H3 Phase Research Articles

Microstructure Modified Ni-Rich Cathode Material By Microscale Concentration Gradient for High Energy Density Li-Ion Batteries

A concentration gradient (CG) Li[Ni0.9Co0.05Mn0.05]O2 cathode in which Li[Ni0.94Co0.038Mn0.022]O2 at the particle core is encapsulated by a 1.5-μm-thick CG shell with the outer surface composition Li[Ni0.841Co0.077Mn0.082]O2 is synthesized using a differential co-precipitation process. The CG at the microscale combined with the radial texturing of the refined primary particles in the CG shell layer protracts the detrimental H2 → H3 phase transition1,2, providing with the rise to sharp changes in the unit cell dimensions. This protraction, confirmed by in-situ X-ray diffraction and transmission electron microscopy, allows effective dissipation of the internal strain generated upon the H2 → H3 phase transition, markedly improving cycling performance and thermochemical stability as compared to those of the conventional without CG Li[Ni0.9Co0.05Mn0.05]O2 cathode3-6. The compositionally partitioned cathode delivers a specific capacity of 229 mAh g-1 and exhibits capacity retentions of 88% after 1,000 cycles in a pouch-type full cell (cf. 68% for the conventional cathode). Thus, the proposed cathode material provides an opportunity for the rational design and development of a wide range of multi-functional 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 100% depth of discharge state.Reference U.-H. Kim, S.-T. Myung, C. S. Yoon and Y.-K. Sun ACS Energy Lett. 2017, 2, 1848-1854.H.-H. Ryu, K.-J. Park, C. S. Yoon and Y.-K. Sun Chem. Mater. 2018, 30, 1155.Y.-K. Sun, S.-T. Myung, B.-C. Park, J. Prakash, I. Belharouak and K. Amine. Nat. Mater. 2009, 8, 320.U.‐H. Kim, E.‐J. Lee, C. S. Yoon, S.‐T. Myung and Yang‐Kook Sun Adv. Energy Mater. 2016, 6, 1601417.D.-W. Jun, C. S. Yoon, U.-H. Kim and Y.-K. Sun Chem. Mater. 2017, 29, 5048.U.-H. Kim, S.-T. Myung, C. S. Yoon and Yang-Kook Sun ACS Energy Lett. 2017, 2, 1848.

Comparison of Ni-Rich Li[Ni1 −x−YCoxAly]O2(NCA) Cathodes for Lithium-Ion Batteries Depending on Extent of Microcracks

Ni-rich layered cathode materials Li[Ni1 − x − yCoxMy]O2 (M = Al and Mn are denoted as NCA and NCM, respectively) are typical cathodes for the batteries of electric vehicles (EVs). In particular, Tesla, one of the most promising EVs producers, uses the Panasonic NCA cathode in its Model S. For the wide adoption of EVs, driving range need to be raised by the increase in energy density of lithium ion batteries (LIBs). However, developing LIBs which have high energy density with stable cycle life is mainly hampered by the cathode materials.The general strategy for increasing the reversible capacity of NCM and NCA cathodes is to increase the relative Ni fraction in their composition. However, it is well acknowledged that increasing the Ni composition in NCM cathodes leads to the deterioration in both cycling performance and thermal stability.1 The fast capacity fading of Ni-rich NCM cathode is mostly attributed to the H2–H3 phase transition at the charged state, inducing an anisotropic lattice volume change generating internal microcracks in the particles.2 The extent of microcrack, which is a crucial factor dictating a performance of cathode material, increases as Ni fraction increases. Similar to NCM cathodes, Ni-rich NCA cathodes are also supposed to suffer from capacity fading and thermal instability problems owing to microcracks. To apply Ni-rich NCA cathodes in high-energy batteries for EVs, it is important to establish the capacity-cycling stability-thermal stability relationship depending on the extent of microcracks.Based on the results in NCM cathodes, we investigated the comparative study of NCA cathodes whose Ni fraction are 80%, 88%, and 95% to assess their cycling stability and to determine the capacity limit that can be attained by Ni-rich NCA cathodes without substantial sacrifice of the cycling stability. In addition, The extent of microcracks during charging and discharging was focused to identify the capacity-fading mechanism of these Ni-rich NCA cathodes. The results can provide the technical and scientific basis for further material development of these cathodes. Reference s 1. H.-J. Noh, S. Youn, C. S. Yoon and Y.-K. Sun, J. Power Sources, 2013, 233, 121.2. H.-H. Ryu, K.-J. Park, C. S. Yoon and Y.-K. Sun, Chem. Mater. 2018, 30, 1155.

Effect of Nb doping on the behavior of NCA cathode: Enhanced electrochemical performances from improved lattice stability towards 4.5V application

Although the Ni-rich LiNi0.8Co0.15Al0.05O2 (N85CA) material has been used as the commercial cathode of electric vehicle battery, the instability of the layered structure and the short lifespan under high voltage cycling prevent its practical application to higher energy density. Herein, Nb-doped N85CA material, with enhanced structural stability and improved cycle stability, is reported to work at 4.5V. In particular, Nb0.5%-N85CA shows the best electrochemical performance, with a discharge capacity of 200.2 mAh/g at 0.1C, and a retention of 94.19% after 100 runs at 0.5 C. As a comparison, the data of the pristine N85CA are 210.2 mAh/g and 69.46%, respectively. Such an obvious increase in the capacity retention is caused by the promoted lattice stability provided by the strong Nb–O bond that inhibits the H2–H3 phase transition in the electrochemical process. The incorporation of Nb5+ ions further increases the interslab thickness that enable a rapid migration of Li ions (2.23 × 10−10 S cm−1 vs. 8.32 × 10−11 S cm−1), and the rate behavior of the cathode is also enhanced. Furthermore, Nb0.5%-N85CA material displays a specific capacity of 156.2 mAh/g at 5 C, while that of the pristine N85CA is 123 mAh/g. The results strongly indicate that such Nb-doped N85CA material has high potential as a cathode candidate, especially for high energy applications.

Concentration Gradient-Driven Aluminum Diffusion in a Single-Step Coprecipitation of a Compositionally Graded Precursor for LiNi0.8Co0.135Al0.065O2 with Mitigated Irreversibility of H2 ↔ H3 Phase Transition.

LiNi1-x-yCoxAlyO2 (NCA) possessing a nano-/micro hierarchical architecture delivers a high specific capacity of 200 mAh/g with an upper cutoff voltage of 4.4 V. However, the structural reconstruction due to the irreversibility of the H2 ↔ H3 phase transition at higher voltage increases the initial irreversible capacity loss and charge-transfer impedance and reduces the performance at higher C-rates. Structural and electrochemical stability can be achieved by reducing the nickel content and increasing the electrochemically inactive aluminum at the surface. Nonetheless, getting an aluminum concentration gradient in NCA-(OH)2 is difficult owing to the difference in the solubility constant and reaction kinetics of Al(OH)3 compared to that of NiCo-(OH)2. Hence, we have exploited the high diffusion of nano-Al(OH)3 driven by the concentration gradient of Al across the hierarchical hydroxide structure and synthesized LiNi0.8Co0.135Al0.065O2 (NCA) with reduced Ni and increased Al at the surface. The process of formation of a concentration gradient was analyzed by X-ray diffraction, Fourier transform infrared spectroscopy, and cross-sectional elemental mapping. The concentration-graded NCA exhibited superior electrochemical performance compared to its pristine counterpart. The graded NCA shows excellent reversibility of the H2 ↔ H3 phase, leading to low impedance development, confirming the reduced surface reconstruction during the initial cycles. Therefore, the specific capacity of graded NCA is 65% higher than that of pristine NCA at 10 C. Both in half-cell and in full-cell configurations, the graded NCA exhibited superior first cycle reversibility and specific capacity. Specifically, in the full-cell configuration, the capacity retention of graded NCA is 91.5%, while that of pristine NCA is 83% after 150 cycles when cycled between 3 and 4.3 V. Further, the capacity loss reduces to 1% even after 500 cycles when the upper cutoff voltage is reduced to 4.2 V in the case of graded NCA.

Suppressing H2–H3 phase transition in high Ni–low Co layered oxide cathode material by dual modification

Dual modification can effectively stabilize layered structure, improve cycling stability, amend reaction kinetics and facilitate Li+ transport for the application of high Ni cathodes in EVs.

Investigating the Effects of Magnesium Doping in Various Ni-Rich Materials

As lithium ion battery technology expands into more demanding applications such as electric vehicles, attention has shifted towards nickel-rich positive electrode materials, namely LiNi1-x-yMnxCoyO2 (NMC) and LiNi1-x-yCoxAlyO2 (NCA).1 Aims to improve energy density and reduce costs of NMC and NCA can be achieved by increasing the Ni content of the material, but at the cost of shorter cell lifetimes. Besides Al, Co and Mn, research has tried substituting Ni with many different metals.1 Mg has repeatedly come up as a beneficial dopant from positive electrode material doping studies that try to improve material cycling performances. Mg has been shown to improve cycling performances of various positive electrode materials including LiCoO2 (LCO), NMC, and NCA when doped into the material in small amounts.1–4 Doping the material with Mg slightly reduces the specific capacity of the material since Mg is an inactive constituent, but it has been shown that a Mg content of as low as 1 mol% can improve cycling.3 LiNiO2 (LNO) and some other Ni-rich materials undergo phase transitions as Li gets removed during charging. In particular, when the material transitions from the H2 phase to the H3 phase at low Li, it experiences a large volume change. It is believed that this contributes to a poor lifetime of the material. Mg doping has been shown to suppress these phase transitions and related volume changes.2,4 Mg has also been shown to reduce the amount of Ni migration to the Li layer.1 As research continues to increase the Ni content of NMC and NCA, the compositions will invariable converge towards LNO. Additionally, efforts are being made to reduce the Co content of Ni-rich materials due to cost and sourcing issues.4 Co is believed to reduce the amount of Ni in the Li layer and also to stabilize cycling performance, which are similar benefits that Mg imparts. This work studies the effect of Mg doping in LiMO2 (M = Ni, Ni+Al, Ni+Co+Al, Ni content > 0.8) at a dopant level of less than 5 mol%. Structural and electrochemical characterization of the materials was carried out to understand how Mg affects the materials and whether the presence of Al or Co influences the dopant effects of Mg. Figure 1 shows the initial half-cell voltage vs capacity and differential capacity vs voltage curves for a series of LiNi0.88-xCo0.09Al0.03MgxO2 (x = 0, 0.01, 0.02, 0.04) materials, showing a trend of how Mg affects the electrochemical performance of the materials. (1) Kim, J.; Lee, H.; Cha, H.; Yoon, M.; Park, M.; Cho, J. Adv. Energy Mater. 2018, 8, 1–25. (2) Sasaki, T.; Godbole, V.; Takeuchi, Y.; Ukyo, Y.; Novák, P. J. Electrochem. Soc. 2011, 158, A1214–A1219. (3) Huang, B.; Li, X.; Wang, Z.; Guo, H.; Xiong, X. Ceram. Int. 2014, 40, 13223–13230. (4) Li, H.; Cormier, M.; Zhang, N.; Inglis, J.; Li, J.; Dahn, J. R. J. Electrochem. Soc. 2019, 166, A429–A439. Figure 1

Insight into Performance Degradation of Ni-Rich Layered Cathode Materials

As a promising cathode material for high-energy-density lithium-ion batteries, Ni-rich lithium transition metal oxides, LiNi1-x-yMnxCoyO2, suffer from a number of problems, such as inferior cycling stability, poor safety, and gas generation, which greatly hinder their application in practical batteries. Despite much effort made previously, the mechanism for the above problems is still not well understood. Selecting LiNi0.80Co0.10Mn0.10O2 (NCM811) as an example, in this work we studied performance degradation of the Ni-rich layered cathode materials by cycling and float-charging Li/NCM811 cells in high voltage range (4.5 V and 4.7 V, respectively). It is found that nearly all the above problems can be linked to the oxidation of lattice oxygen occurring in the capacity region with respect to the H2→H3 phase transition. In addition to contributing to overall capacity, the oxidation of lattice oxygen results in irreversible loss of oxygen from the cathode material by means of oxygen evolution and relative reactions between the active oxygen evolution intermediates and electrolyte solvents. It is the loss of oxygen that results in irreversible layered-spinel-rocksalt phase transition, secondary particle crack, and capacity loss. The oxidation of lattice oxygen is an intrinsic property of the Ni-rich layered cathode materials at high state-of-charge states (corresponding to high potentials), which takes place not only on the surface but also in the bulk of the NCM materials. It is suggested that the priority for future research on such cathode materials should give to suppression of the oxygen evolution, and development of the anti-oxygen electrolytes that are chemically stable against the active oxygen evolution intermediates. Figure 1

Capacity Fading Mechanism of Ni-Rich NCA Cathode for Lithium-Ion Batteries

To satisfy the energy density requirement, layered Ni-rich LiMO2 (M = Ni, Co, and Mn (NCM) or Ni, Co, and Al (NCA)) cathodes have been developed extensively for the last two decades because of their high theoretical capacity of 275 mAh g-1.1,2 Currently, Li[Ni0.8Co0.15Al0.05]O2 and Li[Ni0.6Co0.2Mn0.2]O2 cathodes are adapted for EVs (Tesla Model 3 and GM Bolt) that have a driving range of 380 km (236 miles) for a single charge, which is still short of the recommended threshold. To further increase the driving range, the Ni content in the cathodes should be increased gradually so that the energy density of the LIBs is increased. However, it is well acknowledged that the increase in the Ni concentration of LiMO2 leads to an almost linear increase in the reversible capacity but a proportional decrease in the cycling performance and thermal safety.3,4 The fast capacity fading of Ni-enriched NCMs is ascribed to the H2–H3 phase transition in the highly charged state of approximately 4.2 V Li/Li+, inducing an anisotropic lattice volume change that generates internal microcracks in the cathode particles.4 The microcracks facilitate electrolyte infiltration into the particle interior along the grain boundaries. The penetrated electrolyte accelerates surface degradation of the primary particles through the reaction of unstable Ni4+ with electrolyte to form NiO-like rocksalt impurity phases, and thereby increases the cell impedance.4 The currently deployed cylindrical full cell based on the NCA cathode (Li[Ni0.76Co0.14Al0.1]O2) and graphite anode demonstrates poor long-term cycling performance (capacity retention of 50% after 2000 cycles) when cycled at 100% depth of discharge (DOD). On the other hand, the full cell demonstrates an outstanding Li intercalation stability at DOD of 60% during the same cycling period, indicating that the DOD should be limited to 60% for long-term cycling.5,6 However, limiting the DOD during cycling reduces the energy density of the battery significantly, and consequently, increases the EV’s weight and battery cost. Currently, Tesla, which is one of the most prominent EV producers, uses the Panasonic Li[Ni0.84Co0.12Al0.04]O2 cathode in its Model S and Model X. However, with the increasing demand for higher energy density and the ever-increasing cost of raw cobalt, which has more than doubled in the last five years, increasing the Ni content in the cathode active material has become mandatory. In this regard, we performed a systematic comparative study of the standard Li[Ni0.8Co0.16Al0.04]O2 (herein after denoted as NCA80), Li[Ni0.88Co0.10Al0.02]O2 (NCA88), and Li[Ni0.95Co0.04Al0.01]O2 (NCA95). Based on this result, we explore the fundamental battery performance and capacity fade mechanisms of the three NCA cathodes to understand the impact of the increased Ni content on the electrochemical performance of the highly Ni-enriched NCA cathodes. References (1) Y.-K. Sun, S.-T. Myung, H.-S. Shin, Y. Bae, C. S. Yoon, J. Phys. Chem. B 2006, 110, 6810–6815. (2) Y.-K. Sun, S.-T. Myung, B.-C. Park, J. Parakash, I. Belharouak, K.Amine, Nat. Mater. 2009, 8, 320–324. (3) C. S. Yoon, M. H. Choi, B.-B. Lim, E.-J. Lee, Y.-K. Sun, J. Electrochem. Soc. 2015, 162, A2483–A2489. (4) H.-H. Ryu, K.-J. Park, C. S. Yoon, Y.-K. Sun, Chem. Mater. 2018, 30, 1155–1163. (5) S. Watanabe, M. Kinoshita, T. Hosokawa, K. Morigaki, K. Nakura, J. Power Sources 2014, 260, 50–56. (6) S. Watanabe, M. Kinoshita, T. Hosokawa, K. Morigaki, K. Nakura, J. Power Sources 2014, 258, 210–217.

Unraveling the Role of Al Substitution in Anionic Oxygen Activity of Ni-Rich Layered Oxide Cathodes

Ni-based layered oxides have attracted great attention for the development of high energy and high capacity Li-ion cathodes. The most popular Ni-based cathodes, LiNi x Mn y Co1-x-y O2 (NMC) and LiNi x Co y Al1-x-y O2 (NMC), are indeed derivatives of LiNiO2 by partially substituting Ni with Co, Mn or Al. The current trend to develop high-Ni and low-Co or Co-free layered Ni-based oxide cathodes, makes a strong push to the limit of Ni content in such layered oxides, leading to many common properties with their parent compound, LiNiO2. In fact, lithium nickel oxide often exists off-stoichiometry because of the presence of Ni2+ in the Li layer, which blocks the Li+ diffusion pathway upon lithiation/delithiation. Therefore, a second metal of a minimal content, such as Co or Al, which can somehow facilitate the formation of layered structure is still needed, although they are not strongly favored due to cost and sacrifice upon the use of inactive Al. In this work, a number of Al substituted lithium nickel oxides, LiAl x Ni1-x O2 (0 ≤ x ≤ 0.20), have been synthesized via a solid state reaction, showing good R m layered structure. Along with their parent LiNiO2, the oxygen activity in these compounds have been studied using combined synchrotron X-ray absorption spectroscopy (XAS), resonant inelastic X-ray scattering (RIXS) and differential electrochemical mass spectrometry (DEMS). The goal of this research is to elucidate the role of Al on the anionic oxygen activity in Ni-based layered oxides, including both reversible anionic oxygen redox and irreversible oxygen release during the electrochemical process. The electronic structure of transition metal and O was investigated at different states of charge to obtain a complete picture of cationic and anionic redox during the charge/discharge process. Beyond the redox chemistry, the effect of Al substitution on the phase transformation at highly delithiated states, dominated by H2 to H3 phase transformation for LiNiO2, as well as thermal stability was carefully studied. We hope this work helps provide a comprehensive picture of the role of Al substitution on the overall performance of Ni-based layered oxide cathodes.

Problems and their origins of Ni-rich layered oxide cathode materials

Ni-rich layered oxides, LiNixCoyMnzO2 (NCM) and LiNixCoyAlzO2 (NCA) with x ​+ ​y ​+ ​z ​= ​1 and x ​≥ ​0.8, are regarded to be the best choice for the cathode material of high energy Li-ion batteries due to their combined advantages in capacity, working potential and manufacture cost. However, their application in practical Li-ion batteries is hindered by two essential problems of (1) performance degradation and (2) safety hazard over the whole life of battery. Performance degradation behaves as declines in battery's capacity and working voltage as well as the battery's swelling and impedance growth; Safety hazard arises from thermal runaway under abuse conditions such as overcharging, overheating, and electric shorting. It appears that nearly all problems can be ultimately attributed to the loss of oxygen, especially caused by the oxidation of lattice oxygen in H3 phase where the capacities are contributed by both of the Ni and O redox couples. In this review, the problems and their origins of Ni-rich layered oxides are overviewed, and the solutions attempted to mitigate these problems are outlined.

Degradation Mechanism of Highly Ni-Rich Li[NixCoyMn1-x-y]O2 Cathodes with x > 0.9.

A series of Ni-rich Li[NixCo(1-x)/2Mn(1-x)/2]O2 (x = 0.9, 0.92, 0.94, 0.96, 0.98, and 1.0) (NCM) cathodes are prepared to study their capacity fading behaviors. The intrinsic trade-off between the capacity gain and compromised cycling stability is observed for layered cathodes with x ≥ 0.9. The initial specific capacities of LiNiO2 and Li[Ni0.9Co0.05Mn0.05]O2 are 245 mAh g-1 (91% of the theoretical capacity) and 230 mAh g-1, and their corresponding capacity retentions are 72.5% and 88.4%. However, the capacity retention characteristic deteriorates at an increasingly faster rate for x > 0.95, in contrast with the nearly linear increase of specific capacity. The fast capacity fading stems from the chemical attack of the cathode by the electrolyte infiltrated through the microcracks, resulting from the mechanical instability inflicted by the anisotropic internal strain caused by the H2 ⇆ H3 phase transition. Thus, the capacity fading of the NCM cathodes for x > 0.9 critically depends on the extent of the H2 → H3 phase transition. Retardation or protraction of the H2 ⇆ H3 phase transition by engineering the microstructure should improve the cycle life of these highly Ni-enriched NCM cathodes.

Understanding of performance degradation of LiNi0.80Co0.10Mn0.10O2 cathode material operating at high potentials

Inferior cycling stability, poor safety, and gas generation are long lasting problems of Ni-rich LiNi0.80Co0.10Mn0.10O2 (NCM811) cathode material. Although much effort has been made, mechanisms for the above problems are poorly understood. Studying the cycling and float-charging characteristics of Li/NCM811 cells in high voltage conditions (4.5 V and 4.7 V, respectively), in this work we find that nearly all known problems with NCM811 material can be attributed to the oxidation of lattice oxygen occurring in the capacity region corresponding to H2 → H3 phase transition. While contributing to overall capacity, the oxidation of lattice oxygen results in a loss of oxygen through oxygen evolution and relative reactions between active oxygen evolution intermediates and electrolyte solvents. It is the loss of oxygen that results in irreversible layered-spinel-rocksalt phase transition, secondary particle cracking, and performance degradation. The conclusions of this work suggest that the priority for further research on NCM811 material should give to the suppression of oxygen evolution, followed by the use of the anti-oxygen electrolyte being chemically stable against the active oxygen evolution intermediates.

Nano-Rod Gradient Li[Ni0.81Co0.06Mn0.13]O2: Preserving Cathode Structure for Long-Term Cycling (1000 Cycles) Li-Ion Batteries

To address the growing demand for both energy density and cycling stability, a layered nanorod gradient (NRG) Li[Ni0.81Co0.06Mn.0.13]O2 cathode was synthesized with a high Ni-content bulk and a radially columnar nano-rod comprised surface and benchmarked against the widely used constant concentration (CC) Li[Ni0.82Co0.14Al0.04]O2. The NRG cathode delivered a discharge capacity of 225 mAh g-1, equivalent to ~ 830 Wh kg-1 at the cathode level, with 91%capacity retention in half-cells after 100 cycles (210 mAh g-1 and 79 % for CC cathode) and 88% capacity retention in full-cells after 1000 cycles (56 % for CC cathode). Through a combination of in-situ and time-resolved X-ray diffraction (XRD), cross-section scanning electron microscopy imaging (SEM), and high-resolution transmission electron microscopy (HR-TEM), we confirm that the exceptional electrochemical performance of the NRG material, both in energy density and cycling performance, is attributed to its radially columnar concentration graded nano-rods at the surface. These nano-rods function as a buffer to diminish abrupt stress from the high Ni-content bulk during the H2 → H3 phase transition by suppressing crack propagation to preserve particle coherency, enabling reversibility of the cathode particle. This is important because electrolyte infiltration into the reactive Ni-rich bulk and subsequent formation of the electrically insulating rocksalt nano-structure (NiO) along the cracks are prevented, thereby minimizing impedance increase during long-term cycling. Furthermore, the nano-rods also enhance the thermal stability by delaying the layered to rocksalt phase transition on the surface through an increased concentration of Mn on the exterior.

Phase Transformation Behavior and Stability of LiNiO2 Cathode Material for Li-Ion Batteries Obtained from In Situ Gas Analysis and Operando X-Ray Diffraction.

Ni-rich layered oxide cathode materials, in particular the end member LiNiO2 , suffer from drawbacks such as high surface reactivity and severe structural changes during de-/lithiation, leading to accelerated degradation and limiting practical implementation of these otherwise highly promising electrode materials in Li-ion batteries. Among all known phase transformations occurring in LiNiO2 , the one from the H2 phase to the H3 phase at high state of charge is believed to have the most detrimental impact on the material's stability. In this work, the multistep phase transformation process and associated effects are analyzed by galvanostatic cycling, operando X-ray diffraction, and in situ pressure and gas analysis. The combined results provide thorough insights into the structural changes and how they affect the stability of LiNiO2 . During the H2-H3 transformation, the most significant change occurs in the c-lattice parameter, resulting in large mechanical stress in LiNiO2 . As for electrochemical stability, it suffers strongly in the H3 region. Oxygen evolution is observed not only during charge but also during discharge and found to be correlated with the presence of the H2 and H3 phases. Taken together, the experimental data improve the understanding of the degradation processes and the inherent instability of LiNiO2 in Li-ion cells when operated above around 75 % state of charge.

Microstructure‐Controlled Ni‐Rich Cathode Material by Microscale Compositional Partition for Next‐Generation Electric Vehicles

AbstractA multicompositional particulate Li[Ni0.9Co0.05Mn0.05]O2 cathode in which Li[Ni0.94Co0.038Mn0.022]O2 at the particle center is encapsulated by a 1.5 µm thick concentration gradient (CG) shell with the outermost surface composition Li[Ni0.841Co0.077Mn0.082]O2 is synthesized using a differential coprecipitation process. The microscale compositional partitioning at the particle level combined with the radial texturing of the refined primary particles in the CG shell layer protracts the detrimental H2 → H3 phase transition, causing sharp changes in the unit cell dimensions. This protraction, confirmed by in situ X‐ray diffraction and transmission electron microscopy, allows effective dissipation of the internal strain generated upon the H2 → H3 phase transition, markedly improving cycling performance and thermochemical stability as compared to those of the conventional single‐composition Li[Ni0.9Co0.05Mn0.05]O2 cathodes. The compositionally partitioned 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 cathode). Thus, the proposed cathode material provides an opportunity for the rational design and development of a wide range of multifunctional cathodes, especially for Ni‐rich Li[NixCoyMn1‐x‐y]O2 cathodes, by compositionally partitioning the cathode particles and thus optimizing the microstructural response to the internal strain produced in the deeply charged state.

(Invited) Microstructure Controlled Ni‐Rich NCM Cathode Material By Microscale Compositional Partition for Li-Ion Batteries

A multi‐compositional particulate Li[Ni0.9Co0.05Mn0.05]O2 cathode in which Li[Ni0.94Co0.038Mn0.022]O2 at the particle center is encapsulated by a 1.5‐μm‐thick concentration gradient (CG) shell with the outermost surface composition Li[Ni0.841Co0.077Mn0.082]O2 is synthesized using a differential coprecipitation process. The microscale compositional partitioning at the particle level combined with the radial texturing of the refined primary particles in the CG shell layer protracts the detrimental H2 → H3 phase transition, which normally occurs abruptly in Ni‐enriched Li[NixCoyMnz]O2 (NCM) cathodes in the deeply charged state giving rise to a sharp change in the unit cell dimensions. This protraction, which is confirmed by in‐situ X‐ray diffraction and transmission electron microscopy, allows effective dissipation of the internal strain generated upon the H2 → H3 phase transition, markedly improving cycling performance and thermochemical stability as compared to those of the conventional single‐composition Li[Ni0.9Co0.05Mn0.05]O2 cathode. The compositionally partitioned cathode delivers a discharge capacity of 229 mAh g‐ 1 and exhibits capacity retentions of 92% after 100 cycles in a coin‐type half cell (cf. 85% for the conventional cathode) and 88% after 1,000 cycles in a pouch‐type full cell (cf. 68% for the conventional cathode). Thus, the proposed cathode material provides an opportunity for the rational design and development of a wide range of multi‐functional 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.

Structural Relaxation of LixNi0.874Co0.090Al0.036O2 after Lithium Extraction down to (x ≤ 0.12)

Structural relaxation experiments have been carried out on Lix(Ni0.874Co0.090Al0.036)O2 (Co-9.0 NCA) after the lithium extraction down to x ≤ 0.12. X-ray diffraction has been made during the relaxation process, which were analyzed by the Rietveld method assuming two phases (H2 and H3) co-existence with symmetry. For the lithium extraction to x = 0.12, Co-9.0 NCA seems to show single H2 phase for all the relaxation period, while H2 and H3 coexistence is detected for x = 0.09 or 0.06. Even for the smaller lithium concentration region, H2 phase is predominant rather than H3. Comparing with the previous study of LiNiO2 and Co-3.1 NCA, very small H2-H3 transition has been observed during the relaxation time, which is due to the relatively larger c-length in H3 phase for highly extracted region (x = 0.06).

Correlation between long range and local structural changes in Ni-rich layered materials during charge and discharge process

Ni-rich cathode materials can deliver higher energy density with a lower cost compared with other conventional layered oxide materials. However, the cycling performance of Ni-rich materials needs further improvement, and the irreversible phase transformation related to the cell failure is not fully understood yet. Although an H1H2H3 structural change process is revealed, a more specific explanation about the difference of these hexagonal phases is needed for deeper comprehension and further targeted modification of Ni-rich materials. In this work, the different phase transformation mechanisms of LiNi0.6Co0.2Mn0.2O2 (NCM622) and LiNi0.8Co0.1Mn0.1O2 (NCM811) are investigated by C X-ray diffraction (XRD) measurement, and the relationship between structural change and capacity degradation is discussed, showing that H3 phase can be harmful for cycling. In addition, 6Li solid state nuclear magnetic resonance (ss-NMR) experiments are carried out for detecting the Li chemical environment at different state of charge, and the data can be associated with structural change obtained from in operando XRD results. From the analysis mentioned above, a new explanation of H1/H2/H3 is given, and a relationship between long range and local structural change has been put forward for the first time.

Structural Relaxation of Lix(Ni0.874Co0.090Al0.036)O₂ after Lithium Extraction down to x = 0.12.

A structural relaxation study has been carried out on Lix(Ni0.874Co0.090Al0.036)O2 after the electrochemical lithium was extraction down to x = 0.12. These relaxation analyses have been carried out using X-ray diffraction coupled with the Rietveld analysis, assuming two phases (H2 and H3) and co-existence with symmetry. The mole fraction of the H3 phase seemed not to vary largely during the relaxation time. As for the lattice constants, both H2 and H3 phases gradually increased the a-axis with the relaxation time. On the other hand, H2 and H3 phases increased and decreased the c-axis, respectively. The results are compared with that of previously reported Ni-rich NCA.

Facilitating the Operation of Lithium-Ion Cells with High-Nickel Layered Oxide Cathodes with a Small Dose of Aluminum

Layered oxide cathodes with a high Ni content of >0.6 are promising for high-energy-density lithium-ion batteries. However, parasitic electrolyte oxidation of the charged cathode and mechanical degradation arising from phase transitions significantly deteriorate the cell performance and cycle life as the Ni content increases. We demonstrate here a significantly prolonged cycle life with superior cell performance by substituting a small-dose of Al (2 mol %) for Ni in LiNi0.92Co0.06Al0.02O2; the capacity retention after operating a full cell fabricated with graphite anode for 1000 cycles increases from 47% to 83% on going from the Al-free LiNi0.94Co0.06O2 to the Al-doped LiNi0.92Co0.06Al0.02O2 cathode. Through in situ X-ray diffraction, we provide the operando evidence that the Al-doping tunes the H2–H3 phase transition process from a two-phase reaction to a quasi-monophase reaction, minimizing the mechanical degradation. Furthermore, secondary-ion mass spectrometry reveals considerably suppressed transitio...