Preparation, Structure And Properties Of Ni-Al Layered Double Hydroxides For Zn/Ni Batteries Cathode Material

Ni-Al layered double hydroxides （Ni-Al LDHs）, also known as stable α-Ni（OH）₂dopedwith Al, follows the α（II）/γ（III） cycle and the crystal lattice doesn’t obviously expand orshrink during charge-discharge process. It also has higher specific discharge capacity anddischarge voltage plateau than β-Ni（OH）₂. Those advantages make it a promising substitute ascathode active material for Zn/Ni batteries. The specific energy and cycle life of the batteriesshould be enhanced markedly. Ni-Al LDHs is often synthesized by chemical co-precipitationmethod. However, the samples obtained normally have small specific surface areas, wideparticle distribution ranges and not ideal electrochemical performances. Moreover, lowelectronic conductivity of Ni-Al LDHs results in poor discharge performance at high rates. Onthe other hand, the matching properties of Ni-Al LDHs electrode with alkaline compoundelectrolyte and Zn anode need to be researched also. Aiming at those problems, thedissertation used orthogonal experiments to study the influence of main factors in thesynthesis process on the electrochemical performances of Ni-Al LDHs. Spray technique wasapplied to treat the gel precipitate intermediate to improve the physical properties andelectrochemical performances of the product. Ni-Al LDHs sample doped with Co and La wasprepared also. Finally, the performances of Zn/Ni battery with Ni-Al LDHs cathode materialwere studied. The main results and conclusions obtained were as follows:1. The influence of main factors in the synthesis process with chemical co-precipitationmethod on the specific discharge capacity of Ni-Al LDHs product was studied by orthogonalexperiments. The experimental results showed that the influence order from great to smallwas raw material ratio nNi²⁺: nAl³⁺, drying temperature of gel precipitate, grinding size of thepowder, reaction temperature, pH value and aging temperature of mother solution. Thepreferable synthesis condition was that the raw material ratio nNi²⁺: nAl³⁺was85:15, reactiontemperature was45℃, pH value was10.5, aging time was36h, drying temperature was140℃, and grinding size was600mesh. Further research results showed that the raw materialratio affected the crystal structure and crystallinity of the product. Reducing the material ratiocould enhance the crystallinity of the product and the discharge voltage plateau of electrode.Drying temperature affected the crystallinity and intercalation content of water molecular between the layer plates. Elevating the drying temperature could enhance the electrochemicalactivity, discharge specific capacity of the product and discharge voltage plateau of theelectrode. Grinding size of the powder influenced the particle size distribution and averageparticle size. Reducing the grinding size could enhance the electrochemical activity anddischarge specific capacity of the final sample.2. Spray technique was used to treat the gel-like intermediate in the synthesis process ofNi-Al LDHs with chemical co-precipitation method. Comparing with that obtained by normalmethod, the powder sample had more narrow particle distribution range and highercrystallinity, and the specific surface area increased from6.8m²/g to14.1m²/g.Electrochemical testing results showed that the sample had higher electrochemical activityand discharge specific capacity. The electrode reaction was more reversible and showed bettercycle stability. The highest and average specific discharge capacity of the sample reached347.5mAh/g and337.9mAh/g at0.5C rate, respectively. Meanwhile, the capacity retentionrate was95.4%after100cycles.3. Ni-Al LDHs doped with both Co and La was synthesized by chemicalco-precipitation method. The sample obtained consisted of loose spherical particles with sizebelow5μm, while its specific surface area reached132.5m²/g. The crystal structure retained αphase well after charge-discharged for100cycles. CV testing results showed that theelectrode had better reversibility and the proton diffusion coefficient reached5.7×10^-9cm²/s.Charged-discharge testing results showed that the sample had higher electrochemical activityand its average discharge capacity attained311.5mAh/g at1C rate. The electrode had bettercycle stability, lower charge but higher discharge voltage plateau, which was moreoutstanding at higher rate. The sample doped with Co only synthesized with the same methodhad irregular particle shape, a much small specific surface area of5.4m²/g and wider particledistribution range. Meanwhile, the electrochemical activity was lower, and the reversibility ofthe electrode reaction was poorer. Neither the discharge voltage plateau nor the cycle stabilitywas ideal. As for the sample doped with La only, though it had quite similar physicalproperties and electrochemical activity to that of the sample doped with both Co and La, butits specific discharge capacity was obviously lower.4. Zn/Ni experimental battery was manufactured with Ni-Al LDHs sample synthesized by spray technique as cathode active material, and the charge-discharge cycling performancewas studied. Experimental results showed that the average discharge capacity of the batteryreached308.6mAh/g （calculated with the mass of cathode active material）, which wasobviously higher than that with β-Ni（OH）₂. And, the middle discharge voltage was about57mV higher than that of the latter. Although the discharge voltage plateau reduced remarkablyat5C rate, the middle voltage was still above1.60V. Further experimental results showed thatthe cycle performance of the battery was affected by the electrolyte used greatly. With7mol/L KOH electrolyte, the discharge capacity reduced quickly and fluctuated obviously after40cycles. While with3.2mol/L KOH⁺1.8mol/L K₂CO₃⁺1.8mol/L KF compoundelectrolyte, the cycle performance was improved markedly and the capacity retention rateexceeded90%after120cycles. Slight decrease of the charge and discharge performances wasfound with the compound electrolyte, because the impedance of charge transfer step and thepolarization of anodic oxidation and cathodic reduction increased. Further experimentalresults showed that the dissolution of anode active material ZnO affected the dischargecapacity, cycle life and charge-discharge performance distinctly. Using the compoundelectrolyte inhibited the dissolving of ZnO and enhanced the cycle life of battery significantly.