Understanding the failure mechanism towards developing high-voltage single-crystal Ni-rich Co-free cathodes

Benefited from its high process feasibility and controllable costs,binary-metal layered structured LiNi_(0.8)Mn_(0.2)O_(2)(NM)can effectively alleviate the cobalt supply crisis under the surge of global electric vehicles(EVs)sales,which is considered as the most promising nextgeneration cathode material for lithium-ion batteries(LIBs).However,the lack of deep understanding on the failure mechanism of NM has seriously hindered its application,especially under the harsh condition of high-voltage without sacrifices of reversible capacity.Herein,singlecrystal LiNi_(0.8)Mn_(0.2)O_(2) is selected and compared with traditional LiNi_(0.8)Co_(0.1)Mn_(0.1)O_(2)(NCM),mainly focusing on the failure mechanism of Cofree cathode and illuminating the significant effect of Co element on the Li/Ni antisite defect and dynamic characteristic.Specifically,the presence of high Li/Ni antisite defect in NM cathode easily results in the extremely dramatic H2/H3 phase transition,which exacerbates the distortion of the lattice,mechanical strain changes and exhibits poor electrochemical performance,especially under the high cutoff voltage.Furthermore,the reaction kinetic of NM is impaired due to the absence of Co element,especially at the single-crystal architecture.Whereas,the negative influence of Li/Ni antisite defect is controllable at low current densities,owing to the attenuated polarization.Notably,Co-free NM can exhibit better safety performance than that of NCM cathode.These findings are beneficial for understanding the fundamental reaction mechanism of single-crystal Ni-rich Co-free cathode materials,providing new insights and great encouragements to design and develop the next generation of LIBs with low-cost and high-safety performances.

Mechanical densification synthesis of single-crystalline Ni-rich cathode for high-energy lithium-ion batteries

The intergranular microcracking in polycrystalline Ni-rich cathode particle is led by anisotropic volume change and stress corrosion along grain boundary,accelerating battery performance decay.Herein,we have suggested a simple but advanced solid-state method that ensures both uniform transition metal distribution and single-crystalline morphology for Ni-rich cathode synthesis without sophisticated coprecipitation.Pelletization-assisted mechanical densification(PAMD)process on solid-state precursor mixture enables the dynamic mass transfer through the increased solid-solid contact area which facilitates the grain growth during sintering process,readily forming micro-sized single-crystalline particle.Furthermore,the improved chemical reactivity by a combination of capillary effect and vacancyassisted diffusion provides homogeneous element distribution within each primary particle.As a result,single-crystalline Ni-rich cathode with PAMD process has eliminated a potential evolution of intergranular cracking,thus achieving superior energy retention capability of 85%over 150 cycles compared to polycrystalline Ni-rich particle even after high-pressure calendering process(corresponding to electrode density of~3.6 g cm^(-3))and high cut-off voltage cycling.This work provides a concrete perspective on developing facile synthetic route of micron-sized single-crystalline Ni-rich cathode materials for high energy density lithium-ion batteries(LIBs).

Synthesis and properties of single-crystal Ni-rich cathode materials in Li-ion batteries

Single-crystal Ni-rich cathode material LiNi0.88Co0.09Al0.03O2(SC) was synthesized by a high-temperature solid-state calcination method. Physicochemical properties of primary and delithiated SC samples were investigated by X-ray diffractometry, X-ray photoelectron spectroscopy, and transmission electron microscopy. Electrochemical performance was characterized by long-term cycling, cyclic voltammetry, and in-situ impedance spectroscopy. The results indicated that high temperature rendered layered oxides to lose lithium/oxygen in the interior and exterior, and induced cationic disordering. Besides, the solid-phase synthesis process promoted phase transformation for electrode materials, causing the coexisting multi-phase in a single particle. High temperature can foster the growth of single particles, but it caused unstable structure of layered phase.

Suppressed Internal Intrinsic Stress Engineering in High-Performance Ni-Rich Cathode Via Multi layered In Situ Coating Structure

LiNi_(x)Co_(y)Al_(z)O_(2)(NCA)cathode materials are drawing widespread attention,but the huge gap between the ideal and present cyclic stability still hinders their further commercial application,especially for the Ni-rich LiNi_(x)Co_(y)Al_(z)O_(2)(x>0.8,x+y+z=1)cathode material,which is owing to the structural degradation and particles'intrinsic fracture.To tackle the problems,Li_(0.5)La_(2)Al_(0.5)O_(4)in situ coated and Mn compensating doped multilayer LiNi_(0.82)Co_(0.14)Al_(0.04)O_(2)was prepared.XRD refinement indicates that La-Mn co-modifying could realize appropriate Li/Ni disorder degree.Calculated results and in situ XRD patterns reveal that the LLAO coating layer could effectively restrain crack in secondary particles benefited from the suppressed internal strain.AFM further improves as NCA-LM2 has superior mechanical property.The SEM,TEM,XPS tests indicate that the cycled cathode with LLAO-Mn modification displays a more complete morphology and less side reaction with electrolyte.DEMS was used to further investigate cathode-electrolyte interface which was reflected by gas evolution.NCA-LM2 releases less CO_(2)than NCA-P indexing on a more stable surface.The modified material presents outstanding capacity retention of 96.2%after 100 cycles in the voltage range of 3.0-4.4 V at 1C,13%higher than that of the pristine and 80.8%at 1 C after 300 cycles.This excellent electrochemical performance could be attributed to the fact that the high chemically stable coating layer of Li_(0.5)La_(2)Al_(0.5)O_(4)(LLAO)could enhance the interface and the Mn doping layer could suppress the influence of the lattice mismatch and distortion.We believe that it can be a useful strategy for the modification of Ni-rich cathode material and other advanced functional material.

The mechanism of side reaction induced capacity fading of Ni-rich cathode materials for lithium ion batteries

Ni-rich cathode materials show great potential of applying in high-energy lithium ion batteries,but their inferior cycling stability hinders this process.Study on the electrode/electrolyte interfacial reaction is indispensable to understand the capacity failure mechanism of Ni-rich cathode materials and further address this issue.This work demonstrates the domain size effects on interfacial side reactions firstly,and further analyzes the inherent mechanism of side reaction induced capacity decay through comparing the interfacial behaviors before and after MgO coating.It has been determined that LiF deposition caused thicker SEI films may not increase the surface film resistance,while HF erosion induced surface phase transition will increase the charge transfer resistance,and the later plays the dominant factor to declined capacity of Ni-rich cathode materials.This work suggests strategies to suppress the capacity decay of layered cathode materials and provides a guidance for the domain size control to match the various applications under different current rates.

Plasma assisted synthesis of LiNi_(0.6)Co_(0.2)Mn_(0.2)O_(2) cathode materials with good cyclic stability at subzero temperatures

Layered Ni-rich cathode materials,LiNi_(0.6)Co_(0.2)Mn_(0.2)O_(2)(NCM622),are synthesized via solid reaction assisted with a plasma milling pretreatment,which is resulted in lowering sintering temperatures for solid precursors.The plasma milling pretreated NCM622 cathode material sintered at 780℃(named as PM-780)demonstrates good cycling stability at both room and subzero temperatures.Specifically,the PM-780 cathode delivers an initial discharge capacity of 171.2 mAh g^(-1) and a high capacity retention of 99.7%after 300 cycles with current rate of 90 mA g^(-1) at 30℃,while stable capacities of 120.3 and 94.0 m Ah g^(-1) can be remained at-10℃and-20℃in propylene carbonate contained electrolyte,respectively.In-situ XRD together with XPS and SEM reveal that the NCM622 cycled at-10℃presented better structural stability and more intact interface than that of cathodes cycled at 30℃.It is also found that subzero temperatures only limit the discharge potential of NCM622 without destroying its structure during cycling since it still exhibits high discharge capacity at 30℃after cycled at subzero temperatures.This work may expand the knowledge about the low-temperature characteristics of layered cathode materials for Li-ion batteries and lay the foundation for its further applications.

Advances of high-performance LiNi_(1-x-y)Co_(x)M_(y)O_(2) cathode materials and their precursor particles via co-precipitation process

Layered LiNi_(1-x-y)Co_(x)M_(y)O_(2)(M=Mn or Al)is a promising cathode material for lithium-ion batteries due to its high specific capacity and acceptable manufacturing cost.However,the polycrystalline LiNi_(1-x-y)Co_(x)M_(y)O_(2) cathode material suffers from disordered orientation of primary particles and poor geometric symmetry of secondary particles,which severely hampers the migration of Lit ions.Furthermore,the resulting anisotropy accelerates the disintegration of the secondary particle structure,significantly affecting the electrochemical performance of the polycrystalline cathode.In spite of less grain boundary,the single-crystal LiNi_(1-x-y)Co_(x)M_(y)O_(2) cathodes still suffer from severe microcracks generated by repeated planar gliding during cycling,which poses a great challenge to the cycling stability of single-crystal materials.It's worth noting that the microstructure of the cathode material is mainly inherited from its precursor.Therefore,it is necessary to deeply understand the influence of the microstructure of Ni_(1-x-y)Co_(x)M_(y)(OH)2 on the electrochemical properties of LiNi_(1-x-y)Co_(x)M_(y)O_(2) cathode materials,so as to optimize the production process of preparing high-performance cathode precursors.In this review,we summarize recent advances in the research and development of Ni-rich cathode precursor materials.Firstly,the challenges faced by the Ni-rich hydroxide precursor materials are presented,including the effect of primary particle morphology and arrangement on the electrochemical performance of cathode materials,the influence of secondary particle morphology on lithium insertion reactions in cathode,and the effect of particle size on the microcracking of single-crystal particles.Secondly,the presentation of the conventional co-precipitation reactor,the mechanism of precursor particle growth,and the influence of coprecipitation parameters are described in detail.Finally,the strategies are systematically discussed to solve the challenges of hydroxide precursors,such as the innovation and optimization on reactants,synthesis processes,and reaction equipment.To obtain satisfactory high-quality precursor materials,future work will require an in-depth understanding of the reaction mechanism,combined with simulation techniques such as flow field theory calculations to guide the synthesis of precursors.This review provides a comprehensive analysis of the current progresses on the producing technologies of highperformance cathode precursors and offers prospects for future industry developments.

Amorphous Zr(OH)4 coated LiNi0.915Co0.075Al0.01O2 cathode material with enhanced electrochemical performance for lithium ion batteries

LiNiCoAlO（NCA） with Zr（OH）coating is demonstrated as high performance cathode material for lithium ion batteries（LIBs）. The coated materials are synthesized via a simple dry coating method of NCA with Zr（OH）powders, and then characterized with scanning electron microscopy（SEM）, transmission electron microscopy（TEM） and X-ray photoelectron spectroscopy（XPS）. Experimental results show that amorphous Zr（OH）powders have been successfully coated on the surface of spherical NCA particles, exhibiting improved electrochemical performance. 0.50 wt% Zr（OH）coated NCA delivers a capacity of 197.6 mAh/g at the first cycle and 154.3 mAh/g after 100 cycles with a capacity retention of 78.1% at 1 C rate. In comparison, the pure NCA shows a capacity of 194.6 mAh/g at the first cycle and 142.5 mAh/g after 100 cycles with a capacity retention of 73.2% at 1 C rate. Electrochemical impedance spectroscopy（EIS） results show that the coated material exhibits a lower resistance, indicating that the coating layer can efficiently suppress transition metals dissolution and decrease the side reactions at the surface between the electrode and electrolyte. Therefore, surface coating with amorphous Zr（OH）is a simple and useful method to enhance the electrochemical performance of NCA-based materials for the cathode of LIBs.

Effect of particle micro-structure on the electrochemical properties of LiNi_(0.8)Co_(0.1)Mn_(0.1)O_(2) cathode material

Ni-rich layered material is a kind of high-capacity cathode to meet the requirement of electric vehicles.As for the typical LiNi_(0.8)Co_(0.1)Mn_(0.1)O_(2) material,the particle formation is significant for electrochemical properties of the cathode.In this work,the structure,morphology,and electrochemical performance of LiNi_(0.8)Co_(0.1)Mn_(0.1)O_(2) secondary particles and single crystals were systematically studied.A lower Ni^(2+)/Ni^(3+)molar ratio of 0.66 and a lower residual alkali content of 0.228wt%were achieved on the surface of the single crystals.In addition,the single crystals showed a discharge capacity of 191.6 mAh/g at 0.2 C(~12 mAh/g lower than that of the secondary particles)and enhanced the electrochemical stability,especially when cycled at 50℃ and in a wider electrochemical window(between 3.0 and 4.4 V vs.Li+/Li).The LiNi_(0.8)Co_(0.1)Mn_(0.1)O_(2) secondary particles were suitable for applications requiring high specific capacity,whereas single crystals exhibited better stability,indicating that they are more suitable for use in long life requested devices.

Synthesis of high-capacity LiNi_(0.8)Co_(0.1)Mn_(0.1)O_2 cathode by transition metal acetates

LiNi0.8Co0.1Mn0.1O2 cathode was synthesized using transition metal acetates under different synthesis conditions. Simultaneous thermogravimetric–differential scanning calorimetry–derivative thermogravimetric analysis was applied to investigating the mixture of transition metal acetates. X-ray powder diffraction and charge–discharge test were adopted to characterize the as-prepared LiNi0.8Co0.1Mn0.1O2. The mixture of transition metal acetates undergoes dehydration and decomposition during heating. All the examined LiNi0.8Co0.1Mn0.1O2 samples have a layered structure with R3 m space group. LiNi0.8Co0.1Mn0.1O2 samples prepared with different lithium sources under different synthesis conditions exhibit very different charge–discharge performances. The sample synthesized via the procedure of sintering at 800 °C after heating lithium carbonate and transition metal acetates at 550 °C achieves a highest capacity of 200.8 m A·h/g and an average capacity of 188.1 mA ·h/g in the first 20 cycles at 0.2C.

Pr doped single-crystal LiNi_(0.5)Mn_(0.3)Co_(0.2)O_(2)cathode enables high rate capability and cycle stability for lithium ion batteries

The massive application of single crystal(SC)ternary cathode material LiNi_(1-x-y)Mn_(x)Co_(y)O_(2)is largely restricted by the unsatisfactory rate capability which is caused by the sluggish Li+diffusion and structural instability.Herein,Pr^(3+),a large radius ion is introduced to single-crystal LiNi_(0.5)Mn_(0.3)Co_(0.2)O_(2)to enhance Li^(+)conductivity and structural stability.With 0.4%Pr doping,the Li(Ni_(0.5)Mn_(0.3)Co_(0.2))_(0.996)Pr_(0.004)O_(2)cathode displays a capacity retention of 79.72%at 10 C,and a 98.17%capacity retention after 50 cycles at 25°C and 96.3%capacity retention after 50 cycles at 55°C within a 3.0–4.5 V voltage window.Electrochemical impedance spectroscopy confirms that the Pr doping can effectively lower the charge-transfer resistance and facilitate the transportation of Li^(+)on the surface of LiNi_(0.5)Mn_(0.3)Co_(0.2)O_(2).The Direct current internal resistance result implies that the structure of the Pr-doped cathode particles is more stable during cycling.In addition,differential scanning calorimetry measurements measurement combined with in situ X-ray diffraction confirms the thermo-stabilization effect of the Pr dopant.

Strategies of Removing Residual Lithium Compounds on the Surface of Ni-Rich Cathode Materials

Ni-rich cathode materials have become one of the most promising cathode materials for advanced high-energy Li-ion batteries(LIBs)owing to their high specific capacity.However,Ni-rich cathode materials are sensitive to the trace H2O and CO2 in the air,and tend to react with them to generate LiOH and Li2COg at the particle surface region(named residual lithium compounds,labeled as RLCs).The RLCs will deteriorate the comprehensive performances of Ni-rich cathode materials and make trouble in the subsequent manufacturing process of electrode,including causing low initial coulombic efficiency and poor storage property,bringing about potential safety hazards,and gelatinizing the electrode slurry.Therefore,it is of considerable significance to remove the RLCs.Researchers have done a lot of work on the corresponding field,such as exploring the formation mechanism and elimination methods.This paper investigates the origin of the surface residual lithium compounds on Ni-rich cathode materials,analyzes their adverse effects on the per-formance and the subsequent electrode production process,and summarizes various kinds of feasible methods for removing the RLCS.Finally,we propose a new research direction of eliminating the lithium residuals after comparing and summing up the above.We hope this work can provide a reference for alleviating the adverse effects of residual lithium compounds for Ni-rich cathode materials'industrial production.

Improving cycling performance of the NaNiO_(2)cathode in sodium-ion batteries by titanium substitution

O3-type layered oxide cathodes,such as NaNi_(0.5)Mn_(0.5)O_(2),have garnered significant attention due to their high theoretical specific capacity while using abundant and low-cost sodium as intercalation species.Unlike the lithium analog(LiNiO_(2)),NaNiO_(2)(NNO)exhibits poor electrochemical performance resulting from structural instability and inferior Coulomb efficiency.To enhance its cyclability for practical application,NNO was modified by titanium substitution to yield the O3-type NaNi_(0.9)Ti_(0.1)O_(2)(NNTO),which was successfully synthesized for the first time via a solid-state reaction.The mechanism behind its superior performance in comparison to that of similar materials is examined in detail using a variety of characterization techniques.NNTO delivers a specific discharge capacity of∼190 mAh g^(−1)and exhibits good reversibility,even in the presence of multiple phase transitions during cycling in a potential window of 2.0−4.2 V vs.Na^(+)/Na.This behavior can be attributed to the substituent,which helps maintain a larger interslab distance in the Na-deficient phases and to mitigate Jahn–Teller activity by reducing the average oxidation state of nickel.However,volume collapse at high potentials and irreversible lattice oxygen loss are still detrimental to the NNTO.Nevertheless,the performance can be further enhanced through coating and doping strategies.This not only positions NNTO as a promising next-generation cathode material,but also serves as inspiration for future research directions in the field of high-energy-density Na-ion batteries.

An ultrasound-triggered cation chelation and reassembly route to one-dimensional Ni-rich cathode material enabling fast charging and stable cycling of Li-ion batteries

Ni-rich oxides,LiNixMnyCozO2(NMC),are among leading candidates for cathode materials in Li-ion batteries.However,they are mostly fabricated by coprecipitation approach under complex conditions,which usually produces large secondary particles composed of aggregated primary particles.Undesirable cation mixing and crack propagation upon cycling block ion and electron transport,result in fast capacity fading and poor rate capability.Herein,we present an ultrasound-triggered cation chelation and reassembly route for synthesizing one-dimensional precursor with homogeneous element distribution at the atomic level.This process is accomplished by the synergistic combination of ultrasound and surfactant,which can disperse reactants and remove hydration shells surrounding cations so as to accelerate chelating reaction,and then separate and assemble chelates into one dimensional structure.The whole synthesis time is only 20 min(8.9 min of ultrasonic working time)in an open vessel under natural ambient conditions.One-dimensional LiNi0.6Mn0.2Co0.2O2 has a high reversible capacity(184 mAh·g^−1 at 0.1 C)and long cycling stability(95.1%and 82.4%capacity retention for 100 and 1000 cycles,respectively).The short charging time of 76 s is realized at super high current rate of 20 C,which is very important to improve the competitiveness of electric vehicles relative to fuel vehicles.Our synthetic approach can provide a general strategy for the growth of mixed-metal-EDTA chelate precursors by changing the feeding ratio of Ni2+,Mn2+and Co2+cations in reaction for fabricating NMC cathode materials with other compositions.

Recent advances in Ni-rich layered oxide particle materials for lithium-ion batteries

Ni-rich layered oxides with chemical formula of LiNixCoyMnzO2 or LiNixCoyAl2O2(x+y+z=1,x≥0.6)have been considered as promising cathode materials for 1让hium-ion batteries(LIBs)because of their high specific capacity(≥180mAhg^-1)and acceptable manufacture cost.However,the problems associated with high Nicontent severely restrict their large-scale applications.In this review,we summarize the recent advances in Ni-rich layered oxide particle materials for LIBs.We begin with the introduction of the structure,redox mechanism,and problems of Ni-rich layered oxides,mainly including residual lithium compounds,gas evolution,rock-salt phase formation,microcrack of particles,dissolution of transitionmetal ions,and thermal runaway.Then,four strategies(primary particle engineering,surface coating,doping,concentration gradient design)toward solving the problems of Ni-rich layered oxides will be systematically discussed with the emphasis on structure-performa nee relati on ships.To achieve satisfied comprehensive performance and accelerate large-scale applications of Ni-rich layered oxides,the combination of two or more strategies(particle engineering and surface/bulk stabilization techniques)w让h synergistic effects is necessary in future works.This review would promote further research and application of high-performance Ni-rich layered oxide particle materials for LIBs.

Mechanism exploration of enhanced electrochemical performance of single-crystal versus polycrystalline LiNi_(0.8)Mn_(0.1)Co_(0.1)O_(2)

Single-crystal LiNi_(0.8)Mn_(0.1)Co_(0.1)O_(2)(SC-811),which offers better cycle performance compared to the polycrystalline counterpart,has received great attention.We report herein the synthesis of SC-811 with a grain size of 2-4μm by washing and reheating method,which compares with conventional polycrystalline LiNi_(0.8)-Co_(0.1)Mn_(0.1)O_(2)(PC-811).The discharge capacity of SC-811 reaches 152.1 mAh·g^(-1)after 100 cycles(86.7%capacity retention)at 1.0 C,which is much better than that of PC-811(130.2 mAh·g^(-1),73.8%capacity retention).By using multiscale characterization,the results unveil that SC-811 can not only improve the reversibility of the H2-H3 phase transitions,suppress the generation of micro-cracks and phase transformations,but also mitigate the undesired side reactions between electrode and electrolyte.Besides,the Li-O bond of SC-811 is longer than that of PC-811,which is conducive to the de-intercalation of Li-ions,thereby enhancing the structural stability.This finding provides an impressive strategy to sustain structural stability and improve the cycling life of Ni-rich layered cathodes.

A low-carbon strategy for revival of degraded single crystal LiNi_(0.6)Co_(0.2)Mn_(0.2)O_(2)

Single crystal LiNi_(0.6)Co_(0.2)Mn_(0.2)O_(2)is currently widely used due to the outstanding cycle stability and safety.However,its sensitivity to the environment and the high residual alkali makes the electrochemical performance and processing property severely degraded after long-term storage,especially for the Ni-rich single crystal material.Therefore,it is highly urgent to develop a cost-effective strategy for the revival of degraded Ni-rich cathode materials.Here,a low-carbon strategy is proposed to revive the degraded single crystal LiNi_(0.6)Co_(0.2)Mn_(0.2)O_(2)(SCNCM622)through water washing.The solid-liquid reaction mechanism of SCNCM622 and water was revealed and the strong dependence of the recovery effect on the washing time was clarified.Under optimized conditions,the sample with a washing time of 24 h shows 31.2%reduction in viscosity,18.4%improvement in discharge capacity,15.3%enhancement in cycle life,and excellent rate performance compared to the blank sample.Therefore,this strategy can achieve higher utilization of single crystal Ni-based cathode materials with a lower cost.