Study On The Energy Storage Characteristics Of Lead-free Ceramics

Because lead-containing materials are prone to cause environmental problems,the development of clean new energy sources has become the focus of our current research,which requires us to accelerate the exploration of energy storage equipment with superior performance.In the current common energy storage system,dielectric capacitors can simultaneously deliver fast charge-discharge rate and high power density and high stability,and are widely used in new energy trams,portable mobile devices and other fields.The dielectric materials used in energy storage capacitors mainly include linear dielectrics（LD）,ferroelectrics（FE）,antiferroelectrics（AFE）and relaxor ferroelectrics（RFEs）.Among these materials,linear dielectrics have small dielectric constants,not suitable for high energy storage devices;ferroelectric materials usually have large Pmax and moderate electric field resistance,but their Pr is large,resulting in low energy storage density and low energy storage efficiency.Relatively speaking,relaxor ferroelectrics have good energy storage capacity due to their large saturation polarization value;antiferroelectric materials have attracted much attention because of their large polarization rate,small residual polarization,and unique double hysteresis loop caused by AFE-FE phase transitions.At present,the widely used antiferroelectric material is lead lanthanum zirconium titanate stannate（PLZST）-based ceramic material,which has a large energy storage density.However,lead is toxic and unfriendly to the environment.Lead AFE energy storage materials have become the current hot spot.Among various lead-free dielectric ceramics,0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃（0.94NBT-0.06BT）material has a larger maximum polarization,so it is considered as a good candidate material for the preparation of environmentally friendly dielectric capacitors,but its high remanent polarization value seriously limits the improvement of energy storage density and efficiency.Silver niobate（AgNbO₃）lead-free antiferroelectric ceramics have excellent energy storage performance and have become one of the most promising materials to replace lead-based antiferroelectric energy storage ceramics.However,the current preparation of AgNbO₃is mainly a solid-phase preparation method,which has many defects.Therefore,based on the questions raised above,we mainly carried out the following three tasks:1.In order to obtain lead-free dielectric materials with good energy storage properties in a wide temperature range,we introduced La³⁺into sodium bismuth titanate-barium titanate(Na_0.5Bi_0.5TiO₃-BaTiO₃),and prepared[(Bi_0.5Na_0.5)_0.94Ba_0.06]（1-1.5x）La_xTiO3（x=0,0.04,0.08,0.12,0.14）lead-free relaxor ferroelectric ceramics.With the introduction of La³⁺,the dielectric relaxation properties of the ceramics are more pronounced due to the enhanced cationic disorder at the A-and B-sites.Therefore,the P-E loop becomes slim and the remanent polarization（P_r）decreases.Furthermore,since the mobility of La³⁺as a cation in the ceramic is lower than that of the A-site cation,the grain size decreases with the increase of La³⁺content,which leads to an increase in the breakdown strength（E_b）.Therefore,we obtained a large recoverable energy density(W_rec)of 1.92 J/cm³and a high energy efficiency（η）of 85.7%in the ceramic with a La³⁺content of 12 mol%.More importantly,even at200°C and a low driving electric field of 155 k V/cm,the W_recandηof this ceramic are still as high as 1.2 J/cm³and 89.4%,indicating the ceramic has good temperature stability.2.At present,the preparation of AgNbO₃is mainly by the solid-phase preparation method,but this method requires multiple ball milling and high-temperature sintering,which is cumbersome and expensive.Moreover,due to the thermodynamic instability of Ag₂O in the high-temperature sintering process,it is difficult to control the reduction of Ag⁺ions and the oxidation of metallic silver.In addition,the crystal grains of pure AgNbO₃ceramics prepared by solid-state method are large,which greatly limits the improvement of the breakdown field strength of ceramics.In contrast,the powder synthesized by the hydrothermal method has small particle size,good dispersibility and high activity,which is conducive to the preparation of fine-grained ceramics to obtain high E_b,and the preparation process is relatively simple.At the same time,with the increase of E_Aand E_F,the energy storage density is also significantly improved.Therefore,a large recoverable energy density(W_rec)of 3.34 J/cm³and a high energy efficiency（η）of 54.5%were obtained in the ceramic when the powder preparation time was 20 h.They also exhibit excellent discharge performance with a discharge time of about 0.032μs and a power density of 118.64 MW/cm³,showing good potential for practical applications.More importantly,even at 160°C and a high driving electric field of 250 k V/cm,the W_recandηof this ceramic are still as high as 3.11J/cm³and 60.8%,indicating the ceramic has good temperature stability.3.Although the ordinary hydrothermal method is conducive to the preparation of powders with high activity,small particle size,and ceramics with excellent performance,the preparation time of powders is long,which reduces the preparation efficiency.In contrast,the microwave hydrothermal method can greatly reduce the reaction holding time,improve the experimental efficiency,and heat evenly,and the prepared powder also has high activity and small particle size.Therefore,a large recoverable energy density(W_rec)of 3.82 J/cm³and a high energy efficiency（η）of 49.8%were obtained in the ceramic when the powder preparation time was 6 h.At the same time,the ceramic also exhibits good fatigue characteristics and frequency stability.