Difficulties, strategies, and recent research and development of layered sodium transition metal oxide cathode materials for high-energy sodium-ion batteries

Energy-storage systems and their production have attracted significant interest for practical applications.Batteries are the foundation of sustainable energy sources for electric vehicles(EVs),portable electronic devices(PEDs),etc.In recent decades,Lithium-ion batteries(LIBs) have been extensively utilized in largescale energy storage devices owing to their long cycle life and high energy density.However,the high cost and limited availability of Li are the two main obstacles for LIBs.In this regard,sodium-ion batteries(SIBs) are attractive alternatives to LIBs for large-scale energy storage systems because of the abundance and low cost of sodium materials.Cathode is one of the most important components in the battery,which limits cost and performance of a battery.Among the classified cathode structures,layered structure materials have attracted attention because of their high ionic conductivity,fast diffusion rate,and high specific capacity.Here,we present a comprehensive review of the classification of layered structures and the preparation of layered materials.Furthermore,the review article discusses extensively about the issues of the layered materials,namely(1) electrochemical degradation,(2) irreversible structural changes,and(3) structural instability,and also it provides strategies to overcome the issues such as elemental phase composition,a small amount of elemental doping,structural design,and surface alteration for emerging SIBs.In addition,the article discusses about the recent research development on layered unary,binary,ternary,quaternary,quinary,and senary-based O3-and P2-type cathode materials for high-energy SIBs.This review article provides useful information for the development of high-energy layered sodium transition metal oxide P2 and O3-cathode materials for practical SIBs.

Modification strategies improving the electrochemical and structural stability of high-Ni cathode materials

With the increasing spotlight in electric vehicles,there is a growing demand for high-energy-density batteries to enhance driving range.Consequently,several studies have been conducted on high-energy-density LiNi_(x)Co_(y)Mn_(z)O_(2)cathodes.However,there is a limit to permanent performance deterioration because of side reactions caused by moisture in the atmosphere and continuous microcracks during cycling as the Ni content to express high energy increases and the content of Mn and Co that maintain structural and electrochemical stabilization decreases.The direct modification of the surface and bulk regions aims to enhance the capacity and long-term performance of high-Ni cathode materials.Therefore,an efficient modification requires a study based on a thorough understanding of the degradation mechanisms in the surface and bulk region.In this review,a comprehensive analysis of various modifications,including doping,coating,concentration gradient,and single crystals,is conducted to solve degradation issues along with an analysis of the overall degradation mechanism occurring in high-Ni cathode materials.It also summarizes recent research developments related to the following modifications,aims to provide notable points and directions for post-studies,and provides valuable references for the commercialization of stable high-energy-density cathode materials.

Research on Preparation and Electrochemical Performance of the High Compacted Density Ni-Co-Mn Ternary Cathode Materials

The high compacted density LiNi0.5-xCo0.2Mn0.3MgxO2 cathode material for lithium-ion batteries was synthesized by high temperature solid-state method, taking the Mg element as a doping element and the spherical Ni0.5Co0.2Mn0.3 (OH)2, Li2CO3 as raw materials. The effects of calcination temperature on the structure and properties of the products were investigated. The structure and morphology of cathode materials powder were analyzed by X-ray diffraction spectroscopy (XRD) and scanning electronmicroscopy (SEM). The electrochemical properties of the cathode materials were studied by charge-discharge test and cyclic properties test. The results show that LiNi0.4985Co0.2Mn0.3 Mg0.0015O2 cathode material prepared at calcination temperature 930°C has a good layered structure, and the compacted density of the electrode sheet is above 3.68 g/cm3. The discharge capacity retention rate is more than 97.5% after 100 cycles at a charge-discharge rate of 1C, displaying a good cyclic performance.

Low-temperatures synthesis of CuS nanospheres as cathode material for magnesium second batteries

Rechargeable magnesium batteries(RMBs),as one of the most promising candidates for efficient energy storage devices with high energy,power density and high safety,have attracted increasing attention.However,searching for suitable cathode materials with fast diffusion kinetics and exploring their magnesium storage mechanisms remains a great challenge.Cu S submicron spheres,made by a facile low-temperature synthesis strategy,were applied as the high-performance cathode for RMBs in this work,which can deliver a high specific capacity of 396mAh g^(-1)at 20 mA g^(-1) and a remarkable rate capacity of 250 m Ah g^(-1)at 1000 mA g^(-1).The excellent rate performance can be assigned to the nano needle-like particles on the surface of Cu S submicron spheres,which can facilitate the diffusion kinetics of Mg^(2+).Further storage mechanism investigations illustrate that the Cu S cathodes experience a two-step conversion reaction controlled by diffusion during the electrochemical reaction process.This work could make a contribution to the study of the enhancement of diffusion kinetics of Mg2+and the reaction mechanism of RMBs.

Perspectives in Electrochemical in situ Structural Reconstruction of Cathode Materials for Multivalent-ion Storage

Multivalent-ion(such as Zn^(2+),Mg^(2+),Al^(3+))batteries are considered as a prospective alternative for large-scale energy storage.However,the main problem of cathode materials for multivalent-ion batteries is the sluggish diffusion of multivalent ions.Many cathode materials will self-adjust under electrochemical conditions to achieve the optimal state for multivalent-ion storage.In this review,the significant role of electrochemical in situ structural reconstruction of cathode materials is suggested.The types,basic characteristics,and formation mechanisms of reconstructed phases have been systematically discussed and commented.The most important insight we pointed out is that the cathode materials with loose structures after in situ electrochemical activation are conducive to the reversible diffusion of multivalent ions.Moreover,several crucial issues of electrochemical activation and reconstruction were further analyzed and discussed.The challenges and future perspectives are presented in the final section.

Regeneration of spent LiFePO4 as a high-performance cathode material by a simultaneous coating and doping strategy

With the number of decommissioned electric vehicles increasing annually,a large amount of discarded power battery cathode material is in urgent need of treatment.However,common leaching methods for recovering metal salts are economically inefficient and polluting.Meanwhile,the recycled material obtained by lithium remediation alone has limited performance in cycling stability.Herein,a short method of solid-phase reduction is developed to recover spent LiFePO4 by simultaneously introducing Mg2+ions for hetero-atom doping.Issues of particle agglomeration,carbon layer breakage,lithium loss,and Fe3+defects in spent LiFePO4 are also addressed.Results show that Mg2+addition during regeneration can remarkably enhance the crystal structure stability and improve the Li+diffusion coefficient.The regenerated LiFePO4 exhibits significantly improved electrochemical performance with a specific discharge capacity of 143.2 mAh·g^(−1)at 0.2 C,and its capacity retention is extremely increased from 37.9%to 98.5%over 200 cycles at 1 C.Especially,its discharge capacity can reach 95.5 mAh·g^(−1)at 10 C,which is higher than that of spent LiFePO4(55.9 mAh·g^(−1)).All these results show that the proposed regeneration strategy of simultaneous carbon coating and Mg2+doping is suitable for the efficient treatment of spent LiFePO4.

Selenium-doped cathode materials with polyaniline skeleton for lithium-organosulfur batteries

Sulfur-containing polymer(SCP)is considered as an outstanding cathode material for lithium-sulfur batteries.However,undesirable soluble polysulfides may shuttle in electrolyte,concluding long-chain Li_(2)S_(n)(n>4)and short-chain Li2Sn(n≤4),as well as the sluggish conversion kinetics are yet to be solved to enhance the performance of lithium-sulfur batteries.Here Se-doped sulfurized polyaniline with adjusted sulfur-chain-S_(x)-(x≤6)contribute to ensure the absence of long-chain polysulfides,and the skeleton with quinoid imine can endow strongly adsorption towards short-chain polysulfides by the reversible transition between deprotonated/protonated imine(-NH^(+)=and-N=),which offer double insurance against suppressing“shuttle effect”.Furthermore,Se atoms are doped into sulfurized polysulfides to accelerate the redox conversion and take a frontier orbital theory-oriented view into catalytic mechanism.Se-doped sulfurized polyaniline as active materials for lithium-organosulfur batteries delivers good electrochemical performance,including high rate,reversible specific capacity(680 mA h g^(-1)at 0.1 A g^(-1)),and lower capacity decay rate only of 0.15%with near 100%coulomb efficiency during long-term cycle.This work provides a valuable guiding ideology and promising solution for the chemistry-oriented structure design and practical application for lithium-organosulfur batteries.

Defective layered Mn-based cathode materials with excellent performance via ion exchange for Li-ion batteries

Defective layered Mn-based materials were synthesized by Li/Na ion exchange to improve their electrochemical activity and Coulombic efficiency.The annealing temperature of the Na precursors was important to control the P3-P2 phase transition,which directly affected the structure and electrochemical characteristics of the final products obtained by ion exchange.The O3-Li_(0.78)[Li_(0.25)Fe_(0.075)Mn_(0.675)]O_(δ) cathode made from a P3-type precursor calcined at 700℃ was analyzed using X-ray photoelectron spectrometry and electron paramagnetic resonance.The results showed that the presence of abundant trivalent manganese and defects resulted in a discharge capacity of 230 mAh/g with an initial Coulombic efficiency of about 109%.Afterward,galvanostatic intermittent titration was performed to examine the Li^(+) ion diffusion coefficients,which affected the reversible capacity.First principles calculations suggested that the charge redistribution induced by oxygen vacancies(OV_(s))greatly affected the local Mn coordination environment and enhanced the structural activity.Moreover,the Li-deficient cathode was a perfect match for the pre-lithiation anode,providing a novel approach to improve the initial Coulombic efficiency and activity of Mn-based materials in the commercial application.

Enhanced Electrochemical Performances of Ni Doped Cr_(8)O_(21)Cathode Materials for Lithium-ion Batteries

Cathode materials,nickel doped Cr_(8)O_(21),were synthesized by a solid-state method.The effects of Ni doping on the electrochemical performances of Cr_(8)O_(21) were investigated.The experimental results show that the discharge capacities of the samples depend on the nickel contents,which increases firstly and then decreases with increasing Ni contents.Optimized Ni_(0.5)Cr_(7.5)O_(21)delivers a first capacity up to 392.6 m Ah·g^(-1)at 0.1C.In addition,Ni doped sample also demonstrates enhanced cycling stability and rate capability compared with that of the bare Cr_(8)O_(21).At 1 C,an initial discharge capacity of 348.7 m Ah·g^(-1)was achieved for Ni_(0.5)Cr_(7.5)O_(21),much higher than 271.4 m Ah·g^(-1)of the un-doped sample,with an increase of more than 28%.Electrochemical impedance spectroscopy results confirm that Ni doping reduces the growth of interface resistance and charge transfer resistance,which is conducive to the electrochemical kinetic behaviors during charge-discharge.

Organic cathode materials for rechargeable magnesium-ion batteries:Fundamentals, recent advances, and approaches to optimization

Rechargeable magnesium-ion batteries(MIBs) are favorable substitutes for conventional lithium-ion batteries(LIBs) because of abundant magnesium reserves, a high theoretical energy density, and great inherent safety. Organic electrode materials with excellent structural tunability,unique coordination reaction mechanisms, and environmental friendliness offer great potential to promote the electrochemical performance of MIBs. However, research on organic magnesium battery cathode materials is still preliminary with many significant challenges to be resolved including low electrical conductivity and unwanted but severe dissolution in useful electrolytes. Herein, we provide a detailed overview of reported organic cathode materials for MIBs. We begin with basic properties such as charge storage mechanisms(e.g., n-, p-, and bipolartype), moving to recent advances in various types of organic cathodes including carbonyl-, nitrogen-, and sulfur-based materials. To shed light on the diverse strategies targeting high-performance Mg-organic batteries, elaborate summaries of various approaches are presented.Generally, these strategies include molecular design, polymerization, mixing with carbon, nanosizing and electrolyte/separator optimization.This review provides insights on exploring high-performance organic cathodes in rechargeable MIBs.

CoSnO_(3)/C nanocubes with oxygen vacancy as high-capacity cathode materials for rechargeable aluminum batteries

Rechargeable aluminum batteries(RABs)are attractive cadidates for next-generation energy storage and conversion,due to the low cost and high safety of Al resources,and high capacity of metal Al based on the three-electrons reaction mechanism.However,the development of RABs is greatly limited,because of the lack of advanced cathode materials,and their complicated and unclear reaction mechanisms.Exploring the novel nanostructured transition metal and carbon composites is an effective route for obtaining ideal cathode materials.In this work,we synthesize porous CoSnO_(3)/C nanocubes with oxygen vacancies for utilizing as cathodes in RABs for the first time.The intrinsic structure stability of the mixed metal cations and carbon coating can improve the cycling performance of cathodes by regulating the internal strains of the electrodes during volume expansion.The nanocubes with porous structures contribute to fast mass transportation which improves the rate capability.In addition to this,abundant oxygen vacancies promote the adsorption affinity of cathodes,which improves storage capacity.As a result,the CoSnO_(3)/C cathodes display an excellent reversible capacity of 292.1 mAh g^(-1) at 0.1 A g^(-1),a good rate performance with 109 mAh g^(-1) that is maintained even at 1 A g^(-1) and the provided stable cycling behavior for 500 cycles.Besides,a mechanism of intercalation of Al^(3+)within CoSnO_(3)/C cathode is proposed for the electrochemical process.Overall,this work provides a step toward the development of advanced cathode materials for RABs by engineering novel nanostructured mixed transition-metal oxides with carbon composite and proposes novel insights into chemistry for RABs.

Influences of transition metal on structural and electrochemical properties of Li[Ni_xCo_yMn_z]O_2(0.6≤x≤0.8) cathode materials for lithium-ion batteries

Li[NixCoyMn2]O2（0.6≤x≤0.8） cathode materials with a typical hexagonal α-NaFeO2 structure were prepared utilizing a co-precipitation method.It is found that the ratio of peak intensities of（003） to（104） observed from X-ray diffraction（XRD）increases with decreasing the Ni content or increasing the Co content.The scanning electron microscopy（SEM） images reveal that the small primary particles are agglomerated to form the secondary ones.As the Mn content increases,the primary and secondary particles become larger and the resulted particle size for the Li[Ni（0.6）Co（0.2）Mn（0.2）]O2 is uniformly distributed in the range of100-300 nm.Although the initial discharge capacity of the Li/Li[NixCoyMn2]O2 cells reduces with decreasing the Ni content,the cyclic performance and rate capability are improved with higher Mn or Co content.The Li[Ni（0.6）Co（0.2）Mn（0.2）]O2 can deliver excellent cyclability with a capacity retention of 97.1%after 50 cycles.

Influence of pH value and chelating reagent on performance of Li_3V_2(PO_4)3/C cathode material

The Li3V2（PO4）3/C composite cathode material was synthesized via sol-gel method using three different chelating agents （citric acid, salicylic acid and polyacrylic acid） at pH value of 3 or 7. The crystal structure, morphology, specific surface area and electrochemical performance of the prepared samples were investigated by X-ray diffraction （XRD）, scanning electron microscopy （SEM）, cyclic voltammetry （CV）, electrochemical impedance spectroscopy （EIS） and galvanostatic charge/discharge test. The results show that the effects of pH value on the performance of the prepared materials are greatly related to the chelating agents. With salicylic acid or polyacrylic acid as the chelating reagent, the structure, morphology and electrochemical performance of the samples are greatly influenced by the pH values. However, the structure of the materials with citric acid as the chelating agent does not change as pH value changes, and the materials own uniform particle size distribution and good electrochemical performance. It delivers an initial discharge capacity of 113.58 mA·h/g at 10C, remaining as high as 108.48 mA·h/g after 900 cycles, with a capacity retention of 95.51%.

Grinding sol gel synthesis and electrochemical performance of mesoporous Li_3V_2(PO_4)_3 cathode materials

Li3V2（PO4）3 precursor was obtained with V2Os.nH2O , LiOH＇H2O, NH4H2PO4 and sucrose as starting materials by grinding-sol-gel method, and then the monoclinic-typed Li3Vz（PO4）3 cathode material was prepared by sintering the amorphous Li3V2（PO4）3. The as-sintered samples were investigated by X-ray diffraction （XRD）, transmission electron microscopy （TEM）, N2 adsorption-desorption and electrochemical measurement. It is found that Li3Vz（PO4）3 sintered at 700 ℃ possesses good wormhole-like mesoporous structure with the largest specific surface area of 188 cmZ/g, and the smallest pore size of 9.3 nm. Electrochemical test reveals that the initial discharge capacity of the 700 ℃ sintered sample is 155.9 mA.h/g at the rate of 0.2C, and the capacity retains 154 mA.h/g after 50 cycles, exhibiting a stable discharge capacity at room temperature.

Effect of Mn-doping on performance of Li_3V_2(PO_4)_3/C cathode material for lithium ion batteries

Li3V2-2/3xMnx（PO4）3（0≤x≤0.12） powders were synthesized by sol-gel method. The effect of Mn2＋-doping on the structure and electrochemical performances of Li3V2（PO4）3/C was characterized by XRD, SEM, XPS, galvanostatic charge /discharge and electrochemical impedance spectroscopy（EIS）. The XRD study shows that a small amount of Mn2＋-doped does not alter the structure of Li3V2（PO4）3/C materials, and all Mn2＋-doped samples are of pure single phase with a monoclinic structure （space group P21/n）. The XPS analysis indicates that valences state of V and Mn are ＋3 and ＋2 in Li3V1.94Mn0.09（PO4）3/C, respectively, and the citric acid in raw materials was decomposed into carbon during calcination, and residual carbon exists in Li3V1.94Mn0.09（PO4）/C. The results of electrochemical measurements show that Mn2＋-doping can improve the cyclic stability and rate performance of these cathode materials. The Li3V1.94Mn0.09（PO4）3/C cathode material shows the best cyclic stability and rate performance. For example, at the discharge current density of 40 mA/g, after 100 cycles, the discharge capacity of Li3V1.94Mn0.09（PO4）3/C declines from initial 158.8 mA·h/g to 120.5 mA·h/g with a capacity retention of 75.9%; however, that of the Mn-undoed sample declines from 164.2 mA·h/g to 72.6 mA·h/g with a capacity retention of 44.2%. When the discharge current is increased up to 1C, the intial discharge capacity of Li3V1.94Mn0.09（PO4）3/C still reaches 146.4 mA·h/g, and the discharge capacity maintains at 107.5 mA·h/g after 100 cycles. The EIS measurement indicates that Mn2＋-doping with a appropriate amount of Mn2＋ decreases the charge transfer resistance, which is favorable for the insertion/extraction of Li＋.

Effects of chromium doping on performance of LiNi_(0.5)Mn_(1.5)O_4 cathode material

In order to improve the cycle and rate performance of LiNi0.5Mn1.5O4, LiCr2 Ni0.5 Mn1.5 O （0≤Y≤0.15） particles were Y -Y -Y 4 synthesized by the sucrose-aided combustion method. The effects of Cr doping in LiNi0.5Mn1.5O4 on the structures and electrochemical properties were investigated. The samples were characterized by X-ray diffractometry （XRD）, scanning electron microscopy （SEM）, cyclic voltammetry （CV）, galvanostatic charge-discharge test and electrochemical impedance spectrum （EIS）. The results indicate that the LiCr2 Ni0.5 Mn1.5 O possess a spinel structure and small particle size, and LiCr0.2Ni0.4Mn1.4O4exhibits Y -Y -Y 4 the best cyclic and rate performance. It can deliver discharge capacities of 143 and 104 mA·h/g at 1C and 10C, respectively, with good capacity retention of 96.5% at 1C after 50 cycles.

Synthesis and electrochemical properties of LiNi_(0.87)Co_(0.10)Mg_(0.03)O_2 cathode materials

A Co-Mg co-substituted LiNi0.87Co0.10Mg0.03O2 cathode material was prepared by a co-precipitation method. The prepared LiNi0.87Co0.10Mg0.03O2 exhibits excellent electrochemical properties, such as initial discharge capacities of 202.6 mA.h/g and 190.5 mA.h/g at 0.2C and 1C rate, respectively, in operating voltage range of 3.0-4.3 V （versus Li^＋/Li）. The capacity retentions are 96.1% and 93.4% at 0.2C and 1C, respectively, after 50 cycles. Moreover, the cycle performance of the sample was investigated in a 053048-type square Li ion battery. This type of battery can keep 81.7% of initial capacity after 500 charge-discharge cycles at 1C rate, which is close to that of commercial LiCoO2 battery. Therefore, the as-prepared material is capable of such high energy applications as portable product power.

Synthesis of Ni_(0.8)Co_(0.1)Mn_(0.1)(OH)_2 precursor and electrochemical performance of LiNi_(0.8)Co_(0.1)Mn_(0.1)O_2 cathode material for lithium batteries

Spherical and homogeneously mixed metal hydroxide Ni0.8Co0.1Mn0.1（OH）2 precursor was successfully synthesized by co-precipitation method in a simple and small vessel with the volume of 1L.The conditions of synthetic process including amount of chelating agent,stirring speed and temperature were studied.LiNi0.8Co0.1Mn0.1O2 samples were obtained by calcinating the precursors.The crystal structure,morphology and electrochemical properties were investigated by X-ray diffraction（XRD）,scanning electron microscopy（SEM）,charge-discharge test,AC impedance and cyclic voltammetry.In the voltage range of 2.8-4.3 V,the initial discharge capacities of LiNi0.8Co0.1Mn0.1O2 at 0.1C and 1C rates were 199 and 170 mA·h/g,respectively.After 80 cycles at 1C,the discharge capacity retention was 92%,suggesting its promising application as the cathode material for Li-ion batteries.

Effect of Mg doping on electrochemical performance of Li_3V_2(PO_4)_3/C cathode material for lithium ion batteries

Li3Mg（2x）V（2-2x）（PO4）3/C（x=0,0.05,0.1,0.2） composites were synthesized by carbothermic reduction,using a self-made MgNH4PO4/MgHPO4 compound as Mg-doping agent.X-ray diffraction（XRD）,scanning electron microscope（SEM）,electrochemical performance tests were employed to investigate the effect of Mg doping on Li3V2（PO4）3/C samples.The results showed that a proper quantity of Mg doping was beneficial to the reduction of charge transfer resistance of Li3V2（PO4）3/C compound without changing the lattice structure,which led to larger charge/discharge capacity and better cycle performance especially at high current density.Li3Mg（2x）V（2-2x）（PO4）3/C sample with x=0.05 exhibited a better performance with initial charge/discharge capacity of146/128 mA·h/g and discharge capacity of 115 mA·h/g at 5C,while these two figures were 142/118 mA·h/g and 90 mA·h/g respectively for samples without Mg doping,indicating that a proper amount of doped Mg can improve the electrochemical performance of LVP sample.All of these proved that,as a trial Mg dopant,the synthesized MgNH4PO4/MgHPO4 compound exhibited well doping effect.

Influence of synthesis temperature on electrochemical performance of polyoxomolybdate as cathode material of lithium ion battery

In order to improve the electrochemical performance of polyoxomolybdate Na3[AlMo6O24H6]（NAM） as the cathode material of lithium ion battery, the NAM materials with small particle size were synthesized by elevatingthe synthesistemperaturein the solution.The as-prepared NAM materials were investigated by FT-IR, XRD, SEM and EIS. Their discharge-charge and cycle performance were also tested. The resultsshowthat the particle size decreasesto less than10μm at the temperature ofhigher than 40℃.When synthesized at 80℃,the NAMwiththe smallest particle size （-3μm）exhibitsthe best electrochemical performance such ashigh initial discharge capacity of 409 mA·h/gandcoulombic efficiency of 95% in the first cycle at 0.04C.