High-Voltage Electrolyte Additive for High-Energy Lithium-Ion Batteries

Increasing demand of high-energy lithium-ion batteries for being adopted in electric vehicles and energy storage systems drives the development of high-voltage and high-capacity cathode together with functional electrolyte with high anodic and thermal stabilities. Lithium-rich layered material of xLi₂MnO₃-(1-x)Li(Ni, Co, Mn)O₂  is an attractive cathode material because of their higher capacity than 200 mAh/g. Its performance however is often difficult to be achieved, in particular, at high voltage operation (> 4.3 V vs. Li/Li⁺), due to severe oxidative decomposition of conventional electrolyte consisting of LiPF₆ salt and non-aqueous carbonate-based organic solvents. Operation of the cathode at higher voltage than 4.6 V can lead to a further increase in the capacity. Extensive research efforts have been made to develop appropriate electrolyte components with high anodic stability but yet to be established. We have been screening and evaluating a number of fluorinated carbonates as electrolyte additives for high-performance operation of 4.8 V Li_1.2Mn_0.525Ni_0.175Co_0.1O₂ cathode. The use of a low fraction additive comprises low cost, compared to solvent. Here we present the first report on a new fluorinated carbonate as a high-performance electrolyte additive for 4.8 V Li_1.2Mn_0.525Ni_0.175Co_0.1O₂ cathode operated at a wide temperature range. The SEI formation mechanism, composition and stability, and their relation to high-voltage cycling performance, and cycling performance of full-cells are discussed.

The Li_1.2Mn_0.525Ni_0.175Co_0.1O₂ cathode material was synthesized at 900 ^oC in air using the carbonate coprecipitate precursor. The crystal structure of coprecipitate precursor and cathode material were identified by X-ray diffraction analysis, measured in the 2θ range of 10 - 80^o with the scan rate of 2^o/min. Lithium coin cells, consisted of Li_1.2Mn_0.525Ni_0.175Co_0.1O₂ as a working electrode, a lithium foil as counter electrode and the electrolyte of 1M LiPF₆/EC:EMC (3:7 volume ratio) with 5 wt% additive of fluorinated carbonate was assembled in the Ar-filled glove box. The 2016 coin half- and full-cells were evaluated for their cycling ability at C/5 rate between 2.5 and 4.8 V. AC impedance spectra were also collected during cycling. For characterization of solid electrolyte interface (SEI) composition, attenuated total reflection FTIR combined with X-ray photoelectron spectroscopic (XPS) analyses were conducted.

Figure 1a compares the cycling ability of Li_1.2Mn_0.525Ni_0.175Co_0.1O₂ cathode without and with additive. With additive, the initial charge and discharge capacities are 350 and 256 mAh/g, respectively, with initial coulombic efficiency of 73%. The cathode delivers the capacity retention of 89% with the discharge capacity of 227 mAh/g at the 50^th cycle. On the contrary, without additive, inferior capacities of 222 – 156 mAh/g and capacity retention of 70% over 50 cycles are observed. The use of additive is found to be very effective in enabling high-voltage cycling performance of Li_1.2Mn_0.525Ni_0.175Co_0.1O₂ cathode. Our spectroscopic surface chemistry studies suggest that with additive, the cathode surface is effectively passivated with a stable SEI layer with maintained surface cathode structure (Figure 1b-iii), leading to a suppressed change in charge transfer resistance with cycling. On the contrary, the occurrence of surface structural degradation by the formation of dissolvable Mn²⁺proably followed by oxygen loss is observed when cycled without additive (Figure 1b-ii). Further discussion of the SEI formation mechanism and stability, their correlation to interfacial resistance and cycling performance, and the cycling performance of full-cells would be presented in the meeting.

Acknowledgements: This research was financially supported by the Korean Ministry of Education and National Research Foundation (2012026203) and by the Ministry of Trade, Industry & Energy (A0022-00725).