Pouch-type Full Cells Research Articles

Crack-free single-crystalline Ni-rich layered NCM cathode enable superior cycling performance of lithium-ion batteries

Ni-rich layered oxides are extensively employed as a promising cathode material in lithium ion batteries (LIBs) due to their high energy density and reasonable cost. However, the hierarchical structure of secondary particles with grain boundaries inevitably induces the structural collapse and severe electrode/electrolyte interface parasitic reactions as the intergranular crack arises from the anisotropic shrink and expansion. Herein, the single-crystalline LiNi0.83Co0.11Mn0.06O2 (SC-NCM) with primary particles of 3–6 μm diameter is developed and comprehensively investigated, which exhibits superior cycling performance at both room temperature and elevated temperature (55 °C) as well as significantly improved structural integrity after long-term cycling. Remarkably, the SiO-C||SC-NCM pouch-type full cell with a practical loading (8.7 mAh cm−2) delivers a capacity retention of 84.8 % at 45 °C after 600 cycles at a current rate of 1C (1C = 200 mA g−1), retaining a high specific energy density of 225 Wh/kg. Using a combination of X-ray photoelectron spectroscopy, time-of-flight secondary-ion mass spectrometry and scanning transmission electron microscopy, we reveal that SC-NCM particles with micron-sizes effectively mitigate the undesired electrode/electrolyte side interactions and prevent the generation of intergranular cracks, thereby alleviating irreversible structural degradation. The strategy of developing single-crystalline micron-sized particles may offer a new path for maintaining the structural stability and improving cycling life of Ni-rich layered NCM cathodes even under high temperature.

(Invited) Progress in High-Capacity Concentration Gradient Cathode for Next-Generation Electric Vehicles

The ability of Li-ion batteries (LIBs) to provide portable high-density energy sources with outstanding cycle life has led to their deployment in recent electric vehicles (EVs). For wider consumer acceptance of EVs, however, the current state-of-the-art LIBs face formidable technological challenges, including concerns related to the battery cost, durability, and driving range. Resolving these hurdles requires substantial improvements in energy density, cycle life, and safety of current LIBs. Compared to the most widely accepted anode, graphite, cathodes suffer from inferior capacity, poor cycle life, thermal characteristics, and high cost. As a result, high-energy cathodes enabling a long cycle life and reliable safety need to be developed. Among them, a compositionally graded cathode material in which concentrations of the transition metals continuously varied from the particle center to the surface appears to be the most promising since the graded cathodes have demonstrated remarkable improvements over cathodes with single uniform composition, not only in lifetime and safety, but also in battery power due to the superior Li+ diffusion kinetics. In this presentation, we review the most recent and promising results concerning Li[Ni1-x-yCoxAly]O2 (NCA) and Li[Ni1-x-yCoxMny]O2 (NCM) cathodes. One example, concentration gradient Li[Ni0.9Co0.05Mn0.05]O2 cathode delivers a discharge capacity of 229 mAh g-1 and exhibits capacity retention of 88% after 1000 cycles in a pouch-type full cell (compared to 68% for the conventional NCM cathode). The proposed concentration gradient cathodes provide an opportunity for the rational design and development of a wide range of multifunctional cathodes, especially for Ni-rich NCM cathodes, by compositionally partitioning the cathode particles and thus optimizing the microstructural response to the internal strain produced in the deeply charged state.

Simultaneous MgO Coating and Mg Doping of Co-Free Cathode Material for High-Voltage Sodium-Ion Batteries

Over the past two decades, advancements in Li-ion battery (LIB) technologies have contributed immensely to changing our daily lives. Recently, another type of energy storage system, sodium-ion batteries (SIBs), have received significant attention mainly because of the abundance of sodium resources in the Earth's crust, which could thereby greatly reduce costs. Encouraged by previously conducted research activities in this field, we have adopted an effective strategy for simultaneous MgO coating and Mg doping to produce substantially improved high voltage stability compared to the previously reported results in the literature on Na[Ni0.5Mn0.5]O2 cathodes. The MgO coating layer effectively suppressed the unfavorable side reactions during cycling while the partial Mg doping into the bulk Ni sites improved the structural stability by moderating the extent of the irreversible multiphase transformation. As a result, the combination of a MgO coating with Mg doping provides enhanced electrochemical performance and structural stability of Na[Ni0.5Mn0.5]O2 within the voltage range of 2.0–4.2 V. The practical acceptability of the simultaneous MgO coating and Mg doping of the Na[Ni0.5Mn0.5]O2 cathode was obviously verified using scaled-up pouch-type full cells with hard carbon anodes. Moreover, the use of earth's abundant and inexpensive Mg and Na elements, and a simple practical strategy are highly desirable for developing high-energy and low-cost SIBs. Although the practical use of the MgO-NM55 cathode in SIBs will require further work, the methodology used in this study will be helpful in developing an efficient design of high performance cathode materials for SIBs.

Ethylene Carbonate-Free Electrolytes for High-Nickel Layered Oxide Cathodes

Layered lithium nickel oxide (LiNiO2), with a specific charge-storage capacity of almost 250 mA h g-1, an operating voltage of 3.8 V vs. Li+/Li, and a high packing density of above 3.4 g cm-3, offers a very high energy density approaching the limits of layered oxide cathode materials for lithium-ion batteries. However, despite early interests in the community, practical application of LiNiO2 was soon dismissed for a multitude of problems. This includes (i) poor thermal-abuse tolerance triggering battery thermal runaway at high states of charge, and (ii) severely restricted reversible capacity of < 200 mA h g-1 for desired cycle/calendar life owing to a high surface reactivity with conventional ethylene carbonate (EC)-based electrolytes and not fully reversible rhombohedral H2/H3 phase transition at > 4.2 V vs. Li+/Li. In spite of recent efforts on stabilizing ultrahigh-nickel layered oxides (Ni > 0.9) during battery operation through compositional tuning, the poor thermal stability and cyclability over a broad Li content range remain challenges of LiNiO2-based cathodes. Here we design nonaqueous electrolytes with exclusively aprotic acyclic carbonate solvents (such as ethyl methyl carbonate, EMC) tailored to an ultrahigh-nickel layered oxide cathode (LiNi0.94Co0.06O2). Common lithium-conducting salts (e.g., lithium hexafluorophosphate, LiPF6) and interphase-forming additives (e.g., vinylene carbonate, VC) are adopted in two model electrolyte systems. Assembled graphite|LiNi0.94Co0.06O2 pouch-type full cells in the two EC-free electrolyte systems show excellent long-term cycling performance of ~ 80% capacity retention after 1,000 charge-discharge cycles at 25 oC (1C, 2.5 – 4.2 V), notably superior to the same cells in the baseline EC-containing electrolyte (1.0 M LiPF6/EC-EMC(3:7)+2%VC). Through time-of-flight secondary-ion mass spectrometry coupled with lithium isotopic labelling and in situ X-ray diffraction, we demonstrate suppressed unwanted electrode-electrolyte reactions and irreversible structural degradation of LiNi0.94Co0.06O2 at highly delithiated states in the EC-free electrolytes during battery operation. In addition, through differential scanning calorimetry, we also show significantly inhibited self-heating of the ultrahigh-Ni layered oxide cathode at full charge in the EC-free electrolytes, outperforming charged LiNi0.8Co0.15Al0.05O2 in the EC-containing electrolyte. In summary, we present a facile approach of nonaqueous EC-free electrolytes to address the long-standing challenges of LiNiO2-based cathode materials for safer and long-life high-energy-density lithium-ion batteries.

High-performance Ti-doped O3-type Na[Tix(Ni0.6Co0.2Mn0.2)1-x]O2 cathodes for practical sodium-ion batteries

Ti-doped O3-type Na[Tix(Ni0.6Co0.2Mn0.2)1-x]O2 is studied as a high-performance cathode material for practical sodium-ion batteries. The effect of partial Ti doping on the structural and electrochemical properties of O3-type Na[Ni0.6Co0.2Mn0.2]O2 materials is investigated by varying the Ti content (x) in Na[Ni0.6Co0.2Mn0.2]O2 from 0 to 0.01 to 0.03. Scanning electron microscopy images show that upon doping, the primary particles aggregate and form densely packed secondary particles, yielding enhanced mechanical strength and high tap density of ∼2.3 g cm−3. The compact morphology of the particles effectively minimizes the void volume for possible electrolyte penetration that usually leads to unwanted side reactions. In addition, partial doping of Ti in the transition-metal layer greatly improve the structural stability. By taking both morphological and structural advantages, the O3-type Na[Ti0.03(Ni0.6Co0.2Mn0.2)0.97]O2 cathode demonstrate the great enhancements of battery performances in terms of capacity, cycle retention, rate capability, and thermal properties. Pouch-type full cells assembled by combining the present cathodes with hard carbon anodes show good practical applicability, with an outstanding cycle retention of 77% over 400 cycles. The results of this study may open up a new avenue for designing and developing suitable transition metal oxide cathodes for high-performance sodium-ion batteries.

Co-Free Cathode Material with Simultaneous MgO Coating and Mg Doping for High-Voltage Sodium-Ion Batteries

To simultaneously address the inherent limitations of the capacity and cycling instability of Na[Ni0.5Mn0.5]O2 cathodes, it is necessary to concurrently stabilize particle surfaces and crystal structure. Encouraged by previously conducted research activities in this field, we have adopted an effective strategy for simultaneous MgO coating and Mg doping to produce substantially improved high voltage stability compared to the previously reported results in the literature on Na[Ni0.5Mn0.5]O2 cathodes. The MgO coating layer effectively suppressed the unfavorable side reactions during cycling while the partial Mg doping into the bulk Ni sites improved the structural stability by moderating the extent of the irreversible multiphase transformation. As a result, the combination of a MgO coating with Mg doping provides enhanced electrochemical performance and structural stability of Na[Ni0.5Mn0.5]O2 within the voltage range of 2.0–4.2 V. The practical acceptability of the simultaneous MgO coating and Mg doping of the Na[Ni0.5Mn0.5]O2 cathode was obviously verified using scaled-up pouch-type full cells with hard carbon anodes. Compared with previously reported similar cathode materials, the proposed MgO-NM55 cathode showed great competitiveness for high-voltage stability in SIBs. Moreover, the use of earth's abundant and inexpensive Mg and Na elements, and a simple practical strategy are highly desirable for developing high-energy and low-cost SIBs. Although the practical use of the MgO-NM55 cathode in SIBs will require further work, the methodology used in this study will be helpful in developing an efficient design of high performance cathode materials for SIBs. References J.-Y. Hwang, S.-T. Myung and Y.-K. Sun, Soc. Rev., 2017, 46, 3529–3614. J.-Y. Hwang, C. S. Yoon, I. Belharouak and Y.-K. Sun, J. Mater. Chem. A, 2016, 4, 17952.P.-F. Wang, H.-R. Yao, X.-Y. Liu, J.-N. Zhang, L. Gu, X.-Q. Yu, Y.-X. Yin and Y.-G. Guo, Adv. Mater., 2017, 29, 1700210.S. Komaba, N. Yabuuchi, T. Nakayama, A. Ogata, T. Ishikawa and I. Nakai, Inorg. Chem., 2012, 51, 6211.

A Mg-Doped High-Nickel Layered Oxide Cathode Enabling Safer, High-Energy-Density Li-Ion Batteries

High-nickel layered oxide cathodes with a Ni content of >90% show substantial potential for next-generation lithium-ion batteries (LIBs) due to their high capacity and lower cost. However, they are plagued by rapid capacity decay and poor thermal stability, which hamper their practical viability. We present here Li0.98Mg0.02Ni0.94Co0.06O2 (NC-Mg) with 2% Mg doping, aiming to provide a strategic guideline for solving the issues. The Mg2+ ions occupy the lithium layer and are proposed to act as pillar ions, which substantially enhance the structural reversibility and reduce the anisotropic lattice distortion upon cycling, thereby greatly improving the electrochemical and thermal stability of NC-Mg compared to the undoped LiNi0.94Co0.06O2 (NC). Specifically, NC-Mg delivers 214 mA h g–1 with a capacity retention of 80.1% after 500 cycles in pouch-type full cells, much higher than the retention of NC (56.3%). A discharge capacity of 158 mA h g–1 at 10C rate demonstrates its remarkable rate capability. Addition...

Multi-walled carbon nanotube-wrapped SiP2 as a superior anode material for lithium-ion and sodium-ion batteries

SiP2 has a high specific theoretical capacity of 2902 mA h g-1 as an anode material for lithium-ion batteries and 1788 mA h g-1 for sodium-ion batteries, respectively, but demonstrates very poor cycling stability in the entire voltage range (0–2 V). Here, we report high performance SiP2 anodes for lithium-ion batteries and sodium-ion batteries with good cycling stability by wrapping SiP2 with multi-walled carbon nanotube (MWCNT) on the surface of SiP2 via a ball milling prociess. MWCNT provides a robust network to connect fragment SiP2 with surrounding electrical conductor and serves as an effective conductive agent. MWCNT-wrapped SiP2 shows a reversible capacity of 1622 mA h g-1 after 100 cycles at 0.5 A g−1 in lithium-ion batteries and 925 mA h g-1 after 200 cycles at 0.2 A g−1 in sodium-ion batteries. In addition, rate capability is significantly enhanced. Furthermore, both coin-type and pouch-type full cells are assembled using SiP2 anodes which are coupled respectively with a cathode of LiFePO4 for lithium-ion batteries and a cathode of Na2/3Ni1/3Mn2/3O2 for sodium-ion batteries, and supplied as a reliable energy source for electronic uses to evaluate practical applications.

Correlating Gas Evolution and Oxygen Release to the Electrochemical Full-Cell Performance of Lithium-Rich Layered Oxides

Lithium- and manganese-rich layered oxides, also referred to as high energy NCM (HE-NCM), are a promising class of materials for applications in future Li-ion batteries. Due to their high specific discharge capacity (~250 mAh/g) and the high operating voltage, the energy density of next-generation batteries could be increased using these materials [1]. However, these materials exhibit several problems that hinder their commercialization such as low first cycle coulombic efficiency, poor rate capabilities, as well as a significant voltage-hysteresis between charge and discharge accompanied by a rapid voltage decay [2]. So far it is well known that the first cycle activation is a crucial step and strongly influences the further cycling performance of the material [3]. It has been shown by on-Line electrochemical mass spectrometry (OEMS) that during initial cycling oxygen is released from the cathode active material, leading to a chemical surface degradation, forming a spinel-type surface layer [4, 5]. A similar mechanism has been recently reported for Ni-rich NCM materials, where it has been proposed that the release of singlet oxygen might lead to rapid reactions with the electrolyte, leading to rapid capacity fading [6]. In this study, we want to examine possible effects causing the fast capacity decay of Graphite//HE-NCM full-cells and propose strategies how to overcome these issues. Therefore, we will first examine the gas evolution by OEMS in order to obtain information about electrolyte and LiPF6 decomposition. In a further step, we will then correlate the gas-evolution to the electrochemical performance using coin and pouch type full-cells. From these experiments, we aim to get a profound understanding about the influence of different effects onto cell-fading. Figure 1 shows the gas evolution during the first two cycles for HE-NCM with the composition x Li2MnO3 • 1-x LiMO2 (M = Ni, Co, Mn). It can be seen that the oxygen release also induces CO2 evolution, initiating at the same potential. In this study the role of critical parameters as temperature, oxygen release, as well as material and electrolyte composition and the effects on the electrochemical performance will be discussed. Therefore, we will show OEMS experiments and correlate the gas evolution to distinct degradation mechanisms leading to capacity fading of these materials. Literature: [1] M. M. Thackeray, S.-H. Kang, C. S. Johnson, J. T. Vaughey, R. Benedek and S. A. Hackney, J. Mater. Chem. 17, 3112-3125 (2007). [2] J. R. Croy, M. Balasubramanian, K. G. Gallagher and A. K. Burrell, Acc. Chem. Res. 48, 2813-2821 (2015). [3] J. R. Croy, K. G. Gallagher, M. Balasubramanian, Z. Chen, Y. Ren, D. Kim, S.-H. Kang, D. W. Dees and M. M. Thackeray, J. Phys. Chem. C 117, 6525-6536 (2013). [4] B. Strehle, K. Kleiner, R. Jung, F. Chesneau, M. Mendez, H. A. Gasteiger and M. Piana, J. Electro chem. Soc., 164(2), A400 (2017). [5] T. Teufl, B. Strehle, P. Müller, H. A. Gasteiger and M. A. Mendez, manuscript in preparation. [6] R. Jung, M. Metzger, F. Maglia, C. Stinner and H. A. Gasteiger, J. Electrochem. Soc., 164(7), A1361 (2017). Acknowledgement: The authors want to thank BASF SE for their financial support of this work. Figure 1: Gas evolution of HE-NCM during the first two cycles, the upper panel shows the potential curves, the middle panel the oxygen release and the lower panel the CO2 evolution. Data were measured between 2.0 and 4.8 V using a Li-CE, the first charge was carried out at C/10, afterwards the cell was cycled with C/5. Figure 1

Resolving the Degradation Mechanism of the O3-Type Layered Oxide Cathode Surface through the Nano-Size Aluminum Oxide Coating for High-Energy Density Sodium-Ion Batteries

The increasing concerns regarding the rapid depletion of fossil fuels and their environmental footprint have prompted a strong worldwide interest in the exploration of inexpensive and eco-friendly energy storage technologies.1 Layered sodium oxides, particularly O3-type layered compounds, have been extensively studied due to their adequate capacity and well-established chemistry which was inherited from LIB technologies. So far, various chemical compositions have been introduced through different transition metal substitutions (Ni, Co, Mn, and Fe) such as Na[NixFeyMnz]O2 or Na[NixCoyMnz]O2.2-4 A surface-modified O3-type Na[Ni0.6Co0.2Mn0.2]O2 cathode was covered by Al2O3 nanoparticle using a simple dry ball-milling route. The nanoscale Al2O3 particles (∼15 nm in diameter) densely covering the spherical O3-type Na[Ni0.6Co0.2Mn0.2]O2 cathode particles effectively minimized parasitic reactions with the electrolyte while assisting Na+ migration. The proposed Al2O3 coated Na[Ni0.6Co0.2Mn0.2]O2 cathode showed a high specific capacity of 151 mA h g−1, as well as improved cycling performance and rate capability in a half cell. Furthermore, the Al2O3 coated cathode was scaled up to a pouch-type full cell using a hard carbon anode that exhibited a superior rate capability and capacity retention of 75% after 300 cycles with a high energy density of 130 W h kg−1. In addition, the post-mortem surface characterization of the cathodes from the long-term cycled full cells cooperated with identifying the specific mechanism of the surface reaction with the electrolyte and the reason for its subsequent degradation and showed that the nano-size Al2O3 coating layer was effective at resolving the degradation pathways of the cathode surface from hydrogen fluoride (HF) attack. References J.-Y. Hwang, S.-T. Myung and Y.-K. Sun, Soc. Rev., 2017, 46, 3529–3614.J.-Y. Hwang, S.-T. Myung, D. Aurbach and Y.-K. Sun, Power Sources, 2016, 324, 106–112. J.-Y. Hwang, S.-M. Oh, S.-T. Myung, K. Y. Chung, I. Belharouak and Y.-K. Sun, Nat. Commun., 2014, 6, 6856. J.-Y. Hwang, C. S. Yoon, I. Belharouak and Y.-K. Sun, J. Mater. Chem. A, 2016, 4, 17952–17959.

Self-Assembled Porous Micro-Spherical LiFePO4 Synthesized by Spray-Drying for Pouch-Typed Cell

Self-Assembled porous micro-spherical LiFePO4 have been successfully obtained through spray drying route. The prepared LiFePO4 is higher tap of 1.4 g cm–3 thank to its self-assembled porous structure. The studies of electrochemical performances were conducted at full cell level. The results show that the assembled pouch-typed full-cells with as-obtained LiFePO4 are of excellent rate capability and cycle life. As a result, the as-prepared LiFePO4 cathode material with self-assembled porous structure is capable of large-scale applications.

Synthesis of LiFePO4 by In Situ Polymerization Restriction Method for Pouch-Typed Cell

We report a two-step approach to synthesize nanosized LiFePO4C composite by an in situ polymerization restriction method. First step, the Fe2O3@polyaniline is fabricated with in situ polymerization method. Secondly, the as-obtained Fe2O3 coated with polyaniline is mixed with LiH2PO4, followed by carbothermal reduction process. The outer polyaniline shell coating Fe2O3 can restrict the aggregation of particles and serves as carbon-containing precursor. The studies of electrochemical performances were carried out at full cell level. The results show that the assembled pouch-typed full-cells with as-obtained LiFePO4 present excellent rate capability and cycle life.

Electrochemical properties of high-energy-density Li[Ni0.5Co0.2Mn0.3]O2 cathode at full-cell level

Layered lithium transition metal oxide LiNi0.5Co0.2Mn0.3O2 cathode materials were successfully obtained through a simple carbonate co-precipitation method. Monodispersed Li[Ni0.5Co0.2Mn0.3]O2 spherical agglomeration consisted of nanoscale secondary particles enable material to be higher energy and powder density. Pouch-typed cells were assembled to investigate electrochemical performance of Li[Ni0.5Co0.2Mn0.3]O2 at level of full-cell. The results show that the assembled pouch-typed full-cells present higher capacity, excellent rate capacity, and cycle life, which is due to its monodispersed uniform morphology with nanoscale secondary particles.

Synthesis of LiFePO4 nanoparticles and electrochemical studies at full-cell level

LiFePO4 nanoparticles have been successfully obtained by a solid-phase synthesis method using nanoscaled FePO4 as starting materials. Three-electrode system using as-prepared LiFePO4 as cathode was assembled to monitor the variation of voltage and impedance in the anode or cathode. The pouch-typed cells with prepared LiFePO4 were assembled to investigate electrochemical performance at level of full-cell. The results show that the assembled pouch-typed full-cells present higher power density and favorable cycle life.

Comparative study on hydrothermally synthesized LiMnxFe1–xPO4 (x = 0∼1) cathode at full-cell level

Olivine structure LiMn x Fe1–xPO4 (x = 0∼1) cathode materials were successfully synthesized through hydrothermal method. All obtained samples present monodispersed nanorod morphology with different particle sizes. After identifying specific capacity of as-synthesized cathode materials with half-cell, pouch-typed cells were assembled to investigate electrochemical performance of LiMn x Fe1–xPO4 (x = 0∼1) at level of full-cell. The results show that the assembled pouch-typed full-cells with as- obtained LiFePO4 present excellent rate capability, high-low temperature capacity and cycle life. But even present higher operating potential of 4.1 V vs. Li, LiMnPO4 show unacceptable electrochemical properties at level of full-cell, which presents a big challenge for the practical applications of LiMnPO4.

Synthesis and electrochemical properties of Li-rich Li[Ni0.5Co0.2Mn0.3]O2 for pouch-typed cell

Layered transition metal oxide LiNi x Co y MnzO2 cathode materials with different Li amount were successfully synthesized via co-precipitation method. Monodispersed Li[Ni0.5Co0.2Mn0.3]O2 and Li-rich Li1.1[Ni0.5Co0.2Mn0.3]O2 spherical agglomeration consisted of secondary particles, which is favorable for the higher tap-density of materials, can be easily obtained. The pouch-typed cells with obtained materials were assembled to investigate electrochemical performance at level of full-cell. The results show that the assembled pouch-typed full-cells with Li-rich sample present higher capacity, better rate capability and cycle life.

Pouch-typed cell from Li-rich LiFePO4 prepared by low-cost carbothermal reduction method

Lithium iron phospho-olivine cathode material with optimized lithium amount for lithium-ion batteries was successfully prepared from low cost Fe2O3 as raw materials by thermal reduction method. The as-obtained material showed a reversible discharge capacity of 153.8 mAh g–1 in the voltage window of 2.0–4.2 V at half-cell level. The pouch-typed cells with prepared Li1.05FePO4 were assembled to investigate electrochemical performance at level of full-cell. The results show that the assembled pouch-typed full-cells present better rate capability and cycle life. The low-cost approach reported here firstly sheds light on application of mass production of olivinestructured LiFePO4 at level of full-cell.

Long-Life Nickel-Rich Layered Oxide Cathodes with a Uniform Li2ZrO3 Surface Coating for Lithium-Ion Batteries

As nickel-rich layered oxide cathodes start to attract worldwide interest for the next-generation lithium-ion batteries, their long-term cyclability in full cells remains a challenge for electric vehicles. Here we report a long-life Ni-rich layered oxide cathode (LiNi0.7Co0.15Mn0.15O2) with a uniform surface coating of the cathode particles with Li2ZrO3. A pouch-type full cell fabricated with the Li2ZrO3-coated cathode and a graphite anode displays 73.3% capacity retention after 1500 cycles at a C/3 rate. The Li2ZrO3 coating has been optimized by a systematic study with different synthesis approaches, annealing temperatures, and coating amounts. The complex relationship among the coating conditions, uniformity, and morphology of the coating layer and their impacts on the electrochemical properties are discussed in detail.

Solution Synthesis of Iodine-Doped Red Phosphorus Nanoparticles for Lithium-Ion Battery Anodes

Red phosphorus (RP) is a promising anode material for lithium-ion batteries due to its earth abundance and a high theoretical capacity of 2596 mA h g-1. Although RP-based anodes for lithium-ion batteries have been reported, they were all in the form of carbon-P composites, including P-graphene, P-graphite, P-carbon nanotubes (CNTs), and P-carbon black, to improve P's extremely low conductivity and large volume change during cycling process. Here, we report the large-scale synthesis of red phosphorus nanoparticles (RPNPs) with sizes ranging from 100 to 200 nm by reacting PI3 with ethylene glycol in the presence of cetyltrimethylammonium bromide (CTAB) in ambient environment. Unlike the insulator behavior of commercial RP (conductivity of <10 -12 S m-1), the conductivity of RPNPs is between 2.62 × 10-3 and 1.81 × 10-2 S m-1, which is close to that of semiconductor germanium (1.02 × 10-2 S m-1), and 2 orders of magnitude higher than silicon (5.35 × 10-4 S m-1). Around 3-5 wt % of iodine-doping was found in RPNPs, which was speculated as the key to significantly improve the conductivity of RPNPs. The significantly improved conductivity of RPNPs and their uniform colloidal nanostructures enable them to be used solely as active materials for LIBs anodes. The RPNPs electrodes exhibit a high specific capacity of 1700 mA h g-1 (0.2 C after 100 cycles, 1 C = 2000 mA g-1), long cycling life (∼900 mA h g-1 after 500 cycles at 1 C), and outstanding rate capability (175 mA h g-1 at the charge current density of 120 A g-1, 60 C). Moreover, as a proof-of-concept example, pouch-type full cells using RPNPs anodes and Li(Ni0.5Co0.3Mn0.2)O2 (NCM-532) cathodes were assembled to show their practical uses.

Capacity fading behavior of Ni-rich layered cathode materials in Li-ion full cells

We report the failure mechanism of a Ni-rich layered cathode material (LiNi0.8Co0.1Mn0.1O2) in Li-ion full cells. Laminated pouch-type full cells were cycled 300 times, after which the cells exhibited 16.3% capacity fading. After 1, 100, 200, and 300cycles, each full cell was disassembled and the cathode was investigated via a half-cell test, X-ray diffraction (XRD), and field emission scanning electron microscopy (FE-SEM). XRD analysis showed loss of the active lithium source in the cathode, and FE-SEM observation exhibited traces of mechanical failure in the Ni-rich secondary particles. Particle strength measurements on the cathode materials by applying compressive force demonstrated that the mechanical strength of particles significantly weakened after full-cell cycling. Thus, the capacity of the Ni-rich cathode may deteriorate during full-cell cycling because of decreases in compressive particle strength. This issue should be considered in materials preparation processes.