1,3-Propanesultone as an effective functional additive to enhance the electrochemical performance of over-lithiated layered oxides

Abstract

Over-lithiated layered oxides (OLOs) are one of the promising positive electrode (PE) materials, however, the poor cycle-life of OLOs has to be resolved in order to make OLO cells available. In this work, several kind of additives are investigated to enhance the interfacial stability and the most efficient additive is 1,3-propanesultone (PS). According to spectroscopic analysis, it is found that the surface film derived from PS is effective in suppressing both metal dissolution and undesired reactions of the electrolyte on the PE, which results in remarkably enhanced cycle performance of the OLO electrode.

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Introduction

Lithium-ion batteries (LIBs) are attractive prospective energy storage devices, since they possess high energy, excellent cycle performance, and good rate capability.^1–3 Although LIBs have superior energy density compared to other battery types, there is a need to further improve the energy density of current-generation LIBs for use in electric vehicles.^4,5 In this respect, over-lithiated layered oxides (OLOs) are attractive positive electrode (PE) material candidates, based on their high operating potential (over 4.5 V) and remarkably large specific capacity (over 250 mA h g⁻¹).^6–8 These outstanding properties are beneficial for the overall energy density of the LIBs and will drive the development of high energy-demand applications. However, there are some drawbacks to these high operating potentials, such as electrode deterioration by undesired electrolyte decomposition and severe metal dissolution.^9,10 Proper control of interface is necessary to realize the expected performance benefits of the OLO PEs because the instability of the interface between the PE and electrolyte results in poor cycle performance.^11,12

In this work, we investigated the influence of various functional additives on the interfacial stability between the electrolyte and PEs. Although additive-based approaches have previously been reported for the stabilization of the interfaces of negative electrodes (NEs)^13–15 to the best of our knowledge, only limited literature is available to demonstrate the effect of additives on the electrochemical performance of OLO PEs operating at high potential (>4.5 V, vs. Li/Li⁺).¹⁶ Furthermore, verification on the role of these additives has not been actively performed, even though this would provide helpful insights into the design of effective electrolyte systems for OLOs. Therefore, it is believed that this study provides clues to the selection of appropriate electrolyte systems for OLO-PE-based LIBs through the fine characterization of the surface chemistry of the OLO PEs in various electrolytes.

Result and discussion

Additive candidates are categorized according to chemical functionality: vinylene carbonate (VC, with olefin groups), succinonitrile (SN, with nitrile groups), and 1,3-propanesultone (PS, with sultone groups) (Fig. 1a). According to the linear sweep voltammetry analysis shown in Fig. 1b, all additives decompose earlier than the base electrolyte. In particular, the electrolytes containing additives showed pronounced increases in the oxidation current (corresponding to the electrochemical reaction of the additives) until 4.3 V followed by stabilization of the oxidation current, whereas considerably continuous increase in the oxidation currents is observed in the electrolyte without any additive. This means that the selected additives may effectively cover the surface of the PE before the main solvent decomposes during the initial charge process.†

Fig. 1 (a) Chemical structure of additive candidates, (b) linear sweep voltammetry (LSV) of electrolyte with additives.

The effect of additives on the cycle performance was investigated with coin-type half-cells containing electrolytes with 5 wt% VC, SN, or PS additions (Fig. 2a–c). The initial specific capacity and shape of the voltage profiles of the various cells are nearly identical (around 200 mA h g⁻¹), however, the capacity retention of cells with different additives diverge as the number of cycle increases. In fact, some cells show even worse cycle results than those with no additive. The cells with PS additive and the additive-free cells show a good retention rate after 30 cycles (PS: 93.9%, additive-free: 91.6%) compared to the other cells incorporating electrolyte additives (86.4% for VC and 65.1% for SN). The voltage profiles obtained with the different additives at 30^th cycles show quite different behaviors: the cells cycled with VC and SN show larger polarizations than the corresponding analogues, resulting in a lower remaining specific capacity. Similar but distinct behaviors were observed in the graphite/OLO full cells shown in Fig. 2d. The PS-containing cell holds more than 83.5% of its initial capacity after 80 cycles, whereas the others show relatively rapid degradation. The cell without any additive showed 73.9% capacity retention, and the cells with VC and SN additives exhibited retentions of 55.7% and less than 30%, respectively. This indicates that the choice of additive is a significant factor determining the electrochemical performance of OLO cathodes, because the additive changes the interfacial chemistry.

Fig. 2 (a) Voltage profiles at initial cycle (half-cell), (b) voltage profiles at 30^th cycles (half-cell), (c) half-cell performance of electrolyte with additives, (d) full-cell performance of electrolyte with additives.

The surface morphology of fresh and cycled PEs was analyzed to determine the reason for the dependence of the electrochemical behavior on additive type, as shown in Fig. 3a. No noticeable morphological changes were observed after 50 cycles, regardless of the electrolyte composition. Unlike the morphology results, the results of the ICP-MS analysis show a great dependence of the stability of the PEs on the different additives. Each of the electrolytes was retrieved from the fully charged cells and stored at an elevated temperature (60 °C) for 7 days (Fig. 3b). The behavior during high-temperature storage could be directly correlated to the cycle performance of cells: the amount of metal dissolution is inversely proportional to the capacity retention. The cell containing the PS additive shows the lowest metal dissolution (under 20 ppm) among all of the stored samples.

Fig. 3 (a) SEM analysis after 50^th cycles depending on additives (fresh electrode, cathode with no additive and cathode with PS), (b) ICP-MS analysis of electrolytes after storage tests at 60 °C.

To investigate the interfacial chemistry, PEs exposed to different additives were analyzed by FT-IR after the 1st cycle (Fig. 4a). The common components of SEI (solid electrolyte interface) from the main solvents (EC, EMC, and LiPF₆) were observed from the IR spectra: polycarbonate (C=O, 1807 cm⁻¹ and 1776 cm⁻¹), alkyl carbonate (RCO₂R, 1714 cm⁻¹, 1653 cm⁻¹, 1560 cm⁻¹, 1539 cm⁻¹ and 1404 cm⁻¹), and lithium carbonate (Li₂CO₃, 1362 cm⁻¹).^17–19 In addition, independent transmittance signals were also found to depend on additive type: the electrolyte with VC added has a distinct peak of the polymeric olefin group at 1508 cm⁻¹,^17,20,21 the electrolyte with SN shows polymeric C–N connectivity at 1165 cm⁻¹,^17,21 and finally the electrolyte with PS shows an alkyl sulfonate group (RSO₂R) at 1020 cm⁻¹.^17,22,23 For further analysis, XPS experiments were performed (Fig. 4b) on cycled PEs with PS treatments. The surface of PEs exposed to PS clearly indicates the existence of alkyl sulfonate groups at 169.5 eV, which is apparent evidence for the formation of a surface film on the PE.²⁴ In addition, FT-IR spectra of the cathode obtained after the 50^th cycle provide evidence to verify the effect of the PS additive on the interfacial stability between the electrolyte and OLO cathode. In contrast to the cells cycled with VC and SN, for which the signals for the main surface film components observed in the initial cycles disappeared (polymeric olefin at 1508 cm⁻¹ and polymeric C–N at 1165 cm⁻¹), the cell cycled with PS still clearly contains the alkyl sulfonate component formed by electrochemical oxidation of PS after 50 cycles. This difference in the FT-IR spectra can be attributed to the continuous electrolyte decomposition for the cells cycled with VC and SN. In particular, it is speculated that the surface film derived from the VC and SN is not stable enough to suppress further electrolyte decomposition, resulting in accumulation of the decomposed products on the cathode surface. This leads to the disappearance of the surface film component signals in the FT-IR spectra after 50 cycles. Consistent results were also obtained in the XPS analysis of the cathode cycled with PS: alkyl sulfonate chemical functionality is still present on the cathode surface even after 50 cycles (Fig. 4d). The voltage profiles of the cells with VC and SN after cycling is another evidence to support our speculation and it is shown in Fig. 2b. The cells cycled with VC and SN showed considerable polarization in their voltage profiles, which is induced probably by increase in interfacial resistance as a result of accumulation of the electrolyte decomposition products on the cathode surface.^25–27 Furthermore, recent reports provide informative clues to support this speculation. First, the possible reason of accelerated degradation of a cell consisting of OLO and graphite was found to be the rise in impedance with voltage fading at the positive electrode due to electrolyte decomposition at the cathode–electrolyte interface.^28,29 Second, the VC additive was found causing poor cycle life, especially at a high operating voltage, because irreversible oxidation of VC caused the cathode surface to be covered with the decomposition products of VC, which may disturb the facile charge transport at the cathode–electrolyte interface.³⁰ These results indicate that the compatibility of the additive for each positive electrode is a very important for determining the cycle performance of the cell. Therefore, the enhanced cycle performance of the OLO is attributed to the enhanced interfacial stability obtained by the alkyl sulfonate component of the surface film. It is concluded that the surface stability of the OLO cathode is strongly influenced by the functional additives, which directly affect the electrolyte decomposition and metal dissolution by forming surface films on the PEs.

Fig. 4 (a) FT-IR analysis of cathodes after 1st cycle (full-cell), (b) XPS analysis of cathodes after 1st cycle (full-cell), (c) FT-IR analysis of cathodes after 50^th cycles (full-cell), (d) XPS analysis of cathode after 50^th cycles (full cell).

Conclusions

Three kinds of functional additives: vinylene carbonate (VC), succinonitrile (SN), and 1,3-propanesultone (PS) were incorporated into the electrolytes of electrochemical cells to examine their compatibility with the over-lithiated layered oxide (OLO). According to an analysis of the surface properties by spectroscopic tools, electrochemical performance is highly dependent on chemical functionality in additives. The most effective additive out of three is PS, and the cells with a PS additive show excellent capacity retention after full-cell evaluation (83.5% remaining discharge capacity at 80^th cycles) and a low degree of metal dissolution (under 20 ppm). The alkyl sulfonate group originated from PS is effective to inhibit electrolyte decomposition and metal dissolution, resulting in a remarkable enhancement of the cycle performance of the OLO electrode.

Acknowledgements

This work was supported by a grant from the Fundamental R&D Program for Technology of World Premier Materials funded by the Ministry of Trade, Industry and Energy, Republic of Korea (Project number: 10037921).

Notes and references

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Footnote

† The battery-grade reference electrolyte and electrolytes containing additives were provided by PanaxEtec (Korea). They were used as received without further purification. The anodic stability of the additives was determined by linear sweep voltammetry (LSV) using a glassy carbon as a working electrode and lithium foil as a counter and reference electrode, respectively.

The anode and cathode for the electrochemical testing of the additives were prepared as follows. To prepare the anodes, a mixture of graphite (Poscochemtech, Korea), carbon black (Super P), carboxymethyl cellulose (Cellogen, DKS), and styrene-butadiene rubber (SBR, BM 400B, Zeon) were mixed in a weight ratio of 96 : 1 : 1 : 2 and finely dispersed in distilled water. The resulting slurry was coated on a piece of copper foil, and this electrode plate was dried in a vacuum oven at 120 °C for 12 h. The loading and electrode density were fixed at 8.45 mg cm⁻² and 1.50 g cm⁻³, respectively. To prepare the cathode, a mixture of over-lithiated oxides (0.5Li₂MnO₃·0.5LiNi_0.4Co_0.2Mn_0.4O₂, Ecopro, Korea), polyvinylidene fluoride (KF1100, Kureha), and carbon black (Super P) were mixed in a weight ratio of 92 : 4 : 4 and dispersed in N-methyl-2-pyrrolidone. The resulting slurry was coated on a piece of aluminum foil, and this electrode plate was dried in a vacuum oven at 120 °C for 12 h. The loading and electrode density were fixed at 14.45 mg cm⁻² and 2.95 g cm⁻³, respectively.

Galvanostatic discharge–charge cycling was performed using both a coin-type cell (2032, for half-cell test) and a pouch-type cell (34 mm × 50 mm in size, for full-cell test). The half-cell and full-cell were assembled from anodes of Li metal and graphite, respectively, a cathode, a separator, and an electrolyte mixture of ethyl carbonate (EC)–ethyl methyl carbonate (EMC) = 1 : 2 (vol%), 1 M LiPF₆, and 5% of each additive (VC, SN and PS). The half-cells were galvanostatically charged to 4.6 V vs. Li/Li⁺ and discharged to 3.0 V vs. Li/Li⁺ repeatedly at a constant current of 0.5 C at room temperature using a TOSCAT-3100 charge/discharge unit (TOYO). The full-cells were galvanostatically charged to 4.5 V and discharged to 3.0 V repeatedly at a constant current of 0.5 C at room temperature using a TOSCAT-3100 charge/discharge unit.

The surface morphologies of the PEs were observed using FESEM on a Quanta 3D FEG (FEI). The surface characterization of the cycled PE was performed using FT-IR (VERTEX 70, Bruker) in the attenuated total reflection mode and XPS (K alpha, Thermo-Scientific) in a N₂ atmosphere in a dry room with a dew point around −60 °C. To quantify the degree of metal dissolution, the three-electrode beaker cells fabricated with the various electrolyte–additive mixtures were fully charged to 4.6 V (vs. Li/Li⁺). PEs were retrieved from each cell and stored in a corresponding fresh electrolyte–additive solution at 60 °C for 7 days. The remaining supernatants were collected and the metal content of the electrolyte was analyzed using ICP-MS (Aurora M90, Bruker).

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