Li-ion Full Cells Research Articles

Controlled formation of BNb3O9 nanobelts as superior host material for high performance electrochemical energy storage

Ultra-stable anode materials for lithium-ion batteries are increasing in demand as highly sustainable energy storage system with excellent charge transport becomes unprecedentedly important. Here, a novel Li-ion full cell, utilizing BNb3O9 hollowed-out nanobelts as the typical intercalation anode and commercial LiFePO4 as the cathode in organic electrolyte, is reported. Moreover, the hollowed-out framework of BNb3O9 nanobelts, prepared by an adjustable-voltage electrospinning method, exhibits prominent cyclic stability (116.1 mAh g−1 at 700 mA g−1 (12C) over 1000 cycles) and outstanding rate capability (126.8 mAh g−1 at 900 mA g−1) in half cell. The key finding is that the formation mechanism of BNb3O9 hollowed-out nanobelts can be thoroughly discussed by scanning electron microscope. The contribution of the external pseudocapacitance largely affects the rate capability, which is determined via kinetic analysis. Furthermore, the lithium storage mechanism and structural evolution of BNb3O9 are investigated through in-situ X-ray diffraction. Such preeminent electrochemical performance and distinctive morphology endow BNb3O9 hollowed-out nanobelts with potentials as anode materials for lithium-ion batteries.

Probing lithiation and delithiation of thick sintered lithium-ion battery electrodes with neutron imaging

As lithium-on (Li-ion) batteries increase in their prevalence and the range of applications expands, there is a need to understand and exploit the limits of electrochemical performance. Probing the internal processes in Li-ion batteries provides insights into the electrochemical characteristics of the cells as well as information necessary for rational cell design. In this manuscript an in situ method, neutron imaging, is applied to Li-ion full cells to track the lithiation/delithiation processes in the electrodes during discharge. The electrodes comprise thick sintered films of only active material, which improves the number of discrete points that are mapped for net changes in neutron intensity through the electrode. The lithiation/delithiation processes are qualitatively consistent with calculations of the Li+ concentration and discharge profiles of the cells. These results illustrate that neutron imaging can experimentally probe and confirm limitations in the electrochemical performance of Li-ion full cells, particularly those with thick electrodes.

Carbon Dioxide As Electrolyte Additive for Silicon-Based Lithium Ion Full Cells

Following up the great success of lithium ion batteries (LIBs) in the field of consumer electronics in the recent decades, LIBs are today also widely used as energy storage devices for various types of electric vehicles (EVs). However, it is believed that a substantial breakthrough of the electromobility can only take place, if EVs are able to achieve driving ranges of more than 500 km and, therefore, gaining greater consumer acceptance. To achieve these driving ranges, the energy content of the used LIBs needs to be increased to values of 350 Wh/kg and 750 Wh/L at the cell level. These improvements can be achieved by replacing the state-of-the-art graphite anode by high-capacity anode materials like silicon (Si), which is able to provide a nearly 10 times higher specific capacity compared to graphite.[1] However, due to huge volume changes of up to 300% upon lithiation/de-lithiation, Si-based anodes often suffer from poor cycling performance related to cracking and pulverization of the Si particles and continuous solid electrolyte interphase (SEI) re-formation. Typically, these issues are addressed by decreasing the size of the active material particles to the nanoscale (i.e. nanoparticles, nanowires or thin films) or the fabrication of composites like intermetallic Si phases or Si/carbon composite materials to decrease the overall volume changes.[2] Although these strategies are reported to be effective to improve the performance of Si electrodes, it was revealed that the dominating failure mechanism of full cells containing Si-based negative electrodes lies in the consumption active lithium from the cathode related to continuous electrolyte reduction at the Si negative electrode. The addition of appropriate electrolyte additives such as fluoroethylene carbonate (FEC) or vinylene carbonate (VC) can significantly improve the capacity retention and Coulombic efficiency of Si-based full cells by forming a more stable SEI on the Si negative electrode.[3, 4] Recently, Krause et al. demonstrated that the addition of carbon dioxide in the form of dry ice significantly improved the performance of Si-based full cells and even outperformed the most commonly used additive FEC.[5] However, the exact working mechanism of the carbon dioxide additive and especially the reductive decomposition reactions including the resulting morphology and/or chemical composition of the formed SEI layer are still uncertain. Within this study, thin film Si electrodes prepared via magnetron sputtering are employed as model electrodes (i.e. no binder or conductive additive) therefore, possible effects of the carbon dioxide can be correlated to the Si active material. These Si thin film electrodes are coupled with NMC-111 positive electrodes to Si/NMC-111 full cells, which are used for the electrochemical investigations. As the solubility of carbon dioxide in the organic carbonate based electrolytes is rather low, diethylpyrocarbonate is added to the baseline electrolyte, as pyrocarbonates are known as carbon dioxide generators and, therefore, the carbon dioxide is released in-situ within the assembled full cell. The addition of the diethylpyrocarbonate electrolyte additive leads to significantly improved performance of the Si/NMC-111 full cells in terms of Coulombic efficiency and capacity retention full cells during prolonged cycling compared to the baseline electrolyte. The chemical composition of the SEI layer formed on the Si negative electrodes is investigated by means of X-ray photoelectron spectroscopy (XPS). The XPS investigations reveal that the improved performance of the carbon dioxide containing cells is related the improved passivation of the negative electrode by the formation of mainly lithium carbonate at the Si surface. Reference s : [1] D. Andre, H. Hain, P. Lamp, F. Maglia, B. Stiaszny, Future high-energy density anode materials from an automotive application perspective, Journal of Materials Chemistry A, 5 (2017) 17174-17198. [2] M.N. Obrovac, V.L. Chevrier, Alloy Negative Electrodes for Li-Ion Batteries, Chemical Reviews, 114 (2014) 11444-11502. [3] R. Nölle, A.J. Achazi, P. Kaghazchi, M. Winter, T. Placke, Pentafluorophenyl Isocyanate as an Effective Electrolyte Additive for Improved Performance of Silicon-Based Lithium-Ion Full Cells, ACS Applied Materials & Interfaces, 10 (2018) 28187-28198. [4] M. Klett, J.A. Gilbert, S.E. Trask, B.J. Polzin, A.N. Jansen, D.W. Dees, D.P. Abraham, Electrode Behavior RE-Visited: Monitoring Potential Windows, Capacity Loss, and Impedance Changes in Li1.03(Ni0.5Co0.2Mn0.3)0.97O2/Silicon-Graphite Full Cells, Journal of The Electrochemical Society, 163 (2016) A875-A887. [5] L.J. Krause, V.L. Chevrier, L.D. Jensen, T. Brandt, The Effect of Carbon Dioxide on the Cycle Life and Electrolyte Stability of Li-Ion Full Cells Containing Silicon Alloy, Journal of The Electrochemical Society, 164 (2017) A2527-A2533.

A Practical Li-Ion Full Cell with a High-Capacity Cathode and Electrochemically Exfoliated Graphene Anode: Superior Electrochemical and Low-Temperature Performance

Lithium-ion batteries (LIBs) are widely adopted power sources for portable digital products and electrical vehicles (EV). However, the high cost associated with raw materials and manufacturing and low-temperature performance are primary concerns for large-scale deployment of LIBs, especially for EV and grid-scale storage applications. Herein, a novel lithium-ion full cell assembled by 3 μm-LiNi0.6Co0.2Mn0.2O2 (LNCM-3) cathode and electrochemically exfoliated graphene (EG) anode was fabricated, generating an energy density of ∼278 Wh kg–1, which is calculated on the basis of the design of commercial 18650 LIBs. Besides, it also exhibits outstanding rate capability, superior low-temperature electrochemical performance and ultralong cycle life (no capacity fading within 300 cycles at temperatures from 25 to −30 °C at 180 mA g–1). The proposed novel full cell configuration will enable the next-generation high performance batteries for EV applications.

Spray-Printed and Self-Assembled Honeycomb Electrodes of Silicon-Decorated Carbon Nanofibers for Li-Ion Batteries.

Directional, micron-scale honeycomb pores in Li-ion battery electrodes were fabricated using a layer-by-layer, self-assembly approach based on spray-printing of carbon nanofibers. By controlling the drying behavior of each printed electrode layer through optimization of (i) the volume ratio of fugitive bisolvent carriers in the suspension and (ii) the substrate temperature during printing, self-assembled, honeycomb pore channels through the electrode were created spontaneously and reliably on current collector areas larger than 20 cm × 15 cm. The honeycomb pore structure promoted efficient Li-ion dynamics at high charge/discharge current densities. Incorporating an optimum fraction (2.5 wt %) of high-energy-density Si particulate into the honeycomb electrodes provided a 4-fold increase in deliverable discharge capacity at 8000 mA/g. The spray-printed, honeycomb pore electrodes were then investigated as negative electrodes coupled with similar spray-printed LiFePO4 positive electrodes in a full Li-ion cell configuration, providing an approximately 50% improvement in rate capacity retention over half-cell configurations of identical electrodes at 4000 mA/g.

Comprehensive Aging Analysis of Volumetric Constrained Lithium-Ion Pouch Cells with High Concentration Silicon-Alloy Anodes

In this research, twenty-four high capacity (1360 mAh) NMC622/Si-alloy Li-ion full pouch cells with high silicon-alloy content (55%) are cycle aged under seven different cycling conditions to study the effect of different stressors on the cycle life of Si-anode full cells, among which are the effect of ambient temperature, Depth of Discharge (DoD) and the discharge current. The cells are volumetrically constrained at an optimal initial pressure to improve their cycle life, energy and power capabilities. Furthermore, the innovative test setup allows measuring the developed pressure as a result of repeated (de-)lithiation during battery cycling. This uniquely vast testing campaign on Si-anode full cells allows us to study and quantify independently the influence of different stress factors on their cycle life for the first time, as well as to develop a new capacity fade model based on an observed linear relationship between capacity retention and total discharge capacity throughput.

A Self‐Healing Room‐Temperature Liquid‐Metal Anode for Alkali‐Ion Batteries

AbstractGiven the high energy density, alkali metals are preferred in rechargeable batteries as anodes, however, with significant limitations such as dendrite growth and volume expansion, leading to poor cycle life and safety concerns. Herein a room‐temperature liquid alloy system is proposed as a possible solution for its self‐recovery property. Full extraction of alkali metal ions from the ternary alloy brings it back to the binary liquid eutectic, and thus enables a self‐healing process of the cracked or pulverized structure during cycling. A half‐cell discharge specific capacity of up to 706.0 mAh g−1 in lithium‐ion battery and 222.3 mAh g−1 for sodium‐ion battery can be delivered at 0.1C; at a high rate of 5C, a sizable capacity of over 400 mAh g−1 for Li and 60 mAh g−1 for Na could be retained. Li and Na ion full cells with considerable stability are demonstrated when pairing liquid metal with typical cathode materials, LiFePO4, and P2‐Na2/3[Ni1/3Mn2/3]O2. Remarkable cyclic durability, considerable theoretical capacity utilization, and reasonable rate stability present in this work allow this novel anode system to be a potential candidate for rechargeable alkali‐ion batteries.

Preparation of ternary phase Li4Ti5O12/anatase/rutile nanocomposites with defects and their enhanced capability for lithium ion storage

Developing anode materials that have high safety and high performance is essential for their application in Li ion batteries. Ternary phase Li4Ti5O12/rutile/anatase (N-LTO/A/R) nanocomposites that have abundant phase boundaries and structure defects are prepared via solvothermal synthesis and following calcination under an Ar/NH3 atmosphere. With the introduction of multiple phases and defects, the as-prepared ternary phase nanocomposites display excellent lithium storage capability in a Li half-cell as well as in full-cell batteries. The nanocomposites deliver a reversible specific capacity of 149.0 mAh g−1 at a current density of 10 A g−1 and maintain about 85% of the specific capacity after 800 cycles at a higher rate of 5 A g−1 for the Li half-cell. Even in the Li ion full-cell, the as-prepared nanocomposites deliver specific capacities of 210 and 124 mAh g−1 at current densities of 0.05 and 1 A g−1, respectively. The extraordinary electrochemical performance of the N-LTO/A/R nanocomposites is attributed to the nanostructures, phase boundaries, presence of structure defects, and larger specific surface area, which favor facilitating lithium ion diffusion and electron transfer. This study shows that the ternary phase nanocomposites are very promising as a high-performance anode in lithium ion batteries.

Performance and Failure Mechanisms of Si Based Li-Ion Full Cells

Lithium based chemistry is on the forefront of battery technology and many efforts are done to optimise Li-ion batteries in terms of lifetime and energy density. Many alternative Li chemistries arose during the last decade, which could potentially replace classical rocking chair batteries. Today, most of these technologies are still in their infancy and it is difficult to foresee if the corresponding technology can be commercialised. Current Li-ion batteries rely on intercalation host materials and the capacity is limited by the positive host materials delivering a specific capacity of around 120-200 mAh/g. Graphite, the state of the art negative electrode in commercial cells, delivers a moderate capacity of 320-350 mAh/g. A two to three fold increase in specific capacity of the negative electrode would have a profound effect on the specific capacity of a full Li-ion cell. Si is a possible contender to substitute graphite in the next generation of Li-ion batteries. It undergoes an alloying reaction with Li at low potentials resulting in a high theoretical capacity of about 3600 mAh/g or 8500 mAh/cm3. This high level of lithiation causes enormous volume changes. It comes with the price of electrode detachment or pulverisation and an instable SEI layer, which cannot keep intimate contact with the Si surface upon these volume changes. Hence, Si negative electrodes suffer from severe inefficiencies and charge fade.1,2 Various morphologies have been synthesised with the aim to better accommodate the volume expansions. Furthermore, several binding agents, electrolyte additives and salts have been studied to increase the cyclability of Si based electrodes.3,4 Most of the synthesised Si structures are very costly and would result in a low tap density and hence a low volumetric capacity. In addition, the vast majority of studies have been performed on half-cells, where a Li counter electrode acts as a sheer endless Li reservoir. Unfortunately, this Li reservoir can mask several issues of the Si electrode and it has been shown that the performance and failure mechanisms are different in full cells.5,6 This study concerns the performance and failure mechanism of full cells with layered oxide positive electrodes and µ-Si negative electrodes. The Si electrodes consisted of commercial available battery grade silicon from Elkem (Silgrain ®, e-Si 400), graphite and carbon black and an aqueous Na-CMC binder. Layered oxides such as LCO and NCA were synthesised by a citric acid aided wet chemical route. Performance and failure mechanisms of the cells were analysed by a toolbox of electrochemical methods and post mortem analysis using electron microscopy coupled with energy dispersive X-ray spectroscopy and electron energy loss spectroscopy. 1 D. Larcher, S. Beattie, M. Morcrette, K. Edström, J.-C. Jumas and J.-M. Tarascon, J. Mater. Chem., 2007, 17, 3759. 2 N. Nitta, F. Wu, J. T. Lee and G. Yushin, Mater. Today, 2015, 18, 252–264. 3 M. Ashuri, Q. He and L. L. Shaw, Nanoscale, 2016, 8, 74–103. 4 C. Erk, T. Brezesinski, H. Sommer, R. Schneider and J. Janek, ACS Appl. Mater. Interfaces, 2013, 5, 7299–7307. 5 S. D. Beattie, M. J. Loveridge, M. J. Lain, S. Ferrari, B. J. Polzin, R. Bhagat and R. Dashwood, J. Power Sources, 2016, 302, 426–430. 6 N. Dupré, P. Moreau, E. De Vito, L. Quazuguel, M. Boniface, A. Bordes, C. Rudisch, P. Bayle-Guillemaud and D. Guyomard, Chem. Mater., 2016, 28, 2557–2572.

Positive – Positive Symmetric Cell Study on the Compatibility of Different Electrolyte Additives with Li[Ni0.6Mn0.2Co0.2]O2

Electrode/electrolyte compatibility plays a critical role to increase the energy density and longevity of lithium-ion cells. A sound understanding of the interactions between electrode and electrolyte allows us to design better electrolyte formulation with more suitable additives. Nevertheless, the study of electrode/electrolyte interaction is difficult to carry out in a full lithium-ion cell due to the interactions between positive and negative electrodes, such as the migration of oxidation products and dissolved transition metal to the negative electrode1, and the effect of different negative electrodes on the surface layer composition of positive electrode2. In recent years, there has been quite a few experimental techniques developed aiming to solve this problem3,4. Among them, symmetric cells are excellent test vehicles where performance is exclusively determined by the compatibility of the electrode of interest with the electrolyte of choice. In this study, the compatibility of two electrolyte additives, vinylene carbonate (VC) and prop-1-ene-1,3-sultone (PES), with uncoated polycrystalline Li[Ni0.6Mn0.2Co0.2]O2 were studied using positive-positive symmetric cells with electrodes hand-made from powder. Each symmetric cell was constructed with one fresh electrode and one delithiated electrode prepared using a coin-cell half cell. These symmetric cells were then cycled at 40oC with 10 cycles of C/10 followed by 1 cycle of C/30 repeatedly between ±0.9 V, ±0.8 V, ±0.7 V, ±0.6 V, ±0.5 V, which correspond to half cell voltages of 4.39 V, 4.35 V, 4.29 V, 4.22 V and 4.15 V vs. Li/Li+, respectively. It was found that for all symmetric cells cycled with the chosen voltage cut-offs, the cells containing VC showed better capacity retention compared to those with PES. This is surprising because in full Li-ion cells operated above 4.3 V, the opposite was found to be true by Lin Ma et al. 5 The symmetric cell building method in this work can also be used to study electrode/electrolyte compatibility when machine-made electrodes are not readily available to researchers. References A. J. Smith, J. C. Burns, D. Xiong, and J. R. Dahn, J. Electrochem. Soc., 158, A1136 (2011)E. Björklund, D. Brandell, M. Hahlin, K. Edström, and R. Younesi, J. Electrochem. Soc., 164, A3054–A3059 (2017)D. J. Xiong et al., J. Electrochem. Soc., 164, A340–A347 (2017)C. Shen et al., J. Electrochem. Soc., 164, A3349–A3356 (2017)Lin Ma et al, J. Electrochem. Soc. 161, A2250-A2254, (2014). Figure 1

Mesoporous Mn-Sn bimetallic oxide nanocubes as long cycle life anodes for Li-ion half/full cells and sulfur hosts for Li-S batteries

Mesoporous Mn-Sn bimetallic oxide (BO) nanocubes with sizes of 15–30 nm show outstanding stable and reversible capacities in lithium ion batteries (LIBs), reaching 856.8 mAh·g–1 after 400 cycles at 500 mA·g–1 and 506 mAh·g–1 after 850 cycles at 1,000 mA·g–1. The preliminary investigation of the reaction mechanism, based on X-ray diffraction measurements, indicates the occurrence of both conversion and alloying–dealloying reactions in the Mn-Sn bimetallic oxide electrode. Moreover, Mn-Sn BO//LiCoO2 Li-ion full cells were successfully assembled for the first time, and found to deliver a relatively high energy density of 176.25 Wh·kg–1 at 16.35 W·kg–1 (based on the total weight of anode and cathode materials). The superior long-term stability of these materials might be attributed to their nanoscale size and unique mesoporous nanocubic structure, which provide short Li+ diffusion pathways and a high contact area between electrolyte and active material. In addition, the Mn-Sn BOs could be used as advanced sulfur hosts for lithium-sulfur batteries, owing to their adequate mesoporous structure and relatively strong chemisorption of lithium polysulfide. The present results thus highlight the promising potential of mesoporous Mn-Sn bimetallic oxides for application in Li-ion and Li-S batteries.

Robust erythrocyte-like Fe2O3@carbon with yolk-shell structures as high-performance anode for lithium ion batteries

To address the issues of sluggish electronic transport and large volume variation of Fe2O3 anode nanomaterials caused by repeated Li+ insertion/extraction, herein we engineer an erythrocyte-like Fe2O3 with thin mesoporous carbon into yolk-shell architectures, in which the erythrocyte-like Fe2O3 is well confined inside the thin mesoporous carbon. In this design, the erythrocyte-like Fe2O3 cores with high specific capacity together with the permeable thin carbon shells could not only greatly facilitate the fast diffusion for both ion and electron but also retain a thin and stable solid electrolyte interface (SEI) layer on its surface, resulting in excellent rate capability and cycling stability. Meanwhile, the hollow cavity between carbon shell and Fe2O3 core provides extra free space for Fe2O3 expansion, thus preserving the structural integrity. By virtue of their advantageous structural features, the elegant void-containing mesoporous carbon-encapsulated erythrocyte-like Fe2O3 electrodes exhibit superior performance including large capacity, high rate capability and long cycle stability. More importantly, when coupled with commercial LiCoO2 cathode, the assembled Li-ion full cell demonstrates a high reversible capacity of 615 mA h g−1 after 100 cycles at 1C with capacity retention of 83.4% as well as outstanding rate capability (675 mA h g−1 at 4C with capacity retention of 83% from 0.2C to 4C), demonstrating significant potential for future practical applications.

Safety Enhanced, Block Copolymer Derived Nanoporous Battery Separators

A battery separator physically and electronically separates the anode and cathode but permits Li+ transport between them. Battery separators play a critical role in battery safety by providing, e.g., thermal shutdown functionality in abusive events, or, by preventing lithium metal dendrites or other defects in the cell from penetrating the membrane which can cause internal short circuit leading to uncontrolled thermal runaway, as in the case of the Boeing 787 battery fire incident [1]. One of the challenges with designing safe battery separators is the trade-off between mechanical robustness and porosity/transport properties. For example, high separator tortuosity is good for dendrite resistance but can lead to higher separator resistance [2]. Separator design is further complicated by additional constrains including tolerance to abuse conditions, stable at high voltages (e.g., > 4V), chemically inert to other cell materials, and low cost to meet the performance and cost targets. There is a need to tailor design battery separators that enable high energy and power with engineered functionality for safety, without adversely affecting cost. Here we present ADA efforts, in collaboration with university researchers, to develop a nanoporous battery separator that is highly tunable in safety characteristics (e.g., thermal shutdown temperature) and is derived from functionalized block copolymers (polyolefins) with low cost precursors [3, 4, 5]. In particular, we will present a hybrid separator concept where the rapid thermal shutdown-capable nanoporous separator developed is hybridized with a highly porous, mechanically and thermally robust separator. This approach renders a hybrid separator technology that affords high energy density, high rate capability, long cycle life and unprecedented safety characteristics for the next generation Li-ion batteries. Figure 1 shows scanning electron microscopy (SEM) images of the synthesized nanoporous polyethylene (NPE) membranes, an average 25 mm thickness, revealing the desired bicontinuous cubic morphology and percolating pore architecture. Our membranes possess a narrower pore size distribution and a high porosity, as intended based on our chemistry design selection, compared with the commercial separator. Figure 2 shows separator impedance as a function of temperature, for NEP Gen1, NPE Gen2, and a commercial separator. The onset of the impedance rise corresponded to 80°C, 115°C and 130°C for NPE Gen2, NPE Gen1, and the commercial separator, respectively. The increase of the separator impedance is a result of the separator thermal shutdown, i.e., micropore closing due to the melting/shrinking of the separator, which impedes the ionic motion. The drastic separator impedance increase associated with the separator thermal shutdown is responsible for providing the safety feature to the otherwise normal operation of Li-ion batteries. The enhanced thermal shutdown features seen in the NPE separators will render earlier, much more rapid and effective thermal shutdown at abusive events (e.g., short circuit, overcharge, etc.), which leads to much improved safety charasteristics for Li-ion batteries. Figure 3 shows charge/discharge voltage vs. specific capacity for a representative graphite/LiCoO2 (LCO) full cell using a NPE-based hybrid separator. The full cell displayed a good discharge specific capacity of ~ 120 mAh/g (LCO active mass). Figure 4 shows a rate capability test. The NPE-based hybrid separator demonstrated superior rate capability than the commercial separator at C rates ≥ 5C. At 20C rate, the NPE hybrid separator based Li-ion cells demonstrated an impressive rate capability of > 73% capacity retention, 2.5 times that of the commercial counterpart (29%). Figure 5 shows discharge capacity retention vs. cycle number of the Li-ion full cells using NPE-based hybrid separator and a commercial separator (control). The NPE hybrid separator based Li-ion full cells showed excellent cycle life performance with capacity retention of 88% after 300 cycles. It is noticed that the NPE-based hybrid separator demonstrated very compatible cycle life performance as the commercial separator. This result illustrates an excellent chemical and electrochemical compatibility of the NPE-based hybrid separator. Our study shows that the strategy of preparing block copolymer derived nanoporous separators provides a very promising and powerful tool to allow exquisite tuning of performance and safety features (e.g., thermal shutdown temperature) of the nanostructured separator. The NPE-based hybrid separator technology will afford an unprecedented performance and safety characteristics for the next generation Li-ion batteries. References National Transportation Safety Board Incident Report, NTSB/AIR-14/01, PB2014-108867, Jan 7, 2013.Pankaj Arora and Zhengming (John) Zhang, Rev. 104, 4419-4462, (2004).Louis M. Pitet, Mark A. Amendt, and Marc A. Hillmyer, AM. CHEM. SOC. 132, 8230–8231 (2010).Bielawski, C. W.; Grubbs, R. H. Polym. Sci. 32, 1–29 (2007).Pitet, L. M.; Hillmyer, M. A. Macromolecules, 42, 3674–3680 (2009). Figure 1

Effect of Electrolyte Compositions on Cycling Performance of Li-Ion Full Cells with Si-Graphite Composite Electrodes

The demand for high energy densities in Li-ion batteries (LIBs) necessitates exploring bottlenecks of cell performance. Layered oxide positive electrodes, such as LiNixCoyMn1-x-yO2 (NCM), and silicon-graphite composite (Si-Gr) negative electrodes in a LIB undergo a series of complex structural and compositional changes during electrochemical cycling. The induced changes have a direct influence on the energy and power output from a battery and a profound dependence on the cycling or aging conditions. The promise of multifold enhancement in energy densities of LIBs using Si based negative electrodes is accompanied by the limited stability of the electrode-electrolyte interface. In commercial LIBs, the solid electrolyte interphase (SEI) layer on the graphite negative electrode facilitates Li+ ion conduction and minimizes reduction of the electrolyte. In cells with Si- containing negative electrodes, the large volume changes of Si during cycling results in continuous degradation of the SEI layer. The formation of a new SEI on the newly-exposed Si surfaces during subsequent cycling causes further trapping of Li+ ions, contributing to performance loss. In the present study, full cells with NCM 523 cathode, Si-Gr (containing 15 wt% Si) anode, and a baseline electrolyte (1.2 M LiPF6 in Ethylene Carbonate/Ethyl Methyl Carbonate, 3:7 w/w) were tested under calendar life ageing and cycle life ageing protocols between 3 and 4.1 V. The cycle-life cells showed ~92% capacity loss after 100 cycles, while the calendar-life cells showed ~13% capacity loss; the reasons for this difference will be highlighted during the presentation.1 In order to further improve capacity retention we studied the effect of various additives to the baseline electrolyte, and also examined EC-free electrolyte systems. Advanced diagnostic techniques, including X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and scanning electron microscopy (SEM) were used to examine the effect of these additives on the SEI layer.2,3 Insights from electrochemical response in conjunction with post-mortem characterization studies have fostered deeper understanding of the charge transfer mechanisms and the properties of the interfacial layers in the blended negative electrodes. Fundamental knowledge of the nature of this dynamic SEI layer is essential to design better surface films and achieve stable electrochemical cycling. Support from David Howell, Brian Cunningham, and Peter Faguy of the U.S. Department of Energy’s Office of Vehicle Technologies is gratefully acknowledged. This work was performed under the auspices of the US Department of Energy, Office of Vehicle Technologies, Hybrid and Electric Systems, under Contract No. DE-AC02-06CH11357 K. Kalaga et al., manuscript in preparation (2017). K. Kalaga, I.A. Shkrob, R.T. Haasch, C. Peebles, J. Bareño, D.P. Abraham, “Auger Electrons as Probes for Composite Micro and Nanostructured Materials: Application to Solid Electrolyte Interphases in Graphite and Silicon-Graphite Electrodes”, J. Phys. Chem. C 2017, 121, 23333−23346. J. Bareño, I.A. Shkrob, J.A. Gilbert, M. Klett, D.P. Abraham, “Capacity Fade and Its Mitigation in Li-ion Cells with Silicon-Graphite Electrodes”, J. Phys. Chem. C 121 (2017), pp 20640-20649.

Horsetail-derived Si@N-doped carbon as low-cost and long cycle life anode for Li-ion half/full cells

Silicon (Si) nanomaterials attract tremendous attentions when applied as anodes for lithium ion batteries (LIBs) owing to their high capacity, however, they usually suffer from low electronic conductivity and large volume expansion during the charge/discharge processes, resulting in the poor cycling stabilities. In this study, horsetails (HTs) are explored as low cost and renewable resources for deriving Si materials for Li-ion half/full cells for the first time. Moreover, Si@N-doped carbon nanocomposites are fabricated by polypyrrole (PPy) coating and pyrolysis carbonization of PPy at 400 °C for 2 h in Ar atmosphere, and they show enhanced long cycle stabilities for Li-ion half cells (1047.1 mAh g−1 at 0.5 A g−1 after 450 cycles, and 750 mAh g−1 at 1 A g−1 after 760 cycles). Furthermore, the Si@N-doped carbon//LiCoO2 Li-ion full cells are assembled successfully, which display high discharge capacity (705 mAh g−1 at 0.5 A g−1 after 100 cycles). The high performance of Si@N-doped carbon nanocomposite could be attributed to the synergistic effect of high capacity of nanoscale sized Si and long cycle stability of N-doped carbon. It is believed the HTs-derived Si@N-doped carbon nanocomposite has great prospects for practical applications due to low cost and sufficient source of HTs.

New Electrode and Electrolyte Configurations for Lithium-Oxygen Battery.

Cathode configurations reported herein are alternative to the most diffused ones for application in lithium-oxygen batteries, using an ionic liquid-based electrolyte. The electrodes employ high surface area conductive carbon as the reaction host, and polytetrafluoroethylene as the binding agent to enhance the oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) reversibility. Roll-pressed, self-standing electrodes (SSEs) and thinner, spray deposited electrodes (SDEs) are characterized in lithium-oxygen cells using an ionic liquid (IL) based electrolyte formed by mixing lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt and N,N-diethyl-N-(2-methoxyethyl)-N-methylammonium bis(trifluoromethanesulfonyl)imide (DEMETFSI). The electrochemical results reveal reversible reactions for both electrode configurations, but improved electrochemical performance for the self-standing electrodes in lithium-oxygen cells. These electrodes show charge/discharge polarizations at 60 °C limited to 0.4 V, with capacity up to 1 mAh cm-2 and energy efficiency of about 88 %, while the spray deposited electrodes reveal, under the same conditions, a polarization of 0.6 V and energy efficiency of 80 %. The roll pressed electrode combined with the DEMETFSI-LiTFSI electrolyte and a composite Lix Sn-C alloy anode forms a full Li-ion oxygen cell showing extremely limited polarization, and remarkable energy efficiency.

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.

Design of Positive Electrodes for Li-Ion Full Cells with Silicon

Wound pouch cells with a Li(Ni0.6Mn0.2Co0.2)O2 (NMC622) positive electrode and graphite/Si alloy negative electrode are shown to have significantly better performance than equivalent cells with a LiCoO2 (LCO) positive electrode. Silicon containing NMC622 full cells have better performance with a 4.35 V upper cutoff voltage (UCV) than with a 4.25 V UCV. The addition of small amounts (≤8 wt%) of high surface area NMC622 to LCO-based positive electrodes in single layer pouch cells with graphite/Si alloy negative electrodes improves cycling performance and coulombic efficiency. Adding fluoroethylene carbonate (FEC) further improves the performance of cells showing the benefits of NMC622 and FEC can be combined. High precision calorimetry and cycling, Archimedes volume measurements, and cross-section scanning electron microscopy support the hypothesis of interplay between the parasitic reactions at the positive and negative electrodes. The NMC622 positive electrode is proposed to stabilize the Si alloy in the negative electrode via a CO2 release mechanism, though further confirmation of the mechanism is needed.

Effects of Commonly Evolved Solid-Electrolyte-Interphase (SEI) Reaction Product Gases on the Cycle Life of Li-Ion Full Cells

In this work, it is found that cycling full cells in Ar, versus static conditions, significantly reduces their rate of capacity fade, which is attributed to the removal of gaseous reaction products from the cell. The presence of gases evolved in situ can account for a large fraction of the total capacity fade exhibited by the static cells tested. In attempt to understand the role of specific gaseous species commonly evolved during cycling, LiMn2O4 and LiCoO2 based full cells were further cycled in Ar-H2, Ar-CO, Ar-CO2, and Ar-CH4 at reactive gas compositions between 0.5% and 5% and flow rates high enough to rapidly remove any evolved gas. The different electrodes were sensitive to different gases, the effects of gas composition were non-linear, and the effect of mixing gases was not additive. An attempt was made to correlate the degree of capacity fade with the amount of transition metal deposited on the anode, as measured by secondary ion mass spectrometry, but a strong correlation was not found.

Thermal Behavior of Ni-Rich Layered Oxide Cathode Materials during Cycling of 20 Ah-Scale Li-Ion Batteries

Ni-rich layered oxide cathode materials have been extensively investigated as a cathode material for lithium-ion batteries because of their higher reversible capacity and lower cost than lithium cobalt oxide. However, most of them have been examined using lab-scale coin cells, and consequently, there is a limit to fully understanding the failure mode of Ni-rich layered oxides in commercial high energy Li-ion batteries. In this paper, the pouch-type 20 Ah-scale Li-ion full cells are fabricated using Ni-rich layered oxides as a cathode and graphite as an anode. Then, the electrochemical performances and thermal behaviors of Ni-rich layered oxides are compared to investigate their failure mode in large-scale Li-ion cells. The cell skin temperature is measured to observe the thermal behavior of Ni-rich layered oxides. A strong correlation between capacity retention, cell temperature variation, and internal cell impedance is observed in Ni-rich layered oxides. Moreover, an increase in the internal cell impedance during cycling is attributed to interfacial degradation of Ni-rich layered oxides rather than their bulk structural deformation. Therefore, we need to focus on the interfacial stability of Ni-rich layered oxide materials to improve their electrochemical performance for large-scale Li-ion batteries.