Cellulose-based Separator Research Articles

Anti-dissolving cellulose-based separator for dendrite-free rechargeable aluminum batteries

Rechargeable aluminum batteries (RABs) are noteworthy due to their environmental sustainability and cost-effectiveness. However, its practical applications suffer from wilder growth of anode dendrites and greater physicochemical instability of separator triggered by corrosive ionic liquid electrolytes (ILs). This work produces a core-shell structured CCP@TiO2 separator by introducing TiO2 nanolayer onto commercial cellulose paper (CCP) to improve its stability by tackling the challenge of cellulose dissolution in ILs. By combining experimental and simulation studies, CCP@TiO2 separator exhibits excellent anti-dissolving property and interfacial polarization effect, not only provides the stable anion flux and high anion transfer number of 0.29, but also facilitates the uniform distribution of the ions and electric charges at the separator/electrolyte/anode interface, thereby promoting dendrite-free Al deposition. Thus, the assembled graphite//Al battery maintains a higher discharge capacity of 90.3 mAh g−1 after 1000 cycles at 0.2 A g−1, compared to the 82.4 mAh g−1 residual of that with glass fiber (GF) separator. Furthermore, the battery with secondary use CCP@TiO2 separator still maintains 98 % capacity retention. Such separator engineering of insolubility of cellulose in ILs and inhibition of dendrite growth, provides an unparalleled pathway for promising alternative to GF separator and stable operation, sustainable development of RABs.

Tuning the surface charge of rice straw-derived cellulose nanofibril membrane separator for electrochemical performance enhancement of supercapacitors

The electrochemical performance of supercapacitors has often overlooked the effect of surface charge on cellulose-based separators. Cellulose nanofibrils (CNFs) produced by 2,2,6,6-tetramethylpipridine-1-oxyl (TEMPO) oxidation possess a high anionic surface charge due to the presence of carboxylate groups, which could affect the transport of electrolyte ions. In this work, the surface of CNFs is modified with a cationic polyelectrolyte, namely polydiallydimethylammonium chloride (PDADMAC), to yield a nearly-zero surface charge CNF membrane derived from rice straw. The surface modification of CNFs using 20 wt% PDADMAC results in CNF-M2, with a surface charge of +5.3 mV, notable porosity (64 %), excellent electrolyte uptake (225 %), and improved ionic conductivity (5.0 mS cm−1). A symmetric supercapacitor assembled with CNF-M2 as a separator, exhibits enhanced specific capacitance (185.3 F g−1 at 0.1 A g−1), energy density (37.1 Wh kg−1 at a power density of 0.24 kW kg−1), and is able to maintain 100 % capacitance retention over 10,000 cycles in 1.0 M Na2SO4 aqueous electrolyte solution. This surface modification leads to 1.2–1.4 times increase in energy and power densities compared to the unmodified CNF membrane. Thus, the nearly-zero surface charge of the modified CNF membrane holds promise as a separator that elevates the performance of supercapacitors.

Eco-Friendly Lithium Separators: A Frontier Exploration of Cellulose-Based Materials.

Lithium-ion batteries, as an excellent energy storage solution, require continuous innovation in component design to enhance safety and performance. In this review, we delve into the field of eco-friendly lithium-ion battery separators, focusing on the potential of cellulose-based materials as sustainable alternatives to traditional polyolefin separators. Our analysis shows that cellulose materials, with their inherent degradability and renewability, can provide exceptional thermal stability, electrolyte absorption capability, and economic feasibility. We systematically classify and analyze the latest advancements in cellulose-based battery separators, highlighting the critical role of their superior hydrophilicity and mechanical strength in improving ion transport efficiency and reducing internal short circuits. The novelty of this review lies in the comprehensive evaluation of synthesis methods and cost-effectiveness of cellulose-based separators, addressing significant knowledge gaps in the existing literature. We explore production processes and their scalability in detail, and propose innovative modification strategies such as chemical functionalization and nanocomposite integration to significantly enhance separator performance metrics. Our forward-looking discussion predicts the development trajectory of cellulose-based separators, identifying key areas for future research to overcome current challenges and accelerate the commercialization of these green technologies. Looking ahead, cellulose-based separators not only have the potential to meet but also to exceed the benchmarks set by traditional materials, providing compelling solutions for the next generation of lithium-ion batteries.

Cellulose-based separator woven by double-layer-configuration fibers for high S-load Li-S batteries

Lithium-sulfur (Li-S) battery with high theoretical energy density, has been the most promising next-generation energy storage devices. However, the capacity of the high S-loaded cathode decays rapidly due to the shuttling of soluble polysulfide intermediates between the two electrodes, which seriously inhibits its development. In this work, polyacrylonitrile (PAN) and cellulose composite film (PAN@CE) is designed, which is woven by double-layer configuration fiber. The outer layer of fiber is mainly composed of cellulose, and the inner layer is mainly PAN. From DFT calculations and experiments, the double-layer configuration enriched with nitrogen and oxygen groups can effectively inhibit the shuttle of soluble polysulfides. Meanwhile, the interlaced network structure of PAN@CE improves the electrolyte wettability and provides abundance transport paths for ions. In addition, the film also has good thermal stability. Hence, the cycling stability of Li-S batteries with PAN@CE as separator has been greatly improved. Cellulose-based Li-S battery with 9.1 mg S loading and an E/S ratio of 6.2 μL mg−1 is assembled. Its initial area capacity is 5.67 mAh cm−2. After 200 long cycles, the area capacity keeps above 4.0 mAh cm−2.

Facile fabrication of regenerated cellulose-based separators for high-performance lithium-ion batteries by regulating degrees of polymerization

Cellulose-based separators have great application prospects in the field of lithium-ion batteries (LIBs) due to their excellent wettability and thermal stability. However, most current cellulose-based separators come from high-cost nanocellulose and bacterial cellulose. Herein, regenerated cellulose (RC) separators were prepared from dissolving pulp with different degrees of polymerization (DPs) by using the NaOH/urea/thiourea dissolution system as well as a nonsolvent-induced phase separation method. The results showed that the DP of cellulose had a significant influence on the mechanical properties, pore structure, and electrochemical properties of the resultant RC separator. An appropriate increase in the DP could improve the mechanical strength, porosity, and ionic conductivity of the separator. The RC separator with a DP of 599 exhibited the best performance with a porosity of 56.1 %, an average pore size of 305 nm, an electrolyte uptake of 339 %, a tensile strength of 38.3 MPa, and an ionic conductivity of 1.88 mS·cm−1. The lithium-ion battery prepared with the optimal RC separator had a specific capacity of 156.55 mAh/g for 100 cycles at a current density of 0.5 C and a coulombic efficiency of more than 96 %, which was a clear advantage over the commercially available Celgard2400 and cellulose separators. This work makes contributions to the development of high-performance LIBs separators from cellulose.

Cellulose nanofibril separator from Coffea arabica waste for supercapacitor applications

Biomass derived cellulose-based separators has gained popularity in the field of supercapacitors as they are highly capable of preventing short circuiting due to their increased mesoporous nature and these membranes are found to perform well with excellent electrochemical properties. In our research work, we have adopted the 2,2,6,6–tetramethylpiperidine–1–oxyl (TEMPO)-mediated oxidation and developed an innovative cellulose membrane by employing coffee pulp as raw material. The prepared biopolymer separator explicitly showed a necessary porosity of 55% with uniformly distributed pores which facilitates the movement of ions easily without any obstruction. It also showed a high thermal stability at low thermal degradation temperature of 321 ℃ and an increased uptake of electrolyte about 266% with a voluminous value of 3 mS cm−1 for ionic conductivity. These separators are found to be environmentally friendly and they could be processed at minimal cost. When comparing our separators with the commercially available separators our cellulose separators are found to perform well and this is proved through the obtained results and we suggest that it could be an excellent alternative to celgard separator in the field of supercapacitor applications.

Strategy to Increase the Efficiency of Battery Systems Equipped with Cellulose-Based Separators

Strategy to Increase the Efficiency of Battery Systems Equipped with Cellulose-Based Separators

H2O/HF Scavenging Mechanism in Cellulose-Based Separators for Lithium-Ion Batteries with Enhanced Cycle Life.

Lithium-ion batteries (LIBs) are increasingly being integrated into the transportation industry due to their high energy density, durability, and low cost. With the growing demand for transportation and other emerging applications, there is a concurrent rise in concern over LIB material sourcing and recycling. This urges the development of LIBs with extended cycle lifespans. One mechanism of capacity fading in LIBs is the dissolution of transition metals into the electrolyte after the cathode is etched with hydrofluoric acid (HF). HF is readily generated by the hydrolysis of the LIB electrolyte salt, lithium hexafluorophosphate (LiPF6), which makes minimizing moisture in the electrolyte a priority in manufacturing. In this study, a nonwoven, cellulose-based separator (CBS) is introduced as an alternative separator for battery technologies to scavenge residual water and HF from the electrolyte. The CBS is shown to be capable of scavenging varying amounts of water from the electrolyte based on different drying processes of the CBS, and a mechanism for this water scavenging is identified based on the materials present in the CBS. In addition, the chemical and electrochemical performance of the CBS in real battery conditions is investigated. Results suggest an effective H2O/HF scavenging capability in the CBS that allows LIB coin cells to have over 17% higher capacity retention than those with conventional separators. Furthermore, studies of the industrially manufactured, commercially relevant cylindrical and pouch cells show remarkable 761 and 103% improvements in the 60% capacity lifetime, respectively. The environmental friendliness, low cost, and easy application empowered by the cycle life improvements shown in this work make this nonwoven CBS a promising candidate for improving industry-level LIB performance.

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.

Industrial Waste Derived Separators for Zn-Ion Batteries Achieve Homogeneous Zn(002) Deposition Through Low Chemical Affinity Effects.

Designing a cost-effective and multifunctional separator that ensures dendrite-free and stable Zn metal anode remains a significant challenge. Herein, a multifunctional cellulose-based separator is presented consisting of industrial waste-fly ash particles and cellulose nanofiber using a facile solution-coating method. The resulting fly ash-cellulose (FACNF) separators enable a high ion conductivity (5.76 mS cm-1) and low desolvation energy barrier of hydrated Zn2+. These features facilitate fast ion transfer kinetics and inhibit water-induced side reactions. Furthermore, experimental results and theoretical simulations confirm that the presence of fly ash particles in FACNF separators effectively accommodate the preferential deposition of Zn(002) planes, due to the weak chemical affinity between Zn(002) plane and fly ash, to mitigate dendrite formation and growth. Consequently, the utilization of FACNF separators causes an impressive cycling performance in both Zn||Zn symmetric cells (1600h at 2mA cm-2/1 mAh cm-2) and Zn||(NH4)2V10O25 (NVO) full cells (4000 cycles with the capacity retention of 92.1% at 5 A g-1). Furthermore, the assembled pouch cells can steadily support digital thermometer over two months without generating gas and volume expansion. This work provides new insights for achieving crystallographic uniformity in Zn anodes and realizing cost-effective and long-lasting aqueous zinc-ion batteries (AZIBs).

Enhanced ion transport in ultrathin regenerated cellulose supercapacitor separators

This work reports the fabrication of regenerated cellulose-based separators with efficient ion transport properties, high porosity, and excellent electrolyte wettability using vacuum freeze-dried cellulose membranes.

The Effect of Cellulose Separator Water-Scavenging on Cycle Life in Lithium-Ion Batteries

The presence of water inside a lithium ion (Li-ion) battery causes several interconnected chemical mechanisms that lead to material degradation including transition metal dissolution. [1-4]. As a result, cell performance is reduced, and the cell capacity rapidly fades.To mitigate transition metal dissolution caused by trace water, research groups have proposed various approaches to scavenge and neutralize the water within different components of the cell [4-6]. These methods include using a dehydratable molecular sieve within the cathode active material powder [6], direct dosing of the electrolyte with a water scavenging additive [5], and introducing a metal organic framework with water scavenging properties by mixing it with a polymer binder to create a film for use as a separator. Results from these studies show promise in improving the cycling stability of cells under abuse conditions, such as elevated temperature and high water content in the electrolyte. While these water scavenging techniques show clear benefits to cell capacity retention, they come with the trade-off of higher material cost, more complex production processes, and lower energy density, which have limited their widespread adoption in Li-ion batteries.This study focuses on the use of “cellulose”, a cheap, abundant and naturally dehydrating biopolymer, as a separator material. Cellulose-based separators have been used in Li-ion batteries and shown to be advantageous for capacity retention of the cells. The benefits of the cellulose separators have been attributed to their superior wettability, uniform pore size distribution, high porosity, and low electrical resistance [7-10]. Despite their well-known hydrophilicity, their water scavenging capabilities have not been thoroughly evaluated.In this work, we present new insights into the interaction of water with cellulose-based separator. The water scavenging properties of the cellulose separator are investigated both outside of the battery using the Karl-Fischer Coulometric Titration technique, and inside of the battery through cycling tests. As shown in Figure 1, replacing the conventional polymer-based separator with a cellulose-based nonwoven separator resulted in a significant improvement in cycle life. Furthermore, the water scavenging mechanism of cellulose-based nonwoven separator is studied using surface chemistry characterizations, suggesting water scavenging by the naturally occurring hydrogen bonding sites of cellulose. Additional discussion on drying conditions and the impact of other fiber types are also provided.

A facile blow spinning technique for green cellulose acetate/polystyrene composite separator for flexible energy storage devices

The search of sustainable gadgets, such as the portable electronics and wearables, have sparked the need for more sustainable and environment friendly constituent elements (e.g., electrode materials, separators, and green electrolytes) and low-cost scalable fabrication techniques. Herein, a facile and scalable blow spinning technique is proposed for the synthesis of a cellulose-based separator for flexible energy storage devices. A cellulose acetate and polystyrene (CA:PS) based composite separator is synthesized for the first time for flexible supercapacitors by exploiting the blow spinning technique. Different combinations of CA:PS were synthesized, and electrochemical performances of the devices were evaluated. A sweat simulation solution is used as green electrolyte for the development of symmetrical carbon yarn-based supercapacitors. The influence on the device performances of pristine carbon yarn, activated carbon yarns and PEDOT functionalized carbon yarns, electrodes were compared. Specific capacitances of 2.8 Fg−1 and 33 Fg−1 were obtained for pristine carbon and PEDOT functionalized carbon fibers respectively. The fabricated devices exploiting the composite separator exhibited good washing stability up to 30 cycles and capacitance retention of 95% up to 1000 charge/discharge cycles.

“Pore‐Hopping” Ion Transport in Cellulose‐Based Separator Towards High‐Performance Sodium‐Ion Batteries

AbstractSodium‐ion batteries (SIBs) have great potential for large‐scale energy storage. Cellulose is an attractive material for sustainable separators, but some key issues still exist affecting its application. Herein, a cellulose‐based composite separator (CP@PPC) was prepared by immersion curing of cellulose‐based separators (CP) with poly(propylene carbonate) (PPC). With the assistance of PPC, the CP@PPC separator is able to operate the cell stably at high voltages (up to 4.95 V). The “pore‐hopping” ion transport mechanism in CP@PPC opens up extra Na+ migration paths, resulting in a high Na+ transference number (0.613). The separator can also tolerate folding, bending and extreme temperature under certain circumstances. Full cells with CP@PPC reveal one‐up capacity retention (96.97 %) at 2C after 500 cycles compared to cells with CP. The mechanism highlights the merits of electrolyte analogs in separator modification, making a rational design for durable devices in advanced energy storage systems.

"Pore-Hopping" Ion Transport in Cellulose-Based Separator Towards High-Performance Sodium-Ion Batteries.

Sodium-ion batteries (SIBs) have great potential for large-scale energy storage. Cellulose is an attractive material for sustainable separators, but some key issues still exist affecting its application. Herein, a cellulose-based composite separator (CP@PPC) was prepared by immersion curing of cellulose-based separators (CP) with poly(propylene carbonate) (PPC). With the assistance of PPC, the CP@PPC separator is able to operate the cell stably at high voltages (up to 4.95 V). The "pore-hopping" ion transport mechanism in CP@PPC opens up extra Na+ migration paths, resulting in a high Na+ transference number (0.613). The separator can also tolerate folding, bending and extreme temperature under certain circumstances. Full cells with CP@PPC reveal one-up capacity retention (96.97 %) at 2C after 500 cycles compared to cells with CP. The mechanism highlights the merits of electrolyte analogs in separator modification, making a rational design for durable devices in advanced energy storage systems.

Preliminary analysis of the cellulose-based battery separator

The worldwide energy demand is increasing due to the development of advanced technologies in various industries. Therefore, alternative and safe energy sources should be implemented in order to keep the environment clean and sustainable. Aluminum-air batteries have been recognized as a potential energy storage system candidate for the post lithium era. Unfortunately, serious corrosion of the anode in the aqueous aluminum-air battery will reduce the energy density. In this work, a novel idea of using cellulose-based separators for the Al-air battery is proposed to achieve a greener and cheaper battery. It was constructed by applying aluminum plate as an anode, activated carbon as an air–cathode, manganese oxide as electrocatalyst and sea water as the electrolyte. The performance of the self-fabricated water hyacinth cellulose nanofibrils separator is compared with the filter paper. The results showed that the performance of the water hyacinth cellulose nanofibrils separator with polyethylene glycol 400 binder in the ratio of 85 wt%:15 wt% is better than that of the commercial filter paper. The total discharge time is about 307.85 min using 10 mA of discharge current. The cellulose separator can be optimized to make a better aluminum-air battery.

A bacterial cellulose-based separator with tunable pore size for lithium-ion batteries

Bacterial cellulose (BC) lithium-ion batteries separators possess outstanding thermal dimensional stability and electrolyte wettability, but theirs nano diameter and high aspect ratio lead to poor porosity and pore size uniformity of dense BC separators, limiting the Li+ transmission in the separators. In this paper, chitosan (CS) with different molecular weight was grafted onto BC (named OBCS), and a high-performance OBCS separator with excellent pore structure and tunable pore size was prepared by simple suction filtration. The spacing and dispersion uniformity of OBCS were improved by the CS grafted on BC surface, thus improving the pore structure and porosity of OBCS separators. The results showed that the obtained OBCS separators not only have excellent physicochemical properties, but also exhibit higher electrochemical performances than the commercial polypropylene (PP) separator. This work provides a new feasible strategy for improving the pore structure and porosity of nanocellulose separators.

Investigation of Cellulose-Based Separators for Secondary Lithium Metal Batteries

Lithium-ion batteries (LIBs) have been the predominant energy storage technology for a variety of applications such as portable electronic devices and wireless power tools. However, the rising demand for emerging technologies such as long-range electric vehicles and grid-level energy storage and delivery has drastically increased the necessity for low-cost LIBs with enhanced performance and safety.Improvements in the modern LIB technology can be achieved through improvements in different, individual components of the battery. Among the key components of the battery, the separator plays a vital role. To date, polypropylene (PP)/polyethylene (PE) membranes have been used as separators in LIBs due to their desired electrochemical stability. However, these separator materials suffer from low thermal stability, which results in their deformation or decomposition at elevated temperatures upon high charging rates1. A dangerous consequence of this material degradation is an electrical short, leading to an aggressive discharge of the battery and subsequent fire. Moreover, PP/PE separators possess relatively low electrolyte wettability and an expensive and eco-unfriendly fabrication process. Hence, alternative separator materials for future LIB technology are indispensable.Among alternative separator materials, cellulose is a promising candidate2. Cellulose is derived from biomass which is one of the most abundant and renewable resources on Earth. It also is non-toxic and has high mechanical and chemical stability. Additionally, with an initial decomposition temperature of 270°C, cellulose offers a major advantage in thermal stability compared to its polymeric competitors3. The thermal and electrochemical stability, electrolyte wettability, and performance of cellulose-based separators in LIBs have been studied3,4. However, those reports mostly consider the conventional LIB electrode materials– Li transition metal oxide and graphite; and focus on the separator/electrolyte compatibility. Therefore, to be considered for future generations of high-performance Li-based batteries, cellulose-based separators must be investigated in batteries with new electrode chemistries.This work presents new insights on the interaction of cellulose-based separator and metallic Li – the leading candidate for future anodes. Coin cells were prepared using various cathode materials, Li metal anode, and a commercial cellulose-based separator. The cycling performance of the cells was tested at different C rates. Results were compared with the cycling performance of the coin cells with similar electrodes but a commercial PP/PE separator. Comparable discharge profiles were observed in the two groups of cells, but the cellulose separator hampered the charging process. Additional electrochemical analysis suggested an undesired interaction between the cellulose-based separator and metallic Li. To further understand this interaction, various protective coatings on the separator were investigated, the results of which suggest a mechanical degradation in the cellulose separator during cycling and consequently a soft short. These results are expected to provide a new understanding regarding the stability of cellulose-based separators in Li metal-containing batteries, which can help with their implementation in the next generation of Li-based batteries with enhanced performance and safety.

Statistical Approach to Design Zn Particle Size, Shape, and Crystallinity for Alkaline Batteries

The modern alkaline primary battery is composed of a highly alkaline, gelled electrolyte-zinc powder anode dispersion (slurry), a cellulose-based separator, and an electrolytic manganese dioxide (EMD) cathode pellet – typically arranged in a cylindrical cell geometry. This battery system has its origins in the 1950s and thus has had decades to be researched and matured and it is known that the limiting performance of this battery system is concentrated at the slurried zinc anode. This had led many researchers to focus on the anode for improvement with most previous engineering efforts targeting enhanced performance, capacity, and stability of this material. Due to the various material fabrication processes involved with manufacturing industrial zinc powder, there exists an array of unique zinc particle size, shape, and crystallinity combinations that distinguish industrial zinc powders from one another. These industrial zinc powders are typically produced through trial-and-error processes using existing “rules of thumb”. Consequentially, it is possible that something has been missed and a data-driven approach could elucidate the optimum combination of zinc particle properties.In this study, we investigate the effect of Zn particle size, shape, and degree of crystallinity on the achievable capacity and corrosion current. Various types of zinc powders were sieved into size fraction ranges and chemically and thermo-chemically treated to control their crystallinity. The procedure for producing zinc powders of various sizes, shapes, and crystallinities led to a 64-powder Zn matrix that was then electrochemically analyzed by linear sweep voltammetry to determine the corrosion current and galvanostatically discharged to determine the achievable capacity. These experiments were done in a newly-designed, custom-built electroanalytical cell that will be described in the talk. Following the data collection, a 4k factorial statistical analysis was performed. This analysis identified which of the controlling variables have a statistically significant influence on the achievable capacity and corrosion. The statistical model was also able to provide guidance regarding the preferred particle shape. This information can be used to develop actionable procedures for battery manufacturers to create cells that are more stable, longer lasting, and have higher energy densities.

Chemical foaming integrated polydopamine hybridization towards high-performance cellulose-based separators for ultrastable and high-rate lithium metal batteries

High-performance cellulose-based separators are promising owing to biodegradation and electrolyte affinity. Nevertheless, strong hydrogen bonding interactions among cellulose nanofibers usually result in dense rather than porous membranes. Uncontrolled dendrite growth is also an obstacle for application of lithium metal anodes in the batteries. Herein, a novel strategy, i.e. chemical foaming integrated polydopamine hybridization, is developed to induce abundant pores and to improve electrolyte/lithium metal anode interfaces. The resultant cellulose-based separator has a high ionic conductivity of 0.81 mS/cm and can effectively suppress dendrite growth on lithium anodes. The coin cells, assembled by LiFePO4 cathode and lithium anode, behave superior cyclic stability at 5 C, and a high specific capacity of 130.7 m Ahg−1 is retained after 300 cycles.