Study On The Preparation And Properties Of A New Cathode Material Li₂MnSiO₄for Lithium-ion Batterries

As a new cathode material for lithium batteries, lithium manganese silicate(Li2MnSiO4) is able to achieve two lithium insertion/extraction theoretically, whichmeans it has a high theoretic specific capacity of330mAh/g, nevertheless, it also showsadvantages such as abundance and low cost of raw materials, good safety, andenvironmental friendliness, thus it has attracted increasing interest to researchers fromhome and abroad in recent years. However, the extremely low electronic conductivityand bad cycling stability of this cathode have greatly limited its development andapplication. Aimed at solving the existing problems of the Li2MnSiO4material, in-situcarbon coating and nano technique were adopted successfully and its specific capacityand cycling capability were greatly improved, Li2MnSiO4/C nanocomposites weresynthesized by sol-gel method using lithium acetate, manganese acetate and tetraethylorthosilicate as main starting materials, and adipic acid, glucose andresorcinol-formaldehyde were used as carbon sources, respectively. LiF and Cr3+dopingwere introduced and their influence on the structure and electrochemical properties ofthe Li2MnSiO4/C nanocomposites were also investigated.A Li2MnSiO4/C nanocomposite was prepared by sol-gel method using adipic acidas both chelating agent and carbon source, the impacts of synthesis conditions to thestructure, micro morphology and electrochemical properties of the Li2MnSiO4/Cmaterial were also investigated. Under the optimal synthesis condition, theLi2MnSiO4/C composite synthesized by adipic acid assisted sol-gel method delivers amuch higher initial discharge capacity of167.3mAh/g compared to that without addingcarbon source, which only has a discharge capacity of20mAh/g, however, the cyclingability needs to be improved.A carbon coated Li2MnSiO4nanocomposite was prepared by a glucose assistedsol-gel method, under the optimal synthesis condition, the Li2MnSiO4/C nanocompositehas a high initial discharge capacity and superior rate capability, the intial dischargecapacity is253.4mAh/g (exclude coated carbon) at a current density of10mA/g, whichmeans about1.53Li+could transfer during the intercalation/deintercalation process, andthe intial discharge capacities were194.7,164.1and120.9mAh/g（include coatedcarbon）at80,160and320mA/g, respectively. The impacts of the calcinationtemperature and time, the dosage of glucose to the preparation of the Li2MnSiO4/Cmaterial by glucose assisted sol-gel method were investigated systematically. Therelationships between the structure and the electrochemical properties of theas-synthesized Li2MnSiO4/C nanocomposite were also furtherly studied, aftermodification by in-situ carbon coating and nano technique, the Li2MnSiO4/C compositehas a smaller nanoparticle size and uniform coated carbon layer, and the electronic conductivity as well as the lithium-ion diffusion coeffecient of the composite are greatlyenhanced, thus the Li2MnSiO4/C nanocomposite exhibits excellent electrochemicalproperties. The TEM tests further indicate that the existing of amorphous phase in thecomposite not only influences the initial charge capacity of the Li2MnSiO4material, butalso has a potential impact on the structural stability of the material, which may lead tothe bad cycling ability of the material.A Li2MnSiO4/C nanocomposite was prepared by a resorcinol-formaldehydeassisted sol-gel method for the first time, when the carbon content in the composite ishigh（~30wt%）, it has stable cycling capacity, the capacity retention of the material is90.7%over50cycles, the TEM and ex-situ XRD tests were used to uncover therelationships between the structure and cycling ability of the Li2MnSiO4/C compositeand it indicates that the stable cycling capacity of the material may be ascribed to thegood crystallinity and thick coated carbon layer, which keep the crystalline structure ofLi2MnSiO4almost unchanged during cycling. The impacts of the calcinationtemperature and the content of resorcinol to the structure, micro morphology andelectrochemical properties of the Li2MnSiO4/C nanocomposite were studiedsystematically, it shows that the as-synthesized products have better crystallinity andlarger particle size with the increase of calcination temperature, accordingly, theelectrochemical properties of the products are firstly enhanced and then weakened. Thecontent of resorcinol has a close relationship with the initial discharge capacity andcycling ability of the as-synthesized material, with the increase of the carbon content inthe material, the initial discharge capacity was firstly enhanced and then dropped, whenthe carbon content reaches~30wt%, the material has stable cycling ability.LiF doped, Cr3+doped Li2MnSiO4/C materials were prepared by glucose assistedsol-gel method, compared to that without doping, LiF doping is not able to improve thecapacity and the cycling ability of the as-synthesized material, however, Cr3+doping hasa slight impact on the initial capacity of the Li2MnSiO4/C material, but it can apparentlyimproved the cycling ability of the Li2MnSiO4/C material.