Gel Polymer Electrolyte Membrane Research Articles

Highly efficient suppression of zincate ion crossover in zinc–air batteries using selective membrane PVA-KOH/ZIF-8 gel polymer electrolytes

Rechargeable zinc–air batteries have attracted considerable attention in the industries due to their high energy density, affordability, and safety. However, these batteries still have limitations in their lifetime because of the issue of zincate ion (Zn(OH)42−) crossover. To suppress this issue, a new PVA-KOH-based gel polymer electrolyte membrane incorporating zeolitic imidazolate framework-8 (ZIF-8) nanoparticles was developed as a selective membrane. The influence of different-sized ZIF-8 nanoparticles synthesized with three different zinc salts on thermal, physical, and electrochemical properties was investigated. The PVA-KOH/ZIF-8 membrane significantly reduced the diffusion coefficient of zincate ions to 3.27 × 10−6, 2.01 × 10−6, and 1.99 × 10−6 cm2 min−1 by the incorporation of ZIF-8, synthesized with Zn(NO3)2, Zn(OAc)2, and ZnSO4, respectively. Because of the pore apertures of ZIF-8 that are equivalent to Zn(OH)42−, the PVA-KOH/ZIF-8 selective membrane could block Zn(OH)42−and allowed OH− to pass through the membrane. Moreover, a large external hydrophilic surface was achieved by the influence of smaller-sized particles, which improved the retention of electrolytes. As a result, the ionic conductivity of the PVA-KOH/ZIF-8 membrane increased up to 164.27 mS cm−1 when added 10 wt% ZIF-8, synthesized by ZnSO4. Furthermore, this membrane showed the highest value of battery life cycle of 89 cycles at a current density 10 mA cm−2 and the discharge capacity of 396 mAh g−1. These results indicated that the development of a PVA-KOH/ZIF-8 membrane has a high potential and suitability for use as a new class of separators in rechargeable zinc–air batteries.

Controlling the Potential of Affordable Quasi‐Solid Composite Gel Polymer Electrolytes for High‐Voltage Lithium‐Ion Batteries

AbstractThis study focuses on the development of a composite gel polymer electrolyte membrane (CGPEM) as a solution to address safety concerns arising from the reactivity of lithium metal and the formation of dendrites. The CGPEM integrates a solid polymer matrix, solid‐electrolyte LSiPS (Li10SiP2S12), with a plasticizer that countering the performance decline caused by sulfide solid electrolyte (SSE) interactions with the cathode. Poly ethylene oxide (PEO) emerges as a promising polymer matrix due to its flexibility, cost‐effectiveness, eco‐friendliness, solvability for Li‐salt, mechanical processing adaptability, adhesive strength, and ionic conductivity. Conductivity and processability of CGPEM were optimized through meticulous adjustment of liquid plasticizer concentration. The CGPEM′s chemical and electrochemical stability were systematically investigated using in‐situ electrochemical impedance spectroscopy (EIS) and distribution of relaxation times (DRTs). A lithium metal battery is constructed against a high voltage cathode and newly developed CGPEM. Impressively, the cell exhibited outstanding performance, maintaining a discharge capacity of around 146.22 mAh/g after 200 cycles, retaining 86.38 % of its initial capacity. The formation of a LiF‐rich interface layer near the lithium surface, a vital element in curbing CGPE degradation and dendritic growth, resulted in enhanced overall cell performance.

Mo-MXene-filled gel polymer electrolyte for high-performance quasi-solid-state zinc metal batteries

The production of MXenes from the MAX phase typically requires hydrofluoric acid etching, a process with significant toxicity and severe environmental implications. Given the wide-ranging applications of MXenes, developing a large-scale, environmentally benign chemical etching process is highly desirable. The goal of this study was to establish selective electrochemical aluminum etching (E-etching) from the Mo3AlC2 MAX phase using an innovative 1-ethyl-3-methylimidazolium bis(trifluoromethyl sulfonyl)imide (EMITFSI) and zinc trifluoromethane sulfonate (Zn(OTF)2) battery electrolyte formulation. The E-etching technique effectively removes the “A” layer from the Mo3AlC2 MAX phase at 60 °C using a TFSI−-based electrolyte. This method demonstrates that the resulting MXene terminal (Tx) contains O and OH groups. Subsequently, the resulting Mo2CTx MXene was embedded into a gel polymer electrolyte (GPE) as an inorganic filler. The Mo2CTx/EMITFSI/Zn(OTF)2/poly(vinylidene fluoride-co-hexafluoropropylene) (PVHF) GPE membrane displayed excellent ionic conductivity, Zn2+ transference number, and galvanostatic Zn plating/stripping compatibility with Zn anodes. Finally, the performance of a zinc-metal battery using a belt-like CaV6O16⋅3H2O cathode was evaluated as a full-cell assembly of CaV6O16⋅3H2O//Mo2CTx/EMITFSI/Zn(OTF)2/PVHF//Zn. This assembly showcased an impressive capacity of 155 mA h g−1, excellent durability (99%), coulombic efficiency approaching 100%, and excellent self-life over 200 h.

A promising approach to prepare gel electrolyte and electrocatalyst derived from natural long-staple cotton for high-performance flexible Zn-air battery

Due to the high energy density and risk-free operation, flexible zinc–air batteries (ZABs) have the potential to be energy sources for wearable electronic devices. Nonetheless, the performance of quasi-solid-state gel electrolytes diminishes progressively throughout the ion-exchange process, primarily due to water evaporation on the exposed cathode side, which shortens the useful lifetime of ZABs. Herein, we propose a distinctive gel polymer electrolyte membrane comprising cotton fiber (CF) from natural long-staple cotton and polyvinyl acetate (PVA) to effectively extend the battery lifespan, which is due to that the addition of CF improves surface wettability, liquid absorption and retention, and ionic conductivity (1.53 × 10−2 S cm−1) compared with single PVA electrolyte membrane. We also prepare a composite electrocatalyst (Co-N-C@GCF-950) by pyrolyzing the precursor of ZIF-67 in situ grown on the cotton fiber at 950 ℃. The flexible ZAB assembled with Co-N-C@GCF-950 as the cathode electrocatalyst and the CF-PVA membrane as the gel electrolyte shows a high open-circuit voltage of 1.5 V, and a stable charge-discharge voltage gap of 0.8 V for 11-h lifespan at a current density of 2 mA cm−2 under various bending conditions, which guides the future way of the application potential of environmental cotton fibers in the multiaspects for flexible metal-air batteries.

Multifunctional electrospun PVDF-HFP gel polymer electrolyte membrane suppresses dendrite growth in anode-free li metal battery

It is widely accepted that high electrolyte consumption due to decomposition leads to rapid capacity fading in anode-free Li metal batteries (AFLMBs). Here, we report an electrospun gel polymer electrolyte (GPE) membrane, unpeeled-off and integrated with its Cu foil collector (Cu@GPE), prepared using a novel method. The Cu@GPE provides bifunctional applications; as electrolytes and an artificial solid-electrolyte interphase (ASEI). Interestingly, the strong electrostatic interaction between the fiber sheet and the Cu foil collector reconstructs the Cu surface structure, confirmed by grazing angle X-ray diffraction, which positively influences the fluidity of deposited Li. X-ray absorption, depth profile of X-ray photoelectron, Raman and infrared spectroscopies analyses showed the formation of pure β-phase PVDF-HFP as ASEI in which the F elements are aligned toward the Cu surface. These lead to a uniform Li-ion flux and regulate the Li nucleation due to the strong binding between Li and electronegative C-F functional groups promoting uniform Li deposition. The Cu@GPE|Li cell achieved outstanding performance even at a high current density of up to 5 mA cm−2 (coulombic efficiency, 97.14%) after 200 cycles relative to the GPE prepared by the conventional method (cGPE) (90.08%) after 100 cycles. Coupling with LiNi0.33Mn0.33Co0.33O2 (NMC), the Cu@GPE|NMC AFLMB delivered superior performance than Cu|cGPE|NMC.

Effect of polymer blending on the electrochemical properties of porous PVDF/PMMA membrane immobilized with organic solvent based liquid electrolyte

Abstract In this paper, experimental studies on blend gel polymer electrolyte membranes comprising of poly(vinylidene fluoride) (PVDF) and poly(methyl methacrylate) (PMMA) saturated with 0.1 M liquid electrolyte of sodium tetrafluoroborate (NaBF4) in the mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) are presented. Membranes are prepared by phase inversion technique. The effect of blending on the ionic conductivity, electrochemical stability window, ionic transference number, and cation transport number has been investigated using complex impedance spectroscopy, linear sweep voltammetry and DC polarization technique. Ion dynamics in the electrolyte membranes has also been investigated using dielectric studies. The optimized electrolyte membrane with composition PVDF:PMMA (95:5) + 0.1 M NaBF4 + EC + DEC shows highest ionic conductivity of 0.6 mS cm−1 which follows Vogel–Tamman–Fulcher (VTF) behavior with temperature. The membrane shows an electrochemical stability window of 3.5 V and sodium ion transport number as ∼0.33.

Anion Receptor Enhanced Li Ion Transportation for High-Performance Lithium Metal Batteries.

High-potential lithium metal batteries (LMBs) are still facing many challenges, such as the growth of lithium (Li) dendrites and resultant safety hazards, low-rate capabilities, etc. To this end, electrolyte engineering is believed to be a feasible strategy and interests many researchers. In this work, a novel gel polymer electrolyte membrane, which is composed of polyethyleneimine (PEI)/poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) cross-linked membrane and electrolyte (PPCM GPE), is prepared successfully. Due to the fact that the amine groups on PEI molecular chains can provide the rich anion receptors and strongly pin the anions of electrolytes and thus confine the movement of anions, our designed PPCM GPE owns a high Li+ transference number (0.70) and finally contributes to the uniform Li+ deposition and inhibits the growth of Li dendrites. In addition, the cells with PPCM GPE as a separator behave the impressive electrochemical performances, i.e., a low overpotential and an ultralong and stable cycling performance in Li∥Li cells, a low overvoltage of about 34 mV after a stable cycling for 400 h even at a high current density of 5 mA/cm2, and, in Li∥LFP full batteries, a specific capacity of 78 mAh/g after 250 cycles at a 5 C rate. These excellent results suggest a potential application of our PPCM GPE in developing high-energy-density LMBs.

Flexible Aluminum-Air Battery Based on Ionic Liquid-Gel Polymer Electrolyte.

There is an urgent demand to develop high-performance flexible batteries for a wide range of contemporary emerging fields, including flexible electronics, wearable sensors, and implantable medical devices. However, the inherent safety and stability issues of traditional organic liquid-based electrolytes make their application in flexible batteries unsatisfactory. Therefore, exploring gel electrolytes with superior ionic conductivity and safety is considered to be the key to the development of flexible batteries. In this paper, two types of high-quality ionic liquid-based gel polymer electrolyte membranes (PVDF-ILs) are created by a conventional solution-casting method, which are further integrated into flexible aluminum-air batteries to guide the interface and process research, and the related discharge properties of two ionic liquid-based electrolyte membrane (PVDF-[C4mpyr]Cl, PVDF-[BMIM]Cl) in different bending states are discussed. The results show that PVDF-ILs have a rich pore structure and interwoven skeleton network, leading to relatively high ionic conductivity (2.97 × 10-3 S cm-1). Moreover, two types of batteries can meet the needs of flexibility, although there is a slight loss of power density under various bending conditions. In general, a PVDF-[C4mpyr]Cl-based flexible aluminum-air battery is suitable for the working conditions of high power and low bending angle, while the PVDF-[BMIM]Cl-based flexible aluminum-air battery is favored for microwatt low-power devices with high flexibility requirements.

LiNi0.5Mn1.5O4-Hybridized Gel Polymer Cathode and Gel Polymer Electrolyte Containing a Sulfolane-Based Highly Concentrated Electrolyte for the Fabrication of a 5 V Class of Flexible Lithium Batteries.

The design and fabrication of lithium secondary batteries with a high energy density and shape flexibility are essential for flexible and wearable electronics. In this study, we fabricated a high-voltage (5 V class) flexible lithium polymer battery using a lithium nickel manganese oxide (LiNi0.5Mn1.5O4) cathode. A LiNi0.5Mn1.5O4-hybridized gel polymer cathode (GPC) and a gel polymer electrolyte (GPE) membrane, both containing a sulfolane (SL)-based highly concentrated electrolyte (HCE), enabled the fabrication of a polymer battery by simple lamination with a metallic lithium anode, where the injection of the electrolyte solution was not required. GPC with high flexibility has a hierarchically continuous three-dimensional porous architecture, which is advantageous for forming continuous ion-conduction paths. The GPE membrane has significant ionic conductivity enough for reliable capacity delivery. Therefore, the fabricated lithium polymer pouch cells demonstrated excellent capacity retention under continuous deformation conditions. This study provides a promising strategy for the fabrication of scalable and flexible 5 V class batteries using GPC and GPE containing SL-based HCE.

Tough and redox-mediated alkaline gel polymer electrolyte membrane for flexible supercapacitor with high energy density and low temperature resistance

To meet the practical application requirements of the flexible supercapacitors, the improvements of the toughness and energy density attract more and more attention. Herein, a physically cross-linked double network alkaline gel polymer electrolyte (AGPE) membrane based on PVA and kappa-carrageenan is developed. In this AGPE membrane, KOH plays a dual role including carrier donator and ion cross-linker, which makes the AGPE membrane show the unique improvement of ionic conductivity (0.31 S/cm) and mechanical properties (tensile strength of 1.62 MPa and tensile elongation of 670%) at the same time. In order to improve energy density, the redox-active mediator p-phenylene diamine has been introduced in the AGPE membrane, endowing the fabricated supercapacitor with high electrode specific capacitance (741 F/g) and energy density (18.3 Wh/kg) with a maximum power density of 800 W/kg. Meanwhile, the supercapacitor can exhibit great flexibility and toughness, and maintain stable capacitance performance under bending and stretching state. Moreover, the supercapacitor shows an excellent low temperature resistance and it is able to maintain 82% of its room-temperature capacitance even at -40 °C. This investigation offers a strategy to prepare gel polymer electrolyte membrane with high adaptive ability of deformation and low temperature, which has potential application in various flexible, portable and wearable electronics. • Dual role KOH can improve conductivity and mechanical properties simultaneously. • PPD can signally increase the capacitance and energy density of EDLC supercapacitor. • Supercapacitor shows ideal electrochemical properties under deformation and coldness.

Cardanol-Derived Epoxy Resins as Biobased Gel Polymer Electrolytes for Potassium-Ion Conduction.

In this study, biobased gel polymer electrolyte (GPE) membranes were developed via the esterification reaction of a cardanol-based epoxy resin with glutaric anhydride, succinic anhydride, and hexahydro-4-methylphthalic anhydride. Nonisothermal differential scanning calorimetry was used to assess the optimal curing time and temperature of the formulations, evidencing a process activation energy of ∼65–70 kJ mol–1. A rubbery plateau modulus of 0.65–0.78 MPa and a crosslinking density of 2 × 10–4 mol cm–3 were found through dynamic mechanical analysis. Based on these characteristics, such biobased membranes were tested for applicability as GPEs for potassium-ion batteries (KIBs), showing an excellent electrochemical stability toward potassium metal in the −0.2–5 V voltage range and suitable ionic conductivity (10–3 S cm–1) at room temperature. This study demonstrates the practical viability of these biobased materials as efficient GPEs for the fabrication of KIBs, paving the path to increased sustainability in the field of next-generation battery technologies.

Shielding the electrostatic attraction by design of zwitterionic single ion conducting polymer electrolyte with high dielectric constant

According to Coulomb's law, besides the design of bulky anion, increase in dielectric constant can also reduce the electrostatic attraction force, both of which are benefit for ion dissociation. Here, we report a zwitterionic single ion conducting polymer electrolyte by chemically grafting two propanesulfonyl(trifluoromethanesulfonyl)imide groups to the nitrogen of poly(biphenyl piperidium) (PBP-g-Li(PSI)2). One of side-chain groups burdens negative charge to counterbalance the positive charge of quaternary ammonium of main chain, and the other one provides lithium ion to conduct charge. Poly(biphenyl piperidinium) grafted with one lithium propanesulfonyl(trifluoromethanesulfonyl)imide (PBP-g-LiPSI) is synthesized for contrast. It is discovered that the dielectric constant of PBP-g-Li(PSI)2 is 22.3 at 2 MHz which is 3.5 times that of PBP-g-LiPSI, and the ionic conductivity of PBP-g-Li(PSI)2 gel polymer electrolyte membrane is 2.41 × 10−4 S cm−1 at 25 °C, nearly twice that of PBP-g-LiPSI gel polymer electrolyte membrane. The dipole structure of bonded bis(sulfonyl)imide anion and quaternary ammonium cation can shield the electrostatic attraction between lithium cation and immobilized anion as confirmed by Li 1s XPS and 7Li NMR. Zwitterionic polymer electrolyte not only withstands galvanostatic cycling in lithium symmetric cell at ±0.5 mA cm−2@2 mAh cm−2 for 2000 h, but also performs stably in lithium metal secondary battery.

Materials for Metal-Air Batteries

Metal-air batteries have received significant attention lately due to their inherent safety, potentially high capacity and use of relatively inexpensive materials. We will present results from our work on a Zn-air battery using novel gel polymer electrolyte membranes and metal organic frameworks as masking agents for the negative electrode. The membrane enhances capacity due to higher ionic conductivity and water retention ability, while the metal organic framework improves cyclic stability of the Zn air battery. In this presentation, we will discuss the use of Polyacrylic acid/Polyacrylamide gel polymer electrolyte and MOF/Zn negative electrode.

Studies on ionic liquid based nanocomposite gel polymer electrolyte and its application in sodium battery

A nanocomposite gel polymer electrolyte system comprising porous membrane of poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP)/TiO2 immobilizing a liquid electrolyte of sodium trifluoromethanesulfonate (NaCF3SO3) in 1-butyl-3-methylimidazolium trifluoromethanesulfonate (BMImCF3SO3) ionic liquid is reported in the paper. The porous gel polymer electrolyte membranes are prepared by phase-inversion technique. The electrolyte membranes are characterized by various physical and electrochemical techniques. The TiO2 nanoparticles display interaction with the anion (CF3SO3−) of the liquid electrolyte and enhance amorphicity in the polymer network. The optimized membrane displays maximum room temperature ionic conductivity as ~0.4 mS cm−1 with improved electrochemical stability window of ~4.3 V and Na+ ion transport number as ~0.27. A proto-type sodium battery using the optimized membrane is fabricated and characterized. The battery delivers a stable open circuit potential of ~2.2 V and shows a discharge capacity ~200 mA h g−1 at 25 mA g−1 drain current.

Optimization and Preparation of a Gel Polymer Electrolyte Membrane for Supercapacitors

AbstractSupercapacitors have great potential in the hybrid fuel automobile market because of their high power density and fast charge and discharge, in which the working electrolyte of supercapacitors is the key to determine their performance. Optimization and preparations of poly(vinylidene difluoride)‐hexafluoropropylene/poly(methyl methacrylate) (PVDF‐HFP/PMMA) gel polymer electrolyte membranes for supercapacitors based on Design Expert 8.0 software are described. The range of conditions is determined by monofactor analysis. The optimal preparation conditions are obtained by response surface simulation optimization.

A study of low-temperature solid-state supercapacitors based on Al-ion conducting polymer electrolyte and graphene electrodes

Gel electrolytes currently draw considerable interest for flexible supercapacitors. Conventional hydrogel electrolytes find limited suitability at low/high temperatures as they contain immense water. This hinders their application in supercapacitors under a harsh environment. Herein, a novel gel polymer electrolyte (GPE) membrane based on the PVdF-HFP/EMITf/Al(Tf)3 system is prepared. The optimized GPE membrane exhibits a high ionic conductivity of ~1.6 × 10−3 S cm−1 at room-temperature with a high value of ~0.8 × 10−3 S cm−1 retained at a freezing temperature of −20 °C. The prepared GPE membrane also offers a wide electrochemical stability window (~5.6–4.2 V) in the temperature range of −20 to 60 °C. The supercapacitor cells designed with GPE membrane and graphene nano-platelet electrode display excellent capacitive performance (323.9 F g−1 at 2 V) and cycling stability (over 50000 cycles) at room-temperature. At −20 °C, the supercapacitor cells still maintain promising capacitive performance and outstanding cycling stability. Moreover, the designed flexible supercapacitors also offer remarkable performance under various bending conditions and maintain low-temperature tolerance. Consequently, it is believed that the low-temperature tolerance GPE membrane based on the PVdF-HFP/EMITf/Al(Tf)3 system possesses potential applications in flexible supercapacitors under harsh environments.

Gel Polymer Electrolyte Membranes Boosted with Sodium-Conductive β-Alumina Nanoparticles: Application for Na-Ion Batteries

Electrolytes, the crucial and indispensable component of a battery system, dominate the interfacial chemistry and the overall performances of the battery. In this work, a poly(vinylidene fluoride-c...

Defect Electrocatalysts and Alkaline Electrolyte Membranes in Solid-State Zinc-Air Batteries: Recent Advances, Challenges, and Future Perspectives.

Rechargeable zinc-air batteries (ZABs) have attracted much attention due to their promising capability for offering high energy density while maintaining a long operational lifetime. One of the biggest challenges in developing all-solid-state ZABs is to design suitable bifunctional air-electrodes, which can efficiently catalyze the key oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) electrochemical processes. The other one is to develop robust electrolyte membranes with high ionic conductivity and superb water retention capability. In this review, an in-depth discussion of the challenges, mechanisms, and design strategies for the defect electrocatalyst and the electrolyte membrane in all-solid-state ZABs will be offered. In particular, the crucial defect engineering strategies to tune the ORR/OER catalysts are summarized, including direct controllable strategies: 1) atomically dispersed metal sites control, 2) vacancy defects control, and 3) lattice-strain control, and the indirect strategies: 4) crystallographic structure control and 5) metal-carbon support interaction control. Moreover, the most recent progress in designing electrolyte membranes, including polyvinyl alcohol-based membranes and gel polymer electrolyte membranes, is presented. Finally, the perspectives are proposed for rational design and fabrication of the desired air electrode and electrolyte membrane to improve the performance and prolong the lifetime of all-solid-state ZABs.

Development of a thin flexible Li battery design with a new gel polymer electrolyte operating at room temperature

In this study, we develop a new Li-metal battery design merging with IoT requirements, mainly the low thickness, the thermal stability and the flexibility. To reach these specifications, we firstly prepared an efficient gel polymer electrolyte (GPE) composed of a PVDF-HFP polymer network, a LiFSI: Pyr13FSI liquid binary solution and a lithium montmorillonite Li-MMT clay. The as-synthesized material exhibits a high ionic conductivity (0.48 mS cm−1 at 25 °C) and a good thermal stability, up to 140 °C. In parallel, a new battery design with an optimized ratio of packaging to active material thickness is developed. In this design, copper foils act both as current collector and as battery casing, decreasing the overall cell thickness. Li metal batteries are realized using the developed GPE material and this new battery design. The cell thickness is 360 and 760 μm for single side and double-sided architectures respectively. These batteries show well functioning under high bending and exhibit a good cycling ability with a remaining capacity higher than 85% after more than 200 cycles at 25 °C. Thanks to the combination of the original Cu packaging and the flexible GPE membrane, developed Li-metal batteries exhibit promising properties to merge with the new IoT requirements.

Ultrathin, Compacted Gel Polymer Electrolytes Enable High‐Energy and Stable‐Cycling 4 V Lithium‐Metal Batteries

AbstractBatteries with lithium metal as anodes and gel polymer membranes as electrolytes are promising, because of their high energy densities and improved safety. To achieve metal battery energy densities that are comparable to liquid‐electrolyte batteries, ultrathin and lightweight gel electrolytes with wide electrochemical stability windows are desired. However, it is challenging to make gel polymer electrolyte membranes with comparable thicknesses to commercial polymer electrolyte separators (<20 μm), because of the porous configuration and increased risk of short‐circuiting cells. Here, we report on a 10 μm‐thick compacted gel polymer electrolyte (TCGPE), demonstrated to have a robust chemical interaction between SiO2 nanoparticles and poly(vinylidene fluoride‐co‐hexafluoropropylene) (PVDF‐HFP) chains that are activated via an in situ sol‐gel reaction. The developed TCGPE exhibits an extremely wide electrochemical stability window of 5.5 V vs. Li/Li+ and prevents the batteries from short‐circuiting even at a high current density of 1.0 mA cm−2. Moreover, Li/TCGPE/LiNi0.5Co0.2Mn0.3O2 shows an excellent capacity retention of 99.8 % after 180 cycles at 1 C rate.