(Invited) Lithium Batteries and Its Diagnosis System

The rapid development of mobile electronics, electric-vehicles and intermittent power generation relies on the availability of advanced electrochemical energy storage systems such as high capacity lithium ion batteries (LIBs). Although the state-of-the-art commercial LIBs have impressive electrochemical performance, they are suffering from relative low capacities due to the usage of host materials for storing lithium ions safely and reproducibly. The host material for LIB anode is dominated by graphite with a theoretical capacity of 372 mAh g-1. To further improve the capacity and energy density of LIBs, revolutionary changes in electrode materials, electrolytes, additives, binders and cell design are much needed. Alloying anode materials like Si, Ge and Sn have much higher theoretical capacities (4200, 1600 and 990 mAh g-1 for Si, Ge and Sn, respectively) as compared to graphite. However, a high capacity inevitably leads to a large electrode volume change during lithiation and de-lithiation process (e.g. 300% for Si-based anodes). Such a huge volume change can cause electrode pulverization, detachment of active materials from conductive additives, unstable solid-electrolyte interphase, and eventually result in a rapid, permanent capacity loss. Although Si-based anode has caught much research attention, formidable challenges remain to develop an electrode fabrication method that is efficient, low-cost, scalable, highly reproducible, and self-assembled, thus boosting the commercialization potential of Si-based anodes. As inspired by the hollow fiber membrane technology that has been successfully commercialized in the last decades to provide billions of gallons of purified drinking water worldwide, we report the use of carbonized asymmetric membrane electrodes (fiber and thin film types) that contain silicon, silicon monoxide, germanium, tin dioxide or vanadium (V) oxide to significantly increase LIB electrode capacity while maintaining a long cycling life and an excellent rate performance. These asymmetric membrane electrodes are fabricated using a modified reverse-osmosis membrane technology, the phase-inversion method combined with dip-coating and carbonization. The asymmetric membrane electrodes have a thin, nanoporous top layer (up to several microns) and a thick, macroporous bottom layer (up to 200 µm). The top layer can prevent fractured anode materials from falling away, while the bottom layer can efficiently accommodate the large volume changes of high capacity alloying anodes. 90% capacity of thin film asymmetric membrane at 1000 mAh g-1 containing Si nanoparticles can be retained after 200 cycles with an initial capacity loss of ~ 30%. Combining phase inversion with sol-gel chemistry, this method can be extended to obtain SnO2 and V2O5 asymmetric membranes. Thin film SnO2 asymmetric membrane electrode demonstrates a specific capacity of 500 mAh g-1 a current density of 280 mA g-1 (~0.5C) with >96% capacity retention after 400 cycles. When the current density is increased from 28 to 560 mA g-1, more than 65% capacity can be retained. The V2O5 electrode can be cycled at 0.5C with a capacity of 160 mAh g-1 for 380 times with ~100% capacity retention. The same method can be employed to stabilize micron-size Ge and Si alloying anodes with >80% capacity retention in 100 cycles. Most recently, our group has successfully fabricated various fibrous Si and Sn asymmetric membrane electrodes whose electrochemical performance is compared to thin film Si and Sn asymmetric membrane electrodes. e.g. ~90 % initial capacity at ~1,200 mAh g-1 can be maintained in 300 cycles applying a current density of 400 mA g-1 with an initial capacity loss less than 15%. These asymmetric membrane electrodes have been systematically characterized using extensive instruments spanning from scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS), Raman spectroscopy, powder X-ray diffractometer (PXRD), thermogravimetric analyzer (TGA), surface area analyzer (BET method) to X-ray photoelectron spectroscopy (XPS), as well as various electrochemical tests such as cycling and rate performance, cyclic voltammetry, and electrochemical impedance spectroscopy, purposing to understand the relationship between structure, chemical composition and electrochemical properties for these asymmetric membrane electrodes.

Electrochemical impedance spectroscopy (EIS) is one of the most important non-destructive diagnosis tools in lithium-ion battery (LIB) technology. EIS is routinely used for characterization of battery materials, optimization of manufacturing processes, and understanding of degradation mode. However, interpretation of EIS spectra is not a straightforward procedure. Typical EIS data of LIB have heavily overwrapped capacitive semicircles in Nyquist (complex impedance) plot because LIBs’ anodes and cathodes both involve several conduction processes. Widely used method for information separation is least-squares fitting based on discrete equivalent circuit model. However, this method requires a priori assumption on the number of capacitive processes. Destructive EIS measurement methods, such as insertion of reference electrode and symmetry cell construction, are powerful for separation of cathode/anode information. But these involve meticulous work, and battery structure or degradation state often make it difficult to obtain an accurate measurement result.Recently, distribution of relaxation times (DRT) analysis has increasingly used for EIS interpretation. This analysis treats an EIS spectrum as a continuous function of relaxation time, or time constant of infinitesimal RC equivalent circuit element. DRT analysis can distinguish several different capacitive processes without the need of prior model assumption. Application field of DRT analysis currently focuses on solid oxide fuel cell, but several LIB studies have been reported for LiFePO4 cathode system[1] and LiMn2O4-LiNixCoyMnzO2 cathode system[2]. Here we demonstrate that DRT analysis is a powerful tool for degradation analysis of LIBs with LiCoO2(LCO)-based cathode and graphite anode, and provide experimental verification of information obtained by DRT analysis.Commercial-size prototype LIBs with LCO-based cathode and graphite anode are used for this work. Charge-discharge cycling tests of the cells are performed at different temperature conditions. EIS measurements are conducted at fully charged state of every fifty cycles, with frequency range of 1 MHz to 10 mHz. For better separation of different processes, the EIS spectra are obtained at 23 deg C, 10 deg C and 0 deg C in a climate chamber. DRT is calculated by MATLAB with in-house code, which is based on Fourier transfer methods utilizing window function preprocessing[3]. To understand DRT results in physical viewpoints, several electrode analyses of disassembled cells are conducted at initial and the cycled state. Figure 1 shows Nyquist plots and the corresponding DRT plots of (a) low-temperature and (b) high-temperature cycled LIBs obtained at 10 deg C. In Nyquist plot, all spectra exhibit the shape of two depressed capacitive semicircles. At a first glance, both low-temperature cycling and high-temperature cycling appear to increase the size of low-frequency semicircles. But closer look shows that low-temperature cycling cause “merge” or heavier overwrap of two semicircles, whereas high-temperature cycling retains the separated feature. These characteristics are further clarified by DRT analysis. In each cell, we can identify four different peaks in DRT plots. The high-temperature cycled cell shows gradual increase in the lowest-frequency (1) peak, whereas the peak remains almost the same intensity in the low-temperature cycled cell and the middle-frequency (2) peak increases significantly. No change is observed for the (3) peak during the cycle test. Both cycling condition cause the increase in the highest-frequency (4) peak. Further teardown analyses verify that these DRT peaks can be attributed to different electrode degradation phenomena, which are related to cycling temperature. These results demonstrate the ability of DRT analysis as a degradation diagnostics tool for LIBs with various kinds of material systems. [1] J. P. Schmidt, T. Chrobak, M. Ender, J. Illig, D. Klotz, E. Ivers-Tiffée, J. Power Sources, 196 5342 (2011). [2] B. Stiaszny, J. Ziegler, E. Krauß, J. P. Schmidt, E. Ivers-Tiffée, J. Power Sources, 251 439 (2014).[3] H. Schichlein, A. C. Müller, M. Voigts, A. Krügel, E. Ivers-Tiffée, J. Appl. Electrochem., 32 875 (2002). Figure 1

Electrochemical impedance spectroscopy (EIS) is strongly requested for analysis of the battery health. The capacity and the internal resistance of large-scale lithium ion batteries (LIBs) become higher and lower, respectively, for the application to electric vehicles and large-scale power storage systems. EIS using conventional FRA – potentiostat systems is not easy to measure the impedance of the LIB because of its low internal resistance. In our previous study, application of square wave potential for input signals of EIS was investigated in simple electrochemical reaction to verify a new technique called “Square-potential/current electrochemical impedance spectroscopy (SP-EIS, SC-EIS)” which is a method for EIS without using the FRA systems [1]. And then, we applied SC-EIS to evaluate a state of commercial LIB [1]. In this study, introduction of SC-EIS to diagnosis technology of laminated LiB was investigated.A commercially available laminated LIB with a nominal capacity of 5 Ah was used. For LIB module investigation, one module was assembled using four cells (two series cells, two parallel cells). High precision bipolar power supply and conventional large-scale charge-discharge test system were mainly used for input signal source. On the basis of the technique of fourier transform in ref. [2][3], EIS was carried out using square current input at SOC = 50 %. Frequency of square current was 0.5, 5.0, 50 Hz. Amplitude of peak to peak and sampling frequency were 800 mA and 100 kHz, respectively.Figure 1 shows input current waveform at 50 Hz, output voltage waveform, and Nyquist plots of 5Ah LIB using bipolar power supply and large scale charge – discharge test system. Dots show the results of SC-EIS. Open symbols show the reference results of conventional impedance spectroscopy. The waveform generated by charge-discharge system is not ideal square shape compared with the waveform generated by bipolar power supply. Thus, effect of waveform on the EIS was investigated. In the case of ideal square waveform generated by bipolar power supply, fine Nyquist plots could be obtained by means of SC-EIS as well as that by means of the FRA system even in the case of LIBs. 200th harmonic could be measured at frequency of (0.5 Hz, 5 Hz, 50 Hz). Only using these three frequencies, very wide range of 0.5 Hz – 10 kHz could be measured by using this method.In the case of not ideal square waveform generated by large-scale charge-discharge test system, fine Nyquist plots up to 3 kHz could be obtained by means of SC-EIS as well as that by means of the FRA system even in the case of LIBs. In usual, frequency response up to 2 kHz is needed for diagnosis technology of LIBs. Thus, this technique is could be applied to cell checking system only using not expensive instruments. We also applied this technique to cell module. As a result, we successfully obtained fine Nyquist plots of both module and each single cell by once measuring. Moreover, some error in the modules could be picked out by using this measurement technique successfully.From these results, new techniques called “Square- Current Electrochemical Impedance Spectroscopy (SC-EIS)” demonstrated to be suitable method for EIS without using FRA systems. The SC-EIS must be a method of great candidate for diagnosis technology of battery management systems.

Electrochemical impedance spectroscopy (EIS) was used to study the micro-overcharge cycle damage mechanism of Lithium-ion batteries (LIBs). Micro-overcharge cycle experiments of LIBs were carried out, and the capacity fading of LIBs under different charging cut-off voltages were analyzed. It was found that the capacity fading rate of LIBs increased with the rising of overcharge cut-off voltages and the increasing of cycle numbers. The EIS results show that the main damage pattern of LIBs during micro-overcharge cycle is the active lithium loss when the cut-off voltage is between 4.3 V and 4.4 V. Lithium loss accounts for more than 80% damage proportion when LIBs cycling for more than 20 cycles.

Capacity fading mechanisms and state of health prediction of commercial lithium-ion battery in total lifespan

INTRODUCTION Hard carbons prepared by pyrolysis of various polymer precursors have been tested for lithium ion intercalation and were found having higher reversible capacities than that of graphite (1, 2). However the main disadvantage of using hard carbon as anode material for lithium ion battery is higher irreversible capacity due to absorption of lithium ions on the internal surface of nanopores formed by disordered single layer graphene sheets (3).Madou et al. used SU-8 photoresist to prepare carbon films on silicon wafer for lithium insertion (4). Their electrical characteristics were found similar to that of glassy carbon while their reversible capacity was measured to be ~ 220 mAh/g (5).In this study SU-8 photoresist derived carbon films were prepared on stainless steel (SS) wafer as substrates. The choice of SS wafer (resistivity: 73 µΩ-cm) as a substrate is more apt as the same is used in the commercial lithium ion battery. Galvanostatic charge discharge of assembled Swagelok cell within 0.01-3 V was performed to measure the lithium insertion capacities. EXPERIMENTAL SU-8 2005 (Microchem Corp., MA) was spin coated at 3000 rpm for 30 s on dehydrated single side polished SS substrates (MTI Corp., USA). After 10 min relaxation, films were soft baked at 950C for 2 min. After cooling to room temperature films were exposed to UV light followed by post exposure bake at 95 0C for 3 min to complete the crosslinking. Hard bake of samples was carried out at 1500C to reduce the internal residual stresses generated due to shrinkage during crosslinking.As-prepared SU-8 films were then carbonized at 9000C in the inert atmosphere using two step pyrolysis. Thus prepared SU-8 derived carbon film on SS and lithium foil (Sigma Aldrich) were used as working and counter electrode respectively. 1M solution of LiPF6in a 1:1 v/v mixture of ethylene carbonate and diethyl carbonate was used as an electrolyte and glass microfiber filter (Whatman Filters) was used as separator. Packed Swagelok cells were soaked for 12 h before Galvanostatic cycling at C/10 rate to measure intercalation capacity of SU-8 derived carbon films as anode. RESULTS Cyclic voltammetry Cyclic voltammogram recorded at 1 mV/s is shown in Fig.1a. Broad reduction peak at 0.5 V corresponds to reduction of passive layer formation by electrolyte decomposition and other reduction peak near 0 V corresponds to absorption of Li ions in hard carbon. Galvanostativc charge discharge cycling Galvanostatic discharge and charge cycles experiment was carried out at C/10 rate (Fig. 1b). Initial discharge capacity was very high compare to other cycles due to electrolyte decomposition voltage near to 0.4 V (6). Reversible capacity was found to be ~400mAh/g which was higher than that of graphite and also as reported earlier (5). Electrochemical impedance spectroscopy (EIS) Fig. 1c shows the Niyquist plot recorded under ac amplitude of 10 mV over the frequency range of 10 mHz-100 KHz.The charge transfer resistance was found to be less than half as compared to reported values (5) earlier that further suggests the superior performance of the designed architecture (SU-8 photoresist derived carbon on SS substrate).Fig.1 (a) Cyclic voltammetry of SU-8 derived carbon film on SS substrate; (b) Galvanostatic charge discharge curve for SU-8 derived carbon films on SS substrate at C/10 rate; (c) EIS plot for SU-8 derived carbon film after 10 charge/discharge cycles SUMMARY Photoresist (SU-8) derived carbon films were prepared on SS as substrates and then tested for lithium ion intercalation. Galvanostatic charge/discharge experiments shows higher reversible capacity (400 mAh/g) as compared to graphite and previous values reported. EIS measurements also supplement these findings that may be of potential use for anode materials for Lithium ion battery. REFERENCES 1. A. Piotrowska et al., J. Anal. Appl. Pyrolysis, 102, 1 (2013).2. T. Zheng et al., J. Electrochem. Soc., 142, 2581 (1995).3. Y. Liu et al., Carbon, 34, 193 (1996).4. B. Y. Park et al., J. Electrochem. Soc., 152, J136 (2005).5. G.T. Teixidor et al., J. Power Sources, 183, 730 (2008).6. S. Zhang et al., Electrochem. Solid State Lett., 4, A206 (2001).

Si provides an effective approach to achieving high-energy batteries owing to its high energy density and abundance. However, the poor stability of Si requires buffering with graphite particles when used as anodes. Currently, commercial lithium-ion batteries with Si/graphite composite anodes can provide a high energy density and are expected to replace traditional graphite-based batteries. The different lithium storage properties of Si and graphite lead to different degrees of lithiation and chemical environments for this composite anode, which significantly affects the performance of batteries. Herein, the interplay between Si and graphite in mechanically mixed Si/graphite composite anodes is emphasized, which alters the lithiation sequence of the active materials and thus the cycling performance of the battery. Furthermore, performance optimization can be achieved by changing the intrinsic properties of the active materials and external operating conditions, which are summarized and explained in detail. The investigation of the interplay based on Si/graphite composite anodes lays the foundation for developing long-life and high-energy batteries. The abovementioned experimental methods provide logical guidance for future research on composite electrodes with multiple active materials.

Capacity fade mechanism of CoSb 3 intermetallic compound

Battery life is a critical and challenging issue for battery development and applications. The capacity of lithium-ion batteries reduces during continuous discharge-charge cycling. This capacity fade or loss occurs due to different mechanisms which are associated with side reactions that occur in these batteries [1]. The objective of this work is to use electrochemical impedance spectroscopy (EIS) to study the effects of pulse charging on the capacity and cycling characteristics of lithium-ion cells and compare it with conventional charging method at constant current-constant voltage (CCCV). Implementing conventional charging and pulse charging at an average of 0.5C and 1C on 18650 Lithium Iron Phosphate batteries separately provides varied results in capacity after few cycles. Equivalent circuit models are extracted from the measured EIS data for both of the charging methods to provide detailed results on capacity and battery performance. Reference: [1] P. Arora, R. E. White, and M. Doyle, “Capacity Fade Mechanisms and Side Reactions in Lithium‐Ion Batteries,” J. Electrochem. Soc. , vol. 145, no. 10, pp. 3647–3667, Oct. 1998. Figure 1

Graphite is the anode material commonly used in commercial lithium-ion batteries (LIB). The practical capacity of graphite is already close to its theoretical limit, so alternative anode materials are needed to further increase the energy density of LIB. For this purpose, high-capacity active materials with stable charge-discharge performance are needed. Among the different materials, metal-alloy, metal-oxides and sulphides are studied because they undergo alloying and/or conversion reaction to give capacities larger than that of graphite (> 370mAh/g).1,2,3. However, these materials exhibit large volume change during lithium insertion and extraction, which typically leads to fast capacity fading with cycling.4,5 In this study, we focus on high-capacity SnS material for lithium-ion applications. Both Sn and S can react with Li, resulting in a theoretical capacity of 1109 mAh/g (6.25 e-per SnS). To buffer the large volume change during charge and discharge, SnS-C composite is prepared. The carbon composite can also increase the electrical conductivity of the material by forming interconnected conductive network. A one-pot co-precipitation method is developed to synthesize the SnS-C composite from SnCl2, Na2S and organic precursors. The precipitates were then heat-treated in N2atmosphere to carbonize the organic materials. X-ray diffraction (XRD) shows that the materials are phase-pure with an orthorhombic crystal structure (JCPDS #65-3812). The materials were made into electrodes and tested with lithium metal counter electrode comparing with that of SnS (purchased from Sigma-Aldrich). Figure 1 (a) and (b) show the charge-discharge behavior of SnS and SnS-C composite. Initial capacities of SnS and SnS-C are similar (about 900 mAh/g at 100mA/g). The charge-discharge curves show two regions of reaction: <1V (which corresponds to Sn/Li alloying) and 1.5-2.5V (which corresponds to Li/S reaction). With cycling, SnS shows significant loss in capacity, whereas SnS-C composite can maintain stable cycling performance with about 700 mAh/g at 250 mA/g after 100 cycles (Fig. 1(c)). Cycle efficiency of SnS-C is also significantly better than SnS (Fig. 1(d)). The carbon composite introduced during the preparation process significantly improved the cycle performance. After 55 cycles, only the Sn/Li reaction below 1V is observed for the SnS sample (Fig. 1(a)). On the other hand, SnS-C composite still show both Sn/Li and Li/S reactions (Fig. 1(b)). This suggests that in addition to providing electrical conductivity and buffering for volume expansion, the carbon composite is able to facilitate recombination of SnS after charging. Further work on the effect of cycle performance on carbon content, as well as the mechanical properties of the materials are underway and will be presented at the meeting. Acknowledgement This research is sponsored by the Innovation Technology Funding (ITS/363/13) managed by Innovation and Technology Commission from the Government of the Hong Kong SAR.

Different percentages of nanoparticles graphene (G) were mixed with nano-micron sized silicon (Si) particles as follows: 10, 20, 30 and 40 wt% graphene to silicon ratios. The crystal structure of pure Si powder pattern has cubic phase SEM, TEM/SAED and XPS equipments were implemented to study the surface properties of the prepared G@Si composites. Cyclic voltammetry (CV) measurement for the G@Si cell revealed two broad cathodic peaks, related to the deposition of Li2O thin layer on Si particles and the lithiation process of Si to form lithium silicide. Meanwhile, the oxidation of LixSi into Si and Li ionis confirmed by the anodic strong peak at 0.56 V. Electrochemical impedance spectroscopy (EIS) measurements revealed high interfacial resistance ~ 1825 Ω for pure Si anode in comparison with that of G@Si composite anode. It is concluded that graphene acts as a conductive shielding pathway to inhibit the large volume change and minimize the capacity fading during successive galvanostatic cycling of G@Si composite anode materials versus Li/Li+. Accordingly, the specific discharge capacity of 30%G@Si cell delivered about 1240 and 900 mAhg−1 for 1st and 100th charge–discharge cycles, respectively.

Depletion of fossil fuels and concerns over CO2 emission have driven the development of electric vehicles (EVs) with high-energy efficiencies and low emissions. Lithium-ion rechargeable batteries, compared to lead acid, nickel cadmium, nickel metal hydroxide, and other popular rechargeable batteries, are considered as the most promising candidates for EVs for their high operating voltage and high energy density. Silicon nanowires are considered as lithium-ion battery anodes for their ultra high capacity at 4200 mAh·g-1 (10× higher than conventional graphite anode), as well as stress accommodation for reversible lithiation and delithiation. Silicon nanowires were fabricated via metal assisted electroless etching, and conductive nickel monosilicides ohmic contacts were created via simple one-step thermal annealing procedure between nanowires and nickel electrodes for integration. Composite anodes were prepared from electrolessly fabricated silicon nanowires for lithium-ion batteries, and an addition of only 15 % silicon nanowires results in a two-fold increase in reversible capacities for 15 cycles. Silicon anodes with hydride, methylated and siloxane surface terminations were prepared and tested in lithium-ion cells; another silicon anode was cycled with 5 % trimethoxymethylsilane. Analyses showed methylated and siloxane terminations lead to passivated surfaces, and hydride-terminated nanowires were relatively more reactive with electrolytes. The addition of silane additive results in more OPFx compounds and Si-O-Si bonds at the silicon surface with significantly higher capacities (3287 mAh·g-1). AFM nano-indentation analyses also showed a significant increase in contact stiffness with silane additive, and the increase in contact stiffness may improve the anode’s ability to withstand large volume changes. Although the chemical composition of the SEI is altered with silane additives, performance improvements were mainly associated mechanical effects.

With a view to minimize the unavoidable large volume changes of tin based Cu6Sn5 alloy anodes, a composite Cu6Sn5/graphite anode has been prepared via. a mechanical alloying process and subsequently coated with disordered carbon through pyrolysis of PVC. Phase pure products with better crystallinity and preferred surface morphology were obtained, as evident from PXRD and SEM respectively. Upon electrochemical charge-discharge, the intermetallic Cu6Sn5 alloy-graphite composite anode was found to exhibit an enhanced initial discharge capacity of 564 mAh g−1 followed by significant capacity fade (>20%) especially after five cycles. On the other hand, carbon coated Cu6Sn5 alloy-graphite composite demonstrated promising electrochemical properties such as steady reversible capacity (∼200 mAh g−1), excellent cycle performance (<5% capacity fade) and high coulombic efficiency (∼98%) via. significant reduction of volume changes. The carbon coating offers buffering and conductive actions on the anode active material and thereby enhances the electrochemical behavior of carbon coated Cu6Sn5 alloy/graphite composite anode material.

Proposal of novel equivalent circuit for electrochemical impedance analysis of commercially available lithium ion battery

Impact of fast charging and low-temperature cycling on lithium-ion battery health: A comparative analysis