Ni-rich Materials Research Articles

Removing the Intrinsic NiO Phase and Residual Lithium for High-Performance Nickel-Rich Materials

Layered Ni-rich materials for lithium-ion batteries exhibit high discharge capacities but degraded cyclability at the same time. The limited cycling stability originates from many aspects. One of the critical factors is the intrinsic insulating residual lithium compounds and the rock-salt (NiO) phase on the surface of particles. In this work, LiNi 0.8 Co 0.1 Mn 0.1 O 2 material is etched with a trace amount of boric acid and used as a model to demonstrate the influences of weak acid treatment on the surface phase regulations. After the etching process, the pH of the material is reduced from 12.08 to 11.82, along with a lower cation mixing degree and promoting electrochemical performances. Corresponding measurements demonstrate that weak acids such as H 3 BO 3 can also etch the NiO phase on the surface to adjust the surface of the particles to a pure layered structure. This process improves the lithium-ion diffusion and electron transport in the interface between material and electrolyte, consequently leading to better cycling performance and rate capability. This study provides a novel strategy and comprehensive understanding of acid modification and surface phase regulation process of Ni-rich cathode materials for lithium-ion batteries.

Unraveling the nonlinear capacity fading mechanisms of Ni-rich layered oxide cathode

In order to improve the electrochemical performance and security of lithium-ion batteries, it is crucial to understand the structure and interface evolution behavior during charge-discharge cycling. Here, we investigate the dominant mechanism of nonlinear capacity fading of Ni-rich electrode materials under different charge-discharge rates. Despite the high initial discharge capacity of low-rate charge-discharge, the cumulative release of stress at the end of the cycle may rapidly reduce its capacity. During the high-rate cycling, the rapid decay of initial capacity is mainly caused by the thickening of CEI and the interfacial side reactions on account of electrolyte decomposition. Pulverization caused by microcrack expansion and intensified side reactions between the exposed new interface and the electrolyte, is the main contributor to the end-of-cycle capacity decay. One of the key causes of nonlinear capacity fading is the change of dominant mechanism. The proposed work provides new insight into the capacity decay and diving of lithium-ion batteries during cycling.

Revealing the Relationships between Washing/Recalcination Processes and Structure Performance of Ni-Rich Layered Cathode Materials

Ni-rich layered oxides are the most promising cathode materials for Li-ion batteries due to their high specific capacity and reasonable cost. Unfortunately, undesired residual Li compounds (RLCs) tend to form on the surface of Ni-rich materials, causing severe limitations to their commercialization. In this work, water washing and subsequent recalcination strategies were adopted to eliminate surface RLCs as well as guarantee the cycling stability of LiNi0.83Co0.11Mn0.06O2 materials. The washing/recalcination processes not only induced the migration of Li+ and the redox of Ni2+/Ni3+ but also contributed to the variation of the specific surface area. Combined with the electrochemical properties, we found that these structure evolutions showed different impacts on the performance as the recalcination temperatures changed. Taking capacity and cycling stability into account, the optimal recalcination condition was selected. More importantly, the relationships between washing/recalcination processes and structure performance were established. This work reinforces the understanding of modified Ni-rich materials and motivates the development of advanced cathodes for batteries.

Morphology-controllable synthesis and excellent electrochemical performance of Ni-rich layered NCM622 as cathode materials for lithium-ion batteries via glycerin-assisted solvothermal method

High performance Ni-rich layered materials are widely regarded as valuable energy materials due to their good electrochemical performance and low preparation cost. The design of reasonable morphology and size is one of the effective ways to improve the structural stability of lithium-ion battery cathode materials. Here, a simple glycerin solvothermal strategy is introduced to achieve this goal. Furthermore, the influence of microstructure on the electrochemical properties and structural stability of Ni-rich layered cathode materials is studied. In particular, the cathode material LiNi0.6Co0.2Mn0.2O2-50% glycerin (NCM-5) in the form of microspheres shows the best electrochemical performance and cyclic stability in all samples. After 100 cycles, the capacity retention rate is 79.25%. It is confirmed that the specific morphology and reasonable size can effectively alleviate the structural degradation and phase transition of particles after long-term cyclic testing.

Tantalum Oxide Coating of Ni-rich Cathode Active Material via Atomic Layer Deposition and its Influence on Gas Evolution and Electrochemical Performance in the Early and Advanced Stages of Degradation

Detrimental side-reactions of Ni-rich cathode active materials (CAMs) with the electrolyte have historically impeded the extension of the utilized voltage window to higher upper cut-off voltages. Doping and coating approaches are studied widely to further improve these materials and to reduce the intensity of bulk and surface degradation but suffer from poor control of film thickness and homogeneity, leading to partial doping of the bulk. We herein report the singular effect of a tantalum oxide (Ta2O5) thin film on Li[Ni0.8Mn0.1Co0.1]O2 (NMC811), generated by atomic layer deposition, offering the possibility of a high-level homogeneity and thickness control. After chemical analysis using X-ray photoelectron spectroscopy the composition of the deposited thin film is identified as a lithium tantalate chemistry (LiTaO3). At an early degradation stage, clear improvements directly attributed to the coating, such as suppressed exothermic side-reactions (−51%), reduced released gas amounts (−14.8%) and less charge-transfer resistance growth (2× lower) are observed. However, at an advanced degradation stage, the materials show similar cycle life, as well as similar gassing behavior and an even higher charge-transfer resistance growth as compared to the uncoated material. This study highlights the necessity of bulk stabilization and identifies the effect of surface coatings on undoped NMC811 without any doping influence.

Fluorine-free water-based Ni-rich positive electrodes and their performance in pouch- and 21700-type cells

In recent years substantial progress has been made in the lab-scale aqueous processing of cathode electrodes containing Ni-rich active materials such as LiNi0.83Co0.12Mn0.05O2. Now it has to be proven in production-scale that such an environmentally-friendly, water-based process will deliver cells with competitive performance. We demonstrate the upscaling of a carboxymethyl cellulose and styrene-butadiene rubber based electrode-coating recipe from 250 mL lab-scale to 10 L production-scale by coating a double-sided electrode with 140 m length, very homogeneous mass loading, and excellent roll to roll processability. Several challenges such as the control and stabilization of slurry pH and fine adjustment of the mixing and coating time have been solved. The organic- and H2O-based pilot scale cathode electrodes with areal capacities of 3.0 mAh cm−2 were characterized in pouch- and 21700-type cylindrical cells. The cells were cycled at room temperature and 45 °C for up to 2000 cycles at 1C/1C. Pouch cells containing H2O-based electrodes show a high capacity retention of 79.3% after 1000 cycles. Similarly, H2O-based electrodes built into 21700-type cells with 3.5 Ah capacity achieved 80% after 1000 cycles at 1C/1C cycling.

Mn Interdiffusion Mobility Controlled By Simple Drying Process for Cobalt Free Core-Shell Ni Rich Cathode Material in Lithium Ion Batteries

Lithium-ion batteries (LIBs) become an essential part of many portable devices and even electric vehicles than ever before. Among the various cathode materials, Ni rich layered cathode material has big attention because it has higher specific capacity and energy density. However, at a delithiated state, unstable Ni4+ leads to oxygen release and structural degradation and as a result, irreversible phase transition from R3m to electrochemical inactive Fm3m is observed. So, various strategy is applying to overcome this limitation like three-component system (NCM) or surface coating. However, NCM still cannot solve that problem perfectly and surface coating makes a problem like reducing specific capacity caused by insulating coating material (Al2O3, MnO2...etc) And also, this day, Co free structure of Ni rich cathode material has received more great attention as an alternative to NCM because of its hazardous toxicity and increasing price of Co. At this perspective, many studies have demonstrated Core-Shell or FCG (Full Concentration Gradient) structure can make better structural stability without Co metal than surface coating, which has insulated coating materials. In addition, core shell structure is easier to control the composition than FCG structure. However, in Core-Shell structure, interdiffusion of shell metal to core deteriorate the stability of Core-Shell material, so it needs very delicate heat treatment. And generally, to prevent interdiffusion from traditional core-shell structure, lower synthesis temperature or high valence metal dopant would be needed and that leads lower initial specific capacity or additional doped metal. So, we prevent the Mn interdiffusion by changing valence state of Mn in precursor through simple convection drying process not vacuum drying without decreasing synthesis temperature and additional dopant. Atomic interdiffusion in layered metal oxide structures follows the atomic migration through octahedral and tetrahedral sites. In case of various stated Mn, higher valence state Mn has higher energy barrier to migrate between each Oh and Td sites. Based on this theory, surface Mn rich shell can be oxidized easily under convection oven drying and highly oxidized Mn will be remained better than lower state Mn during high temperature calcination. These more remained Mn in shell can protect the particle surface from electrolyte attack and also higher valence state Mn makes slightly more Ni2+ due to thermodynamic stability and charge balance of Mn on the surface. And that Ni2+ in Li slab (Cation mixing) acts as a pillar to suppress irreversible phase transition of Ni rich materials. Through this surface passivation, phase transition (layered to rock salt ) propagation surface to bulk can be blocked. Furthermore, mechanical pulverization of secondary particle is also prevented because of less permeating electrolyte into bulk structure. Therefore, Li ion diffusion will not be sluggish after cycling as observed by GITT. In addition, more clear Ni rich core assure high specific capacity and faster electrochemical reaction kinetic without severe capacity fading. And also, in Full cell test (using graphite as anode), convection oven dried sample has better structure stability and that means our strategy for core-shell LiNi0.97Mn0.03O2 sample can be good candidate to alternate conventional cathode active material for Lithium ion battery practically. As prepared materials, atomic distribution was observed by EDS and XPS analysis. This study suggests useless of vacuum drying and more cost effective way to prepare Co free Core Shell LiNi0.97Mn0.03O2 cathode material for Lithium Ion Batteries.

The Role of Long Lifetime Li-Ion Cells in a Sustainable Future

State-of-the-art Li-ion cells can have decades of lifetime (>40 years) and tremendous cycle life greater than 10000 cycles1. Such incredible cells greatly exceed the goal of 80% capacity retention after 800 cycles, promoted by some as sufficient for electric vehicles (EVs). However, by 2030 it is projected that more than 90% of all Li-ion batteries will be used to power vehicles2, with very few remaining for energy storage from intermittent renewables which must displace fossil fuels for power generation. The batteries in electric vehicles will represent a vast amount of energy storage capacity, which can be harnessed using vehicle to grid (V2G) technology. Batteries with that can maintain 80% of their capacity after a mere 800 cycles are unsustainable since most of the charge-discharge cycles in V2G will occur when the EV is stationary. For this reason, ultra-long-lived cells are a critical component of a sustainable future.In this talk, we will discuss the important role of graphite material and upper cut-off voltage (UCV) on the lifetime of NMC811 cells, and how avoiding volume contraction in Ni-rich materials yields cells that should be eminently suitable V2G applications3,4. Additionally, we will discuss the results of a two-year study on the impact of C-rate, depth of discharge (DoD), UCV, and temperature on the lifetime of NMC811 cells. Finally, we will demonstrate that NMC cells with a low UCV, with an emphasis on Ni-rich materials, yield greater energy throughput compared to their higher energy counterparts due to their prolonged lifetime. The $/kWh metric is surely important when selecting a battery technology, but the levelized cost of energy over the battery lifetime is more important when batteries last for many decades. References Harlow, J. E. et al. A Wide Range of Testing Results on an Excellent Lithium-Ion Cell Chemistry to be used as Benchmarks for New Battery Technologies. Journal of The Electrochemical Society 166, A3031–A3044 (2019). Pillot, C. The Rechargeable Battery Market and Main Trends 2018-2030. https://rechargebatteries.org/wp-content/uploads/2019/02/Keynote_2_AVICENNE_Christophe-Pillot.pdf (2019). Eldesoky, A. et al. Impact of Graphite Materials on the Lifetime of NMC811/Graphite Pouch Cells: Part I. Material Properties, ARC Safety Tests, Gas Generation, and Room Temperature Cycling. Journal of The Electrochemical Society 168, 110543 (2021). Eldesoky, A. et al. Impact of Graphite Materials on the Lifetime of NMC811/Graphite Pouch Cells: Part II. Long-Term Cycling, Stack Pressure Growth, Isothermal Microcalorimetry, and Lifetime Projection. Journal of The Electrochemical Society 169, 010501 (2022).

Advances in the Aqueous Processing of Ni-Rich Positive Electrodes

A sustainable and environmental friendly production process of the battery components is a crucial factor for a successful contribution of Li-ion batteries to a more sustainable energy supply. For carbon-based anodes the eco-friendly, water-based processing is already widely established, whereas the same procedure still poses several challenges for cathode materials, especially those with high Nickel content. One major challenge is the detrimental side-reaction of Ni-rich cathode active materials (CAM) with water, creating hydroxide and carbonate species which increase the pH of the paste to above 12. Consequently, the Al foil is corroded during the coating process. Therefore, it is essential to control the paste’s pH before casting the paste on the substrate Al foil. This step becomes more challenging the longer the electrode mixing and coating procedure takes. On top of this, a promising way of improving the aqueous-based processing is to identify a suitable water-based binder system to improve the electrochemical cycling stability.We present the application of a variety of different water-based binders, all used in combination with carboxymethyl cellulose (CMC) as thickening agent. The paste pH is controlled by adding H3PO4. Electrodes with industry relevant areal capacities of up to 3 mAh/cm2 are electrochemically evaluated in full pouch cells. The long-term cycling stability determined with 1 C charge/discharge was highly dependent on the binder system. The best electrodes reached an excellent capacity retention of up to 88% after 1000 cycles. The NMP-based reference electrode reached 91%.The investigation of the aged cells via post mortem analysis suggests that the main source of cell aging is the cathode electrode. According to scanning electron microscopy data, no stronger cracking of the secondary CAM particles and no enhanced dissolution of transition metals were observed for the aqueous cathodes. However, a stronger rise in charge-transfer impedance was observed for the aged, water-based cathodes. This suggests that the formation of a blocking surface layer is the major reason for a stronger performance decay with increasing cycle number.AcknowledgmentThe presented work was financially supported by the project DigiBattPro4.0 BW (3–4332.62-IPA/69) by the Ministry of Economic Affairs, Labour and Housing Baden-Württemberg and by the project DigiBattPro4.0 BMBF funded by the German federal ministry of education and research Baden-Württemberg (03XP0374D).

Using Synchrotron Techniques, Investigation of Electrochemical Interfaces in Ni-Rich NMC and Sulfide Electrolytes in All-Solid-State Lithium Metal Batteries

a Nano-electrochemistry Laboratory, Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology, Taipei 106, Taiwan, R.O.C b National Synchrotron Radiation Research Center (NSRRC), Hsinchu, 30076, Taiwan, R.O.CIn all-solid-state lithium metal batteries enable long cyclability of high voltage oxides cathode persistent problem for the large scale application as their underprivileged interfacial steadiness in contrast to sulfide solid-state electrolyte. In this context, the interfaces of the solid electrolyte and Ni-rich NMC811 active material are looked upon as interfacial chemical responses induced by delithiation. In this study, we monitor the impedance progress at the unstable electrode|electrolyte interface due to the electrochemical interfacial response and help us understand the complex nature of reactivity and degradation kinetics with the solid-solid interface redox decomposition, which makes decoupling each effect difficult. we investigated the interfacial phenomenon between LPSC and high voltage cathode NMC811. The effects of spontaneous retort by the side of the interface were separated, and the intrinsic electrochemical decomposition of LPSC was quantified. Moreover, we show that the notch of interfacial degradation surges and the presence of oxidation mechanisms. At the higher delithiation stage, the cathode might twitch structural defenselessness and oxygen utter and resulting in further stark degradation. This complex kinetic degradation behavior was investigated at the solid-solid interface in a delithiation NMC811 and SSE based on the local oxidation state of NMC811, and LPSC SE interfacial chemical response. In this work, we used various characterization techniques to investigate the interfacial phenomenon between LPSC|NMC811 combining EIS and advanced synchrotron techniques such as sXAS, XPS, XRF-XANES mapping, and In-situ Raman spectroscopy. Keywords: delithiation, Ni-rich cathode, Sulfide-solid-state electrolyte, interfacial reaction, Synchrotron XPS, XRF.

Alleviating Anisotropic Volume Variation at Comparable Li Utilization during Cycling of Ni-Rich, Co-Free Layered Oxide Cathode Materials

Driven by demand for greater energy densities, Ni-rich cathode materials, such as lithium nickel cobalt manganese (NCM) and nickel cobalt aluminum (NCA) oxides, with compositions approaching the lithium nickel oxide (LiNiO2) end-member have been investigated intensively. While such compositions are targeted assuming the redox activity of nickel will lead to higher capacities, the role of even small amounts of Mn and Co in these systems is of great importance. To raise considerations about the role of Mn and Co, operando X-ray diffraction has been used to resolve the structure–electrochemistry relationships in a series of Ni-rich NMX (LiNi1–yMnyO2, y = 0.25, 0.17, 0.10, 0.05) cathode materials. To ensure a meaningful comparison, the upper cutoff potential was varied as a function of the Mn content in the material to ensure comparable states of delithiation and thereby provide a capacity-normalized comparison of the structural evolution. During the first cycle all materials deliver a specific charge capacity exceeding 230 mAh g–1, corresponding to a residual Li content of x(Li) ≈ 0.15, and exhibit a structural evolution free of any first-order phase transitions. Monitoring the structural parameters of the materials during cycling shows that Mn substitution substantially reduces the magnitude of expansion/contraction of lattice parameters even when comparable amounts of Li are removed from the structure and more significantly also reduces the anisotropy of the volume changes. Thus, these Co-free, Ni-rich materials hold promise as high-capacity cathodes with good structural and mechanical stability.

Inhibition of Gas-evolved electrolyte decomposition in cylindrical Li-ion battery cells of Ni-rich layered oxide with a dry coating process without post thermal annealing

Although Ni-rich layered oxide materials widely used as the cathode of lithium-ion batteries (LIBs) possess several interesting properties e.g., high energy density, they encounter severe capacity degradation and intrinsic poor thermal stability concerning the Ni-concentration. Herein, a core-shell-like architecture has been achieved by employing the cost-effective dry particle fusion method without an energy-consuming thermal annealing process with alumina (Al2O3) nanoparticles over LiNi0.8Mn0.1Co0.1O2 or NMC811 and resulting in an intact layer with an average shell thickness of 150–200 nm. Such NMC811@ Al2O3 core-shell exhibits excellent cycling stability compared with pristine NMC811. The chemical lithium diffusion coefficient, as calculated from a galvanostatic intermittent titration technique (GITT) infers that the introduction of Al2O3 coating slightly reduces the lithium diffusivity; however, it limits the active material dissolution, as perceived from post-ICP-OES analysis of the counter electrode. Besides, such modified systems exhibit a significant delay in onset potential related to surface layer decomposition and lattice oxygen release. Therefore, the present investigation concludes that the Al2O3 shell in the NMC811@ Al2O3 not only offers superior electrochemical performance but also exhibits good thermal stability. It could achieve thru a cost-effective and scalable dry-fusion method.

Nanoscale surface modification to suppress capacity fade of Ni-Rich layered oxide material at high cut-off voltage

Ni-rich layered oxide is a promising cathode material for future advanced lithium-ion batteries (LIBs). However, the nickel-rich layered cathode material exhibits insufficient high-voltage cycling stability ascribing to the phase transformation, surface side reactions, and lattice oxygen evolution. This work developed an effective inorganic coating strategy to construct LiNiO2/Na1-xNi1-yPO4 surface hybrid coating layer and Na bulk doping simultaneously by a simple wet-chemical method, improving the high-voltage performance of LiNi0.8Co0.1Mn0.1O2 (NCM) effectively. It is found that the surface oxygen vacancy and surface residual lithium on NCM reacts with NaNiPO4 precursors at 500 °C to form LiNiO2 inner layer in addition to the Na1-xNi1-yPO4 outer layer, which is structurally coherent to the layered lattice of NCM. In the constructed hybrid layer, layered phase LiNiO2 firmly fixes on the NCM surface. At the same time, highly chemical stable Na1-xNi1-yPO4 with lattice tunnels can enable lithium-ion transport and act as a protecting layer to inhibit surface side reactions. In addition, Na bulk doping could enlarge lithium-ion diffusion channels and stabilize lattice structure. All these advantages contribute to improving the electrochemical performance of NCM. The results show that the surface treatment NCM cathode exhibit excellent cycling stability with a higher capacity retention of 88 % after 200 cycles under 25 °C and 86 % after 100 cycles under 55 °C, both at 1C. This method provides a feasible strategy to promote the high-voltage cycling performance of electrode material, thus improving the service life, energy density, and safety performance of Li-ion batteries.

Optimization of Sr-doping boosting the structural stability for single crystalline LiNi0.8Co0.1Mn0.1O2 cathode to enhance its electrochemical performance at elevated voltage and temperature

Single crystalline (SC) Ni-rich cathodes have been considered the potential cathodes for the next generation lithium-ion batteries. Unfortunately, some inevitable defects, such as severe irreversible phase transition, microcracks formation and transition metal dissolution at high voltage, hinder its commercial applications. Herein, a high stable modified SC-LiNi0.8Co0.1Mn0.1O2 cathode is prepared by using Sr doping strategy, which is helpful to tackle the above challenges simultaneously. The introduction of Sr atoms can occupy the transition metal sites of the material, resulting in the increase of Li/Ni exchange energy and the improvement of oxygen layer stability. Meanwhile, Sr doping can promote the growth of primary particles and reduce the agglomeration of SC particles. The optimized doped sample of 0.5Sr-NCM exhibits pronounced improvements in pouch-type full cell with a remarkable capacity retention of 86.6% after 1200 cycles at 1C between 2.8 and 4.35 V (45 °C), while the pristine sample only shows a capacity retention of 80.8% after 798 cycles. The mechanism of enhanced electrochemical properties for the doped sample is revealed by the characterization analyses and density functional theory calculation. This work provides a facile strategy to regulate the bulk structure of SC Ni-rich materials, which may promote the development of high-energy-density lithium-ion batteries.

Synergistic modification of Ni-rich full concentration gradient materials with enhanced thermal stability

Major challenge hindering the large-scale applications of Ni-rich cathode materials (CAMs) lies on the poor cycle (especially under elevated temperature or high cutoff voltage) and thermal stability due to the highly reactive Ni4+. Herein, the full concentration strategy is combined with Ti pillar and Li2ZrO3 (LZO) coating modification, on which a high-performance CAM with elevated kinetics and stability, CGTZ-1, is obtained. It proves that both cycle and thermal stability can be greatly enhanced by the concentration gradient design and the LZO coating. Whereas the promoted Li+ diffusion coefficient is largely attributed to the Ti pillar. The optimal resultant CAM shows high capacity retention of 88.1% after 200 cycles under 55°C, while that of pristine is only 32.1%. More importantly, it also shows a high thermal release temperature of 261.5°C (vs 222.3°C of the pristine), which demonstrates the effectiveness of this synergistic modification strategy.

Regulating electrode-electrolyte interphases and eliminating hydrogen fluoride to boost electrochemical performances of Li/NCM811 batteries

Li metal batteries (LMBs) coupled with Ni-rich materials are promising candidates for next-generation high-energy-density lithium batteries. However, due to the inferior stability of cathode/solid-electrolyte interphases (CEI/SEI) and aggressive chemistry of Ni-rich cathode materials, Li metal anode (LMA) and LiPF6-based carbonate electrolytes pose a threat to the achievement of high-energy-density LMBs. Herein, 1-(trimethylsilyloxy)cyclohexene (TMSCH) with functional groups (C=C, Si-O) as a novel multi-functional electrolyte additive is employed to regular the CEI/SEI films on the both Ni-rich cathode materials (LiNi0.8Co0.1Mn0.1O2, NCM811) and LMA surface, and scavenge detrimental hydrogen fluoride (HF) to enhance electrochemical performances of the Li/NCM811 batteries. Furthermore, the theoretical calculation, electrochemical measurements and physical characterizations demonstrate the feasibility and effectiveness of the strategy. Specially, the TMSCH-derived SEI film exhibits a hierarchical structure consisting of an organic-rich outer layer and a LiF-rich inner layer. Accordingly, the microcracks generation of NCM811 secondary particles, transition metal ions dissolution, lithium dendrite growth, parasitic reactions and HF generation of electrolyte are effectively addressed. As a result, with existence of the TMSCH additive, the capacity retention of Li/NCM811 batteries at 1C are remarkably enhanced from 68.2% to 84.9% at 30 °C and 64.2% to 85.8% at 45 °C. Furthermore, the Li/Li symmetric cells with TMSCH can stably cycle over 1000 h at 0.2 mA cm−2 and exhibit super-low overpotential of 75 mV at 1 mA cm−2.

Towards Ni-rich layered oxides cathodes with low Li/Ni intermixing by mild molten-salt ion exchange for lithium-ion batteries

In the conventional synthesis of layered transition metal oxides, the high-temperature process not only causes lithium loss due to evaporation, but also facilitate the inevitable cation mixing of Li+ and Ni2+, resulting in severe shifts in their stoichiometric ratios and thus structure instability. Herein, we report a universal ion-exchange method to prepare Ni-rich layered oxide materials LiNi0.85Co0.06Mn0.09O2 (NCM85) with low Li/Ni intermixing at a low reaction temperature of 300 °C, where sodium-based layered oxides (NaNi0.85Co0.06Mn0.09O2) were used as precursors for ion exchange with Li+ ion in lithium molten salts. By studying the effects of ion-exchange time and additional heat treatment on the structure and performance of the layered oxide materials, we established the processing-structure-performance relationships for Ni-rich NCM85 materials. Moreover, we further demonstrated the universality of this ion-exchange method and processing-structure-performance relationships for other layered oxide materials including LiNi0. 5Co0.2Mn0.3O2 and LiNi0.5Mn0.5O2. We anticipate that such universal method of ion exchange and universal processing-structure-performance relationships can guide the rational design and synthesis of other layered oxide materials for lithium-ion batteries.

Advanced single-crystal layered Ni-rich cathode materials for next-generation high-energy-density and long-life Li-ion batteries

The studies of Ni-rich cathode materials have been the top priority of research because of the high energy density and fair cycling life. However, suffering from severe crack generations and side reactions, the traditional polycrystal (PC) Ni-rich material displayed structural/electrochemical fade during cycling. Compared with PC, single-crystal (SC) Ni-rich materials exhibited excellent structural stability and cycling performance, benefiting from the limited side reaction and gas generation. In this review, the authors not only compared the structural evolution and electrochemical failure mechanisms between PC and SC, but also summarized the synthesis methods and characterization techniques of SC, which provides universal insights into the development of Ni-rich cathode materials.

Co-precipitation synthesis of nickel-rich cathodes for Li-ion batteries

The preparation of Ni-rich cathode materials is challenging due to the Ni2+ ion sensitivity to oxidation during synthesis. The synthesis conditions during the manufacture of Ni-rich materials such as LiNi0.8Mn0.1Co0.1O2 (NMC811) therefore require stringent control. The co-precipitation step, applied in the synthesis of the metal hydroxide precursor, determines the secondary particle assembly formation, where it is typically desirable to produce uniform, spherical, ∼10μm-diameter structures. A stirred tank reactor is often employed to maintain a constant temperature of 60 °C and a controlled pH of between 10.5 and 11.5 in an inert atmosphere to maintain a high Ni2+/Ni3+ ion ratio. This promotes the formation of an NMC hydroxide precursor (NixMnyCoz(OH)2) which is typically milled with a lithium salt and calcined to form LiNixMnyCozO2 with a layered α-NaFeO2 crystalline structure. This review outlines some of the critical synthetic parameters for the formation of spherical secondary assemblies of metal hydroxide precursors for nickel-rich layered cathodes.

Effects of Oxygen Pressurization on Li+/Ni2+ Cation Mixing and the Oxygen Vacancies of LiNi0.8Co0.15Al0.05O2 Cathode Materials.

Ni-rich cathode materials are a low-cost and high-energy density solution for high-power lithium-ion batteries. However, Li+/Ni2+ cation mixing and oxygen vacancies are inevitably formed during the high-temperature calcination process, resulting in a poor crystal structure that adversely affects the electrochemical performance. In this work, the LiNi0.8Co0.15Al0.05O2 cathode material with a regular crystal structure was prepared through oxygen pressurization during lithiation-calcination, which effectively solved the problems caused by the high calcination temperature, such as oxygen loss and a reduction of Ni3+. The co-effect of oxygen pressure and calcination temperature on the properties of Ni-rich materials was systematically explored. Oxygen pressurization increased the redox conversion temperature, thus promoting the oxidation of Ni2+ and reducing Li+/Ni2+ cation mixing. Moreover, due to the strong oxidizing environment provided by the elevated calcination temperature and oxygen pressurization, the LiNi0.8Co0.15Al0.05O2 material synthesized under 0.4 MPa oxygen pressure and a calcination temperature of 775 °C exhibited few oxygen vacancies, which in turn suppressed the formation of microcracks during the electrochemical cycling. An additional feature of the LiNi0.8Co0.15Al0.05O2 material was the small specific surface area of the particles, which reduced both the contact area with the electrolyte and side reactions. As a result, the LiNi0.8Co0.15Al0.05O2 material exhibited remarkable electrochemical performance, with an initial discharge capacity of 191.6 mA h·g-1 at 0.1 C and a capacity retention of 94.5% at 0.2 C after 100 cycles.