefin membrane are harvested. The lithium‐ion cell assembly with the membrane separator exhibits a discharge capacity reaching 142 mAh g−1 and an excellent capacity retention of 90.7%. This present study provides an innovative and promising approach for the fabrication of high‐performance battery separator by electrospinning.Highlights PVdF@PMMA membranes are prepared by a layer‐to‐layer electrospinning. In the membranes, fiber orientation in adjacent layers is kept at a certain angle. Hot‐pressing further boosts mechanical performance of the membranes. Membranes with different angle of fibers show various mechanical strengths.

Dendrite-suppressed Li deposition enabled by surface-tailored carbon-based current collectors for high-rate and stable Li-metal batteries

Lithium (Li)-metal batteries (LMBs) are considered as one of the most promising next-generation batteries due to the exceptionally low redox potential and high specific capacity of the Li metal anode. However, their practical application remains challenging due to problems such as Li dendrites growth and poor Coulombic efficiency (CE) during cycling. Among the various approaches proposed to solve these challenges, the introduction of 3D current collectors has been proven effective for suppressing the growth of dendritic Li. Nevertheless, the surface properties of 3D current collectors have been somewhat overlooked in existing studies. In this work, we assessed three distinct carbon nanotube (CNT)-based current collectors (namely, CNT, oxidized CNT (ONT), and reduced ONT (rONT)) to investigate the effects of their surface properties and compositions on the Li deposition process and, hence, the electrochemical stability of the resulting LMBs. The pristine CNT-based current collector exhibits a poor electrolyte wettability, thus resulting in a rapid decline in CE over successive cycles. In contrast, the ONT current collector shows enhanced electrolyte wettability with lithiophilic surface, which significantly improves the interfacial kinetics and cycling stability. Moreover, after the subsequent reduction process, the rONT current collector exhibits a higher electrical conductivity than the ONT current collector while maintaining the favorable electrolyte wettability. Consequently, the Li|rONT cell delivers a more stable cycling performance than the Li|ONT cell at high current densities. These results demonstrate that the electrochemical performances of the LMBs can be significantly improved by suitably modifying the surface characteristics of the current collectors.

Benefits of Femtosecond Laser 40 MHz Burst Mode for Li-Ion Battery Electrode Structuring.

In Li-ion batteries, ion diffusion kinetics represent a limitation to combine high capacity and a fast charging rate. To bypass this, textured electrodes have been demonstrated to increase the active surface, decrease the material tortuosity and accelerate the electrolyte wetting. Amongst the structuring technologies, ultrashort pulse laser processing may represent the key option enabling, at the same time, high precision, negligible material deterioration and high throughput. Here, we report a study on the structuring of electrodes with both holes and grooves reaching the metallic collector. Electrochemical models emphasize the importance of hole and line dimensions for the performances of the cell. We demonstrate that we can control the hole and line width by adjusting the applied fluence and the repetition rate. In addition, results show that it is possible to drill 65 µm-deep and ~15 µm-wide holes in nearly 100 µs resulting in up to 10,000 holes/s. To further reduce the takt time, bursts of 40 MHz pulses were also investigated. We show that bursts can reduce the takt time by a factor that increases with the average power and the burst length. Moreover, at comparable fluence, we show that bursts can shorten the process more than theoretically expected.

Phosphorylated cellulose nanofibers establishing reliable ion-sieving barriers for durable lithium-sulfur batteries

The shuttle effect is among the most characteristic and formidable challenges in the pursuit of high-performance lithium-sulfur (Li-S) batteries. Herein, phosphorylated cellulose nanofibers (pCNF) are intentionally engineered to establish an ion-sieving barrier against polysulfide shuttling and thereby improve battery performance. The phosphorylation, involving the grafting of phosphate groups onto the cellulose backbone, imparts an exceptional electronegativity that repels the polysulfide anions from penetrating through the separator. Moreover, the electrolyte wettability and Li+ transfer can be significantly promoted by the polar nature of pCNF and the facile Li+ disassociation. As such, rational ion management is realized, contributing to enhanced reversibility in both sulfur and lithium electrochemistry. As a result, Li-S cells equipped with the self-standing pCNF separator demonstrate outstanding long-term cyclability with a minimum fading rate of 0.013% per cycle over 1000 cycles at 1 C, and a decent areal capacity of 5.37 mA h cm−2 even under elevated sulfur loading of 5.0 mg cm−2 and limited electrolyte of 6.0 mL g−1. This work provides a facile and effective pathway toward the well-tamed shuttle effect and highly durable Li-S batteries.

Preparation of GO/Diatomite/Polyacrylonitrile Functional Separator and Its Application in Li-S Batteries.

Lithium-sulfur (Li-S) batteries have received extensive attention due to their numerous advantages, including a high theoretical specific capacity, high energy density, abundant reserves of sulfur in cathode materials, and low cost. Li-S batteries also face several challenges, such as the insulating properties of sulfur, volume expansion during charging and discharging processes, polysulfide shuttling, and lithium dendritic crystal growth. In this study, a composite of a porous multi-site diatomite-loaded graphene oxide material and a PAN fiber membrane is developed to obtain a porous and high-temperature-resistant GO/diatomite/polyacrylonitrile functional separator (GO/DE/PAN) to improve the electrochemical performance of Li-S batteries. The results show that the use of GO/DE/PAN helps to inhibit lithium phosphorus sulfide (LPS) shuttling and improve the electrolyte wetting of the separator as well as the thermal stability of the battery. The initial discharge capacity of the battery using GO/DE/PAN is up to 964.7 mAh g-1 at 0.2 C, and after 100 cycles, the reversible capacity is 683 mAh g-1 with a coulombic efficiency of 98.8%. The improved electrochemical performance may be attributed to the porous structure of diatomite and the layered composite of graphene oxide, which can combine physical adsorption and spatial site resistance as well as chemical repulsion to inhibit the shuttle effect of LPS. The results show that GO/DE/PAN has great potential for application in Li-S batteries to improve their electrochemical performance.

Direct wetting of Li7La3Zr2O12 electrolyte with molten Li anode and its application in solid-state lithium batteries

Garnet Li6.5La3Zr1.5Ta0.5O12 (LLZTO) is a promising solid electrolyte for solid-state batteries (SSBs) owing to its high ionic conductivity and high chemical/electrochemical stability. However, the solid-to-solid interface between LLZTO electrolyte and Li anode restricts its application. In this work, an in-situ integration of LLZTO electrolyte with Li anode is proposed to solve this problem. Superior to reported works, the LLZTO obtained by our self-consolidation method can be directly wetted by molten Li without any surface treatment. The self-consolidated LLZTO exhibits a dense microstructure consisting of grains and interstitial phases among them, both of which are accessible to molten Li. Removal of surface contaminants on molten Li rather than that on LLZTO is a prerequisite for direct wetting. Benefiting from direct wetting, an intimate interfacial contact between the LLZTO and Li anode is achieved and the interfacial resistance is significantly reduced. The Li symmetric cell exerts a stable Li plating/stripping cycle for 1000 h at 0.20 mA cm−2 and the solid-state battery paired with LiCoO2 cathode exhibits a capacity retention of 90.0 % at 0.2 C after 200 cycles at room temperature. This work highlights the surface chemistry of electrolyte and electrode, which is crucial for designing their integration and application.

Defect-Enhanced Lithium Storage Performance of Nanostructured Mesoporous LiFePO4 for a High-Power Lithium-Ion Battery

Mesoporous materials have received growing interest, particularly as electrode materials for lithium-ion battery applications since they provide short transportation length for Li ion and electrons, and favour electrolyte wettability. Such unique features are highly beneficial for improving the electrochemical performance of olivine LiFePO4 as this material has intrinsically low electronic and ionic conductivities, which otherwise would affect the storage performance. In addition, this sluggish kinetic brings about huge polarisation specifically at high current rates, resulting in poor energy efficiency. In order to overcome such kinetic issues, we present here a facile soft template-solvothemal method to synthesise mesoporous LiFePO4. Such mesoporous LiFePO4 is made of well interconnected nanograins (20–30 nm) which exhibits excellent storage performance and long-term cycling stability. In particular, the material shows improved storage performance at high rates with significantly less polarisation and clear signature of voltage plateaus for both Li ion insertion-extraction processes. In comparison with the LiFePO4 synthesized by the soft template method, the mesoporous LiFePO4 demonstrates excellent storage performance. This is attributed to the 2-D diffusion of both Li ions and electrons along b- and c-axes consistent with the 2-D Li ions transport reported previously for LiFePO4 single crystal.

Limitations of Polyacrylic Acid Binders When Employed in Thick LNMO Li-ion Battery Electrodes

Polyacrylic acid (PAA) is here studied as a binder material for LiNi0.5Mn1.5O4 (LNMO) cathodes for lithium-ion batteries. When the LNMO electrodes are fabricated with an active mass loading of ∼10 mg cm−2 (∼1.5 mA h cm−2), poor discharge capacity and short cycle life is obtained in full-cells with graphite electrodes. The electrochemical results with PAA are compared with a commonly used water-based binder, sodium carboxymethyl cellulose (CMC), which shows better electrochemical performance. The main cause for these problems in PAA based cells is identified to be the high internal resistance in the initial cycles, caused by factors such as contact resistance, inhomogeneous binder distribution and poor electrolyte wetting of the active material.

Nano-alumina@cellulose-coated separators with the reinforced-concrete-like structure for high-safety lithium-ion batteries

Nano-alumina@cellulose-coated separators with the reinforced-concrete-like structure for high-safety lithium-ion batteries

Achieving highly stable sodium metal batteries with self-adapting and high-ionic-mobility ceramic fiber membranes

Tough issues like sodium (Na) dendrite growth and poor anode reversibility hinder the practical application of sodium metal batteries (SMBs) with moderate liquid electrolytes. To settle these problems, using a smart self-adapting Al2SiO5 ceramic fiber (CF) membrane is demonstrated to enable homogeneous Na depositions and inhibit the dendritic growth. This inorganic membrane itself has superb thermal stability, high ionic mobility (Na+ transference number: 0.65) and electrolyte wettability over traditional glass fiber (GF) or polymeric ones, guaranteeing the low voltage polarization (14 mV) and long-cyclic lifetime (over 600 h) in symmetric cells testing. Notably, aluminous components in CF membranes would interact with F-based molecules in the electrolyte phase, thereby releasing some Al3+ species that can be electrochemically deposited onto the anodic interface. The packed (+)Na3V2(PO4)3|CF|Na(−) full SMBs exhibit far superior cyclic stability (capacity retention over 78.7 % after 600 cycles at 1C) than other counterparts. The in-situ detection/postmortem analysis reveal that Al/F-based inorganics formed in as-built SEI layers play a vital role in Na metal anode protection. This work may provide a viable strategy to overcome the constraints of high-energy SMBs in practical applications.

Dendrite‐Free Lithium Metal Batteries Enabled by Coordination Chemistry in Polymer‐Ceramic Modified Separators

AbstractIssues with lithium dendrite growth and dead lithium formation limit the practical application of lithium metal batteries, especially under high current conditions where uneven temperature distribution leads to serious safety concerns. Herein, In situ assembly of polydopamine (PDA) and aluminum nitride (AlN) coatings on polypropylene (PP) separator is introduced to address these challenges. The AlN particles are encapsulated by PDA, and the functional groups in PDA form Al‐O coordination bonds with Al3+, which promote uniform Li+ flux and reduce the migration barrier of Li+, thereby enabling dendrite‐free lithium deposition. In addition, the designed PDA@AlN@PP separator exhibits excellent electrolyte wettability, enhanced mechanical performance, and stable thermal resistance, providing a uniform thermal distribution and serving as a robust barrier against dendrite penetration. As a result, symmetric Li||Li cells (over 1800 h at 1 mA cm−2 and 1 mAh cm−2) and Li||Cu cells (over 600 cycles at 0.5 mA cm−2 and 0.5 mAh cm−2, coulombic efficiency over 98%) demonstrate outstanding long cycle performance and high coulombic efficiency. Moreover, the corresponding Li||LiFePO4 cells exhibit a high specific capacity of 91.3 mAh g−1 at 5 C. This work provides a new approach for designing functionalized separators for high‐performance lithium metal batteries.

Enabling Further Organic Electrolyte Infiltration of Cellulose-Based Separators via Defect-Rich Polypyrrole Modification for High Sodium Ion Transport in Sodium Metal Batteries.

Sodium metal batteries (SMBs) have high-density and cost-effective characteristics as one of the energy storage systems, but uncontrollable dendrite growth and poor rate performance still hinder their practical applications. Herein, a nitrogen-rich modified cellulose separator with released abundant ion transport tunnels in organic electrolyte was synthesized by in situ polymerization of polypyrrole, which is based on the high permeability of cellulose in aqueous solution and the interfacial interaction between cellulose and polypyrrole. Meanwhile, the introduction of abundant structural defects such as branch chains, oxygen-containing functional groups, and imine-like structure to disrupt polypyrrole conjugation enables the utilization of conductive polymers in composite separator applications. With the electrolyte affinity surface on, the modified separator exhibits reinforced electrolyte uptake (254%) and extended electrolyte wettability, thereby leading to accelerated ionic conductivity (2.77 mS cm-1) and homogeneous sodium deposition by facilitating the establishment of additional pathways for ion transport. Benefiting from nitrogen-rich groups, the polypyrrole-modified separator demonstrates selective Na+ transport by the data of improved Na+ transference number (0.62). Owing to the above advantages, the battery assembled with the modified separators exhibits outstanding rate performance and prominent capacity retention two times that of the pristine cellulose separator at a high current density under the condition of fluorine-free electrolyte.

Carbon cloth modified by direct growth of nitrogen-doped carbon nanofibers and its utilization as electrode for zero gap flow batteries

The synthetic procedure and characterization of carbon nanofibers (CNFs) grown on carbon cloth (CC) are explored in this study, with a focus on their potential application as electrodes in vanadium redox flow batteries (VRFBs). CC offers an attractive platform for surface modification owing to its conductive properties and three-dimensional architecture, while the N-doped CNFs formed by nitrogen (N) rich composition of melamine precursor enhance wettability of electrolyte and redox reactivity of vanadium ions. Electrochemical assessments reveal that NCC electrodes significantly increase voltage efficiency (VE) and capacity retention in VRFBs compared to bare CC (BCC) electrodes. Notably, NCC demonstrates a VE of 65.9%, surpassing the 55.9% of BCC electrodes. Additionally, NCC maintains superior capacity retention under varying current densities, a crucial factor for VRFBs. Long-term stability tests over 1000 cycles highlight the NCC electrode's durability, with only a minimal decrease in VE. Post-experiment analysis confirms the structural integrity of the CNFs on the CC electrodes, validating their resilience in VRFB operations. In summary, the study introduces a novel approach for fabricating N-doped CNFs on CC, resulting in electrodes that significantly boost VRFB performance in terms of efficiency, capacity retention, and stability, marking a notable advancement in flow battery technology.

Multilayer polyethylene separator with enhanced thermal properties for safe lithium-ion batteries

The separator plays an important part in battery safety and performance. Polyolefin separators are widely used in commercial Lithium-ion batteries (LIBs), owing to their excellent properties, but they suffer from serious thermal shrinkage and poor electrolyte wettability. Thus, a multilayer separator (ASPESA) is developed by coating two thin layers of low-density polyethylene (LDPE) and Al2O3 on both sides of a polyethylene membrane using a facile and environmentally friendly casting technique. The ASPESA separator demonstrates a shutdown function at 120 °C and shows enhanced thermal stability under 185 °C, with a small thermal shrinkage of 1%. Meanwhile, the LDPE and Al2O3 layers can improve the electrolyte wettability and electrolyte uptake (407.23%). The multilayer ASPESA separator delivers an excellent cycle performance in LiFePO4||Li cells with a discharge capacity of 144.5 mAh g−1 after 900 cycles, with a high-capacity retention of 98.9% (compared to the 5th cycle). Therefore, the multilayer ASPESA separator has great utilization potential as a high-safety separator in LIBs.

Electrospun of polyvinyl alcohol composite hydrogel nanofibers prepared by in-situ polymerization: A novel approach to fabricate hydrogel nanofiber membrane for lithium-ion batteries

The increasing demand for high-performance lithium-ion batteries has propelled the exploration of advanced materials to overcome limitations associated with commercially available polyolefin-based separators. This study introduces a pioneering approach involving the synthesis of a crosslinked hydrogel nanofiber membrane (PDs) composed of polyvinyl alcohol (PVA) and N, N-dimethylacrylamide (DMAAm) through the combination of the photo-polymerization with electrospinning process. To address key drawbacks observed in existing separators—namely, low porosity, inadequate thermal stability, and insufficient electrolyte wettability—the porosity of the PDs hydrogel nanofibers was systematically controlled through both heating drying (PD12-HD) and freeze-drying (PD12-FD) methods. The separator’s properties and performance were thoroughly examined, focusing on its chemical, mechanical, thermal, swelling, morphological, conductivity, and electrochemical properties. Comprehensive characterization of the PD12-FD separator revealed remarkable attributes surpassing those of a commercial counterpart (Celgard). The PD12-FD separator demonstrated exceptional mechanical strength (16.3 MPa), robust thermal stability (no shrinkage or deformation at 170 °C), high porosity (84.66 %), substantial electrolyte uptake capacity (672.86 %), superior ionic conductivity (3.405 mS/cm), and high lithium-ion transfer number (tLi+ = 0.678). The improvement is attributed to the cross-linked structure formed between PVA and DMAAm. Importantly, coin cells assembled with the PD12-FD separator exhibited outstanding electrochemical performance. Even after 150 cycles at 1C, the PD12-FD separator maintained a capacity above 95 % (145.48 mAh/g), surpassing both PVA and Celgard separators. This study proposes a novel methodology for fabricating hydrogel nanofiber separators and introduces a customizable synthesis approach for producing separators with enhanced performance and controlled characteristics.

A dendrite-blocking polyimide-meta-aramid separator with ultrahigh strength and thermostability for high-security lithium-ion battery

Commercial polyolefin-based separators broadly applied in lithium-ion batteries (LIBs) seriously affect the safety and development of LIBs due to the poor thermostability and electrolyte wettability caused by its low melting point and non-polar structure. In this work, a simple self-adsorption crosslinking technology is invented to fabricate polyimide-meta-aramid (PI-MA) separators with superior thermostability, wettability and safety. This technology utilizes the hydrogen bond adsorption and MA self-adhesive effect to realize the close encapsulation of MA on the surface of PI nanofiber and form a compact crosslinking structure. Specifically, the PI-MA separator has no significant deformation at 300 °C, while PP has completely melted and contracted at 200 °C. The PI-MA exhibits an electrolyte contact angle as low as 6.1° and the faster electrolyte diffusion rate in both horizontal and vertical directions. Moreover, in the long-term stripping/plating characterization, the lithium symmetric cell with PI-MA exhibits the stable cycle performance at a current density of 2 mA cm−2 for 1000 h. Using LiNi0.8Co0.1Mn0.1O2 (NCM811) as cathode, the cell with PI-MA has a capacity retention rate of 82.1 % in the 300-cycle test at 1C, much higher than 58.3 % of PP separator. Compared with the cell assembled with PP separator exploding at 56 min, the cell with PI-MA discharged stably to the cut-off voltage within 160 min at 120 °C.