New high-voltage electrolyte additive supports high energy density and stability in LMNC Li-ion battery; 2x energy density over LiCoO2

18 December 2014

Discharge capacity and cycle numbers for LMNC cathode with and without DFDEC in the electrolyte. Pham et al. Click to enlarge.

A team led by researchers at Chungnam National University (S. Korea) has developed a novel high-voltage electrolyte additive, di-(2,2,2 trifluoroethyl)carbonate (DFDEC), for use with the promising lithium-rich layered composite oxide high-energy cathode material xLi₂MnO₃·(1-x)LiMO₂ (M = Mn, Ni, Co).

In a study reported in the Journal of the Electrochemical Society, the team, led by Professor Seung-Wan Song, operated a 0.6Li₂MnO₃·0.4LiNi_0.45Co_0.25Mn_0.3O₂ (Li_1.2Mn_0.525Ni_0.175Co_0.1O₂, LMNC) cathode at 2.5–4.8 V with 5 wt% of the fluorinated linear carbonate DFDEC as an additive. The cathode with DFDEC-enhanced electrolyte outperformed that in electrolyte only, delivering a high capacity of 250 mAhg⁻¹ with an excellent charge-discharge cycling stability at the rate of 0.2C. A full cell based on the LMNC cathode and graphite anode successfully demonstrated doubled energy density (∼278 Wh kg⁻¹) compared to ∼136 Wh kg⁻¹ of a commercialized cell of graphite/LiCoO₂ as well as an excellent cycling stability.

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Proposed interfacial reaction and SEI formation mechanisms of the Li_1.2Mn_0.525Ni_0.175Co_0.1O₂ cathode during high-voltage (4.8 V) cycling in (top right) electrolyte only and (bottom right) with DFDEC additive.

The researchers determined that with the use of DFDEC, the cathode surface is effectively passivated by a stable SEI composed of DFDEC decomposition products, which inhibit a detrimental metal dissolution and structural cathode degradation. Pham et al. Click to enlarge.

Enabling the high-energy and safety lithium-ion battery requires the development of high-capacity and high-voltage cathode, high-capacity anode and accordingly functional electrolyte with high voltage stability, interfacial compatibility with electrodes and safety. Li-rich layered composite oxides, represented xLi₂MnO₃·(1-x)LiMO₂ (M = Mn, Ni, Co), has been appealing as high-energy cathode materials because of a possible high specific capacity as much as or higher than 250 mAhg⁻¹ on the high-voltage operation above 4.6 V vs. Li/Li⁺, which can provide enhanced energy density compared to ∼136 Wh kg⁻¹ by LiCoO₂ in a commercial battery. Cycling stability, rate capability and safety are however often arduous to achieve at high-voltage operation (4.3 V vs. Li/Li⁺), due to structural instability at a highly charged (Li+- deintercalated) state and voltage fade with cycling, and in particular, serious oxidative decomposition of conventional electrolyte.

Enabling the high-energy lithium-ion battery with high-voltage cathode relies on an electrolyte breakthrough and the SEI stabilization. We have been searching for and screening a number of fluorinated linear carbonates as high-voltage electrolyte additives … The use of an additive rather than solvent comprises a low cost. … In this work, we report high-performance of the half-cell and full-cell with 4.8 V LMNC cathode and a novel electrolyte additive of di-(2,2,2 trifluoroethyl)carbonate (DFDEC) for the first time for a high-energy lithium-ion battery.

Cycling in electrolyte without the additive resulted in a structural degradation from surface to bulk, particle cracking, metal dissolution and oxygen loss, all of which degrades performance.

The team’s surface chemistry studies indicated that DFDEC leads to the formation of a stable SEI consisting of its decomposition, together with electrolyte decomposition products, which let the cathode withstand high-voltage operation to 4.8 V.

Resources

• Hieu Quang Pham, Kyoung-Mo Nam, Eui-Hyung Hwang, Young-Gil Kwon, Hyun Min Jung, and Seung-Wan Song (2014) “Performance Enhancement of 4.8 V Li_1.2Mn_0.525Ni_0.175Co_0.1O₂ Battery Cathode Using Fluorinated Linear Carbonate
  as a High-Voltage Additive,” Journal of The Electrochemical Society, 161 (14) A2002-A2011 doi: 10.1149/2.1141412jes]