Advanced and Emerging Negative Electrodes for Li-Ion Capacitors: Pragmatism vs. Performance

Recent advancements in an electrification of transportation sectors and adoption of renewable energies have increased a demand for energy storage devices with both high power density and energy density as current technologies including Li-ion battery (LIB) and electrical double layer capacitor (EDLC) can deliver either high energy or high power density. To bridge the gap between LIB and EDLC, Li-ion capacitors (LICs) was proposed by Amatucci in 2011[1] by employing a LIB-type electrode as an anode and a EDLC-type electrode as a cathode in carbonate based organic electrolytes containing Li-salts. Li-ions are stored in anode via intercalation process and anions are stored on cathode surface via adsorption. LIC is promised to combines the advantages of two entities by delivering high energy density, high power density, long cycle life, and low self-discharge.[2,3] However, the main challenge in LIC is the disparity in charge storage kinetics arising from the different charge storage mechanism in anode and cathode. To increase Li-ion kinetics in LIB-type electrode, many electrode-engineerings including nanostructuring active materials[4], making composites with carbonaceous nanomaterials[5–8], and utilizing electronically conductive sulfides[9] and nitrides[10] showed promising results. However, in spite of the considerable number of studies focusing on increasing power density of LIB-type electrode, only a few reports investigate the effects of electrolytes on the LIC performances.[11,12] The influences of anions in electrolytes on the LIC performance are indisputable as charge storage of the EDLC-type cathode sololy depends on the anion-adsorptions. Herein, this study explores the roles of different anions in Li-salts, including lithium hexfluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), and lithium bis(trifluoromethanesulfonyl)imide (LiC2F6NO4S2, LiTFSI), on the LIC performance. The prepared electrolytes were tested in a LIC consists of spinel lithium titanate anode (Li4Ti5O12, LTO) and activated carbon cathode (AC). The present study aims to highlight the important parameters to be considered when designing an optimized electrolyte system for LIC applications. [1] G.G. Amatucci, F. Badway, A. Du Pasquier, T. Zheng, J. Electrochem. Soc. 148 (2001) A930 [2] K. Naoi, S. Ishimoto, J. Miyamoto, W. Naoi, Energy Environ. Sci. 5 (2012) 9363 [3] W. Zuo, R. Li, C. Zhou, Y. Li, J. Xia, J. Liu, Adv. Sci. 4 (2017) 1 [4] L. Kong, C. Zhang, J. Wang, W. Qiao, L. Ling, D. Long, Sci. Rep. 6 (2016) 21177 [5] H. Kim, M.-Y. Cho, M.-H. Kim, K.-Y. Park, H. Gwon, Y. Lee, K.C. Roh, K. Kang, Adv. Energy Mater. 3 (2013) 1500 [6] J. Wang, H. Li, L. Shen, S. Dong, X. Zhang, RSC Adv. 6 (2016) 71338 [7] L. Yan, X. Rui, G. Chen, W. Xu, G. Zou, H. Luo, Nanoscale. 8 (2016) 8443 [8] Q. Wang, Z.H. Wen, J.H. Li, Adv. Funct. Mater. 16 (2006) 2141 [9] K. Kato, F.N. Sayed, G. Babu, P.M. Ajayan, 2D Mater. 5 (2018) 25016 [10] R. Wang, J. Lang, P. Zhang, Z. Lin, X. Yan, Adv. Adv. Funct. Mater. 25 (2015) 2270 [11] T. Zhang, B. Fuchs, M. Secchiaroli, M. Wohlfahrt-mehrens, S. Dsoke, Electrochim. Acta. 218 (2016) 163 [12] S. Liu, S. Liu, K. Huang, J. Liu, Y. Li, D. Fang, H. Wang, Y. Xia, J. Solid State Electrochem. 16 (2012) 1631

Li ion battery (LIB) and electrochemical capacitor (EC) are considered as the most widely used energy storage systems (ESSs) because they can produce a high energy density or a high power density, but it is a huge challenge to achieve both the demands of a high energy density as well as a high power density on their own. A new hybrid Li ion capacitor (HyLIC), which combines the advantages of LIB and Li ion capacitor (LIC), is proposed. This device can successfully realize a potential match between LIB and LIC and can avoid the excessive depletion of electrolyte during the charge process. The galvanostatic charge-discharge cycling tests reveal that at low current, the HyLIC exhibits a high energy density, while at high current, it demonstrates a high power density. Ragone plot confirms that this device can make a synergetic balance between energy and power and achieve a highest energy density in the power density range of 80 to 300 W kg−1. The cycle life test proves that HyLIC exhibits a good cycle life and an excellent coulombic efficiency. The present study shows that HyLIC, which is capable of achieving a high energy density, a long cycle life and an excellent power density, has the potential to achieve the winning combination of a high energy and power density.

We develop zirconium-templated NiO/NiOOH nanosheets on nickel foam and polypyrrole-embedded in exfoliated carbon fiber cloth as complementary electrodes for an asymmetric battery-type supercapacitor device. We achieve high volumetric energy and power density by the modification of commercially available current collectors (CCs). The modified CCs provide the source of active material, actively participate in the charge storage process, provide a larger surface area for active material loading, need no additional binders or conductive additives, and retain the ability to act as the CC. Nickel foam (NF) CCs are modified by use of a soft-templating/solvothermal treatment to generate NiO/NiOOH nanosheets, where the NF is the source of Ni for the synthesis. Carbon-fiber cloth (CFC) CCs are modified by an electrochemical oxidation/reduction process to generate exfoliated core-shell structures (ECFC). Electropolymerization of pyrrole into the shell structure produces polypyrrole embedded in exfoliated core-shell material (PPy@rECFC). Battery-type supercapacitor devices are produced with NiO/NiOOH@NF and PPy@rECFC as positive and negative electrodes, respectively, to demonstrate the utility of this approach. Volumetric energy densities for the full-cell device are in the range of 2.60-4.12 mWh cm-3 with corresponding power densities in the range of 9.17-425.58 mW cm-3. This is comparable to thin-film lithium-ion batteries (0.3-10 mWh cm-3) and better than some commercial supercapacitors (<1 mWh cm-3).1 The energy and power density is impressive considering that it was calculated using the entire cell volume (active materials, separator, and both CCs). The full-cell device is highly stable, retaining 96% and 88% of capacity after 2000 and 5000 cycles, respectively. These results demonstrate the utility of directly modifying the CCs and suggest a new method to produce high volumetric energy density and power density storage devices.

Micro Li-ion capacitor with activated carbon/graphite configuration for energy storage

In today’s nanoscale regime, energy storage is becoming the primary focus for majority of the world’s and scientific community power. Supercapacitor exhibiting high power density has emerged out as the most promising potential for facilitating the major developments in energy storage. In recent years, the advent of different organic and inorganic nanostructured materials like nano carbons, metal oxides, nanosheets of graphene, and conducting polymers has enabled high-performance-fabricated devices. A review of different carbon-based materials used in the fabrication of electrodes for electrochemical capacitors is presented in this paper. Along with materials used, a brief overview of different types of supercapacitors depending on charge storage mechanism is also been discussed. Materials summary including applications have been provided through the exhaustive analysis of the literature. Keeping nano-architecture electrodes in view, a summary of different technologies considering the integration of metal oxide into carbon nanofibers, carbon fiber papers, graphene/reduced graphene oxide, and SWCNTs/MWCNTS has been presented in this work. The specific capacitance in the range of 40–300 F/g had been reported in the literature for the EDLC (electric double-layer capacitors) supercapacitors. In contrast to this, carbon nanomaterials-based metal-oxides supercapacitors (CNMO-SC) have emerged as the new promising candidate which possess large specific capacitance (> 100 F/g), high energy density, and cost effectiveness. Hence, a review of certain types of carbon nanomaterials has also been reported here.

Three-dimensional porous carbon nanofiber loading MoS2 nanoflake-flowerballs as a high-performance anode material for Li-ion capacitor

Developing dielectric capacitors with both a high power density and a high energy density for application in power electronics has been a long-standing challenge. Glass-ceramics offer the potential of retaining the high relative permittivity of ceramics and at the same time of exhibiting the high dielectric breakdown strength and fast charge/discharge rate of glasses, thus producing concurrently high power and energy densities in a single material. In this work, glass-ceramics are fabricated to achieve simultaneously high power and energy densities, high efficiency, and thermal stability by tuning the glass crystallization process via a suitable nucleating agent and a high oxygen partial pressure. Under the same practical charge-discharge test conditions, the as-prepared glass-ceramics combine the high energy density of ceramics and ultrafast discharge rate of glasses, producing the highest power density among glass- and ceramic-based dielectric materials. This work demonstrates the significant potential of achieving both high power and energy densities in glass-ceramics by optimizing the glass crystallization process.

Potassium pre-inserted K1.04Mn8O16 as cathode materials for aqueous Li-ion and Na-ion hybrid capacitors

The Li-ion capacitor (LIC) is composed of a lithium-doped carbon anode and an activated carbon cathode, which is a half Li-ion battery (LIB) and a half electrochemical double-layer capacitor (EDLC). LICs can achieve much more energy density than EDLC without sacrificing the high power performance advantage of capacitors over batteries. LIC pouch cells were assembled using activated carbon (AC) cathode and hard carbon (HC) + stabilized lithium metal power (SLMP®) anode. Different cathode configurations, various SLMP loadings on HC anode, and two types of separators were investigated to achieve the optimal electrochemical performance of the LIC. Firstly, the cathode binders study suggests that the PTFE binder offers improved energy and power performances for LIC in comparison to PVDF. Secondly, the mass ratio of SLMP to HC is at 1:7 to obtain the optimized electrochemical performance for LIC among all the various studied mass ratios between lithium loading amounts and active anode material. Finally, compared to the separator Celgard PP 3501, cellulose based TF40-30 is proven to be a preferred separator for LIC.

Advanced binder-free electrode based on core–shell nanostructures of mesoporous Co3V2O8-Ni3V2O8 thin layers@porous carbon nanofibers for high-performance and flexible all-solid-state supercapacitors

Li-ion batteries (LIB) are now widely used in various areas such as electric vehicle (EV) or mobile phone. However, there are some issues of LIB like high-cost, safety, and energy density. Therefore, battery that has higher energy density and high power density is strongly required. Dual carbon battery (DCB), which consists of graphitic carbon for both positive and negative electrodes, is attracting much interest because of superior rate property, low cost, and high discharge potential. In this battery, for charge process, Li ions intercalate into anodic graphite, and counter anions in electrolyte also intercalate into cathodic graphite, and for discharge process, de-intercalation is occurred in both electrodes. Therefore, charge and discharge scheme is similar with that of capacitor, however, by using intercalation reaction, much higher energy storage capacity is expected. In our previous study, we investigated dual carbon battery using LiPF6 and demonstrated that discharge is observed at potential from 4.0 to 5V with 90mAh/g-carbon on the full cell and by using the high concentration electrolyte of 3 M LiPF6 in DMC, energy density is also much increased. Since it is reported that intercalation potential increases with increasing ionic size of anion, we investigated electrochemical intercalation property of bis(trifluoromethanesulfonyl) imide (TFSI) anion in this study. Effects of electrolyte on intercalation of TFSI anion into graphitic carbon was investigated and it was found that intercalation of TFSI anion was observed in propylene carbonate (PC) based electrolyte at room temperature. Among the electrolytes examined, the largest intercalation capacity was achieved in PC-EMC(1:1). By using 3MLiTFSI/PC-EMC (1:1), discharge capacity of 110mAh/g was stably sustained over 50 cycles, however, coulombic efficiency is around 90%. Further detail study was performed on intercalated structure of TFSI anion. Comparing with PF6 anion, intercalation potential is almost the same, however, the capacity is larger (90 mAh/g for PF6, and 110 mAh/g for TFSI). Therefore, it was found that the intercalation of TFSI anion is also usable for cathodic reaction in dual carbon battery. Stage structure of intercalated TFSI- is also studied by using in-situ XRD and it was confirmed that the distance of (002) plane of graphitic carbon was expanded by increasing potential suggesting that electrochemical intercalation of TFSI anion is occurred. From the distance of (002) plane, stage 1 structure seems to be observed at 5.2V with 0.79 nm distance which is little larger than that of PF6- (0.77nm). Although expansion of electrode is little larger, TFSI anion is also promising anion for dual carbon battery from high potential plateau (5V) and large capacity (110mAh/g).

Functionalized interfacial cover design toward pure silicon anode for high power density lithium–ion capacitor

The high demand on fast rechargeable batteries and supercapacitors combined with the limited resources of their active materials (e.g. Li and Co) motivate the exploration of sustainable energy storage systems such as Zn-ion hybrid supercapacitors. MXenes are two-dimensional materials with outstanding properties such as high conductivity and capacitance which enhance their performance in energy storage devices. Herein, we report on the use of freestanding Mo1.33CTz–Ti3C2Tz mixed MXene films in Zn-ion hybrid supercapacitors. The mixed MXene films are prepared from pristine MXene suspensions using a one-step vacuum filtration approach. The mixed MXene delivers capacities of about 159 and 59 mAh/g at scan rates of 0.5 and 100 mV/s, respectively. These capacity values are higher than the pristine MXene films and previously reported values for MXene electrodes in Zn-ion supercapacitors. Furthermore, the electrodes offer a promising capacity retention of about 90% after 8,000 cycles. In addition, the mixed MXene features energy densities of about 103 and 38 Wh/kg at power densities of 0.143 and 10.6 kW/kg, respectively. Insights into the effect of electrode thickness on rate performance and the mechanism of charge storage are also discussed. This study opens a venue for the use of Mo1.33CTz–Ti3C2Tz mixed MXene electrodes in sustainable energy storage systems with high energy density and power density.

Most of the research in development of next generation and high-performance Li-ion batteries has focused on active materials (either in the cathode or anode side). This might sound obvious as active materials are the main component responsible for storing charge in batteries, and therefore, any improvement in them, will directly impact the overall performance of the battery. However, when looking at the cell level, passive components such as current collectors, conductive additives, binders, and separators constitute around 20 wt.% of each battery cell. These so-called passive components while do not directly contribute to the charge storage, their ratio, composition, and structure has a profound effect on the overall performance of the active material and therefore the battery cell. This is mainly due to their importance in charge transport inside the electrode, processing of electrodes, and also cycle life the battery cells. Despite this, they usually receive less attention in development of high power and energy density batteries. If these components are engineered properly or substitute with alternative new materials, potentially, one can improve the performance (energy and power metrics) of a Li-ion battery cell by up to 20%; a number that industry still strives to achieve through modification of active materials which usually is a more complex issue with slower development timeframes. In this talk, we take a closer look at the role of passive components in Li-ion batteries and discuss the possibilities that novel nanomaterials can offer as next generation passive components, both from the standpoint of improvements in metrics of battery cells as well as electrode processing and manufacturing in Li-ion batteries. We will also discuss the cost and safety advantages these materials can offer when used as passive components of the cells and present results and give examples that demonstrate how nanomaterials can in fact act as semi-passive components where they can substitute multiple conventionally used passive components and serve multiple purposes even if directly do not contribute to charge storage. Taking such an approach in development of battery electrodes/cells can potentially pave the way for development of batteries with higher power and energy densities using the current commercially available active materials, and further enable the use of high-capacity materials that degrade quickly and therefore, cannot be used in commercial cells using current technologies.

Fast development of portable electronic devices (such as a smart phone, iPad, and Galaxy Note) and electrified vehicles (such as EVs and HEVs) require better and smaller lithium ion batteries as their energy storages. What is needed for the cathode of the lithium ion battery is a material capable of a higher volumetric energy density as well as gravimetric energy density. However, not only the development of cathode material but also the commercialization of high energy density anode material has been limited due to cathode material’s much lower energy density compared to anode material. In this regard, Li-rich material (Li2MnO3-LiMO2, M=Ni, Mn and Co) that has 150 % higher energy density than commercialized LiCoO2 is the unique next generation cathode material to solve the problem of cathode material’s low energy density. In the past decade, many researches have focused on the surface stabilization method of this material to improve its intrinsic problems of poor cycle/ rate capability and such surface treatment methods have been demonstrated in a few examples, including AlF3 coating and spinel heterostructures, yielding noticeable improvements in rate and cycle ability. Although such surface treatments showed improved rate and cycle performances, achieving the high volumetric energy density and long-term cycle life which are more critical factors for the commercialization of Li-rich material still remained unsatisfied. In general, Li-rich material’s reversible capacity is related with its particle size since the Li2MnO3 phase in large Li-rich material, which is a major component to realize high energy density, is not fully activated in low voltage range below 4.6 V. For this reason, many previous papers reported their results with only small primary particle size below 100nm. The high surface area induced by the small particle size increases side reactions with electrolyte resulting poor long-term cycle life as well as lowers the volumetric energy density. Note that Li-rich material’s deteriorations are generated from its surface and continue to affect its cycling capability and the deteriorations can be divided into two mechanisms: one is side reactions with electrolyte, and another is the phase collapse. The side reactions with electrolyte at its high working voltage yield active material attack speiceses such as HF and electrolyte exhaustion causing Li-rich material’s fast discharge capacity decay during cycles. Furthurmore, it is hardly show long-term cycle life with over 200 cycles since the phase transition from layered structure to spinel-like and NiO rock salt is generated on the surface of active material, and followed by expention of this transition into the bulk. For instance, AlF3 coated Li-rich LNCMO has abrupt capacity fade after 100 cycles, although it showed good rate and cycle capability within 100 cycles. Hence, new active material design, rather than simple surface modification methods such as coating or doping, is required to solve above previous limits.The new material design needs to start with a simple question: how to minimize the damages of surface deteriorations such as side reasction with electrolyte and phase trasnsition on bulk material and maximize volumetric energy density. The most effective solution for the question is increasing the primary particle size with the stable activation method, since the volumetric energy density of cathode material is maximized from synthesis of micron secondary particles consisted of primary particles by using co-precipitation method in industry. Here we demonstrate a novel approach for lithium storage, which is a material design of a secondary structure which consists of large flake shaped primary particle (hundred nanometers x mictrometers) with a novel activation method using simple chemical treatment to achieve superior long-term cycle life with high volumetric energy density. In this design, the large primary particle effectively reduced its surface area producing markedly decreased surface instability reaction as well as high tap density. Interstingly, the chemical approach activated only surface Li2MnO3 phase of large primary particle and the very surface activation effectively overcame the activation problem, which is limit of large primary particle have. This novel concept is very meaningful in that it is the first and unique method to achieve cathode material’s high volumetric energy density with long-term cycle life. As a result, this novel designed material affords remarkable battery performance with extremely high cycle retention during 400 cycles and high volumetric energy density ever reported before.