Modification And Electrochemical Properties Of Li-rich Mn-based Oxides

Layered lithium–rich transition–metal oxides,noted as x Li₂Mn O₃·?1–x?Li MO₂?LLO?,has many advantages,such as abundant reserves,high theoretical capacity,good cycling performance and low price.In this paper,the precursor of transition metal salt was prepared by chemical coprecipitation method.Afterwards,LLO was fabricated by the precursors and lithium carbonate at high temperature.Then,substitution and surface modification were performed to the as–prepared LLO.For the LLO prepared by chemical coprecipitation method,the specimen exhibits the best layer structure with a particle size of 300–400nm when the uncalcined transition metal precursor and lithium carbonate were prepressed under10 MPa.Then,treatment of LLO with 0.2M concentration of nitric acid was implemented.After acid corrosion,the layered structure of the m aterial gradually decreased,the ratio of spinel phase increased.When corrosion time reached 4h,the maximum cycle specific capacity of the material at 0.1C ratio was 231 m Ah/g,which was higher than that of the material without acid corrosion?201m Ah/g?.At the same time,the specific capacity of the material was 123.6m Ah/g under the 2C times ratio.The Ti–doped LLO was synthesized by high temperature calcination.With the increase of doping ratio,the content of the spinel phase in the material decreases first and then increases.It shows that the addition of TiO₂ can regulate the phase composition of the LLO,and when the amount of Ti doping x=0.025,almost 90%of the Mn element exists in the form of Mn⁴⁺.In this case,the particle size was the smallest,being 100–200nm.The first Kulun efficiency was 76.91%,and the initial discharge specific capacity reaches 244.9m Ah/g.However,the cycle performance is relatively poor in this case.Surface modification of LLO by hydrothermal method did not change the crystal structure and phase composition of the material,but the grain grew after modification.However,composition change without any grain growth was observed for the materials modified by pre–coating.The materials before and after substitution and surface modification showed the same structural characteristics After 2wt.%YSZ surface modification the first coulombic efficiency of the material was 67.9%;the maximum discharge specific capacity reached 239 m Ah/g;the discharge specific capacity of the material decreased with the increase of YSZ mass percentage,but the capacity retention rate decreased.Through surface modification,the high specific capacity of the material increased.Under 2C condition,the specific capacity of the material modified by 2wt.%YSZ is nearly 123.1m Ah/g compared with that of YSZ.The first charge discharge efficiency of the surface modified material by pre–coating method was 70%,and the maximum discharge specific capacity reached 304.1m Ah/g.Under 0.1C condition,the specific capacity of the material 25Ti2 YSZ is nearly204 m Ah/g.