Modification On 0.5Li₂MnO₃·0.5LiNi_1/3Co_1/3Mn_1/3O₂ Electrode/Electrolyte Interface By Electrolyte Additives

Along with the vigorous promotion of electric vehicles and large-scale energy storage stations, it is extremely necessary to develop lithium-ion battery cathode materials with high-capacity, security and stability. In recent years, lithium-rich, manganese-rich layered oxides have been a hotspot in the field of lithium ion battery cathode materials owing to its excellent discharge capacity (250～280 mAh/g), low cost and good safety performance, etc. However, this material is also facing several serious problems:there is a large irreversible capacity loss during first cycling, and the material structure would transform from lamellar to spinel during cycling, leading to decreased discharge platform and increased polarization, in addition, since the cut-off voltage of this material is as high as 4.6 - 4.8 V, the decomposition of traditional electrolyte inevitably happens during cycling, forming a passivation film on the cathode surface which worsens the electrochemical performance of the material. In this paper, three additives tris (trimethylsilyl) borate (TMSB), tri (2,2,2 - trifluoroethyl) borate (TTFEB) and cyclohexylbenzene (CHB) are adopted to modify the interface between 0.5Li2MnO3·0.5LiNi1/3Co1/3Mn1/3O2 and electrolyte, then a series of electrochemical tests are used to investigate the effects on the material’s electrochemical properties, furthermore, a variety of characterization methods are adopted to investigate mechanisms of these three additives.The discharge capacity of 0.5Li2MnO3·0.5LiNi1/3Co1/3Mn1/3O2 increased when 0.5 mass% TMSB was added to electrolyte. TMSB would decompose and deposit into the the surface film of positive electrode during cycling, reducing the impedance of the surface film and mitigating the polarization of the material. Furthermore, after adding 0.5 mass% TMSB, the cycling performance of the material increased significantly:the irreversible capacity was only 21.72 mAh/g and the coulombic efficiency is above 99% after 100 cycles at 55℃. Furthermore, the film resistance was almost constant after 200 cycles in electrolyte with 0.5 mass% TMSB, demonstrating that the stability of cathode surface film was dramastically improved. The results of XPS and FTIR tests showed that the content of LiF and lithium alkyl carbonates in the cathode surface film were significantly reduced when 0.5 mass% TMSB was added. The boron atom in TMSB could combine with the anions of LiF, Li2O and Li2O2, promoting the dissolution of inorganic contents in the surface film, thereby reducing the film resistance and polarization, maintaining surface film stability, improving the cycle stability of the material. In addition, TMSB could coordinate with F- of HF, inhibiting the destruction of material and cathode surface film by HF.A new boron-based anion receptor TTFEB could improve the stability of 0.5Li2MnO3·0.5LiNi1/3Co1/3Mn1/3O2/electrolyte interface, thereby improving the electrochemical properties of the material. The discharge capacity decreased to 110 mAh/g after 400 cycles at 25℃ in the electrolyte without additive, however, after adding 0.5 mass% TTFEB, the discharge capacity could maintain at 130 mAh/g after 400 cycles. The capacity of the material rapidly decreased to 88.5% and the coulombic efficiency decreased dramatically when cycled in the electrolyte without additive at 55 ℃, however, the coulombic efficiency has been above 98% at 55℃ when adding 0.5 mass% TTFEB. Both XPS and FTIR tests demonstrated that TTFEB could combine with F-、O22-、O2- and other anions, which could suppress the reaction between O22-/O2-and electrolyte. And it could promote the dissolution of LiF、Li2O and Li2O2, improving the conductivity of the film, reducing polarization, finally improving the cycling stability of the material.CHB may polymerize prior to electrolyte on the surface of 0.5Li2MnO3·0.5LiNi1/3Co1/3Mn1/3O2, forming a polymer film with compact structure and good conductivity which could be effectively hinder the contact between electrolyte and material, thereby inhibiting oxidative decomposition of the electrolyte and improving the cycle stability of the material. After 400 cycles at 25℃, the capacity of battery with the blank electrolyte faded to 71.65 mAh/g, however, after adding 0.1 mass% CHB, the capacity improved significantly:the capacity after 400 cycles at 25℃ was 95.93 mAh/g. the material would transform from layered structure to spinel structure, which would damage the material. The SEM test showed that there were many cracks on the material surface which cylced in electrolyte without CHB, however, when the material cycled in electrolyte with 0.1 mass% CHB, there were less cracks on the material surface. The polymerization of CHB would generate a high polymer film covering on the surface of the material, effectively inhibiting the broken of the material, then improving the cycle life of it.