C-rate Performance Research Articles

A Novel Self-Binding Composite Separator Based on Poly(tetrafluoroethylene) Coating for Li-Ion Batteries.

In this study, a novel composite separator based on polytetrafluoroethylene (PTFE) coating layers and a commercial polyethylene (PE) separator is developed for high performance Li-ion batteries. This composite separator is prepared by immersing a PE separator directly into a commercial PTFE suspension to obtain a self-binding PTFE/PE/PTFE tri-layered structure. Then, the as-prepared composite separator is further treated with a H2O2/H2SO4 solution to enhance its electrolyte affinity. The results show that the coating layer, consisting of close-packed PTFE particles, possesses a highly ordered nano-porous structure and an excellent electrolyte wettability property, which significantly enhance the ionic conductivity of the composite separator. Due to the presence of the PTFE-based coating layer, the composite separator exhibits better thermal stability compared with the PE separator, reaching the thermal-resistant grade of commercial ceramic-coated separators. By using different separators, CR2032-type unit half-cells composed of a Li anode and a LiFePO4 cathode were assembled, and their C-rate and cycling performances were evaluated. The cell assembled with the composite separator was proven to have better C-rate capability and cycling capacity retention than the cell with the polyethylene separator. It is expected that the composite separator can be a potential candidate as a coating-type separator for high-performance rechargeable Li-ion batteries.

Micro-scale graded electrodes for improved dynamic and cycling performance of Li-ion batteries

Li-ion battery cathodes based on LiFePO4 are fabricated by a layer-by-layer spray printing method with a continuous through thickness gradient of active material, conductive carbon, and binder. Compared with cathodes with the more usual homogeneous distribution, but with the same average composition, both C-rate and capacity degradation performance of the graded electrodes are significantly improved. For example at 2C, graded cathodes with an optimized material distribution have 15% and 31% higher discharge capacities than sprayed uniform or conventional slurry cast uniform cathodes, and capacity degradation rates are 40–50% slower than uniform cathodes at 2C. The improved performance of graded electrodes is shown to derive from a lower charge transfer resistance and reduced polarization at high C-rates, which suggests a more spatially homogeneous distribution of over-potential that leads to a thinner solid electrolyte interphase formation during cycling and sustains improved C-rate and long-term cycling performance.

A new strategy of PVDF based Li-salt polymer electrolyte through electrospinning for lithium battery application

Polyvinylidene fluoride (PVDF) ultrafine fibers with different proportions of lithium nitrate (LiNO3) were fabricated by an electrospinning device. The processing parameters are optimized to 19 wt% PVDF to get a bead free structure. Scanning electron microscope (SEM) and atomic force microscope (AFM) showed the uniform and interconnected porous structure. With the addition of 2 wt% LiNO3, the fiber diameter of the electrospun membrane decreased from 371 to 222 nm. Furthermore, the addition of LiNO3 into the nanofibrous membrane enhanced the ionic conductivity from 0.97 × 10−3 S cm−1 to 1.61 × 10−3 S cm−1 at room temperature after soaking with 1 M LiPF6 (lithium hexafluoro-phosphate) in ethylene carbonate (EC) and diethyl carbonate (DEC) in (1:1 wt%). Compared with the conventional Celgard and pristine PVDF membrane, the salt doped PVDF membranes showed higher electrochemical stability window and lower interfacial resistance. The electrospun membrane separators (ES) were assembled into Lithium cobalt oxide (LiCoO2) as cathode and lithium metal as an anode. The salt doped membrane showed superior discharge, C-rate and stable cycle performance than the commercial Celgard membrane.

Development of new pyrazole-based lithium salts for battery applications – Do established basic design concepts really work?

This work is focused on applying structural concepts and basic chemical principles to model two N-heterocyclic lithium salts, based on trifluoromethyl substituted pyrazolide anions. An easily upscalable preparation method without difficult purification steps was also developed. In a comparative study, the physicochemical properties of the two new lithium salts were investigated, particularly the effect of an additional BF3-group at the nitrogen atom. In comparison to non-substituted lithium pyrazolide, the BF3-addition led to a strong improvement of thermal and electrochemical stability, ionic conductivity, as well as better C-rate and cycling performance. Furthermore, the anodic stability of Al current collectors was investigated and compared to commercial lithium salts, namely LiPF6 and lithium bis((trifluoromethyl)sulfonyl)imide (LiTFSI). Possible mechanisms that lead to the presented improvements are discussed.

Gel polymer electrolyte based on p(acrylonitrile-maleic anhydride) for lithium ion battery

A polymer matrix of poly(acrylonitrile-maleic anhydride) (P(AN-MAH)) is used to prepare a novel porous membranes by phase inversion. The membranes are immersed into liquid electrolyte to form gel polymer electrolyte (GPE) used in lithium ion batteries (LIBs). The results indicate that P(AN-MAH) is successfully synthesized and the membrane of P(AN-MAH) copolymer with adding mass ratios of AN: MAH = 4:1 exhibits superior pore structure and better thermal stability compared with that of pure PAN membrane, which endows the corresponding GPE with excellent comprehensive performances: the considerable ionic conductivity (3.03 × 10−3 S cm−1 at room temperature), fine compatibility between GPE and electrode, eminent lithium transference number (tLi+ = 0.57), and high enough electrochemical stability potential (up to 5.4 V (vs. Li/Li+)). Accordingly, in constant current charging/discharging cycle, the prepared battery for the GPE also presents excellent initial discharge capacity, C-rate and cycling performances.

Rate Performance of Silicon Alloy in Blended Lithium-Ion Battery Anode with Graphite

Incorporation of silicon based materials to graphite anode has been a promising strategy to boost anode specific capacity. Due to the distinct difference between open circuit potential (OCP) of the two material, participation of silicon alloy was in later stage of delithiation under slow C-rate condition, as shown by our previous study. 1 The present work expands silicon alloy contribution study into C-rate performance combing both experimental and Multi-Scale Multi-Dimensional modeling2 efforts. Diffusion overpotential drastically increases at high C-rates as diffusion coefficient of silicon alloy is smaller by one to two orders of magnitude than that of graphite1. We are able to track evolution of Si alloy exchange current density along charging and discharging process, and quantify contribution of silicon alloy in different C-rates. A critical C-rate level is also suggested, depending on blending ratio with graphite, beyond which enhancement of silicon alloy is diminished. Cao, L.; Yang, C.; Santhanagopalan, S.; Inman, K.; Pesaran, A., Evaluation of Silicon Alloy Contribution in Blended Lithium-Ion Battery Anode with Graphite. Meeting s 2017, MA2017-01 (1), 27.Kim, G.-H.; Smith, K.; Lawrence-Simon, J.; Yang, C., Efficient and Extensible Quasi-Explicit Modular Nonlinear Multiscale Battery Model: GH-MSMD. Journal of The Electrochemical Society 2017, 164 (6), A1076-A1088.

Studying the Effects of Carbon Coatings on the Electrochemical Performance of LiNi1/3Co1/3Mn1/3O2

Over the past few years LiNi1/3Co1/3Mn1/3O2 (NCM) has become the material of interest in lithium ion batteries because of its low cost, low toxicity and higher energy density than the commercial LiCoO2. NCM can achieve practical capacitiy of ~170 mAh/g at room temperature up-to 4.3 V vs Li/Li+ cut-off potential. If a cut-off potential of more than 4.3 V vs Li/Li+ is applied to achieve a higher discharge capacity, structural instability of the layered NCM structure suffers mechanical stress and phase changes. This deteriorates the crystal structure and causes oxygen release. Furthermore, high cut-off potentials facilitate the dissolution of metal ions into the electrolyte. The dissolved ions can migrate through the separator to the anode and deposit at the surface which worsens the anode performance and cause accelerated capacity fading.(1, 2) If the cell is exposed to higher temperatures, the electrolyte starts to decompose and the products lead to further reactions with the NCM. This results in deterioration of the cycling performance and rate capability. To overcome these issues, different methods and techniques have been applied by different research groups such as doping, coating or addition of additives in electrolytes. Herein, coating of the active material has an advantage because it can stop the direct contact between cathode and electrolyte. This is known to stop dissolution of ions into the electrolyte as well as other side reactions.(3) Different materials have been coated on NCM such as oxides (TiO2, ZrO2, Al2O3), lithium containing compounds (LiFePO4, Li3PO4), carbon and solid state electrolytes (LiTaO3).(4) Among those, a carbon coating can increase the electronic conductivity and improve the C-rate performance while allowing sufficient lithium-ion diffusion through the macro-porous structure.(5) In addition, the performance at higher temperatures as well as improved Coulombic efficiencies during formation might be realized with such coatings. To realize a carbon coating, pyrolysis of e.g. sugar or poly-vinyl alcohol have been used in air atmosphere. Pyrolysis in inert atmosphere leads to the decomposition of the active material due to oxygen release from the NCM structure and the subsequent reaction with the carbon precursor, which happens at temperatures below 400 °C. There are other studies which used physical methods such as microwave plasma chemical vapor deposition (MPCVD) which coated crystalline carbon on NCM. To the best of our knowledge, there is no electrochemical study on carbon coated active material at temperatures higher than room temperature. In this study a carbon coating on NCM was realized using a hydrothermal method. Stirring was applied to achieve a uniform precursor coating from a formalin-phenol polymer reaction. Post-annealing of the organic precursor coating was used for carbonization. Different temperatures for post-annealing were used to combust the organic precursors in air in order to create different characteristics of the coating with regard to coating thickness and surface chemistry. The synthesized powders were analyzed with regard to their thermal properties during annealing, surface morphology and chemistry, as well as coating thickness, and the possible influence on the active material by means of thermogravimetric analysis (TGA), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and powder X-ray diffraction (XRD). Electrochemical performance was investigated in Li-metal cells using pristine NCM material as reference. The thickness and properties of the coating vary with the annealing temperature which consequently affected the electrochemical performance. Therein, higher annealing temperatures improved the combustion of the organic precursor resulting in a decreased coating thickness and lower internal cell resistance. Here, annealing temperatures higher than 1000 °C could not be applied due to irreversible degradation of the active material. To study the effect on the formation Li-ion cells were build and the Coulombic efficiencies were compared by cycling the cells to an upper cut-off voltage of 4.3 V at different temperatures. Figure 1

In situ constructed Ag/C conductive network enhancing the C-rate performance of Si based anode

Abstract Poor intrinsic electrical conductivity as well as considerable volume change during lithium alloying/dealloying process has been a critical defect for high theoretical capacity silicon-based anodes. In our work, we demonstrate the synthesis design of multiscale recombined dendritic Si/Ag/C anode for high energy density LIBs via compositing bulky silicon with uniformly distributed Ag NPs, followed by a carbon source PDA (polydopamine) coating step. Here Ag NPs are generated by an in situ redox reaction between Ag+ and PDA, no need for additional reducing agents. According to the characterization analysis, the robust porous Si/Ag/C structure can provide channels for fast Li+ diffusion and electron conduction, promoting the formation of a thinner and more stable SEI film. As a result, the Si/Ag/C composite anode still yields a relatively high residual capacity of 1422.1 mAh g−1 after 100 cycles at 0.2 A g−1. In addition, it remains 633.1 mAh g−1 after 500 cycles at a high current density of 8 A g−1.

Checkerboard deposition of lithium manganese oxide spinel (LiMn2O4) by RF magnetron sputtering on a stainless steel in all-solid-state thin film battery

All-solid-state thin film lithium batteries (TFLBs) are the most competitive low-power sources to be applied in various kinds of micro-electro-mechanical systems and have been draw a lot of attention in academic research. In this paper, the checkerboard deposition of all-solid-state TFLB was composed of thin film lithium metal anode, lithium phosphorus oxynitride (LiPON) solid electrolyte, and checkerboard deposition of lithium manganese oxide spinel (LiMn2O4) cathode. The LiPON and LiMn2O4 were deposited by a radio frequency magnetron sputtering system, and the lithium metal was deposited by a thermal evaporation coater. The electrochemical characterization of this lithium battery showed the first discharge capacity of 107.8 μAh and the capacity retention was achieved 95.5% after 150 charge-discharge cycles between 4.3V and 3V at a current density of 11 μA/cm2 (0.5C). Obviously, the checkerboard of thin film increased the charge exchange rate; also this lithium battery exhibited high C-rate performance, with better capacity retention of 82% at 220 μA/cm2 (10C).

The high performances of SiO2-coated melt-blown non-woven fabric for lithium-ion battery separator

This paper reports a kind of composite separator which is prepared by adopting polypropylene melt-blown non-woven fabric as base material, and coating nano-SiO2 particles on its unilateral side. This paper also observes the morphology of separators and character the pore size, thermal stability, and electrochemical performance of separators. Results show that: filling nano-SiO2 particles into polypropylene melt-blown non-woven fabric could improve the thermal stability, ionic conductivity, and pore size distribution of polypropylene melt-blown non-woven fabric, especially, the C-rate performance. When the coating solution reaches 15 wt.%, the c-rate performance of separator reaches a higher value of 152 mAh.g−1 at 0.2 C and 89 mAh.g−1 at 5 C, which was more excellent than c-rate performance of other composite separators and pp membrane. Such kind of designing used new preparation process and will to be a promising separator for power lithium-ion battery.

Porous graphene current collectors filled with silicon as high-performance lithium battery anode

Despite the massive success for high energy density, the charge–discharge current rate performance of the lithium-ion batteries are still a major concern owing to inherent sluggish Li-ion kinetics. Herein, we demonstrate three-dimensional porous electrodes engineered on highly conductive graphene current collectors to enhance the Li-ion conductivity, thereby c-rate performance. Such high-quality graphene provides surface area for loading a large amount of electrochemically active material and strong adhesion with the electrode. The synergism of porous structure and conductive current collector enables us to realize high-performance new-generation silicon anodes with a high energy density of 1.8 mAh cm−2. Further, silicon electrodes revealed with excellent current rates up to 5C with a capacity of 0.37 mAh cm−2 for 500 nm planar thickness.

Rapid formation of polyimide nanofiber membranes via hot-press treatment and their performance as Li-ion battery separators.

We describe a new strategy to prepare thermally- and electrochemically-stable polyimide (PI) nanofiber membranes by the hot-press treatment of polyamic acid (PAA) nanofiber sheets in situ and examine their performance as Li-ion battery separators. Typical thermal imidization of PAA to PI membranes using sequential high temperature treatments in an oven takes a long time, but our method readily completes this conversion process at a mild temperature in 30 min while generating a high probability of inter-nanofiber imidization. Along with the improved electrolyte uptake capability and uniform distribution of the pore size and porosity caused by the dense and compact arrangements, the hot-press-induced PI membrane exhibits relatively thin sheets and a much greater mechanical strength than the membrane prepared by the thermal treatment. Subsequently, these PI-based membranes are installed in Li-ion full coin cells as battery separators whose C-rate (charging and discharging) performances are comparable to a commercial polyethylene (PE) separator. In addition, the highly improved thermal stabilities of these PI separators over PE separators are observed during thermal shrinkage and hot-box tests. Overall, our strategy can allow for the manufacture of diverse PI-based membranes with minimal preparation time and cost that can be utilized in high power portable devices requiring thermal and electrochemical stability.

Enhanced wettability and thermal stability of nano-SiO2/poly(vinyl alcohol)-coated polypropylene composite separators for lithium-ion batteries

To improve the electrolyte wettability and thermal stability of polypropylene (PP) separators, nano-SiO2/poly(vinyl alcohol)-coated PP composite separators were prepared using a simple but efficient sol–gel and dip-coating method. The effects of the tetraethoxysilane (TEOS) dosage on the morphology, wettability, and thermal stability of the composite separators were investigated using Fourier-transform infrared spectroscopy, scanning electron microscopy, and contact-angle measurements. All the composite separators gave a smaller contact angle, higher electrolyte uptake, and lower thermal shrinkage compared with the PP separator, indicating enhanced wettability and thermal stability. Unlike the case for a traditional physical mixture, SiOC covalent bonds were formed in the coating layer. The composite separator with a TEOS dosage of 7.5wt% had a unique porous structure combining hierarchical pores with interstitial voids, and gave the best wettability and thermal stability. The ionic conductivity of the composite separator containing 7.5wt% TEOS was 1.26mS/cm, which is much higher than that of the PP separator (0.74mS/cm). The C-rate and cycling performances of batteries assembled with the composite separator containing 7.5wt% TEOS were better than those of batteries containing PP separators.

Multi-functional nanowall arrays with unrestricted Li+ transport channels and an integrated conductive network for high-areal-capacity Li–S batteries

Multifunctional nanowall arrays with “all-in-one” capabilities in suppressing PS dissolution, improving Li+/e− transport and retention of its structural integrity during repeated cycling were designed to improve the cycling stability and C-rate performance of high-areal-capacity Li–S batteries.

Cu doped LiNi0.5Mn1.5−xCuxO4 (x = 0, 0.03, 0.05, 0.10, 0.15) with significant improved electrochemical performance prepared by a modified low temperature solution combustion synthesis method

A series of Cu-doped LiNi0.5Mn1.5−xCuxO4 (x = 0, 0.03, 0.05, 0.10, 0.15) spinel samples have been successfully prepared using a modified low temperature solution combustion synthesis method. X-ray diffraction(XRD) and infrared spectroscopy(FT-IR) analysis are used to characterize the phase structure. Scanning electron microscopy(SEM) is used to observe the microstructure of the products. The electrochemical performance are studied by galvanostatic charge-discharge testing, cyclic voltammetry(CV) and electrochemical impedance spectroscopy(EIS). No obvious secondary phases were observed in XRD patterns of as-synthesized LiNi0.5Mn1.5−xCuxO4 powders. The samples have a combination structure of ordered and disordered space group and the order degree increases with the increase of Cu doping content obtained from FT-IR spectra. The electrochemical performances show that although the specific capacity decreases with the Cu-doping content, the cycle-life both at room temperature and 55°C and the C-rate performance are remarkably improved. The factors of stable structure, grain refinement, better crystallinity and lower charge transfer resistance lead to the excellent performance of Cu-doped samples.

Difference of rate performance between discharge and charge reactions for bismuth fluoride electrode in lithium-ion battery

The conversion-based BiF3 is a promising cathode material for lithium-ion batteries due to its high theoretical capacity (302mAhg−1). Nanocomposites of BiF3 and carbon (BiF3/C) are known to improve the electrochemical performance by increasing the electronic conductivity of the electrode. Here we investigate the electrochemical performance of BiF3/C at high C-rates. In particular, we newly investigate the difference of high C-rate performance between discharge and charge reactions. The discharge and charge capacities in the first cycle were almost the same at 0.1C. In contrast, the discharge capacity was higher than charge capacity at 10C. Further, during cycling at 10C, the charge capacity drastically decreased, but the discharge capacity remained high. The rate performance of the discharge reaction was higher than that of the charge reaction, especially after cycling.

A high-performance and environment-friendly gel polymer electrolyte for lithium ion battery based on composited lignin membrane

A biodegradable composite polymer membrane is fabricated by synthesizing polyvinylpyrrolidone (PVP) on the matrix of lignin, and then the corresponding gel polymer electrolyte (LP-GPE) is further prepared by absorbing the liquid electrolyte. The morphology, mechanical property, and thermal stability of the composite lignin-PVP membrane and the electrochemical properties of LP-GPE are investigated. The results of the investigation present that the mechanical property of the membrane is remarkable improved (670%) and the composite membrane exhibits a better thermal security. For electrochemical properties, a high ionic conductivity of 2.52 × 10−3 S cm−1 at room temperature, excellent lithium-ion transference number of 0.56, and outstanding electrochemical stability of LP-GPE are confirmed. Moreover, the C-rate performance and capacity retention based on Li/LP-GPE/LiFePO4 cell are superior to that of the commercial Celgard 2730 cell. Consequently, all these results demonstrate that LP-GPE can be applied as a novel electrolyte for lithium ion battery with high-performance, low-cost, and environment-friendly properties.

A novel separator material consisting of ZeoliticImidazolate Framework-4 (ZIF-4) and its electrochemical performance for lithium-ions battery

A novel film based on zeoliticimidazolate framework-4 (ZIF-4) as a separator is prepared by hydrothermal method and a stable electrode-supported separator is synthesized by blade-coating technology. The ZIF-4 separator with the thicknesses of about 60 μm is coated on Li[Ni1/3Co1/3Mn1/3]O2 electrode with a slurry of ZIF-4, PVDF and amount of N-methyl-2-pyrrolidone (NMP). It is found that the ZIF-4 separator has good thermal stability and cycle performance than conventional polypropylene (PP) separator. And it has larger liquid electrolyte uptake, higher retention, higher ionic conductivity and lower interfacial resistance. The C-rate performance of ZIF-4 separator is superior than PP separator, and it enhances the safety of lithium-ions battery. Therefore, the ZIF-4 separator is a promising material to improve stability and safety of lithium-ions battery.

Hierarchy concomitant in situ stable iron(II)−carbon source manipulation using ferrocenecarboxylic acid for hydrothermal synthesis of LiFePO4 as high-capacity battery cathode

The iron precursor of lithium iron phosphate (LiFePO4) is highly prone to oxidation to Fe3+ during the hydrothermal synthesis. The Fe3+ impurities in LiFePO4 restrict the conduction path of Li+ ions in LiFePO4, which negatively affect the cell performance. In this paper, we report that ferrocenecarboxylic acid possessing an extremely stable Fe2+ species and as carbon source has been used successfully to suppress Fe3+ impurities in LiFePO4. The X-ray diffraction results reveal that Li2CO3 first reacts with (NH4)2HPO4 to form Li3PO4 at low temperatures, which above 160°C further reacts with ferrocenecarboxylic acid to give LiFePO4. The infrared spectroscopy, nuclear magnetic resonance, mass spectrometry, and elemental analysis results show that the carbon sources during calcination of LiFePO4 are derived from polymers through sequential [4+2] cycloaddition reactions of cyclopentadiene and 1,3-cyclopentadiene-1-carboxylic acid during decomposition of ferrocenecarboxylic acid. The electron paramagnetic resonance results show that the percentage of Fe3+ in the synthesized LiFePO4 is as low as 0.5 mol%. A plausible reaction mechanism for the hydrothermal synthesis of LiFePO4 is also proposed. The as-synthesized LiFePO4 shows an orthorhombic olivine with a discharge capacity of 158mAhg−1 at a discharge rate of 0.1C. The cell also shows excellent C-rate and cycle-life performances.

Hybrid Battery System to Achieve High Power and High Energy for Automotive Applications

Although lithium-ion batteries (LiBs) have superior power and energy density than the other type of batteries, generally, a LiB is specially customized to achieve its specific requirements such as long-range mileage for EV or high power for HEV. This is because it is difficult to meet both high power and high energy density with a single type of battery. For example, high energy batteries for EVs that have thick electrodes may undergo charging problem such as lithium metal plating at low temperature. Accordingly, several studies were conducted to simultaneously improve both power and energy performance of LiBs. Using altered composition of electrode materials, electrolyte, and separator, LiBs could have improved energy retention or power performance [1-3]. But, preceding studies could not achieve both energy and power performance for vehicle applications at the same time. In order to resolve this problem, Kotz et al. [4] proposed a concept of a hybridized battery which combined a high energy battery with a high power capacitor within a single system. They reported that the internal parallel hybridization of those two components could achieve higher power and higher energy than each component operates independently. In the present study, we propose a new concept of battery system to achieve both high power and high energy at once by externally hybridizing a high power LiB with a high energy LiB in a single system, so called “hybrid battery (HB)”. To investigate the feasibility of the concept, we construct a mathematical model of a HB system as shown in figure 1. Three LiBs are connected in parallel. Two of them are high power LiBs and the other is high energy LiB. Conducting computational simulations with the developed model, basics characterization (OCV, resistance, C-rate performance) of the hybrid battery system is investigated for the first time. Then, we examine the performance of the hybrid battery under various operating conditions. Also, charging/discharging interactions among LiB members are monitored. To validate the present HB concept with the developed mathematical model, an actual hybrid battery system is constructed by using three automotive LiBs. Basic characterization and simulated driving test shows a good agreement with the theoretically predicted result. Through the present study we confirm the validity of the HB concept for automotive applications.