Short communicationMultilayer electrolyte cell: A new tool for identifying electrochemical performances of high voltage cathode materials
Graphical abstract
Introduction
The high operation voltage of the positive electrode can be beneficial for electric vehicle (EV) applications because it can reduce the number of cells in a pack and thereby lower overall costs. The LiNi0.5Mn1.5O4 spinel (~ 4.7 Vvs.Li) is one of the promising high voltage positive electrode materials for next generation Li-ion batteries with a good power performance. However, a major challenge for the implementation of LiNi0.5Mn1.5O4 for EV applications is the instability of standard electrolytes at high voltages (> 4.5 Vvs.Li) [1]. In addition, LiNi0.5Mn1.5O4 spinel suffers from the “Mn dissolution” problem that is detrimental to the cycle life of full-cells with graphite negative electrodes [2], [3], [4], [5]. The poor cycle life observed in full-cells has been understood to occur by the loss of active Li+ through continuous SEI formation (electrolyte reduction) promoted by Mn reduced on top of the graphite's surface [4], [6], [7]. Unfortunately, the impact of Mn dissolution on the overall capacity fading in the full-cell has not been gauged properly because it is difficult to separate the influence of reductive and oxidative electrolyte decomposition using traditional cell designs (i.e. coin cells).
In this study, a multilayer electrolyte cell (MEC) was designed and developed as a new tool for investigating electrode/electrolyte interfacial reactions in a battery system. The MEC consists of two liquid electrolytes (L.E.) separated by a solid electrolyte (S.E.) which selectively transports Li+ ions illustrated in Fig. 1 (a). This design offers the benefit of isolating the individual positive and negative electrode/electrolyte interfacial reactions in a single cell which cannot be realized by conventional cell designs (i.e. coin cell). Here, the stability of battery performance using the MEC was examined using LiFePO4 as the positive electrode material. In addition, attempts to identify the origin of capacity fading and the impact of Mn dissolution in a LiNi0.5Mn1.5O4/graphite full cell were studied and the potential applications for MEC were demonstrated. Finally, we identify the benefits of MEC as a tool for future electrode/electrolyte interface or materials studies.
Section snippets
Experimental methods
Fig. 1(b) shows a schematic diagram of the MEC. The cell was made with a polypropylene body which contains L.E., electrodes, and current collectors (stainless steel rods at each side). A dense ceramic S.E., Li1 + x + yTi2-xAlxP3-ySiyO12 (LTAP, OHARA, Inc.), with a thickness of 150 μm was placed between the L.E.s and completely sealed in order to prevent any electrolyte and any potential reduction or oxidation products to crossover while selectively transporting Li+ ions. Both regions of L.E. used 1 M
Results and discussion
Fig. 1 illustrates a schematic diagram and design of the MEC. In Fig. 1(a), traditional L.E.s have their highest occupied molecular orbital (HOMO) located above the electrochemical potential of the LiNi0.5Mn1.5O4spinel positive electrode. They also have their lowest unoccupied molecular orbital (LUMO) located below the potential of graphite. As a result, both electrolyte oxidation and reduction will occur in the LiNi0.5Mn1.5O4/L.E./graphite full-cell. However, the passivation layer (SEI) formed
Conclusion
The MEC successfully demonstrated its capability of blocking the migration of Mn2 + ions and eliminating the effect of the “Mn dissolution” problem on the cell's electrochemical performances. This result instructs that incorporation of a S.E. in the Li-ion battery cell may solve the Mn dissolution problem. In addition, the MEC can provide additional benefits as a tool for future electrode/electrolyte interface studies with following examples; a) Since the MEC is capable of employing different
Acknowledgements
This work was supported by Funding Opportunities for Research Commercialization and Economic Success (FORCES) at Indiana University Purdue University Indianapolis (IUPUI).Authors would like to thank Bob R. Powell and Mark W. Verbrugge in Chemical & Materials Systems Laboratory in GM R&D for many helpful discussions.
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Capacity degradation in commercial Li-ion cells: The effects of charge protocol and temperature
2019, Journal of Power SourcesCitation Excerpt :For the electrochemical active materials, structural disordering, surface/loss reconstruction, metal dissolution, and SEI formation can all lead to capacity loss and likely occur in parallel during cycling. More recently published evidence shows that there is an interaction or “crosstalk” between the anode and cathode which is mediated by the electrolyte and exists in full Li-ion batteries; these complex interactions result in a significant negative impact on the long-term performance of Li-ion batteries [15–27]. The anode/cathode interaction can be divided into categories including (1) the dissolution of metal ions from the cathode that migrate to the anode surface, catalyzing SEI formation on the anode [15,20,24–26], and (2) electrolyte decomposition products generated at the largely delithiated cathode/electrolyte interface that diffuse/migrate to the anode surface, which can also result in precipitate/SEI formation [16,18,19,21,22].