A study of methyl phenyl carbonate and diphenyl carbonate as electrolyte additives for high voltage LiNi0.8Mn0.1Co0.1O2/graphite pouch cells
Introduction
Rechargeable lithium ion batteries (LIBs) are widely used in portable electronics. However, safer, longer-lasting LIBs with higher energy density and lower cost need to be developed in order to meet the increasing demand for applications such as electric vehicles and large scale stationary energy storage [1], [2]. One way to achieve higher energy density is increasing the specific capacity and average potential of the cells [3]. Therefore, looking for new positive electrode materials that have high energy density is important.
The layered lithium Ni-Mn-Co oxides Li1+x(NiyMnzCo(1-y-z))1-xO2 (NMC) are considered to be promising positive electrode materials [4], [5], [6], [7]. The Ni-rich oxide LiNi0.8Mn0.1Co0.1O2 (NMC811) can deliver a high capacity of ∼200 mAh g−1 with an average discharge potential of ∼3.8 V (vs. Li+/Li) over a narrow potential range of ∼3–4.3 V (vs. Li+/Li), which has drawn great interest [8], [9], [10], [11]. However, cells cycled under high-voltage conditions can suffer electrolyte oxidation which can causes gas generation and impedance growth among other problems, thus causing capacity fading during cycling [12], [13]. The use of electrolyte additives has been shown to be one of the most effective ways to improve cycle life, calendar life and safety of Li-ion batteries [14].
Phenyl carbonates, such as methyl phenyl carbonate (MPC) and diphenyl carbonate (DPC), are very promising electrolyte additives. Petibon et al. [15] showed that adding phenyl carbonates to Li[Ni0.33Mn0.33Co0.33]O2/graphite cells could provide similar cycling performance to cells containing 2% vinylene carbonate (VC) but that cells with phenyl carbonates showed lower impedance after cycling tests. Another advantage is the low price of DPC compared with VC. The results suggested that the use of phenyl carbonates as additives can help yield Li-ion cells with long lifetime, high power density and reduced manufacturing cost. Recently, in a major study of over 110 electrolyte additive combinations tested in NMC111/graphite pouch cells, Wang et al. [16] showed that ternary electrolyte additive blends containing one or more of VC, methylene methane disulfonate (MMDS) and tris(-trimethyl-silyl)-phosphite (TTSPi) gave additional improvements to cell performance under challenging conditions. Table 1 shows selected ternary additive combinations that were used as additives in the electrolytes in this work.
In this work, two phenyl carbonates including MPC and DPC were studied as electrolyte additives in NMC811/graphite pouch cells. Fig. 1(a) and (c) showed the molecular formula and structures of MPC and DPC, respectively. Experiments were carried out using Ultra High Precision Coulometry (UHPC) [17] an automated storage system [18], an in situ gas evolution apparatus [19], electrochemical impedance spectroscopy (EIS) as well as long-term cycling tests. Gas evolution during formation and cycling, coulombic efficiency (CE) and charge end point capacity slippage as well as EIS spectra before and after UHPC cycling were examined and compared. The results obtained clearly demonstrate significant improvements in electrochemical performance of cells cycled to 4.3 V when phenyl carbonate electrolyte additives are used.
Section snippets
Experimental
1 M LiPF6 ethylene carbonate (EC)/ethyl methyl carbonate (EMC) (3:7 wt% ratio, BASF) was used as the control electrolyte in the studies reported here. To this electrolyte, the additives vinylene carbonate (VC, BASF, 99.97%), diphenyl carbonate (DPC, Aldrich, 99.97%), methyl phenyl carbonate (MPC, Alfa Aesar, 97.20%), 1, 5, 2, 4-dioxadithiane-2, 2, 4, 4-tetraoxide (MMDS, Tinci Materials Technology, >98.7%), and/or tris(trimethylsilyl) phosphite (TTSPi, Aldrich, ≥95.0%) were added either singly
Results and discussion
Fig. 1(b) and (d) show the differential capacity (dQ/dV) vs. V curves of NMC811/graphite pouch cells with different concentrations of VC, MPC, and DPC during the formation cycle to the first degassing point (3.5 V). The peaks in (dQ/dV) vs. V curves can help to determine at which potential the additives initially react with the partially lithiated graphite. Fig. 1b shows that control cells have a peak at about 2.9 V, corresponding to a potential of ∼ 0.8 V vs. Li/Li+, which relates to the
Conclusions
The two phenyl carbonate electrolyte additives DPC and MPC were studied in NMC811/graphite pouch cells. The results of storage, high precision coulometry, AC impedance spectroscopy, gas measurements and long-term cycling experiments at 4.3 V were compared and considered. The conclusions are summarized as follows:
- 1.
Both DPC and MPC showed no obvious reduction peak during formation. Compared with control cells, both DPC and MPC decreased the impedance after formation. DPC decreased the gas
Acknowledgment
The authors acknowledge the support of this work by 3M Canada and NSERC under the auspices of the Industrial Research Chairs program. Wenda Qiu acknowledges the support by grants from National Basic Research Program of China (973). The authors thank Dr. Jing Li of BASF for providing most of the solvents, salts and additives used in this work.
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