High Energy Density Li-ion Batteries Research Articles

Enhancing the High-Voltage Cycling Stability of Nickel-Rich Oxide Cathode Using Functional Electrolyte

Increasing needs of electric vehicles and grid-based energy storage systems for the reduction of carbon dioxide gases require higher energy density lithium (Li)-ion batteries than commercial batteries. The energy density of Li-ion batteries can increase by increasing the specific capacity of cathode material, which can increase by raising the charge cut-off voltage. Among various cathode materials, nickel-rich layered oxides of Li(Ni1 –x –y Co x Mn y )O2 (NCM, 1–x–y ≥ 0.5) are promising high-capacity cathode materials for high-energy density Li-ion batteries because of higher capacity and lower cost than nickel-lean oxides. However, the limited anodic instability of the conventional carbonate-based organic electrolytes at the voltage higher than 4.2 V versus Li/Li+ and instability of cathode-electrolyte interface at highly charged state makes high-voltage charge of nickel-rich oxide cathode difficult. In order to mitigate the electrolyte and interface issues, we have been developing new functional electrolytes that provides high anodic stability above 4.3 V and highly stable cathode-electrolyte interface. High-voltage electrochemical and interfacial reaction studies and their correlation to cycling performance would be discussed in the meeting. Acknowledgements: This research was supported by Ministry of Trade, Industry & Energy (R0004645) and Creative Human Resource Development Consortium for Fusion Technology of Functional Chemical/Bio Materials of BK Plus program by Ministry of Education of Korea.

The Role of Zr Doping in Stabilizing Li[Ni0.6 Co0.2 Mn0.2 ]O2 as a Cathode Material for Lithium-Ion Batteries.

Ni-rich layered LiNi1-x-y Cox Mny O2 systems are the most promising cathode materials for high energy density Li-ion batteries (LIBs). However, Ni-rich cathode materials inevitably suffer from rapid capacity fading and poor rate capability owing to structural instability and unstable surface side reactions. Zr doping has proven to be an effective method to enhance the cycle and rate performances by stabilizing the structure and increasing the Li+ diffusion rate. Herein, effects of Zr-doping on the structural stability and Li+ diffusion kinetics are thoroughly investigated in LiNi0.6 Co0.2 Mn0.2 O2 (LNCM) cathode material using atomic-resolution scanning transmission electron microscopy imaging, XRD Rietveld refinement, and density functional theory calculations. Zr doping mitigates the degree of cation mixing, decreases the structural transformation, and facilitates Li+ diffusion resulting in improved cyclic performance and rate capability. Based on the obtained results, an atomistic model is proposed to explain the effects of Zr doping on the structural stability and Li+ diffusion kinetics in LNCM cathode materials.

Dandelion-like Mesoporous SiOx@C Nanoparticles for High-Capacity and Ultralong Lifespan Lithium-Ion Battery Anode

Silicon suboxide (SiOx, 0˂x˂2) is one of the promising anode materials for lithium-ion batteries (LIBs) due to its high specific capacity, low operating voltage (˂0.4 V) and rich natural resource [1]. SiOx converts to Si nanoparticles and inert components (lithium oxide and lithium silicate) during initial lithiation process, where the Si nanoparticles can reversibly store lithium ions and the inert components act as buffer matrix to accommodate the large volume variation of nano-sized Si. Compared to pure Si, SiOx electrode shows relatively good cycling stability while maintaining high specific capacity, thus is a promising anode material for high energy density Li-ion batteries. Nevertheless, bulk SiOx electrode cannot withstand long-term cycling due to the inevitable volume variation upon lithium uptake and removal. In addition, the poor intrinsic electrical conductivity of SiOx often limits the capacity utilization [2]. In this work, we prepared mesoporous SiOx@C nanoparticles with unique “dandelion flower” structure by a facile microemulsion method combined with a subsequent carbon coating process. The SiOx particles display a dandelion flower like porous structure with large void space and specific surface area. A uniform carbon layer wholly encapsulates the SiOx backbone, forming a 3D mono-dispersed SiOx@C particle with particle size of ~70 nm. The abundant mesopores inside particle can accommodate the volume expansion of SiOx during lithiation and lower the diffusion-induced stresses, which prevent from the electrode pulverization and thus maintain the structural integrity of SiOx@C electrode during repeated cycles. The 3D carbon network not only effectively facilitates the fast electron transport throughout the whole SiOx particle and thereby improving the electrode reaction kinetics, but also homogenizes the local current density of active particles and hence decreases the mechanical stress induced by volume change of SiOx. Benefiting from these advantages, the prepared SiOx@C anode exhibits ultrahigh reversible capacity (1261 mAh g-1 at 0.1 A g-1), ultralong lifespan (635 mAh g-1 at 2 A g-1 after 1000 cycles with retention of 88.6%), as well as superior rate capability (293 mAh g-1 at 10 A g-1). [1] Journal of Power Sources, 2017, 363: 126-144. [2] J. Mater. Chem. A, 2014, 2, 3521-3527.

Electrolyte Development and Its Application for Fast Charging of Li-Ion Batteries

Energy density, cost and safety are, more than ever, the most significant barriers to overcome in order to increase the wide acceptance of Li-ion batteries (LIBs) in electric vehicles (EVs).1 The U.S. Department of Energy (DOE) has set ultimate goals for battery electric vehicles (BEVs), which include reducing the production cost of the battery pack to $150/kWh, increasing the electrical range of the battery to 300 miles, and decreasing the charging time to 15 minutes or less.2 Increasing electrode thickness, and hence increasing active material loading, is an effective way to achieve these energy density and cost targets.3 The caveat, however, is that thicker electrodes fail during fast charging. One of the solutions for enabling extreme fast charging while retaining most of the battery energy can be achieved through the significant enhancement of the Li-ion mass-transport in electrolytes such that enough Li ions are available for intercalation in graphite. The mass transport of Li-ions can be evaluated by two macroscopic characteristic values: 1) the Li ions ionic conductivity that is related to the total flux of charge carriers, 2) the Li ion transference number that is related to the fraction of the total current that is carried by Li ions. An electrolyte with both higher Li ions conductivity and transference numbers is ideal for higher Li ions transport, and hence would be a step toward realizing cells with higher charging rates. The fast charging performance of high-energy density (NMC811/graphite) Li-ion cells iss studied when different lithium salts were used in the electrolyte. The new electrolyte shows both higher ionic conductivity and Li-ion transference number compared to LiPF6 electrolyte. During a 12-minute fast charging step, cells with LiPF6 electrolyte reached the cut-off voltage in 4.2 minutes, which is much earlier compared to 7.4 minutes for the new electrolyte. The new electrolyte showed 13% capacity improvement in the first cycle of the 12-minute charge. The capacity retention was also significantly higher at 87.7% after 500 cycles with less lithium plating observed. (1) Needell, Z. A.; et al. Potential for Widespread Electrification of Personal Vehicle Travel in the United States. Nat. Energy 2016, 1 (9). (2) Howell, D.; et al. Enabling Fast Charging: Enabling Fast Charging: A Technology Gap Assessment. 2017, No. October. (3) Du, Z.; et al. Understanding Limiting Factors in Thick Electrode Performance as Applied to High Energy Density Li-Ion Batteries. J. Appl. Electrochem. 2017, 47 (3), 405–415. Figure 1

Engineering the Core–Shell-Structured NCNTs-Ni2Si@Porous Si Composite with Robust Ni–Si Interfacial Bonding for High-Performance Li-Ion Batteries

A new strategy has been innovatively proposed for wrapping the Ni-incorporated and N-doped carbon nanotube arrays (Ni-NCNTs) on porous Si with robust Ni-Si interfacial bonding to form the core-shell-structured NCNTs-Ni2Si@Si. The hierarchical porous silicon core was first fabricated via a novel self-templating synthesis route based on two crucial strategies: in situ thermal evaporation of crystal water from the perlite for producing porous SiO2 and subsequent magnesiothermic reduction of porous SiO2 into porous Si. Ni-NCNTs were subsequently constructed based on the Ni-catalyzed tip-growth mechanism and were further engineered to fully wrap the porous Si microparticles by forming the Ni2Si alloy at the heterojunction interface. When the prepared NCNTs-Ni2Si@Si was evaluated as the anode material for Li-ion batteries, the hierarchical porous system in the Si core and the rich void spaces in carbon nanotube arrays contributed to the remarkable accommodation of volume expansion of Si as well as the significant increase of Li+ diffusion and Si utilization. Moreover, the Ni2Si alloy, which chemically linked the Ni-NCNTs and porous Si, not only provided good electronic contact between the Si core and carbon shell but also effectively prevented the CNTs' detachment from the Si core during cycling. The multifunctional structural design rendered the whole electrode highly stable and active in Li storage, and the electrochemically active NCNTs-Ni2Si@Si electrode delivered a high reversible capacity of 1547 mAh g-1 and excellent cycling stability (85% capacity retention after 600 discharge-charge cycles) at a current density of 358 mA g-1 (0.1 C) as well as good rate performance (778 mAh g-1 at 2 C), showing great potential as an efficient and stable anode for high energy density Li-ion batteries.

Precursor pre-oxidation enables highly exposed plane {010} for high-rate Li-rich layered oxide cathode materials

Precursor synthesis technology is a key factor to improve the electrochemical properties of Li-rich layered oxide (LLO) cathode materials. However, the effect of precursor preparation strategies on the morphology and electrochemical properties of cathode materials has not been clearly elucidated. In this work, an LLO cathode material (denoted as LMNC-300) with highly exposed {010} planes and a lower amount of surface residual lithium is reported via pre-oxidation of a precursor at 300 °C. This LMNC-300 cathode material exhibits an initial Coulombic efficiency of 91.7% with a discharge capacity of 277.2 mA h g−1 at 0.1 C, an excellent high-rate capability with a discharge capacity of 175.1 mA h g−1 even at 5.0 C and a good cycling performance with a capacity retention of 92.6% after 200 cycles at 0.5 C. In addition, a pouch cell consisted of the LMNC-300 cathode and a commercial graphite anode presented good cycling stability with a capacity retention of 81.6% after 500 cycles at 2.0 C. The outstanding electrochemical performance can be ascribed to an accelerated Li+ diffusion dynamics. Therefore, the method reported in this work will be significant to understanding the effects of preparation strategies of transition metal precursors on the electrochemical performance of Li-rich layered cathode materials for high-energy density Li-ion batteries.

High–Speed electron beam curing of thick electrode for high energy density Li-ion batteries

Electron beam curing is demonstrated as a promising method for high speed, low cost and environmentally friendly battery electrode manufacturing. This work reports transfer of this process to pilot scale equipment and evaluation of electrochemical performance in prototype 1.5 Ah pouch cells. Thick LiNi0.5Mn0.3Co0.2O2 (NMC532) composite electrodes with an areal loading of 25 mg cm−2 (∼4 mAh cm−2) are successfully cured at a line speed of 500 feet per minute at 275 keV. Compared to the NMC532 cathode processed via a conventional coating method, the electron beam cured electrodes show higher capacity fade in the first 100 cycles, but similar fade rate afterwards. Further improvement strategies are proposed and discussed. This work demonstrates that electron beam curing is a promising method for manufacturing thick battery electrodes at high speeds and low capital/operation cost.

Material structure and chemical bond effect on the electrochemical performance of black phosphorus-graphite composite anodes

Black phosphorus (BP) is a little-studied but promising next-generation anode material for high-energy density Li-ion and Na-ion batteries. To understand the relationship between material property and electrochemical performance, black phosphorus-graphite (BP-G) composite materials are synthesized with two different BP-G molar ratios. This study demonstrates that the crystal structure and PC bond of BP-G composites are affected by the BP-G molar ratio, which are directly correlated to cycle stability and the lithiation/delithiation process. BP0.9G1 shows a medium-range ordered structure, retaining some features of BP and having fewer or weaker PC bonds. In contrast, BP0.3G1 has an amorphous-like structure with robust PC bonds, which is helpful in withstanding the large volume change upon lithiation or delithiation. As a result, fracture and pulverization of particles, which are clearly seen in BP0.9G1 electrode, are scarcely observed in the cycled BP0.3G1 electrode. The possibility of mechanical failure in the BP-based electrode induced by BP's hydrophilic nature and the formation of Cu3P is also discussed. This work not only contributes to improving the current understanding of phosphorus-based anodes, but also provides direct evidence of fracture and pulverization of BP particles during cycling.

Role of Polyacrylic Acid (PAA) Binder on the Solid Electrolyte Interphase in Silicon Anodes

To obtain high-energy density Li-ion batteries for the next-generation storage devices, silicon anodes provide a viable option because of their high theoretical capacity, low operating potential ve...

Double-helix-superstructure aqueous binder to boost excellent electrochemical performance in Li-rich layered oxide cathode

Li-rich layered oxides are highlighted as promising candidates for the next-generation cathode materials in Li-ion batteries, but suffer from poor cycling stability and voltage fading during the cycling process. Currently, less efforts are devoted to addressing these issues through exploring novel binders. In this work, we explore an aqueous binder of xanthan gum with double helix superstructure containing abundant charged functional groups. This unique structure is beneficial to the occurrence of significant mechanical interlocking effect and formation of chemical bond with particles’ surface, which can effectively suppress electrode exfoliation, particle pulverization and structural degradation during cycling. As a consequence, the electrode with xanthan gum still delivers the discharge capacity of 275.6 mAh g−1 with the retention of 98.4% after 200 cycles at 0.1C rate. Accordingly, the mid-point discharge voltage remains 87% (about 2.3 mV/per cycle drop). The current study provides a useful insight into how to design and develop superior binders based on charged double-helix superstructure for potential low-cost and environmentally friendly aqueous processing of high energy density Li-ion batteries.

Erratum to: Lithium-ion full cell with high energy density using nickel-rich LiNi0.8Co0.1Mn0.1O2 cathode and SiO-C composite anode

A high-energy-density Li-ion battery with excellent rate capability and long cycle life was fabricated with a Ni-rich layered LiNi0.8Mn0.1Co0.1O2 cathode and SiO−C composite anode. The LiNi0.8Mn0.1Co0.1O2 and SiO−C exhibited excellent electrochemical performance in both half and full cells. Specifically, when integrated into a full cell configuration, a high energy density (280 Wh·kg−1) with excellent rate capability and long cycle life was attained. At 0.5C, the full cell retained 80% of its initial capacity after 200 charge/discharge cycles, and 60% after 600 cycles, indicating robust structural tolerance for the repeated insertion/extraction of Li+ ions. The rate performance showed that, at high rate of 1C and 2C, 96.8% and 93% of the initial capacity were retained, respectively. The results demonstrate strong potential for the development of high energy density Li-ion batteries for practical applications.

Unveiling the Origin of Superior Electrochemical Performance in Polycrystalline Dense SnO2 Nanospheres as Anodes for Lithium-ion Batteries

Development of feasible electrode materials is significant to realize high energy density Li-ion batteries (LIBs). Tin(IV) oxide, in particular, has a number of merits including higher theoretical capacity compared with graphite (1493 mAh g–1), low cost, and environmental friendliness. Nevertheless, huge volume changes and subsequent pulverization usually resulted in poor capacity retention of SnO2, where various nanostructures have been adopted to overcome its intrinsic limitations. Here we introduce the new insights into employing polycrystalline dense SnO2 nanospheres (NSs), rather than its hollow structures, as high-performance electrode for LIBs. Contrary to the previous notions, polycrystalline dense SnO2 NSs can exhibit highly stable cycle retention characteristics (1009.9 mAh g–1 after 300 cycles at 0.5 A g–1) as well as considerable rate capabilities (349 mAh g–1 at 5.0 A g–1), even superior to those of polycrystalline hollow SnO2 NSs. Based on the in situ TEM analyses and electrochemical/postmor...

Problem and Simple Solutions of Layered Cathode Materials

Layered cathode materials, including Ni-rich and Li-rich lithium transition metal oxides, are one class of the most promising cathode materials for high energy density Li-ion batteries because of their high capacity and low cost. However, the Ni-rich and Li-rich cathode materials face two same problems of (1) oxygen evolution at high potentials, and (2) significant amount of Li residues in the forms of Li2O, LiOH, and Li2CO3 on the particle surface. Most of known concerns with these two types of cathode materials can be attributed to these two problems, such as irreversible phase transition from hexagonal to cubic phase, intergranular cracking with repeated cycling, transition metal ion dissolution, as well as electrolyte degradation that consequently results in impedance increase and volumetric swelling of the batteries. Since these two problems are the intrinsic properties of the layered lithium transition metal oxides, there are no perfect solutions that can completely solve these two problems. What one can do is to minimize the negative effects caused by these two problems. By selecting Ni-rich LiNi0.80Mn0.10Co0.10O2 (NMC811) as an example, in this presentation we demonstrate two simple strategies: (1) removing Li residues in the cathode slurry-coating step, and (2) in-situ eliminating the released active oxygen in the battery. Both strategies are simple without need of changing the existing procedures, and show different degree improvements on the cycling stability and rate capability of the Li/NMC811 cells. Our presented strategies are believed to be applicable to all other Ni-rich and Li-rich layered cathode materials.

In Situ Focused Ion Beam Scanning Electron Microscope Study of Microstructural Evolution of Single Tin Particle Anode for Li-Ion Batteries.

Tin (Sn) is a potential anode material for highenergydensity Li-ion batteries because of its high capacity, safety, abundance and low cost. However, Sn suffers from large volume change during cycling, leading to fast degradation of the electrode. For the first time, the microstructural evolution of micrometer-sized single Sn particle was monitored by focused-ion beam (FIB) polishing and scanning electron microscopy (SEM) imaging during electrochemical cycling by in situ FIB-SEM. Our results show the formation and evolution of cracks during lithiation, evolution of porous structure during delithiation and volume expansion/contraction during cycling. The electrochemical performance and the microstructural evolution of the Sn microparticle during cycling are directly correlated, which provides insights for understanding Sn-based electrode materials.

Weakened Capacity Fading of Li-Rich Cathode via Aqueous Binder for Advanced Lithium Ion Batteries

Poor capacity stability during long cycle and poor adhesion under high potential hinder the Lithium-manganese rich layered oxides cathode from applications, and necessary strategy for this is clearly needed. In this work, a novel aqueous binder (lithium poly (acrylic acid), Li-PAA) is introduced for stabilized cyclic performance of Li1.15Ni0.17Co0.11Mn0.57O2 materials. Contrasting with the commercial ones (polyvinylidene fluoride, PVDF), this binder is beneficial to enhance the connection between the electrode and current collector, which can improve cycle stability. In the meanwhile, optimization mechanisms are revealed via detailed measurements, which indicate the suppressed formation of spinel phase during cycling. As a result, the electrode with Li-PAA still delivers the specific capacity of 254.7 mAh g−1 with the retention of 90.77% after 200 cycles at 0.1C, higher than that of electrode with PVDF (223.6 mAh g−1,76.21%). The current subject study provides a conducive insight to develop an excellent binder based on Li-PAA for potential low-riced and environmentally friendly high energy density Li-ion batteries.

Concentrated Electrolytes for Enhanced Stability of Al-Alloy Negative Electrodes in Li-Ion Batteries

Replacing graphite with alloying Al negative electrodes would allow for the development of high energy density Li-ion batteries. However, large volume changes associated with the alloying/dealloying process often result in pulverization of the electrode and rapid capacity fade during cycling due to the continuous formation of solid electrolyte interphase (SEI) layers and loss of electronic contact. In this study, we report that increasing salt concentration in the electrolyte to > 5 mol dm−3 led to enhanced capacity retention during cycling of Li-Al half-cells, which was accompanied by nearly constant impedance for the Al electrode in lithium bis(fluorosulfonyl)imide (LiFSI)/dimethyl carbonate (DMC) 1:1.1 (mol/mol) superconcentrated electrolyte. X-ray photoelectron spectroscopy (XPS) revealed that a potential hold in the superconcentrated electrolyte formed an SEI layer with a greater LiF concentration than in standard 1 mol dm−3 solution. This was supported by Raman spectroscopy of LiFSI solutions in DMC, supplemented with density functional theory calculations, which showed an increased driving force for the reduction of FSI− anions to form LiF from Li+-coordinated DMC complexes with increasing salt concentration. Therefore, the enhanced capacity retention and stability can be attributed to the stability of LiF-rich SEI layers which limit carbonate reduction and charge transfer impedance growth.

Temperature dependent flux balance of the Li/Li7La3Zr2O12 interface

The potential for safe, high energy-density Li-ion batteries has motivated the development of the solid electrolyte Li7La3Zr2O12 (LLZO) to physically stabilize the Li-electrolyte interface. Although dense LLZO is a relatively hard ceramic, it has been observed that above a certain critical current density (CCD), Li metal can still propagate through both polycrystalline and single crystalline LLZO. However, reported values of CCD are still well below the current density regimes relevant to applications like electric vehicles (≥3 mA cm−2). The relationship between CCD and temperature was studied using recently developed methods for achieving consistently low interfacial impedances without interfacial coatings that can complicate the analysis of what controls interface stability. By analyzing the flux of Li+ ions at the Li-electrolyte interface, it is hypothesized here that solid-state diffusivity of Li in the Li electrode may be a governing mechanism that controls the CCD. These results demonstrate an improvement for dendrite-free cycling in LLZO up to ∼1 mA cm−2 at room temperature and ∼7 mA cm−2 at 100 °C without the need for coatings to achieve low interface resistances (∼10 Ω cm2). Furthermore, the presented analysis may provide additional insight on the role of Li diffusivity in Li propagation through ceramic electrolytes.

Non-flammable organic liquid electrolyte for high-safety and high-energy density Li-ion batteries

With increased energy density of rechargeable lithium-ion batteries for powering smart phones and electric vehicles and for their long range use, battery safety becomes more important than ever. This aspect motivated us to develop non-flammable liquid electrolyte that removes the risk of battery fire and explosion, which is urgently needed. Battery energy density and performance however should not be sacrificed to achieve just the safety. Here we report for the first time a rational design of non-flammable carbonate-based organic liquid electrolyte to satisfy safety, energy density and performance simultaneously. Our novel electrolyte, composed of 1 M lithium hexafluorophosphate salt and propylene carbonate and fluorinated linear carbonate co-solvents, at unmeasurable flash point does not fire representing non-flammable safe batteries but permits high-voltage stability to enable high-voltage charge of lithium-rich layered oxide cathode up to 5.0 V, high-energy density of 856 Wh per kg of cathode active mass and stable charge-discharge cycling performance of full-cell with graphite anode, in contrast to rapid performance fade of flammable conventional electrolyte system. The discovery of non-flammable carbonate-based organic liquid electrolyte opens up a new avenue to high-safety and high energy-density lithium-ion batteries for electric vehicles and advanced energy-storage applications.

Scale-up production of high-tap-density carbon/MnOx/carbon nanotube microcomposites for Li-ion batteries with ultrahigh volumetric capacity

Electrode materials with high tap density are important for their practical application to achieve high volumetric energy density Li-ion batteries. In this work, large-scale fabrication of microsized carbon/MnOx/carbon nanotube composites by a low-cost spray-drying method is reported. The carbon/MnOx/carbon nanotube is characteristic of microspherical morphology consisting of interconnected carbon nanotubes and nano-MnOx with an exterior carbon coating. The micropacking enables the composite to exhibit a tap density as high as 2.54 g cm−3. When evaluated as anode materials for Li-ion batteries, the high-tap-density carbon/MnOx/carbon nanotube electrode delivers a specific capacity of 927 mAh g−1 after 100 cycles, corresponding to an ultrahigh volumetric capacity of 2355 mAh cm−3. The carbon network also has dual-functions of excellent electrical conductivity and good mechanical elasticity for the MnOx to sustain superior rate and cycling properties. The scale-up production method provides a reliable practical avenue of fabricating high-tap-density metal-oxide materials for high-performance Li-ion batteries.

Hierarchical nanostructured silicon-based anodes for lithium-ion battery: Processing and performance

Silicon anodes are promising for high energy density Li-ion batteries but are prone to capacity fading with cycles due to volume change and stress buildup leading to failure of electrodes. This study presents an innovative approach for realizing a high capacity silicon anode based on easily available metallurgical low-grade polycrystalline silicon powder. A three-step technique comprising of etching to create silicon nanofibers on particles, rapidly freezing the homogeneous mixture of Si nanofibers and superconducting carbon black in furfuryl alcohol solution, and then converting the furfuryl alcohol to carbon is used to achieve hierarchical nanostructure and improved performance. This approach ensures that the porous structure is filled with and separated by superconducting carbon black and is covered with a conformal and uniform conducting carbon coating. This approach improves the conductivity of the electrode and offers the flexibility for managing the volume expansion during lithiation and delithiation. The resultant electrode shows a reversible specific capacity of ∼778 mA h g−1 even after 200 cycles and coulombic efficiency of 99.6%.